## Thursday, February 09, 2006

### Andy's public lecture

Andy Strominger has just presented himself as an extremely entertaining populariser of physics. In a crowded lecture hall of the Science Center at Harvard University, he spoke about string theory after Lisa Randall introduced him and explained some of his extraordinary achievements.

At the beginning, he explained why contradictions are physicists' friends. The resolution of many contradictions led to new frameworks in physics. The last contradiction, in between QFT and GR, leads to string theory. There is also an experiment that confirmed each new theory that resolved a previously existing contradiction. In the case of string theory, the experiment had a question mark behind it. However, Andy showed a picture of the ATLAS detector inside the LHC and explained some potential signatures.

Andy had to explain quantum mechanics - everything is wiggly - and general relativity - the space is curved and you can't say what is a straight line which is why you end up orbitting the Sun. When he introduced black holes, he drew a caricature of George W. Bush probing the black hole horizon, and explained that if you really want to get rid of something, you throw it to the black hole. Most of the audience - but not everyone - violently applauded. ;-)

There were physicists, their families, medicine Nobel prize winners, Massachusetts lawyers, janitors, cameramen, New England attorneys, and many others in the audience.

Then the black hole evaporates and nothing is left. This is what had to happen with the weapons of mass destruction, Andy conjectured, and such an event is very troubling for a physicist because things should not be disappearing without any trace. He explained what determinism is about and how melting of icecubes can be reverted. Although Andy used to be ambiguous about the black hole information paradox, he presented the mainstream string theory answer - that the information comes out because it is encoded in the strings.

Strings were explained as loops that you can see under the microscope, and string theory was described as a pot of soup that has virtually all good ideas that have been found in theoretical physics of the last 30 years. Eleven-dimensional supergravity that was found to be equivalent to strongly coupled type IIA string theory was his example how the good ideas tend to unify at the level of string theory, which is one of the reasons why we think that we are on the right track.

Andy answered many questions and some of them have been very frequent questions that follow many similar talks. What is a string made of (nothing else), is it made of matter (no, you would get an F), is it made of waves of space (no), was the Big Bang a black hole (if it were, we are still in it), what can string theory say about the origin of the Universe (not much so far), and so forth.

Andy also drew a picture of a staircase, with string theory being the next step before an infinite sequence that follows, and admitted that most string theorists would disagree with this picture because in their opinion, string theory is the end. Lisa confirmed this disclaimer immediately by saying that by definition, we will call all other steps a part of string theory. Andy recalled a debate with Steven Weinberg. Weinberg said that people also thought that the Nile could not have an end when they were looking for it - and suddenly, they found the end. Andy does not buy it. (Let us assume that Andy buys that the Nile has an end, but he doubts that theoretical physics does.)

Andy emphasized that there is "nothing nothing" inside the black hole and that the equations of general relativity break down everywhere inside the black hole - which I would describe as a controversial statement but one that has simplified his situation after several questions.