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Stephen Hawking: 'There are no black holes'
Notion of an 'event horizon', from which nothing can escape, is incompatible with quantum theory, physicist claims.
In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form.
“There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “The correct treatment,” Hawking says, “remains a mystery.”
Hawking posted his paper on the arXiv preprint server on 22 January1. He titled it, whimsically, 'Information preservation and weather forecasting for black holes', and it has yet to pass peer review. The paper was based on a talk he gave via Skype at a meeting at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in August 2013 (watch video of the talk).
Fire fighting
Hawking's new work is an attempt to solve what is known as the black-hole firewall paradox, which has been vexing physicists for almost two years, after it was discovered by theoretical physicist Joseph Polchinski of the Kavli Institute and his colleagues (see 'Astrophysics: Fire in the hole!').
In a thought experiment, the researchers asked what would happen to an astronaut unlucky enough to fall into a black hole. Event horizons are mathematically simple consequences of Einstein's general theory of relativity that were first pointed out by the German astronomer Karl Schwarzschild in a letter he wrote to Einstein in late 1915, less than a month after the publication of the theory. In that picture, physicists had long assumed, the astronaut would happily pass through the event horizon, unaware of his or her impending doom, before gradually being pulled inwards — stretched out along the way, like spaghetti — and eventually crushed at the 'singularity', the black hole’s hypothetical infinitely dense core.
霍金自我否定稱黑洞不存在！
But on analysing the situation in detail, Polchinski’s team came to the startling realization that the laws of quantum mechanics, which govern particles on small scales, change the situation completely. Quantum theory, they said, dictates that the event horizon must actually be transformed into a highly energetic region, or 'firewall', that would burn the astronaut to a crisp.
This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe — whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place.
Beyond the horizon
Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.
In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon.
Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out 'Hawking radiation'. In that case, the event horizon would, in theory, become smaller than the apparent horizon. Hawking’s new suggestion is that the apparent horizon is the real boundary. “The absence of event horizons means that there are no black holes — in the sense of regimes from which light can't escape to infinity,” Hawking writes.
“The picture Hawking gives sounds reasonable,” says Don Page, a physicist and expert on black holes at the University of Alberta in Edmonton, Canada, who collaborated with Hawking in the 1970s. “You could say that it is radical to propose there’s no event horizon. But these are highly quantum conditions, and there’s ambiguity about what space-time even is, let alone whether there is a definite region that can be marked as an event horizon.”
Although Page accepts Hawking’s proposal that a black hole could exist without an event horizon, he questions whether that alone is enough to get past the firewall paradox. The presence of even an ephemeral apparent horizon, he cautions, could well cause the same problems as does an event horizon.
Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape).
If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre. Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were.
“It would be worse than trying to reconstruct a book that you burned from its ashes,” says Page. In his paper, Hawking compares it to trying to forecast the weather ahead of time: in theory it is possible, but in practice it is too difficult to do with much accuracy.
Polchinski, however, is sceptical that black holes without an event horizon could exist in nature. The kind of violent fluctuations needed to erase it are too rare in the Universe, he says. “In Einstein’s gravity, the black-hole horizon is not so different from any other part of space,” says Polchinski. “We never see space-time fluctuate in our own neighbourhood: it is just too rare on large scales.”Raphael Bousso, a theoretical physicist at the University of California, Berkeley, and a former student of Hawking's, says that this latest contribution highlights how “abhorrent” physicists find the potential existence of firewalls. However, he is also cautious about Hawking’s solution. “The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls,” he says. "But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance."
