Excitations of torelon

和宇航员对话想问的问题小学英语作文全文共6篇示例，供读者参考篇1Questions I'd Like to Ask an AstronautEver since I was a little kid, I've been fascinated by space and the idea of exploring the unknown vastness that lies beyond our planet. The thought of venturing out into the cosmos, leaving Earth's cozy embrace, and experiencing the wonders of the universe firsthand is both thrilling and daunting. If given the chance to have a conversation with an astronaut, there are so many questions I would love to ask them.First and foremost, I would inquire about their journey to becoming an astronaut. What inspired them to pursue this extraordinary career path? Was it a lifelong dream, or did the passion develop later in life? I'd be curious to learn about the rigorous training process they underwent, the challenges they faced, and the sacrifices they had to make along the way. Becoming an astronaut is no easy feat, and I'd love to gain insight into the dedication and perseverance required to achieve such a lofty goal.Next, I would ask them to describe the experience of liftoff and the moments leading up to it. Can they even put into words the mixture of excitement, fear, and anticipation they must have felt as the countdown reached zero and the powerful rockets ignited beneath them? I imagine the sheer force and magnitude of the launch must be utterly indescribable. I'd love to hear their personal accounts of those heart-pounding moments when they left Earth's embrace and embarked on their cosmic voyage.Once in space, the questions would only multiply. What was their first impression upon gazing at our beautiful, blue planet from afar? Did it fill them with a newfound appreciation for the fragility and preciousness of our world? I'd be curious to know if the experience of seeing Earth from such a unique vantage point changed their perspective on life or instilled a deeper sense of responsibility towards protecting our planet.I would also inquire about the challenges of living and working in the microgravity environment of space. How did they adapt to the absence of gravity, and what tasks or everyday activities proved to be the most difficult? Did they experience any unexpected physical or psychological effects during their time in space? Understanding the realities of life in such an alien environment would be fascinating.Inevitably, the conversation would turn to the future of space exploration and the role humans might play in it. I'd ask for their thoughts on the potential for long-term human habitation on other planets or moons, such as Mars or the lunar surface. What challenges and obstacles would need to be overcome to make such endeavors possible? And what would be the broader implications and benefits for humanity if we were to establish a permanent presence beyond Earth?Furthermore, I'd be curious to hear their perspectives on the search for extraterrestrial life. Do they believe we are alone in the universe, or is the existence of other intelligent life forms a real possibility? If alien life were discovered, what might that mean for our understanding of the cosmos and our place within it?Beyond the scientific and technological aspects, I would also inquire about the personal and emotional impact of their experiences in space. Did the breathtaking vistas and the vastness of the universe instill a sense of humility or insignificance? Or did it perhaps reinforce the idea that篇2Questions I'd Love to Ask an AstronautEver since I was a little kid, I've been fascinated by space exploration and astronauts. I used to stare up at the night sky in awe, imagining what it would be like to float weightlessly among the stars. Astronauts are like real-life superheroes to me - brave men and women who venture into the unknown depths of the cosmos.If I ever got the chance to talk to an astronaut, there are so many questions I would ask them. I've tried to imagine what their incredible experiences must be like, but I'm sure the reality is far beyond what I can even picture in my mind.First off, I would ask them what it felt like when they first left the Earth's atmosphere and saw our beautiful planet from space. Did it take their breath away? Did they feel a sense of wonder at how tiny and fragile Earth looks from that vantage point? I've seen the amazing photos, but I can only imagine howmind-blowing it must be to witness that view with your own eyes.Speaking of photos, I'd love to know what it's like to look out into the inky blackness of space and see stars and galaxies that we can barely make out from Earth with our best telescopes. Is it as breathtaking as it seems, or do astronauts almost take thatincredible view for granted after a while since it's part of their daily routine in space?I'm also super curious about what the experience of being in a rocket during launch feels like. Is it terrifying, exhilarating, or a mixture of both? Do astronauts get intense adrenaline rushes or does their training help them stay calm through all those intense g-forces? I can't even fathom what it must feel like to be strapped into a rocket and then blasted off into the sky.Another thing I've always wondered about is how astronauts adapt to living in zero gravity conditions for extended periods. Is it really disorienting at first, or do they get used to it quickly? What's it like to be able to float around freely instead of being weighed down by gravity? I imagine even simple tasks like eating and drinking must be totally different experiences.I'd be really eager to learn about the day-to-day routines and schedules that astronauts follow on long space missions. How do they balance work responsibilities with personal time and rest? Do they get opportunities to do fun activities like watch movies or read books? Or does the sheer wonder of being in space trump the need for typical entertainment?On a more serious note, I'm really interested in learning about the vital scientific research and experiments astronautsconduct while in space. Whether it's studying things like radiation's effects on the human body or unlocking mysteries about the formation of the universe, their work is crucial to expanding our knowledge. I'd ask what types of studies they're working on and how those further humanity's understanding of our place in the cosmos.Another thing I'm dying to know is what astronauts enjoy the most about being in space, and what are the biggest challenges or things they struggle with. Is the exhilarating feeling of weightlessness addictive, or do they end up missing gravity after a while? What's the hardest part about being confined to a relatively small space station for months at a time? I imagine being an astronaut takes an immense mental and physical toll, in addition to all the rewards.I'd probably have to fight back happy tears if I got to ask astronauts what their most awe-inspiring or profound moments have been while in space. Whether it's witnessing a spectacular cosmic event, enjoying a deepened appreciation for our planet, or just being floored by the majesty of the universe around them, I'm sure they must have experiences that change their perspectives forever. Those stories could be life-changing for us Earthlings to hear.Finally, I might ask if spending time in space and seeing Earth from that ultra-rare vantage point has affected their philosophies or ways of looking at life. Does being an astronaut fundamentally transform a person's worldview or make them think about our existence in新的ways? I'd be honored to gain that type of perspective and insight.Those are just a few of the bajillion questions I'd love to ask an astronaut if given the chance. While I'll probably never get to experience space exploration myself, I'll always be in awe of those who do. Talking to a real-life spacefarer would be a dream come true and one of the coolest experiences imaginable for us starry-eyed Earth kids. Astronauts are篇3Questions I'd Love to Ask an AstronautEver since I was a little kid, I've been fascinated by space and the idea of exploring the unknown vastness that lies beyond our Earth. The thought of venturing into the great cosmic frontier has always filled me with a sense of wonder and excitement. And who better to learn about this incredible journey than from the brave men and women who have actually experienced it firsthand – the astronauts?If I ever had the chance to meet and chat with an astronaut, there are so many questions I'd love to ask them. First and foremost, I'd be curious to know what inspired them to pursue this extraordinary career path. Was it a lifelong dream or a passion that developed later in life? What motivated them to take on such a daring and challenging profession?I'd also be eager to learn about the rigorous training process they had to undergo. How did they prepare both physically and mentally for the harsh conditions of space travel? What were some of the most difficult or grueling aspects of their training? And how did they cope with the immense stress and pressure that must have accompanied such intense preparation?I'd want to know about the challenges and difficulties they faced during their missions. Were there ever any harrowing moments or close calls that tested their resolve and training? How did they handle the intense confinement of living and working in such a small space for extended periods? And what was it like to perform complex tasks and experiments in the unique environment of zero gravity?Naturally, I'd be incredibly curious about the sights and experiences that left the most lasting impressions on them. What was the most breathtaking celestial phenomenon they witnessed?Did they have any profound or life-changing realizations while gazing down at our fragile planet from above? And were there any humorous or lighthearted moments that helped break the tension or monotony of their missions?Beyond the personal experiences, I'd be eager to pick their brains about the broader implications and future of space exploration. What do they see as the most promising avenues for future research and discovery? Do they believe manned missions to other planets or even beyond our solar system could become a reality in our lifetimes? And what role do they think the private sector and commercial space industry might play in driving these endeavors forward?As someone who has spent countless hours gazing up at the night sky and dreaming of the wonders that lie among the stars, the opportunity to speak with an astronaut would be an absolute dream come true. Their firsthand experiences and insights would not only satisfy my boundless curiosity but also provide invaluable inspiration and guidance for anyone aspiring to follow in their footsteps.While the chances of such an encounter may be slim, I can't help but imagine the captivating stories and incredible tales an astronaut would have to share. From the breathtaking beauty ofEarth suspended in the inky blackness of space to the challenges and triumphs of exploring the great unknown, their perspectives and wisdom would undoubtedly leave a lasting impact on my life and further fuel my passion for space exploration.So, to any astronauts out there reading this, know that there are countless dreamers and stargazers like myself who would jump at the chance to pick your brains and learn from your extraordinary experiences. Your journeys have not only expanded the boundaries of human knowledge but also captured the imagination of countless people around the world. And for that, we are forever grateful and in awe of your bravery and accomplishments.篇4Talking to an Astronaut: My List of QuestionsEver since I was a little kid, I've been fascinated by space. I have a telescope that I use to gaze up at the stars and planets on clear nights. I read every book about space I can get my hands on from the library. And my biggest dream is to become an astronaut someday and travel to the Moon or even Mars!That's why I was so excited when my teacher told our class that we would have a very special visitor coming to speak with us- a real astronaut who has actually been to space! I could hardly sit still thinking about all the questions I would ask. After lots of thought, here is my list of questions I hope to ask the astronaut when they visit:What is it like to launch into space aboard the rocket? I've seen videos of rocket launches and it looks both terrifying with all that fire and shaking, but also amazing as you blast off the ground. Does it feel as intense as it looks? Are you strapped in very tightly?Once you get into space, what does it feel like being in zero gravity and floating around? Does it make you feel dizzy or sick at first not having any up or down? Or is it fun to push off and float wherever you want? I've heard some astronauts have a hard time at first until they get their "space legs."How do you go to the bathroom in space? This might seem like a silly question, but I'm really curious! Is it difficult having to go to the bathroom while floating around? Do you have to be strapped into a special toilet? I've heard astronauts have to be really careful or it could be a mess!What is your daily routine like on a space mission? Do you have set times to wake up, exercise, eat meals, do experiments, etc? Or is it more of a go-with-the-flow situation since you're in adifferent environment? I imagine it's really different from normal life on Earth.What is your favorite space food to eat? I've seen videos of astronauts squeezing food out of tubes and adding water to rehydrate freeze-dried ice cream and snacks. Does the food taste pretty good or is it more to just get nutrition? Do you get tired of eating the same things over and over?How do you stay entertained during long stretches of time in the spacecraft? Do you watch movies, read books, play games with your crewmates? I can't imagine how I'd stay occupied being confined to such a small space for weeks or months at a time. I'd probably go stir crazy!What does the Earth look like from space? I've seen beautiful pictures of the Earth from the International Space Station, but I wonder how it appears with your own eyes in person seeing our whole planet floating in the blackness of space. Is it breathtaking or does it start to look normal after a while?What training did you have to go through to become an astronaut? I know it's extremely competitive and difficult, with lots of tests of your mental and physical abilities. Did you have to practice living in simulations of space conditions? Learn all kindsof science and technical skills? I'll have to work really hard if I want that job someday!What is your favorite or most amazing experience you had during a spaceflight? Seeing a sunrise from orbit? Doing a spacewalk? Watching a rocket launch from space? I can only imagine how incredible everything must look and feel so different from anything on Earth.What advice would you give a kid like me who dreams of becoming an astronaut? Besides working hard in school at all my science and math classes, what should I be doing now to help achieve that goal? Read certain books? Attend space camps? Look into special programs or internships? I'll do whatever it takes!I have so many other questions too, like what the re-entry through the atmosphere feels like, what items you wish you had brought with you, seeing other planets and stars up close, and so on. But those are my top 10 burning questions for now that I'm hoping to ask the astronaut visitor.I know that being an astronaut is one of the most difficult and dangerous professions in the world. You have to be brave, determined, highly intelligent, and physically fit. But that's my ultimate dream job - to leave Earth and travel amongst the starsand planets. Our astronaut visitor has made that dream a reality, and I'll get to hear all about it straight from their mouth! I can't wait to shake their hand, have them sign my books, and learn everything I can from someone who has experienced the incredible frontier of space. It's going to be out of this world!