Strings attached
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But is that physics? That seemed to have been the caveat raised by Richard Feynman, Nobel laureate physicist, against the string theory, popularly known as the Theory of Everything. Though the experts, not particularly fond of hypes, don’t like the name that much, the string theory is one of the attempts to unify the two seemingly irreconciliable concepts — relativity and quantum mechanics — which between them explain everything from an apple’s fall to a picture’s formation on a TV screen. Albert Einstein, always a seeker of unity in diversity, spent several decades of his life in an successful chase after such a single concept that can explain all physical phenomena — the Holy Grail of physics. So why did Feynman, whose Nobel Prize was in recognition of his success in unifying Einstein’s first version of relativity (special relativity) and quantum mechanics, not like string theory? In one of the interviews during the last years of his life, Feynman explained this dislike as the perennial strife between the old the new. “I have noticed when I was younger that a lot of old men in the field couldn’t understand new ideas very well… such as Einstein not being able to take quantum mechanics,” said he. “I am an old man now, and these are new ideas, and they look crazy to me, and they look like they are on the wrong track.”
This excuse of age is, perhaps, metaphorical, for both Einstein and Feynman had solid reasons behind their objections to the new ideas sweeping their fields. Quantum mechanics, for all its experimental success, is full of common sense-defying tenets, oddities that haven’t been resolved yet. And the string theory, whatever promise it may hold, hasn’t been tested in the labs. Adherents of the concept, Feynman believed, had been too enticed by its mathematical beauty to harbour any scepticism about its validity.
Was Feynman right in his opposition to string theory? The question seems to be a valid one this year, especially as meetings and symposia are commemorating the 20th anniversary of the concept in its mature form. “Feynman was almost always right,” says Prof. Andrew Strominger, a leading string theorist based at the Harvard University, US, who is on a brief visit to Calcutta. “I think he was probably speaking the truth. He didn’t believe in string theory because he was too old,” he adds with a chuckle.
According to string theory, the ultimate constituents of matter aren’t various point-like particles. Instead, the world is made of strings: one-dimensional objects whose modes of vibration make them look like the elementary particles. This makeover in our understanding came in 1984 when Prof. John Schwarz from the California Institute of Technology, US, and Prof. Michael Green, now at the Cambridge University, UK, showed that this eliminated some irksome problems from the path towards combining relativity and quantum mechannics.
But even that success wasn’t enough. Physicists found that the postulated framework needed, at least, 10 dimensions to hold sway. Our known world has got only four — three (left-right, front-back and up-down) of space, and one of time — so, where’s the other six? Also, it didn’t describe electrons and protons, the particles of the real world. It’s this divorce from reality inherrent in string theory that irked Feynman, who thought physicists’ job was to analyse Nature to decipher its beauty, rather than impose a beautiful structure on it by believing that it follows complex mathematics.
In 1984, Strominger (then at the University of California, Santa Barbara), along with Dr Philip Candelas of the University of Texas at Austin and Prof. Edward Witten from the Princeton University, discovered a way of getting rid of the extra six dimensions of string theory. They invoked ideas propounded by Dr Eugene Calabi (Pennsylvania University) and Dr Shing-tung Yau (Harvard University) to show that those dimensions aren’t seen in the real world because they are compactified — curled up to points. Other than dispensing with the excess dimensions, the new framework had a bonus: it predicted the various kinds of forces that operate in Nature, including gravity. However, theoretical successes like these have strings attached to them. Experimental confirmation still eludes string theory.
“In science a theory earns prestige and recognition only when it predicts something and then that is experimentally verified,” says Srominger. As an example, he cites Einstein’s General Theory of Relativity (GTR) explaining the excess amount of rotation (in space) of the orbit of Mercury around the sun. Being closest to the sun, and also because of its proximity to the other planets, Mercury is subjected to strong gravitational influences. As a result, the planet’s elliptical path around the sun is supposed to rotate.
However, in 1845, French mathematician Leverrier showed that the rotation was more than what would be expected if gravity’s course was dictated by Isaac Newton’s formula. Why this anomaly? Various hypotheses, including the existence of till-then-undiscovered planets, were put forward, but they did not pan out. “Finally, though GTR explained it beautifully, I think it didn’t impress anyone, except Einstein himself,” says Strominger. According to him, it was only after the 1919 experimental confirmation, by British astronomer Arthur Eddington, of the GTR’s prediction that the sun would bend starlight coming to the earth that “people started believing GTR is correct.”
Why is it so difficult for the string theory to make predictions that can be tested in a laboratory? The answer, explains Strominger, lie in the extremely tiny sizes of the strings. “We don’t know how big they are,” comments Strominger. “They may be very small and so very hard to see.” To probe matter at more and more deep level you need to crush it with higher and higher energies. This requires more and more powerful particle accelerators. Which is why some say that to see the strings you need an accelerator the size of a galaxy!
“Thre’s some discussion on how big the strings are,” says Strominger. “There are some suggestions that they are big enough so that we will see them at the new accelerator now being built near Geneva, the Large Hadron Collider (LHC). But I think we have to be extraordinarily lucky for things to work out that way.”
