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Gian Giudice 談為何我們的宇宙可能處於刀鋒

Gian Giudice: Why our universe might exist on a knife-edge

 

Photo of three lions hunting on the Serengeti.

講者:Gian Giudice

2013年5月演講,2013年10月在TEDxCERN上線

 

翻譯:洪曉慧

編輯:朱學恒

簡繁轉換:洪曉慧

後制:洪曉慧

字幕影片後制:謝旻均

 

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關於這場演講

發現希格斯玻色子最大的驚喜是什麼?就是沒有驚喜。Gian Giudice闡述一個理論物理學問題:如果希格斯場以超密度狀態存在意味著所有原子物質的崩潰,會發生什麼情形?藉由充滿機智、引人入勝的演講,Giudice描繪出一個嚴酷的命運-以及為什麼我們目前還不需要擔心。(攝於TEDxCERN)

 

關於Gian Giudice

Gian Giudice是理論物理學家,對粒子物理學及宇宙學新知的探索貢獻良多。

 

為什麼要聽他演講

Gian Giudice是CERN(歐洲核子研究組織)理論物理組永久成員。他的研究主題包括超對稱、額外維度及暗物質,曾提出描述宇宙最初階段的理論,將目前對粒子世界的知識應用在較小的距離。

 

Giudice為暢銷書《A Zeptospace Odyssey》作者,致力於將大型強子對撞機的複雜作用傳遞給大眾。為了讓一般讀者瞭解,這本書闡述了建造LHC(大型強子對撞機)的創新需求,以及需藉此證明的理論。藉由希格斯玻色子的發現,Giudice發現一個令人驚訝的結果:如果標準模型繼續維持在非常短的距離,宇宙必定處於臨界狀態,接近可能導致所有原子物質崩潰的相變。幸運的是,在TED演講中,他說明這將發生於十分遙遠的未來。

 

Gian Giudice的英語網上資料

Book: A Zeptospace Odyssey

 

[TED科技‧娛樂‧設計]

已有中譯字幕的TED影片目錄(繁體)(簡體)。請注意繁簡目錄是不一樣的。

 

Gian Giudice 談為何我們的宇宙可能處於刀鋒

 

去年七月四日,藉由大型強子對撞機實驗,我們發現了希格斯玻色子(又稱上帝粒子);這是歷史性的一天。毫無疑問地,從此之後,人們記憶中的七月四日不再是發表《獨立宣言》的日子,而是發現希格斯玻色子的日子。好吧,至少對CERN(歐洲核子研究組織)來說。

 

但對我來說,那天最大的驚喜就是沒有驚喜。在理論物理學家看來,希格斯玻色子能巧妙解釋某些基本粒子如何獲得質量,但似乎並非令人滿意且完整的答案,許多問題尚未得到解答。希格斯玻色子不具備和其他基本粒子相同的美感、對稱性和優雅,因此大多數理論物理學家認為希格斯玻色子無法提供完整解釋。我們期待伴隨希格斯玻色子出現的新粒子和新現象,然而目前為止,LHC(大型強子對撞機)測量結果並未顯示新粒子或新現象存在的跡象。

 

當然,這並非蓋棺論定的結論。到了2015年,LHC將使進行碰撞的質子能量翻倍,這種更強烈的碰撞將使我們能進一步探索粒子世界,我們肯定能得到更多資訊。

 

但目前,既然我們尚未發現新現象存在的證據,我們不妨假設目前已知的粒子,包括希格斯玻色子,是自然界中僅有的基本粒子,甚至擁有比我們目前所知高出許多的能量,我們看看這個假設會讓我們發現什麼。我們會發現宇宙令人驚訝和有趣之處,為了解釋我的觀點,首先我得告訴各位何謂希格斯粒子。為了說明,我們必須回到宇宙大爆炸後百億分之一秒。根據希格斯理論,那一刻宇宙中發生了一件驚天動地的大事-時空經歷相變過程,十分類似水在攝氏零度以下結冰的相變過程,但這個例子中的相變並非物質內部分子排列的變化,而是時空構造發生改變。

