霍金的伟大不在于他提出黑洞理论，在勇于自我批评， 但。。。

公孙明 03/17/2018

霍金的伟大，不在于他提出黑洞理论，在于他勇于自我批评，但更伟大的是，他从21岁起就已是个渐冻人，却能够克服种种困难生存到76岁。

他对生命的尊重，热爱，和执着，以及那种永不退缩的毅力和与逆境斗争的精神，才是值得我们学习的对象。

以研究「黑洞」着名的物理学家霍金，2014年1月22日在网路上发表一篇名为《黑洞的信息保存与气象预报》（Information Preservation and Weather Forecasting for Black Holes）的最新论文：

Information Preservation and Weather Forecasting for Black Holes

https://genius.com/Stephen-hawking-information-preservation-and-weather-forecasting-for-black-holes-annotated

S. W. Hawking

(Submitted on 22 Jan 2014)

Abstract

It has been suggested [1] that the resolution of the information paradox for evaporating black holes is that the holes are surrounded by firewalls, bolts of outgoing radiation that would destroy any infalling observer. Such firewalls would break the CPT invariance of quantum gravity and seem to be ruled out on other grounds. A different resolution of the paradox is proposed, namely that gravitational collapse produces apparent horizons but no event horizons behind which information is lost. This proposal is supported by ADS-CFT and is the only resolution of the paradox compatible with CPT. The collapse to form a black hole will in general be chaotic and the dual CFT on the boundary of ADS will be turbulent. Thus, like weather forecasting on Earth, information will effectively be lost, although there would be no loss of unitarity.

∗ Talk given at the fuzz or fire workshop, The Kavli Institute for Theoretical Physics, Santa Barbara, August 2013

Some time ago [2] I wrote a paper that started a controversy that has lasted until the present day. In the paper I pointed out that if there were an event horizon, the outgoing state would be mixed. If the black hole evaporated completely without leaving a remnant, as most people believe and would be required by CPT, one would have a transition from an initial pure state to a mixed final state and a loss of unitarity. On the other hand, the ADS-CFT correspondence indicates that the 7evaporating black hole is dual to a unitary conformal field theory on the boundary of ADS. This is the information paradox.

Recently there has been renewed interest in the information paradox [1]. The authors of [1] suggested that the most conservative resolution of the information paradox would be that an infalling observer would encounter a firewall of outgoing radiation at the horizon.

There are several objections to the firewall proposal. First, if the firewall were located at the event horizon, the position of the event horizon is not locally determined but is a function of the future of the spacetime.

Another objection is that calculations of the regularized energy momentum tensor of matter fields are regular on the extended Schwarzschild background in the Hartle-Hawking state [3, 4]. The outgoing radiating Unruh state differs from the Hartle-Hawking state in that it has no incoming radiation at infinity. To get the energy momentum tensor in the Unruh state one therefore has to subtract the energy momentum tensor of the ingoing radiation from the energy momentum in the Hartle-Hawking state. The energy momentum tensor of the ingoing radiation is singular on the past horizon but is regular on the future horizon. Thus the energy momentum tensor is regular on the horizon in the Unruh state. So no firewalls.

For a third objection to firewalls I shall assume that if firewalls form around black holes in asymptotically flat space, then they should also form around black holes in asymptotically anti deSitter space for very small lambda. One would expect that quantum gravity should be CPT invariant. Consider a gedanken experiment in which Lorentzian asymptotically anti deSitter space has matter fields excited in certain modes. This is like the old discussions of a black hole in a box [5]. Non-linearities in the coupled matter and gravitational field equations will lead to the formation of a black hole [6]. If the mass of the asymptotically anti deSitter space is above the Hawking-Page mass [7], a black hole with radiation will be the most common configuration. If the space is below that mass the most likely configuration is pure radiation.

Whether or not the mass of the anti deSitter space is above the Hawking-Page mass the space will occasionally change to the other configuration, that is the black hole above the Hawking-Page mass will occasionally evaporate to pure radiation, or pure radiation will condense into a black hole. By CPT the time reverse will be the CP conjugate. This shows that, in this situation, the evaporation of a black hole is the time reverse of its formation (modulo CP), though the conventional descriptions are very different. Thus if one assume quantum gravity is CPT invariant, one rules out remnants, event horizons, and firewalls.

Further evidence against firewalls comes from considering asymptotically anti deSitter to the metrics that fit in an S1 cross S2 boundary at infinity. There are two such metrics: pe- riodically identified anti deSitter space, and Schwarzschild anti deSitter. Only periodically identified anti deSitter space contributes to the boundary to boundary correlation func- tions because the correlation functions from the Schwarzschild anti deSitter metric decay exponentially with real time [8, 9]. I take this as indicating that the topologically trivial periodically identified anti deSitter metric is the metric that interpolates between collapse to a black hole and evaporation. There would be no event horizons and no firewalls.