So, in the absence of experimental proofs, how far away do the string theorists think they are from the Holy Grail? “We don’t know, it’s entirely possible that everything we’ve been doing for the last 20-30 years is barking up the wrong tree,” comments Strominger. “Decades from now we might be considered merely as a bunch that got mad about a fascinating idea. But, of course, I think that’s very unlikely. If that were likely, I wouldn’t be working on it. It’s true that we won’t know for sure that string theory is correct until somebody makes a prediction using it and then it’s experimentally verified. And that could take a very long time. It’s one of the biggest question, if not the biggest question, that humanity has posed. We should not expect it to be answered overnight. It’s a process which has been going for many centuries. It can also continue for many centuries.”
Einstein’s goal was to prove that God had no choice in creating the Universe; it had to run only on scientific principles. In their initial euphoria physicists thought string theory would achieve that goal — it would show why the fundamental particles of matter had the masses, or in some cases electrical charges, that they did. It is known that if those masses and charges were slightly different, the cosmos would also have been a vastly different place. Will string theory ever explain why those particles have the masses and charges that they do? “One extreme point of view, which was the prevalent one in the mid-1980s but no longer so common, is that the theory will solve all our problems, but that hasn’t happened.”
So why was there so much of hope in the first place? “That’s a very good question,” laughs Strominger. “Even though I was one of those who began in the mid-80s, I was not among those who thought string theory would solve all our problems. I would be very happy if it solved all our problems. It’s more than enough for me if it solves one of our problems.”
 Old-age problem: Richard Feynman didn’t appreciate the string theory |
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According to Strominger, string theory is viewed as the final theory of Nature — “the l-a-s-t step in the path that began with Aristotle and Plato.” But he thinks that is implausible. “My view is that there’s a good chance that it’s a step forward, but I’m not sure if it’s really a step forward,” he comments. “Our inability to calculate the masses or charges of the fundamental particles of Nature from string theory is one reason that I don’t think it’s going to be the last step. I think there’s something else needed.”
What could be that? Having failed — so far, at least — to calculate the masses and charges of the particles that make this world, many physicists have embraced what is known as the ‘anthropic principle’, the idea that the parameters of the world ought to be what they are, or else the cosmos would not have harboured life. Should our presence in this Universe obviate the need to probe its charactteristics? Isn’t that a failure of science, a turnaround from the Einsteinian dream? Experts, including Prof. David Gross, director, Kavli Institute for Theoretical Physics, US, and one of the Nobel winners in physics this year, think so.
“The notion that the anthropic principle is needed to explain some things in the world around us is logically possible, it may be needed to explain some of the things that we hope to explain by the string theory,” says Strominger. “I don’t think there’s something logically wrong with the anthropic principle. However, I also don’t think that there’s very much to be gained from the discussion of it. Of late, there’s has been a lot of discussions on it.”
To move forward, Strominger argues, the string theorists should work on the problems that they are able to make a progress on. For example, he says, they should concentrate on explaining why the dark energy that is enhancing the cosmic expansion rate is so small. “We aren’t producing ideas on that,” he obser-ves. “Maybe later we will find an angle that will enable us explain why the Universe is the way it is. We may or may not succeed in this endeavour. Maybe the parameters of our world are ultimately anthropically determined. My point is that we shouldn’t embrace that idea before exploring other options.”
Although there’s no direct evidence that string theory is correct, Strominger points out, “we’ve a number of signposts that it’s on the right track.” What are they? String theory, Strominger explains, has shown beautiful connections with various branches of mathematics. During the early 1990s physicists ended up with five varieties of string theory, and then it was shown that they were essentially various incarnations of a single theory. Like the Calabi-Yau compactifications achieved by Strominger and others, he and Dr Cumran Vafa pulled off another feat in the mid-1990s. They showed that string theory calculations yielded exactly the same value of an attribute of black holes as those obtained through other computations earlier. This success came as a shot in the arm of string theorists to silence their critics.
Another thing has been very encouraging, according to Strominger. “String theory is an amalgam of almost all the ideas that have come out of theoretical physics in the last 30 years,” says he. “It’s happened that some people had studied new theories as alternatives to string theory, but at the end those turned out to be string theory in disguise. String theory has no competitors, because they have been gobbled up and turned into string theory.”
Strominger may not be entirely right. There are other theories that have the same goal as that of the string theory, the most notably among them is the ‘loop quantum gravity’. This alternative visualises spacetime as made of tiny units (with holes in between where there is no space at all!), rather than a continuous whole. In which way is string theory ahead of look quantum gravity? “There are three major problems,” explains Strominger. “One is the problem of finding a theory that in principle can unify all the forces and particles in Nature. The second one is consistently putting together quantum mechanics and Einstein’s General Relativity. The third problem is the mathematical explanation of the black hole attribute that we computed. String theory has mathematically solved all the three problems. We don’t know if Nature avails herself of the solutions provided by string theory. But, on principle, string theory is capable of resolving all three of the riddles. Loop quantum gravity hasn’t solved any one of the three problems.”
The failure of the string theory in the last 20 years to come closer to testable reality has been disappointing, admits Srominger. “But even though it’s disappointing, we shouldn’t go home,” he says. “I mean, it’s not a reason to think that the theory is wrong. Even though some things have been disappointing, other aspects of this theory have worked out in a much more beautiful and satisfying way that we wouldn’t have imagined in 1984.”
Isn’t this limbo exciting in some ways? Is he planning to relate his feelings to laymen by writing a popular science title, as some of his peers have done? “Oh, no,” retorts Strominger. But why not? “Because doing research is too much fun. It’s too exciting.”
