A reader emails to ask if I can make sense of this announcement from the European Space Agency:
Scientists funded by the European Space Agency have measured the gravitational equivalent of a magnetic field for the first time in a laboratory. Under certain special conditions the effect is much larger than expected from general relativity and could help physicists to make a significant step towards the long-sought-after quantum theory of gravity.
Just as a moving electrical charge creates a magnetic field, so a moving mass generates a gravitomagnetic field. According to Einstein's Theory of General Relativity, the effect is virtually negligible. However, Martin Tajmar, ARC Seibersdorf Research GmbH, Austria; Clovis de Matos, ESA-HQ, Paris; and colleagues have measured the effect in a laboratory.
Unfortunately, the answer to "can you explain this?" is "Not really." I'll offer some opinions below the fold, though.
The effect they're trying to measure is related to the general relativistic effect called "frame dragging," which I also don't understand, but which involves small changes in the gravitational field of a rotating object. There's a well-known space experiment with the wonderfully poetic name "Gravity Probe B" (honestly, it's like they're not even trying) that's designed to measure this effect for the Earth.
In this case, the rotating object is a ring of superconducting material that they spin up to a very high rotation rate (6500 rpm) very, very quickly. While the rotation rate in increasing, there should be a small graviational effect due to the accelerating motion of the "Cooper pairs" of electrons inside the supercondutor. (The BCS theory of superconductivity involes electrons "pairing up" through the mediation of the solid lattice, in order to form composite bosons that then undergo Bose-Einstein Condensation. If that doesn't make sense, I've got a hand-wave for it that I can post at some later time.).
They can detect the change in gravity with three sets of accelerometers placed near the rotating superconductor. If gravity gets stronger or weaker, that should show up as a slight acceleration of any mass nearby. It's a very slight acceleration-- measured in millionths of the acceleration due to gravity at the Earth's surface-- but it should be there, and be measurable. And, indeed, they seem to see such an acceleration at roughly the times and places that they expect to find an acceleration. It's not clear to me why they only seem to see an effect right at the transition temperature for the superconductor (or even why the temperature is changing during the runs), but I didn't read the paper all that closely.
There's a PDF pre-print on the ESA site (link in the right column of the page linked above) describing the experiment in detail, and they certainly seem to have gone about this in a responsible manner. They repeat the experiment at different temperatures, with different samples (niobium, where they expect a large effect; lead, where thy expect a small effect; and a ceramic high-temperature superconductor, where they expect no effect), and different directions of rotation, and they see acceleration where they expect acceleration, and don't see it where they don't expect it. Reading through the paper, it doesn't look like kookery-- they've done the right tests, and I can't think of any obvious cross-checks they've left out.
But, boy, their data (shown in the image above, click for a slightly larger view) don't make me want to stand up and cheer. The peaks they highlight (marked with arrows) don't obviously stand out from the other peaks in the signal, that are presumably noise. There are peaks there, and they show up in the two accelerometers where they ought to, and not so much in the one where they shouldn't, but... Before I go declaring this proof positive of quantum gravity, I'd sure like to see some more data, and better data. In particular, I'm a little troubled by the reference signal (the bottom data trace), which seems to have spikes in the same places as the biggest spikes in the real signal-- that might be an illusion, and ought to be accounted for in their analysis (the top two traces are the difference between the signals from the accelerometers with a real signal, and the reference signal which should just be mechanical noise), but it makes me uneasy.
Of course, this is the problem with gravity-- it's so damn weak that any experiment looking for gravitational effects is stuck with these crappy, tiny signals (hey to Jeff and Jeff). We'll just have to wait for either some improvements in accelerometer technology, or for some clever person to come up with another way of doing the experiment that generates a cleaner signal.
As for the impact, if it holds up, this would be a big deal. But that's still a big "if" at this point.
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Is this really just GR or some new patina on Podkletnov?
Just a nickpicky comment: calling this "quantum gravity" doesn't seem like the word you want to use (third to last paragraph.)
I'm wondering how close this is to gravitational waves as in LIGO. It seems to me a gravitational wave ought to be a set of oscillating gravitomagnetic and gravitoelectric fields in analogy to (classical) electrodynamics. I don't think LIGO (or any related experiment) has measured an actual signal yet. (Of course there always the indirect dectection of gravitational waves in a binary pulsar.)
Aaron: Is this really just GR or some new patina on Podkletnov?
The pre-print linked from the press release page takes pains to distinguish it from Podkletnov-- the magnitude is much smaller, and there was something about the direction, as well. I'm too lazy to look it up, but they are careful to say that it's not the same thing.
Dave: Just a nickpicky comment: calling this "quantum gravity" doesn't seem like the word you want to use (third to last paragraph.)
I got the quantum gravity connection from their press release. They don't claim it as proof positive, but they say it could be modelled by heavy gravitons, and there's something about how it might be a step toward a quantum theory of gravity.
A: I'm wondering how close this is to gravitational waves as in LIGO.
No clue.
Not terribly close, I don't think, but I don't understand it well enough to say.
Thanks, Dave! I was hoping someone on Mixed States would pick this up and you did a damn find job of this.
s/this.$/it.$/
Why does Dave get all the credit?