 

在相變過程中,空間被某種物質填充,我們稱之為希格斯場。或許我們無法看見這種物質,但它具有物理現實性,它時時環繞在我們四周,就像我們在這個房間裡呼吸的空氣。某些基本粒子與這種物質發生交互作用,於過程中獲得能量,這個固有能量就是所謂的粒子質量。藉由發現希格斯玻色子,LHC最後證明這種物質確實存在,因為希格斯玻色子是這種物質的組成要素。簡言之,這就是希格斯理論的重點。

 

但故事內容比這有趣得多。藉由研究希格斯理論,理論物理學家發現-並非藉由實驗,而是藉由數學的力量-希格斯場並非只以我們目前觀測到的形式存在,正如物質可以液態或固態存在。因此希格斯場,即填充整個時空的物質,亦可以兩種狀態存在。除了已知的希格斯場,尚存在第二種狀態的希格斯場,其密度比我們目前觀察到的高數十億倍。另一種希格斯場狀態的存在衍生了一個潛在問題:因為根據量子力學,兩種狀態的轉換是可行的,即使兩種狀態間存在能量障壁,這種現象恰如其分地被稱為量子隧道。由於量子隧道的存在,我可以從這個房間消失,出現在隔壁房間,也就是穿牆而過。但別指望我真的能在各位面前表演這個把戲,因為我穿牆而過的可能性微乎其微,在我成功之前,各位得等待很長一段時間。但相信我,量子隧道是真實現象,已在許多系統中觀測到,例如穿隧二極體,一種使用於電子產品的元件,因量子隧道奇蹟而得以運作。

 

我們回到希格斯場。如果超密度希格斯狀態確實存在,那麼由於量子隧道效應,此狀態中的氣泡可在某個時間於宇宙中某個地點突然出現。這類似燒開水時,水的內部產生氣泡,然後膨脹,從液體變成氣體。同樣地,超密度希格斯狀態的氣泡會因量子隧道而出現,這種氣泡將以光速膨脹,佔據整個空間,將希格斯場從原本狀態變成新狀態。

 

這有什麼問題嗎?是的,這是個大問題。在日常生活中,我們或許不會意識到,但希格斯場強度對物質結構來說十分重要。只要希格斯場強度高上幾倍,我們將看見原子開始萎縮,原子核內的中子開始衰變,原子核開始分裂,氫將成為宇宙中唯一可能存在的化學元素。而希格斯場,超密度狀態的希格斯場,其密度不僅比目前所知的大上數倍,而是大上數十億倍。如果時空被這種希格斯狀態填充,所有原子物質都將崩潰,任何分子結構都不可能存在,亦不存在生命。

 

因此我質疑,未來希格斯場是否可能經歷相變過程,並且-藉由量子隧道,轉變成這種糟糕的超密度狀態?換句話說,我問自己,我們宇宙中的希格斯場命運將會如何?回答這個問題所需的關鍵要素是希格斯玻色子的質量。LHC實驗中發現,希格斯玻色子質量約126 GeV,若以常用單位表示,這是很小的數值,因為這相當於10^–22克。但以粒子物理單位來說則相當大,因為這相當於整個DNA分子結構的重量。

 

因此藉由來自LHC的資訊,我和幾位CERN的同事合作,計算我們的宇宙藉由量子隧道轉變成超密度希格斯狀態的可能性,我們發現一個十分有趣的結果。我們的計算顯示希格斯玻色子質量的測量值十分特殊,正好相當於使宇宙在不穩定狀態中維持平衡的數值。希格斯場存在於持續至今的不穩定狀態中,但它終將瓦解。因此根據這些計算,我們就像露營者無意中將帳篷搭在懸崖邊,最終希格斯場將經歷相變,物質將自行崩解。

