解读“上帝”的意志：七种万有理论候选者

http://www.newscientist.com/arti ... rything.html?page=1

来源：新科学家（newscientist）杂志官网
作者：迈克尔·马歇尔（Michael Marshall）


这篇文章已重新整理过目前科学界所认可的可能的万有理论的候选理论！

                               
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深入上帝的思维之中

“万有理论”是几乎所有的科学家都魂牵梦绕的至高梦想。一旦它被发现了，那么它将彻底地解释整个宇宙，在最基础的层次上的运作模式以及包含所有我们对于自然界的认识。它还将能够回答像暗物质是什么、为什么时间只有一个流逝的方向（仅仅一个维度）和重力是如何运作的这些（物理学上）亘古的难题。这也就不难理解斯蒂芬·霍金（Stephen J. Hawking）以他的一句名言来形容这样的一个理论——那将是“人类理性的终极胜利——因为届时，我们将能洞晓上帝的思维”。

不过，神学家也不必为此每晚睡不安稳。尽管开展了几十年的努力，但（有关方面的）理论的进展还是非常缓慢。许多物理学家只局限于发展“量子重力”理论来尝试统一量子力学与广义相对论——完成万有理论的先决条件。但是如今除了已有的一个或两个相互竞争的理论能被观测证据检验的优势之外，现在我们已有一些丰沛的理论候选者针对这些问题的不同部分提供解答而且还提供了好些宝贵的（万有理论的）线索或许是可行的也说不定！

                               
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以下就是其中的一些领跑者！


弦理论（String Theory）

                               
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这或许就是最广为人知的候选理论，也是目前投入最多的一个研究方向。这个理论表示我们所观察到的基本粒子其实根本不是粒子，而是一些极其微小的弦。只不过因为它们是如此之小，所以从我们的科学仪器中看到是一些微小的、点状的粒子。

更重要的是，弦理论的数学模型也依赖时空的额外维度（来阐明），而这些都是一般人类不能直接接触的东西。

这些都是从根本上的颠覆性的想法！但是很多理论家发现弦的处理方式是如此地优雅并已提出了许多在基础层次上的变数貌似能解决目前许多各式各样的宇宙学上的难题。然而，弦理论存在两个主要的挑战必须先克服以好说服科学界接受弦理论是万有理论的不二选择！

首先，弦论学家目前还未能使它的一些预测被（观测所）证实。所以弦理论目前来说还仅仅是一个（假设性）理论。

其次，弦理论含有太多不同的“版本”，任何一个都有可能是正确的，它们之间差异性太少了。为了解决这个问题，一些物理学家开始提出了一个更广泛的框架并之称为M-理论，至此，这许多不同的弦理论版本得以结合为一。

但这也带出了它自身的另一个问题。取决于我们如何设置它的初始条件，M-理论可以描述任何10^500个宇宙。不过，一些物理学家却认为这是多重宇宙的存在最好的证明，但其他人则认为它经不起（观测和实验的）检验。


环圈量子重力（Loop Quantum Gravity）

                               
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尽管没有太多的曝光率，环圈量子重力是迄今为止弦理论唯一的直接竞争对手。

它的基本概念是时空并非如我们所想的那样是连续的。时空本质上是由相等于10^-35公尺的微小组块（空间微粒）所构成，然后通过一些链接把时空结合起来并造就了我们的日常所感受到的经验。当这些链接不断地纠缠成穗带或扭节时，它们就产生了基本粒子。

环圈量子重力已经产生了一些对真实世界的一些效应的尝试性预测，也为宇宙的诞生（的解释）带来了一线曙光。但它的拥护者目前还未能把重力融入到他们的理论中。并且和弦论一样，对找到一个能检验它的实验我们还有一段距离要走。

Causal Dynamical Triangulations (CDT)

                               
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这个我们称之为“因果动态三角剖分”的理论乍看之下仿佛和上面的环圈量子重力有些相似。正如环圈量子引力理论把空间细分成微小的“构建模块”，CDT也假设空间是由一些微小的构建模块——这一次是叫做pentachorons的四维模块所组成的.

pentachorons可以粘连在一起然后产生宇宙的大尺度结构——也就是我们在现实中看到的一个有三维空间和一维时间的宇宙一样。到目前为止一切还算顺利，但CDT有一个最致命的缺点：那就是还不能解释物质的存在。

