物理抽象概念

物理抽象概念最广泛被接受关于时间的物理理论是爱因斯坦的相对论。

在相对论中，时间与空间一起组成四维时空，构成宇宙的基本结构。

时间与空间都不是爱因斯坦爱因斯坦绝对的，观察者在不同的相对速度或不同时空结构的测量点，所测量到时间的流逝是不同的。

广义相对论预测质量产生的重力场将造成扭曲的时空结构，并且在大质量（例如：黑洞）附近的时钟之时间流逝比在距离大质量较远的地方的时钟之时间流逝要慢。

现有的仪器已经证实了这些相对论关于时间所做精确的预测，并且其成果已经应用于全球定位系统。

另外，狭义相对论中有“时间膨胀”效应：在观察者看来，一个具有相对运动的时钟之时间流逝比自己参考系的（静止的）时钟之时间流逝慢。

就今天的物理理论来说时间是连续的，不间断的，也没有量子特性。

但一些至今还没有被证实的，试图将相对论与量子力学结合起来的理论，如量子重力理论，弦理论，M理论，预言时间是间断的，有量子特性的。

一些理论猜测普朗克时间可能是时间的最小单位。

史提芬·霍金（Stephen W. Hawking）史提芬·霍金（Stephen W. Hawking）根据史提芬·霍金（Stephen W. Hawking）所解出广义相对论中的爱因斯坦方程式，显示宇宙的时间是有一个起始点，由大霹雳（或称大爆炸）开始的，在此之前的时间是毫无意义的。

而物质与时空必须一起并存，没有物质存在，时间也无意义。

时间定义：人类在生活中总结出时间的观念，其根源来自于日常生活中事件的发生次序。

当然人们在生活中得到的绝不仅仅是事件发生次序的概念，同时也有时间间隔长短的概念，这个概念来源于对两个过程的比较——比如两件事同时开始，但一件事结束了另一件事还在进行，我们就说另一件事所需的时间更长。

这里我们可以看到，人们运用可以测量的过程来测量抽象的时间。

在物理学中也是类似，时间是通过物理过程来定义的，首先在一个参考系（要求是惯性系，或者是非惯性系，但过程发生的空间范围无穷小）中，取定一个物理过程，设其为时间单位，然后用这个过程和其他过程比较，以测定时间。

但测量时间（即上述比较过程）必须有同时性概念。

过程开始有一个同时性问题，过程结束也有一个同时性问题——最简单的例子：我们要求运动员在发令枪开枪同时起跑，同时计时员开始计时，并在运动员抵达终点线时计时员必须同时停止计时。

这个问题具体见各类相对论书籍。

同时性问题，使得古典牛顿力学、狭义相对论和广义相对论有着不同的“时间”。

直观概念告诉我们：任何人在事件是否同时上是可以达成一致意见的（也许某些人会欺骗别人，造成类似侦探小说或政治小说中的情形，不过这是人类“高智商”的表现，我们完全可以用测量用的仪器来代替：），所以我们之后不说人，而说观察者）在相对论中，观察者的运动状态引起同时性的变化，或者说观察者1以v1运动，认为同时的两件事，以v2运动的观察者2可能会认为不同时——这导致时间测量的相对性。

从数学上说，古典牛顿力学中时间参数只有一个，所有参考系共享此时间参数。

这其实就是假设所有参考系，所有空间位置可以共享同一个同时性定义。

而狭义相对论认为不同参考系就不同时了，即不同参考系有各自的时间参数t，其间关系由洛伦兹变换决定。

广义相对论认为不同地点也会不同时，广义相对论中关于时间有比较复杂的内容，参见广义相对论书籍。

当然请注意：严格说这不是简简单单的“认为”，而是基于两个假设：狭义相对论是光速不变原理。

广义相对论是引力本质为时空弯曲等。

而这两个假设得到了实验的广泛验证。

上面我们说完了时间间隔测量的问题。

但前面也提到：时间的先后次序是人们在日常生活中对时间的第一印象。

古典牛顿力学中，这一点很容易理解：我们有唯一的时间参数t，所以任意两事件（一个发生在t1，另一个发生在t2）也就有确定的先后次序。

那么相对论中呢？相对论中同地两事件先后顺序的确定的，这可以从洛伦兹变换直接看出。

但可以肯定，相对论中不同地两事件的先后次序也是随参考系（我很愿意这么说：仪器的运动状态不同，这样能够把事情的本质说出来）不同而不同的。

但这里就有一个问题：会不会有可能在参考系1中事件a先于事件b发生，且事件a的发生影响了事件b的发生（最极端的情况，使得b 无法发生，譬如一个孩子杀死了他年轻的祖父），而在参考系2中正好反过来？如果是这样，物理学乃至一切原理中最重要的一个基本原理——因果律将轰然倒塌。

