最新研究显示黑洞并不存在

诸平

This artist's concept depicts a supermassive black hole at the center of a galaxy. The blue color here represents radiation pouring out from material very close to the black hole. The grayish structure surrounding the black hole, called a torus, is made up of gas and dust. Credit: NASA/JPL-Caltech

这幅由艺术家绘制的图像描绘了一个位于星系中心的超大质量黑洞。图中蓝色区域是物质进入黑洞时所产生的辐射。围绕黑洞的灰色结构被称为环面，由气体和尘埃组成。

黑洞（Black hole）是现代广义相对论中，宇宙空间内存在的一种超高密度天体，由于类似热力学上它是完全不反射光线的黑体，故名为黑洞。于1969年由美国物理学家约翰·阿提·惠勒命名。

“黑洞是时空曲率大到光都无法从其视界逃脱的天体。[1-3]    

黑洞是由质量足够大的恒星在核聚变反应的燃料耗尽而“死亡”后，发生引力坍缩产生的。黑洞的质量极其巨大，而体积却十分微小，它产生的引力场极为强劲，以至于任何物质和辐射在进入到黑洞的一个事件视界（临界点）内，便再无力逃脱，甚至目前已知的传播速度最快的光（电磁波）也逃逸不出。

黑洞无法直接观测，但可以借由间接方式得知其存在与质量，并且观测到它对其他事物的影响。借由物体被吸入之前的因高热而放出紫外线和X射线的“边缘讯息”，可以获取黑洞存在的讯息。推测出黑洞的存在也可借由间接观测恒星或星际云气团绕行轨迹取得位置以及质量。[4] 

2013年11月30日，两名中国科学家首次制造出可以吸收周围光线的人造电磁“黑洞”。这个黑洞可以在微波频率下工作，预计不久后它就能够吸收可见光，一种把太阳能转化为电能的全新方法可能因此产生。

2014年1月24日英国著名科学家斯蒂芬霍金教授再次以其与黑洞有关的理论震惊物理学界。他在发表的一篇论文中承认，黑洞是不存在的，不过“灰洞”的确存在[5-6]   。该说法在学术界反响不一。科学家最新研究理论显示，当黑洞死亡时可能会变成一个“白洞”，它并非像黑洞吞噬邻近所有物质，而是喷射出之前黑洞捕获的所有物质。[7] 

　　作为宇宙中最黑暗质量最密集的物质，黑洞点燃了不少人的想像力，为众多科幻小说和电影提供了精彩的素材和设定。但来自美国科学家的一项新研究可能要让这些人失望了：这名理论物理学家经过数学计算得出结论——黑洞根本就不存在。相关论文分别发表在著名的预印本网站ArXiv和《物理快报B》（Physics Letters B）杂志上。

　　“得出这个结论后，即便我本人都感到十分震撼。”提出这一理论的美国北卡罗来纳大学教堂山分校（UNC-Chapel Hill in the College of Arts and Sciences）理论物理学教授劳拉·梅尔西尼—霍顿（ Laura Mersini-Houghton）这样描述自己的感受。她说：“科学家们研究这个问题已经超过了50年，而这个解决方案给了我们许多新的思考。”

　　经典理论认为，黑洞是宇宙中存在的一种超高密度天体，由一个质量足够大的恒星在能量耗尽后因引力坍缩形成。其中心是一个密度无限大、时空曲率无限高、体积无限小的奇点。围绕在奇点四周的是一片空空如也的区域，这便是黑洞视界。一个恒星形成黑洞的过程就像是把一个地球大小的天体压缩成一个花生大的小球。根据爱因斯坦的相对论，黑洞会吞噬邻近宇宙区域的所有光线和任何物质，只要进入黑洞视界就有去无回。

　　但这种解释与另一种基本理论产生了冲突：量子力学认为，任何物理演化过程都应满足因果律，即信息是守恒的，没有信息能从宇宙中永远消失。不少科学家试图使用数学的方法来让两种理论形成统一，但都无功而返。自此，关于黑洞中的信息是否丢失的问题就成了一个谜。

