Greetings from Toronto, where I’m visiting UofT to talk about dark energy, the arrow of time, and other obsessions of mine. Which has prevented me from as yet writing the long-awaited second installment of “Unsolicited Advice,” the one that will tell you how to choose a graduate school. It is that time of year, after all.
In the meantime, check out this nice post at Anthonares on Lunar laser ranging. The Apollo astronauts, during missions 11, 14, and 15, were sufficiently foresighted to bring along reflecting corner mirrors and leave them behind on the Moon’s surface. Why would they do that? So that, from down here on Earth, we can shoot lasers at the lunar surface and time how long it takes for them to come back. Using this data we can map the Moon’s orbit to ridiculous precision; right now we know where the Moon is to better than a centimeter. This experiment, called Lunar laser ranging, teaches us a lot about the Moon, but it also teaches us about gravity. The fact that we can pinpoint the location of the Earth’s biggest satellite and keep track of it over the course of years provides us with a uniquely precise test of Einstein’s general relativity.
You might think that general relativity is already pretty well tested, and it is, but clever folks are constantly inventing alternatives that haven’t yet been ruled out. One example is DGP gravity, invented by Gia Dvali, Gregory Gabadadze, and Massimo Porrati. This is a model in which the observable particles of the Standard Model are confined to a brane embedded in an infinitely large extra dimension of space. Unlike usual models with compact extra dimensions, the extra dimension of the DGP model is hidden because gravity is much stronger in the bulk; hence, the gravitational lines of force from an object on the brane like to stay on the brane for a while before eventually leaking out into the bulk.
The good news about the DGP model is that it makes the universe accelerate, even without dark energy! This is one of the things that I talked about at my colloquium yesterday, and I hope to post about in more detail some day. The better news is that it is potentially testable using Lunar laser ranging! The claim is that the DGP model predicts a tiny perturbation of the Moon’s orbit, too small to have yet been detected, but large enough to be within our reach if we improve the precision of existing laser ranging experiments. People are hot on the trail of doing just that, so we may hear results before too long.
Not to get too giddy, the bad news about DGP is that it may be a non-starter on purely theoretical grounds. There are claims that the model has ghosts (negative-energy particles), and also that it can’t be derived from any sensible high-energy theory (see Jacques’s post). I haven’t examined either of these issues very closely, although I hope to dig into them soon. Maybe if I could quite traveling and sit down and read some papers.
No gravitation theory uses composition. The Nordtvedt effect (lunar laser ranging) nulls. Other Equivalence Principle (EP) violations fail: Hyper-bound (30% of stoichiometric mass as gravitational binding energy), hyper-spinning (equatorial 20% of lightspeed), hyper-magnetized (10^8 tesla), superconducting neutronium binary pulsars orbit as expected.
Properties must couple to translation or rotation for measurable EP violation. Only proper orthochronous Lorentz symmetries and discrete symmetries (not charge parity) are generators. Measureable EP parity violation is allowed by theory (disjoint non-overlap of metric vs. teleparallel gravitation) and prior observation.
Do left and right hands fall identically?
Parity can be calculated for any mass distribution with overall finite moments of inertia and countable points therein. Point composition is empirically EP-inert. Only spatial distribution is active.
alpha-Quartz crystallizes in parity space groups P3(1)21 (right-handed screw axes) or P3(2)21 (left-handed screw axes). Single crystal quartz is intensely parity divergent with increasing radius; emergent scale = 0.323 nm diameter sphere. Eötvös balance test masses are 2 cm in diameter or 2.4×10^24 larger in volume.
Single crystal alpha-quartz is commercially grown. Gold plated solid sphere P3(1)21 vs. P3(2)21 single crystal test masses are indistinguishable. Parity Eötvös experiments are perfect classical nulls. EP parity violation is explicit. Chemical PVED searches limit divergence to under 10^(-10) difference/average, 1000 times the detection limit.
Is there an EP parity violation? Somebody should look,
http://www.mazepath.com/uncleal/qz3.pdf
‘… clever folks are constantly inventing alternatives…’
Sean, thanks for that link to the DGP paper, and thanks for calling them “clever folks”, not “free-thinkers” etc.