所以这个问题是非常重要的。

让我们严格叙述这个问题：事件a发生，并发出信号（广义的信号，涵盖一切可以影响到b的方式，但由于a，b不同地，这个信号就需要一定时间的传播），影响b。

另一个参考系中正好相反。

值得庆幸的是：可以用洛伦兹变换证明，只要信号速度不超过光速（最多使用光，光速），信号就不可能先于b的发生传递到b所在位置。

另外说一句：狭义的另一个假设：任何物理系中物理定律有着相同形式，也是广义相对论所服从的。

换句话说，参考系1中对一个物理过程加以测量，得到l1=v1t1。

参考系2中加以测量同样也会得到l2=v2t2，尽管可能l1，v1，t1和l2，v2，t2都不相等。

当然严格说这个例子不合适，因为v的定义位置矢量导数。

但是对一些复杂的物理学定律，如麦克斯韦方程组，这个假设就很重要了。

时间箭头自古以来人类就知道时间是不可逆的，人出生，成长，衰老，死亡，没有反过来的。

玻璃瓶掉到地上摔破，没有破瓶子从地上跳起来合整的。

从经典力学的角度上来看，时间的不可逆性是无法解释的。

两个粒子弹性相撞的过程顺过来反过去没有实质上的区别。

时间的不可逆性只有在统计力学和热力学的观点下才可被理论地解释。

粗略地说，热力学第二定律说在一个封闭的系统中（我们可以将宇宙看成是最大的可能的封闭系统）熵只能增大，不能减小。

宇宙一直在朝熵增大的方向演化，这给出了宏观的时间箭头。

下面说说时间箭头。

在以上的讨论中，我们从时间间隔和先后次序两方面讨论了时间，却忽略了时间很重要的一个特性：时间箭头。

子曰：逝者如斯夫，不舍昼夜。

人生百年，逝去就没有重生的余地。

但覆水难收的又何尝仅仅是人生！物理学理论告诉我们：凡是与热现象相关的物理过程，都是不可逆的。

这里的不可逆，不是绝对意义上的不可恢复，而是说：这些物理过程产生的结果不可能在不造成其他影响的情况下完全恢复。

这就是大名鼎鼎的热力学第二定律。

下面给出两个热力学第二定律的表述：1.低温热源不可能将热量自发传递给高温热源（或不可能从低温热源将热量传递给高温热源，并不产生其他变化）时间的表现时间的表现2.不可能从单一热源吸热完全转化为机械功，并不引起其他任何变化。

可以证明两表述等价。

后一个表述有明显的工程痕迹——这来源于对蒸汽机一类将热转化为功的工程机械的研究。

这些研究大都与当时那个工业革命的时代相联系，在今天已经没有太多纯理论的价值，但却有一种东西，虽然主流研究已经基本绝迹，还是有非专业学者前仆后继地加以研究，那就是第二类永动机。

第二类永动机是这样一种机器——给它一定能量，让它开始运行，接下来它可以将由于摩擦等耗散因素耗散掉的能量全部吸收，接着再将这些能量投入回机械的能量循环。

这样的一个永动机如果造出，就意味着我们有办法用今天开采出的能源维持机械的永恒运动（因为一切耗散掉的能量都可以重新利用），使得世界以现今的能耗速度运行到世界末日！但热力学第二定律很明确地告诉我们：这是不可能的。

耗散掉的能量（内能）绝不能完全转化为耗散前的形式（机械功），这破坏了无论古典牛顿力学还是相对论中的，基本原理的无时间方向性。

那么这是为什么呢？熵为了理解这一点，我们必须引入熵的概念。

由于在经典热力学中，引入熵的概念需要很多技术性内容，这里不加赘述，可以参见任何热学教本。

这里只给出熵的一个性质：任何绝热（也就是孤立，不被外界所影响）热力学过程，只要初始状态和末态是平衡态（经典热力学中熵对平衡态才能定义，对于这一点的误解，曾导致了热寂说），末态的熵一定大于初态的熵。

简单说，孤立体系向着熵增加的方向发展。

注意，一般热学书中会说：不可逆绝热过程熵増，可逆绝热过程熵是不变的。

但其实可逆过程不是真实存在的过程——真实存在的宏观过程，只要其中分子有热运动，过程就是不可逆的。

（在超流等现象中，存在可逆宏观过程，但这时超流部分没有热运动）但熵究竟是什么？玻尔兹曼用一个公式告诉了我们S=klnw，其中k为常数，w为热力学概率（关于物质的分子、原子运动的量，在经典热力学的情况表征体系混乱程度的量）。

