本篇為根本級科普文章要謝謝紐約時報
A Black Hole Mystery Wrapped in a Firewall Paradox
By DENNIS OVERBYE August 19, 2013
This time, they say, Einstein might really be wrong.
A high-octane debate has
broken out among the world’s physicists about what would happen if you
jumped into a black hole, a fearsome gravitational monster that can
swallow matter, energy and even light. You would die, of course, but
how? Crushed smaller than a dust mote by monstrous gravity, as
astronomers and science fiction writers have been telling us for
decades? Or flash-fried by a firewall of energy, as an alarming new
calculation seems to indicate?
-
Alicia DeSantis/The New York Times
An unexpected paradox involving black holes pits
two basic tenets of modern science against one another: the theory of
quantum mechanics, which governs subatomic particles, and Einstein’s
theory of general relativity, which explains how gravity works.
-
Jim Wilson/The New York Times
Leonard Susskind.
-
Jim Wilson/The New York Times
Raphael Bousso.
This dire-sounding debate
has spawned a profusion of papers, blog posts and workshops over the
last year. At stake is not Einstein’s reputation, which is after all
secure, or even the efficacy of our iPhones, but perhaps the basis of
his general theory of relativity, the theory of gravity, on which our
understanding of the universe is based. Or some other fundamental
long-established principle of nature might have to be abandoned, but
physicists don’t agree on which one, and they have been flip-flopping
and changing positions almost weekly, with no resolution in sight.
“I was a yo-yo on this,” said one of the more prolific authors in the field,
Leonard Susskind of Stanford. He paused and added, “I haven’t changed my mind in a few months now.”
Raphael Bousso, a theorist at the University of California, Berkeley, said, “I’ve never been so surprised. I don’t know what to expect.”
You might wonder who cares,
especially if encountering a black hole is not on your calendar. But
some of the basic tenets of modern science and of Einstein’s theory are
at stake in the “firewall paradox,” as it is known.
“It points to something
missing in our understanding of gravity,” said Joseph Polchinski, of the
Kavli Institute for Theoretical Physics in Santa Barbara, Calif., one
of the
theorists who set off this confusion.
Down this rabbit hole are
many of the jazzy magical mysteries of modern physics: Black holes. The
shortcuts through space and time called wormholes. Quantum entanglement,
also known as spooky action at a distance, in which particles separated
by light-years can still instantaneously appear to remain connected.
The reward for going down this hole could be a new understanding of why
we think we live in a universe with space and time at all, with suitably
unpredictable consequences. After all, if Einstein hadn’t been troubled
a century ago by logical inconsistencies in the Newtonian universe, we
might not have GPS systems, which rely on his theory of general
relativity to keep time, in our pockets today.
Falling Bodies
Black holes are the most
extreme predictions of Einstein’s theory, which describes how matter and
energy warp the geometry of space and time the way a heavy sleeper
causes a mattress to sag. Too much matter and energy in one place could
cause space to sag so far that the matter inside it would disappear as
if behind a magician’s cloak, collapsing endlessly to a point of
infinite density known as a singularity. Einstein thought that idea was
ridiculous when it was pointed out to him at the time, in 1916, but
today astronomers agree that the universe is speckled with such dark
monsters, including beasts lurking in the hearts of most galaxies that
are millions and billions of time more massive than the Sun. Many of
them resulted from the collapse of dead stars.
General relativity is based
on what Einstein later called his “happiest thought,” that a freely
falling person would not feel his weight. It is known simply as the
equivalence principle; it says that empty space looks the same
everywhere and to everyone.
One consequence of this
principle is that an astronaut would not feel anything special happening
when he fell through the point of no return, known as the event
horizon, into a black hole. Like a bungee jumper, he would feel
weightless then and all the way until he hit the bottom, which could
take seconds or years depending on how big the hole was, and he would be
stretched like a noodle by tidal forces and then crushed into a speck.
