Friday, August 10, 2018

CPT Symmetry and Gravitation

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Recorded: 3/28/2018 Released: 8/10/2018

Jim and Randy discuss what happens when CPT symmetry is applied to gravitation. CPT symmetry -- what happens to a theory when you reverse the sign of the charge, the handedness of a particle, and the direction of time evolution all at the same time -- is a basic tenet of the standard model. Massimo Villata has applied this symmetry to gravitation and has derived consequences for the way in which antimatter particles interact with gravity and various cosmological conclusions that follow from that.



1. The papers we read for this program:
2. I found these papers in the footnotes to Alberto Vecchiato's Variational Approach to Gravity Field Theories.

3. Please visit and comment on our subreddit, and if you can help us keep this going by contributing to our Patreon, we'd be grateful.

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Recorded: 3/3/2018 Released: 7/25/2018

Randy talks to Jim about retrocausality in quantum physics -- how does the future affect the past? In particular, they talk about the ideas of Huw Price and Ken Wharton on using temporal boundary conditions to constrain the wave function through its initial and final boundary conditions, effectively creating quantum harmonics in the time domain. They also discuss what this means in terms of the de Broglie-Bohm hypothesis, the multiple worlds interpretation, and Yakir Aharonov's interpretation in Quantum Paradoxes.



1. The papers we read for this program:
2. Books discussed in the program:

3. Huw Price also wrote a book about the philosophy of time called Time's Arrow and Archimedes' Point that, according to the plane ticket I was using as a bookmark, I last read in 2003.

4. Please visit and comment on our subreddit, and if you can help us keep this going by contributing to our Patreon, we'd be grateful.

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Thursday, August 2, 2018

Physics Frontiers: Most Popular Episodes

Most popular episodes, by downloads/month after 4 months.

2017 The Physics of Time Travel
Vacuum Fluctuations and the Casimir Effect
f(R) theories of gravity

Physics Frontiers Index

Wednesday, July 25, 2018

Quantum Paradoxes by Aharonov and Rohrlich

You might think I like Quantum Paradoxes by Yakir Aharonov and Daniel Rohrlich. I mean, I started a podcast about it. I might even finish it someday.

This book explores the meaning of quantum mechanics through paradoxical thought experiments. It uses a few standard ones, like Schrodinger's* cat, and a lot of interesting variations of the double slit experiment and electron diffraction. The first eight chapters motivate mainly how quantum mechanics works using paradoxes. The last ten chapters motivate Aharonov and Rohrlich's interpretations.

I am very enamored of the format.

Each chapter follows a formula. After a short preamble, a paradox is presented in detail. The paradoxes are presented as thought experiments, first. This means that a detailed, if not physically possible, experiment is described, and then its physics discussed. The physics leads to two possible interpretations, such as: there is no physical difference in the dynamics of an electron on either side of a charged capacitor, but quantum mechanics predicts a phase shift in the wave function of the electron. How can that be?

Next, aspects of the physics are discussed in mathematical detail. In this case, what is the relationship between the gauge and the phase of the wave function. This leads to a choice, clarification, or reconciliation. The most interesting part of this for me has been the use of modular variables to clear up some points that have to do with the use of gauges, although the general set-up of the interference experiments Aharonov and Rohrlich are discussing requires a bit of careful reading. Sometimes, a section or two follows with implications and real, physical experiments.

The second half of the book deals with the interpretation of quantum mechanics in the context of weak measurements. I really don't have a great idea about how to explain a weak measurement, but the two important facets are: (1) they allow you to measure the wave function without (completely) destroying it and (2) they are only approximations to the wave function. Aharonov and Rohrlich mainly deal with their own interpretation, and (a) the Copenhagen interpretation (a favorite among users of quantum mechanics) and (b) the many-worlds hypothesis (a favorite among string theorists). Mainly, I think, because these are their main competitors.

Their own interpretation has to do with temporal boundary conditions, which is very appealing to me because it's compatible with the block universe idea of relativity, at least conceptually. It's very important to remember that every fundamental physical theory must be compatible with every other fundamental physical theory -- if two theories that should apply to a situation don't, you have a paradox. So, any interpretation of quantum mechanics must be compatible with relativity. This hasn't been a problem with the theory -- quantum electrodynamics is exactly the integration of quantum mechanics and special relativity. It has been a major problem with interpretations, and the authors detail some of those problems in the book.

