Why are we so concerned about quantum mechanics, as it relates to black holes? Well, the existence of Hawking radiation presents an interesting problem to black hole physicists.
우리가 양자역학에 대해 그렇게 주의를 기울이는 이유는 블랙홀과 연관이 있어서 겠지요? 호킹 복사가 실제로 있다면 블랙홀 물리학자들에게는 흥미로운 문제를 일으킬 것이다.
Classical black holes can be characterized by three numbers; their mass, angular momentum, and charge. This is a concept called the no-hair theorem which is a way of saying that information, which is what we mean by hair in this instance, can't escape from a black hole.
블랙홀의 고전적인 특징은 질량, 각운동량 그리고 전하 였다. 이 세가지를 제외하곤 블랙홀은 아무런 특징을 가지지 못한다는 무모의 정리(no-hair theorem)에 따르면 어떤 정보도 밖으로 새어 나오지 못한다.
But quantum mechanics tells a completely different story, which has led to one of the biggest unsolved problems of our time, the black hole information paradox.
하지만 양자역학은 전혀 다른 이야기를 내놓고 있다. 이시대의 가장 큰 풀지못한 문제로 블랙홀 정보 모순이다.
In addition to quantum mechanics enabling processes like Hawking radiation, it also throws a bit of a wrench into theories that try to combine quantum mechanics with general relativity.
양자역학은 호킹복사 같은 현상을 설명하는데 사용될 뿐만 아니라 일반 상대론과 엮어 새 이론의 가능성을 제공한다.
In particular, quantum mechanics requires information to be preserved. So in order for astrophysicists to have a complete description of a black hole, they will have to find a way to explain the so-called black hole information paradox.
특히 양자역학은 정보 보존의 원칙을 기반으로 한다. [불확정성 원리에 따르면 위치변화와 속도변화, 혹은 에너지 변화량과 시간 변화량은 각각 결정될 수 없으나 두 정보의 곱은 상수다.] 물리학자들이 블랙홀을 정확히 기술하기 위해선 블랙홀 정보 모순을 설명할 방법을 찾아내야 한다.
Information can be defined as something which is an answer to a question. When physicists talk about information, they're generally asking questions about the characteristics and state of something. So when we ask about the mass of a star, or we measure what its spectral output is, we're generating information.
'정보'란 어떤 의문점에 대해 답을 주는 단서라고 할 수 있다. 물리학자들이 정보를 언급하는 때는 보통 대상의 특징이나 상태에 대한 의문을 제기하는 경우다. 별의 질량을 알아보는 경우나 방출 스펙트럼을 알고 싶어 조사하면서 정보를 생산한다.
Now imagine that you're the commander of a scientific research vessel, that's in orbit around a black hole, collecting measurements about the properties of the black hole. You can very easily deduce the mass of the black hole by observing, measuring, and timing how long it takes to orbit the black hole at a given distance.
과학 탐사선을 타고 블랙홀의 특성을 측정하기 위해 인근 궤도를 돌고 있다고 해보자. 블랙홀의 질량을 알아내고 측정하는 일은 궤도 반경과 공전시간을 측정하면 되므로 쉽다. [뉴튼 만유인력으로도 충분하다.]
You can also measure the spin of the black hole by comparing the size of the innermost stable circular orbit (ISCO) to the size of the event horizon.
또한 사건 지평선의 크기를 알려면 ISCO의 크기를 비교해 보면 된다.
And by dropping a couple of test charges and observing their behaviour, you can tell the charge of the black hole. Scientists think that most black holes have zero charge anyway.
시험 전하를 떨어트려서 어떻게 움직이는지 관찰하며 블랙홀의 전하를 알수 있다. 대부분 과학자들은 블랙홀이 전하를 띄지 않다고 여기는 중이기는 하다.
So as the commander, you've collected all this data, but oh! no you've accidentally crossed the event horizon. Luckily, you and your crew have survived but the future is bleak.
탐사대장으로 이런 정보를 모두 수집해오고 있던중에 예기치못하게 사건의 지평선을 넘어버렸다고 치자. 운좋게 탐사대가 살아 남았더라도 그 미래는 암울하다.
Given the fuel that you have in your reserve, you can only postpone your collision with the singularity by a few hours, maybe a few days at most. Is there any way that you could send what you learned back to a distant observer or does the event horizon prevent information from escaping?
보유중인 연료로는 몇시간 가량 어쩌면 몇일 정도 특이점에 충돌을 지연시킬 수 있었을 뿐이다. 그간 수집한 정보들을 먼 외부로 전송할 방법이 있을까? 사건의 지평선이 이 정보가 빠져나가는 것을 막지는 않을까?
Classical physics, that is to say general relativity, has very bad news for you. There's no way for you to transmit your data across the event horizon. But being a good spaceship captain, you are brushed up on your quantum mechanics and you know about two important principles; quantum determinism and the principle of reversibility.
고전 물리학은, 그러니까 일반 상대론을 말하는 거다만, 아주 비관적이다. 사건 지평선넘어 정보를 내보낼 방법이 없다. 하지만 양자역학을 갈고닦은 유능한 선장이라면 두가지 양자역학의 원리로 무장 했을 것이다. 양자 확정성과 가역성.