At the event horizon there would be “no drama,” in the lexicon — at
least in the physical sense, as opposed to the intellectual trauma of
knowing you were not ever going home. Things or people went in, they got
crushed to infinite density and disappeared. That was the traditional
view of black holes.
Things got more
interesting, however, in 1974 when Stephen Hawking, the British
cosmologist, stunned the world by showing that when the paradoxical
quantum laws that describe subatomic behavior were taken into account,
black holes would leak particles and radiation, and in fact eventually
explode, although for a hole the mass of a star it would take longer
than the age of the universe.
This was a breakthrough in
combining general relativity, the gravity that curves the cosmos, with
quantum theory, which describes the microscopic quirkiness inside it,
but there was a big hitch. Dr. Hawking concluded that the radiation
coming from a black hole would be completely random, conveying no
information about what had fallen into it. When the black hole finally
exploded, all that information would be erased from the universe
forever. “God not only plays dice with the universe,” Dr. Hawking said
in 1976 in a riposte to Einstein’s famous doubts about the randomness of
quantum theory, “he sometimes throws them where they can’t be seen.”
Particle physicists cried
foul, saying that this violated a basic tenet of modern science and of
quantum theory, that information is always preserved. From the material
in the smoke and flames of a burning book, for example, one could figure
out whether it was the Bible or the Kama Sutra; the same should be true
of the fizz and pop of black holes, these physicists argued. A 30-year
controversy ensued.
The Firewall Paradox
Now, however, some
physicists say that Dr. Hawking might have conceded too soon. “He had
good reason,” said Dr. Polchinski, “but he gave up for the wrong
reason.” Nobody, he explained, had yet figured out exactly how
information does get out of a black hole.
That was the task that four
researchers based in Santa Barbara — Ahmed Almheiri, Donald Marolf, and
James Sully, all from the University of California, Santa Barbara, and
Dr. Polchinski of the Kavli Institute set themselves a year ago. The
team (called AMPS, after their initials) found, to their surprise, that
following the known laws of physics would lead to a contradiction, the
firewall paradox.
Their calculations showed
that having information flowing out of a black hole was incompatible
with having an otherwise smooth Einsteinian space-time at its boundary,
the event horizon. In its place would be a discontinuity in the vacuum
that would manifest itself as energetic particles — a “firewall” —
lurking just inside the black hole.
Being incinerated as you
entered a black hole would certainly contradict Einstein’s dictum of no
drama. If this were true, you would in fact die long before the
bungee-jumping ride ever got anywhere close to the bottom. The existence
of a firewall would mean that the horizon, which according to general
relativity is just empty space, is a special place, pulling the rug out
from under Einstein’s principle, his theory of gravity, and modern
cosmology, which is based on general relativity. This presented the
scientists with what Dr. Bousso calls the “menu from hell.” If the
firewall argument was right, one of three ideas that lie at the heart
and soul of modern physics, had to be wrong. Either information can be
lost after all; Einstein’s principle of equivalence is wrong; or quantum
field theory, which describes how elementary particles and forces
interact, is wrong and needs fixing. Abandoning any one of these would
be revolutionary or appalling or both.
Quantum Vows
The firewall argument
hinges on one of the weirder aspects of quantum physics, the action
called entanglement. As Einstein, Boris Podolsky and Nathan Rosen
pointed out in 1935,
quantum theory predicts that a pair of particles can be connected in
such a way that measuring a property of one — its direction of spin, say
— will immediately affect the results of measuring the other one, even
if it is light-years away.
Einstein used this “spooky
action at a distance” to suggest the absurdity of quantum mechanics, but
such experiments are now done in labs every day. You can’t use it to
send a message faster than light, because the correlation shows up only
when the two experimenters get together and compare their respective
results. But it plays a crucial role in quantum computing and
cryptography — and, it turns out, in explaining how information encoded
in the Hawking radiation gets out of a black hole.
Consider two particles
(let’s call them Bob and Alice) that have been radiated by a black hole.