篇5Questions I'd Ask an AstronautIf I ever got the chance to talk to a real astronaut, there are so many questions I would want to ask them! Being an astronaut and going to space seems like the coolest job in the entire universe. I can't even imagine what it would be like to leave Earth and float around in zero gravity. I bet astronauts have some amazing stories to tell. Here are some of the top questions I would love to ask an astronaut:What does it feel like when the rocket takes off?I've seen videos of rocket launches, and it looks totally intense with all the fire and smoke blasting out. Does it feel as powerful as it looks when you're strapped in ready for liftoff? Is it scary or exciting or both? I remember going on a roller coaster once and getting that feeling of butterflies in my stomach on the first big drop. I can only imagine that feeling gets multiplied by amillion on a rocket launch! Do your insides feel like they're getting scrambled up?What is your favorite thing about being in space?There are so many parts of being in space that seem amazing to me. Is it the incredible views of Earth from the windows? The experience of floating around in microgravity and not feeling your own weight? Setting off on spacewalks and being outside the spaceship with only your suit between you and the vacuum? Maybe it's just the pride and excitement of few people getting to go to space? I want to know what the astronaut's favorite part is.Do you ever get homesick or miss things from Earth?As great as being in space seems, I imagine it would also be really hard in a lot of ways. You're in a tiny space for months, eating dried food, working long hours, and not getting to see your family or pet your dog. Don't astronauts ever just want a break from the routine to go walk in the park, eat a pizza, and sleep in their own bed? What kinds of things from normal Earth life do they miss the most?Have you ever gotten space sickness? What's that like?I read that a lot of astronauts feel sick and throw up when they first get to space because their inner ear gets confused by the lack of gravity cues. That sounds like it would be the worst! Is space sickness kind of like having a really bad stomach flu where you can't keep anything down? Do you eventually get used to it after being up there for a while? I'd hate to be in a confined space feeling sick with nowhere to go!How do you go to the bathroom in space?This one might be a little gross, but I really want to know! Without gravity, how does, you know...everything not just float away? Do you have to use special zero-gravity space toilets? What if you get sick and have a bathroom emergency in the middle of an hours-long spacewalk? Sorry if these questions are maybe too personal, but inquiring young minds have got to know!What's the food like up there?From what I've seen, it doesn't look that appetizing to eat squeeze tubes of applesauce and bite vacuum-sealed pouches. But I guess food has to be packaged in a way that it won't make a mess in zero-g. Is it all just tasteless mush and freeze-dried ice cream? Or do you get any good meals up there? Do you have afavorite space snack or meal? Being stuck with only yucky food for months would definitely be one of the downsides for me.What's your scariest experience or closest call in space?Being an astronaut seems like such an adventure, but also extremely dangerous when you think about all the things that could potentially go wrong millions of miles from Earth. Have they ever had any terrifying malfunctions or close calls with danger? Did emergency training kick in to save the day? I'm sure astronauts have some incredible stories of times when things went wrong but their skills and keeping a cool head got them through it. I want to hear those heart-pounding tales!Is outer space as cool and fun as it seems?At the end of the day, this is the biggest question I have. Being an astronaut and working in space just seems like the most awesome thing a person could ever do. But maybe there are some not-so-exciting parts that make it more like a regular job too. I want to know if living and working in space is asmind-blowingly amazing as I'm picturing it. Or if the realities of the day-to-day work take some of the novelty away after a while.I just hope I'll get to find out for myself someday if I work really hard and make my dreams come true!So those are the main things I would ask if I was face-to-face with an astronaut. Of course, I'm sure I'd have about a million other questions pop into my head once I was there too. It would be like getting to interrogate a real-life superhero or action movie character about their incredible job. An astronaut's stories and experiences have got to be endlessly fascinating. If I ever get the chance, you can be sure I'm going to fill them with so many questions they'll be begging for a break from this curious kid!篇6A Conversation with an AstronautHave you ever looked up at the night sky and wondered what it would be like to travel to space? I certainly have! Getting the chance to talk to a real astronaut would be a dream come true. If I ever got that amazing opportunity, there are so many questions I would love to ask them about their incredible experiences.First off, I would want to know all about what it feels like to actually blast off into space aboard a rocket ship. Does it make your stomach feel funny from the g-forces? Is it scary or exhilarating? I've seen videos of rocket launches, but I can onlyimagine how loud and powerful it must seem in person as those huge engines ignite. I bet the shaking and vibrations are crazy!Once they made it up into orbit, I'd ask them to describe in detail what the Earth looks like from up in space. We've all seen beautiful photos, but it's got to be breathtaking to actually witness it with your own eyes. Does the planet really look relatively small against the vastness of space? Can you make out continents, clouds, and geographic features? I've heard astronauts say the view is life-changing.Speaking of being in orbit, I'd be super curious to know what it feels like to be in a constant state of freefall and weightlessness. The astronaut could demonstrate some fun zero-gravity tricks like letting a ball of floating water hang in the air. I'd ask if it's hard to eat, sleep, and do normal daily activities when you're weightless. Do they ever get nauseated or seasick from the sensation? It seems like it would take some getting used to!If the astronaut had done a spacewalk before, I'd have a million questions about that too. What does it feel like to step outside into the vacuum of space? Is it scary being attached to the spacecraft by just a tether? Looking down and seeing the entire planet below you must be both amazing and terrifying at the same time. I can't even comprehend how cold it must be outthere too - well below freezing! The astronaut could show me what their bulky spacesuit looks like and describe having to rely on it to stay alive.I'd definitely ask the astronaut if they've ever seen any bizarre sights or had any unusual experiences while in space that even they can't explain. You always hear rumors of astronauts reporting unknown objects or strange phenomena. I'd try to get them to spill the beans on any weird stories! Even if nothing too crazy happened, I bet they still have some interesting tales of equipment failures, close calls, or otherworldly sights.For an astronaut who has been to the Moon, I'd ask them to share what that was like in great detail. Hearing firsthand what it's like to actually walk on the lunar surface would be simply amazing. Did they get to look back at the Earth in the sky? How tough was it to maneuver around in their bulky spacesuit on such low gravity? What did they think when they first stepped foot on an entirely different world? I'd love to hear any other cool stories or fun facts about their moonwalk.No conversation would be complete without asking the astronaut about the intense training process they had to go through just to have the opportunity to go to space. I'd ask them to describe some of the craziest simulations or tests theyendured to prepare their bodies and minds. Things like extreme hot and cold scenarios, survival training, living in simulated space habitats, and all the classroom learning they had to undergo first. It must have taken an incredible amount of hard work and dedication!Finally, I'd make sure to ask the astronaut plenty of questions about what first got them interested in space exploration and what inspired their journey to becoming an astronaut. Was it something they dreamed about sincebeing a little kid? Who were their heroes or role models growing up? What advice would they give to young students today who hope to one day work for NASA or go to space themselves? Getting encouragement and wisdom straight from an astronaut would be incredibly motivating.Can you imagine how cool it would be to shake the hand of someone who has traveled to space and experienced the wonders of the cosmos firsthand? An astronaut would have seen and done things that few humans ever will. I'll never forget getting that once-in-a-lifetime chance to ask them anything I wanted about their incredible journey. It would be a day I'd cherish forever!。

介绍太空探索英语作文Space exploration has been a fascinating topic for many and it is a field that continues to grow and evolve with each passing year. Here is an essay on space exploration that you might find interestingTitle The Wonders of Space ExplorationSpace exploration is a field that has captured the imagination of people all around the world. From the first human landing on the moon to the ongoing search for extraterrestrial life the mysteries of the cosmos have always been a source of intrigue and wonder. This essay will delve into the various aspects of space exploration including its history current missions and the potential future of this exciting field.The History of Space ExplorationThe journey into space began in the early 20th century with the development of rocket technology. The Soviet Union made history in 1957 by launching the first artificial satellite Sputnik 1 into orbit. This was followed by the United States first successful satellite launch Explorer 1 in 1958. The race to space intensified culminating in the historic Apollo 11 mission in 1969 where Neil Armstrong and Buzz Aldrin became the first humans to set foot on the moon.Current Missions and AchievementsToday space exploration is no longer limited to the moon. Various space agencies and private companies are working on a range of missions. NASAs Mars Rover missions have been instrumental in gathering data about the red planets geology and potential for past life. The Hubble Space Telescope has provided us with breathtaking images of distant galaxies and nebulae expanding our understanding of the universe.The International Space Station ISSOne of the most significant collaborative efforts in space exploration is the ISS. This space station orbits Earth and serves as a research laboratory for scientists from around the world. It has been continuously occupied since November 2000 and has facilitated numerous scientific experiments including those related to human health and the effects of longterm space travel.The Search for Extraterrestrial LifeThe quest to find extraterrestrial life is another exciting aspect of space exploration. Missions like the Kepler Space Telescope have identified thousands of exoplanets some of which are in the habitable zones of their stars. The search for microbial life within our solar system particularly on Mars and the icy moons of Jupiter and Saturn is ongoing.The Future of Space ExplorationLooking ahead space exploration is poised to make even greater strides. Plans for manned missions to Mars the establishment of lunar bases and the potential for space tourism are all on the horizon. The development of new technologies such as reusable rockets and advanced propulsion systems will make space travel more efficient and accessible. ConclusionSpace exploration is not just about scientific discovery it is about pushing the boundaries of human knowledge and capability. It inspires generations to dream big and strive for a future where the stars are within our reach. As we continue to explore the cosmos we learn more about our place in the universe and the potential for life beyond Earth.This essay has provided a brief overview of the captivating world of space exploration highlighting its historical milestones current endeavors and the promising future that lies ahead. The cosmos holds endless possibilities and our journey to uncover its secrets is both thrilling and essential to our understanding of the universe.。

重力的束缚英语作文Title: The Shackles of Gravity。

Gravity, the unseen force that binds us to the Earth, has long been a subject of fascination and inquiry for scientists and thinkers alike. Its influence permeates every aspect of our existence, shaping the very fabric of our universe and defining the limits of our exploration. In this essay, we delve into the profound implications of gravity, exploring its role in both constraining and enabling human endeavors.At its core, gravity is the force of attraction that exists between all objects with mass. First elucidated by Sir Isaac Newton in the 17th century through his law of universal gravitation, gravity governs the motion of celestial bodies and dictates the trajectories of projectiles on Earth. From the graceful orbit of the moon around our planet to the majestic dance of galaxies in the cosmos, the influence of gravity extends across vast cosmicdistances.Yet, while gravity bestows upon us a sense of stability and order, it also imposes formidable constraints on our aspirations for space exploration and travel. The immense gravitational pull of celestial bodies, particularly that of Earth, presents formidable challenges for spacecraft attempting to break free from their gravitational grasp. The energy requirements for overcoming Earth's gravity are substantial, necessitating the development of powerful propulsion systems and innovative engineering solutions.Moreover, the gravitational forces exerted by massive celestial bodies create gravitational wells, which can trap objects within their gravitational fields. These gravitational wells pose significant obstacles to interstellar travel, as navigating out of them demands tremendous amounts of energy and precise trajectory calculations. As a result, human exploration of distant stars and galaxies remains a distant dream, constrained by the formidable shackles of gravity.However, despite the challenges posed by gravity, humanity has demonstrated remarkable ingenuity in devising strategies to transcend its limitations. The advent of space exploration technologies, such as multi-stage rockets and ion propulsion systems, has enabled humans to venture beyond the confines of Earth's atmosphere and explore the vast expanse of space. Mission such as the Apollo moon landings and the ongoing exploration of Mars stand as testament to humanity's indomitable spirit and determination to overcome the constraints of gravity.Furthermore, advances in theoretical physics have opened new avenues for understanding the nature of gravity and its potential manipulation. The theory of general relativity, proposed by Albert Einstein in the early 20th century, revolutionized our understanding of gravity by describing it as the curvature of spacetime caused by the presence of mass and energy. This groundbreaking theory has paved the way for speculation about exotic concepts such as wormholes and warp drives, which could potentially allowfor rapid interstellar travel by circumventing the conventional limitations imposed by gravity.In conclusion, gravity represents both a fundamental force of nature and a formidable barrier to human exploration and progress. While its influence serves to tether us to the Earth and impose constraints on our ambitions for space travel, humanity's ingenuity and resilience offer hope for overcoming these limitations in the future. Through continued scientific inquiry and technological innovation, we may one day unlock the secrets of gravity and embark on voyages to the farthest reaches of the cosmos, liberated from the shackles of gravity's embrace.。