他用统计方法证明了，平衡态下这个公式给出了前面所说的熵。

也就是说，在经典热力学的意义下，熵意味着事物朝混乱的方向发展。

当然需要指出的是，并非所有情况下，这种发展都可以称之为“混乱”。

比如宇宙从远古的浓汤状态演化到现今的星系结构。

Physical abstractionThe most widely accepted theory about the time of the physical is Einstein's theory of relativity. In the theory of relativity, time and space to make up the four-dimensional space-time, constitute the basic structure of the universe. Time and space are notEinsteinEinsteinAbsolute, and the observer at different relative speeds or different spatial and temporal structure of the measuring point, the measured lapse of time is different. General relativity predict the quality of the gravity field will distort the space-time structure, and in the large mass (for example: a black hole) near the clock time goes by slower than clock time in places farther distance massive passage of time. Existing instruments have confirmed these relativistic done about the time accurate predictions, ,and the results have been applied to the Global Positioning System. In addition, the special theory of relativity, time dilation "effect: the observer, a clock of the relative motion of the clock of the passage of time than their reference system (still) the slow passage of time.Physical theory is a continuous, uninterrupted, there is no quantum properties. But has yet to be confirmed, trying to combine the theory of relativity and quantum mechanics, quantum theory of gravity, string theory, M theory, prophecy time discontinuous quantum properties. Some theoretical speculation that the Planck time might be the smallest unit of time.Steven • Hawking (Stephen W. Hawking)Steven • Hawking (Stephen W. Hawking)According to Einstein's equations of general relativity Steven • Hawking (Stephen W. Hawking) solution, the time of the universe is a starting point, or by the Big Bang (Big Bang) began this Time is meaningless. The material time and space, must work together to co-exist, there is no material existence, time is meaningless.Time Definition: human life summed up the concept of time, from its roots in the order of occurrence of the events of daily life. Of course, people get more than just an event in life occurs concept of order, but also the concept of interval length, this concept comes from the comparison of the two processes - such as two things at the same time, but one thing to the end The other thing is still going on, we say another thing needed for a longer time. Here we can see that people can measure measuring abstract time.Is also similar in physics, time is defined by physical processes, first of all, in a reference system (requirements inertial system, or a non-inertial reference frame, but the spatial extent of the process occurs infinitely small), whichever is given a physical process, set its unit of time, and then use this process and other processes compared to the measurement time.Measuring time (i.e., the above-mentioned comparison process) must have the concept of simultaneity. The process begins with a simultaneous end of the process, a simultaneity problem - the most simple example: We require athletes starting at the same time the starting gun shot, simultaneously begin timing timekeeper, and athletes arrive at the finish line timekeeper must stop timing.The this specific see all kinds of relativistic books. Issues, classical Newtonian mechanics, special relativity and general relativity has a different "time". Intuitive concept tells us: anyone in the events are on can agree on (maybe some people will deceive others, resulting in similar detective novels or political novel situations, but this is the performance of mankind "highly intelligent" We measurement instrument can use instead :), so we do not say, say observers) caused by simultaneous changes in the theory of relativity, the state of motion of the observer or observers v1 movement that while two things, the v2 sports observer may not at the same time - which led to a relative time measurement.Mathematically speaking, the time parameters of classical Newtonian mechanics is only a reference system to share this time parameter. In fact, this is the assumption that all reference systems, all the spatial location can share the same simultaneity defined.And the special theory of relativity that different reference system is not at the same time, different reference systems each time parameter t, during which the relationship is determined by the Lorentz transformation. General relativity that different locations will not at the same time, about the time in general relativity complex, , see the books of the general theory of relativity.Course Note: Strictly speaking, this is not simple, "think", but is based on two assumptions: the special theory of relativity is the speed of light the same principle. General relativity is gravitational nature of time and space bending. These two assumptions have been extensive verification of the experiment.Above finished the interval measurement problem. But also mentioned earlier: the priorities of the time, people in their daily lives, the first impression of the time. Classical Newtonian mechanics, which is easy to understand: we have only the time parameter t, so any two events occurred at t1 (a, another t2) will determine priorities. So the theory of relativity? The orderdetermined by the theory of relativity in the same place in both events, which can be seen directly from the Lorentz transformation. To be sure, however, that the theory of relativity in a different manner and the sequence of events is also with the reference system (I am willing to say: the state of motion of the instrument, so that the nature of the matter to say it) vary. But here there is a question: will likely reference system in the event a b occur prior to the event, and the event of the occurrence of a impact (the most extreme circumstances of the occurrence of the event b such that b can not occur, such as a child killed his young grandfather), while the reference system just turn? If this is the case, the most important physics even all principle a basic principle - the law of causality collapsed. Therefore, this problem is very important. Let rigorous description of