Bob left it eons ago, as it began leaking radiation; quantum
entanglement theory dictates that in order for the black hole to keep
track of what information it has been transmitting, Bob out there has to
be entangled with Alice, who just left.
But that scenario competes
with another kind of entanglement, between particles on either side of
the event horizon, the black hole’s boundary. If space is indeed smooth,
as Einstein postulated, and if quantum field theory is correct, Alice
must be entangled with another particle, Ted, who is just inside the
black hole.
But quantum theory forbids
promiscuous entanglements. In the language of quantum information, Alice
can marry either Bob or Ted, but not both, even if the second marriage
happens inside the black hole where most of us can’t see it.
Alice should have a
consistent explanation of the universe, Dr. Polchinski explained, “just
as we ourselves must, even though we are inside the cosmic horizon.”
Meanwhile, physicists have
more reason than ever to think that information cannot be lost. A
celebrated 1997 paper by Juan M. Maldacena of the Institute for Advanced
Study describes nature as a kind of hologram, in which the information
about what happens inside a volume of three-dimensional space, for
example, is encoded in quantum equations on its two-dimensional
boundary, the way a 3-D image is encoded on the face of your bank card.
Mark Van Raamsdonk, a young
theorist at the University of British Columbia, likes to use a spookier
analogy to describe this, namely the chip that controls a Matrix-like
video game. (Feel free to insert your own woo-woo music here.)
The discovery that the
information needed to describe what happens in some volume is
proportional to the area enclosing that volume is the strangest and most
far-reaching consequence of
Dr. Hawking’s discovery that black holes explode, and is still wreathed in mystery.
Dr. Maldacena’s universe is
often portrayed like a can of soup, in which galaxies, black holes,
gravity, stars and so forth, including us, are the soup inside, while
the information to describe them resides, like a label, on the outside.
Think of it as gravity in a can. The equations that represent the label
are deterministic and there is no room in them for information to be
lost, implying that information in the universe inside is also
preserved.
Which leaves the firewall
as the only way to stop the illegal marriage of Alice and Ted, Dr.
Polchinski said — an odious solution because it contravenes the basic
principle of general relativity.
Recently a new way of
solving the firewall conundrum and of answering that haunting question
has attracted a lot of attention, although no consensus. Dr. Maldacena
and Dr. Susskind have proposed that Einstein could come to his own
rescue via one more far-out notion in modern physics: wormholes.
In 1935 Einstein and Rosen
found that, mathematically anyway, black holes could come in pairs
connected by shortcuts through space — then known as Einstein-Rosen
bridges, now known as wormholes. A wormhole would not be traversable by
any means we now know about, ruling out time travel and other violations
of relativity, despite the dreams of science fiction writers and
interstellar pioneers.
In effect, what these
theorists were saying was that without the phenomenon of entanglement,
space-time would have no structure at all. Or as Dr. Maldacena put it,
“Spooky action at a distance creates space-time.” If true, this insight
would be a step toward a longtime dream of theorists of explaining how
space and time emerge from some more basic property of reality, in this
case, bits of quantum information. The theorist
John Wheeler, of Princeton, who had coined the term “black hole,” called this concept “it from bit.”
Taking this idea seriously,
Dr. Maldacena and Dr. Susskind proposed that a similar kind of wormhole
arrangement existed between the black hole in the AMPS case and its
Hawking radiation. Instead of a tunnel snaking through hyperspace and
opening at the maw of another black hole, the wormhole would split into a
zillion spaghetti-like strands ending on each of the pieces of Hawking
radiation. That would mean that Bob, the Hawking particle in the cartoon
version of the theory mentioned above, might be light years away from
the event horizon, but he would still be connected to the interior of
the black hole, as if there were a doorway in New Jersey that opened up
into a basement in Manhattan.
