最精妙理论也难解释“暗物质”

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What Is Dark Matter？

Even the Best Theories Are Crumbling

最精妙理论也难解释“暗物质”

Korey Haynes 科丽·海恩斯

Dark matter research is unsettling. Scientists were unnerved when they first noticed that galaxies don’t rotate by the same physics as a spinning plate. The stars at a galaxy’s edge rotate faster than expected. And their motion can only be explained by a lot of invisible matter that we can’t see.

That was exciting more than unsettling when the field was new and ideas were plentiful and had yet to be proven wrong. Researchers consolidated the possibilities into two main camps， complete with clever acronyms： MACHOs （Massive Compact Halo Objects） and WIMPs （Weakly Interacting Massive Particles）.

MACHOs are the less exotic possibility. You and I don’t glow or reflect light terribly well， so it’s perfectly reasonable to suggest that space and galaxies contain lots of stuff that we simply can’t see because they are literally dark and we don’t have a big enough flashlight.

Except we can detect some of those objects out there because they’re so massive that they bend light around them. They do exist， and we know they’re there despite their darkness. And yet there’s just not enough of them to make the galaxy-rotation math work.

So the astrophysics community mostly moved on to WIMPs. Rather than big objects， maybe the universe is full of little things we can’t see. These would be swarms of objects like atoms that just don’t reflect or absorb light or any other kind of electromagnetic energy， unlike all the matter we can touch and measure and see around us on Earth. We do know that neutrinos exist： tiny， mostly mass-less particles that barely interact with the universe around them. The problem there is that they’re mostly mass-less. We can’t figure out how there are enough of them to make up the 84 percent of the universe’s matter that we can’t see.

So maybe dark matter is a different object we haven’t observed at all yet， something called a neutralino.

But we’ve been looking for them for a while. We’ve built incredibly sensitive， bizarre instruments to look for them. It includes the Large Hadron Collider， one of the most expensive science experiments ever built. And we haven’t found them. We haven’t found the WIMPs themselves， and we haven’t found convincing evidence that they exist.

For decades， a few rogue scientists have stood hopefully at the edge of respectability， offering their theory called Modified Newtonian Dynamics， or MOND. Essentially， it says that physics doesn’t work as we know it at the largest scales. It says we’ve been drawing the wrong conclusions， and dark matter isn’t required to explain the universe. No one has managed to develop a theory of MOND that adequately explains the universe around us， but it occasionally gains converts simply because the competing theory of dark matter has a glaring flaw： we can’t find it.

Perhaps we’re wrong about something in the standard model that defines how the tiniest particles in the universe behave and interact， and dark matter exists， but in a very different form than we’re expecting. Or perhaps we are wrong about the laws of gravity.

Or perhaps， maybe even tomorrow， an experiment will turn up a neutralino exactly where researchers say it should be. The LHC team will discover a new particle. Science is hard， and seen against the long story of scientific progress， we only started looking for dark matter yesterday. Until something changes， well have to rest uneasy with the unsettling possibility that physics as we know it might be very wrong.

暗物质研究让人不安。科学家首次发现星系不是按照与转盘相同的物理特性转动时，他们感到不知所措。处于星系边缘的恒星的旋转速度高于预期。而它们的运动只能用许多我们看不见的隐形物质来解释。

当这个领域是新鲜的、想法层出不穷并且尚未被证伪的时候，局面让人兴奋多于不安。研究人员将各种可能性整合为两大阵营，配以巧妙的首字母缩写：MACHO（晕族大质量致密天体）和WIMP（大质量弱相互作用粒子）。

MACHO是不那么怪异的一种可能性。你我都不能很好地发光或反光，那么完全有理由认为太空和星系中包含许多我们根本看不见的东西，因为它们简直就是一团漆黑，而我们没有足够大的手电筒。

只不过我们能够察觉那里存在那样一些物体，因为它们的质量极大，以至于让周围的光线发生弯曲。它们的确存在，尽管漆黑一片，我们还是知道它们就在那里。可是它们的数量并不足以让与星系自转有关的数学运算成立。

于是，天体物理学界大多转而支持WIMP理论。也许宇宙中充斥着我们看不见的小东西，而不是大物体。它们会是大批大批像原子这样的物体，根本不反射或吸收光线以及其他任何一种电磁能，这一点不同于我们在地球上周遭能够触摸、测量并看到的所有物质。我们的确知道中微子的存在，它是几乎不与周围宇宙发生相互作用的微小的、基本无质量的粒子。问题在于它们基本上是没有质量的。我们捉摸不透它们的数量怎么足以构成我们看不到的84%的宇宙物质。

因此，也许暗物质是我们根本未曾观察到的另一种物体，是被称为中性微子的一种东西。

但是我们寻找它们有一段时间了。我们造出了极为敏感、奇形怪状的仪器来寻找它们。其中包括大型强子对撞机（LHC），这是有史以来建造的最昂贵的科学实验器材之一。可是我们尚未找到它们。我们没有找到WIMP本身，也没有找到令人信服的证据证明它们存在。

几十年来，少数特立独行的科学家满怀希望地站在声名毁于一旦的边缘，提出了名为“修正牛顿动力学”或简称MOND的理论。究其根本，它说物理学在最大尺度上不像我们了解的那样发挥作用。它说，我们一直在得出错误的结论，其实解释宇宙无需暗物质。还没有人设法形成一套充分阐释我们周遭宇宙的MOND理论，不过它偶尔会收获新的皈依者，纯粹缘于与之相对的暗物质理论有一个显眼的瑕疵：我们找不到它。

或许我们对界定宇宙最微小粒子如何起作用和相互作用的标准模型中某些东西的理解是错误的，暗物质的确存在，不过是以截然不同于我们预期的方式存在。又或许我们对万有引力定律的理解是错误的。

再或者，也许就在明天，一项实验会让中性微子现身于研究人员所说的它应该在的地方。LHC团队会发现一种新的粒子。科学是艰深的，而且从科学进步的漫漫历史来看，我们寻找暗物质不过是前几天才开始的。只要情况还没有发生改变，我们就不得不寝食难安地接受一个令人不安的可能性：我们所了解的物理学可能是大错特错的。（李凤芹译自美国《发现》月刊网站9月21日文章）