对于宇宙的起源，这是所有人类文明都很关心的问题。在过去一个世纪，宇宙学有了飞跃的发展。这主要得力于天文观察技术的大幅进步，提供了大量的数据。另一方面，科学家们又建立了一些可以量化的理论模型。现在的宇宙学主要应用的，一个是粒子物理的标准模型（ Standard Model of particle physics ），另一个就是广义相对论。 1.目前的宇宙学与相对论对于真空有不同的假设 不过，现在的宇宙学与相对论之间在概念上并非完全一致。例如在 Alan Guth ( 阿兰 · 古斯 ) 的暴胀理论里，宇宙膨胀的动力来源主要是用广义相对论来解释 。但他这个理论的假设与相对论对于 “ 真空 ” 的看法又有着一些分歧。在目前的宇宙学里面，它的基本假设是宇宙中的能量和物质来自“真空”中的量子扰动（ quantum fluctuation ）。 “ 真空 ” 只是一个“场”的基态。它并非空无一物的，而是具有很特殊的物理性质。（详细的介绍请看我的上一篇博文《了解宇宙起源的关键：真空是什么？》 Link ： http://blog.sciencenet.cn/home.php?mod=spaceuid=226454do=blogid=1067514 ）。这种 “ 真空非空 ” 的概念会与相对论产生矛盾。相对论的第一假设（ 1 st postulate ）就是 “ 相对性原理 principle of relativity” 。就相对论而言， “ 真空 ” 应该是空的。否则它就会形成一个宇宙中的静止参照体系（ resting frame ），我们在理论上就可以量到所有惯性系相对于这个静止参考系的运动。这样一来，相对论的第一假设（即所有的惯性系都是相等的）就会被打破。 有人可能说，目前宇宙学用的是广义相对论，而非狭义相对论。但是，广义相对论和狭义相对论的基本假设应该是一致的，因为前者必须建立在后者的理论基础上。所以，广义相对论也不可能违背狭义相对论的第一假设。 对于“真空”是否是空的这个问题，爱因斯坦并没有很清楚地表态。他关于 “ 以太 ” 的看法是曾有些反复的。在 1905 年当他首先 提出狭义相对论的时候，爱因斯坦是明显地认为 “ 以太 ” 并不存在。可是到他提出广义相对论时，他对于 “ 真空 ” 的看法已经有所改变。 1920 年爱因斯坦在 Leiden 大学给过一篇 “ 以太与相对论 ” 的演讲。在其中，他就公开表明不能否定 “ 以太 ” 的存在。而且，他认为把 “ 真空 ” 当做一个空无一物的空间是不符合力学的基本事实 。他认为在广义相对论里面，空间是有某种物理性质的。因此可以把它当作一种新的 “ 以太 ” 。 。不过，他认为这种新的 “ 以太 ” 与旧的 “ 以太 ” 观念有别，它并非一种有质量的运动介质，因此它不会形成一个静止的参考系。所以，这种新的 “ 以太 ” 不会否定他的狭义相对论的第一假设。 2.目前的证据能否说明宇宙是否有一个静止的参考系？ 从以上的讨论可见，现在争论的焦点已经不限于 “ 真空”是否是空的，而是我们能不能从实验上证明宇宙有或者没有一个静止的参考系？ 爱因斯坦提出狭义相对论是基于迈克耳孙－莫雷实验 （ Michelson-Morley experiment ）。让我们重新来看看 这个实验是否证明了宇宙中没有静止参考系。该实验原来的设计，只是要检验：假如光是由一种叫 “ 以太 ” （ aether ）的介质传播的话，我们能不能观察到 “ 以太 ” 这种物质与地球的相对运动。实验的结果是否定的（ null ）；也就是说，实验无法观察到 “ 以太 ” 与地球的相对运动；光的传导在任何一个惯性系里都是一样的。这个结果完全符合了爱因斯坦提出的相对性原理。但是迈克耳孙－莫雷实验只观察了光（或者电磁波）的传播，而不是对于所有物理运动的观察。因此，这个实验只能说明光的传递符合了相对性原理，而并不能说明除了光的传递以外，其它的物理相互作用（例如重力，核力等等）也都符合相对性原理。所以，迈克耳孙－莫雷实验并没有直接地证明宇宙中没有一个静止的惯性系。这个实验当然也无法检验 “ 真空 ” 是否真的是空的。 当然， “ 以太 ” 理论在 20 世纪初被抛弃不仅仅是因为迈克耳孙－莫雷实验的结果。它还有一些别的原因，包括 “ 以太 ” 理论难以解释星体何以能够毫无阻力地穿透这种介质。而且，这种假设的 “ 以太 ” 介质有些相互矛盾的物理特性。在 19 世纪的时候， “ 以太 ” 是假设存在于所有物质以外的空间。要满足这个条件， “ 以太 ” 就必须是一种具备高度流动性的物质（气体或者液体）。但是， “ 以太 ” 又被假设为光的传递介质，但光是一种高频率的横波，只有固态的介质才能传递这样的波。这样 “ 以太 ” 就必须是固态的。这与前面的要求相反。 那么，现代的物理学提出了 “ 真空 ” 非空的假设，它是否也会碰到 19 世纪 “ 以太 ” 面对的难题呢？答案是否定的。现在的 “ 真空 ” 概念与 “ 以太 ” 有些根本的不同。首先， “ 真空 ” 是充塞着整个宇宙而非只是存在于物质以外的空间。其次，构成物质的原子是由基本粒子组成的。而粒子只是 “ 真空 ” 的激发波。因此，物质在 “ 真空 ” 中运动不会感受到阻力。 严格来说，迈克耳孙－莫雷的实验结果并没有否定真空介质的存在。爱因斯坦 1905 年提出的相对论文章大量使用了麦克斯韦的电磁学理论 。我在上一篇博文中已经指出，麦克斯韦理论假设了 “ 真空 ” 是一种电介质（ dielectric medium ）。因此爱因斯坦可以说已经间接地接受了这种假设。事实上，迈克耳孙－莫雷的实验结果也没有直接违背 “ 真空 ” 是一种电介质的假设。因为根据麦克斯韦方程导出的光的运动方程为 . 这个方程是符合洛仑兹协变的（ Lorentz covariant ）。这就是说，如果把方程中使用的时间和空间从一个坐标系 (x, y, z, t) 用洛仑兹转换（ Lorentz transformation ）转换到另外一个坐标系 (x’, y’, z’, t’) ，光的传导方程看上去是不变的。因此，迈克耳孙－莫雷实验只是证明了光的传导是洛仑兹协变的，而并没有说明 “ 真空 ” 是不是空的。 3.宇宙微波背景辐射（CMB）可否作为一个静止参考系？ 在今天，我们对于宇宙的观测要远比爱因斯坦时候深入得多。在最近几十年，许多关于宇宙起源的观测都是从分析宇宙微波背景辐射（ Cosmic Microwave Background ，简称 CMB ）而来。在二十世纪 60 年代，美国贝尔实验室的彭齐亚斯（ Arno Penzias ）和威尔逊（ Robert Wilson ）在偶然间发现了宇宙微波背景辐射。许多物理学家认为 CMB 是宇宙大爆炸后遗留下来的辐射波，它可以为我们研究早期的宇宙提供许多宝贵的信息。因此，科学家用各种各样的实验手段来观察 CMB 。除了一些地面的观察以外，还进行了三个卫星观测计划，即 COBE （ Cosmic Background Explorer, 宇宙背景探测卫星， 1989-1993 ）， WMAP （ Wilkinson Microwave Anisotropy Probe ，威尔金森微波各向异性探测器， 2001-2010 ）和 Planck （普朗克卫星， 2009-2013 ），搜集了大量的数据。根据科学家的分析，这种宇宙微波背景辐射有着非常均匀的空间分布，它与我们观察得到的宇宙物质分布也大致吻合。这就提供了一个可能性：这种宇宙微波背景辐射可不可以作为我们宇宙的一个静止参考系？ 根据现在的卫星观测，我们可以准确地量度到地球与 CMB 的相对运动，甚至可以计算出地球相对于 CMB 运动的速度约为 3.7x10 5 m/s 。主持 COBE 项目的物理学家乔治 · 斯穆特（ George Smoot ）就因为这项研究 CMB 的工作而获得 2006 年的诺贝尔奖。在他获颁诺贝尔奖的演讲中，他就提到过 CMB 作为一个静止参考系的可能性 。