Because of this wormhole
connection, Dr. Maldacena explained, “Ted and Bob are the same.” So the
result is sort of like the happy ending of one of those screwball
romantic comedies that involve mistaken identity and the handsome
vagabond turns out to be the prince in disguise; Alice can marry Ted who
is really Bob and the bonds of matrimony extend smoothly across the
edge of the black hole.
Entangled Theories
Dr. Maldacena and Dr.
Susskind admit that the wormhole hypothesis is still a work in progress.
Few of their colleagues are convinced yet that it has been formulated
in sufficient detail, let alone that it can solve the firewall paradox.
“All I can say,” Dr. Susskind said in an e-mail on the eve of a firewall
workshop next week at the Kavli Institute where wormholes and
everything else will surely be scrutinized, “is that no one has a
completely solid case and that certainly includes me. Time will tell.”
Dr. Polchinski said, “My
current thinking is that all the arguments that we are having are the
kind of arguments that you make when you don’t have a theory.” We need a
more complete theory of gravity, he concluded.
“Maybe ‘space-time from entanglement’ is the right place to start,” he wrote. “I am not sure.”
Dr. Bousso, who has been
e-mailing with Dr. Maldacena, is skeptical that the wormholes will
eliminate firewalls. “My own view is that it’s time to move on, accept,
and actually understand firewalls,” he said. After all, he added,
there’s no principle of nonviolence in the universe, except for
Einstein’s equivalence principle, which says the black hole’s horizon is
not a special place. But maybe it is, after all.