a r X i v :c o n d -m a t /0611266v 2 [c o n d -m a t .s t r -e l ] 16 N o v 2006Elementary Excitations of Quantum Critical 2+1D AntiferromagnetsZaira Nazario †and David I.Santiago †,⋆†Department of Physics,Stanford University,Stanford,California 94305⋆Gravity Probe B Relativity Mission,Stanford,California 94305(Dated:February 6,2008)It has been proposed that there are degrees of freedom intrinsic to quantum critical points that can contribute to quantum critical physics.We point out that this conclusion is quite general below the upper critical dimension.We show that in 2+1D antiferromagnets skyrmion excitations are stable at criticality and identify them as the critical excitations.We found exact solutions composed of skyrmion and antiskyrmion superpositions,which we call topolons.We include the topolons in the partition function and renormalize by integrating out small size topolons and short wavelength spin waves.We obtain correlation length exponent ν=0.9297and anomalous dimension η=0.3381.PACS numbers:75.10.-b,75.40.Cx,75.40.Gb,75.40.-sQuantum phase transitions have been a subject of the-oretical and experimental exploration since the pioneer-ing work of John Hertz[1].Since then,quantum criti-cal behavior has been understood and studied as arising from quantum fluctuations of the order parameter[1,2].In this traditional approach the quantum transition is studied via the Wilson renormalization group in which fluctuations of the order parameter are taken properly into account.It is said that quantum phase transitions follow the Landau-Ginzburg-Wilson paradigm (LGW).It has recently been suggested that quantum critical points will have properties that cannot be obtained from LGW order parameter fluctuations alone[3,4].In partic-ular,it was suggested that quantum critical points will have low energy elementary excitations intrinsic to the critical point whose fluctuations will contribute and mod-ify the critical properties.It was postulated that these excitations will be fractionalized[3,4].That some quantum critical points have elementary excitations different from those of each of the phases it separates can be inferred quite generally.We concentrate in relativistic quantum critical points,but we emphasize that these physics can take place in other systems.For such a system,which we take to be an antiferromagnet,we are interested in the N`e el magnetization Green’s func-tion,or staggered magnetic susceptibility.In the ordered phase the transverse Green function or susceptibility corresponds to spin wave propagation and it has a nonanalyticity in the form of a pole corresponding to such propagation:n (−ω,− k )· n (ω, k ) =Z (ω, k )c 2k 2+∆2−ω2+G incoh (ω, k ).Here A (ω, k )is between 0and 1,and the incoherent back-ground G incoh vanishes at long wavelengths and smallfrequencies,∆is the gap to excitations in the disordered phase.That this Green’s function has a pole means that triplet or triplon spin waves are low energy eigenstates of the disordered antiferromagnet.For 2+1D antiferro-magnets,and in general for antiferromagnets below the upper critical dimension,the quasiparticle pole residue A vanishes as the system is tuned to the quantum crit-ical point[5].At criticality,triplon excitations have no spectral weight and thus triplons cannot be elementary excitations of the system.On the other hand right at criticality the response function below the upper critical dimension (below which η=0,while above η=0)has nonanalyticities that are worse than polesn (−ω,− k )· n (ω, k ) =A ′12elementary excitations of the quantum mechanical phases away from criticality,the spin waves,cannot even be ap-proximate eigenstates at criticality as they are absolutely unstable.The quantum critical point is a unique quantum me-chanical phase of matter,which under any small per-turbation becomes one of the phases it separates.It is a repulsive fixed point of the renormalization group.As far as the transition from one quantum mechanical phase to the other is continuous,and both phases have different physical properties,the critical point will have its unique physical properties different from the phases it separates.The properties of the critical point follow from the critical Hamiltonian H (g c )(g c is the critical coupling constant),which will have a unique ground state and a collection of low energy eigenstates which are its elementary exci-tations.These low energy eigenstates are different from those of each of the phases as long as we are below the upper critical dimension.As a matter of principle,all quantum critical points below the upper critical dimen-sion will have their intrinsic elementary excitations .We have seen that below the upper critical dimension,the excitations of the stable quantum phases of the sys-tem become absolutely unstable and decay when the sys-tem is tuned to criticality.The question comes to mind immediately:what could they be decaying into?When one tries to create an elementary excitation of one of the phases,it will decay immediately into the elementary ex-citations of the critical point.The critical excitations will be bound states of the excitations of the stable phases the critical point separates.These bound states could be fractionalized as conjectured by Laughlin[3]and Senthil,et.al.[4],but they need not be in all cases.These criti-cal degrees of freedom are responsible for corrections to the LGW phase transition canon[4].The intrinsic quan-tum critical excitations contribute to the thermodynam-ical and/or physical properties of the quantum critical system.Now we turn to a specific model in order to identify what the critical excitations are.We concentrate on 2+1dimensional short-range Heisenberg antiferromagnets in a bipartite lattice.These are described by the O (3)nonlinear sigma model augmented by Berry phases[7,8]Z = D n δ( n 2−1)e −S ES E =iS B + β0dτd 2 x ρsc 2 .(2)where ρs ≡JS 2is the spin stiffness,and the spin-wave velocity c =2√2d 2 x (∂i n )2=4πρs .(3)These solitons are called skyrmions[12].Skyrmions are of a topological nature as they are char-acterized by the integer winding numberq =1|w |2+1,n 3=1−|w |21+n 3.(5)In terms of w the nonlinear σ-model action isS E [w ]=2Λ(1+|w |2)2=2Λ(1+|w |2)2,(6)where z =x +iy and z ∗=x −iy is its conjugate,and Λ/g Λ=ρs .g Λis the microscopic Goldstone coupling constant defined at the microscopic cutoffscale Λ[5,6].The classical equations of motion which follow by sta-tionarity of the classical action are∂20w−4∂z ∂z ∗w =2w ∗3 When the system N`e el orders, n,or equivalently w,willacquire an expectation value: n a =δ3a, w =0.As mentioned above,besides Goldstones,there arestatic skyrmion configurations[11,13]:w= q i=1λ/(z−a i)whose topological invariant(4)in terms of the stere-ographic variable,w,isq=1(1+|w|2)2.(7)This configuration can easily be checked to have charge q and energy4πqΛ/gΛ.λq is the arbitrary size and phase of the configuration and a i are the positions of the skyrmions that constitute the multiskyrmion config-uration.Similarly,the multiantiskyrmion configuration can be shown to be w= q i=1λ∗/(z∗−a∗i)with charge −q and energy4πqΛ/gΛ.We now investigate whether skyrmions and anti-skyrmion configurations are relevant at the quantum critical point.As mentioned above,their classical energy is4πΛ/gΛ,which is independent of the size of the skyrmionλ.On the other hand,in real physical systems there are quantum and thermalfluctuations. These renormalize the effective coupling constant of the nonlinear sigma model and makes it scale dependent. To one loop order the renormalized coupling constant isgµ=µ1−(gΛ/2π2)(1−µ/Λ).(8)Since the skyrmion has an effective sizeλ,spin waves of wavelength smaller thanλrenormalize the energy of the skyrmion via the coupling constant renormalization lead-ing to an energy and Euclidean action S E=βE,which are now scale dependent through the scale dependence of the coupling constant at scaleµ=1/λ.If the system is at temperature T=1/β,this temperature sets the size of the skyrmion to be the thermal wavelengthλ=β.The skyrmion Euclidean action is thenS E=8πβgΛ 1−gΛβΛ(9)Having obtained the Euclidean action for skyrmions (9),we now study its low temperature limit in the N`e el ordered phase and at the quantum critical point.Ac-cording to the one loop renormalized coupling constant (8),the quantum critical point occurs when the renor-malized spin stiffness(ρs(µ)∝µ/gµ)vanishes at long wavelengths(µ→0):gΛ=g c=2π2.Since the skyrmion gap is4πµ/gµ,the critical point corresponds to skyrmion gap collapse.When in the N`e el ordered phase,gΛ<g c, the skyrmion Euclidean action(9)is infinite.Therefore, the probability for skyrmion contributions is suppressed exponentially at low temperatures,vanishing at zero tem-perature.Skyrmions are gapped and hence irrelevant to low temperature physics in the N`e el ordered phase.At the quantum critical point gΛ=g c=2π2,the skyrmion Euclidean action isS E=4z−an+ λ∗2 (11)whereλis the size of the configuration,θandϕare the arbitrary directions of the configurations,a is the ar-bitrary position of the configuration and n is an integer. This configuration is topologically trivial because it has q=0as obtained from(4).On the other hand,it is composed of arbitrary superpositions of equal numbers of skyrmions and antiskyrmions with q=±n,i.e.the precise superpositions we need to sum over in the path integral for the q=0sector.Since this configurations is made of topologically nontrivial skyrmions and anti-skyrmions,we dub it a topolon.While the argument of the tangent is obviously a sum of an n skyrmion and an n antiskyrmion,it appears to not be a fully general one as all the skyrmions are at the same position.By starting with a fully general skyrmion configuration and making a change of variables to an effective“center of mass”coor-dinate,it follows that the results are the same as having all skyrmions at the same place.The topolon with spatial and temporal sizeλhas Eu-clidean action(6)S t E= λ0dτ8πgΛ(λΛ)2n+O(g0Λ)(12)The partition function including topolon configurations is given by Z= ∞n=0Z n where4Z 0=D νD ν∗(1+|w (n )t+ν|2)2d 2a4πdλg µd 3x∂µν∂µν∗g Λd 3x∂µν∂µν∗×1− 1−µ2π2+ 1−µ3(e 8π/g Λ−1)ρs (µ)=µg Λ×1− 1−µ2π2+ 1−µ3(e 8π/g Λ−1)β(g )=µ∂g2π2+g β(g )∼µ−1(g c −g µ)1/β′(g c ).(14)The correlation length exponent is ν=−1/β′(g c )≡−(dβ/dg |g =g c )−1.The correlation length exponent with topolon contributions evaluates to ν=0.9297.The d =2+ǫexpansion of the O (N )vector model,whichagrees with the 1/N expansion for large N ,gives ν=0.5[16].We note that our value is larger than the accepted nu-merical evaluations of critical exponents in the Heisen-berg model,ν=0.71125[15],but about as close to this accepted Heisenberg value than the 2+ǫexpansion or the 1/N expansion.We conjecture that the difference between our value and the Heisenberg value is real and attributable to quantum critical degrees of freedom.Goldstone renormalizations of the ordering direction σ=n 3,and hence of the anomalous dimension η,are no-toriously inaccurate.The one loop approximation leads to a value of η=2,thousands of percent different from the accepted numerical value of η≃0.0375[15].The large N approximation,which sums bubble dia-grams,is a lot more accurate.To order 1/N one obtains η=8/(3π2N )≃0.09for N =3.We now calculate the value of ηfrom topological nontrivial configurationsn 23 =Z =1− n 21+n 22=1−4|w |231−µ/ΛZ ∂Z3(e 8π/g −1)For the anomalous dimension at the quantum critical point we obtain η(g c )=0.3381.On the other hand,we have seen that spin wave contributions tend to give quite large and nonsensical values of η.In fact so large as to wash out the momentum dependence of the propagator.Hence,to calculate η,spin wave contributions prove to be tough to control.Our calculation gives a value quite larger than the accepted numerical value.We have re-cently calculated [17]the unique value of ηthat follows from quantum critical fractionalization into spinons and find η=1.While our value obtained from topolons is far from 1,it is a lot closer than the accepted numerical Heisenberg value and the 1/N value.[1]J.A.Hertz,Phys.Rev.B 14,1165(1976).[2]lis,Phys.Rev.B 48,7183(1993).[3]R. ughlin,Adv.Phys.47,943(1998); B. A.Bernevig,D.Giuliano and ughlin,An.of Phys.311,182(2004).[4]T.Senthil,A.Vishwanath,L.Balents,S.Sachdev andM.P.A.Fisher,Science 303,1490(2004);Phys.Rev.B 70,144407(2004).[5]S.Chakravarty,B.I.Halperin and D.R.Nelson,Phys.Rev.B 39,2344(1989);A.V.Chubukov,S.Sachdev and J.Ye,Phys.Rev.B 49,11919(1994).[6]A.M.Polyakov,Phys.Lett.B 59,79(1975);E.Br´e zinand J.Zinn-Justin,Phys.Rev.Lett.36,691(1976);E.Br´e zin and J.Zinn-Justin,Phys.Rev.B 14,3110(1976).[7]F.D.M.Haldane,Phys.Rev.Lett.61,1029(1988).[8]S.Sachdev,Low Dimensional Quantum Field Theoriesfor Condensed Matter Physicists ,Proc.of the Trieste Summer School 1992(World Scientific,Singapore,1994).[9]T.Dombre and N.Read,Phys.Rev.B 38,7181(1988);E.Fradkin and M.Stone,Phys.Rev B 38,7215(1988);5X.G.Wen and A.Zee,Phys.Rev.Lett.61,1025(1988).[10]N.Read and S.Sachdev,Phys.Rev.B42,4568(1990).[11]A.A.Belavin and A.M.Polyakov,JETP Lett.22,245(1975).[12]T.Skyrme,Proc.Royal Soc.London A260,127(1961).[13]D.J.Gross,Nucl.Phys.B132,439(1978).[14]J.Goldstone,Nuovo Cimento19,154(1961);Y.Nambuand G.Jona-Lasinio,Phys.Rev.122,345(1961);J.Goldstone,A.Salam,and S.Weinberg Phys.Rev.127,965(1962).[15]M.Campostrini et.al.,Phys.Rev.B65,144520(2002).[16]J.Zinn-Justin,Quantum Field Theory and Critical Phe-nomena,Fourth Edition,Oxford Univ.Press,Oxford, UK(2002),Chapter31,Section4.[17]Z.Nazario and D.I.Santiago,arXiv:cond-mat/0606386(2006).。

关于音叉效应的作文音叉效应是指当一个音叉被敲击或者摇动时，附近的其他音叉也会开始发出相同的频率的声音。

这种现象可以通过共振来解释，即当一个物体的固有频率与另一个物体的外部激励频率相匹配时，就会发生共振现象。

音叉效应在日常生活中有很多应用，比如音乐演奏、声波传播等。

英文回答：The phenomenon of tuning fork effect refers to the situation where when one tuning fork is struck or shaken, nearby tuning forks also start to emit sounds at the same frequency. This can be explained by resonance, which occurs when the natural frequency of one object matches the external excitation frequency of another object. The tuning fork effect has various applications in daily life, such as in music performance and sound wave propagation.For example, in a choir or orchestra, musicians often use tuning forks to ensure that all instruments or voicesare in tune. When one tuning fork is struck, the sound waves it produces can cause the air molecules around it to vibrate at the same frequency. These vibrations then travel through the air and reach other tuning forks, causing themto vibrate and emit sounds at the same frequency. Thishelps musicians to achieve harmony and synchronization in their performances.Another example is the use of tuning forks in medical diagnostics. Doctors often use a tuning fork to test a patient's hearing or to assess nerve function. When the tuning fork is struck, it produces a specific frequency of sound. If the patient's hearing or nerve function is normal, they will be able to hear the sound and perceive the vibrations. However, if there is a problem with their hearing or nerve function, they may not be able to hear the sound or perceive the vibrations properly. This canindicate the presence of an underlying medical condition.中文回答：音叉效应是指当一个音叉被敲击或者摇动时，附近的其他音叉也会开始发出相同的频率的声音。