I don't want to go into more detail, but if you want to get more detail, then over the next thirty-four-odd weeks, I discuss each chapter with a friend of mine in a podcast. Contact me for the address if you're not already subscribed.

So, I just love this book. It's a great way to not only explore quantum mechanics, but to explore what it means to be an interpretation of quantum mechanics in a rigorous and technical, but not exceedingly technical (to a physicist). If you have the mathematical background to play with differential equations, or even the intellectual fortitude to not be scared of them, I highly recommend this book. If you don't have that knowledge, then check out the podcast. It'll probably be more than enough for you.

Friday, July 6, 2018

Tunneling Time

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Recorded: 11/25/2017 Released: 7/6/2018

Jim and Randy discuss the tunneling time problem: just how long does quantum tunneling take? No definitive answer to this question exists, but people have been trying to answer it for at least eighty years -- with answers that span from instantaneous to subluminal. In this episode, we discuss several different ideas and how experiments at ETH-Zürich have helped clarify the issue.



1. The papers we read for this program:
2. The group responsible for the attoclock measurements at ETH-Zürich, including Landsman and Keller.

3. Please visit and comment on our subreddit, and if you can help us keep this going by contributing to our Patreon, we'd be grateful.

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Friday, June 15, 2018

Post-Newtonian Gravitation

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Recorded: 11/12/2017 Released: 6/8/2018

Randy and Jim discuss the Parameterized Post-Newtonian Framework, a generalized way to compare metric theories of gravity to experiment in a standardized way. In this episode we discuss several theories of gravity and how they hold up under the light of experimental data.


A Guide to the Parameters

  • γ (gamma) - Coupling of matter to curvature, GR = 1 , Newton = 0
  • β (beta) - Linearity of superposition, GR = 1 - Superposition linear
  • ξ (xi) - Preferred location effects, GR = 0 - Spatially homogeneous
  • α1 (alpha) - Preferred frame effects, GR = 0 - Lorentz invariant
  • α2 (alpha) - Preferred frame effects, GR = 0 - Lorentz invariant
  • α3 (alpha) - Preferred frame effects, GR = 0 - Lorentz invariant
  • ζ1 (zeta) - Momentum changes, GR = 0 - Momentum conserved
  • ζ2 (zeta) - Momentum changes, GR = 0 - Momentum conserved
  • ζ3 (zeta) - Momentum changes, GR = 0 - Momentum conserved
  • ζ4 (zeta) - Momentum changes, GR = 0 - Momentum conserved



1. The paper we read for this program (only sections 3 and 4):
2. My review of Will's book. This paper serves as something of an update of it. A true update is scheduled to come in December 2018.

3. Related Episodes of Physics Frontiers:

4. If you have any information about good packages for numerical relativity for Randy, please leave them in the comments.

5. Our subreddit.

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Thursday, May 24, 2018

The Consistent Histories Interpretation of Quantum Mechanics

Recorded: 10/29/2017 Released: 5/24/2018

Jim and Randy investigate the Consistent Histories interpretation of quantum mechanics. This highly logical interpretation was conceived of by Robert Griffiths and is based on bundling possible histories for a particle together and only using those histories that are consistent with the measurements we perform to winnow out the possible states of the particle. Although Griffiths calls this "Copenhagen done right," the interpretation is based on the idea the quantum particles have definite values for observables in the intervening space.



1. The papers we read for this program:

2. Griffith's book Consistent Quantum Theory, which we mention in the podcast because he continually refers to it in the podcast.

3. My review of Speakable and Unspeakable in Quantum Mechanics by John Bell, which includes the paper in which he formulates Bell's Theorem -- the testable version of the EPR Paradox, which is based on the first tractable formulation of the paradox by David Bohm. I also recently reviewed a book on wave function realism, which seems to be a response to a response to Bell.

4. I haven't quite got the Patreon I promised Randy in this episode up and running, but it is at least limping.

5. Our subreddit.