星空浩淼
发表文章数: 1743
武功等级: 九阳神功
(第五重)
内力值: 617/617
我曾经跟昌海兄请教过的一个问题及专家答案
yinhow在“sage”的跟帖里面提供了一个好网址，刚才进去看了看“Frequently Asked Questions in Physics”，没想到里面也有我迷惑过的问题。引力，特别引力量子化方面我是外行，越是外行越好奇，越有疑问。其中有个问题是我曾经在信上请教过昌海兄。虽然我这辈子没有机会研究黑洞等，但有些问题自己就冒出来：例如既然黑洞吞食一切，那传播引力的引力子又是如何跑出来的？我自己猜想这是因为虚引力子可以超光速前进（这与下面专家答案差不多）。这个问题还跟我的背景有关：一方面由于我硕士毕业论文是做关于意大利人的喇叭天线超光速实验理论解释的，结论是我发现人家实验结论有问题，物理上不可能成立。可惜当事人死不承认。我也很奇怪他们那种犯有低级理论错误的论文居然能在PRL上发表——事后果真发现有十几篇comment反驳。不过世界各国都能重复的截止波导光子隧穿超光速实验却是有相当的可信度。说到理论解释，我猜想隧穿光子相当于虚光子，可以超光速但不违背因果律——这之前我刚好又研究过时间是否可以作为力学量的问题（这是我在J.Phys.A上发表的处女作），从资料得知人们研究tunneling time的最后结论是隧穿粒子可以是超光速的。在“Frequently Asked Questions in Physics”中我能收获最大的是关于虚粒子如何传播吸引力的问题（可惜有些地方我觉得还不够清楚，将来有时间和大家在这里讨论一下。
附：
How does the gravity get out of the black hole?
Purely in terms of general relativity, there is no problem here. The gravity doesn't have to get out of the black hole. General relativity is a local theory, which means that the field at a certain point in spacetime is determined entirely by things going on at places that can communicate with it at speeds less than or equal to c. If a star collapses into a black hole, the gravitational field outside the black hole may be calculated entirely from the properties of the star and its external gravitational field before it becomes a black hole. Just as the light registering late stages in my fall takes longer and longer to get out to you at a large distance, the gravitational consequences of events late in the star's collapse take longer and longer to ripple out to the world at large. In this sense the black hole is a kind of "frozen star": the gravitational field is a fossil field. The same is true of the electromagnetic field that a black hole may possess.
Often this question is phrased in terms of gravitons, the hypothetical quanta of spacetime distortion. If things like gravity correspond to the exchange of "particles" like gravitons, how can they get out of the event horizon to do their job?
Gravitons don't exist in general relativity, because GR is not a quantum theory. They might be part of a theory of quantum gravity when it is completely developed, but even then it might not be best to describe gravitational attraction as produced by virtual gravitons. See the physics FAQ on virtual particles for a discussion of this.
Nevertheless, the question in this form is still worth asking, because black holes can have static electric fields, and we know that these may be described in terms of virtual photons. So how do the virtual photons get out of the event horizon? Well, for one thing, they can come from the charged matter prior to collapse, just like classical effects. In addition, however, virtual particles aren't confined to the interiors of light cones: they can go faster than light! Consequently the event horizon, which is really just a surface that moves at the speed of light, presents no barrier.
I couldn't use these virtual photons after falling into the hole to communicate with you outside the hole; nor could I escape from the hole by somehow turning myself into virtual particles. The reason is that virtual particles don't carry any information outside the light cone. See the physics FAQ on virtual particles for details.
持之以恒就是胜利
| 发表时间：2004-10-10, 13:10:08 |
 作者资料
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星空浩淼
发表文章数: 1743
武功等级: 九阳神功
(第五重)
内力值: 617/617
Re: 我曾经跟昌海兄请教过的一个问题及专家答案
补充一下：这个问题我记得昌海兄的回答比较完善，总的来说跟这里的专家回答差不多，只是分成了几次才考虑成熟的，最开始也是先从广义相对论的弯曲时空观点来看的。我注意到在那个网站里，专家对某一个问题的回答也经常多次更新，并且每次更新可能是不同的人。
另外我后来还跟yinhow讨论过这个问题。我当时有个想法是：如果天体坍塌成黑洞之后，传递引力相互作用的（虚）引力子从天体传出来的概率比原来减小，那么天体坍塌成黑洞之后，它的引力质量有可能不再等于它的惯性质量——这个，纯属瞎猜而已。
持之以恒就是胜利
| 发表时间：2004-10-10, 13:37:46 |
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yinhow
发表文章数: 727
武功等级: 弹指神通
(第二重)
内力值: 469/469
Re: 我曾经跟昌海兄请教过的一个问题及专家答案
有一个更为专业的论坛, 星空兄不妨去看看:
http://www.lns.cornell.edu/spr/
| 发表时间：2004-10-10, 20:42:23 |
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