 

因此這就是人類滅絕的方式嗎?我不認為如此。我們的計算顯示,希格斯場的量子隧道效應不太可能發生在未來10^100年間。這是相當長的時間,甚至比義大利組織一屆穩定的政府所花的時間還長。

 

(笑聲)

 

儘管如此,到時我們早已不復存在。大約50億年後,太陽將變成紅巨星,膨脹程度遠至地球軌道,地球將徹底毀滅。一兆年後,如果暗能量持續獲得補給,空間膨脹維持目前速率,你甚至無法看見自己的腳趾頭,因為周遭的一切都以超光速膨脹,因此我們實在不太可能親眼目睹希格斯場崩潰。

 

但我對希格斯場之轉變感興趣的原因在於我想提出這個問題:為何希格斯玻色子的質量如此特殊?為何它正好使宇宙處於相變臨界點?理論物理學家經常問「為什麼」問題,勝於詢問某種現象如何發生。理論物理學家總是對某種現象為何如此呈現感興趣,我們認為這些「為什麼」問題可給予我們關於自然界基本原則的線索。事實上我的問題的可能答案開闢了新宇宙-如字面上的意思。根據推測,我們的宇宙僅是無數泡泡組成的多重宇宙泡泡中的一個泡泡,每個泡泡代表不同的宇宙,蘊含不同的基本常數和物理定律。在這種脈絡下,你可討論尋找希格斯粒子確定質量的可能性。解開這個奧秘的關鍵或許在於多重宇宙的統計特性,如同海灘上沙丘的情形。一般來說,你預期可在海灘上找到任何坡度的沙丘,然而沙丘的坡度通常介於30至35度左右,原因很簡單:因為風使沙粒堆積,重力使其滑落,結果是大部分沙丘形成接近坍塌臨界值的坡度,多重宇宙中希格斯玻色子質量的情況雷同。在大部分多重宇宙泡泡中,希格斯粒子的質量接近希格斯場導致宇宙崩塌的臨界值,因為存在兩項競爭因素,如同沙丘的例子。

 

我的故事沒有結局,因為我們仍不知道故事的結局。這是發展中的科學,為了解開這個奧秘,我們需要更多資料,衷心希望LHC很快就能為這個故事加入新線索。僅僅一個數字,希格斯玻色子的質量,我們就能從中得到許多資訊。我從一個假設開始:遍布宇宙的所有已知粒子甚至存在於我們尚未探索的領域。藉此,我們發現瀰漫在時空中的希格斯場或許正站在刀鋒上,隨時可能導致宇宙坍塌。我們發現這或許是一個提示:我們的宇宙只是多重宇宙這片廣大沙灘上的一顆砂粒。

 

但我不知道我的假設是否正確。這就是物理運作的方式:一個簡單的測量結果,可使我們邁向對宇宙產生全新認知的康莊大道,或導致我們走進死胡同。但無論結果如何,我可以肯定一件事:這段旅程將充滿驚喜。

 

謝謝。

 

(掌聲)

 

以下為系統擷取之英文原文

About this talk

The biggest surprise of discovering the Higgs boson? That there were no surprises. Gian Giudice talks us through a problem in theoretical physics: what if the Higgs field exists in an ultra-dense state that could mean the collapse of all atomic matter? With wit and charm, Giudice outlines a grim fate -- and why we shouldn't start worrying just yet.
 
About Gian Giudice
Gian Giudice is a theoretical physicist who has contributed greatly to our present understanding of particle physics and cosmology.
 
About the transcript
So last year, on the Fourth of July, experiments at the Large Hadron Collider discovered the Higgs boson. It was a historical day. There's no doubt that from now on, the Fourth of July will be remembered not as the day of the Declaration of Independence, but as the day of the discovery of the Higgs boson. Well, at least, here at CERN.
 
