## Thursday, December 21, 2017

### Exoplanets

Episode 19 - Exoplanets has been lost. We continue with our original numbering as we attempt to fix it.
Go to Episode 20 - Time Crystals for the next episode.

### Time Crystals

Jim talks to Randy about structures that are periodic in time like crystals are periodic in space. This idea came from Frank Wilczek in 2012, and was realized just recently, providing an extraordinary turn-around time from theory to observation.

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Notes:

1. The papers we read for this program:

- Quantum Time Crystals, Wilczek,, F.
*Physical Review Letters***109**, 160401 (2012) [arXiv] - Discrete Time Crystals: Rigidity, Criticality, and Realizations,Yao, Potter, Potirniche, and Vishwanath.
*Physical Review Letters***119**030401 (2017) [arXiv] - Observation of Discrete Time Crystal, Zhang, Hess, Kyprianidis, Becker, Lee, Smith, Pagano, Potirniche, Potter, Vishwanath, Yao, and Monroe.
*Nature***543**217 (2017) [arXiv] - Observation of discrete time-crystalline order in a disordered dipolar many-body system, Choi, Choi, Landig, Kucsko, Zhou, Isoya, Jelezko, Onoda, Sumiya, Khemani, von Keyerlingk, Yao, Demler, and Lukin.
*Nature***543**221 (2017) [arXiv]

2. Our subreddit.

------------------------------------------------

The following post was made to the arXiv_plus subreddit about these papers:

This was an interesting one. Frank Wilczek hypothesized, here, that there would be structures that were periodic in time the way crystals are periodic in space. The wave functions, in time, would be similar to the Bloch functions of condensed matter. The character of the wave functions would be a little like solitons, with an attractive nonlinearity balanced by uncertainty-related dispersion. His original model was to look at coupled superconducting rings. The coupling would repeatedly and periodically reproduce the same state.

In Discrete time crystals: rigidity, criticality, and realizations, Yao, et al., showed that the ground state wave function cannot have the periodicity required -- but an excited state could. What you would need to do is produce a Hamiltonian that did three things, successively: orient the system, order the system, and finally randomly disorder it.

Two groups wasted no time at all producing these excited "time crystals," simultaneously publishing in Nature about a year ago. One group looked at what I'd think of as a very artificial system, a few optically-trapped, ultra-cold atoms. In this case each of the effects was programmed by laser interactions. The other group used a real crystal: diamond with nitrogen vacancies at room temperature.

Both groups successfully reproduced the phenomena of Yao's paper. The nature of the Hamiltonians, if I'm free to interpret them, is a successive Zeeman interaction to align the spins of the atoms, an exchange term like the Heisenberg Hamiltonian, and a diffusion term.

There is some similarity to spin echos here, but the effects are much more coherent.

## Wednesday, December 6, 2017

### The 2T Physics of Itzhak Bars

Randy tells Jim about a theory that complements other theories of fundamental physics based upon a phase space symmetry between the 4-position and the 4-momentum of a particle. The upshot of the theory is that there should be a second time dimension and a fourth space dimension, both macroscopic in extent, and the physics we see are 4D projections from the larger 6D space-time.

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Notes:

1. The papers we read for this program:

- A two Time Universe?, by Tom Siegfried on phys.org. This is a popular article on the theory.
- The Standard Model as a 2T Physics Theory, Itzhak Bars. AIP Conference Proceedings 903, 550 (2006) [arXiv]
- Gravity in 2T Physics, Itzhak Bars, Physical Review D77, 125027 (2008) [arXiv]

2. Itzhak Bars lecture on 2T Physics.

3.

*Extra Dimensions in Space and Time*, a book containing a longer, less technical (almost popular) description of 2T physics by Itzhak Bars. That takes up about half the book, the other half is John Terning discussing more traditional extra dimensions in string theory. My review on PhysicsFM is here.

4. Our subreddit.

## 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.

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Notes:

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.