“We discuss the idea that the accelerated Universe could be the result of the gravitational leakage into extra dimensions on Hubble distances rather than the consequence of non-zero cosmological constant.” – http://arxiv.org/abs/hep-th/0005016
This is the kind of investigation which is respectable; it’s a risk-taking scientific model that could be wrong. Abstract stringy-type models of unobservables (which take no real risk of being wrong) are boring. Let’s hope the Lunar ranging produces fuzzy results which shore up funding for extra dimensional theorists a bit longer. 😉
How’s this for another non-starter (probably?)—a measurable gravitomagnetic field produced by a superconductive gyroscope. The report is from the European Space Agency.
From your Chris W’s reference,
Physical existence of gravitons aside, Gravity Probe B was two pairs of anti-parallel superconductive 4300 rpm gyroscopes free falling in a 0.7742 rpm housing. No anomaly vs. housing or reference star was detected over 352 days of free fall. Cf: Podkletnov.
Binary pulsars orbit to General Relativity specs within observational error. A pair of 1.4 solar mass superconductng gyroscopes with equatorial surface velocities of 20% lightspeed bearing huge magnetic fields and observed over months gave no anomalies,
http://relativity.livingreviews.org/Articles/lrr-2006-2/
http://relativity.livingreviews.org/Articles/lrr-2005-7/
Do left and right hands locally vacuum free fall identically? Is there a chiral pseudoscalar vacuum background in the mass sector? A parity Eötvös experiment compares calculated opposite parity space group otherwise exactly identical single crystal test masses: same chemical composition, density, shape, mass, nuclear properties… for every classical and quantum physics observable. The atomic lattices are non-superposable mirror images that calculate to within 10^(-15) relative of maximal theoretical parity divergence for 1 cm diameter alpha-quartz test masses. There is no basis for non-null output other than EP parity violation by chiral mass distributions.
EP parity violation could be 1000X detection sensitivity without contradicting prior observation in any venue at any scale. One is amazed that academic physicists refuse to look. Parity violation is the only allowed EP violation at 10^(-13) difference/average sensitivity.
Unfortunately some people did not know that such proposals were on the table, while they spoke sourly about things, while the rigours of testability on models were being used.
Varney’s work for instance. 🙂
Just two more points.
Weak field measures have not in any way dissuaded one from see applicable understanding using Grace, and in the time variable arena, our views have changed?
That doesn’t go away.
Lagrangian perspectve on Earth sun relations hasn’t changed either, and hence our views of that gravitational perspective as well?
The “high energy sector” (strings) if seen within the context of being wrapped(Coleman-De Luccia instanton), would still implore and I speculate, the need for gravitational perspective? If weak, still consistent with high energy perspectives?
I very much liked the way Adams et al formulated the situation. Instead of the usual Distler-style gleeful trashing of other people’s work, they say “Conversely, any experimental support for the DGP model, or measured negative signs for anomalous quartic gauge boson couplings at future accelerators, would constitute direct evidence for the existence of superluminality and macroscopic non-locality unlike anything previously seen in physics…” and they leave it to you to judge how likely you think that is….also, when reading JD’s little piece, make sure not to miss Nima A-H firmly giving JD a taste of his own medicine……
Tom Murphy (UCSD) has a new lunar laser ranging experiment running on the Apache Point 3.5m in New Mexico. Pictures of first light are at:
http://physics.ucsd.edu/~tmurphy/apollo/first_lt.html
Besides the importance of testing theories of gravity, yada yada yada, how freakin’ cool is it to be shooting laser beams at the moon!
With Time Variable measures, progress pushing perspective, is still very exciting. LIGO, and ideas of “resonance curve?” are just “continued ways” that we are exercising our mind when we look at the “spacetime fabric.” :)Oui! Non?
Wonderful indeed, that 35 years in duration, and we can find “new uses” for these experiments.
There are plans for building radio telescopes on the moon that, combined with telescopes on earth, will allow astronomers to do interferometry. Lunar ranging measurements will then become much more accurate.
The most accurate way to determine relative distances on Earth is not via the GPS system, but rather by doing observations of distant quasars using the VLBI system of radio telescopes, see here.
On “Gauss’s mountain,” more on name.
A “quick” historical summation is always nice when looking at this “graduation process” of the interferometer, to present day aspirations in astronomical proportions. Thanks Count
How much bigger is LIGO then it’s counterpart in Michelson interferometer. Kip Thorne as Wheeler’s protege, contributed greatly as a well as a proportional measure, according to the size of LIGO?
Wonderful.