a rXiv:h ep-ph/57120v221J ul26HUTP-05/A0030UTAP-530RESCEU-10/05Dynamics of Gravity in a Higgs Phase Nima Arkani-Hamed a ,Hsin-Chia Cheng a ,b ,Markus A.Luty a ,c ,d ,Shinji Mukohyama a ,e ,Toby Wiseman a a Jefferson Laboratory of Physics,Harvard University Cambridge,Massachusetts 02138b Department of Physics,University of California Davis,California 95616c Physics Department,Boston University Boston,Massachusetts 02215d Physics Department,University of Maryland College Park,Maryland 20742∗e Department of Physics and Research Center for the Early Universe The University of Tokyo,Tokyo 113-0033,Japan Abstract We investigate the universal low-energy dynamics of the simplest Higgs phase for gravity,‘ghost condensation.’We show that the nonlinear dynam-ics of the ‘ghostone’field dominate for all interesting gravitational sources.Away from caustic singularities,the dynamics is equivalent to the irrota-tional flow of a perfect fluid with equation of state p ∝ρ2,where the fluid particles can have negative mass.We argue that this theory is free fromcatastrophic instabilities due to growing modes,even though the null energy condition is violated.Numerical simulations show that solutions generally have singularities in which negative energy regions shrink to zero size.We exhibit partial UV completions of the theory in which these singularities are smoothly resolved,so this does not signal any inconsistency in the effective theory.We also consider the bounds on the symmetry breaking scale M in this theory.We argue that the nonlinear dynamics cuts offthe Jeans instability of the linear theory,and allows M <∼100GeV.1IntroductionIs general relativity the correct description of gravity at long distances and times? Certainly,there are good reasons for thinking that this is the case.Experimentally, gravity has been probed at distance scales ranging from10−1mm(in short-range force experiments)to at least1014cm(the size of the solar system).Theoretically, general relativity is the unique Lorentz-invariant theory of massless spin2,and its conceptual elegance is beyond question.However,the situation is far less clear on cosmological distance and time scales.Structure formation,galaxy rotation curves and gravitational lensing,and the accelerating expansion of the universe cannot be explained by general relativity coupled to known matter.These anomalous effects are conventionally attributed to‘dark matter’and‘dark energy.’However,given the fact that the observed effects are purely gravitational,it makes sense to ask whether they may have a common origin in a modification of gravity in the infrared.These considerations have led to a revival of interest in consistent infrared modifications of gravity[1,2,3,4,5,6,7].In the present paper,we further investigate the model of Ref.[5],‘ghost conden-sation.’This can be viewed as the universal low-energy dynamics associated with the simplest Higgs phase for gravity,arising when Lorentz symmetry is broken sponta-neously.Breaking of Lorentz symmetry is of course ubiquitous.For example,time-dependentfields in cosmology define a preferred frame.However,these solutions are not the ground state of the theory:they carry energy density and dilute away as the universe expands.Any‘modification of gravity’induced by such solutions becomes relevant only at scales of order the horizon.We are instead interested in the situation where Lorentz symmetry is broken inflat spacetime,allowing nontrivial modification of gravity inside the horizon.This means that the symmetry breaking sector has peculiar properties;in particular,the stress-energy tensor must vanish in the ground state:Tµν =0.(1.1) Spontaneous breaking of Lorentz symmetry gives rise to a gapless scalar excitation analogous to the Nambu-Goldstone bosons that arise from spontaneous breaking of internal symmetries.‘Ghost condensation’gives rise to a single such mode,and is in this sense the minimal model of spontaneous breaking of Lorentz symmetry.We refer to the scalar mode as a‘ghostone boson.’In analogy with the Higgs phase for gauge theory,the ghostone mode mixes with the graviton,modifying gravity in a nontrivial manner.Ref.[5]studied this theory and analyzed the modification of gravity in the weak-field limit.The dynamics is governed by a consistent effective theory defined by the scale M where the symmetry is broken.It was shown that the ghostone mode gives a possible new origin for dark energy and dark matter.Ref.[8]showed that the ghostone mode may also be the inflaton,leading to interesting testable consequences.In the present paper,we further investigate the dynamics of this theory.We show that nonlinearities dominate the dynamics of the ghostone sector for all gravitational sources of interest.In particular,the time scale for the onset of nonlinear dynamics for afixed gravitational source is precisely the infall(or orbit)time associated with the source.Away from singularities,the nonlinear solutions are equivalent to the gradientflow of afluid with equation of stateρ2p=3,we discuss the nonlinear dynamics of the ghost condensate.We discuss the time scales and present thefluid picture.In section4,we show the numerical simulations of the nonlinear evolutions and discuss the resolution of caustic singularities.In section 5,we discuss the bounds of this theory.This includes mass and energy accretion in slow-moving objects,gravitational lensing and energy loss from moving objects. We do not claim to have a complete understanding of the dynamics,so this section is intended to be preliminary and provocative.In section6,we briefly discuss the possibility that ghost condensate may constitute the dark matter.We show that the initial growth of the density perturbations in the linear regime is identical to that of the standard cold dark matter.Whether it can form the correct structure depends on the details of the nonlinear evolution which is left for future investigations.Our conclusions are presented in section7.2Review of the Linear Theory2.1Effective TheoryWhat is a Higgs phase for gravity?It is easiest to answer this question in linearized general relativity,where we expand the metric aboutflat spacegµν=ηµν+hµν(2.1)and keep only terms quadratic in hµν.In this theory,thefields hµνare closely analo-gous to gaugefields with gauge transformation lawδhµν=−(∂µξν+∂νξµ),(2.2)whereξµare the generators of infinitesmal diffeomorphismsxµ→xµ+ξµ.(2.3) We want to consider the case where the time diffeomorphisms generated byξ0are spontaneously broken.This means that time diffeomorphisms are realized nonlinearly in the effective theory containing only the ghostonefield.The minimal model contains a single real ghostonefieldπthat shifts under time diffeomorphisms:δπ=−ξ0.(2.4)Note thatπnaturally has units of time.We now write the most general effective Lagrangian invariant under these symmetries.This contains the Einstein Lagrangian,and additional terms constructed from the invariantsΣ=˙π−1(˙h ij−∂i h0j−∂j h0i+2∂i∂jπ).(2.6)2The leading terms areL eff=L E+M4 12h00 2−α12M2K2+O(π3) .(2.7) (Note that we have built in the fact thatflat space is a solution by not writing any linear terms in the Lagrangian.)In the limit where we turn offgravity,we see that the ghostone mode has dispersion relationα k4ω2=A conventional effective Lagrangian for this theory isL=+1φ=0.This has solutions withφ=nµxµfor any constant 4-vector nµ.If nµis timelike,we can choose the time direction so that the solution isφ=ct.(2.11)This is a solution for any constant c.Atfirst sight it may appear that these are obviously not candidate ground states of the theory,but the situation is actually more subtle.Suppose we expand influctuations about this solutionφ=ct+π.(2.12)Note that under time diffeomorphisms,πtransforms asδπ=−ξ0/c,(2.13)just like the ghostone mode considered above.Expanding the Lagrangian to quadratic order inπ,onefinds that thefluctuations forπhave good time and space kinetic terms.This means that the solution is stable under localfluctuations for any value of c!The reason for this is that the theory has a conserved current associated with the shift symmetryJµ=∂µφ.(2.14)Solutions with c=0have a constant nonzero charge density.Local excitations cannot change the total charge,so configurations with lower energy cannot be reached. However,when we turn on gravity,solutions with c=0will cause the universe to expand,and the charge will dilute away.Lorentz invariance is therefore not broken spontaneously in this theory.Consider instead an effective Lagrangian of the formL eff=M4P(X),X=∂µφ∂µφ.(2.15) Note thatφhas dimensions of length(or time),so that X is dimensionless.This omits only terms with more than one derivative acting onφ,such as(We see that small perturbations are stable provided2c2P′′(c2)+P′(c2)>0,P′(c2)>0.(2.17) (Note that a conventional kinetic term P(X)=+12(X−1).(2.22)Stability in the linearized theory requires higher-derivative terms to give a nonzero spatial kinetic term,for example∆L eff=−αM2φ)2=−αM2αM2M2−,T J∼M2Pl M2ScreeningExponentially fallingPotentialFig.2.A schematic illustration of the screening effect for gauge theories in the Higgs phase.For M<∼10MeV,T J is longer than the lifetime of the universe and there is clearly no constraint from the Jeans instability.2.4Negative EnergyEven for time shorter than the Jeans time scale T J,stability is an issue because the ghostone energy can be negative.Recall that the ground state X=1is the boundary of the stability region.Expanding to higher orders inπ,wefind interaction terms such asL eff=M4 12˙π( ∇π)2+···−α˙π 1/2.(2.27) Quantum-mechanically,there arefluctuations at all length scales.Nonetheless,Ref.[5] showed that there is no quantum instability in the effective theory using a scaling ar-gument.If we scale energy by E→sE,the quadratic kinetic terms are left invariant−1Anti−ScreeningOscillating PotentialFig.3.A schematic illustration of the anti-screening effect for gravity in the Higgs phaseif we scalet→s−1t,(2.28)x→s−1/2 x,π→s1/4π.With this scaling the cubic operator˙π( ∇π)2in Eq.(2.26)scales as s1/4and is therefore (barely!)irrelevant.All other operators are even more irrelevant,showing that there is a regime of low energies where the expansion is under control.2.5Preview of Nonlinear EffectsThe arguments above show that the effects of the nonlinear terms are under control at low energies and smallfield amplitudes.However,in the presence of large classical gravitational sources the nonlinear terms can become important.In fact,the time scale for the ghostonefield near a classical source is just the gravitational infall time of the source.This can be understood from the form of the stress-energy tensor.In the approximation where the Lagrangian is L=P(X),the stress-energy tensor has the formTµν∝−P(X)gµν+2P′(X)uµuν.(2.29)whereuµ=∂µφ.(2.30) This has the form of a stress-energy tensor for a perfectfluid with4-velocity uµ.1 The fact that uµis a gradient means that theflow is irrotational:∂[µuν]=0implies ∇× u=0.The equations of motion for the ghostonefield follow from the conservation of the stress-energy tensor∇µTµν=0,which are intepreted as the conservation of energy and momentum in thefluid.In the presence of a classical gravitational source, afluid clearly responds on a time scale given by the infall time,and therefore so does the ghostone mode.As it will be shown in the next section,this is exactly the time scale where the nonlinear term becomes important.For smallfluctuations about the minimum X=1,P(X)can be approximated byP(X)≈12Σ2,(2.31)where an overall contribution to the cosmological constant has been removed.We can read offthe equation of state of thefluid from the energy-momentum tensor.It isp=ρ21The defining property of a perfectfluid is that at each point there is a frame in which the stress-energy tensor has the form Tµν=diag(−ρ,p,p,p).Such phenomena were also observed in some other scalarfield theories[11].Near these singularities,the higher-derivativeα( ∇2π)2term can no longer be neglected. This term contributes positive gradient energy,and wefind in numerical simulations that it generically smoothly resolves the caustic singularity,giving rise to a‘bounce’with outgoingπwaves and positive and negative regions ofΣnear the would-be caustic.The fact thatΣ(and henceρ)can be negative brings up again the question of the stability of the theory.As discussed above,regions withΣ<0modes with sufficiently long wavelengths are unstable.In numerical simulations,wefind that negative energy regions tend to shrink while the amplitude ofΣgrows inside the region.This can be understood from thefluid picture,since this is valid in the limit where we neglect the( ∇2π)2term,which is a good approximation away from caustic singularities.In this picture theΣ<0region consists offluid particles with negative mass.It is therefore clear that energy(mass)cannotflow across the boundary of theΣ<0region,since the boundary consists of particles with vanishing mass.The boundary can move however,and in theΣ<0region the positive pressure favors large gradients and causes theΣ<0region to shrink.Numerical simulations show that someΣ<0regions continue to shrink until they exit the regime of validity of the effective theory.These singularties need to be resolved in a more fundamental theory.Similar conclusion was also obtained in Ref.[12]which studied the two-dimensional case.However,we show that the total energy inside theΣ<0regions formed in astrophysical situations is very small,and does not lead to any observable consequences provided the singularities are regulated in a smooth way.We will discuss a partial UV completion to do this,and present numerical evidence that it works.The nonlinear dynamics affects the bounds on M derived in the linear theory. The Jeans instability in the linear theory gives a bound M<∼10MeV if we require that there is no exponential growth of the oscillatory potential within the age of the universe.However,the nonlinear dynamics is expected to cut offthe instability, and may weaken this bound.We argue below that the strongest bound comes from gravitational lensing due to regions of positive and negative energy produced by the Jeans instability.Demanding that the random walk of light rays due to the lensing does not smear out the observed CMB anisotropies gives the bound M<∼100GeV.We also consider other possible bounds on the ghost condensate from the grav-itational sector.The modifications of the gravitational potential are small due to velocity effects[13,14].We also consider energy loss in the nonlinear theory,as well as energy stored in would-be caustic singularities.Wefind that these effects are negligible,and we believe that the theory is safe for M<∼100GeV.3Nonlinear DynamicsWe now turn to the nonlinear dynamics of the theory.The nonlinear dynamics is veryrich and complex,and we emphasize that we do not claim a complete understandingin this work.It is therefore important to keep in mind that there is a simple limit ofthis theory,independent of the details of the nonlinear dynamics[5].The Ghostonesector naturally couples to matter only through gravity.(Gravitationally induceddirect couplings to standard modelfields are easily seen to be negligibly small.)Themaximum value of the gravitational energy in the Ghostone energy is of order M4,which does not affect even cosmology if M<∼(M Pl H0)1/2∼10−3eV.Such low values of M are still very interesting for cosmology,since the ghost may be a source of darkenergy and dark matter[5]and may drive inflation[8].Another general point to keep in mind in the following is that the modificationsof gravity vanish in regions whereΣ=0(X=1)and we neglect the( ∇2π)2term. This is because in this limit,the Lagrangian in‘unitary gauge’φ=t(φ=0)isL=√where∇2Φ=T00(X−1)=˙π−12Σ2−α2π2T NL∼√we have T2NL∼r3/2/R S,which is the Kepler relation.This is a very direct way of seeing that the nonlinear effects become important on the gravitational time scale.Solving Eq.(3.7)forπ,we obtain| ∇π|∼πΦ≪1.(3.9) That is,ghostone amplitudes are small for weak gravity,in agreement with our as-sumptions.On the other hand,in the linear approximation theα( ∇2π)2becomes comparable to˙π2at T Lin∼ML2.As a result,the nonlinear evolutions completely dominates for T NL<∼T Lin∼ML2,orΦL2>∼1M src M Pl(X−1)2=18Σ2gµν+Σuµuν ,(3.13)2whereuµ=∂µφ.(3.14) Note that uµis nonzero and timelike everywhere.This has the form of the stress-energy tensor for a perfectfluid with4-velocity uµ.Because uµis the gradient of a scalar,theflow of thefluid is irrotational.The conservation of the stess-energy tensor∇µTµν=0gives the equation of motion for the ghostonefield,and also gives the Euler equation for thefluid.ForΣ≪1we can read offthe density and pressure.(3.15)ρ=M4Σ,p=12M4This establishes the equivalence of the ghostone theory without theα( ∇2π)2term to the irrotationalflow of a perfectfluid2.It is also insightful to understand the equivalence directly in terms of the equa-tions of motion.For the ghostonefield,the equations of motion have the form of a conservation law˙Σ= ∇·[Σ ∇π],(3.16) whereΣis the charge density.For afluid made of particles carrying the conserved charge,the current is J=Σ v,so we identifyv=− ∇π.(3.17) Again we see that thefluidflow is irrotational.In thisfluid picture,the equations of motion are satisfied simply due to the fact that thefluid particles carry their charges with them.It remains only to satisfy the relation betweenΣandπ:Σ=˙π−1=− ∇(Φ+Σ),(3.19)Dt4πG Nρcorresponding to this equation of state is˜L J∼M P l/M2, where c s= ρ/M4is the sound velocity.Intriguingly,this agrees with the Jeans length (2.25)in the linear theory up to a constant of order unity.This equation of state ignores the k4 term,while the linear analysis does not take into account the nonlinear termΣ2in p.Hence,it is not a priori clear whether these two Jeans length should be the same or not.Nonetheless,they agree.whereD+ v· ∇(3.20)∂tis the time derivative along the particle worldline(also called the convective or Lagran-gian derivative).Eq.