　　1974年，霍金通过量子力学的方法得出结论：黑洞不仅能够吸收黑洞外的物质，同样也能以热辐射的方式向外“吐出”物质。而这种量子力学现象，就被称为霍金辐射。

　　物理学家组织网2014年9月25日报道称，新研究中梅尔西尼—霍顿描述了一种全新的方案。她和霍金都同意，当恒星因自身的引力发生坍塌时会产生霍金辐射。但梅尔西尼—霍顿认为，发出这种辐射后，恒星的质量也会不断地发生损失。正因为如此，当这些恒星坍缩时就不可能达到形成黑洞所必须的质量密度。她认为，垂死的恒星在发生最后一次膨胀后，就会爆炸，然后消亡，奇点永远不会形成，黑洞视界也不会出现。根本就不会存在像黑洞这样的东西。

　　其实早在今年年初，霍金就曾通过论文指出在经典理论中黑洞是不存在的，他承认自己最初有关视界的认识是有缺陷的，并提出了新的“灰洞”理论。该理论认为，物质和能量在被黑洞困住一段时间以后，又会被重新释放到宇宙中。

《科技日报》总编辑圈点

　　黑洞这一定义在经过漫长的时间推测后，已经慢慢被人们所接受。然而霍金2014年年初发文否认黑洞的存在，取而代之提出了“灰洞”理论，这在物理学界掀起了不小的波澜。如今，梅尔西尼—霍顿直截了当地称“根本就不会存在像黑洞这样的东西”，这无疑成为又一枚重磅炸弹——尽管梅尔西尼—霍顿远不及霍金出名。当然，想以一己之力推翻既有的理论并不那么容易，需要更多有说服力的证据加以佐证。

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By merging two seemingly conflicting theories, Laura Mersini-Houghton, a physics professor at UNC-Chapel Hill in the College of Arts and Sciences, has proven, mathematically, thatblack holes can never come into being in the first place. The work not only forces scientists to reimagine the fabric of space-time, but also rethink the origins of the universe.

"I'm still not over the shock," said Mersini-Houghton. "We've been studying this problem for a more than 50 years and this solution gives us a lot to think about."

For decades, black holes were thought to form when a massive star collapses under its own gravity to a single point in space – imagine the Earth being squished into a ball the size of a peanut – called a singularity. So the story went, an invisible membrane known as the event horizon surrounds the singularity and crossing this horizon means that you could never cross back. It's the point where a black hole's gravitational pull is so strong that nothing can escape it.

The reason black holes are so bizarre is that it pits two fundamental theories of the universe against each other. Einstein's theory of gravity predicts the formation of black holes but a fundamental law of quantum theory states that no information from the universe can ever disappear. Efforts to combine these two theories lead to mathematical nonsense, and became known as the information loss paradox.

In 1974, Stephen Hawking used quantum mechanics to show that black holes emit radiation. Since then, scientists have detected fingerprints in the cosmos that are consistent with this radiation, identifying an ever-increasing list of the universe's black holes.

But now Mersini-Houghton describes an entirely new scenario. She and Hawking both agree that as a star collapses under its own gravity, it produces Hawking radiation. However, in her new work, Mersini-Houghton shows that by giving off this radiation, the star also sheds mass. So much so that as it shrinks it no longer has the density to become a black hole.

Before a black hole can form, the dying star swells one last time and then explodes. A singularity never forms and neither does an event horizon. The take home message of her work is clear: there is no such thing as a black hole.

The paper, which was recently submitted to ArXiv, an online repository of physics papers that is not peer-reviewed, offers exact numerical solutions to this problem and was done in collaboration with Harald Peiffer, an expert on numerical relativity at the University of Toronto. An earlier paper, by Mersini-Houghton, originally submitted toArXiv in June, was published in the journal Physics Letters B, and offers approximate solutions to the problem.

Experimental evidence may one day provide physical proof as to whether or not black holes exist in the universe. But for now, Mersini-Houghton says the mathematics are conclusive.

Many physicists and astronomers believe that our universe originated from a singularity that began expanding with the Big Bang. However, if singularities do not exist, then physicists have to rethink their ideas of the Big Bang and whether it ever happened.

"Physicists have been trying to merge these two theories – Einstein's theory of gravity and quantum mechanics – for decades, but this scenario brings these two theories together, into harmony," said Mersini-Houghton. "And that's a big deal."

 Explore further: Astrophysicists duo propose Planck star as core of black holes

More information: Mersini-Houghton's ArXiv papers:

— Approximate solutions:arxiv.org/abs/arXiv:1406.1525 
— Exact solutions:arxiv.org/abs/arXiv:1409.1837