New blog

Announcing my new (tentative) blog, Toy Universes.

News on Gravity Probe B – Nov. 2009

Read mission status below:

Closing in on Einstein: Frame-Dragging Clearly Visible

News on Garrett Lisi’s E8 theory

Lisi posted this yesterday over at Physics Forums; I reproduce here:

Hello PF folk.

If you believe the Dirac equation in curved spacetime, and you believe Spin(10) grand unification, then a Spin(3,11) GraviGUT, acting on one generation of fermions as a 64 spinor, seems… inevitable.

Also, it’s pretty.

And it’s up to you whether or not to take seriously or not the observation that this whole structure fits in E8. Personally, I take it seriously. Slides are up for a talk I gave at Yale:

http://www.liegroups.org/zuckerman/slides.html

Best,
Garrett

I am not certain whether it addresses Distler’s previous objections (as I am not certain whether the issue was even settled at that time– see here and here, which goes as far as I could follow. More (older) personal opinions can be found here, here and here in reverse chronological order).

Edit: I forgot to add. I do find the theory beautiful and interesting. I hope it can be properly tested.

Edit: Here are further links that are relevant to this post.

There is no “Theory of Everything” inside E8 by Jacques Distler and Skip Garibaldi.

Here is Distler’s blog entry on his paper.

There was a discussion of Distler and Garibaldi’s paper at Physics Forums some time ago.

There was also a discussion at n-Category Café some time ago.

God said “Let Penrose be” and all was wrong

Roger Penrose Says: Physics Is Wrong, From String Theory to Quantum Mechanics.

News from LIGO

An upper limit on the stochastic gravitational-wave background of cosmological origin
The LIGO Scientific Collaboration & The Virgo Collaboration
Nature 460, 990-994 (20 August 2009)

Abstract

A stochastic background of gravitational waves is expected to arise from a superposition of a large number of unresolved gravitational-wave sources of astrophysical and cosmological origin. It should carry unique signatures from the earliest epochs in the evolution of the Universe, inaccessible to standard astrophysical observations. Direct measurements of the amplitude of this background are therefore of fundamental importance for understanding the evolution of the Universe when it was younger than one minute. Here we report limits on the amplitude of the stochastic gravitational-wave background using the data from a two-year science run of the Laser Interferometer Gravitational-wave Observatory (LIGO). Our result constrains the energy density of the stochastic gravitational-wave background normalized by the critical energy density of the Universe, in the frequency band around 100 Hz, to be <6.9 times 10^{-6} at 95% confidence. The data rule out models of early Universe evolution with relatively large equation-of-state parameter, as well as cosmic (super)string models with relatively small string tension that are favoured in some string theory models. This search for the stochastic background improves on the indirect limits from Big Bang nucleosynthesis and cosmic microwave background at 100 Hz.

Update: Now freely available in the arxiv. [0910.5772]

News from Fermi (formerly GLAST)

[Via Backreaction].

[arxiv:0908.1832]

Testing Einstein’s special relativity with Fermi’s short hard gamma-ray burst GRB090510
Authors: Fermi GBM/LAT Collaborations

Abstract: Gamma-ray bursts (GRBs) are the most powerful explosions in the universe and probe physics under extreme conditions. GRBs divide into two classes, of short and long duration, thought to originate from different types of progenitor systems. The physics of their gamma-ray emission is still poorly known, over 40 years after their discovery, but may be probed by their highest-energy photons. Here we report the first detection of high-energy emission from a short GRB with measured redshift, GRB 090510, using the Fermi Gamma-ray Space Telescope. We detect for the first time a GRB prompt spectrum with a significant deviation from the Band function. This can be interpreted as two distinct spectral components, which challenge the prevailing gamma-ray emission mechanism: synchrotron – synchrotron self-Compton. The detection of a 31 GeV photon during the first second sets the highest lower limit on a GRB outflow Lorentz factor, of >1200, suggesting that the outflows powering short GRBs are at least as highly relativistic as those powering long GRBs. Even more importantly, this photon sets limits on a possible linear energy dependence of the propagation speed of photons (Lorentz-invariance violation) requiring for the first time a quantum-gravity mass scale significantly above the Planck mass.