(3.19)is just Newton’s law for a particle moving in a potential Φ+Σ.Using the identifications Eq.(3.15)we can write− ∇Σ=−13Note that the equivalence between the ghostone andfluid pictures is a kind of duality,since it exchanges a constraint equation with an equation of motion.Noether charge of the original time translation symmetry,which we refer to as the ‘gravitational energy.’This is not the same as the Noether charge associated with the time translation symmetry that is unbroken in the vacuum,which we call the ‘inertial energy.’The inertial energy εis the energy associated with the unbroken time translation symmetry of the Lagrangian Eq.(3.6).(It is also the Hamiltonian density of the system.)Conservation of inertial energy states that˙ε=− ∇· p ,(3.22)whereε=M 4 12Σ( ∇π)2+αM 2( ∇2π) ∇π−˙π ∇( ∇2π) (3.24)is the momentum density.In the linearized approximation,ε=M 4 12M 2( ∇2π)2 ≥0.(3.25)However,in the nonlinear theory the inertial energy is not positive definite due to the second term in Eq.(3.23).The energy density can be negative only in regions where Σ<0.In fact it is easy to see that the energy is unbounded from below.For example,for π=c | x |we have ε=−1M 2 ∇( ∇2π).(3.27)If we neglect theα( ∇2π)2term,this conservation law is taken into account very directly in thefluid picture,to which we now turn.In this approximation,negative energy regions correspond precisely to regions whereΣ<0,so we consider such a region surrounded byΣ>0.5In thefluid picture,the conserved charge is carried by the individual particles,so theΣ<0region consists of particles with negative charge,while the boundary of theΣ<0region consists of particles of vanishing charge.Therefore,there can be noflux of charge across theΣ=0boundary,and the total charge inside the region does not change.On the other hand,theΣ=0boundary can move.Since ∇Σpoints outward at the boundary,the equation of motion forfluid particles Eq.(3.19)implies that the particles on the boundary experience an inward force due to the pressure.Therefore, theΣ<0region tends to shrink.These arguments show that the total shift charge integrated over aΣ<0regionQ= Σ<0d3xΣ(3.28)does not change with time.This can also be seen from the fact that the shift current J vanishes on theΣ=0boundary.The shift charge is not the same as the total inertial energyE= Σ<0d3xε.(3.29)However,if we neglect theα( ∇2π)2term,theflux of inertial energy across theΣ=0 boundary also vanishes,and therefore E also does not change with time.As with thefluid picture,these results hold beyond the approximations made here.This is discussed in the appendix.3.5Caustic SolutionsThefluid picture can be used to understand the structure of the caustic singularties that occur when we neglect theα( ∇2π)2term.We restrict attention to the case Σ=0,where there is no pressure and thefluid particles follow geodesics.In this case,it is clear that there are caustics without theα( ∇2π)2term.It is possible that the pressure resolves the caustics in important situations such as inside galaxy halos made of ghostone dark matter,but we will not consider that here.To introduce the subject of caustics,we consider the gravitational potential due to a uniform sphere of matter with densityρ0.Inside the sphere,the gravitational potential isρ0Φ=,(3.33)Twhere T is a constant that tells us how the initial velocity varies away from x0=0. Solving Eq.(3.31)for x0we obtainxx0=−TFig.4.The1-dimensional‘perfect caustic.’The shaded regions shows the region where we can expand the solution perturbatively about the perfect caustic.Since the velocity is constant along any trajectory,we havexv(x)=v0(x0)=−.(3.36)2(t−T)Note that this solution is scale-free,since T just gives the time to the caustic.Shifting t→t−T,we obtainx2π(x,t)=−=0(3.38)∂x0forfixed t.This gives the time to the caustic for a given value of x0as1t c(x0)=−In a realistic case,the function v0(x0)will be more complicated.We want to expand about the point where the causticfirst forms,i.e.the minimum of t c(x0), which we take to be x0=0.Expanded about this point,v0takes the formv0(x0)=−x06L2Tx30+O(L−3),(3.40)where we have performed a boost so that v0(0)=0.Because of the focussing of the geodesics,this expansion is valid only for|x|≪L t−Tt−T−T3(t−T)4+O(L−3).(3.42)and thereforev(x)=x6L2x3L21We can use Eq.(3.44)to estimate the time and distance scale where theα( ∇2π)2 term becomes important.This will happen whenα∂4xπML2 1/2,(3.46)where∆t=T−t is the time to the caustic.The distance scale where theα( ∇2π)2 term becomes important is therefore∆x∼LM 1/2.(3.47)Note that∆x,∆t≫M−1as long as L,T≫M−1and L/T≪1(i.e.the system is nonrelativistic),so this is within the regime of validity of the effective theory.4Numerical SimulationsIn this section we describe various numerical simulations of the ghostfield which allow us to understand the rather exotic features of its non-linear evolution.The test cases wefirst present assume symmetry to reduce the dynamics to a one dimensional problem,and will see that the various symmetries have different be-haviours.We will discuss how caustics do indeed form in the theory.As mentioned earlier,this is clear forα=0,but one would naively expect non-zeroαto amelio-rate this problem.However,as we will show,this is not the case,and for certain symmetries the‘perfect’caustic remains an attractor.As one might expect,the singular behaviour becomes less strong as one moves from planar through axisymmetry to spherical symmetry.Indeed without any grav-itational potential we will see the planar reduction exhibits singularities,while the spherical theory without potential does not.However,once a gravitationally at-tractive potential is added,all three symmetries become singular under evolution of regular initial data.Clearly assuming symmetry can lead to unphysical behaviours,whilst the phy-sical situation we are ultimately interested,namely structure formation,is expectedto have very little symmetry.Hence this section will conclude with a study of a3-d numerical evolution where no continuous symmetry is present and a moving gravi-tational potential seeds the ghostfield growth.As expected from the1-d examples, we againfind singular caustics do develop,and interestingly appear to take a planar form.4.1One dimensional evolutions:Planar,axial and spherical symmetryWe will initially consider the case of evolution of the ghostfield in the absence of gravitational sources,seeded instead from a local perturbation in the ghostfield itself.Then reducing to planar symmetryπ=π(t,r)and we may write the equation for the ghost decoupled from gravity in a manifestlyflux conservative form,˙H=∂r Σ+1α/M,the second line is simply the definition ofΣand˙x,x′are the time and space derivatives of x.We then use a Crank-Nicholson method to evolve our initial data,which we take to be a Gaussian profile inππ(t=0)=π0e−r2(4.2) withΣ=0initially.As discussed previously,with L=0Σwould remain zero,but the higher derivative term sourcesΣ.In order to make contact with the epoch of structure formation we wish to have moderate initial amplitudes so|π0|<1but is still of order unity.From(4.2)we have chosen units to have initial data with unit spatial variation.We then wish to have L,the ghost length scale,to be L≪1.Naturally numerical methods limit our ability to separate the initial data length scale from the ghost length whilst maintaining accuracy.However we may separate the scale sufficiently to see the asymptotic properties of taking L very small.Thefirstfigures we show,5and6,illustrate the generic behaviour for the planar system for small L,showing both H andΣfor the evolution.We show both the evolution of data for L=0.005and also for L=0.for comparison,and the initial amplitude was taken to beπ0=0.1.As expected L=0.evolution leads to a caustic,whose time for formation scales as t caustic≃1/π0.Adding the higher derivative term changes the evolution dramatically around this time scale,completely smoothing out the ghostfield.However,we seefrom thefigure that the radiated waves scattered by the action of this higher derivative term in fact are attracted back to the origin where they grow and become singular. Obviously since the symmetry is planar,this growth is not due to a measure factor,but rather is due a‘perfect caustic’forming,which as mentioned before cannot be rescued by the higher derivative term we use here.Indeed this perfect caustic formation can be seen locally in detail fromΣnear the singularity.We note that taking sufficiently large L∼√π0when T→∞),or with decreasing L(so that the radiation from the increasingly perfect central caustic region is slower).Thus for small L a cartoon of the evolution is that the initially collapsing ghost field causes the higher derivative term to radiate strongly,but the process of radiation simply refines the collapsing region into the perfect caustic form where it eventually collapses.Suitable modification of the conservative equations of motion(4.1)introduce the geometric measure factor associated with axisymmetry.In this case wefind the behaviour essentially analogous to the planar case.Whilst the singular behaviour appears‘weaker’,taking longer to reach the singularity for the same parameters, the singularity does indeed form.However moving to spherical symmetry wefind a change in behaviour.For spherical symmetry we again take the initial data(4.2)and evolve this in the absence of any gravitational potential.Infigure8we plot the evolution of H for π0=0.1and L=0.005(as shown earlier for the planar case).The behaviour is clearly different with a totally non-singular evolution for all times,a portion of which is shown in thefigure.Whilst for L=0.obviously the spherically symmetric ghost field exhibits the usual caustic singularity,we see the higher derivative term,aided by the geometric measure factor,can now radiate sufficient energy to avoid any later energy build up.So far we have discussed the case of evolution of a local perturbation in the ghost field.We have seen that the symmetry of the initial data has a strong effect on whether the evolution is singular or not.Physically,however,we are interested more。

22.101 Applied Nuclear Physics (Fall 2004)Lecture 22 (12/1/04)Detection of Nuclear Radiation: Pulse Height Spectra_______________________________________________________________________ References:W. E. Meyerhof, Elements of Nuclear Physics (McGraw-Hill, New York, 1967), Sec.3-6. ________________________________________________________________________ We have just concluded the study of radiation interaction with matter in which the basic mechanisms of charged particle, neutron and gamma interactions were discussed separately. A topic which makes use of all this information is the general problem of detection of nuclear radiation detection. Although this subject properly belongs to a course dealing with experimental aspects applied nuclear physics, it is nevertheless appropriate to make contact with it at this point in the course. There are two reasons for this. First, radiation detection is a central part of the foundational knowledge for all students in the department of Nuclear Engineering (soon to be renamed Nuclear Science and Engineering). Even though we cannot do justice to it in view of the limited time remaining in the syllabus, it is worthwhile to make some contact with it, however briefly. Secondly, an analysis of the features observed experimentally in pulse-height spectra of gamma radiation is timely given what we have just learned about the γ-interaction processes of Compton scattering, photoelectric effect and pair production.We first remark that regardless of the type of nuclear radiation, the interactions taking place in a material medium invariably result in ionization and excitation which then can be detected. Heavy charged particles and electrons produce ion pairs in ionization chambers, or light emission (excitation of atoms) in scintillation counters, or electron-hole pairs in semiconductor detectors. Neutrons collide with protons which recoil and produce ionization or excitation. In the case of gammas, all 3 processes we have just discussed give rise to energetic electrons which in turn cause ionization or excitation. Thus the basic mechanisms of nuclear radiation detection involve measuring the ionization or excitation occurring in the detector in a way that one can deduce the energy of the incoming radiation. A useful summary of the different types of detectors and methods of detection is given in the following table [from Meyerhof, p. 107].We now focus on detection of γ radiation. We are concerned with the measurement of two γ rays, at energies 1.37 Mev and 2.75 Mev, emitted from radioactive Na24. The measurements are in the form of pulse-height spectra, number of counts per channel in a multichannel analyzer plotted against the pulse height. Fig. 19.1 shows the results measured by using a Na-I scintillation detector. The spectra consist of two sets of features, one for each incident γ. By a set we mean a photopeak at the incident energy, a Compton edge at an energy approximately 0.25 Mev (m e c2/2) below the incident energy, and two so-called escape peaks denoted as P1 and P2. The escape peaks refer to pair production processes where either one or both annihilation photonsFig. 19.1. Pulse-height spectra of 1.37 Mev and 2.75 Mev γ obtained using a Na-I detector. (from Meyerhof)leave the counter. Thus, P1 should be 0.511 Mev below the incident energy and P2 should be 0.511 Mev below P1. The other features that can be seen in Fig. 19.1 are a peak at 0.511 Mev, clearly to be identified as the annihilation photon, a backscattered peak associated with Compton scattering at θ=π which should be positioned at m e c2/2, and finally an unidentified peak which we can assigned to x-rays emitted from excited atoms.One can notice in Fig. 19.1 that the various peaks are quite broad. This is a feature of scintillation detector, namely, relatively poor energy resolution. In contrast, a semiconductor detector, such Li-drifted Ge, would have much better energy resolution, as can be seen in Fig. 19.2. In addition to the sharper lines, one should notice that the peaks measured using the semiconductor detector have different relative intensities compared the peaks measured by using a scintillation detector. In particular, looking at the relative intensities of P1 and P2, we see that P1 > P2 in Fig. 19.1, whereas P2 > P1 in Fig. 19.2.Fig. 19.2. Same as Fig. 19.1 a semiconductor detector is used. (from Meyerhof)This difference can be explained by noting that the scintillation detector is physically larger than the semiconductor detector, in this case the former is a cylinder 7.6 cm in diameter and 7.6 cm in length, whereas the latter is 1.9 cm in diameter and 0.5 cm in height. Thus one can expect that the probability that a photon will escape from the detector can be quite different in these two cases.To follow up on this idea, let us define P as the probability of escape. In a one-dimensional situation P ~ e−µx , where µ is the linear attenuation coefficient and x is the dimension of the detector. Now the probability that one of the two annihilation gammas will escape is P1 = 2P(1-P), the factor of 2 coming from either gamma can escape. For both gammas to escape the probability is P2=P2. So we see that whether P1 is larger or smaller than P2 depends on the magnitude of P. If P is small, P1 > P2, but if P is close to unity, then P2 > P1. For the two detectors in question, it is to be expected that P is larger for the semiconductor detector. Without putting in actual numbers we can infer from an inspection of Figs 19.1 and 19.2 that P is small enough in the case of the scintillation detector for P1 to be larger than P2, and also P is close enough to unity in the case of the semiconductor detector for P2 to be larger than P1.。