相關介紹：
在這篇名為《黑洞的信息保存與氣象預報》(Information Preservation and Weather Forecasting For Black Holes)的論文中，霍金指出，由于找不到黑洞的邊界，因此黑洞是不存在的。黑洞的邊界又稱“視界”。經(jīng)典黑洞理論認為，黑洞外的物質(zhì)和輻射可以通過視界進入黑洞內(nèi)部，而黑洞內(nèi)的任何物質(zhì)和輻射均不能穿出視界。
霍金的最新“灰洞”理論認為，物質(zhì)和能量在被黑洞困住一段時間以后，又會被重新釋放到宇宙中。他在論文中承認，自己最初有關視界的認識是有缺陷的，光線其實是可以穿越視界的。當光線逃離黑洞核心時，它的運動就像人在跑步機上奔跑一樣，慢慢地通過向外輻射而收縮。
“經(jīng)典黑洞理論認為，任何物質(zhì)和輻射都不能逃離黑洞，而量子力學理論表明，能量和信息是可以從黑洞中逃離出來的?！?BR> 霍金同時指出，對于這種逃離過程的解釋需要一個能夠將重力和其他基本力成功融合的理論。在過去近一百年間，物理學界沒有人曾試圖解釋這一過程。
對于霍金的“灰洞”理論，一些科學家表示認可，但也有人持懷疑態(tài)度。美國卡夫立理論物理研究所的理論物理學家約瑟夫•波爾欽斯基(Joseph Polchinski)指出，根據(jù)愛因斯坦的重力理論，黑洞的邊界是存在的，只是它與宇宙其他部分的區(qū)別并不明顯。
其實，早在2004年霍金就曾做出過類似表示。當年7月21日，霍金在“第17屆國際廣義相對論和萬有引力大會”上指出，黑洞并非如他和其他大多數(shù)物理學家以前認為的那樣，對其周遭的一切“完全吞噬”，事實上被吸入黑洞深處的物質(zhì)的某些信息可能會在某個時候釋放出來。
1976年，霍金稱自己通過計算得出結論，黑洞在形成過程中其質(zhì)量減少的同時，還不斷在以能量的形式向外界發(fā)出輻射。這就是的“霍金輻射”理論。但是，該理論提到的黑洞輻射中并不包括黑洞內(nèi)部物質(zhì)的任何信息，一旦這個黑洞濃縮并蒸發(fā)消失后，其中的所有信息就都隨之消失了。這便是所謂的“黑洞悖論”。
這種說法與量子力學的相關理論出現(xiàn)相互矛盾之處。因為現(xiàn)代量子物理學認定這種物質(zhì)信息是永遠不會完全消失的。30多年來，霍金試圖以各種推測來解釋這一自相矛盾的觀點?；艚鹪硎荆诙粗辛孔舆\動是一種特殊情況，由于黑洞中的引力非常強烈，量子力學在此時已經(jīng)不再適用了。但是霍金的這種說法并沒有讓科學界眾多持懷疑態(tài)度學者信服。
現(xiàn)在看來，霍金終于給了這個當年自相矛盾的觀點一個更具有說服力的答案。霍金稱，黑洞從來都不會完全關閉自身，它們在一段漫長的時間里逐步向外界輻射出越來越多的熱量，隨后黑洞將最終開放自己并釋放出其中包含的物質(zhì)信息。
Stephen Hawking: 'There are no black holes'
Notion of an 'event horizon', from which nothing can escape, is incompatible with quantum theory, physicist claims.
In its stead, Hawking’s radical proposal is a much more benign “apparent horizon”, which only temporarily holds matter and energy prisoner before eventually releasing them, albeit in a more garbled form.
“There is no escape from a black hole in classical theory,” Hawking told Nature. Quantum theory, however, “enables energy and information to escape from a black hole”. A full explanation of the process, the physicist admits, would require a theory that successfully merges gravity with the other fundamental forces of nature. But that is a goal that has eluded physicists for nearly a century. “The correct treatment,” Hawking says, “remains a mystery.”
Hawking posted his paper on the arXiv preprint server on 22 January1. He titled it, whimsically, 'Information preservation and weather forecasting for black holes', and it has yet to pass peer review. The paper was based on a talk he gave via Skype at a meeting at the Kavli Institute for Theoretical Physics in Santa Barbara, California, in August 2013 (watch video of the talk).
Fire fighting
Hawking's new work is an attempt to solve what is known as the black-hole firewall paradox, which has been vexing physicists for almost two years, after it was discovered by theoretical physicist Joseph Polchinski of the Kavli Institute and his colleagues (see 'Astrophysics: Fire in the hole!').
