New Directions in the Foundations of Physics Conference in Washington DC 2013 (part 2)

I continue my presentation of talks from the conference. Today I present:

“What is the alternative to quantum theory?” by John Preskill

Now this is a topic front and center in my area of research and I got very excited about this. Also I have two stories to tell. The first one was from the talk, and the second one from the private discussions.

I don’t think John Preskill needs an introduction, but if you are wondering who he is, it will suffice to say he won the bet against Stephen Hawking on the information loss in black holes.

His talk was around potential experimental tests of quantum mechanics in various settings. For example people play the “big cats” game, or who has the largest “Schrodinger’s cat” in interference experiments pushing the limits with bucky balls, atomic ensembles, uranium atoms, opto-mechanical experiments. Then there is the perennial question of violations of unitarity by gravity. In case you wonder, back holes do not necessarily destroy unitarity as Hawking initially thought he proved, and the best way to see that is by counterexamples from string theory. Sure, they may not be realistic, but only one counterexample is enough to prove Hawking’s initial analysis was faulty. Can unitarity be violated under any circumstances then? This is a different discussion and I’ll cover it later.

Then Preskill talked about attempts to “tweak” the Standard Model maybe by a GRW-type mechanism and he also talked about the renormalization group and the possible emergence of the Standard Model in the infrared domain from chaos in the short distance limit. All those considerations justify the need for experimental tests. We can and should test linearity, unitarity, microcausality, Poincare invariants, gauge invariance and general covariance.

From the theoretical side, questioning those aspects looks like heresy, but from the experimental side we do want to push the limit on testing their validity. For me, if there is one principle I don’t fully understand at this time, it is microcausality (there is a big difference between knowing and understanding). There are reasons to doubt it from string theory. The way to think about this is due to space-time fluctuations, maybe microcausality is not exact but asymptotically exact. Who knows, maybe it decays exponentially. What you really want to know is: “is the property fundamental and exact, or it is only a good approximation in a certain validity range?”.

The answer to those kinds of questions can also come from reconstruction projects, like the project of reconstructing (or deriving) quantum mechanics from physical principles. To this I know the answer and I had a very interesting discussion with John Preskill later in the day. I’ll tell this story in a subsequent post. Please stay tuned.