this problem: Event occurs, and send a signal (a generalized's signal covers everything that can affect the way to b, a, b to the signal you need a certain time of dissemination), , affecting b. Another reference system is the opposite. The good news is: can prove with a Lorentz transformation, as long as the signal speed does not exceed the speed of light (up to the light, the speed of light), the signal can not be prior to the occurrence of b passed to the b location.Also to say: the narrow sense another assumption: any of the Department of Physics of the laws of physics have the same form, but also the general theory of relativity obey. In other words, the reference system of a physical process to be measured, to give L1 = v1t1. The reference system will be measured in the same get l2 = v2t2, despite the possibility of l1, v1, t1 and l2, v2, t2 are not equal. Of course, strictly speaking, this case is not appropriate, because the definition of the position of the v vector derivative. But the complex laws of physics, such as the Maxwell equations, this assumption is very important.Arrow of timeSince ancient times, humans have know time is irreversible, birth, growth, aging, death, and did not turn. The bottles dropped smashes, no broken bottles together the whole jump up from the ground. From the point of view of classical mechanics point of view, the irreversibility of time can not be explained. Elastic collision of two particles smooth over the reversal of the past no practical difference. The irreversibility of time is available only in the point of view of statistical mechanics and thermodynamics theory to explain. Roughly speaking, the second law of thermodynamics in a closed system (the universe as the largest closed system) entropy can only increase, not decrease. The evolution of the universe has been moving in the direction of entropy increase, which gives the macroscopic arrow of time.Here to talk about the arrow of time. In the above discussion, we discussed the time, from the time interval and priorities while ignoring the very important characteristics: the arrow of time. Confucius said: ,lost time, around the clock. Life for centuries, the dead would be no room for rebirth. But is it just spilled a life! Physics theory tells us that: all physical processes related thermal phenomena are irreversible. Irreversible unrecoverable, not in an absolute sense, but rather that: the result of these physical processes is not possible in the case of do not otherwise affect full recovery. This is the famous second law of thermodynamics.Here are two of the second law of thermodynamics expressed:Low heat source is impossible to heat spontaneously passed to the high temperature heat source (or impossible from a low temperature heat source to transfer heat to the high temperature heat source, does not produce other changes)Time performanceTime performance2 impossible from a single heat source endothermic completely converted into mechanical work, and does not cause any other changes.You can prove equivalence of two expressions. After a representation there are obvious signs of engineering - this comes from a class steam engine heat into power engineering machinery. Most of these studies linked to the era of the industrial revolution at that time, not too pure theory of value in today, but there is one thing, although the mainstream research has basically disappeared, or non-professional scholars fought to be studied, and that is the first The two types of perpetual motion. The second type of perpetual motion machine is such a machine - a certain energy to it, it began to run, , then it can be due to friction and other dissipative factors dissipate the energy of all the absorption can then back to mechanical energy input energy cycle. If you create a perpetual motion, it means that we have to maintain the perpetual motion of the mechanical (because all the energy is dissipated can re-use), making the world run to the energy consumption rate mined today's energy end of the world! But the second law of thermodynamics tells us very clearly: this is impossible. Dissipated energy (internal energy) must not be completely converted to the form before the dissipation (mechanical work), which destroys Whether classical Newtonian mechanics or relativity, the basic principles of the direction of time. So this is why?EntropyTo understand this, we must introduce the concept of entropy. Need a lot of technical content in classical thermodynamics, ,the introduction of the concept of entropy, without repeat, can see any thermal textbook. Here give only the entropy of a nature: any adiabatic (that is isolated and not be influenced by the outside world) thermodynamic process, as long as the initial state and the final state equilibrium (classical thermodynamic entrop y defined equilibrium can for it misunderstanding, who led the Heat Death), the entropy of the final state must be greater than the entropy of the initial state. Simply put, the isolated system development toward the direction of entropy increase. Note that the general thermal book would say: irreversible adiabatic process the entropy 増, reversible adiabatic process entropy is constant. In fact, reversible process is not real - real macro process, which molecular thermal motion, the process is irreversible. (Phenomena such as superfluidity, the presence of reversible macro process, but this time there is no thermal motion of the superfluid part)Entropy is exactly what? Boltzmann tells us with a formula the S = klnw, wherein k is a constant, w is the thermodynamic probability (for substances in the amount of movement of the molecules, atoms, and the case of the classical thermodynamic characterization of the amount of the degree of system disorder). He used statistical methods to prove this formula given earlier entropy equilibrium. That is, in the sense of classical thermodynamics, entropy means that things confusing direction development. It should be noted, of course, not all cases, this development can be called the "chaos". Evolution such as soup state of the universe from the ancient to the modern structure of the galaxy.。