Edit: Discussions are also going on over at Physics Forums.

Einstein on axiomatization of physics

Here is the Einstein’s quote that I have previously promissed to post:

If, then, it is true that the axiomatic basis of theoretical physics cannot be extracted from experience but must be freely invented, can we ever hope to find the right way? Nay, more, has this right way any existence outside our illusions? Can we hope to be guided safely by experience at all when there exist theories (such as classical mechanics) which to a large extent do justice to experience, without getting to the root of the matter? I answer without hesitation that there is, in my opinion, a right way, and that we are capable of finding it. Our experience hitherto justifies us in believing that nature is the realization of the simplest conceivable mathematical ideas. I am convinced that we can discover by means of pure mathematical constructions the concepts and the laws connecting them with each other, which furnish the key to the understanding of natural phenomena. Experience may suggest the appropriate mathematical concepts, but they most certainly cannot be deduced from it. Experience remains, of course, the sole criterion of the physical utility of a mathematical construction. But the creative principle resides in mathematics. In a certain sense, therefore I hold it true that pure thought can grasp reality, as the ancients dreamed.

[my italics]

(Einstein, 1954, Ideas and Opinions, quoted from Schweber, “Einstein and Oppenheimer: the meaning of genius”)

I think the above quote by Einstein is remarkable is several ways, specially the privileged role of creativity and freedom of the human mind expressed in mathematical language, but at the same time the recognition of experience as the sole criterion of the physical utility of such mathematical constructions.

(The above quote, according to Schweber, was an observation by Einstein on Hilbert’s program of axiomatization of physics.)

How Far Are We from the Quantum Theory of Gravity?

How Far Are We from the Quantum Theory of Gravity? [arxiv:0907.4238]

R. P. Woodard (University of Florida)

Abstract: I give a pedagogical explanation of what it is about quantization that makes general relativity go from being a nearly perfect classical theory to a very problematic quantum one. I also explain why some quantization of gravity is unavoidable, why quantum field theories have divergences, why the divergences of quantum general relativity are worse than those of the other forces, what physicists think this means and what they might do with a consistent theory of quantum gravity if they had one. Finally, I discuss the quantum gravitational data that have recently become available from cosmology.

Comments: 106 page review article solicited by Reports on Progress in Physics

FQXi prizes: not my time…

FQXi announced the prizes for the Essay Contest on the Nature of Time. Results are here.

My essay was not awarded.

News on Gravity Probe B

Interesting results coming from Gravity Probe B, including a better understanding of the discrepancies between the four gyroscopes, leading to better data on geodetic effect and frame dragging effect in all four gyroscopes.

Also, a complete document “Gravity Probe B Science Results—NASA Final Report” is now available from their site.

Solvay Physics Conference 1927

Unconventional computing

Just received.

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THE SCIENCE AND PHILOSOPHY OF UNCONVENTIONAL COMPUTING (SPUC09)

Cambridge (UK), March 23-25, 2009

SECOND CALL FOR PAPERS

We welcome submissions on topics normally classified under ‘natural computing’ or ‘unconventional computing’ or ‘hypercomputing’ including (but not restricted to) quantum computing, relativistic computing, biology-based computing, analogue computing, and also submissions on the philosophical implications of these new fields for topics including (but again not restricted to) philosophy of mind, philosophy of mathematics, the Church-Turing thesis.

Each presentation should last no more than 30 minutes; a further 10 minutes will be allowed for discussion.

Those wishing to make a presentation should submit by email a 250-word abstract of their paper to Mark Hogarth (mhogarth@cantab.net); enquiries to the same.

Registration fee (yet to be fixed) will be around £100.

Student bursaries are available.