量子爱因斯坦重力（Quantum Einstein Gravity）

                               
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这个想法由来自德国美因茨大学的马丁·路德（Martin Reuter）提出，一个相当不同的研究策略。

在统一重力和量子力学的问题上，一个引人注目的现象是在微观尺度下的重力的表现模式。我们知道，当两个物体间的距离越近，则两者间的相互吸引力就越大；但重力也作用于其自身，所以，（重力）会在极小距离之下开始反馈回路。根据经典理论，重力应该会变得无限强大——这意味着经典理论在什么地方出了问题。

然而，路德找到一个办法就是生成一个“定点”：一个重力不再变强的距离极限。这将有助于解决上述的问题，并带来一个完整的量子化的重力理论。

量子重力图论（Quantum Graphity）

                               
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以上所有的理论都假设时间和空间一开始就存在，然后才试图去建立宇宙的其余部分。量子重力图论——位于加拿大安大略省滑铁卢市的圆周理论物理研究院的马可波罗·卡拉玛拉（Fotini G. Markopoulou-Kalamara）和她的同事们——确试图选择和他们背道而驰。

“当宇宙在大爆炸中形成时”，卡拉玛拉说道，并不存在我们现在所知道的这样的时空。相反，那是由一个抽象的、“节点”的网络所形成的空间，在这里每个“节点”都彼此联系在一起。在创世的一瞬间，这个网络开始崩塌以及一些节点也开始彼此分离，然后形成了我们今天所看到的宇宙。

内相对论（Internal Relativity）

                               
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主要由来自美国麻省理工学院的奥拉夫·德雷尔（Olaf Dreyer）提出。内相对论力图解释广义相对论如何立足于量子世界。

宇宙中的每个粒子有一个称为“自旋”的属性，可以粗略地看作是粒子自身的旋转运动。德雷尔的模型设想有一个独立于物质并随机分布的自旋系统存在。当这个系统达到它的临界温度时，这个自旋会自行排列调整，形成一个有序的格局。

任谁能住在这个自旋系统里的人们都无法直接的看见它，只能看见它对我们的周遭所产生的效应。德雷尔已经表明这将包含一切的时空和物质。他也设法从模型中获得牛顿重力理论：但是，广义相对论却还未出现。

E8

                               
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E8模型图

2007年，物理学家（有时也是冲浪运动员）加勒特·里希（Garrett Lisi）和他的“可能的万有理论”上了各大媒体的头版。

导火线是一篇讨论E8——一个含有8个维度和248个节点的复杂数学图形的论文所引起的。里希展示了所有已知物理的各种基本粒子与作用力可以置于E8模型的节点中，之后它们之间的相互作用关系就很自然的出现了。

一些物理学家猛烈抨击了这篇论文，也有些对它表示谨慎的欢迎。在2008年末，里希获得了一份拨款以继续研究E8模型。

Knowing the mind of God: Seven theories of everything

by Michael Marshall

This story has been edited to clarify that it discusses different approaches being taken to develop a theory of everything.


The"theory of everything" is one of the most cherished dreams of science. If it is ever discovered, it will describe the workings of the universe at the most fundamental level and thus encompass our entire understanding of nature. It would also answer such enduring puzzles as what dark matter is, the reason timeflows in only one direction and how gravity works. Small wonder that Stephen Hawking famously said that such a theorywould be "the ultimate triumph of human reason – for then we should knowthe mind of God".


But theologians needn't lose too much sleep just yet. Despite decades of effort, progress has been slow. Many physicists have confined themselves to developing "quantum gravity" theories that attempt to reconcile quantum mechanics with general relativity – a prerequisite for a theory of everything. But rather than coming up with one or two rival theories whose merits can be judged against the evidence, there is a profusion of candidates that address different parts of the problem andprecious few clues as to which (if any) might turn out to be correct.


Here's a brief guide to some of the front runners.



String theory


This is probably the best known theory of everything, and the most heavily studied. It suggests that the fundamental particles we observe are not actually particles at all, but tiny strings that only"look" like particles to scientific instruments because they are so small.


What's more, the mathematics of string theory also rely on extra spatial dimensions, which humans could not experience directly.


These are radical suggestions, but many theorists find the string approach elegant and have proposed numerous variations on the basic theme that seem to solve assorted cosmological conundrums. However, they have two major challenges to overcome if they are to persuade the rest of the scientific community that string theory is the best candidate for a ToE.