他认为 CMB 是一个很方便理解宇宙膨胀的惯性系。可以把 CMB 的观测当为一种新的 “ 以太 ” 漂移实验。因此，如果有人要把 CMB 的分布当作一个静止参考系，也是可以理解的。不过他自己并不认为 CMB 的存在会直接违背狭义相对论。 4.我们需要设计一个新的实验来解决宇宙学与相对论之间的分歧 根据上面的讨论可知，目前的宇宙学理论与相对论的基本假设有着一些重要的分歧。我们需要设计一个新的实验来检验究竟我们这个宇宙有没有一个静止的参考系。在目的上，这个实验和一百多年前的迈克耳孙－莫雷实验很相似。但是，其使用的手段会大大不同。事实上，迈克耳孙－莫雷实验有一个局限：它使用光来检测不同惯性系的物理运动。不过，光是一种特殊的粒子：它没有静止质量，它的速度永远是 c 。如果我们要检测 “ 真空 ” 是否有一个静止参考系的话，我们应该用有静止质量、其运动速度不必等于 c 的普通粒子来做实验。 那么，这个实验要怎么做呢？最近我在 European Physical Journal ( 欧洲物理杂志 ) 上发表了一篇文章，为这个实验提出了一个设计 。这篇文章得到欧洲物理学界蛮多的重视。今年 3 月，欧洲物理杂志的出版社（ Springer Nature ）把这篇文章作为重点推介（ highlight ）（ Link ： http://www.epj.org/epjplus-news/1218-epjplus-highlight-does-the-universe-have-a-rest-frame ）。在今年 6 月，欧洲物理新闻（ Europhysics News ）又再次把这篇文章选为重点介绍（ Link ： http://epn.eps.org/EPN%2048-3#p=9 ）。如果读者对这篇文章有兴趣，可以点击下面的链接： https://link.springer.com/article/10.1140/epjp/i2017-11402-4 。 我十分希望这个实验能够由中国人自己来做。有兴趣的朋友请跟我联络。 ------------------------------------------------------------------------------ 注 1 ：爱因斯坦 1920 年在 Leiden 大学演讲的摘要： “ To deny the ether is ultimately to assume that empty space has no physical qualities whatever. The fundamental facts of mechanics do not harmonize with this view. … besides observable objects, another thing, which is not perceptible, must be looked upon as real, to enable acceleration or rotation to be looked upon as something real.…the conception of the ether has again acquired an intelligible contental though this content differs widely from that of the ether of the mechanical undulatory theory of light. The ether of the general theory of relativity is a medium which is itself devoid of all mechanical and kinematical qualities, but helps to determine mechanical (and electromagnetic) events.…Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether.According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light,but also no possibility of existence for standards of space and time(measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may betracked through time. The idea of motion may not be applied to it .” 注 2 ： 乔治 · 斯穆特 2006 年获颁诺贝尔物理学奖时演讲的摘要： “ One problem to overcome was the strong prejudice of good scientists who learned the lesson of the Michelson and Morley experiment and special relativity that there were no preferred frames of reference. There was an education job to convince them that this did not violate special relativity but did find a frame in which the expansion of the universe looked particularly simple. More modern efforts to find violations of special relativity look to this reference frame as the natural frame that would be special so that perhaps the suspicions were not fully unfounded. We had to change the name to “the new aether drift experiment”and present careful arguments as the title “aether drift experiment” was too reminiscent of the Michelson and morley ether drift experiment.” 参考文献： A. H. Guth and D. I. Kaiser, Inflationary cosmology: Exploring the universe from the smallest to the largest scales. Science, vol. 307, (5711), pp.884-890, 2005. A. Einstein, Ether and the theory of relativity (speech at Univ. of Leiden, May 5, 1920). From: The Collected Papers of Albert Einstein, the Swiss Years:Writings, 1918-1921, Vol. 7, Translated by Alfred Engel , Ed. M. Janssen et al , Princeton Univ. Press, 2002. A. A. Michelson and E. W. Morley, On the Relative Motion of the Earth and the