우연히 발견한 책인데 현대물리학에 대한 많은 의문을 해소시켜 주었다. 끈이론 연구자들이 어떻게 다른 이론을 배척하는지, 그럼으로써 물리의 발전을 저해하고 있는지에 대해 저자가 얘기할 때, 그의 비분강개가 느껴졌다. 책은 단순한 비분강개만이 아니다. 끈이론의 역사와 현재, 양자중력이론에 대한 다른 접근 방법 소개, 물리와 과학에 대한 저자의 생각 등이 잘 어우러져 있다. 존경 받는 이론물리학자의 하나—하지만 끈이론에서 벗어난 비주류인—로저 펜로즈는 책에 대해 다음과 같은 평을 했다.
“Lee Smolin’s understanding of theoretical physics is unusually broad and deep, and his critical judgements are exceptionally penetrating, so his claim that string theory is responsible for the lack of real progress in fundamental physics for the past quarter of century carries considerable weight. Read this fascinating book and form your own judgement.”
스몰린의 끈이론에 대한 비판은 결국 다음으로 귀결된다. 끈이론은 수학적으로도, 물리적으로도 완결된 이론이 아니며, 앞으로도 그럴 가능성이 거의 없다. 처음에 이런 얘기를 읽었을 때 눈을 의심했다. 아니 그렇게 ‘만물의 이론’이라고 홍보되었고 다들 그렇게 알고 있는데? 결국 이러한 상황은 끈이론 연구자들이 입자물리학의 주류를 차지하면서 다른 접근 방법을 억압한 결과로 나타난 상황일 뿐이다. 저자는 이 책이 끈이론 연구자들에 대한 직접적 공격은 아니며, 다양성이 상실된, 그리하여 정체된 입자물리학계 현재의 상황을 객관적으로 지적하고자 할 뿐이라고 말한다. 하지만 끈이론 연구자들은 정말 아플 것 같다. 이 책을 읽는다면 말이다.
저자는 끈이론이 궁극의 이론일 가능성이 그리 높지 않음을 여러 가지 이유를 들어 설명한다. 판단하는 중요한 지표의 하나는 ‘background independence’이다. ‘배경독립성’이라고 번역될 만한 이 개념은, 시공간 자체가 이론의 결과로 dynamic하게 변하는지를 말한다. 일반상대성 이론은 배경독립성을 갖는 대표적 이론이다. 반면 양자역학은 배경독립적이지 않다. 주어진 시공간에서 이론이 전개되기 때문이다. 끈이론은 양자역학과 중력을 통합하고자 함에도 불구하고 배경독립적이지 않다.
저자는 현대물리학이 해결해야 할 다음의 5가지 문제(양자중력의 문제)를 나열한다(1장).
1. 일반상대론과 양자이론의 통합
2. 양자역학의 기초문제 – 관측과 해석의 문제
3. 여러 입자와 힘들이 통합되어 더 근원적인 실체의 나타남(manifestation)인지 결정
4. 입자물리학 표준모형의 상수들이 어떻게 선택되는지 설명
5. 암흑물질과 암흑에너지 설명
그리고 끈이론이 이러한 질문에 대한 궁극적 해답을 주지 않음을 여러 장에 걸쳐 설명하고 있다. 그 중 한 구절:
What about the first problem in chapter 1, the problem of quantum gravity? Here the situation is mixed. The good news is that the particles carrying the gravitational force come out of the vibrations of strings, as does the fact that gravitational force exerted by a particle is proportional to its mass. Does this lead to a consistent unification of gravity with quantum theory? As I stressed in chapters 1 and 6, Einstein’s general theory of relativity is a background-independent theory. This means that the whole geometry of space and time is dynamical; nothing is fixed. A quantum theory of gravity should also be background-independent. Space and time should arise from it, not serve as a backdrop for the actions of strings.