Sensing with fluorescent nanoparticlesLuca Ba u,a Paolo Tecilla b and Fabrizio Mancin a Received 14th June 2010,Accepted 9th August 2010DOI:10.1039/c0nr00405gFluorescent chemosensors are chemical systems that can detect and signal the presence of selected analytes through variations in their fluorescence emission.Their peculiar properties make themarguably one of the most useful tools that chemistry has provided to biomedical research,enabling the intracellular monitoring of many different species for medical and biological purposes.In its simplest design,a fluorescent chemosensor is composed of a fluorescent dye and a receptor,with a built-in transduction mechanism that converts recognition events into variations of the emission properties of the fluorescent dye.As soon as fluorescent nanoparticles became available,several applications in the field of sensing were explored.Nanoparticles have been used not only as better-performing substitutes of traditional dyes but also as multivalent scaffolds for the realization of supramolecular assemblies,while their high surface to volume ratio allows for distinct spatial domains (bulk,external surface,pores and shells)to be functionalized to a comparable extent with different organic species.Over the last few years,nanoparticles proved to be versatile synthetic platforms for the implementation of new sensing schemes.1.IntroductionFluorescent chemosensors are arguably one of the most useful tools that chemistry has provided to biomedical research.1These molecular systems can detect and signal the presence of selected analytes through variations of their fluorescence emission properties.2The unique role they play in biological studies stems from the combination of few fundamental properties.First,the possibility of integrating specific receptors into the molecular structure of the chemosensor provides a potentially exquisiteselectivity.In addition,their molecular size minimizes physical perturbations to such delicate systems as living cells.Most importantly,the use of fluorescence emission as the analytical signal allows for highly sensitive measurements to be obtained with low-cost,even portable,instrumentation by exploiting different sensing modes,such as intensity,lifetime and polari-zation bined with the high spatial resolution of fluorescence microscopy,these features have enabled the intra-cellular monitoring of many different species for medical and biological purposes.Other important applications have also been pursued,from analytical tools for environmental and biochem-ical assays,to switches and logical gates for molecular informa-tion processing.3Since the seminal work of Tsien in the 1980s,4the design of fluorescent chemosensors has been constantly evolving as more and more sophisticated sensing modes have been explored.In itsa Dipartimento di Scienze Chimiche,Universit adi Padova,via Marzolo 1,35131Padova,Italy.E-mail:Fabrizio.mancin@unipd.it;Fax:+390498275666;Tel:+390498275239bDipartimento di Chimica,Universit adi Trieste,via Giorgeri 1,34127Trieste,Italy.E-mail:ptecilla@units.it;Fax:+390405583903;Tel:+390405583925Luca Ba uLuca Ba uobtained his PhD in 2010at the University of Padova,where he worked on the synthesis of organic molecules and nanosystems for use as intracellular fluorescent sensors,photosensitizers for photody-namic therapy and drug delivery systems.He is currently a post doctoral fellow at the same institution.His research inter-ests lie in the area where organic chemistry and biology meet,his current focus being the synthesis of fluorescent probes for biomedicalapplications.Paolo Tecilla Paolo Tecilla received his PhD in chemistry from the University of Padova with Professor Tonellato in 1989.After post-doctoral work with Professor A.D.Hamilton at the University of Pittsburgh,he took a Lecturer position at the University of Padova.In 1998,he moved to the University of Trieste,where he is now a full Professor of Organic Chemistry.His scien-tific interests are in the field ofsupramolecular chemistry andfocus on the metallocatalysis of hydrolytic reactions,fluorescentchemosensors and model membranes.FEATURE ARTICLE /nanoscale |NanoscaleD o w n l o a d e d o n 21 O c t o b e r 2010P u b l i s h e d o n 21 O c t o b e r 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0N R 00405Gsimplest form,a fluorescent chemosensor is composed of a fluo-rescent dye and a receptor integrated into the same molecule.2A built-in transduction mechanism converts recognition events into variations of the fluorescent dye emission properties.The design of a chemosensor may differ in the level of integration of the two units,in the transduction mechanism or in other subtler details.The receptor and dye units may be part of the same p system,or connected by electronically isolating spacer moieties.2They may even be discrete entities,self-organized through non-covalent interactions.5The sensing scheme may rely on emission quenching by the substrate,activation or deactivation of energy or electron transfer processes,alteration of internal charge transfer (ICT)excited states,or molecular movements resulting in a variation of the distance between two photoactive compo-nents.1The latter effect may cause,in turn,emission quenching,formation of excimers,or switching of distance-dependent energy or electron transfer processes.1More sophisticated designs use antenna groups or multi-dye systems,and may lead to yet other effects such as signal amplification or ratiometric signaling.6As far as the output signal is concerned,intensity measure-ments are by far the most popular choice,mainly because they require relatively inexpensive instrumentation,in contrast with the more demanding lifetime and polarization measurements.1,2Variations in the emission spectrum of a chemosensor may consist either in a general decrease of the intensity (quenching or on-off chemosensors),an increase of the emission (turn-on or off-on chemosensors)or a variation of the spectral shape (ratiometric chemosensors).While off-on behavior is usually preferred over the less sensitive on-off response,ratiometric signaling is by far the most useful.By using the ratio between the emission intensities at two different wavelengths as the analytical signal,the effect of such factors as photobleaching (the progressive decrease of emission due to photoinduced degrada-tion),sensor concentration,polarity of the microenvironment,and fluctuations of the excitation source can be minimized.2The signaling unit of a fluorescent chemosensor is a key component of the assembly,often limiting its practical applica-tion.A huge number of organic dyes,fluorescent proteins andluminescent metal complexes have been used for this purpose over the years.Even the most efficient among conventional fluorophores,however,suffer from some inherent limitations.Photobleaching,limited brightness (the product of extinction coefficient and fluorescence quantum yield)and short lifetimes are serious drawbacks in many applications.7In the context of intracellular measurements,chemical interactions with biomolecules are another cause of concern.8In the last decade,fluorescent nanoparticles have emerged as a new class of fluoro-phores with the potential to overcome these limitations.9Some nanoparticles,such as quantum dots,are intrinsically fluore-scent,while others can be made fluorescent by appropriate doping with fluorescent dyes or luminescent metals.Quantum dots (QDs)are crystals of semiconductor materials a few nanometres in size,with a size-dependent electronic structure which is responsible for some peculiar photophysical properties.7,10Their broad absorption and narrow emission spectrum can be finely tuned by varying the size and composition of the particles,which are also remarkably bright and stable against photobleaching.These features make QDs especially useful both in energy transfer applications and in simultaneous assays exploiting multiple excitation and emission wave-lengths.7,10Organic dyes and luminescent transition metal complexes can be also embedded into silica or polymer nano-particles.9,11The inclusion of dyes in a polymer matrix results in decreased emission quenching and increased photostability due to oxygen and solvent exclusion.11Many of the optical properties of quantum dots can thus be obtained with biocompatible materials and easy,low-cost preparation procedures.As soon as fluorescent nanoparticles became available,several applications in the field of sensing were explored (Fig.1).Nanoparticles were only used,at first,as better-performing substitutes of traditional dyes in biolabeling applications and fluorescence-based assays,12but it soon became clear that these unique objects,combining the properties of extended solids and molecular species,had much more to offer than that.The high amount of reactive sites found on their surface make them ideal multivalent scaffolds for the realization of supramolecular assemblies,while their high surface to volume ratio allows for distinct spatial domains (bulk,external surface,pores and shells)to be functionalized to a comparable extent with different organic species.Over the last few years,nanoparticles proved to be versatile synthetic platforms for the implementation of new chemosensing schemes.2.Sensing quantum dotsIn the most na €ıve approach,a chemosensor would be built by simply sticking a fluorescent dye and a receptor together in the most synthetically straightforward way.Unfortunately,such a strategy is rarely effective without a transduction mechanism in place.The recognition of an analyte by the receptor does not necessarily affect the emission of the fluorophore,unless the analyte itself is a fluorescence quencher.Even so,only the less sensitive on-off response can be obtained.The peculiar physical properties of quantum dots,however,seem to offer a way to exploit this attractively simple approach.The emission of quantum dots originates from the recombination of photo-generated electron-hole pairs,which takes placepredominantlyFabrizio Mancin Fabrizio Mancin received his PhD in Chemistry from the University of Padova with Professor Tonellato in 1999.On the same year he took a Lecturer position at the University of Padova,where he is now asso-ciate Professor.In 2002he spent one year at the University of Toronto as post-doc researcher with Prof.Jik Chin.His research interest is focused on supramolecular chemistry,particularly on the developmentof artificial metallonucleases and on the design of fluorescencemolecular chemosensors.Recently he has broadened his interest to include nanosystems for biomedical applications.D o w n l o a d e d o n 21 O c t o b e r 2010P u b l i s h e d o n 21 O c t o b e r 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0N R 00405Gon the surface of the nanocrystals.As a consequence,the binding of charged species,such as metal cations,close to the surface of quantum dots may affect and even increase their emission.This concept provides the unique possibility to create fluorescent chemosensors by simply grafting ligands to the particles (Fig.1a).The first demonstration of this concept was described by Rosenzweig and his co-workers,who used CdS quantum dots coated with polyphosphate,L -cysteine,or thioglycerol,as water-soluble metal ion sensors.13The nature of the ligands had a dramatic effect on the type and selectivity of luminescence response.Polyphosphate-capped CdS QDs showed no selectivity and displayed an on-off behavior.Thioglycerol-capped QDs were selective towards copper and iron ions over several other transition metal ions with an on-off response.Finally,L -cysteine-capped QDs responded selectively to zinc ions with an off-on behavior and detection limits below 1m M.Similar sensing schemes have been later exploited for several others analytes including metal cations (Fig.2),anions and organic molecules,even if in some cases a simple quenching of the QD emission by the substrate is likely responsible for the observed behaviour.14Sensitivity towards organic molecules was achieved by decorating the QD surface with more sophisticated recognition units:calixarenes were used to bind and signal acetylcholine (Fig.3)14c while cyclodextrins allowed the discrimination of enantiomeric amino acids.14aBesides the modification of surface properties,other mech-anisms typical of molecular chemosensors can also be exploited.For instance,a photoinduced electron transfer (PET)from thecoating molecules to the valence band of an excited QD results in emission quenching.If the electron donor is bound to a substrate,this process may be interrupted and the emission restored.Several groups have recently explored this mechanism.Singaram and co-workers (Fig.4)showed that the addition of a boronic acid substituted viologen to commercially available CdSe/ZnS QDs with carboxylic groups on the surface results in a strong quenching of the emission from the QDs.Likely,electrostatic interactions cause the absorption of the viologen derivative on the anionic surface of the QDs and the achieved proximity allows the viologen to quench the QD emission trough a PET mech-anism.15Subsequent addition of glucose converts theboronicFig.1Representative sensing schemes in fluorescent nanoparticles:(a)Analyte binding to a surface receptor causes spectral changes in a quantum dot by altering its surface electronic structure;(b)analyte binding to a surface receptor affects an energy transfer process from a quantum dot to an organic fluorophore;(c)analyte binding to a nanoparticle embedded chemosensor in the presence of a co-embedded reference dye triggers a ratiometric response in a PEBBLE sensor;(d)ion binding to a nanoparticle embedded ionophore is reported by a co-embedded pH-sensitive dye in a PEBBLE sensor;(e)analyte binding to one surface-grafted chemosensor affects several surrounding units,resulting in signal amplification;(f)receptors and fluorophores are individually grafted to the surface of a nanoparticle in a self-organizedsensor.Fig.2Cu(I )sensing system based on PEGylated CdS clusters:(a)cluster structure;(b)photoluminescence increase upon Cu(I )addition in water,as a consequence of the perturbation of the surface electronic structure of the quantum dots (adapted from ref.14b ).D o w n l o a d e d o n 21 O c t o b e r 2010P u b l i s h e d o n 21 O c t o b e r 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0N R 00405Ggroups into the corresponding glucose boronate esters,which are worse electron donors,at the same time decreasing the affinity of the quencher molecule for the QD surface.As a result,the QD emission increases.Glucose concentrations in the 2–20milli-molar range can be detected in water at pH 7.4.Amine func-tionalized QDs can also be used in the same way:in this case,however,the quenching efficiency of the viologen derivative is lower,probably because of weaker interactions with the posi-tively charged QD surface.Mareque-Rivas and co-workers (Fig.5)used a similar approach to develop a sensing system for cyanide anions.16A 2,20-bipyridine copper(II )complex was adsorbed on the trioctylphosphine oxide coating layer of CdSe QD,resulting in the quenching of the QD emission by an electron transfer from the excited QD to the metal complex.Addition of cyanide ions caused the displacement of the copper(II )ions from the bipy complex,thus restoring the QD emission.Santra and co-workers coated the surface of CdS:Mn/ZnS QDs with azacrown derivatives to obtain a cadmium(II )sensor.In this case,however,the sensitivity was in the millimolar range and several transition metal ions interfered with the cadmium iondetection.17Finally,QDs coated with a urea-bearing ferrocene derivative were used by Callan and co-workers to selectively detect fluoride ions in the sub-millimolar range (detection limit 0.074mM in chloroform).18Apparently,the binding of fluoride to the urea receptors alters the reduction potential of the ferro-cenyl groups causing the interruption of the PET process.A more sophisticated PET-based strategy was devised by Benson and coworkers (Fig.6),who designed a single-molecule biosensor for maltose in the micromolar range.19The authors prepared a chimeric protein composed by amaltose-bindingFig.3CdSe/ZnS QDs for acetyl choline sensing:(a)Schematic repre-sentation of calixarenes coated QDs;(b)fluorescence quenching of QDs in the presence of neurotransmitter compounds and choline (1.6mM,PBS buffer pH 7.4;adapted from ref.14c).Fig.4Working scheme the turn-on glucose sensing system of Singaram et al.:a viologen quencher detaches from the surface of quantum dots upon esterification of pendant boronic acids with glucose (reproduced from ref.15).Fig.5Cyanide sensing QD studied by Mareque-Rivas et al.:(a)QD schematic structure;(b)sensing scheme and naked-eye fluorescent detection of cyanide:the analyte displaces a quenching ion from a surface-grafted ligand,and the emission is restored as a result (vial (i)contains different potentially interfering anions than in the other vials;no Cu(I )complex in vial (a);adapted from ref.16).Fig.6Working scheme of the PET-based maltose sensing system studied by Benson et al.:maltose binding to a surface-grafted MBP protein conjugated with a quencher triggers a conformational change in the protein and restores the emission (reproduced from ref.19).D o w n l o a d e d o n 21 O c t o b e r 2010P u b l i s h e d o n 21 O c t o b e r 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0N R 00405G(MBP)domain and a metallothionein (MT)domain.The MBP-MT protein was selectively conjugated with a ruthenium(II )complex and grafted to a CdSe QD through the cysteine residues of the MT domain.The ruthenium(II )complex quenches the QD emission through a PET process.Maltose binding to the MBP domain causes the protein to undergo a severe conformational change that increases the distance between the Ru(II )complex and the QD,thus interrupting the PET process and restoring the emission.The most largely exploited sensing mechanism,however,is theactivation/deactivation of F €orster resonance energy transfer (FRET)processes (Fig.1b).7Very recently,P.