But for me, the biggest surprise of that day was that there was no big surprise. In the eye of a theoretical physicist, the Higgs boson is a clever explanation of how some elementary particles gain mass, but it seems a fairly unsatisfactory and incomplete solution. Too many questions are left unanswered. The Higgs boson does not share the beauty, the symmetry, the elegance, of the rest of the elementary particle world. For this reason, the majority of theoretical physicists believe that the Higgs boson could not be the full story. We were expecting new particles and new phenomena accompanying the Higgs boson. Instead, so far, the measurements coming from the LHC show no signs of new particles or unexpected phenomena.
 
Of course, the verdict is not definitive. In 2015, the LHC will almost double the energy of the colliding protons, and these more powerful collisions will allow us to explore further the particle world, and we will certainly learn much more.
 
But for the moment, since we have found no evidence for new phenomena, let us suppose that the particles that we know today, including the Higgs boson, are the only elementary particles in nature, even at energies much larger than what we have explored so far. Let's see where this hypothesis is going to lead us. We will find a surprising and intriguing result about our universe, and to explain my point, let me first tell you what the Higgs is about, and to do so, we have to go back to one tenth of a billionth of a second after the Big Bang. And according to the Higgs theory, at that instant, a dramatic event took place in the universe. Space-time underwent a phase transition. It was something very similar to the phase transition that occurs when water turns into ice below zero degrees. But in our case, the phase transition is not a change in the way the molecules are arranged inside the material, but is about a change of the very fabric of space-time.
 
During this phase transition, empty space became filled with a substance that we now call Higgs field. And this substance may seem invisible to us, but it has a physical reality. It surrounds us all the time, just like the air we breathe in this room. And some elementary particles interact with this substance, gaining energy in the process. And this intrinsic energy is what we call the mass of a particle, and by discovering the Higgs boson, the LHC has conclusively proved that this substance is real, because it is the stuff the Higgs bosons are made of. And this, in a nutshell, is the essence of the Higgs story.
 
But this story is far more interesting than that. By studying the Higgs theory, theoretical physicists discovered, not through an experiment but with the power of mathematics, that the Higgs field does not necessarily exist only in the form that we observe today. Just like matter can exist as liquid or solid, so the Higgs field, the substance that fills all space-time, could exist in two states. Besides the known Higgs state, there could be a second state in which the Higgs field is billions and billions times denser than what we observe today, and the mere existence of another state of the Higgs field poses a potential problem. This is because, according to the laws of quantum mechanics, it is possible to have transitions between two states, even in the presence of an energy barrier separating the two states, and the phenomenon is called, quite appropriately, quantum tunneling. Because of quantum tunneling, I could disappear from this room and reappear in the next room, practically penetrating the wall. But don't expect me to actually perform the trick in front of your eyes, because the probability for me to penetrate the wall is ridiculously small. You would have to wait a really long time before it happens, but believe me, quantum tunneling is a real phenomenon, and it has been observed in many systems. For instance, the tunnel diode, a component used in electronics, works thanks to the wonders of quantum tunneling.
 
But let's go back to the Higgs field. If the ultra-dense Higgs state existed, then, because of quantum tunneling, a bubble of this state could suddenly appear in a certain place of the universe at a certain time, and it is analogous to what happens when you boil water. Bubbles of vapor form inside the water, then they expand, turning liquid into gas. In the same way, a bubble of the ultra-dense Higgs state could come into existence because of quantum tunneling. The bubble would then expand at the speed of light, invading all space, and turning the Higgs field from the familiar state into a new state.
 
Is this a problem? Yes, it's a big a problem. We may not realize it in ordinary life, but the intensity of the Higgs field is critical for the structure of matter. If the Higgs field were only a few times more intense, we would see atoms shrinking, neutrons decaying inside atomic nuclei, nuclei disintegrating, and hydrogen would be the only possible chemical element in the universe. And the Higgs field, in the ultra-dense Higgs state, is not just a few times more intense than today, but billions of times, and if space-time were filled by this Higgs state, all atomic matter would collapse. No molecular structures would be possible, no life.
 
