黑洞不存在

霍金宇宙新論
「黑洞不存在」

茫茫宇宙中，傳說可吞噬任何物質的「黑洞」（black hole）到底存在嗎？英國當代物理學大師霍金（Stephen Hawking）窮一生研究希望證實黑洞存在，但他日前決定「打倒昨日的我」，發表論文指所謂能吞噬一切的黑洞，根本不存在，震驚物理學界。他淡然接受一生心血或付諸流水的事實，微笑寫道：「我（對黑洞）的看法改變了。」

黑洞一直是天文物理學上最神秘的領域，它由質量龐大的恒星殆盡後，重力塌縮至無限小的「奇點」（singularity）而成。根據科學家愛因斯坦（Albert Einstein）提出的廣義相對論，它的重力場將強得令任何物質、輻射甚至光線都無法逃逸。而物理學界對黑洞存在與否的關鍵爭拗點，一直在於黑洞的邊緣界線──「事件視界」（event horizon）。
由於「奇點」體積無限小，不可能有空間儲存任何物質和資料，而理論上任何數據物質只要墮進「事件視界」內，均會徹底湮滅，從此消失於宇宙中。但這建基於廣義相對論的黑洞「事件視界」概念，卻與量子力學相矛盾。根據量子力學定律，任何轉化過程均可逆轉，因此應有能量或資訊可從黑洞逃逸。

根本沒有「事件視界」

兩年前有關爭拗更升溫，美國物理學家波爾欽斯基（Joseph Polchinski）團隊發現，依循量子力學定律，「事件視界」必須為高能量的「火牆」將物質燃燒殆盡才墮進黑洞。但問題又來了，若然「火牆」存在，則違反廣義相對論中的「等效原理」──若然物質以自由落體墮進黑洞，將無法在宇宙找到相應的單位。那麼，「事件視界」應毫不起眼，而不能像火牆耀目。
這黑洞「火牆悖論」（firewall paradox）纏繞物理學界兩年後，霍金上周三終於在網上發表論文，拋出兼容兩個理論的新答案：黑洞周圍根本沒有「事件視界」。他解釋，黑洞周圍的量子效應令時空變化太劇烈，因此分界並不清晰，並大膽提出以「表觀視界」（apparent horizon）為真正的黑洞邊界，取代「事件視界」。

「表觀視界」暫存物質能量

「表觀視界」可將企圖逃逸的物質和能量暫存，然後再將物質釋放，但實際物質如何逃離黑洞的機制仍有待進一步研究。不過，雖然訊息難再收復，但訊息某程度上仍然存在，霍金就將相關過程，形容像天氣預報一樣，理論上可行，但實際上很難準確做到。
但霍金宣稱沒有「事件視界」，對解決黑洞存在與否的問題意義非常重大──既然「吞噬」的物質將重回宇宙，換言之，傳統概念說連光線也無法逃逸的所謂黑洞，實際並不存在。霍金在論文中承認：「沒有了『事件視界』，意味（宇宙）沒有黑洞了。」

拿黑洞理論打賭
輸百科全書

愛打賭的霍金多次拿黑洞理論來跟其他科學家賭個不亦樂乎。1997年，他聯同美國理論物理學家索恩（Kip Thorne），跟同是理論物理學家的普雷斯基爾（John Preskill）打賭，結果輸了一套百科全書。
霍金和索恩認為根據廣義相對論，任何物質包括資訊掉入黑洞後，黑洞中產生的輻射都是「新」的，與原來的物質無關；普雷斯基爾卻認為這個命題違反了量子力學，雙方1997年打賭，勝方可自選一套百科全書。
最後霍金在2004年承認輸了，同意訊息不會在黑洞中消失，但會被黑洞撕碎、打破和重整，成為「無用的訊息」，就像燒掉一本百科全書。他向普雷斯基爾奉上一套棒球百科全書，笑道︰「或許我該送他一把（百科全書）灰燼。」

遭學生質疑
比「火牆」更急進

霍金大膽提出「宇宙沒黑洞論」震驚物理學界，但不少人對新見解抱有懷疑，似乎物理學界暫時看來也不會對此全盤接收。
意味沒有「奇點」

美國物理學家波爾欽斯基提出有「火牆」將物質燃燒殆盡才墮進黑洞，他質疑霍金所指「沒有事件視界的黑洞」，究竟能否存在。對霍金有懷疑的，還有霍金的學生布索（Raphael Bousso），他認為恩師的新理論有很多疑點，指「物質可逃出黑洞」的說法似乎比「火牆存在」更急進、引發更多問題。
加拿大物理學家佩奇（Don Page）則認同霍金，認為理論聽來也合理，他說：「儘管你可以說它提出沒有事件視界很急進，但這涉及超高量子條件，我們對時空概念也有分歧，遑論如何界定「事件視界」的區域是否存在吧！」他認同「事件視界」可以不存在，但也擔心「表觀視界」也存在類似「火牆悖論」的問題。
不過有專家認為，若然霍金新理論正確，將顛覆很多物理學界對黑洞的概念，意味黑洞核心內或者根本也沒有「奇點」，物質只暫存在「表觀視界」，最終或以「霍金輻射」（Hawking radiation）釋出。

Stephen Hawking: 'There are no black holes'

Most physicists foolhardy enough to write a paper claiming that “there are no black holes” — at least not in the sense we usually imagine — would probably be dismissed as cranks. But when the call to redefine these cosmic crunchers comes from Stephen Hawking, it’s worth taking notice. In a paper posted online, the physicist, based at the University of Cambridge, UK, and one of the creators of modern black-hole theory, does away with the notion of an event horizon, the invisible boundary thought to shroud every black hole, beyond which nothing, not even light, can escape.

 “There is no escape from a black hole in classical theory, but quantum theory enables energy and information to escape.”

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.

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.

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.

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."