Weekend before last, I went to the 3rd Emergent Quantum Mechanics
Symposium at The Vienna Institute of Technology (TU Wien). This is a
picture of what *other people* at TU Wien were doing

The symposium was even more exciting than the car racing. Or at least it was first conference I'd been to since I stopped being a physicist; and perhaps as a result I was a lot more excited about it than I was at any of the conferences where I had any actual business.

As its name suggests, this conference was about the foundations of quantum mechanics. Moreover, it was motivated by the search for a deeper reality that explains the mathematical rules of quantum mechanics (QM). This is not the kind of thing you would feel an itch for if you think no such reality exists; thus the symposium was dominated by scholars who either have such a realist itch or at list had something to say about how it might or might not be scratched.

I don't have the journalistic skill to write a fair report of the whole thing; there were too many good talks. But I can briefly describe, and link to the things that struck me the most.

### Measuring classical paths for quantum particles

Aephraim Steinberg of the University of Toronto, CA talked about all kinds
of things going on in his lab, but the most significant to me was the (now old)
2011 photon trajectory experiment -- which I had not heard about
until then. Some of the results are shown here:

This plot shows trajectories of photons passing through a double-slit
interferometer; we see two things that are supposed to be forbidden:
* The paths reveal which slit a quantum particle went through, without
destroying interference fringes.
* The paths detail both the position and momentum of the particles in flight.

Of course nothing in this experiment overturns the uncertainty principle, and everything in it can be explained in terms of orthodox non-realist quantum mechanics. The get-out clause in this case is that these paths were calculated from many weak measurements of photon momentum. And the interpretation of that averaging process is up for grabs.

Nonetheless, the paths shown here are those predicted by the de Broglie-Bohm "pilot wave" theory. Those trajectories have often been dismissed as somehow unreal, so it is striking to see them in an experimental plot. The whole point of interpretive debates is to square observed reality with our metaphysical intuitions; plots like the one above serve to hone those intuitions.

### Many interacting classical worlds

Howard Wiseman of Griffith University, AU is the theorist who first
proposed the de Broglie Bohm (dBB) trajectory measurements done by Steinberg
*et al*. He spoke at the symposium, and had a lot to say about the associated
subtleties and difficulties. But his really interesting discussion was about
the many interacting worlds (MIW) theory which he has developed along
with Michael Hall (Griffith), and Dirk-André Deckert (UC Davis).

This is an adaptation of Bohmian mechanics. In that approach, particles have definite trajectories, but are pushed around by the full quantum wavefunction. This wavefunction famously describes a whole multiverse of possible worlds and is not affected by the Bohmian trajectories. For this reason, critics have claimed that those trajectories are superfluous.

Hall, Deckert & Wiseman have come up with a model where classical trajectories are pushed around by other classical trajectories rather than a wavefunction. (The picture above shows a simplified form of this -- the trajectories seem to repel each other). Each of these trajectories is a whole universe, so we still have many worlds. But now there is no wavefunction in sight, so each classical world is an equal and needed member of this multiverse.

This is all quite different from the usual quantum many worlds (QMW) picture, which is all about one big wavefunction. MIW is much closer to the intuitive picture we get from science fiction stories about parallel worlds. In the MIW theory, each of the parallel universes is a distinct, objectively real thing, occupying just one point in phase space. For us who live in such a world, particular events do or do not happen (even if something else happens in a different world). There is no slicing and dicing of a single big wave-function to yield observer-dependent realities.

### Atoms of space time.

Thanu Padmanabhan of Pune University, IN talked about the relation between gravity (and hence spacetime) and thermal physics. Now my grasp of general relativity is weak, and although I find spatio-thermal effects like the Unruh effect and Hawking radiation fascinating, I have no idea what to make of the movement to explain gravity in terms of heat and entropy.

Prof Padmanabhan proposes to clarify that with a simple explanation: gravity is thermal in the same way that hydrodynamics is thermal. That is, like fluids, space-time is made of wee bits who's collective behaviour can be abstracted into a field theory. For fluids that field theory might be the Navier Stokes Equations, for gravity it will be the Einstein Equations of general relativity. But in addition to all that, the statistical physics of the wee bits still gives rise to thermodynamic laws.

In this sense, I suppose, Hawking radiation is like Brownian motion and Johnson noise, a case where the underlying statistical jostling of the wee bits leaks ups through the field-theoretic abstractions. Now if I just had some sort of intuition explaining how the quantum fluctuations at a null surface are like Brownian motion, then I would say I had some vague understanding of this field of physics. And perhaps I could gain such intuition by studying work from places like gloriously named ...

### Quantum Gravity Laboratory

Silke Weinfurtner of the Univeristy of Nottingham, UK talked about
hydrodynamic analogue black-hole systems of the kind she is experimenting with
at the Quantum Gravity Laborotary (QG-Lab). The pictures above and below are
taken from there. She is looking for superradiant amplification of waves by
the vortex pictured.

Unfortunately this amplification had not yet been detected when she spoke, and so I won't actually write about the science here (see the lab's own web-site for some background about how a vortex is an analogue to a rotating black hole). I highlight this talk just because I take childish delight in the thought of this experiment. I was always fascinated by bathtub whirlpools as a kid, and the QG-Lab has a gigantic, continuously operating, fluorescent one as, well as a cracking new binocular imaging system for measuring even tiny ripples. No wonder Silke switched from theory to experiments!

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