天文節目導讀之黑洞訊息的悖論
科學最大的特色是允許有錯，有時候因為錯得精采，激發一班熱切的科學家不眠不休地尋找答案，使科學發展向前邁向一大步。
理論天文組於6月3日(星期三)所研習題目是「黑洞訊息悖論」 ─ 一個爭論了30多年的悖論，史蒂芬霍金到最後不得不向世人承認他自己錯了。小組將透過大家一起觀看其中的科普節目霍金悖論 (Hawking’s Paradox in a black hole)，然後就其中的內容作較深入一點討論。
你帶著問題來，但未必能帶著答案走。小組的目的就是通過一些問題激發大家的參與和討論，看看你能否從中找到你想要的答案。有興趣的朋友，歡迎參加。
題 目： 黑洞訊息悖論
主 持： 鍾偉強、宋維芳
日 期： 2009年6月3日 (星期三)
時 間： 7:00 p.m. – 8:30 p.m.
地 點： 香港天文學會辦事處 (九龍牛頭角創業街36號華基中心610 室 )
費 用： 港幣十元 (作學會辦事處日常運作開支，如水電費等。)
參加辦法： 請以電郵 phononm1@yahoo. com 或 hkas_tag@yahoogroup s.com
理論天文組是香港天文學會轄下的學術小組，自1989年成立至今，無論是學會之會員或非會員都歡迎加入。小組約每兩星期聚會一次，討論的題目廣泛，主要利用一些與天體物理有關的參考書和論文作為研習題目，如恆星演化、廣義相對論、黑洞、優雅的宇宙等。為了讓組員對所討論之題目更深入了解，本組鼓勵組員自行分配題目並輪流負責介紹所定下的討論題目。
注：
由於本組的聚會有可能因天雨關係或講者臨時有要事未能出席而要更改聚會日期，建議有興趣的朋友先登記入理論天文組的電郵組 (http://tech. groups.yahoo. com/group/ hkas_tag)，以取最新的活動消息。
香港天文學會理論天文組天文節目導讀之黑洞訊息的悖論
參考資料: 一段舊新聞
2004年7月21日,英國著名物理學家史蒂芬·霍金在都柏林舉行的“第17界國際廣義相對論和萬有引力大會”上,向學術界宣佈了他對黑洞研究的最新成果。他認爲,黑洞不會將進入其邊界的物體的資訊淹沒,反而會將這些資訊“撕碎”後釋放出去。面對世界各國著名的物理學家,霍金說,“30年來我一直在思考這個問題,現在我有了答案。”
7年前的一場賭博
該假說的提出,說明霍金推翻了他曾提出的“黑洞悖論”。同時也意味著,霍金輸掉了與約翰·普萊斯基爾———美國帕薩迪納加利福尼亞技術研究所的一名理論物理學家———進行的一場曠日持久的較量。
霍金曾經認爲,任何被黑洞所吞噬的物質都將永遠與外部的宇宙完全隔絕。而普萊斯基爾則相信,當一個天體最終變成一顆崩塌的恒星時,它所攜帶的資訊並不會完全被毀滅,甚至還能夠重新恢復。1997年,兩人曾就此打賭,現在霍金提出修正理論,顯然也承認自己輸了這場賭局。
普萊斯基爾表示,“斯蒂芬終於改變了他的立場,我料想他將在這場‘打賭’中輸掉”,他很高興他打賭贏了,雖然“老實說,我聽不懂這些理論。”
而普萊斯基爾的戰利品則是一部百科全書———重達7公斤價值150美元的《終極棒球百科全書》。因爲在英國找不到這本書,霍金爲此特地從美國訂購了送給普萊斯基爾。出版社的老闆透露說霍金原本是要支付郵費的,但是出版社死活不要,因爲大名鼎鼎的霍金在出版社訂購了書,等於是爲他們做了一次顯著的活廣告。
新理論讓科幻迷失望
霍金在1976年提出“黑洞蒸發”理論,指出只要黑洞開始蒸發,所有黑洞內部的狀態和結構的資訊就會被完全破壞,消失殆盡,但是這種理論和量子物理相矛盾,因爲從量子物理學上來看,這些物質必定會以某種形式存在,因此當時霍金的說法也被稱爲“黑洞悖論”。
黑洞究竟會不會將所有東西都給吞噬掉?霍金現在認爲,被黑洞吸入的物質,在經過數十億年或百億年後,當黑洞瓦解時,會逐漸釋出物質或者能量訊息。他說,“古典理論中,黑洞是永遠持續的,物質訊息被認爲保留在其內,難以釋出,不過現在情況改觀,我發現量子效應會讓黑洞以穩定速率輻射(這些訊息)。”
過去霍金爲了解釋其黑洞理論與量子物理學的矛盾,認爲被黑洞吸引的物質是進入另一個宇宙系統中,如科幻小說描述的情節。21日他發表新理論時表示,“並沒有如我過去所想的,有另一個新宇宙,物質訊息一直存在我們這個宇宙間”,“抱歉讓科幻迷失望了,但是如果物質訊息被保存著,那麽就不可能經由黑洞到其他宇宙了。”他說,“很高興解決了這個困擾我30年的問題,雖然這個答案並不比我原本認爲的還令人興奮。”
7年前的一場賭博
該假說的提出,說明霍金推翻了他曾提出的“黑洞悖論”。同時也意味著,霍金輸掉了與約翰·普萊斯基爾———美國帕薩迪納加利福尼亞技術研究所的一名理論物理學家———進行的一場曠日持久的較量。