The absence of event horizons mean that there are no black holes - in the sense of regimes from which light can’t escape to infinity. There are however apparent horizons which persist for a period of time. This suggests that black holes should be redefined as metastable bound states of the gravitational field. It will also mean that the CFT on the boundary of anti deSitter space will be dual to the whole anti deSitter space, and not merely the region outside the horizon.

The no hair theorems imply that in a gravitational collapse the space outside the event horizon will approach the metric of a Kerr solution. However inside the event horizon, the metric and matter fields will be classically chaotic. It is the approximation of this chaotic metric by a smooth Kerr metric that is responsible for the information loss in gravitational collapse. The chaotic collapsed object will radiate deterministically but chaotically. It will be like weather forecasting on Earth. That is unitary, but chaotic, so there is effective information loss. One can’t predict the weather more than a few days in advance.

[1] A. Almheiri, D. Marolf, J. Polchinski, J. Sully, Black Holes: Complementarity or Firewalls?, J. High Energy Phys. 2, 062 (2013)
[2] S. W. Hawking, Breakdown of Predicatability in Gravitational Collapse, Phys. Rev. D 14, 2460 (1976)
[3] M. S. Fawcett, The Energy-Momentum Tensor near a Black Hole Commun. Math. Phys. 89, 103-115 (1983)
[4] K. W. Howard, P. Candelas, Quantum Stress Tensor in Schwarzschild Space-Time, Physical Review Letters 53, 5 (1984)
[5] S. W. Hawking, Black holes and Thermodynamics, Phys. Rev. D 13, 2 (1976)
[6] P. Bizon, A. Rostworowski, Weakly Turbulent Instability of Anti-de Sitter Space, Phys. Rev.
Lett. 107, 031102 (2011)
[7] S. W. Hawking, D. N. Page, Thermodynamics of Black Holes in Anti-de Sitter Space, Commun.
Math. Phys. 87, 577-588 (1983)
[8] J. Maldacena, Eternal black holes in anti-de Sitter, J. High Energy Phys. 04, 21 (2003)
[9] S. W. Hawking, Information Loss in Black Holes, Phys. Rev. D 72, 084013 (2005)

下边是有关霍金对黑洞理论的中文前后解释：

“英国着名物理学家史蒂芬霍金（Stephen Hawking），以其「黑洞理论」闻名于世，更启发现代外太空探险和好莱坞科幻电影，但他日前发表的最新论文却推翻「黑洞理论」，宣称黑洞（Black holes）其实不存在，真实存在的是「灰洞」（Grey holes）。

黑洞指的是宇宙中星体爆炸时，四周会產生强大的引力，并在宇宙空间中產生黑洞，把附近的一切都吞噬，包括光线和物质。由于引力相当强大，被吸入的物质以比光速更快的速度被吸入黑洞，同时永远无法从黑洞边缘（或称「视界」）逃出。

文中表示由于找不到黑洞的边界，因此黑洞根本可被视为不存在。

他还说，物质和能量在被黑洞困住一段时间以后，是可以被重新释放到宇宙中的。他在论文中承认，自己最初有关视界的认识是有缺陷的，光线其实是可以穿越视界的。霍金同时指出，对于这种逃离过程的解释需要一个能够将重力和其他基本力成功融合的理论。在过去近一百年间，物理学界都未曾有人试图解释这一过程。

1976年霍金发表「霍金辐射」理论，该理论认为在「霍金辐射」的影响下，黑洞会浓缩并蒸发。当黑洞消失后，黑洞形成初始最核心的「奇点」也会随之消失。但这种说法与量子力学相关理论出现相互矛盾之处，因为现代量子物理学认定「奇点」是永远不会完全消失的。这便是所谓的「黑洞悖论」。

30多年来，霍金试图以各种推测来解释这一悖论。霍金曾指，黑洞中量子运动是一种特殊情况，由于黑洞中的引力非常强烈，量子力学在此时已经不适用了。但一直以来，他的这种看法并未被物理学界广泛接受。

现在看来，霍金终于给了这个矛盾观点一个更具说服力的答案。霍金称，黑洞从来都不会完全关闭自身，它们在一段漫长的时间里逐步向外界辐射出越来越多的热量，最终黑洞会开放自己并释放出其中包含的物质能量。所以，所谓的黑洞，更像是「灰洞」。

「灰洞」一词本已存在于物理学界。它出现于1993年，美国波士顿大学的布雷彻将「灰洞」定义为：大质量星的坍缩不足以形成黑洞，但有可能形成比典型中子星有较小的半径和较大密度的星体。“