## Saturday, January 08, 2011

### Edward Witten: 70 minutes of string theory

If you have a spare hour and you want to see Edward Witten sitting in an IAS armchair and give you a lecture of modern physics from the 19th century all the way to string theory, continue to read this text.

You will also learn some Italian along the way. Most importantly, "string theory" is "La teoria delle stringhe."

The playlist of "Beautiful Minds, vol. 20", has 5 parts. In Czechia, you couldn't become successful with a half-English program with an English title but in Italy, things are different. ;-) The episode was released in July 2010.

In the first part, Witten discusses how Maxwell's equations predicted light, electromagnetic waves, and their universal speed. The significance of the latter was only appreciated by Einstein. Even more importantly, he developed general relativity a decade later.

If you don't speak Italian, you may want to skip 9:44-12:30 in the first part.

The second part is dedicated to relativistic quantum mechanics and quantum field theory, antimatter, and the Standard Model. He gets to string theory at 3:45 when he adds Einstein's gravity to the quantum framework. At 6:10, an Italian segment begins. Gabriele Veneziano can't be omitted.

The last Italian words are "teoria di tutto" (yes, TOE) and you return to Witten and English after 9:15. He talks about particle physics of the 1960s and the birth of string theory which is, of course, presented as an event in Italy: Regge, Veneziano, Fubini, Virasoro, and too many others. Fundamental equations able to describe the Regge trajectory mess were looked for. Some time is dedicated to the zeroth string revolution - at Rutgers, Joel Shapiro once forced me to invent a concise name for that period of their heroic discoveries. ;-)

In the third part, the amazing richness of string theory is emerging. The Feynman path integral for string theory - the world sheets - is discussed in some detail. String theory automatically tells you how it interacts when you know how the free one propagates - a perturbative explanation of its superior predictive power. At 3:30, another Italian segment begins.

The name of Pierre Ramond appears - so it's not shocking that at 5:19, when Witten returns with English, the discussion is about the supersymmetry. Witten says that string theory is very robust and hard to be deformed. After all, almost nothing works and string theory is just barely able to obey all the conditions. Supersymmetry was nevertheless found, in the string context. Reasons why physicists didn't appreciate string theory are mentioned - extra dimensions looked strange; QCD, and so on.

Witten was in the graduate school when just a handful of people worked on strings. They knew it was miraculous enough. And Witten himself eventually knew that something was going to happen. However, he was bothered that there were no viable left-right-asymmetric (parity-violating) vacua known at the moment. The Green's and Schwarz's discovery changed that, of course.

The fourth part begins with an Italian portion. Comments about the LHC, dark matter, and theory of everything will be comprehensible to everyone. After 2:55, Witten reappears with the picture of string theory after the first superstring revolution. Five string theories existed, and so on. Witten has played a "little bit of a role" :-) in reunifying them. The previous "vices" - such as extra dimensions - were recognized as a blessing in disguise.

In the 1990s, people got broader in their understanding what string theory is all about. Holography and Maldacena's AdS/CFT correspondence are being discussed. Around 11:00, an Italian narrator explains M-theory and its mysterious name.

You will only hear English again in the last, fifth part. Witten says that string theory is probably on the right track - because it predicts gravity while QFT is incompatible with it; and because the consistency holds so miraculously. Two minutes later, the accelerating expansion of the Universe is praised a the biggest recent experimental discovery. The possible Universe clearly follows. Witten is not sure whether the anthropic or "uncalculable" viewpoint is right or whether he likes it; he prefers to be able to calculate everything. But Witten claims that he hasn't consulted when the Universe was created (unless he is just hiding it, of course, haha). So the Universe could work in this way.

At 5:30, you hear the Italian chap again. He speaks about the CERN. At 8:05, Witten returns to the discovery prospects of the LHC, especially about the supersymmetry. He would be delighted, of course: "one half" of his papers depend on SUSY in one way or another. String theory's ability to uncover secrets in other disciplines (via AdS/anything etc.) is mentioned as another major reason to think it's on the right track.

He concludes at 12:00. The main criticism of string theory he may understand is that it is too ambitious, too big a step, so we may always remain uncertain about important things. The circumstantial evidence is convincing, however. Too many things work so that it is unlikely to be a coincidence. And it gives us a good draft of the world.