Friday, November 24, 2017

The Physics of Time Travel

Randy and Jim try to get their heads around how time relates to relativity. Of particular interest is Kurt Goedel's 1949 solution to the field equations that shows closed time-like null geodesics (paths followed by massless particles moving at the speed of light). The subject focuses mainly of some implications of general relativity that obey our intuition locally, but not globally.



1. The papers we read for this program:
  • An Example of a New Type of Cosmological Solutions of Einstein's Field Equations of Gravitation, Kurt Godel (Rev Mod Phys 21 447 (1949)
  • Cauchy Problem in Spacetimes with Closed Timelike Curves, Physical Review D 42, 1915 (1990)
  • Closed Timelike Curves, Bryan W. Roberts
  • Is Physics Consistent with Closed Timelike Curves? John L. Fiedman, Annals of the New York Academy of Sciences
  • Chronology Protection Conjecture, S. W. Hawking, Physical Review D 46, 603 (1992)
  • Time Travel and Time Machines, Chris Smeenk and Christian Wuthrich, Oxford Handbook of Time (Oxford)

2. Kurt Goedel's On Formally Undecidable Propositions of Principa Mathematica and Related Systems, which I jabber on a little too much about in the podcast.

3. A World Without Time: The Forgotten Legacy of Goedel and Einstein by Palle Yourgrau, a popular book on this subject I read a long time ago and I misplaced.

4. James Gleick's Google talk on time travel, and Time Travel: A History, the book it's based on.

5. I dropped about 7 minutes of my recording, which from Randy's comments included a brief discussion of positive probability of backward causation in the Compton effect's path integral formulation. I did include his overview of "Time Travel and Time Machines," after the end of the show which is a philosopher's take on the issue.

6. Our subreddit.

Monday, November 6, 2017

Stochastic Resonance Energy Harvesting

Randy tells Jim about ways in which external vibrations can be used to do useful work in large-scale devices. These processes look at have happens when bistable systems (e.g., a bent cantilever) are subjected to random forcing from the environment.



1. The papers we read for this program:
  • A Piezomagnetoelastic Structure for Broadband Energy Harvesting (APL2009)
  • A Review of the Recent Research on Vibration Energy Harvesting via Bistable Systems (2013)
  • An Application of Stochastic Resonance for Energy harvesting (JSV2014)
  • An Experimental Study of Stochastic Resonance in a Bistable Mechancial Systems (2012)
  • Bistable Vibration Energy Harvesters (2012)
  • Feasibility of Energy harvesting Using Stocahstic Resonance (2014)
  • Towards Broadband Vibration-Based Energy Harvesting

2. Past and future shows:

3. Bing Crosby, Ac-cent-tchu-ate the Positive:

4. Our subreddit.