霍金曾經認爲,任何被黑洞所吞噬的物質都將永遠與外部的宇宙完全隔絕。而普萊斯基爾則相信,當一個天體最終變成一顆崩塌的恒星時,它所攜帶的資訊並不會完全被毀滅,甚至還能夠重新恢復。1997年,兩人曾就此打賭,現在霍金提出修正理論,顯然也承認自己輸了這場賭局。
普萊斯基爾表示,“斯蒂芬終於改變了他的立場,我料想他將在這場‘打賭’中輸掉”,他很高興他打賭贏了,雖然“老實說,我聽不懂這些理論。”
而普萊斯基爾的戰利品則是一部百科全書———重達7公斤價值150美元的《終極棒球百科全書》。因爲在英國找不到這本書,霍金爲此特地從美國訂購了送給普萊斯基爾。出版社的老闆透露說霍金原本是要支付郵費的,但是出版社死活不要,因爲大名鼎鼎的霍金在出版社訂購了書,等於是爲他們做了一次顯著的活廣告。
新理論讓科幻迷失望
霍金在1976年提出“黑洞蒸發”理論,指出只要黑洞開始蒸發,所有黑洞內部的狀態和結構的資訊就會被完全破壞,消失殆盡,但是這種理論和量子物理相矛盾,因爲從量子物理學上來看,這些物質必定會以某種形式存在,因此當時霍金的說法也被稱爲“黑洞悖論”。
黑洞究竟會不會將所有東西都給吞噬掉?霍金現在認爲,被黑洞吸入的物質,在經過數十億年或百億年後,當黑洞瓦解時,會逐漸釋出物質或者能量訊息。他說,“古典理論中,黑洞是永遠持續的,物質訊息被認爲保留在其內,難以釋出,不過現在情況改觀,我發現量子效應會讓黑洞以穩定速率輻射(這些訊息)。”
過去霍金爲了解釋其黑洞理論與量子物理學的矛盾,認爲被黑洞吸引的物質是進入另一個宇宙系統中,如科幻小說描述的情節。21日他發表新理論時表示,“並沒有如我過去所想的,有另一個新宇宙,物質訊息一直存在我們這個宇宙間”,“抱歉讓科幻迷失望了,但是如果物質訊息被保存著,那麽就不可能經由黑洞到其他宇宙了。”他說,“很高興解決了這個困擾我30年的問題,雖然這個答案並不比我原本認爲的還令人興奮。”
參考資料: 引自Wikipedia
Black hole information paradox
From Wikipedia, the free encyclopedia
The black hole information paradox results from the combination of quantum mechanics and general relativity. It suggests that physical information could "disappear" in a black hole, allowing many physical states to evolve into precisely the same state. This is a contentious subject since it violates a commonly assumed tenet of science—that in principle complete information about a physical system at one point in time should determine its state at any other time.
In 1975, Stephen William Hawking and Jacob Bekenstein showed that black holes should slowly radiate away energy, which poses a problem. From the no hair theorem, one would expect the Hawking radiation to be completely independent of the material entering the black hole. Nevertheless, if the material entering the black hole were a pure quantum state, the transformation of that state into the mixed state of Hawking radiation would destroy information about the original quantum state. This violates Liouville's theorem and presents a physical paradox.
