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.

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Notes:

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 8, 2018

The Parameterized Post-Newtonian Framework

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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.

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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


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Notes:

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. 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, May 24, 2018

The Consistent Histories Interpretation of Quantum Mechanics

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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.

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Notes:

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. 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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Tuesday, May 15, 2018

Gravitational Alternatives to Dark Energy

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Recorded: 10/15/2017 Released: 5/15/2018

Jim and Randy discuss the ways in which different modified gravities mimic the observed dark energy in the universe, contra a cosmological constant hypothesis. They talk about various forms of modified gravity theories and the particles associated with their "fifth fields."

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Notes:

1. The papers we read for this program:

2. My review of Will's book, which I talk about a little too much in this podcast.

3. Related Episodes of Physics Frontiers:

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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Tuesday, April 24, 2018

The Quantum Vacuum and the Casimir Effect

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Recorded: 9/16/2017 Released: 4/24/2018

Jim and Randy review two very convincing papers that make the claim that the Casimir effect is due to materials fluctuations and not the zero point energy

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Notes:

1. The papers we read for this program:

2. We discuss an earlier episode on the Casimir Effect and van der Waals forces repeatedly in the linked episode.

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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Saturday, April 14, 2018

The Gravitational Equivalence Principles

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Recorded: 9/10/2017 Released: 4/14/2018

Jim talks to Randy about the different ways in which the equivalence principle of general relativity can be formulated. More than just the equivalence of accelerations, the different possible meanings of the equivalence principle mean different things about how gravity works. From weak to strong, from Einstein's equivalence principle to Schiff's conjecture, the implications of these theories are explored.

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Notes:

1. The papers we read for this program:

2. My review of Will's book, which I talk about a little too much in this podcast.

3. Related Episodes of Physics Frontiers:

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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A review of "Theory of Gravitation Theories" from the arXiv_plus subreddit:

This delightfully named paper explains the relationship between the equivalence principles – the three equivalence principles listed in Will’s book: weak, Einstein, and strong – and the modified gravities that serve as competitors to general relativity (GR) with a cosmological constant in the quest to explain dark energy. The meat of the text centers around showing that a theory of gravity is a cluster of mathematical representations of that theory that are linked in an analogous way as different gauges in electromagnetism, although no general method of transformation is given, and that in order to satisfy the intermediate level of equivalence only one of these representations must satisfy the metric postulates. This ambiguity in representation is reflected in the way in which the additional fields of modified gravities are represented: whether the field is additional “matter” or a coupling that changes the “geometry” is a matter of the representation, not the theory itself.

The metric postulates simply ask a theory to (1) produce a metric gmn that describes the geometry of space-time and (2) admit only stress-energy tensors whose covariant derivative is zero. This is what you have in GR. It has been shown that the weakest version of the equivalence principle, which in its simplest form means that gravitational provides a preferred set of trajectories for small, uncharged particles to take without reference to their mass, is not, in itself, sufficient to establish these metric postulates. Instead, two other postulates are required, both self-explanatory: Local Lorentz Invariance and Local Position Invariance. These three postulates together form the Einstein equivalence principle and, along with some reasonable limitations on the mathematics, are sufficient to create a general class of metric theories of gravity with an additional scalar field (scalar-tensor theories). Each theory itself will be a cluster of the mathematical representations of the same.

One of the take-aways from this paper is that common procedures for determining if a theory satisfies EEP have made an error of checking single mathematical representations in those clusters against the metric postulates or Local Lorentz Invariance and ignoring the equivalent representations. The main example in the text compares the Jordan and Einstein frames in scalar-tensor theories, showing that they are equivalent, but the Einstein representation does not satisfy the metric postulates. However, since the Jordan frame representation of a scalar-tensor theory does satisfy the metric postulates, the theory itself does. This does, however show an issue with the frames that do not satisfy the postulates: changes need to be made in interpreting their metric tensors or geodesic equations, or even the connection in that frame. For example, in moving to the Einstein frame, you add an additional term in the geodesic equations that can be interpreted as:

(1) The gradient of the scalar field

(2) The variation of the masses of elementary particles in space

(3) Changes in the “unit of mass” along the trajectory

Although I’m not sure if (2) and (3) are the same or that the relative masses also change, making a coherent standard impossible.

Unfortunately, according to the authors, only individual mathematical representations of modified gravities exist in the literature, not more general theories that encompass the all of the representations of those theories, and so miscategorizations are likely. More unfortunately, a way forward towards a more abstract definition of individual theory clusters is not given in the text.

Sunday, March 25, 2018

Antimatter Production at a Potential Boundary

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Recorded: 6/17/2017 Released: 3/25/2017

Randy discusses a proposal for the production of antimatter without using supercolliders with Jim. This proposal would have used Klein's paradox for the production of electron-positron pairs at a very sharp, steep potential boundary produced through the Casimir effect.

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Notes:

1. The paper we read for this program:


2. The NASA Institute for Advanced Concepts no longer exists (apparently it closed down in 2007), but there is now a NASA Innovative Advanced Concepts, a program with the same acronym that seems to have a similar purpose (if not, tell me the salient difference in the comments).

3. Our listenership has increased markedly since we recorded this episode last June. We had 1,057 downloads in the first fifteen days of June 2017, and in the first two weeks of March we had 3,117. In both cases there were no podcasts published during the time interval (in the current case because this is my heavy semester at work and it eats into my free time).

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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