Tuesday, April 24, 2018

The Quantum Vacuum and the Casimir Effect

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



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 this episode.

3. Our subreddit.

Saturday, April 14, 2018

The Gravitational Equivalence Principles

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.



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. Our subreddit.


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

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.



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. Our subreddit.

Thursday, March 15, 2018

Gravitational Field Propulsion

Randy introduces Jim to several ways in which people have theorized that gravity can be used to propel an object through space. In order of likelihood and in inverse order of coolness:
  • The Slingshot Effect -- Using a gravitational well to increase the energy available to an object. (In use)
  • The Penrose Process -- Using a rotating black hole to gain energy. (Penrose)
  • Negative Inertia Mass -- If mass has both a negative charge and inertia, then you can build a device that moves perpetually (RL Forward)
  • Induced Gravitational Dipoles -- Spinning half of a sphere will (supposedly) propel it in some direction or other. (Jeong)
  • The Alcubierre Drive -- A process that would sneak around the laws of relativity and allow someone to travel faster than light. (Alcubierre, White)



1. The papers we read for this program:

2. The slingshot effect is in common use and is a topic viable at the junior mechanics level in such books as Taylor's and Morin's.

3. There is a problem defining the center of mass in general relativity, and we've recorded at least one podcast related to the topic (although not directly addressing it) between the recording of this last June and its publication in March. But the problem is with defining mass, not the center of mass. And because of the simplicity of the space-time swimmers, I'm still not sure its as much of a problem as some people do.

4. Popular articles for this program:

5. The video Randy keeps talking about in the podcast he found linking through The Hammock Physicist Blog of Johannes Koelman. You can download it here from the IOP. I haven't yet found the article IOP attached it to.

6. I'll make a video of the non-commutative motion diagrams that I sent Randy and put a link up here.

7. Our subreddit.

Friday, February 23, 2018

The Island of Stability

Randy tells Jim about the Island of Stability. This is a theoretically predicted region (or two, in one calculation) where the isotopes of atoms with heavy nuclei are stable. Jim and Randy talk about the islands, how they are attempting to get there, and some of the calculations people use to predict the stability of elements. Along the way, they mispronounce a lot of words.



1. The papers we read for this program:
2. The book I talked about on fluids that goes into the fluid model of the nucleus is Introduction to the Physics of Fluids and Solids by James S. Trefil, which is apparently no longer in print (I have a Dover edition, I love Dover). My favorite Modern Physics text is Rohlf, mostly because of its focus on scattering, but I taught from Serway, which is more standard.

3. Our subreddit.

Thursday, February 8, 2018

Dark Energy

Randy helps Jim get a handle on dark energy. Why do we need it? Why do we want to believe in it now, but didn't earlier? How many different things could it be? What does it have to do with you? How close are we to knowing anything about it?



1. The papers we read for this program:
  • Carroll, S., "Dark Energy and the Preposterous Universe." [arXiv]
  • Turner, M., "Dark Energy and the New Cosmology." [arXiv]
  • Zwicky, F., "On the Masses of Nebulae and Clusters of Nebulae." Astrophys. J.  86, 217 (1937) [web]
  • Holsclaw, T., Alam, U., Sanso, B, Lee, H., Heitmann, K., Habib, S. and Higdon, D., "New Constraints on the Time-Variation of the Dark Energy Equation of State from Current Supernova Data." [web]
2.I'll review Oyvind Gron's book, Lecture Notes on the General Theory of Relativity soon. I've been meaning to for a couple of months now.

3. Previous Episodes Referenced:
4. Read my review of Cosmic Update, a book about cosmology in the accelerating universe.

5.If you or Dragan Hejdukovic wants to supply us with the background energy that would be predicted from virtual gravitational dipoles, please leave us a nice comment with smilies.

6. Our subreddit.

Sunday, January 21, 2018

Weyl and Quasiparticles

Jim and Randy discuss quasiparticles recently found in condensed matter systems that mirror particles theorized nearly a hundred years ago, but never found in the vacuum. Weyl particles are massless fermions, and once it was hoped that neutrinos would turn out to be this kind of particle, and Majorana fermions have real-valued wave functions and therefore many strange and possibly useful properties.



1. The papers we read for this program:
2.  Popular papers we may have read:
3. Original Papers:
4. I was wrong.  "Weyl" rhymes with "pile," not with "pail."

5. Our subreddit.