Wednesday, April 25, 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.

Book Review Index

  1. The Wave Function: Essays on the Metaphysics of Quantum Mechanics, Ney and Albert, Eds.
  2. Cosmic Update by Adams, Buchert, and Mersini-Houghton
  3. THe Nature of Space and Time by Hawking and Penrose
  4. Extra Dimensions in Space and Time by Bars and Terning
  5. Theory and Experiment in Gravitational Physics by Will
  6. Speakable and Unspeakable in Quantum Mechanics by J.S. Bell

Tuesday, April 24, 2018

Cosmic Update by Adams, Buchert, and Mersini-Houghton

Cosmic Update: Dark Puzzles, Arrow of Time, Future History is the second book in the Multiversal Journeys series run by Farzad Nekoogar and published through Springer. Like its predecessor in the series, Extra Dimensions in Space and Time, this is an accessible, semi-technical discussion about different matters in theoretical physics by experts. In this case, the three main essays are about cosmology, especially: if the universe is expanding due to an unidentifiable force, what does that mean about our physics. All of these topics are perfect topics for Physics Frontiers, and some probably have been and will be.

The first essay, "Dark Energy and Dark Matter Hidden in the Geometry of Space?" by Thomas Buchert describes how gravitational theory is being modified to accommodate the expansion of the universe. In particular, it describes the attempt to look at how the structure we see in the universe aids in creating an apparent cosmological constant. Standard cosmology usually assumes uniform values for the energy density and pressure of the universe, although we know that to be untrue. It's "true enough," they say, "on average." Buchert and coworkers have been looking at how that average model breaks down in the presence of known structure, and what the implications of that structure are, and apparently those nonuniformities might account for the dark energy field and dark matter halos observed by astronomers. The process that does this is the gravitational backreaction against cosmic evolution. Exactly how this works, I'd need to delve into, but it's an interesting way to model what's happening to the cosmos that gives a physical explanation to some ghostly phenomena.

The second essay, "The Arrow of Time in a Universe with a Positive Cosmological Constant Λ" by Laura Mersini-Houghton identifies the direction of thermodynamic processes based on the expansion on the universe. And what apparently happens is that in a de Sitter universe, the gravitational entropy eventually exceeds the matter entropy, and time reverses. Worse, when it happens, there is a "tachyonic instability" from (or by?) "super-Hubble" modes, which results in a violent transition at the boundary. At the conclusion of the chapter Mersini-Houghton says that the result of her theoretical inquiry into the direction of time is that we cannot have a "pure" Λ for dark energy, the cosmological constant has to vary in space and time, in order to avoid a breakdown of general relativity in the infrared regime.

The last of the original essays, "The Future History of the Universe" by Fred Adams is an updated physical eschatology accounting for the presence of dark energy. He discusses the fate of stars of different sizes, black holes, and so on. It's entropically depressing, of course. The universe is young now, in its "stellariferous" era with its fancy stars and pretentious galactic clusters, but in the long run, it's going to be a bleak, black place. In just another 1033 years, though, the universe will be quite unfashionable and enter into the degenerate era, full of brown dwarfs, white dwarfs, blue dwarfs, and any other dwarf that found a way to get out. The scary, lonely thing is that some of these blue dwarfs will have habitable worlds. But there won't be anything out there in the sky for them to see. Going over my notes, I didn't really get where the changes were, except that there were supposed to be difference from what you'd have read in 1995, but it is an interesting discussion.

An added bonus is a reprint of a paper by Lawrence Krauss and Robert Scherrer, "The Return of the Static Universe and the End of Cosmology" that supplements the last essay by saying that there will be a point in future where an observer will not be able to tell that the universe is expanding.

All in all very interesting. It's a little expensive, unlike the next book in the series, Quantum Physics, Mini Black Holes, and the Multiverse: Debunking Common Misconceptions in Theoretical Physics (just out) but if you can get a copy, it's worth a read.

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.



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.