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解疑释惑：什么是反物质？

时间：2016-2-3 15:40作者：update 阅读(2169) 评论: 6

导读：反物质是20世纪物理学领域最激动人心的发现之一。这一概念出现在科幻小说作家们如丹·布朗的作品中(注：在丹·布朗的小说《天使与魔鬼》里，恐怖分子企图从欧洲核子中心盗取0.25克反物质，进而欲炸毁整座梵蒂冈城)许 ...

反物质是20世纪物理学领域最激动人心的发现之一。这一概念出现在科幻小说作家们如丹·布朗的作品中(注：在丹·布朗的小说《天使与魔鬼》里，恐怖分子企图从欧洲核子中心盗取0.25克反物质，进而欲炸毁整座梵蒂冈城)许多人认为反物质还是一个有待证实的科学观点，却不知道，其实反物质每天都在产生。而且，对于反物质的研究会帮助我们了解宇宙是如何运行的。

反物质是由反粒子构成的。科学家们相信，我们已知的每一个粒子都有一个“双胞胎兄弟”，但是它具有相反的电荷，也就是说，任何粒子都可能存在着反粒子。例如，一个电子具有负电荷，但是它的反粒子(我们称之为正电子)与其质量相同但是拥有正电荷。当一个粒子和它的反粒子结合，它们会湮没，消失在一道光中。

英国物理学家保罗·狄拉克在首次尝试将现代物理学的两大经典理论(相对论和量子力学)结合起来时，预测了有这样的粒子存在。狄拉克在相对论方程和量子电动力学的方程中，推导出了低能量的“反物质海洋”的存在，物理学家们至今没有发现这个“海洋”是因为，他们只研究了“海平面以上”的区域。狄拉克设想，所有“正常”能量水平上都被“正常”粒子所占据，但是，当一个粒子从低能量状态跃升到高能量状态的时候，它就变成“正常”粒子，但是它在原来的位置上留下了一个“空位”，这个空位就是我们认为奇怪的，粒子的镜像——反物质粒子。

尽管经历了最初的怀疑，但是粒子-反粒子成对存在的例子很快就被证实了。例如，当宇宙射线击中地球大气层时，就产生了反物质。甚至有证据表明，雷暴中的能量产生反电子，称为正电子。反物质也能在某些放射性衰变中产生，这方面的实际用途，例如，正电子发射X射线层析照相术(PET)，医生利用PET扫描能得出病人体内的详细图像。目前，大型强子对撞机实验也可以产生正物质和反物质。

物理学家预测，根据宇宙大爆炸理论，正物质和反物质应该是对称的。而且，据预测，如果一个粒子和它的反粒子是互换的(这种关系被称为CP守恒)，那么它们的物理定律也应该相同。我们了解的宇宙却没有遵循这样的规则。宇宙几乎都是由正物质组成的，那么与我们的宇宙物质对称的反物质哪儿去了呢?这是迄今为止物理学领域最大的谜团之一。

实验表明，某些放射性衰变过程并不产生等量的粒子与反粒子。但这不足以解释，为什么我们的宇宙中物质和反物质是不对称的。因此，物理学家们包括我自己，在大型强子对撞机和所做的各项实验，以及利用中微子所做的其它实验，如日本的T2K实验，都是在寻找其它过程来揭开这个谜团。

这个世界还有许多未解之谜。科学实验也正在研究，重力对反物质的作用是否与他们影响正物质的方式相同。如果这种对称性被打破，那么就需要对我们的物理学思想进行修订，而这不仅影响粒子物理学，也会影响我们对重力和相对论的理解。

反物质实验能够让我们把对宇宙基本机制的了解应用到新的实验中去。谁都没法预测我们接下来会有什么惊人发现?

（中国科技网张微编译）

英文原文：

Explainer: What is antimatter?

Antimatter was one of the most exciting physics discoveries of the 20th century. Picked up by fiction writers such as Dan Brown, many people think of it as an "out there" theoretical idea – unaware that it is actually being produced every day. What's more, research on antimatter is actually helping us to understand how the universe works.

Antimatter is a material composed of so-called antiparticles. It is believed that every particle we know of has an antimatter companion that is virtually identical to itself, but with the opposite charge. For example, an electron has a negative charge. But its antiparticle, called a positron, has the same mass but a positive charge. When a particle and its antiparticle meet, they annihilate each other – disappearing in a burst of light.

Such particles were first predicted by British physicist Paul Dirac when he was trying to combine the two great ideas of early modern physics: relativity and quantum mechanics. Dirac, however, accepted that the equations were telling him that particles are really filling a whole "sea" of these lower energies – a sea that had so far been invisible to physicists as they were only looking "above the surface". He envisioned that all of the "normal" energy levels that exist are accounted for by "normal" particles. However, when a particle jumps up from a lower energy state, it appears as a normal particle but leaves a "hole", which appears to us as a strange, mirror-image particle – antimatter.

Despite initial scepticism, examples of these particle-antiparticle pairs were soon found. For example, they are produced when cosmic rays hit the Earth's atmosphere. There is even evidence that the energy in thunderstorms produces anti-electrons, called positrons. These are also produced in some radioactive decays, a process used in many hospitals in Positron Emission Tomography (PET) scanners, which allow precise imaging within human bodies. Nowadays, experiments at the Large Hadron Collider (LHC) can produce matter and antimatter, too.

Physics predicts that matter and antimatter must be created in almost equal quantities, and that this would have been the case during the Big Bang. What's more, it is predicted that the laws of physics should be the same if a particle is interchanged with its antiparticle – a relationship known as CP symmetry. However, the universe we see doesn't seem to obey these rules. It is almost entirely made of matter, so where did all the antimatter go? It is one of the biggest mysteries in physics to date.

Experiments have shown that some radioactive decay processes do not produce an equal amount of antiparticles and particles. But it is not enough to explain the disparity between amounts of matter and antimatter in the universe. Consequently, physicists such as myself at the LHC, on ATLAS, CMS and LHCb, and others doing experiments with neutrinos such as T2K in Japan, are looking for other processes that could explain the puzzle.

However, a great many mysteries remain. Experiments are also investigating whether gravity affects antimatter in the same way that it affects matter. If these exact symmetries are shown to be broken, it will require a fundamental revision of our ideas about physics, affecting not only particle physics but also our understanding of gravity and relativity.

In this way, antimatter experiments are allowing us to put our understanding of the fundamental workings of the universe to new and exciting tests. Who knows what we will find?

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