Meanwhile, Dr. Bousso said,
the present debate had raised his estimation, “by another few notches,”
of the “stupendous magnitude” of Dr. Hawking’s original discovery of
the information paradox.
The firewall paradox,” he
said, “tells us that the conceptual cost of getting information back out
of a black hole is even more revolutionary than most of us had
believed.”
關於黑洞，愛因斯坦真的錯了嗎？
他們說，這次愛因斯坦(Einstein)恐怕真的錯了。
全球的物理學家之間爆發了一場炙烈的論辯，爭論的焦點是，
如果你跳進黑洞——這頭令人膽寒，能吞噬物質、能量，甚至連光也不放過的引力怪獸——會發生什麼？當然，你不免一死，但怎麼死呢？是像天文學家和科幻作家
們幾十年來告訴我們的那樣，被強大無比的引力壓縮成比塵埃還細小的微粒？還是像一番驚心動魄的新計算似乎指出的那樣，被一道高能火牆瞬間燒死？
-
Alicia DeSantis/The New York Times
關乎黑洞的一項意想不到的悖論將現代科學的兩項基本宗旨相對立：量子力學理論以及愛因斯坦的廣義相對論。
-
Jim Wilson/The New York Times
斯坦福大學的萊昂納德·薩斯金德。
-
Jim Wilson/The New York Times
加州大學伯克利分校的理論物理學家拉菲爾·鮑索。
去年一年中，這場聽起來很可怕的辯論，製造了大量的論文和
博客文章，以及多場研討會。要緊的問題不是愛因斯坦的名望（他的名望已不可動搖），也不是我們手中的iPhone的效力，但或許是愛因斯坦的引力理論——
廣義相對論的基石，我們對宇宙的認識都基於這一理論。或者一些其他的建立以sic久的自然法則也許會被放棄，但具體是哪一個，物理學家尚無一致意見。他們在這個
問題上的觀點反反覆復，幾乎每周都會改變一次，而且目前看來不會很快有定論。
「在這個問題上，我過去就好像溜溜球一般搖擺不定，」該領域著述頗豐的研究者，斯坦福大學(Standford)的萊昂納德·薩斯金德(Leonard Susskind)說。他停頓片刻後又說道：「最近幾個月來我還沒有改變過主意。」
加州大學伯克利分校(University of California, Berkeley)的理論物理學家拉菲爾·鮑索(Raphael Bousso)說：「我從來沒有感到如此震驚過，真不知道該期待什麼結果。」
或許你會想，誰在乎這事？畢竟遭遇黑洞不是你日程上的待辦事項。但所謂的「火牆悖論」關係到的是現代科學的一些基本宗旨，以及愛因斯坦廣義相對論的一些內容。
「它表明我們對引力的認識存在缺陷，」位於加州聖芭芭拉市
的卡弗里理論物理研究所(Kavli Institute for Theoretical
Physics)的理論物理學家約瑟夫·波爾欽斯基(Joseph Polchinski)說，他是引起這場困惑的理論家之一。
就像《愛麗斯漫遊奇境記》中的兔子洞一般，這裡充滿了現代
物理學中光怪陸離的神奇奧妙：黑洞；穿越時空的被稱為「蟲洞」的捷徑；還有量子糾纏(quantum
entanglement)，它又有「鬼魅般的超距作用」這一稱呼，意思是相隔數光年之遠的粒子仍可表現得瞬息相聯。進入這個魔洞可能得到的回報無法預
料，也許會是我們對一個問題的新認識：我們究竟為什麼會覺得自己所在的宇宙有着時間和空間。這種新認識所能帶來的後果同樣無法預料。畢竟，假如一個世紀前
愛因斯坦不曾對牛頓力學宇宙觀中的不自洽之處感到困惑，我們今天也許就不會有能隨身攜帶的全球定位系統(GPS)，這個系統靠他的廣義相對論原理來計時。