Luminiferous Ether. American J. of Science, vol. 34, pp. 333-345, 1887. D. C. Chang, Why energy and mass can be converted between each other? A new perspective based on a matter wave model. J. Mod. Phys., vol. 7, (04), pp. 395-403, 2016. A. Einstein, Zur elektrodynamik bewegter krper. Ann. Physik, vol. 322, (10), pp. 891-921, 1905. A. Kogut and et al., Dipole anisotropy in the COBE differential microwave radiometers first-year sky maps. Astrophys. J., vol. 419, (1), 1993. G. Hinshaw and et al., Five-year Wilkinson Microwave Anisotropy Probe observations: Data processing, sky maps, and basic results. Astrophys. J. Supp. Series, vol. 180, (2), pp.225-245, 2009. G. F. Smoot, Nobel Lecture: Cosmic microwave background radiation anisotropies: Their discovery and utilization. Rev. Mod. Phys., vol. 79, (4), pp.1349-1379, 2007. D. C. Chang, Is there a resting frame in the universe? A proposed experimental test based on a precise measurement of particle mass. Eur. Phys. J. Plus, vol.132, (3), 140, 2017.
《黑客帝国》Matrix 里有两个世界：真实的物质世界，虚拟的信息世界（在计算机里）。真实的物质世界是由基本粒子组成的。 虚拟的信息世界是由bits组成的。（我个人观点）其实真实的物质世界不太真实，虚拟的信息世界更真实。物质世界和信息世界根本就是一个世界。更确切的说，我们的世界是一个量子信息世界。空间=含有许多许多qubits的集合。基本粒子=qubits的运动激发。也就是说，世间万物（物质世界）是由qubits的激发组成的。 我们生活在一个量子计算机里 Matrix is a story of two worlds: A real material world and a virtual information world (inside computers). The real material world is formed by elementary particles. The virtual information world is formed by bits. (My point of view) in fact, the real material world is not real, the virtual information world is more real. The material world and the information world is actually the same world. To be more precise, our world is a quantum information world: Space = a collection of many many qubits. Vacuum = the ground state of the qubits. Elementary particles = collective excitations of the qubits. In other words, all matter are formed by the excitations of the qubits. We live inside a quantum qubit world 上面的描述十分通俗，但也很贴切。更学术更准确的论述： People have been wondering about the origin of light, electrons, and gravity for very long time. Such a curiosity has driven the development particle physics in last 100 years. In another field --modern condensed matter physics, physicists have been trying to find new types of matter, so that they can use the new materials to make new devices. It turns out that these two seemly unrelated fields have a deep connection. In early days of particle physics, our approach has been dominated by the reductionist approach: to gain a deeper understanding of something, we simply need to divide the thing into small parts. Thus to gain a deeper understanding of photons and electrons and to understand the strange properties of photons and electrons, we need to find a smaller parts of photons and electrons. The different properties of a photon and an electron may be due to their different internal structures. However, this line of thinking may be fundamentally flawed. They are based on the reductionist belief that the space is empty and that things placed in the empty space are divisible. However, there are many examples from condensed matter physics indicating that sometimes this line of thinking doesnot make sense. For example, due to the particle-wave duality in quantumtheory, sound waves in a solid correspond to a kind of particle called phonon. Phonons are