Conference website: http://web.mac.com/mhogarth/Site/SPUC_Conference.html

ORGANISER

Mark Hogarth (Cambridge, UK)

CONFIRMED INVITED SPEAKERS

Selmer Brinsjord (New York, USA))

Jeff Barrett (Irvine, USA)

Philip Welch (Bristol, UK)

Tim Button (Harvard, USA)

Cristian Calude (Auckland, New Zealand))

István Németi (Budapest, Hungry)

Benjamin Wells (San Francisco, USA)

Hajnal Andréka (Budapest, Hungry)

Apostolos Syropoulos (Xanthi, Greece)

Susan Stepney (York, UK)

Bruce MacLennan (Tennessee, USA)

Peter Kugel (Boston, USA)

Mark Sprevak (Cambridge, UK)

Selim Akl (Kingston, Canada)

José Félix Costa (Swansea, UK)

ADVISORY PANEL

Mike Stannett (Sheffield, UK)

John Tucker (Swansea, UK)

Barry Cooper (Leeds, UK)

Sponsored by EPSRC through HyperNet (the Hypercomputation Research Network, EP/E064183/1)

Something I’d like to do if I were younger….

Just received.

…………………………………………………….

Dear Christine,

I am writing to ask for your assistance in drawing the attention of exceptional, highly motivated students to the Perimeter Scholars International (PSI) program.

PSI is an innovative, Masters level course designed to prepare students for cutting-edge research in theoretical physics. It provides a broad overview, allowing students to choose their preferred specialisation, and extensive tuition in formulating and solving interesting problems.

The due date for applications is February 1st: applications received after this date may still be considered but only as long as places remain available.

A number of outstanding lecturers have already signed up to teach, including for example Yakir Aharonov, Phil Anderson, Matt Choptuik, Nima Arkani-Hamed, John Cardy, Ruth Gregory, Michael Peskin, Sid Redner, Xiao-Gang Wen, and a number of Perimeter Institute research faculty. They will be supported by full-time tutors dedicated to the course.

All accepted students will be fully supported.

For further details, see www.perimeterscholars.org.

Thank you in advance for helping us to make this exciting opportunity known as widely as possible.

With my best wishes,

Neil Turok

Director
Perimeter Institute for Theoretical Physics
Waterloo, Ontario, Canada

Black Holes and Loop Quantum Gravity

I have just received this message from the International Loop Quantum Gravity Seminar mailing list:

Valencia, March 26-28, 2009

The Workshop on Black Holes and Loop Quantum Gravity will take place in Valencia, Spain, from the 26th to the 28th of March, 2009. The purpose of the workshop is to bring together researchers working on quantum aspects of black holes, with emphasis on ideas that have originated in loop quantum gravity. A partial list of topics to be covered is as follows:

– Black hole entropy in LQG
– Spin foam approach to black holes
– Singularity resolution and information loss
– Prospects for a detailed description of the Hawking radiation
– Comparison between results from LQG and other approaches

This will be a ‘Discussion Workshop’. Therefore a significant time will be set aside for a critical evaluation of ideas that are being pursued in current research and on finding fertile directions for future work.

Interesting papers by Salisbury et al.

For my record, I list here some interesting papers by Salisbury et al. which cover fundamental questions of relevance on, should I say, “pre-” quantum gravity matters.

The lines of research are summarized as follows:

- preservation of general coordinate transformation and additional gauge symmetries in the transition from a Lagrangian to a Hamiltonian description;
- the nature of observables in classical general relativity, and their potential usefulness in the construction of an eventual quantum theory of gravity;
- construction of diffeomorphism invariants (observables) in general relativity;
- history of constrained hamiltonians.

The papers of relevance are the following:

- Realization in phase space of general coordinate transformations [Phys. Rev. D 27, 740, 1983];

- Gauge transformations in the Lagrangian and Hamiltonian formalisms of generally covariant theories [gr-qc/9612037 = PRDvol55,no2,658,1997]: establishes the general framework in which gauge variables are retained as canonical variables;

- Reduced phase space: quotienting procedure for gauge theories [math-ph/9811029]: describes an alternative algorithm to the Dirac-Bergmann constraint procedure for constructing a self-consistent Hamiltonian model;