自由落體
黑洞是廣義相對論最為極端的理論預言。廣義相對論描述了物
質和能量如何讓時空幾何發生彎曲，就像一個很重的人睡在床上會讓床墊凹下去那樣。物質和能量過度集中在一處，可能導致太空的「凹陷」程度如此之大，以至於
其中的物質會像被魔術師的斗篷隱匿了起來似地消失掉，朝着被稱為「奇點」的無窮大密度點不斷地坍縮。1916年，有人向愛因斯坦指出這種可能時，他認為那
是一個荒謬的想法；但當今的天文學家認同這一觀點：宇宙中四處散落着這種漆黑的怪物，有很多這種黑獸潛藏在大多數星系的中心，它們的質量是太陽的數百萬乃
至數十億倍，許多是死亡恆星塌縮的產物。
廣義相對論來自愛因斯坦後來稱之為自己「最快樂的思想」：正在自由下落的人感覺不到其自身的重量。它被簡單地稱為「等效原理」，意思是沒有物質的空間，在任何地方、從任何觀察角度看上去都是一樣的。
此原理的一條推論是，當一位宇航員跨過名為「事件視界」的
不歸點、進入黑洞時，他自己不會感覺到有任何特殊的事情在發生。就像玩蹦極跳的人一樣，他會有失重感，這種感覺一直持續到他探底的時刻，這段旅程的時間可
以是幾秒鐘也可以是數年，要看黑洞有多大，他也會被潮汐力拉得像根麵條一樣，然後被碾壓成一粒微塵。用行話說來，事件視界之處「沒有戲劇」——至少從物理
學的角度來看，當然不是說知道自己再也不能回家時，所感受到的精神創傷。東西或人掉進去，被碾壓到無窮大密度而消失了，這就是對黑洞的傳統認識。
不過，1974年英國宇宙學家斯蒂芬·霍金
(Stephen
Hawking)讓情況變得更有趣。他的研究結果震撼了世界，他發現，如果把描述亞原子行為、不符合常識的量子物理定律考慮進來的話，那麼黑洞也會泄漏出
粒子和輻射，而且最終會爆發，儘管恆星質量大小的黑洞走完這個過程所需的時間比宇宙目前的年齡還要長久。
把描述致使時空彎曲的引力的廣義相對論，與描述其中微觀奇
異行為的量子理論結合起來，是霍金的一項突破。不過，這裡面也有個大麻煩。霍金的結論是，來自一個黑洞的輻射會是完全隨機的，不包含任何有關落入其中物體
的信息。當黑洞最終爆發時，那些信息將在宇宙中徹底消失。霍金1976年戲仿愛因斯坦對量子理論隨機性的著名質疑說道，「上帝不僅和宇宙玩擲骰子，他有時
候還把骰子擲到看不見的地方。」
粒子物理學家對此表示強烈反對，稱霍金的觀點違背了現代科
學和量子理論的一個基本原則，那就是信息總是被保存下來。比如說，從燒一本書產生的煙與火的成分里，原則上應該能判斷出這本書是基督教《聖經》還是古印度
《愛經》(Kama Sutra)。這些物理學家辯稱，黑洞釋放出的火苗和煙圈按理也應該包含信息。長達30年的論爭因此拉開了帷幕。
2004年，霍金終於承認他原來的觀點錯了，並因此還付了一筆輸了的賭注，這在當時是頭條新聞。
火牆悖論
然而，最近有些物理學家稱，霍金當年恐怕認輸過早了點。 「他提出那個觀點是有好理由的，但他放棄觀點的理由是錯誤的」，波爾欽斯基說，他解釋道，目前還沒人把信息到底會如何從黑洞中出來的問題搞清楚。
而這正是身在聖芭芭拉的四位研究人員給自己下的任務，他們
中的艾哈邁德·艾爾姆赫里(Ahmed Almheiri)、唐納德·馬洛爾夫(Donald Marolf)、和詹姆斯·薩利(James
Sully)來自加州大學聖芭芭拉分校，而波爾欽斯基則來自卡弗里研究所。這個自稱「AMPS」(取自四人姓氏的首字母)的團隊做出了讓自己都吃驚的發
現：從已知的物理定律出發，會得出自相矛盾的「火牆悖論」。
他們的計算表明，如果讓信息從黑洞外流，那麼在黑洞邊界處，原本平滑的愛因斯坦時空（即事件視界）就不可能存在。取而代之的是真空的不連續性，表現為隱藏於黑洞內表的高能粒子——一道「火牆」。
落入黑洞時被燒成灰燼，當然與愛因斯坦「沒有戲劇」的論斷
相矛盾。如果這個結論是對的，那麼你在進入黑洞的蹦極之旅中，早在遠離觸底的地方就沒命了。如果火牆存在，那將會意味着，視界是一個很特殊的地方，而不是