More precisely, if there is an entangled pure state, and one part of the entangled system is thrown into the black hole while keeping the other part outside, the result is a mixed state after the partial trace is taken over the interior of the black hole. But since everything within the interior of the black hole will hit the singularity within a fixed time, the part which is traced over partially might "disappear", never to appear again. Of course, it is not really known what goes on at singularities once quantum effects are taken into account, which is why this theory is conjectural and controversial.
Hawking was convinced, however, because of the simple elegance of the resulting equation which "unified" thermodynamics, relativity, gravity, and Hawking's own work on the Big Bang. This annoyed many physicists, notably John Preskill, who in 1997 bet Hawking and Kip Thorne that information was not lost in black holes.
There are various ideas about how the paradox is solved. Since the 1997 proposal of the AdS/CFT correspondence, the predominant belief among physicists is that information is preserved and that Hawking radiation is not precisely thermal but receives quantum corrections. Other possibilities include the information being contained in a Planckian remnant left over at the end of Hawking radiation or a modification of the laws of quantum mechanics to allow for non-unitary time evolution.
In July 2005, Stephen Hawking published a paper and announced a theory that quantum perturbations of the event horizon could allow information to escape from a black hole, which would resolve the information paradox. His argument assumes the unitarity of the AdS/CFT correspondence which implies that an AdS black hole that is dual to a thermal conformal field theory, is unitary. When announcing his result, Hawking also conceded the 1997 bet, paying Preskill with a baseball encyclopedia (ISBN 1-894963-27-X) 'from which information can be retrieved at will'. However, Thorne remains unconvinced of Hawking's proof and declined to contribute to the award (see Thorne-Hawking-Preskill bet).
On October 28, 2006, The Discovery Channel aired a show called "The Hawking Paradox”. The show explained Hawking's conclusion that one must look at the multiverse as a whole, and that information lost in black holes is saved in parallel universes where no black holes exist.
Black hole information paradox
From Wikipedia, the free encyclopedia
The black hole information paradox results from the combination of quantum mechanics and general relativity. It suggests that physical information could "disappear" in a black hole, allowing many physical states to evolve into precisely the same state. This is a contentious subject since it violates a commonly assumed tenet of science—that in principle complete information about a physical system at one point in time should determine its state at any other time.
In 1975, Stephen William Hawking and Jacob Bekenstein showed that black holes should slowly radiate away energy, which poses a problem. From the no hair theorem, one would expect the Hawking radiation to be completely independent of the material entering the black hole. Nevertheless, if the material entering the black hole were a pure quantum state, the transformation of that state into the mixed state of Hawking radiation would destroy information about the original quantum state. This violates Liouville's theorem and presents a physical paradox.
More precisely, if there is an entangled pure state, and one part of the entangled system is thrown into the black hole while keeping the other part outside, the result is a mixed state after the partial trace is taken over the interior of the black hole. But since everything within the interior of the black hole will hit the singularity within a fixed time, the part which is traced over partially might "disappear", never to appear again. Of course, it is not really known what goes on at singularities once quantum effects are taken into account, which is why this theory is conjectural and controversial.
Hawking was convinced, however, because of the simple elegance of the resulting equation which "unified" thermodynamics, relativity, gravity, and Hawking's own work on the Big Bang. This annoyed many physicists, notably John Preskill, who in 1997 bet Hawking and Kip Thorne that information was not lost in black holes.
There are various ideas about how the paradox is solved. Since the 1997 proposal of the AdS/CFT correspondence, the predominant belief among physicists is that information is preserved and that Hawking radiation is not precisely thermal but receives quantum corrections. Other possibilities include the information being contained in a Planckian remnant left over at the end of Hawking radiation or a modification of the laws of quantum mechanics to allow for non-unitary time evolution.
In July 2005, Stephen Hawking published a paper and announced a theory that quantum perturbations of the event horizon could allow information to escape from a black hole, which would resolve the information paradox. His argument assumes the unitarity of the AdS/CFT correspondence which implies that an AdS black hole that is dual to a thermal conformal field theory, is unitary. When announcing his result, Hawking also conceded the 1997 bet, paying Preskill with a baseball encyclopedia (ISBN 1-894963-27-X) 'from which information can be retrieved at will'. However, Thorne remains unconvinced of Hawking's proof and declined to contribute to the award (see Thorne-Hawking-Preskill bet).
On October 28, 2006, The Discovery Channel aired a show called "The Hawking Paradox”. The show explained Hawking's conclusion that one must look at the multiverse as a whole, and that information lost in black holes is saved in parallel universes where no black holes exist.
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