In a thought experiment, the researchers asked what would happen to an astronaut unlucky enough to fall into a black hole. Event horizons are mathematically simple consequences of Einstein's general theory of relativity that were first pointed out by the German astronomer Karl Schwarzschild in a letter he wrote to Einstein in late 1915, less than a month after the publication of the theory. In that picture, physicists had long assumed, the astronaut would happily pass through the event horizon, unaware of his or her impending doom, before gradually being pulled inwards — stretched out along the way, like spaghetti — and eventually crushed at the 'singularity', the black hole’s hypothetical infinitely dense core.
霍金自我否定稱黑洞不存在！
But on analysing the situation in detail, Polchinski’s team came to the startling realization that the laws of quantum mechanics, which govern particles on small scales, change the situation completely. Quantum theory, they said, dictates that the event horizon must actually be transformed into a highly energetic region, or 'firewall', that would burn the astronaut to a crisp.
This was alarming because, although the firewall obeyed quantum rules, it flouted Einstein’s general theory of relativity. According to that theory, someone in free fall should perceive the laws of physics as being identical everywhere in the Universe — whether they are falling into a black hole or floating in empty intergalactic space. As far as Einstein is concerned, the event horizon should be an unremarkable place.
Beyond the horizon
Now Hawking proposes a third, tantalizingly simple, option. Quantum mechanics and general relativity remain intact, but black holes simply do not have an event horizon to catch fire. The key to his claim is that quantum effects around the black hole cause space-time to fluctuate too wildly for a sharp boundary surface to exist.
In place of the event horizon, Hawking invokes an “apparent horizon”, a surface along which light rays attempting to rush away from the black hole’s core will be suspended. In general relativity, for an unchanging black hole, these two horizons are identical, because light trying to escape from inside a black hole can reach only as far as the event horizon and will be held there, as though stuck on a treadmill. However, the two horizons can, in principle, be distinguished. If more matter gets swallowed by the black hole, its event horizon will swell and grow larger than the apparent horizon.
Conversely, in the 1970s, Hawking also showed that black holes can slowly shrink, spewing out 'Hawking radiation'. In that case, the event horizon would, in theory, become smaller than the apparent horizon. Hawking’s new suggestion is that the apparent horizon is the real boundary. “The absence of event horizons means that there are no black holes — in the sense of regimes from which light can't escape to infinity,” Hawking writes.
“The picture Hawking gives sounds reasonable,” says Don Page, a physicist and expert on black holes at the University of Alberta in Edmonton, Canada, who collaborated with Hawking in the 1970s. “You could say that it is radical to propose there’s no event horizon. But these are highly quantum conditions, and there’s ambiguity about what space-time even is, let alone whether there is a definite region that can be marked as an event horizon.”
Although Page accepts Hawking’s proposal that a black hole could exist without an event horizon, he questions whether that alone is enough to get past the firewall paradox. The presence of even an ephemeral apparent horizon, he cautions, could well cause the same problems as does an event horizon.
Unlike the event horizon, the apparent horizon can eventually dissolve. Page notes that Hawking is opening the door to a scenario so extreme “that anything in principle can get out of a black hole”. Although Hawking does not specify in his paper exactly how an apparent horizon would disappear, Page speculates that when it has shrunk to a certain size, at which the effects of both quantum mechanics and gravity combine, it is plausible that it could vanish. At that point, whatever was once trapped within the black hole would be released (although not in good shape).
If Hawking is correct, there could even be no singularity at the core of the black hole. Instead, matter would be only temporarily held behind the apparent horizon, which would gradually move inward owing to the pull of the black hole, but would never quite crunch down to the centre. Information about this matter would not destroyed, but would be highly scrambled so that, as it is released through Hawking radiation, it would be in a vastly different form, making it almost impossible to work out what the swallowed objects once were.
“It would be worse than trying to reconstruct a book that you burned from its ashes,” says Page. In his paper, Hawking compares it to trying to forecast the weather ahead of time: in theory it is possible, but in practice it is too difficult to do with much accuracy.
