# Discussion Sessions

This page is for the theoretical physics discussion group I used to organise, and may resurrect. The idea of the group was to provide an informal environment to discuss recent papers in theoretical physics which make unexpected or otherwise interesting claims.

The group had a brush with fame when it received this writeup by Louise Mayor, Features Editor at Physics World magazine. Dr. Mayor attended the discussion of the PBR theorem along with Jon Cartwright, author of the May 2013 Physics World article on the subject.

If you would like to be added to the (largely defunct) mailing list please contact me. I'm always on the lookout for interesting papers, so please send anything you think may be of interest.

I have included below the blurbs sent out for previous discussions, including the referenced papers. The blurbs have been changed to the past tense and have had references added as links, but otherwise remain largely unaltered.

## Firewalls

What happens when you fall into a black hole?

The textbook answer is currently ‘not much, at first’. The apparent singularity at the event horizon is a relic of the co-ordinates used by a distant observer, and shouldn't be anything special in the faller's frame.

However, a recent paper by Polchinski et al. argues that careful consideration of quantum mechanical and thermodynamic effects leads to the conclusion that in fact the faller experiences something rather drastic upon crossing the horizon. A readable overview to the paper and its many responses can be found here.

## Heisenberg and the Second Law

This discussion was on a Nature Communications paper with a title which requires no further exposition: “A violation of the uncertainty principle implies a violation of the second law of thermodynamics”. The paper can be found on the arXiv here.

## Semiclassical Spin

This discussion was on a paper by Chuu et al., “Semiclassical Dynamics and transport of the Dirac Spin”.

The discussion was suggested and lead by Martin Gradhand. This was Martin's summary of the paper before the discussion:

“The upcoming discussion group is about a semiclassical picture of the spin magnetic moment.”

The authors argue that the spin magnetic moment can be viewed as the self rotation of an electron wavepacket of the size of the Compton wavelength. The constructed wavepacket neglects negative energy solutions leading to a Berry curvature correction which restricts the minimal size of the wavepacket. Calculating its self-rotating magnetic moment they are able to connect the result to the explicit evaluation of the spin expectation value.

While the mathematical manoeuvre appears valid the question remains whether this gives a new insight to the nature of the electron spin magnetic moment: is the maths making a physical statement that spin can be understood semiclassically, or does it constitute a nice viewpoint only?”

## PBR Theorem

We discussed what has come to be known as the PBR (Pusey, Barrett, Rudolph) theorem, which purportedly disproves claims that the wavefunction merely quantifies our lack of knowledge of a quantum system.

As reading we had the original PBR Nature Physics paper (arXiv version here), plus a less technical overview from the same issue of the journal. We also read a popular account from Physics World.

We then received this mention in the department newsletter:

The theoretical physics discussion group was joined recently by Jon Cartwright, freelance science journalist, and Louise Mayor, features editor of Physics World, for a discussion on the recent PBR theorem of quantum foundations. The theorem was covered by Jon in the May issue of Physics World. After the usual lively debate and discussion - this time starting with “I think therefore I am” and ending with “Does this object live in Hilbert space?” - Dr Mayor wrote this review of the afternoon for the Physics World website.

## Time Crystals

This recent proposal by Frank Wilczek suggests that in certain situations it may be possible to spontaneously break time translation symmetry, leading to a system which exhibits periodic motion in its ground state. As the author puts it, this is “perilously close to fitting the definition of a perpetual motion machine”.

We read a PRL highlight paper covering the three papers which came out simultaneously: quantum time crystals (arXiv version here), classical time crystals (arXiv version here), and a proposed experimental setup for observing the effect (arXiv version here). However, the explicit realizations of time crystals given in the papers received seemingly watertight objections from P Bruno (quantum, experiment). The one-page objections neatly summarize the respective papers then show why the explicit realizations are wrong.

## Breaking the T in CPT

In this discussion we considered a recent experimental paper (arXiv version here) claiming to have observed time reversal symmetry breaking in fundamental particle interactions directly for the first time. Previous experiments have measured CP violation and inferred T from the CPT theorem. In the light of this new result we discussed what it means to break time reversal symmetry, and whether this observation indeed says something about the arrow of time as claimed in the less technical highlight paper.

## Unparticles

We discussed the recently hypothesised 'unparticles' - excitations of a quantum field which look the same on all length and energy scales and which have a superposition of masses. They do not permit a description in terms of particles as we usually imagine them.

Originally proposed in the context of particle physics in 2007, unparticles have this year received attention in the study of cuprate superconductors, where they were invoked to explain the observation that supercurrents appear to exceed what could be carried by electrons alone – the rationale being that if the current isn't carried by particles perhaps it is carried by unparticles.

A readable but superficial introduction to the original work in particle physics is given in the wikipedia entry which formed the suggested reading. You may like to read the original paper by H. Georgi (arXiv version here) or the application to condensed matter by P. W. Phillips which was used to explain violations of Luttinger's theorem as noted in this earlier paper (arXiv version here).