廣義相對論所認為的那個什麼東西都沒有的空間。火牆的存在將動搖愛因斯坦原理的基石，也動搖他的引力理論的基石，因此會動搖建立在廣義相對論之上的現代宇
宙學的基石。用鮑索的話說，這相當於給科學家們提供了一本「地獄菜譜」。如果火牆說正確，那麼處於現代物理學核心地位的三個概念中有一個就必須是錯的。要
麼是信息畢竟可以丟失，要麼是愛因斯坦的等效原理有問題，要麼是描述基本粒子和基本力如何相互作用的量子場論是錯的，需要修正。
量子誓言
火牆論據所依賴的是量子物理中更詭異的一個東西，即被稱為
「糾纏」的作用。正如愛因斯坦、鮑里斯·波多爾斯基(Boris Podolsky)和內森·羅森(Nathan
Rosen)在1935年所指出的，量子理論預言，一對粒子可以有這樣一種關聯方式，不管兩個粒子相距多少個光年，測量其中一個粒子的某種特性，比如自旋，會立刻影響到另一個粒子的測量結果。
愛因斯坦曾用這種「鬼魅般的超距作用」來暗示量子力學的荒
謬，但如今，這種實驗每天都在實驗室中進行。你不能靠它來用比光速還快的速度傳遞信息，因為只有當兩個做實驗者相會，比較各自的結果時，才能看到這種關
聯。但是，糾纏在量子計算和密碼學中起關鍵作用，而且現在看來，糾纏在解釋霍金輻射中隱含的信息如何逃離黑洞上，也很關鍵。
讓我們考慮一下黑洞輻射出的兩個粒子，不妨分別稱它們為鮑伯[Bob]和愛麗絲[Alice]。鮑伯在很久以前就離開了黑洞，也就是在黑洞開始泄露輻射時；量子糾纏理論要求，如果讓黑洞記住自己一直在傳遞什麼信息，遠遠的鮑伯必須與剛離開黑洞的愛麗絲糾纏在一起。
但這種情況會與另一種糾纏競爭，也就是分別位於事件視界（即黑洞邊界）兩邊的粒子之間的糾纏。如果太空像愛因斯坦假設的那樣是平滑的，而且量子場論是正確的，那麼愛麗絲就必須與尚在黑洞內的另一個粒子特德(Ted)糾纏在一起。
但量子理論禁止亂交式的糾纏。把量子信息用通俗的語言來解釋可以這麼說，愛麗絲或者與鮑伯結婚，或者與特德結婚，但不能與兩者都結婚，哪怕與特德的婚姻發生在黑洞裡邊，在我們大部分人都看不到的地方。
波爾欽斯基說，愛麗絲對宇宙必須有一個沒有矛盾的解釋，「就像我們自己必須做的那樣，雖然我們處在宇宙視界中。
與此同時，物理學家有比以往任何時候都多的理由相信，信息
不可能消失。1997年，普林斯頓高等研究院的胡安·M·馬爾達塞納(Juan M.
Maldacena)發表過一篇赫赫有名的論文，把大自然描繪成一種全息圖，其中，比如有關一個三維空間內部發生什麼的信息，是編碼在這個空間的二維界面
上的量子方程中的，就像一個3D圖像被編碼在你的銀行卡上面那樣。
加拿大不列顛哥倫比亞大學(University of
British Columbia)的年輕理論物理學家馬克·范拉姆斯東克(Mark Van
Raamsdonk)喜歡用一個更嚇人的比喻來描述這個概念，即控制類似於《黑客帝國》(Matrix)的視頻遊戲的芯片。（請自己選擇可怕的音樂伴奏。）
霍金發現黑洞爆炸後，其理論最奇怪、也是影響最深遠的後果就是，描述某個空間中發生什麼所需的信息，與把那個空間包含起來的面積成正比，這一發現仍籠罩在神秘的面紗之下。
馬爾達塞納常把宇宙比喻為一個湯罐頭，星系、黑洞、重力、
恆星等，包括我們在內，是裝在罐頭裡的湯，而描述這些物質的信息像是貼在罐頭外面的標籤。你可以把它想像成用罐頭包裝的引力。代表標籤的方程是確定性的，
方程中沒有能讓信息遺失的餘地，這也就意味着，宇宙內部的信息必須被保留。
波爾欽斯基說，這樣一來，火牆就成了阻止愛麗絲與特德非法結婚的唯一辦法，這是一種令人討厭的解決辦法，因為它違反廣義相對論的基本原則。
最近，一個解決火牆難題、以及回答那個縈繞問題的新方法引起了大量的關注，但這個方法沒能讓任命？形成一致的意見。馬爾達塞納和蘇斯金德提出，愛因斯坦可以用在現代物理學中的另一個異想天開的概念實現自救，就是「蟲洞」。