Polchinski, however, is sceptical that black holes without an event horizon could exist in nature. The kind of violent fluctuations needed to erase it are too rare in the Universe, he says. “In Einstein’s gravity, the black-hole horizon is not so different from any other part of space,” says Polchinski. “We never see space-time fluctuate in our own neighbourhood: it is just too rare on large scales.”Raphael Bousso, a theoretical physicist at the University of California, Berkeley, and a former student of Hawking's, says that this latest contribution highlights how “abhorrent” physicists find the potential existence of firewalls. However, he is also cautious about Hawking’s solution. “The idea that there are no points from which you cannot escape a black hole is in some ways an even more radical and problematic suggestion than the existence of firewalls,” he says. "But the fact that we’re still discussing such questions 40 years after Hawking’s first papers on black holes and information is testament to their enormous significance."
相關介紹：
在這篇名為《黑洞的信息保存與氣象預報》(Information Preservation and Weather Forecasting For Black Holes)的論文中，霍金指出，由于找不到黑洞的邊界，因此黑洞是不存在的。黑洞的邊界又稱“視界”。經(jīng)典黑洞理論認為，黑洞外的物質(zhì)和輻射可以通過視界進入黑洞內(nèi)部，而黑洞內(nèi)的任何物質(zhì)和輻射均不能穿出視界。
霍金的最新“灰洞”理論認為，物質(zhì)和能量在被黑洞困住一段時間以后，又會被重新釋放到宇宙中。他在論文中承認，自己最初有關視界的認識是有缺陷的，光線其實是可以穿越視界的。當光線逃離黑洞核心時，它的運動就像人在跑步機上奔跑一樣，慢慢地通過向外輻射而收縮。
“經(jīng)典黑洞理論認為，任何物質(zhì)和輻射都不能逃離黑洞，而量子力學理論表明，能量和信息是可以從黑洞中逃離出來的?！?BR> 霍金同時指出，對于這種逃離過程的解釋需要一個能夠將重力和其他基本力成功融合的理論。在過去近一百年間，物理學界沒有人曾試圖解釋這一過程。
對于霍金的“灰洞”理論，一些科學家表示認可，但也有人持懷疑態(tài)度。美國卡夫立理論物理研究所的理論物理學家約瑟夫•波爾欽斯基(Joseph Polchinski)指出，根據(jù)愛因斯坦的重力理論，黑洞的邊界是存在的，只是它與宇宙其他部分的區(qū)別并不明顯。
其實，早在2004年霍金就曾做出過類似表示。當年7月21日，霍金在“第17屆國際廣義相對論和萬有引力大會”上指出，黑洞并非如他和其他大多數(shù)物理學家以前認為的那樣，對其周遭的一切“完全吞噬”，事實上被吸入黑洞深處的物質(zhì)的某些信息可能會在某個時候釋放出來。
1976年，霍金稱自己通過計算得出結論，黑洞在形成過程中其質(zhì)量減少的同時，還不斷在以能量的形式向外界發(fā)出輻射。這就是的“霍金輻射”理論。但是，該理論提到的黑洞輻射中并不包括黑洞內(nèi)部物質(zhì)的任何信息，一旦這個黑洞濃縮并蒸發(fā)消失后，其中的所有信息就都隨之消失了。這便是所謂的“黑洞悖論”。
這種說法與量子力學的相關理論出現(xiàn)相互矛盾之處。因為現(xiàn)代量子物理學認定這種物質(zhì)信息是永遠不會完全消失的。30多年來，霍金試圖以各種推測來解釋這一自相矛盾的觀點?；艚鹪硎荆诙粗辛孔舆\動是一種特殊情況，由于黑洞中的引力非常強烈，量子力學在此時已經(jīng)不再適用了。但是霍金的這種說法并沒有讓科學界眾多持懷疑態(tài)度學者信服。
現(xiàn)在看來，霍金終于給了這個當年自相矛盾的觀點一個更具有說服力的答案。霍金稱，黑洞從來都不會完全關閉自身，它們在一段漫長的時間里逐步向外界輻射出越來越多的熱量，隨后黑洞將最終開放自己并釋放出其中包含的物質(zhì)信息。