Saturday, April 14, 2018

The Gravitational Equivalence Principles


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

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. Our subreddit.

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.

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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. Our subreddit.

Friday, March 16, 2018

The Nature of Space and Time

Stephen Hawking died two days ago, and I have a copy of The Nature of Space and Time sitting in my review pile waiting its turn. It doesn't have to wait its turn, though, because (1) Stephen Hawking died (did you hear?) and (2) It's short and so easy to review.

The premise of the book is that it is a series of lectures by Hawking and his mentor Roger Penrose. This was "the high point of a six month program held in 1994 at the Isaac Newton Institute for Mathematical Sciences at the University of Cambridge." I'm not sure where Cambridge is or what it's famous for, but I'm sure this was the hottest thing going in that half-year. The book itself is not a debate, only the last of the seven chapters. The first six are lectures alternately by Hawking and Penrose building up to the debate. If the seventh lecture was the high point of the program, the lectures certainly built up the dramatic tension in exactly the way universities don't build up dramatic tension.

The lectures are semi-non-technical. By that I mean that Hawking and Penrose can't help themselves and they put the pretty equations in the text for you to admire, but they aren't used for anything. So, really, they're nothing more than window dressing for the topic at hand. The topics are the classical theory of space and time (what is the future, what is the past, do they always exist?), singularities in space time (and some ideas about them), many varieties of quantum mechanical stuff (quantum black holes, quantum cosmology, quantum gravity), and twistor theory. Not necessarily light subjects, but presented in a way that most anyone should be able to understand.

And here both Hawking and Penrose get a chance to discuss some of their theories, like: why, exactly, would nature abhor a singularity? How exactly does it go about hiding them?

To be honest, the debate doesn't seem to be very heated when you get to it. It's just a restatement of the positions outlined beforehand, mostly. Not much "here's why I'm right and you're wrong." So, if you were waiting to find out who won the debate, I'm sorry: it wasn't that kind of debate.

What I'm thinking about reading:

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.

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Notes:

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?

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Notes:

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.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.

5. Our subreddit.

Tuesday, January 23, 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.

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Notes:

1. The papers we read for this program:
  • Pal, P.B., "Dirac, Majorana, and Weyl Fermions." Am. J. Phys. 79, 485 (2011).
  • Mourik, Zuo, Frolov, Plissard, Bakkers, and Kouwenhoven, "Signatures of Majorana Fermions in Hybrid Superconductor-Semiconductor Nanowire Devices."  Science 336, 1003 (2012). [arXiv]
  • Nadj-Perge, Drozdov, Li, Chen, Jeon, Seo, MacDonald, Bernevig, and Yazsdani, "Observation of Majorana Fermions in Ferromagnetic Atomic Chains of a Superconductor." Science 346, 602 (2014) [arXiv]
  • Xu, et al., "Discovery of a Weyl Fermion State with Fermi Arcs in Niobium Arsenide." Nat. Phys.  11, 748 (2015) [arXiv]
  • Zhang, et al, Signatures of the Adler-Bell-Jackiw Chiral Anomaly in a Weyl Fermion Semimetal." Nat. Comm. 10735 (2016) [arXiv]
2.  Popular papers we may have read:
  • Wilczek, F., "Why are there Analogies between Condensed Matter and Particle Theory?" Physics Today, January 1998, 11.
  • Wilczek, F., "Majorana Returns." Nature Physics 5, 614 (2009)
  • Balents, Leon, "Weyl Electrons Kiss." Physics 4, 36 (2011).
  • "After a Weyl" Nat. Phys.  11, 697 (2015)
  • Bernevig, B.A., "Weyl Electrons Kiss." Nat. Phys. 11, 699 (2015)
3. Original Papers:
  • Weyl, H., "Gravitation and the Electron." ..., 323 (1929).
  • Majorana, "A Symmetric Theory of Electrons and Positrons." Il Nuovo Cimento, 14, 171 (1937).
4. I was wrong.  "Weyl" rhymes with "pile," not with "pail."

5. Our subreddit.

Monday, January 22, 2018

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