1935年，愛因斯坦和羅森發現，至少在數學上，黑洞可以
成對出現，它們由太空中的捷徑連接起來，這種捷徑當時被稱作愛因斯坦-羅森橋，現在被稱作蟲洞。雖然科幻小說作者和星際探索者有過諸多夢想，但用我們目前
已知的所有方法都無法穿越蟲洞，這就排除了穿越時間旅行和其他違背相對論的可能性。
2010年，不列顛哥倫比亞大學的范拉姆斯東克提出，這種蟲洞是量子糾纏的幾何表現。畢竟，這兩種似乎超越了局部空間的現象，都不能用來直接傳遞信息。麻省理工學院的布賴恩·施溫格(Brian Swingle)此前一年也提出過類似的想法。
其實，這些理論學家說的是，如果沒有糾纏現象，時空就根
本不會有結構。或者正如馬爾達塞納所說，「鬼魅般的超距作用創造了時間和空間。」如果真是這樣，這一深刻見解是朝着理論學家的一個長久的夢想邁出的一步。
他們夢想解釋空間和時間如何由已有的一些更基本的性質所產生，在糾纏理論中，更基本的性質是量子信息。普林斯頓大學的理論物理學家約翰·惠勒(John
Wheeler)是「黑洞」這個詞的發明者，他把這個概念稱為「時空源於比特」。
把蟲洞概念進一步延伸，馬爾達塞納和蘇斯金德提出，在
AMPS理論的黑洞和霍金輻射之間存在一種類似蟲洞安排。這種安排不是那種蜿蜒穿越超時空、在另一端與黑洞內部相連的隧道，而是分裂成千絲萬縷的類似於意
大利麵條的連接，每個連接都連到霍金輻射的一個碎片上。這就意味着，在前文中提到的卡通版理論中，雖然霍金粒子鮑勃也許與事件視界相去數個光年之遠，但他
仍與黑洞的內部相連，就好像是在新澤西州有一扇門，打開它能直接通到曼哈頓的一間地下室似的。
馬爾達塞納解釋說，由於有這個蟲洞連接， 「特德和鮑伯是同一個人。」其結果有點像瘋狂浪漫喜劇，儘管其中充滿了身份誤會，但結局皆大歡喜：英俊的流浪漢原來是王子喬裝的。愛麗絲可以跟特德結婚，因為他其實就是鮑勃，婚姻的紐帶順利地延伸到了黑洞之外。
糾纏理論
馬爾達塞納和蘇斯金德都承認，他們的蟲洞假說並不完善。同
行中幾乎沒人相信這個假說的描述足夠詳細，更不用說用它來解決火牆悖論了。一個火牆研討會下周將在卡弗里研究所(Kavli
Institute)舉行，屆時人們肯定會給蟲洞以及所有的理論挑刺兒。蘇斯金德在研討會開幕前給記者的一封電子郵件中表示：「我只能說，還沒有任何人能
給出一個完全紮實的理論，當然也包括我。任何理論只能靠時間來檢驗。」
波爾欽斯基說，「我目前的看法是，我們現在之所以有這麼多的爭論，是因為還沒有一個理論。」他總結稱，我們需要有一個更完整的引力理論。
他寫道，「也許"時空來自糾纏"是一個正確的起點，不過，我不能肯定。」
鮑索一直在用電子信跟馬爾達塞納交流，他對蟲洞能消除火牆
的提法有懷疑。他說，「我本人的看法是，結束爭論的時候到了，我們應該接受火牆的概念，從而開始真正了解火牆。」他補充說，畢竟宇宙中沒有非暴力原則，除
了愛因斯坦的等效原理之外，該原理說黑洞的視界不是一個特殊的地方。但也許它是一個特殊的地方。
不過，鮑索說，目前的爭論讓他對霍金最初發現的信息悖論的「重大意義」的估計又提高了「幾個級別」。
他說，「火牆悖論告訴我們，從一個黑洞里取回信息的概念成本，比我們大多數人認為的更為驚人。」
本文最初發表於2013年8月12日。
翻譯：馬驄、陳亦亭、土土
(Credit: Washington University) 
![[image]](https://lh3.googleusercontent.com/blogger_img_proxy/AEn0k_unqHjSxwexgYFSewsVnmVy6_r4Cavz_v1QeCe1mdt2LGz6L-QJiEOT1YitdL_IrxSgYK9moyJYZ1XcOK1qU7ooZYH9qUiPgjfRKc1kjZv36MoWVztcuiAAXJn_iJFrAHFKCtkdk31M8637OtMYoiGq2AA=s0-d)
James Steinberg