as much particle-like as any other elementary particles. But if we look at phonons closely, we do not see smaller parts that form a phonon. We see the atoms that form the entire solid. The phonons are not formed by those atoms, the phonons are simply collective motions of those atoms. We see that phonons cannot be understood by dividing them into smaller parts and the reductionist approach does not work. This leads us to wonder that are we misguided by the reductionist principle all those years? It should occur to us that photons, electrons, gravitons, etc, may not have smaller parts. Looking for the smaller parts of photons, electrons, and gravitons to gain a deeper understanding of those elementary particles may not be a right approach. Motivated by those considerations, here I will advocate a different view of the elementary particles based on the principle of emergence. In this approach, we view the empty space as a dynamical medium formed by quantum degrees of freedom If there is no degrees of freedom, there will be no space. The simplest local degrees of freedom in quantum theory are qubits. So in this approach, space = a collection of qubits. In condensed matter physics, a collection of qubits is called a lattice spin systems. Here, we will use such a condensed matter terminology and view space itself as a qubit system or a spin system. The deformations of this medium (ie the rearrangements or the collective motions of qubits/spins) are waves, which in quantum theory correspond to various particles. Thus in the emergence approach, we may understand the origin of various particles by understanding the origin of various waves. The different orders of bosons/spins lead to different kinds of waves which in turn lead to different types of particles. In the field of condensed matter physics, we find recently that a new class of quantum states of qubits -- long-range entangled states -- can exist. Amazingly, the collective wave-like excitations in some of those highly entangled quantum states can satisfy the Maxwell equation and Dirac equation , despite the original qubit system has no gauge fields and no Grassman fields! So, those long-range entangled states are the long sought-after ether -- a medium that supports a wave that satisfies the Maxwell equation. This suggests that we can view our vacuum as a long-range entangled state and view the elementary particles as the excitations in such a long-range entangled state. Such a point of view may provides an unification and an origin for all elementary particles. We may try to use such a point view of long-rang entanglement to understand many fundamental issues in theoretical physics. It seems possible that long-range entanglement can unify many fundamental mysteries/wonders, which will totally change our view of universe. People have been studying entanglements from quantum computing and quantum matter points ofview. But the entanglement may have some very fundamental implications to particle physics as well. We have shown that light and electrons can indeed have a unified origin from qubits that form the entire space, provided that those space-forming qubits are in a particular long-range-entangled state -- string-net liquid state. So, such an emergence picture based on long-range entanglements can explain the origin of gauge interaction (such as the electromagneticinteraction) and Fermi statistics, and unify these two seemingly totally unrelated phenomena : Q: Where do light and fermions come from? A: Light and fermions come from the qubits that form the space. Q: Why do light and fermions exist? A: Light and fermions exist because the space-forming qubits form a string-net condensedstate . Q: What are light and fermions? A: Light waves are collective motions of strings and fermions are ends of open strings in the string-net condensed state. The basic picture is: Space = a collection of qubits. No qubits no space. There is nothing between the qubits. Vaccum = ground state of qubits (ie a particular pattern of entanglement of qubits) Motion of qubits (ie deformation of the pattern of entanglement) = waves For right ground state (or for a right pattern of entanglement), the wave happen to satisfy Maxwell equtions, and correspond to the eletromagnetic wave. To form the right pattern of entanglement, the qubits first form string-net and string-nets then form a quantum liquid. The wave that satisfies Maxwell equtions is the string densoty wave. The quanta of the eletromagnetic wave are photons The electrons are ends of strings. If light and electrons can be unified by qubits that form our space, then, can we unify everything under qubits, even all the eight wonders in our universe? Eight wonders in our universe: (1) Locality. (2) Identical particles. (3) Gauge interactions. (4) Fermi statistics. (5) Chiral fermions. (6) Small mass of fermions. (Much less than Planck mass) (7) Lorentz invariance. (8) Gravity. So far, we can unify seven out of eight wonders (1 -- 7) by qubits organized into string-nets. The traditional physics theory of our world (the U(1)xSU(2)xSU(3) standard model) is based on the above 8 strange and random wonders. We just put this 8 fact into our theory by hand and do not ask where they come from. If we assume the space is the ether made of qubits, then we can unify the seven random assumptions into one very simple and reasonable assumption. Qubits unify everything This line of thinking is an unification of matter, interaction, and information ! In fact, quantum theory already unifies matter and information. This is because, acoording to quantum theory and relativity theory Changing information (qubits) - energy - mass - matter. To understand the above relations, we note that - Frequency ~ a property of information - Energy/mass ~ a property of matter In quantum theory Changing information - Frequency Frequency x Planck constant = Energy In relativity Energy/c^2 = mass. So Frequency ~ Energy ~ mass. But can simple qubits (quantum information) really produce/unify the following fascinating and mysterious properties of universe: (1) all interactions are gauge interaction (electromagnetism, strong/weak interation). (2) matter are formed by fermions (electrons, quarks, etc.). Who ordered Fermi statistics in our universe? (3) angular momentum of fermions are fractionalized (spin-1/2) The answer is yes! Information can really unifies matter and interaction. Information and matter are really the same. 