String theory is not currently formulated as a background-independent theory. This is its chief weakness as a candidate for a quantum theory of gravity. We understand string theory in terms of strings and other objects moving on fixed classical background geometries of space that don’t evolve in time. So Einstein’s discovery that the geometry of space and time is dynamical has not been incorporated into string theory. (p. 184)
결국 이 책은 다양성에 대한 옹호이며, 세상(이론물리학계)의 ‘주류’라고 불리는 사람들이 젊은이들을 쥐고 흔드는 상황에 대한 고발이다. 그가 경험한 학계의 일화와 교수 채용 방식의 문제, 과학이 어떠해야 하는가에 대한 의견 등 여러 면에서 곱씹을 것이 많은, 좋은 책이라고 생각한다.
책의 몇몇 구절들:
… The idea that string theory gave us not one theory but a landscape consisting of many possible theories had been proposed in the late 1980s and early 1990s, but it had been rejected by most theorists. As noted, Andrew Strominger had found in 1986 that there was [sic] a huge number of apparently consistent string theories, and a few string theorists had continued to worry about the resulting loss of predictivity, while most of them had remained confident that a condition would emerge that would settle on a unique and correct theory. But the work of Bousso and Polchinski and Stanford group finally tipped the balance. It gave us an enormous number of new string theories, as Strominger had, but what was new was that these numbers were needed to solve two big problems: that is, to make string theory consistent with the observation of a positive vacuum energy and to stabilize the theories. Probably for these reasons, the vast landscape of theories finally came to be seen not as a freak result to be ignored but as a means of saving string theory from being falsified. (p. 158)
… Even if we limit ourselves to theories that agree with observation, there appear to be so many of those that some of them will almost certainly give you the outcome you want. Why not just take this situation as a reductio ad absurdum? That sounds better in Latin, but it’s more honest in English, so let’s say it: If an attempt to construct a unique theory of nature leads instead to 10^500 theories, that approach has been reduced to absurdity. (pp. 158-159)
“I think it’s quite plausible that the landscape is real.”
—MAX TEGMARK (MIT)
… A theory has failed to make any predictions by which it can be tested, and some of its proponents, rather than admitting that, are seeking leave to change the rules so that their theory will not need to pass the usual tests we impose on scientific ideas. (p. 170)
Science was not invented. It evolved over time, as people discovered tools and habits that worked to bring the physical world within the sphere of our understanding. Science, then, is the way it is because of the way nature is—and because of the way we are. (p. 298)
I believe that science is one of those mechanisms [of correction]. It is a way to nurture and encourage the discovery of new knowledge, but more than anything else it is a collection of crafts and practices that, over time, have been shown to be effective in unmasking error. It is our best tool in the constant struggle to overcome our built-in tendency to fool ourselves and fool others. (p. 300)
… We have been trying to do so [make a revolution] with structures and styles of research best suited to normal science. The paradoxical situation of string theory—so much promise, so little fulfillment—is exactly what you get when a lot of highly trained master craftspeople try to do the work of seers. (p. 313)
There is no more earnest or sincere person than 't Hooft. One thing we in the field of quantum gravity love about him is that he is so often there. He comes to many of our meetings, and there you never see him in the halls, politicking with the other prominent attendees. Instead, he comes to every session, something only the young students do. He arrives first thing each morning, impeccably dressed in a three-piece suit (the rest of us are generally in jeans and T-shirts), and he sits in the front row all day and listens to the talks by every single student and postdoc. He doesn’t always comment, and he may even doze off for a minute or two, but the respect he shows by being there for each of his colleagues is impressive. When it’s his turn to speak, he stands up and unpretentiously presents his ideas and results. He knows that his is a lonely road, and I would not be surprised if he resents it. How does a person give up the mantle of leadership, so richly deserved, just because he can’t make sense of quantum mechanics? Imagine what that says about someone’s character. (pp. 318-319)
Over the years, I’ve noticed that a polarized distribution of responses is a strong predictor of future success and influence as a scientist. If some people think X is the future of science and others think X is a disaster, this may mean that X is the real thing, someone who aggressively pushes his or her own ideas and has the talent and perseverance to back them up. An environment that embraces risk takers will welcome such people, but a risk-averse environment will shun them. (p. 342)
In fact, professors with tenure who lose their grant funding because of having switched to a more risky area can quickly find themselves in hot water. They cannot be fired, but they can be pressured with threats of heavy teaching and salary cuts to either go back to their low-risk, well-funded work or take early retirement. (p. 346)