-T.Chou and coworkers developed a ratiometric sensor for potassium ions based on a mixture of two 15-crown-5functionalized CdSe/ZnS QDs of different sizes.20The different size of the QDs makes them suitable as a FRET pair.Potassium ions,when present,are bound in a sandwich-like manner by the crown ether units grafted to different nanoparticles.This results in the activation of an energy transfer process between different particles and in the consequent modification of the emission spectrum of the mixture.Several other sensing schemes involving FRET processes have been designed.The general idea is to use the analyte recognition to modulate the energy transfer process between the QD and a dye appended to its surface.As in the previous example,such modulation will produce a ratiometric response of the sensing system to the analyte.Molecular beacons (fluorescent DNA sensors based on hybridization)are the most explored application.In a typical probe,a QD is functionalized with a DNA sequence comple-mentary to the target sequence,while the sample to be analyzed is conjugated,using standard labeling procedures,with a fluoro-phore whose absorption spectrum overlaps with the emission of the QD.If the target DNA is present,successful hybridization results in the activation of a FRET process where the QD emission decreases and the fluorophore emission becomes stronger.7This strategy has also been exploited in DNA-functionalized gold nanoparticles,with the particles acting as quenchers.21Analyte detection may also be achieved by pursuing the reverse process,i.e.interruption or modulation of FRET due to analyte-induced displacement of a fluorophore from a QD-bound receptor.Such an approach was first explored by Mattoussi and co-workers in 2003using CdSe QDs conjugated with a maltose binding protein (MBP).22A sugar derivative functionalized with a non-emissive dye was bound to the protein resulting in FRET quenching of the QD emission.Upon maltose addition,the quencher is displaced by the protein and the QD emission restored.A 2,4,6-trinitrotoluene (TNT)sensor was also realized in a subsequent work (Fig.7)using a CdSe/ZnS QD,an anti-TNT antibody and a quencher-conjugated TNT analogue.23A similar approach was recently explored by Willner and coworkers (Fig.8)using a boronic acid derivative instead of MBP.24Displacement of dye-labeled galactose or dopamine derivatives from the boronic acid functionalized CdSe/ZnS QDs resulted in the interruption of FRET and gave rise to a ratio-metric response.Submillimolar concentrations of galactose and glucose and micromolar concentrations of dopamine could be detected in water at pH 7.4.Conformational changes of MBP upon maltose binding were also exploited by the Mattoussi group to modulate FRET.25In a work published simultaneously with the previously discussed PET system of Benson et al.,19an engineered MBP was func-tionalized with a Cy3dye and bound to CdSe trough a C-termi-nal pentahystidine fragment.In the resulting nanobiosensor,the FRET process is highly efficient and QD excitation at 300nm results in an emission spectrum containing mainly the Cy3features.Maltose addition increases the donor–acceptor distance resulting in a decrease of the Cy3emission.As an alternative to donor–acceptor distance variations,FRET can also be modulated if the absorption spectrum of the acceptor is modified by analyte recognition.Such changesmayFig.7TNT sensing QDs based on the displacement of a quencher conjugate (TNT-BHQ10)from a QD-bound anti-TNT antibody.(a)Sensing scheme;(b)fluorescence recovery (reproduced from ref.23).Fig.8Competitive analysis of monosaccharides or dopamine using fluorophore-labeled galactose or dopamine and boronic acid function-alized QDs (reproduced from ref.24).D o w n l o a d e d o n 21 O c t o b e r 2010P u b l i s h e d o n 21 O c t o b e r 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0N R 00405Gdecrease or increase the overlap between the QD emission and the acceptor absorption thus modulating the efficiency of FRET.This approach has been explored by conjugating pH-sensitive dyes to QDs.In particular,Nocera and co-workers functional-ized polymer-coated CdSe/ZnS QDs with a pH-sensitive squar-aine dye.26At pH values below 7.5,the overlap between squaraine absorption and QD emission is high and the emission spectrum is dominated by the features of the organic dye.When the pH is raised,squaraine absorption is blue shifted and FRET becomes less efficient:the emission originates predominantly from quantum dots.This results in a remarkable ratiometric behavior that allows the determination of pH with better accu-racy than the dye alone in highly scattering environments.3.Sensing dye-doped particles:PEBBLEsThe unusual optical properties of QDs have attracted great interest and stimulated the development of many ingenious nanosensors,which were discussed in the previous paragraph.However,QDs have only been used in most cases as ‘‘better’’organic dyes,and the same sensing schemes could be,and often have been,exploited with conventional dyes.More complex sensing schemes,exploiting the inherent multivalency of nano-particles to trigger collective and cooperative processes,have been explored using dye-doped nanoparticles.The first approach to the realization of dye-doped sensing nanoparticles was proposed in the late 1990s by Kopelman and co-workers for studies in live cells and dubbed PEBBLE (Probes Encapsulated By Biologically Located Embedding or,more recently,Photonic Explorers for Bioanalysis with Biologically Localized Embedding).8,27In its simplest embodiment,the PEBBLE approach involves the encapsulation of classical fluo-rescent chemosensors into water-soluble polymeric nanoparticles (Fig.1c,d).The semi-permeable and transparent nature of the matrix allows the analyte to interact with a dye that reports the interaction via a change in the emitted fluorescence.This sensing scheme may look the same as that of classical chemosensors,but a closer inspection reveals several distinct advantages.First,the inclusion of chemosensors into nanoparticles minimizes the interaction with cell components such as proteins or membranes.Multifunctional systems implementing complex sensing schemes can also be easily realized.For instance,reference dyes can be added to enable ratiometric sensing,or ionophore/chromoiono-phore combinations can be used in order to take advantage of highly selective but non-fluorescent ionophores.As an example,Kopelman and co-workers 28reported on a ratiometric sensor for intracellular oxygen obtained by including a ruthenium complex [Ru(dpp)3]2+and the dye Oregon Green 488in silica PEBBLEs with diameters ranging from 100to 400nm.In the presence of oxygen,the fluorescence emission of [Ru(dpp)3]2+is strongly quenched while the emission of Oregon Green is not affected,thus allowing the ratiometric determina-tion of oxygen levels.These PEBBLEs were introduced into living cells using gene gun delivery techniques and used for the monitoring of variations of the oxygen level in the cytosol.Similar results were reported by the same research group 29using 120nm diameter ormosil PEBBLEs doped with a platinum porphyrin (platinum(II )octaethylporphine ketone),an oxygen-sensitive dye,and octaethylporphine as a reference dye.Compared to the previous example,this ormosil PEBBLE sensor shows higher sensitivity,a wider linear range and longer excita-tion and emission wavelengths.This last feature is especially important in intracellular measurements,where the background noise from fluorescent biomolecules (‘‘autofluorescence’’)is often a source of confusion.Moreover,these PEBBLE sensors display excellent reversibility and stability with respect to leaching and long-term storage.Other examples of oxygen sensing PEBBLEs have been reported by Kopelman et al.and others using organic polymeric nanoparticles.30The versatility of PEBBLEs was proved by targeting several different analytes.pH is probably one of the most obvious targets,not least because of the large number of pH-sensitive dyes available.Core-shell silica nanoparticles were used by Wiesner and co-workers for ratiometric pH sensing in vitro (Fig.9).31Tetramethylrhodamine,which is pH-insensitive,was covalently included into 50nm silica particles that were sub-sequently coated with a 10nm thick shell doped with the pH-responsive dye fluorescein.The core-shell structure of the particles,whereby sensitive units are excluded from the solvent-inaccessible core and thus prevented from contributing to the background signal,results in increased sensitivity.A different sensing scheme was demonstrated by Wolfbeis and co-workers.32The emission intensity of a fluorescent nano-particle is modulated by the pH-dependent absorption of a polyaniline coating layer.The organic conducting polymer acts as a screen by limiting the amount of exciting light reaching the fluorophores in a variable fashion,depending on the amount of protonated residues.By using fluorescent dyes with absorption centred on either side of the isosbestic point of polyaniline,it was possible to obtain an increase or a decrease of fluorescence with increasing pH depending on the excitation wavelength.With a p K a around 7,polyaniline is especially suitable for pH sensing in a physiologicalenvironment.Fig.9Core-shell silica nanoparticles doped with rhodamine and fluorescein for ratiometric pH sensing:(a)nanoparticle scheme;(b)ratiometric response;(c)false-color ratiometric imaging of pH in various intracellular compartments (inset:overlaid image;adapted from ref.31).D o w n l o a d e d o n 21 O c t o b e r 2010P u b l i s h e d o n 21 O c t o b e r 2010 o n h t t p ://p u b s .r s c .o r g | d o i :10.1039/C 0N R 00405G。

我最喜欢历史用英语写60词左右作文全文共6篇示例，供读者参考篇1My Favorite Subject Is HistoryHistory is the best subject ever! I know a lot of kids think it's boring, but I totally disagree. History is like one huge, epic story spanning thousands of years and covering every part of the world. There's adventure, drama, conflict, discovery, and so much more. I can't get enough of it!One of the coolest things about history is learning about ancient civilizations that seemed almost like fantasy worlds. Like Ancient Egypt with its massive pyramids, mysterious pharaohs entombed with treasures, and cool artifacts decorated with hieroglyphics. Or Ancient Greece with epic legends about heroes and gods. I loved reading the stories of Odysseus's long journey home after the Trojan War. Such amazing adventures!And then there are the epics of Ancient Rome. With its rise from a tiny village to a vast empire stretching across Europe, North Africa and the Middle East. The Romans built impressive engineering marvels like roads, aqueducts and gigantic stadiumscalled Colosseums where they held epic battles and bloodsports. I've watched every movie and documentary about Rome I can find. Gladiator is one of my favorites!Speaking of movies, one era I'm obsessed with is the Middle Ages. It just seems so fascinating and romantic with heroic knights, epic quests, and towering castles. I can't get enough of tales about King Arthur and the Knights of the Round Table or legends of Robin Hood defying the evil Prince John and his villainous henchmen.The Renaissance that followed starting in the 1400s was pretty neat too. With geniuses like Leonardo da Vinci, Michelangelo and Shakespeare revolutionizing art, science and literature. I loved reading about the daring explorers of that age like Christopher Columbus, Ferdinand Magellan and others who sailed across vast oceans to unknown lands. Such courage and thirst for adventure!Another era I'm really into is the Age of Enlightenment in the 1700s. With the American and French Revolutions shaking off monarchy and birthing modern democracy and human rights. It was incredible learning how revolutionaries like George Washington, Thomas Jefferson, Ben Franklin and others stood upto the mighty British Empire to create a brand new nation dedicated to life, liberty and the pursuit of happiness.But those were tame times compared to the chaos and carnage of the 1800s and early 1900s. With the rise of powerful nation states waging massive wars that spanned continents and introduced terrifying new weapons. Just learning about intense conflicts like the American Civil War, World War 1 and World War 2 is mind-blowing. The scale, the death tolls, the heroic actions and horrific atrocities all seem unbelievable.I have to admit, I get a little depressed studying the sheer degree of human suffering, cruelty and oppression that has occurred throughout history. The harsh treatment of enslaved people, the genocide of indigenous populations, deadly racism, the Holocaust, and so many other grave injustices and atrocities are just heartbreaking. But I think it's crucial we learn about the darkness in our past so we never let it happen again.That's why I was really inspired learning about heroic figures who fought against oppression and made sacrifices for freedom and human rights. Leaders like Abraham Lincoln, Susan B. Anthony, Dr. Martin Luther King Jr., Nelson Mandela and Malala Yousafzai. Their courage, vision and tireless activism in the face of daunting challenges is so admirable.Overall, while history definitely has its dark chapters, I still find the entire panorama of human civilization to be utterly fascinating. All the different cultures, empires, inventions, faiths, conflicts and movements across the millennia make for an endlessly compelling story about our species. And of course, history isn't just the past - it's still being written by the people of today.Who knows what awesome, inspiring or tragic events will be added to the chronicles of human history in years to come? I can't wait to learn all about it. In the meantime, I'll just keep devouring every book, documentary and lecture I can on history. It's hands-down the most epic, dramatic andengrossing subject there is. If you haven't already, I highly recommend getting into it! Trust me, you'll be hooked in no time.