If I remember correctly, gravitomagnetism is actually completely different than gravitational radiation. When you split up the GR metric, gravity waves come from the transverse-traceless piece, and I believe that gravitomagnetism represents a completely different mode.
I have no idea how I put down Dave. Anyway.. I'll just be.. uh.. going this way.
I think Jeff thinks your name is Dave, Chad.
Quantum gravity: ah I missed their theory explanation, thanks.
Wow! Thanks for picking this up, Chad. My hopes of an engineering career ended with a thud in calculus 25 years ago, but I've always liked to follow what I could barely understand anyway. For years in my Imagine The World 1,000 Years From Now mode I've pictured the ability to create and control artificial gravity as a source of propulsion ("Ahead warp five, Scotty"). Of course I never expected to see real gains in my lifetime, if it was even theoretically possible, but this article caught my eye.
I'm not sure if your answer tells me if we're poking down that alley or not now, but I appreciate your trying.
Chad,
I do not know what they measure, but their explanation is pretty much nonsense from what I can see.
We know that the photon must have a mass due to Heisenbergs uncertainty principle... What the ...?
The they cite as reference a paper which has mostly zero to do with such a claim.
wolfgang: I do not know what they measure, but their explanation is pretty much nonsense from what I can see.
My background in general relativity is pretty much limited to a "Cosmology and GR for Idiots" class I took as an undergrad, so I have basically no ability to evaluate their theoretical claims. When you start getting into that subject, everything sounds like nonsense to me.
All I can really do is look at what they say about the experimental procedure, and look at the data in general terms. What they say they did sounds like the right stuff, but the data look pretty marginal. But again, gravitational data almost always look marginal.
As someone else said in another forum, "I am so gonna wait for replication on this one."
The paper they cite contains a statement using the energy-time uncertainty relation to say what the best experimental bound on the photon mass could possibly be with all the time in the universe (literally) to measure. I'm guessing the authors of these papers misinterpreted that as saying that the photon actually has to have a mass in that regime.
This brief definition of the gravitomagnetic effect from AIP might help: http://www.aip.org/pnu/1996/split/pnu295-2.htm
There's a graphic here http://www.aip.org/png/html/gravmag.htm , though it seems a bit crude.
I confess to being a little hazy on what "matter currents" are, but it seems the rotation and gravity of Earth have created behaviors in those Cooper pairs consistent with GR predictions. If the results can be verified, this looks like Big News -- gravity has effects analogous to the Faraday effect.
Well, looks like I might be able to put some spin on Faraday's Law when I get around to it in class. (Sorry, I couldn't resist the pun.)
Sorry, I misread the article. It wasn't the Earth's frame-dragging creating the effect, but the gyro's own rotation. So, I'm with Chad here. I'm not so convinced. It could be another cold-fusion hoohah.
Gravity Probe B might provide corroborating evidence, though.
Yeah, the thing is, almost everyone believes that gravitomagnetism exists, but here they're claiming an effect vastly stronger than the standard theory predicts on the basis of data that do not look anything like crystal-clear.
I agree with Dave Bacon about that theory paper, too; playing around with the consequences of Proca electromagnetism and gravity and comparing those to experiment is all very well, but their initial theoretical justifications for using Proca theory in preference to Maxwell and Einstein make no sense at all, as far as I can tell.
It's entirely possible that GR breaks down somehow for Cooper-pair supercurrents on laboratory scales, but it's the kind of thing where you really need extraordinary confirmation.
> It seems to me a gravitational wave ought to be a set of oscillating gravitomagnetic and gravitoelectric fields in analogy to (classical) electrodynamics.
Precisely. The key point is that relativity is a metatheory about how physical theories must behave under a change of reference frame, with electromagnetism as the model.
Mass is the gravitational charge, analogous to the electric charge. (So the "gravitoelectric field" you mention is just the gravitational field.) The differences between gravity and E&M are:
1. Gravity is much weaker.
2. Gravity is only attractive, while electricity can be repulsive.
3. Gravitomagnetic fields have mass, where magnetic fields don't have electric charge. This is the big one.
We all know that a particle increases in mass as its velocity increases. Where is the extra mass stored? In the gravitomagnetic field that is generated by a moving mass.
Why does the mass increase toward infinity as the velocity approaches the speed of light? Because the mass of the gravitomagnetic field is traveling with the particle, and therefore creates its own gravitomagnetic field, which is traveling along with the particle.... The total mass is an infinite sum, which converges for v less than c, but diverges for v equals c. [It converges, in fact, to the familiar m0/sqrt(1-v^2/c^).]
Ack. That's an unusual way of looking at it. Nobody really uses relativistic mass anymore; it's a concept that tends to create more confusion than it solves. Regardless, the relativistic mass formula you give (really it's just energy in slight disguise) holds irrespective of whether gravity's around or not. Perhaps one could come up with some derivation of the formula as you say, but I'm not sure what it would mean.
I'm surprised modulus notation isn't used more; do physicists try their hardest to ignore that little bit of mathematical training?
Nonetheless, I'm a chemist so I'll stand here and stare and occasionally make the right "ooh, aah" noises.