## 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.