篇2My Favorite Subject is HistoryHistory is my absolute favorite subject in school! I just love learning about all the interesting people, places, and events from long ago. It's like getting to travel through time and see the world as it used to be. How cool is that?In history class, we get to read all kinds of fun stories from the past. Last year, we learned about ancient Egypt and the mighty pharaohs who ruled over that civilization. Can you imagine living in a huge pyramid tomb and being treated like a god? We made miniature pyramids out of sugar cubes and it was so much fun stacking them up really tall. I wished I could shrink down and explore the tiny passageways we made!This year, we're studying the colonial era in America. I find it fascinating how the early settlers had to struggle and work so hard just to survive in this new land. We read accounts of children our age helping their families build log cabins, clear fields for planting, and fend off wild animals. My life seems so easy in comparison! Though I don't think I'd want to go a whole winter without bathing like they did back then – yuck!What I love most about history are all the amazing stories of resilience, courage, and perseverance by people who came before us. Like Harriet Tubman, who escaped slavery and then risked her life over and over to help free hundreds of others through the Underground Railroad. Or the pioneers who spent months traveling across harsh landscapes in covered wagons to seek out new opportunities out west. Or the inventors whocreated things that changed the world, like the light bulb, automobiles, and airplanes.Hearing their stories makes me feel braver to take on challenges in my own life. If they could survive such difficult circumstances, surely I can handle whatever obstacles come my way with determination and grit. That's an important lesson history teaches us – our ancestors paved the way for us, and we owe it to them to work hard and make the most of the opportunities their sacrifices afforded us.My friends all tease me for being such a history buff. But I don't care – I think it's the coolest subject ever. Getting to learn about events that actually really happened hundreds or thousands of years ago is just mind-blowing to me. I bet dinosaurs used to roam where our school playground is now. And natives lived in huts on this very same land before modern cities sprung up. History is all around us if we take the time to notice and appreciate it.That's why I always beg my parents to take me to museums and historical sites whenever we go on vacation. You'd think I'd get bored looking at artifact displays or touring old buildings. But not me! Every object has a story behind it, and I imagine who it might have belonged to and how it was used. I run my handsalong structures constructed centuries ago and try to envision the people who built them. What were they like? What did they look like? What were their lives like back then? I wish I could travel back in time just for a day to see for myself!My dream is to become an archaeologist or historian when I grow up so I can go on real digs and discover artifacts that reveal secrets about ancient civilizations. Or better yet, find evidence that rewrites what we know about the past! How amazing would it be to uncover a Viking village in Canada, proving the Norse explorers traveled further than historians realized? Or locate ruins that turn out to be remnants of an advanced society that predated the Mayans and Incas? Unlocking the mysteries of the past could lead to invaluable new understandings about our human origins and development.That's why it's so important that we study and preserve the history we do know about. By learning from past mistakes and accomplishments, we can make smarter choices for our future. Nations that ignore or rewrite the hard lessons their ancestors had to learn often end up tragically repeating the same atrocities. But societies that embrace their full, complex histories – both the good and the bad – are more likely to move forward in a positive direction influenced by the sacrifices of previous generations.So even when my friends say I'm weird for liking history so much, I just shrug it off. Yeah, maybe I am a little weird and nerdy when it comes to learning about the past. But the way I see it, if we forget where we came from and ignore the amazing stories of our ancestors who paved the way for modern life, then we dishonor their struggles and fail to appreciate how篇3My Favorite Thing About HistoryHi there! My name is Jamie and I'm 10 years old. Today I want to tell you all about my favorite thing to learn about in history class - ancient civilizations! I think ancient civilizations are just so fascinating. It's amazing to learn about how people lived thousands of years before we were even born.I especially love learning about ancient Egypt. Can you imagine living back then next to the massive pyramids and the mighty River Nile? The Egyptians had such an incredibly advanced society for the time. Their inventions and accomplishments were way ahead of so many other cultures from the same era.It blows my mind that the ancient Egyptians had ways to preserve bodies after death through mummification. They wouldcarefully remove all the organs, dry out the body, and wrap it up tightly in linen. This allowed the dead person's body to basically last forever! The mummies we've found from ancient Egypt are thousands of years old but still look almost exactly the same as when they were first mummified. It's like the ultimate life hack to beat death and decay! I can't get over how clever the Egyptians were to figure that out.The Egyptian writing system with hieroglyphics is another thing I'm obsessed with learning about. It's so crazy that they had their own alphabet made up of little pictures that each represented a word or sound. Some of the hieroglyphs are pretty easy to figure out, like an eye symbol to represent the word "eye." But others are really tricky, like a picture of reeds to represent the idea of "flourishing." It must have taken the scribes years and years to master writing in hieroglyphs. I've tried my hand at drawing some basic hieroglyphs and let me tell you, it is not easy! The level of artistic skill those Egyptian scribes had is mind-blowing.The discovery of King Tutankhamun's tomb in the 1920s is hands-down one of the most epic stories I've learned about in history class. For over 3,000 years, his tomb filled with glittering golden treasures laid hidden and untouched in the Valley of theKings. Can you imagine how mind-blowing it must have been for those archaeologists to crack open that sealed tomb door and be the first people in over 30 centuries to lay eyes on King Tut's mummy and priceless artifacts? The photos I've seen of all the extraordinary artifacts make me feel like I've uncovered buried riches too. King Tut's death mask made of solid gold and lapis lazuli gemstones is probably the most dazzling thing I've ever seen. I would have fainted dead away!I could go on and on about all the reasons why I find ancient Egypt so captivating. The grand, towering monuments that still stand after thousands of years. The wealthy royals entombed alongside their prized possessions. The beautiful art and jewelry adorned with vibrant colors. The crazy cool system of picture writing that's so different from our alphabet. Basically everything about that ancient civilization gives me such a rush of excitement to learn more!Ancient Egypt is just one example of the amazing ancient cultures I've learned about that have me hooked on studying history. I'm also completely fascinated by ancient Greece and Rome, the Maya and Aztec civilizations, ancient China and its dynasties, and so many others. Digging into how these pivotal ancient societies lived, worked, built cities, invented things,worshipped gods, and laid the foundations for our modern world today is my favorite thing to geek out on.I know a lot of kids find history kind of boring because it's just a bunch of stuff that happened a really long time ago. But to me, it's like getting to travel back in time and see how life was for real people centuries before I existed. Each new civilization I learn about feels like opening up a fresh adventure novel full of fascinating tales, unbelievable accomplishments, and rich, complicated societies. My brain practically overheats from being so energized by all the wonders of the ancient world!Whenever we get assigned essays or projects focused on historical time periods and cultures, I get really excited.篇4My Favorite HistoryHistory is my favorite subject in school! I just love learning about all the cool stuff that happened in the olden days. It's like getting to travel back in time without a magic time machine. My history teacher, Mr. Johnson, makes it really fun and interesting too.One of the historical events I found most fascinating was the Egyptian civilization. Can you believe those ancient Egyptians built massive pyramids that still stand today? That's justmind-blowing to me. And they did it all without modern construction equipment and machinery! The pyramids were tombs for the pharaohs, which were the rulers of ancient Egypt. The most famous pyramids are the Great Pyramids of Giza just outside of Cairo.Inside the pyramids, archeologists have found all sorts of treasures and artifacts from Egyptian life. This includes mummies - preserved bodies wrapped up in cloth. The Egyptians mummified their dead because they believed in an afterlife. They wanted the bodies to be preserved so the souls could use them again in the afterlife. Creepy but also kind of cool!Hieroglyphics are another fascinating part of ancient Egyptian civilization. Hieroglyphics were the writing system the Egyptians used, with symbols and pictures representing words and sounds. It looks like a super hard code to me! The Rosetta Stone was the key that finally allowed historians to decipher the hieroglyphic code in the 1800s.Ancient Greece is another one of my favorite historical civilizations to learn about. I'm in awe of how advanced andinfluential their culture was. The Greeks came up with the first democratic government system and pioneered early disciplines like math, science, philosophy, and theater. Famous Greek thinkers like Socrates, Plato, and Aristotle helped shape the foundations of Western thought and learning.In Athens, people could vote on the laws of their city. This was radical for the time period! Of course, only free male citizens could actually vote. But it was still an early step towards the democratic process. The Greeks also invented the Olympic Games as a way to honor their gods. The first ancient Olympic events included foot races, wrestling, boxing, and chariot racing.Ancient Rome is another civilization from history that I find endlessly intriguing. The Roman Empire stretched across a huge part of Europe, the Middle East, and North Africa at its peak around 2000 years ago. Just imagine - all those diverse lands and peoples all united under one central Roman government!The Romans accomplished some remarkable engineering and architectural feats too. Their roads, aqueducts to supply water, and huge coliseums where gladiators fought are still incredibly impressive even today. And let's not forget the Roman numerals we still use - those originated from the Romans too!I also am amazed by the legends and myths from ancient Rome. Tales of the Trojan Horse from the Trojan War or Romulus and Remus being raised by wolves are stories that have been passed down for thousands of years. And who could forget mythological Roman gods and goddesses like Jupiter, Juno, Mars, and Venus? So imaginative and symbolic.My two other favorite historical events to study are the exploration era when Europeans first came to the Americas, and the settling of the 13 American colonies that became the United States. I find it wild to think that people had to cross vast oceans in tiny ships to reach new unexplored lands back then.Christopher Columbus first making landfall in the Bahamas in 1492 kicked off an entire age of exploration and colonial conquest across the Americas by European nations like Spain, France, and England. It must have been both amazing and terrifying to be one of the first Europeans to set foot in a place like the Americas for the very first time.While the colonization unfortunately led to devastating consequences for Native American peoples, I still find the stories of the earliest English settlements like Jamestown and Plymouth to be really captivating. The struggles of daily colonial life, interactions with indigenous tribes, and eventually the sparksthat led to the American Revolutionary War are all endlessly fascinating history topics to me.Those are just some of the highlights of my favorite historical subjects and eras that we've learned about so far. Who knows what other incredible events and civilizations from long ago we'll get to explore next? Mr. Johnson says in the years ahead we'll dive into periods like the Middle Ages, the Renaissance, ancient China and Japan, and many others. I can't wait!No matter how much time has passed, the stories and accomplishments from past civilizations continue to shape and influence the modern world we live in today. Learning about history gives me a deeper appreciation for how we've篇5My Favorite Subject is HistoryHistory is the best subject ever! I just love learning about all the cool things that happened in the past. It's like getting to travel through time without actually having a time machine. How awesome is that?In history class, we get to learn about ancient civilizations like the Egyptians, Greeks, and Romans. Can you imagine living back then and helping build the Great Pyramids or the Parthenon? Those ancient people were super smart to create such incredible structures without modern technology. I'd love to meet them and ask them how they did it.We also study major events in history like wars, revolutions, and explorations. Last year, we learned all about the American Revolution. The Colonies going up against the mighty British Empire? Talk about an underdog story! I loved reading about brave heroes like George Washington, Paul Revere, and Betsy Ross. It must have been both terrifying and exciting to fight for your country's independence.This year, we're studying the age of exploration and learning about daring adventurers like Christopher Columbus, Ferdinand Magellan, and Neil Armstrong. Can you imagine being one of the first people to cross the vast oceans or even walk on the moon? Those explorers had no idea what they might discover but they went anyway. I think that took a whole lot of courage.My favorite kinds of history lessons are the ones about ordinary people's lives in the past. How did kids go to school back then? What games did they play? What kind of chores didthey have to do? It's fascinating to see how different yet similar life used to be compared to today.Like last week, we learned about what it was like being a pioneer kid living in the American West in the 1800s. They had to do backbreaking farm work, carry water for miles, and study by candlelight after a long day of chores. No electricity, video games, or slides at the park! On the other hand, they had awesome pioneering adventures and lived closer to nature. I'm not sure I could have hacked pioneer life but I have a ton of respect for those tough kids.That's one of the main reasons I love history so much - it teaches us valuable lessons about how to create a better future by understanding the mistakes and triumphs of the past. As they say, those who don't learn from history are doomed to repeat it. I don't want to repeat some of those terrible parts of history, that's for sure!But history also shows us how resilient and remarkable humans can be, even in the darkest of times. Learning about people who bravely fought for freedom, stood up against oppression, and overcame tremendous odds makes me feel inspired. If they could get through such difficult situations, then I know I can tough it out when things get hard too.Most of all, studying history gives me a bigger perspective on the world and my place in it. I'm just a tiny speck in the grand scheme of the centuries and civilizations that came before me. But that makes me want to leave my own positive mark, just like the great leaders, innovators, and unsung heroes I read about. Maybe someday, students will be learning about me and the impact I had! Okay, probably not, but a kid can dream, right?In the meantime, I'll just keep on soaking up as much history knowledge as I can. I might not have all the answers, but at least I can say "hindsight is 20/20!" Thanks, history class, for helping me see the past clearly so I can better understand the present and shape the future. You're the best, and don't ever change!篇6Title: My Favorite HistoryHi, everyone! Today I want to tell you about my favorite subject in school, and that is history! I love history because it teaches us about the past. We learn about famous people like kings, queens, and explorers. We also learn about important events like wars and discoveries. History is like a time machine that takes us on exciting adventures. I enjoy reading about ancient civilizations and their amazing achievements. Historyhelps us understand how the world has changed over time. It's so fascinating! I hope you all love history as much as I do!这篇作文以小学生的语气简单明了地表达了对历史的喜爱，并且提到了一些历史中的重要元素。