道生一，一生二，二生三，三生万物 -- 信息与物质的统一 We see that, in the string-net approach to elementary particles, the qubits are the ultimate things that build up our world. Such an emergence approach has a falsifiable prediction: The U(1)xSU(2)xSU(3) standard model is incomplete. The correct theory must contain an extra discrete gauge theory which leads to new cosmic strings. Refs: 1. arXiv:hep-th/0109120 Origin of Light Xiao-Gang Wen An origin and unification of light and electrons from qubits was proposed. 2. arXiv:hep-th/0302201 Quantum order from string-net condensations and origin of light and massless fermions Xiao-Gang Wen An origin and unification of photons/gluons and leptons/quarks from qubits was proposed. But the SU(2) gluons couple to left-hand and right-hind fermions equally, while in our world the SU(2) gluons couple to left-hand and right-hind fermions differently. 3. http://dao.mit.edu/~wen/talks/LightStory.htm A simple introduction to origin of light from qubits 4. arXiv:hep-th/0507118 Quantum ether: photons and electrons from a rotor model Michael Levin , Xiao-Gang Wen An origin and unification of light and electrons from qubits in more details. 5. The universe is a quantum computer Seth Lloyd The universe is a quantum computer is simply another way to say (抓眼球的表达) quantum theory describes our world. Spirits flying in a mysterious void. Magnetic waist, electric head, sparkles fill the dark sky. Vibrating strings radiate rainbow light. Spreading nets weave into colorful fabric. Neither nonbeing nor being, nonbeing gives birth to being. Both illusive and real, illusive becomes real. Transforming and regenerating not due to divine power. Mother of all matter and root of universe. 2005 summer by Xue Wanqin
http://en.wikipedia.org/wiki/E._T._Whittaker History of science In 1910, Whittaker wrote A History of the Theories of Aether and Electricity , which gave a very detailed account of the aether theories from René Descartes to Hendrik Lorentz and Albert Einstein , including the contributions of Hermann Minkowski , and which made Whittaker a respected historian of science. In 1951 (Vol. 1) and 1953 (Vol. 2), he published an extended and revised edition of his book in two volumes. The second volume contains some interesting historical remarks. For example, it contains a chapter named The Relativity Theory of Poincaré and Lorentz, where Whittaker credited Henri Poincaré and Lorentz for developing special relativity , and he attributed to Albert Einstein 's relativity paper only little importance. He also attributed the formula to Poincaré. In 1984 Clifford Truesdell wrote that Whittaker aroused colossal antagonism by trying to set the record straight on the basis of print and record rather than recollection and folklore and professional propaganda,... On the other hand Abraham Pais wrote that Whittaker's treatment of special relativity shows how well the author's lack of physical insight matches his ignorance of the literature. According to Torretti, Whittaker's views on the origin of special relativity have been rejected by the great majority of scholars, and he cites Born (1956), Houlton (1960,1964), Schribner (1964), Goldberg (1967), Zahar (1973), Hirosige (1976), Schaffner (1976), and Miller (1981). ^ Clifford Truesdell , An Idiot's Fugitive Essays on Science , page 432, Springer ISBN 0-387-90703-3 ^ Pais, Abraham, Subtle is the Lord, 1982 ^ Roberto Torretti, Relativity and Geometry, 1983 A History of the Theories of Aether and Electricity
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