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Notes:

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:

- Physics Frontiers 4: Phononics
- Physics Frontiers 12: Photonic Molecules and Optical Circuits [Metamaterials]
- Physics Frontiers 20: Time Crystals [Coming Soon]

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

4. Our subreddit.

## Saturday, October 21, 2017

### Five Proven Methods of Levitation

Randy shows Jim five different ways in which a body can be levitated: by magnetism, by superconductors, by Lenz' Law, by acoustics, and most recently by thermophoresis.

1. The particular magnetic levitation we talk about is diamagnetic. A very, very large magnetic field induces a large magnetic moment in an object as a reaction. If the diamagnetic component of the object's magnetic susceptibility is larger than its paramagnetic component, the resulting magnetic moment will oppose the field.

Superconductors also expel magnetic fields from their body by the Meissner effect, which makes a "perfect dielectric." This allows the superconductor to levitate.

2. Using Lenz' Law, eddy currents that are induced by time-varying electric fields produce magnetic fields that react with their source, levitating the object.

3. Acoustic levitation occurs when standing sound waves are set up in a region, providing areas where particles can levitate.

4. Optical traps work in a similar way to acoustic levitation with electromagnetic fields instead of pressure fields.

5. Thermophoretic levitation occurs when temperature gradients produce a force that allows small object to float.

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Notes:

1. The papers we read for this program:

- Brandt, E.H., "Levitation in Physics."
*Science***243**349 (1989). - Zuza, Fuisasola, Michelini, and Sani, "Rethinking Faraday's Law for Teaching Motional Electromotive Force."
*European Journal of Physics***33**397 (2012). - Brandt, E.H., "Suspended by Sound."
*Nature***413**474 (2001). - Ashkin, A., "Optical Trapping and Manipulation of Neutral Particles Using Lasers."
*Proceedings of the National Academy of Sciences***94**4853 (1997). - Fung, Usatyuk, DeSalvo, and Chin, "Stable Thermophoretic Trapping of Generic Particles at Low Pressures."
*Applied Physics Letters***110**034102 (2017). [arXiv]

2. Videos of levitating objects:

- Levitating Frog
- Superconducting Levitation
- Lenz' Law
- Acoustic Levitation
- Optical Tweezers
- Thermophoretic Leviatation

3. Our subreddit.

## Saturday, October 7, 2017

### Stochastic Electrodynamics

Is the entire cosmos awash in a sea of invisible energy? Nikola Tesla thought so. And today a few daring theoretical physicists are pioneering the effort to explain the most exotic characteristics of quantum theory by describing the nature of this universal field of energy and its physical consequences. If they’re right, their theory could revolutionize the energy and transportation sectors around the globe, and perhaps even throw open the door to new forms of spaceflight. On this episode of Physics Frontiers, we’ll investigate the theory of stochastic electrodynamics, one of the most intriguing concepts in modern physics and a rising contender to explain the quantum world.

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Notes:

1. The main paper we read for this program: Contribution from Stochastic Electrodynamics to the Understanding of Quantum Mechanics by de la PeÃ±a and Cetto [arXiv]

2. The secondary paper we mentioned in program, predicting spontaneous parametric up-conversion: Non-Locality: The Party May Be Over by Marshall [arXiv]

3. Our subreddit.

## Thursday, September 14, 2017

### Exotic Photon Trajectories in Quantum Mechanics

Jim and Randy discuss strange trajectories observed in triple slit experiments with metallic plates. Photons seem to pass through one slit, come back through the middle slit, and out the third due to their interactions with surface plasmons. There are implications in this experiment about the way in which wavefunctions need to be interpreted in non-relativistic quantum mechanics.

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Notes:

1. The paper we read for this program: Exotic looped trajectories of photons in three-slit interference by MagaÃ±a-Loaiza, et al. [arXiv]

2. Our subreddit.

--------------------------------------------

The following post was made to the arXiv_plue subreddit about this paper:

This paper discusses deviations from the predictions of the Born rule (the interpretation of the probability of an observation being the norm of the convolution of the wave functions of the initial and final states) that are observed when the interference effects of a triple slit experiment are examined. By using a three slit experiment rather than a two slit experiment, the authors follow the procedure of Sinha, et al., that showed a null result in 2010. This procedure looks at all seven combinations of the three slits (in a random order), subtracting the three slit interference pattern from the interference patterns of the other six combinations. The combination of the six double and single slit interference patterns should exactly match the the triple slit interference pattern if the Born rule held.

