Continuing my ad hoc review of Cosmology and Quantum Foundations, I come to Max Tegmark and Simon Saunders, who were the two main champions of Many Worlds Interpretations present at this conference. But before discussing ideas arising from their talks, I want to mention an addendum to the Hidden Variables and de Broglie-Bohm pilot wave theory that I totally coincidentally came across the night after writing the previous post (“Gaddamit! Where’d You Put My Variables”).

## Fluid Dynamics and Oil Droplets Model de Broglie-Bohm Pilot Waves

This is some seriously recent and immature research, but it is fascinating. And really simple to explain so it’s cool. Here’s the link: “*Fluid Tests Hint at Concrete Quantum Reality*“.

You delicately place oil droplets on an immiscible fluid surface (water I suppose) and the droplets bounce around creating waves in the fluid surface. Then, lo and behold! Send an oil droplet through a double slit barrier and it goes through one slit right! Shocking! But then hold on to your skull … after traversing the slit the oil droplet then chaotically meanders around surfing on the wave ripples spreading out from the double slit that the oil droplet was actually responsible for generating before it got to the slits.

Do this for many oil droplets and you will see the famous statistical build-up of interference pattern at a distance radius, but here with classical oil droplets that can be *observed to smithereens* without destroying superposition of the fluid waves, so you get purely classical double slit interference. Just like the de Broglie-Bohm pilot wave theory predicts for the Bohmian mechanics view of quantum mechanics. I say, “jut like” because clearly this is macroscopic in scale and the mechanism of pilot waves is totally different to the quantum regime. Nonetheless, it is a clear condensed matter physics model for pilot wave Bohmian quantum mechanics.

(There is a recent decades trend in condensed matter physics where phenomenon qualitatively similar to quantum mechanics or black hole phenomenology, or even string theory, can be modelled in solid state or condensed matter systems. It’s a fascinating thing. No one really has an explanation for such quasi-universality in physics. I guess, when different systems of underlying equations give similar asymptotic behaviour then you have a chance of observing such universality in disparate and seemingly unrelated physical systems. One example Susskind mentions in his theoretical Minimum lectures is the condensed matter systems that model Majorana fermions. It’s just brilliantly fascinating stuff. I was going to write separate article about this. Maybe later. I’ll just mention that although such condensed matter models have to be taken with a grain of salt, to whatever extent they can recapitulate the physics of quantum systems you have this tantalising possibility of being able to construct low energy desktop experiments that might, might, be able to explore extreme physics such as superstring regimes and black hole phenomenology, only with safe and relatively affordable experiments. I’m no futurist, but as protein biology promises to be the biology of the 21st century, maybe condensed matter physics is poised to take over from particle accelerators as the main physics laboratory for the 1st century? It’d be kinda’ cool wouldn’t it?)

The oil droplet experiments are not a perfect model for Bohmian mechanics since these pilot waves do not carry other quantum degrees of freedom like spin or charge.

Normally I would scoff at this and say, “nice, but so what?” Physics, and science in general, is rife with examples of disparate systems that display similarity or universality. It does not mean the fundamental physics is the same. And in the oil droplet pilot wave experiments we clearly have a hell of a lot of quantum mechanics phenomenology absent.

But I did not scoff at this one.

The awesome thing about this oil droplet interference experiment is that there is a clear mechanism that can recapitulate a lot of the same phenomenology at the Planck scale, and hence offers an intriguing and tantalising alternative explanation for quantum mechanics as an effective theory that emerges from a more fundamental of Plank scale spacetime dynamics (geometrodynamics to borrow the terminology of Wheeler and Misner). Hell, I will not even mention “quantum gravity”, since that’d take me too fa afield, but dropping that phrase in here is entirely appropriate.

The clear Planck scale phenomenology I am speaking of is the model of spacetime as a superfluid. It will support non-dissipative pilot waves, which are therefore nothing less than subatomic gravitational waves of a sort. Given the weakness of gravity you can imagine how fragile are the superpositions of these spacetime or gravitational pilot waves. Not hard to destroy coherent states.

Then, of course, we already have the emerging theory of ER=EPR which explains entanglement using a type of geometrodynamics. If you start to package together everything that you can get out of geometrodynamics then you being to see a jigsaw puzzle filling in that hints maybe the whole gamut of quantum physics phenomenology at the Planck scale can be largely adequately explained using spacetime geometry and topology.

One big gap in geometrodynamics is the phenomenology of particle physics. Gauge symmetries, charges, and the rest. It will take a brave and fortified physicist to tackle all these problems. If you read my blog you will realise I am a total fan of such approaches. Even if they are wrong, I think they are huge fun to contemplate and play with, even if only as mathematical diversions. So I encourage any young mathematically talented physicists to dare to go in to active research on geometrodynamics.

## The Many Worlds Guys

So what about Tegmark and Saunders? Well, by this point I kind of exhausted myself today and forgot what I was going to write about. Saunders mentioned something about frequentist probability having serious issues and that Frequentism could not be a philosophical basis for probability theory. I think that’s a bit unfair. Frequentism works in many practical cases. I don’t think it has to be an over-arching theory of probability. It works when it works.

Same in lots of science. Fourier transforms work on periodic signals, and FT’s can compress non-periodic signals too, but not perfectly. Newtonian physics works bloody well in many circumstances, but is not an all-encompassing theory of mechanics. Natural selection works to explain variation and speciation in living systems, but it is not the whole story, it cannot happen without some supporting mechanism like DNA replication and protein synthesis. You cannot explain speciation using Natural selection alone, it’s just not possible, Natural selection is too general and weak to be a full explanatory theory.

It’s funny too. Saunders seems to undermine a lot of what Tegmark was trying to argue in the previous talk at the conference. Tegmark was explicitly using frequentist counting in his arguments that Copenhagen is no better or worse than Many Worlds from a probabilistic perspective. I admit I do not really know what Saunders was on about. If you can engineer a proper measure than you can do probability. I think maybe Tegmark can justify some sort of MWI space measures. Again, I do not really know much about measure theory for MWI space. Maybe it is an open problem and Tegmark is stretching credibility a bit?

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