Nima’s Talk

Nima Arkani-Hamed may be the most famous and interesting name in the new generation of physicists. I attended his talk today in UMD, and his words are so impressive that even though I can’t understand the details, I feel deeply interested and attracted.

His title is ‘quantum gravity: possible and impossible’. In all, his purpose is to convince us that because of gravity, IR and UV are not totally decoupled as we thought before, gravity can contribute to high dimensional operators in EFT, and that not all IR effective field theory have a consistent quantum gravity theory as its UV completion, no matter it is string theory or not. The most important criteria he uses is the Weak Gravity Conjecture, which says that gravity must be the weakest long range (U(1)) force, and an effective theory should break down even below plank scale, at , where g is the gauge coupling, otherwise this system will decay into a black hole. A sharper form of WGC is that any matter must have . So if an IR theory violate WGC, it can never find a consistent quantum gravity theory as its mother theory.

So what EFT may be on the ‘dead’ list? One most important example is the inflation models which produce gravity wave, such as chaotic inflation. If gravity wave is generated, we require that the scalar field vev must be greater than plank scale. Naively, this can be realized in a 5 dimensional model with a very small 5D gauge coupling and it seems very natural. However, no one can find it in string theory, no matter what form they tried, there is always something, such as diliton, coming out to destroy it. Is there any general reason why they failed? The answer may be yes: it violates week gravity conjecture. There is another model named ‘N-flation’, with N species of scalar field triggering inflation. It is also problematic because it violates species bound which comes from constrain of entropy of a black hole. Moreover, if considering loop correlation of , we have which also leads to violation of WGC for large N. So Nima boldly guess that we can not see gravity wave in CMB, otherwise, it will be a big crisis.

Another interesting model he talked about was Euclidean warm hole. It was proved by Coleman that warm hole theory can be a local theory with global symmetry breaking, which can be realized with axion coupling with gravity. However, as chaotic inflation, we can not find it in string theory. And the obstacle is also WGC. To realize a warm hole, we just need to find a geodesic longer than , which means we have a energy scale higher than plank scale and the corresponding U(1) gauge coupling is weaker than gravity. Further, he said that if we realize it in a AdS/CFT world, it will violate unitarity of CFT, which I do not understand.

Nima also talked about some other models, checking them with WGC. Especially, he mentioned RS. He said we can not ignore the compacted in when we go down from string theory. Otherwise if we take a brane near the UV side with a string on it, the corresponding particle(gauge boson) will have mass which can be larger than . We need the extra-extra dimensions to ‘dilute’ the mass of the particle. I do not understand how either, too ’string’.

Nima is really a good speaker. He makes each point very clear, and goes through the details very smoothly. Speaks fast, writes faster.

Also Sprach David Gross

David Gross was invited to our department for 3 days. During one public lecture, one colloquium and some informal chat, there are something he said which may be interesting to hear. One thing to remind you, Gross is a very conservative reductionist, so you may not like some of his words. Welcome to leave your comments. I just quote them (not word by word) in a random order:

Things I learned these days

• Neother Theorem is the Lagrangian form of Ehrenfest Theorem which is in Hamiltonian form. There are two correspondences (not very exact) :

1. The Lagrangian is invariant under some symmetry corresponds to the Hamiltonian commutes with some operator (symmetry group’s generator)
2. The Neother charge is a time constant corresponds to the expectation value of the symmetry operator is a time constant, actually, the Neother charge is just the expectation value of the generator.

• From Kaplan’s Colloquium (11.29):

1. EW theory must break down because of 4W interaction, just like 4 fermion interaction must break down, which is caused by violating unitarity. And the solution is similar, 4 fermion need a gauge boson to change 4 vertex to 3 vertex, and 4W need a Higgs (or other new particle?) to change 4 vertex to 3 vertex.
2. MSSM has more than 100 parameters! ( I heard this for the first time)
3. Higgs mechnism can be traced back to Schwinger, who proved that massive gauge bosons do not necessarily violate gauge symmetry, by introducing a scalar field, but did not mention symmetry breaking. And there were other guys’ work following this, showing symmetry breaking, until Higgs pointed out the existence of a scalar particle following the referee(highly suspected to be Schwinger)’s suggestion. So came the name ‘Higgs particle’.

• From Tom’s seminar (12.3)

SUSY can be broken explicitly at UV (elementary sector) but emerges accidentally at IR (composite sector). The symmetries at lower energy are more than that at higher energy, which seems blizzard, but not. From 5D view, IR have more gauge symmetry and so more degrees of freedom just because UV and IR are two vacuums separated by the bulk (or domain wall); from 4D view, the different symmetries become global symmetry so it will not take more degrees of freedom, and the low energy global symmetry is always broken at high energy. (Because of gravity)