So, I wonder, is it possible that in the future, the Higgs field will undergo a phase transition and, through quantum tunneling, will be transformed into this nasty, ultra-dense state? In other words, I ask myself, what is the fate of the Higgs field in our universe? And the crucial ingredient necessary to answer this question is the Higgs boson mass. And experiments at the LHC found that the mass of the Higgs boson is about 126 GeV. This is tiny when expressed in familiar units, because it's equal to something like 10 to the minus 22 grams, but it is large in particle physics units, because it is equal to the weight of an entire molecule of a DNA constituent.
 
So armed with this information from the LHC, together with some colleagues here at CERN, we computed the probability that our universe could quantum tunnel into the ultra-dense Higgs state, and we found a very intriguing result. Our calculations showed that the measured value of the Higgs boson mass is very special. It has just the right value to keep the universe hanging in an unstable situation. The Higgs field is in a wobbly configuration that has lasted so far but that will eventually collapse. So according to these calculations, we are like campers who accidentally set their tent at the edge of a cliff. And eventually, the Higgs field will undergo a phase transition and matter will collapse into itself.
 
So is this how humanity is going to disappear? I don't think so. Our calculation shows that quantum tunneling of the Higgs field is not likely to occur in the next 10 to the 100 years, and this is a very long time. It's even longer than the time it takes for Italy to form a stable government.
 
(Laughter)
 
Even so, we will be long gone by then. In about five billion years, our sun will become a red giant, as large as the Earth's orbit, and our Earth will be kaput, and in a thousand billion years, if dark energy keeps on fueling space expansion at the present rate, you will not even be able to see as far as your toes, because everything around you expands at a rate faster than the speed of light. So it is really unlikely that we will be around to see the Higgs field collapse.
 
But the reason why I am interested in the transition of the Higgs field is because I want to address the question, why is the Higgs boson mass so special? Why is it just right to keep the universe at the edge of a phase transition? Theoretical physicists always ask "why" questions. More than how a phenomenon works, theoretical physicists are always interested in why a phenomenon works in the way it works. We think that this these "why" questions can give us clues about the fundamental principles of nature. And indeed, a possible answer to my question opens up new universes, literally. It has been speculated that our universe is only a bubble in a soapy multiverse made out of a multitude of bubbles, and each bubble is a different universe with different fundamental constants and different physical laws. And in this context, you can only talk about the probability of finding a certain value of the Higgs mass. Then the key to the mystery could lie in the statistical properties of the multiverse. It would be something like what happens with sand dunes on a beach. In principle, you could imagine to find sand dunes of any slope angle in a beach, and yet, the slope angles of sand dunes are typically around 30, 35 degrees. And the reason is simple: because wind builds up the sand, gravity makes it fall. As a result, the vast majority of sand dunes have slope angles around the critical value, near to collapse. And something similar could happen for the Higgs boson mass in the multiverse. In the majority of bubble universes, the Higgs mass could be around the critical value, near to a cosmic collapse of the Higgs field, because of two competing effects, just as in the case of sand.
 
My story does not have an end, because we still don't know the end of the story. This is science in progress, and to solve the mystery, we need more data, and hopefully, the LHC will soon add new clues to this story. Just one number, the Higgs boson mass, and yet, out of this number we learn so much. I started from a hypothesis, that the known particles are all there is in the universe, even beyond the domain explored so far. From this, we discovered that the Higgs field that permeates space-time may be standing on a knife edge, ready for cosmic collapse, and we discovered that this may be a hint that our universe is only a grain of sand in a giant beach, the multiverse.
 
But I don't know if my hypothesis is right. That's how physics works: A single measurement can put us on the road to a new understanding of the universe or it can send us down a blind alley. But whichever it turns out to be, there is one thing I'm sure of: The journey will be full of surprises.
 
Thank you.
 
(Applause)

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