Thursday, May 30, 2013

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.

Tuesday, May 28, 2013

New Directions in the Foundations of Physics Conference in Washington DC 2013

This year I attended again the “New Directions” conference and yet again there were many interesting talks. I will gradually present what I learned from this year’s conference, with the caveat that I cannot be comprehensive and cover all talks.

Landauer’s Principle

One talk was by James Ladyman on his refutation of John Norton’s rejection of Landauer’s principle.

Landauer’s Principle states that erasure of information in an irreversible computation creates heat ('s_principle). In prior years, I heard Norton’s talk on rejecting Landauer’s Principle based on his one molecule gas analysis. Refuting Norton’s analysis, Ladyman presented his own arguments, and since Norton was present in the audience, the talk got really interesting in the Q&A session when Norton answered back.

Norton started with a funny joke about a recent Nature paper on this: two identical pieces of paper one which had written a long series of 1s and the other which had a random series of 0 and 1s were burned and the one with the random number generated more heat.

Then he proceeded to find holes in Ladyman’s arguments and rehashed his one molecule gas analysis (which I recalled from prior years). This time there was vigorous disagreement between the audience and Norton was on what constitutes a thermal equilibrium process for a one molecule gas. Basically, Norton’s core argument was that the piston pressing down of a one molecule gas has to be very light and hence it will have violent position fluctuations. In turn this makes a slow compression of the piston impossible and this killed Ladyman's counterarguments.

However, to me the one-molecule gas arguments look flimsy because the arguments hinge on the validity of the thermodynamic limit. And this limit was not proven to my satisfaction. It is highly likely Norton’s arguments will turn to be an artifact of this limit.

Since I am no expert in Landauer’s Principle, I cannot say with certainty who is right and who is wrong. But if I were to bet I’ll put my own money on Landauer’s principle.