- Gauge group and reality conditions in Ashtekar’s complex formulation of canonical gravity [gr-qc/9912085]: discusses Ashtekar’s complex connection approach to gravity;

- Gauge Transformations in Einstein-Yang-Mills Theories [gr-qc/9912086]: discusses gauge symmetries in Einstein-Yang-Mills models;

- The gauge group in the real triad formulation of general relativity [gr-qc/9912087]: discusses a real triad version of canonical gravity;

- Gauge symmetries in Ashtekar’s formulation of general relativity [gr-qc/0004013]: proposes a gauge averaging procedure modeled after an approach of Rovelli’s, though retaining gauge variables and recognizing the essential distinction between time evolution and realizeable canonical gauge symmetries;

- Quantum general invariance and loop gravity [gr-qc/0105097]: preliminary exploration into the construction of diffeomorphism invariants using dynamical field-dependent finite gauge transformations;

- Quantum General Invariance [Proceedings of the Ninth Marcel Grossmann Meeting held in Rome in 2000 ("Quantum general invariance", in Proceedings of the Ninth Marcel Grossmann Meeting, edited by V.G. Gurzadyan, R. T. Jantzen and R. Ruffini, (World Scientific, New Jersey, 2002), 1300-1301)]: continued the exploration of finite gauge transformations;

- The issue of time in generally covariant theories and the Komar-Bergmann approach to observables in general relativity [ gr-qc/0503013 = Phys.Rev. D71 (2005) 124012]: constructs local invariants through the use of intrinsic coordinates. This can be accomplished in the canonical framework in general relativity using Weyl curvature scalars, as was first suggested by Komar and Bergmann. One essential new observation in this work is the recognition that gauge variables become functionals of the non-gauge variables, and consequently in the quantum theory they become subject to fluctuations. In particular, in canonical quantum gravity the light cone is itself fluctuating [the authors supposedly show that there is no conceptual problem for the canonical formulation of generally covariant theories because the mathematical identification of the Hamiltonian as a gauge generator is erroneous (resolution of the time evolution versus gauge problem)];

- Rosenfeld, Bergmann, Dirac and the Invention of Constrained Hamiltonian Dynamics [physics/0701299]: In a paper appearing in Annalen der Physik in 1930 Leon Rosenfeld invented the first procedure for producing Hamiltonian constraints. He displayed and correctly distinguished the vanishing Hamiltonian generator of time evolution, and the vanishing generator of gauge transformations for general relativity with Dirac electron and electrodynamic field sources. Though he did not do so, had he chosen one of his tetrad fields to be normal to his spacetime foliation, he would have anticipated by almost thirty years the general relativisitic Hamiltonian first published by Paul Dirac.

Links will be added later [Edit: almost all included now]. It would be interesting to follow these matters in light of recent advances in canonical quantum gravity. Marcus over at Physics Forums have prepared a selection (actually, an invitation to a poll) of papers published in the arxiv in 2008 in canonical quantum gravity.

On the Nature of Time — essay competition

I have submitted an essay to the FQXi competition. If you are interested in reading it, click here.

Title: On the Nature of Time – Or Why Does Nature Abhor Deadlocks?

Essay Abstract

This essay aims at introducing a novel point of view on the nature of time, inspired by a synthesis of three seemingly unrelated concepts: Bergson’s notion of duration, Dijkstra’s notion of concurrency, and Mach’s notion of inertia.

Edit (June 9th 2009): Apparently, the essays on the nature of time are no longer available at the FQXi site. I have made a very few small corrections and modifications in my essay and a new version is available here (pdf file).

Update on Gravity Probe B

I have just received the news letter below, which I fully reproduce for your convenience. I am particularly glad to read that the program continues, and I am quite interested to learn about their peer-reviewed report, which will be released after the upcoming workshop on “The Nature of Gravity: Confronting Theory and Experiment in Space”. (I could not find the link to the workshop’s homepage).

The news and other information of interest follow below.