First, string theorists have so far struggled to make new predictions that can be tested. So string theory remains just that: a theory.


Secondly, there are just too many variants of the theory, any one of which could becorrect – and little to choose between them. To resolve this, some physicists have proposed a more general framework called M-theory, which unifies many string theories.


But this has its own problems. Depending how you set it up, M-theory can describe any of 10^500 universes. Some physicists argue that this is evidence that there are multiple universes, but others think itjust means the theory is untestable.


Loop quantum gravity


Although it hasn't had the same media exposure, loop quantum gravity is so far the only real rival to string theory.


The basic idea is that space is not continuous, as we usually think, but is instead broken up into tiny chunks 10^-35 metres across. These are then connected by links to make the space we experience. When these links are tangled up into braids and knots, they produce elementary particles.


Loop quantum gravity has produced some tentative predictions ofreal-world effects, and has also shed some light on the birth of the universe. But its proponents have so far struggled to incorporate gravity into their theories. And as with string theory, a true experimental test is still some way off.


CDT


Causal dynamical triangulations looks pretty similarto loop quantum gravity at first glance. Just as loop quantum gravity breaks upspace into tiny "building blocks", CDT   assumes that space-time is split into tiny building blocks – this time, four-dimensional chunks called pentachorons.


The pentachorons can then be glued together to produce a large-scale universe – which turns out to have three space dimensions and one time dimension, just asthe real one does. So far, so good, but there's a major drawback: CDT as it currently stands cannot explain the existence of matter.


Quantum Einstein gravity


This idea, proposed by Martin Reuter of the University of Mainz, Germany, takes a rather different tack.


Part of the problem with unifying gravity and quantum mechanics is what happens togravity at small scales. The closer two objects are to each other, the stronger the gravitational attraction between them; but gravity also acts on itself, and as a result, at very small distances a feedback loop starts. According to conventional theories the force should then become ridiculously strong – this means there’s something wrong with the conventional theories.


However, Reuter has come up with a way to generate a "fixedpoint": a distance below which gravity stops getting stronger. This could help solvethe problem, and lead to a quantum theory of gravity.


Quantum graphity


All the theories above assume that space and time exist, and then try to build upthe rest of the universe. Quantum graphity – the brainchild of Fotini Markopoulouof the Perimeter Institute for Theoretical Physics in Waterloo, Ontario,Canada, and colleagues – tries to do away with them.


When the universe formed in the big bang, Markopoulou says, there was no such thing as space as we know it. Instead, there was an abstract network of"nodes" of space, in which each node was connected to every other. Very soon afterwards, this network collapsed and some of the nodes broke away from each other, forming the large universe we see today.


Internal Relativity


Developed by Olaf Dreyer of the Massachusetts Institute of Technology, internal relativity sets out to explainhow general relativity could arise in a quantum world.


Every particle in the universe has a property called "spin", which can be loosely thought of as what happens to the particle when it is rotated. Dreyer's model imagines a system of spins existing independently of matter and arrangedrandomly. When the system reaches a critical temperature, the spins align, forming an ordered pattern.


Anyone actually living in the system of spins will not see them. All they see aretheir effects, which Dreyer has shown will include space-time and matter. Hehas also managed to derive Newtonian gravity from the model: however, general relativity has not yet emerged.


E8


In2007 the physicist (and sometime surfer) Garrett Lisi made headlines with a possible theory of everything.


The fuss was triggered by a paper discussing E8, a complex eight-dimensional mathematical pattern with 248 points. Lisi showed that the various fundamental particles and forces known to physics could beplaced on the points of the E8 pattern, and that many of their interactions then emerged naturally.


Some physicists heavily criticised the paper, while others gave it a cautious welcome. In late 2008, Lisi was given a grant to continue his studies of E8.

E8模型图让人头晕眼花

就感觉弦理论靠谱一点

楼主好文，有时间读一下。

都很难理解的理论,

一个很棒的译文，为什么关注度如此之少，点个赞。虽然沉了两年，还是希望大家都能来选择一个你心中的那一个理论。我偏向于环圈量子重力（内构）和因果动态三角剖分（表象）之间，但有一些东西和我的想法不一样。E8，可以说是一个完美的结局。但这些理论都是局部性的产生。。看你怎么看了。