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Let's start by unpacking the easy one, reversibility. Reversibility is the idea that physical laws can be used to predict the future or the past of a particular system. If we look at a comet shooting by, not only can we predict where it will be decades from now, but also where it was decades in the past.
In some sense, reversibility also tells us that we can go backwards from one state just as well as we can go forwards. In reality, we know that reversibility is more complex than that. Shattered mugs don't suddenly unshatter themselves. But we'll address that when we discuss entropy.
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On the other hand, quantum determinism is a much stranger idea. Determinism, on its own, is the idea that we can predict the outcome of any interaction if we have sufficient information.
If I throw a ball and you know its direction and how fast it's going, where it lands is predetermined.
But quantum mechanics basically deconstructs the notion of classical determinism because of a concept called the Heisenberg Uncertainty Principle. Since we have limits on what we can measure, we also have limits on what we can predict.
Quantum determinism is then telling us a slightly different version of the story. We can predict with certainty what the probabilities of outcomes will be in quantum mechanics.
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Since the principles of quantum determinism and reversibility are fundamental to quantum mechanics, we've run into a problem. These two principles together mean that information must always be preserved. But the no-hair theorem for black holes, says that information falling into a black hole disappears when it crosses the event horizon.
Many scientists postulate that the information is destroyed somewhere at the interior of the black hole. In the next lesson, we'll delve further into this investigation, examining specific theories which attempt to explain the black hole information paradox.
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So, which is it? Is information preserved by quantum mechanics, or is it destroyed by general relativity? There are many possibilities. The information could be destroyed, it could leak out of the black hole gradually, or maybe information could escape in a powerful explosion.
Perhaps the black hole itself saves the information like a giant hard drive. The sad truth is that we just don't know. Although there are no known laws that unify gravitation with quantum mechanics, many researchers, including Stephen Hawking, now believe that information is preserved somehow and deeply linked to the process of Hawking radiation.
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Some researchers have had other interesting ideas. One such interesting idea is the concept of a new boundary within the event horizon, the information firewall. In 2012, a group of physicists Almheiri, Marolf, Polchinski and Sully, shortened to AMPS, introduced the concept of a boundary of high energy quanta that destroy all incoming information, the AMPS Firewall. The AMPS Firewall cleans up some of the inconsistencies of quantum mechanics by providing a mechanism to destroy or scramble the incoming information.
But some scientists think that the firewall creates more problems than it solves. Present day research, done by Don Page at the University of Alberta, suggests that the information Firewall described by the AMPS group could produce a naked singularity.
If indeed there is a firewall within a black hole, Page and his collaborators demonstrate that the firewall could migrate to a region outside of the event horizon allowing the singularity to become visible to distant observers.
페이지와 그의 동료과학자들은 블랙홀에 실제로 그런 방호벽이 존재한다면 (모든것을 빨아들이는) 특이점을 감안 하건데 그 방호벽은 사건의 지평선 밖으로 이동해서 먼 외부에서 관측됐어야 한다고 주장한다.
We already know how much physicists abhor a naked singularity. One of Page's research collaborators, Misao Sasaki, has said if a firewall exists, not only would an in falling object be destroyed by it, but the destruction could be visible even from the outside.
This is a very complex idea, but it brings up an interesting motivation for physicists. If the firewall idea is right, it means that there is new physics for us to consider. If the firewall idea is wrong, it means that we've uncovered some potential flaws in older physical theories.
Although there is no consensus about how the black hole information paradox will be resolved, there are several leading theories about the fate of information that has fallen into a black hole. One such theory, put forth by Stephen Hawking when he originally described Hawking radiation in 1976, predicts that the outflow particles from the black hole would have unpredictable properties.
So, then what does Hawking radiation actually look like?
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[인터뷰] [커세라 페이지]
What is quantum information?
양자 정보란 무엇인가?
Interview with Dr. Lindsay LeBlanc, Professor at the University of Alberta
앨버타 대학교수 린지 르블랑 박사와 인터뷰
On a quantum level, information is what we call qubit. So, it's storing information, not classically like a one or a zero that you would have in a regular computer, but a quantum system can exist in what's called a superposition. It can hold information as some probability of being in say left, and some probability of being in right. Those labels are completely random, it doesn't matter what they are. But at the same time, you hold two possibilities at once and it is this superposition or this probabilistic holding of information that makes quantum information so much different and actually so much more powerful, because you can actually store a lot more information in one physical thing as opposed to a bit. I don't know a lot about black holes, but I would be interested in looking at how quantum phenomena actually interact in the system. So, we often think of black holes as being a phenomenon of gravity, and gravity and quantum mechanics are two separate things in physics that we think should be connected, but we actually don't have a very good idea of how they are connected. So, black holes is very interesting object that perhaps we could start thinking about what happens to quantum things as they interact in a black hole. So, if you can imagine having a quantum system where you have maybe two particles that are entangled, which means that if you do something to one particle, automatically something happens to the other and there's this intrinsic connection between them that you can't break and so, if you could send one particle into the black hole, you still have the other one and how it behaves should tell you something about what's happening to the one inside this black hole. But black hole, you obviously can't get any information about that. So, yeah, how we would ever do an experiment about that? I don't know but I think it's an interesting problem to think about.
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