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================================
GP-B STATUS UPDATE — September 26, 2008
================================

Since our May 23rd status update, GP-B has continued to make significant progress–fiscal and scientific. NASA funding and sponsorship of the program ends on September 30, 2008, but GP-B has secured alternative funding that will enable our science team to continue working at least through December 2009 in order to complete the data analysis and bring GP-B to a proper close.
The GP-B science team is continuing to make large strides in the data analysis. On Friday, August 29, 2008, the 18th meeting of our external GP-B Science Advisory Committee was held at Stanford to report our progress since the previous SAC meeting in November 2007. The ensuing SAC report to NASA states:

“The progress reported at SAC-18 was truly extraordinary and we
commend the GPB team for this achievement. This has been a heroic
effort, and has brought the experiment from what seemed like a state
of potential failure, to a position where the SAC now believes that they
will obtain a credible test of relativity, even if the accuracy does not
meet the original goal. In the opinion of the SAC Chair, this rescue
warrants comparison with the mission to correct the flawed optics
of the Hubble Space Telescope, only here at a minuscule fraction
of the cost.” –SAC #18 Report to NASA

On October 6-10, six GP-B team members have been invited to present these dramatically improved, interim results at an International Space Science Institute (ISSI) workshop on “The Nature of Gravity: Confronting Theory and Experiment in Space” to be held in Bern, Switzerland. Following the Berne workshop, these improved interim results will undergo a thorough peer-review and vetting; then towards the end of this year, we plan to announce them publicly.

We very much appreciate your continued interest in GP-B, and we will keep you posted on our progress in future status updates.
===================
PREVIOUS GP-B UPDATES
===================
If you wish to read any of our previous updates, our GP-B Web site includes a chronological archive of all the updates/highlights (with photos and drawings) that we have posted over the past 8 years: http://einstein.stanford.edu/highlights/hlindexmain.html

=============================
OTHER LINKS THAT MAY INTEREST YOU
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* Our NEW AND IMPROVED GP-B Web site, http://einstein.stanford.edu contains lots of information about the Gravity Probe B experiment, general relativity, and the amazing technologies that were developed to carry out this experiment.

* Video and/or audio of May 18, 2006 public lecture by Principal Investigator, Professor Francis Everitt, on GP-B. You can view a Flash video of the lecture in your Web browser: http://einstein.stanford.edu/Media/Everitt_Brainstorm-flash.html You can also download either a video or audio only copy of the lecture to an iPod from the Stanford University iTunes U Web site: http://itunes.stanford.edu, This Web page automatically launches the Apple iTunes program on both Macintosh and Windows computers, with a special Stanford on iTunes U “music store,” containing free downloads of Stanford lectures, performances, and events. Francis Everitt’s “Testing Einstein in Space” lecture is located in the Faculty Lectures section. People with audio-only iPods can download the version under the Audio tab; people with 5th generation (video) iPodfs can download the version under the Video tab.

* Visual tour of the GP-B spacecraft and payload from our GP-B Web site: http://einstein.stanford.edu/content/vehicle_tour/index.html

* PDF file containing a 1/20 scale, paper model of the GP-B spacecraft that you can download print out, and assemble: http://einstein.stanford.edu/content/paper_model.

* NASA’s Marshall Space Flight Center also has a series of Web pages devoted to GP-B: http://www.gravityprobeb.com

* The Harvard-Smithsonian Center for Astrophysics (Cambridge) and York University (Toronto), with contributions from the Observatoire de Paris, have been studying the motions of the guide star, IM Pegasi for over a decade. To find out more, visit: http://www.yorku.ca/bartel/guidestar/. In addition, you’ll find information in the Extraordinary Technologies page-Telescope & Guide Star section on our Web site: http://einstein.stanford.edu/TECH/technology1.html#telescope

==========================
ABOUT THE GPB-UPDATE EMAIL LIST
==========================
The email distribution list for this GP-B Weekly Highlights update is maintained on the new Stanford University Mailman lists server.

To subscribe to this list, send an email message to:
gpb-update-join@lists.stanford.edu
The subject and body of the message will be ignored, so it doesn’t matter what you put there.

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Edit: Link to Gravity Probe B Scientific Papers