In 2010, they found that the Born rule held, to one part in 100.

This paper (published in Nature Communications) uses surface plasmons on a gold barrier to "enhance the electromagnetic near-fields" near the slits in a way that increases the likelihood of trajectories that cause violations of the Born rule. These trajectories snake and backtrack through the triple slit apparatus.

The comparisons between the single, double, and triple slit interference patterns indicate evidence for these snaking trajectories. Whether this really indicates evidence that the Born rule fails, I'm not sure.

## Sunday, August 20, 2017

### A Gravitational Arrow of Time

Jim and Randy discuss a cosmological theory that purports to find an explanation for the arrow of time in gravitational theory based on the shape and distribution of matter and how it evolves.

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Notes:

1. The paper we read for this program: Identification of a Gravitational Arrow of Time by Barbour, Kowslowski, and Mercati.

2. Mentioned in this episode: The Physical Basis for the Arrow of Time by H. Dieter Zeh.

3. Mentioned in this episode: Gravity From the Ground Up, B. Shutz.

4. [Update] It came to me in the shower: the word I was looking for is "eschatological."

5. Our subreddit.

## Tuesday, July 18, 2017

### Photonic Molecules and Optical Circuits

Randy tells Jim about photonic molecules, pairs of photons that create bound states like molecules do through a force mediated through an ultracold gas and similar ideas in optical circuits. They also discuss application of the same for quantum computing.

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Notes:

1. The papers we read for this program:

Firstenberg, O., Peyronel, T., Liang, Q.-Y., Gorshkov, A., Lukin, M. and Vuletic, V., "Attractive Photons in a Quantum Nonlinear Medium." Nature 502, 71 (2013)

2. My reservations about this episode. Probably just above this one.

3. Our subreddit.

## Friday, June 30, 2017

### Requirements for Gravitational Theories

In this episode Jim and Randy talk about how to evaluate alternative gravity theories. What sort of things do we want them to explain, what experiments do they have to predict, and what theoretical requirements do they have to meet. This is in some ways a continuation of Episode 9 - f(R) Theories of Gravity, but the discussion is relevant to all attempts to amend gravitational theory.

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Notes:

1. "f(R) Theories of Gravity, by Thomas Sotiriou and Valerio Faraoni". The basis for this discussion and the last program's, as well.

2. The list Randy refers to in the episode. Probably just above this one. 3. Our subreddit.

### List of Requirements for Gravitational Theories

Here it is without fanfare. If you have any questions about any part of it, please comment in the comments:

-------------------------------

To be theoretically consistent and compatible with experiment, a theory of gravitation must:

(1) Predict correct cosmological dynamics

a. Big bang nucleosynthesis

(2) Produce the correct evolution of cosmological perturbations

a. Cosmic microwave background

b. Large scale structure

(3) Have the correct weak-field limits

a. Reproduces Newtonian Mechanics

b. Predicts post-Newtonian experiments in weak field

c. Produces stable solutions

i. Matter-side instabilities (Dolgov-Kawasaki)

1. Ground states should be highly symmetric

ii. Gravity-side instabilities

1. Stable de Sitter solutions

iii. Stability of the first loop in quantum gravity

iv. Stability in the face of inhomogeneous but isotropic perturbations

v. Black hole nucleation

(4) Not contain any ghost fields

(5) Admit a well-posed Cauchy problem

(6) Reasonable theory of gravity waves

-------------------------------

To be theoretically consistent and compatible with experiment, a theory of gravitation must:

(1) Predict correct cosmological dynamics

a. Big bang nucleosynthesis

(2) Produce the correct evolution of cosmological perturbations

a. Cosmic microwave background

b. Large scale structure

(3) Have the correct weak-field limits

a. Reproduces Newtonian Mechanics

b. Predicts post-Newtonian experiments in weak field

c. Produces stable solutions

i. Matter-side instabilities (Dolgov-Kawasaki)

1. Ground states should be highly symmetric

ii. Gravity-side instabilities

1. Stable de Sitter solutions

iii. Stability of the first loop in quantum gravity

iv. Stability in the face of inhomogeneous but isotropic perturbations

v. Black hole nucleation

(4) Not contain any ghost fields

(5) Admit a well-posed Cauchy problem

(6) Reasonable theory of gravity waves

## Friday, June 2, 2017

### f(R) Theories of Gravity

Jim and Randy discuss gravitational theories that modify general relativity by changing the action using a polynomial dependence on the Ricci scalar. Although not physically motivated, some of these theories produce effects similar to those of dark matter, dark energy, and cosmological constants.

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Notes:

1. "f(R) Theories of Gravity, by Thomas Sotiriou and Valerio Faraoni". The basis for this discussion and the next program's, as well.

2. Our subreddit.

## Friday, May 5, 2017

### Vacuum Fluctuations and the Casimir Effect

What are the latest theoretical and experimental developments with the Casimir effect, and are vacuum fluctuations physical, or simply a convenient model for calculating the effects of quantum noise in the fields of matter? On this episode of Physics Frontiers, we explore some of the surprising discoveries that physicists have made unraveling the strange dynamics of mechanical components so small that they can’t even be seen through an optical microscope. And along the way we consider some of the fundamental mysteries of quantum field theory, as applied physics enters the realm of nanotechnology…and approaches exotic new possibilities like quantum levitation.

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Notes:

(1) Materials Perspective on Casimir and van der Waals Interactions by Woods, et al., the basis for this discussion.

(2) Our Subreddit

## Tuesday, March 14, 2017

### Virtual Graviational Dipoles

Randy discusses what the Cosmological implications of a negative gravitational mass would be with Jim. If there were a negative gravitational mass (as opposed to inertial mass), then every time that an electron-positron pair was created in the vacuum, that would create a gravitational dipole. This in turn would create effects similar to dark matter, dark energy, and a cosmological constant -- and this in turn would have an effect on the origin of the universe.

Links:

Virtual Gravitational Dipoles: The Key for the Understanding of the Universe? by Dragan Hajducovic. The paper we discuss in the podcast.

The reddit.

## Wednesday, February 15, 2017

### General Relativity for the Experimentalist

Randy shares a couple of his favorite papers with Jim: discussion about general relativity by engineer and science fiction author Robert L. Forward on how general relativity could be used in a terrestrial environment, including proposals for devices and materials.

Show notes:

Robert L. Forward on Wikipedia.

General Relativity for the Experimentalist in the Proceedings of the IRE (on IEEE Xplore).

Guidelines to Antigravity in the American Journal of Physics (Scitation.org) (alternate).

Previous shows mentioned:

G4V: The Gravitational 4-Vector Formulation of Gravity

Gravitoelectromagnetism

Show notes:

Robert L. Forward on Wikipedia.

General Relativity for the Experimentalist in the Proceedings of the IRE (on IEEE Xplore).

Guidelines to Antigravity in the American Journal of Physics (Scitation.org) (alternate).

Previous shows mentioned:

G4V: The Gravitational 4-Vector Formulation of Gravity

Gravitoelectromagnetism

## Thursday, January 26, 2017

### Pilot Wave Hydrodynamics

In this episode, Randy and Jim discuss pilot wave hydrodynamics, a physical analogy to quantum mechanics. In it, a small droplet bounces atop a fluid, interacting with its own wave. This allows macroscopic experiments which display many of the properties of quantum mechanics, such as self-interference in a doubles slit experiment. This is equivalent to the de Broglie-Bohm interpretation of quantum mechanics that Jim and Randy talked about in the second episode and a little in the main podcast, as well as the quantum interpretations special (that currently needs re-editing).

Show Notes:

The review paper by John W. M. Bush that forms the basis of the discussion.

The videos by Veritasium and Smarter Every Day that we refer to in the podcast that came out while I was editing second episode and just before we recorded this.

## Wednesday, January 4, 2017

### Phononics

Randy tells Jim about the emerging field of Phononics: using quantum particles of heat in materials for information processing in advanced materials.

Notes:

The Review Article that we discussed here.

A reasonable YouTube speech on phononics,

*Prof Ben Eggleton - Phononics, the next wave*.

Discuss on our subreddit.

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