View Full Version : Theoretical Physics Question - Fat Gravitons

11-17-2005, 10:00 AM
This summer I was reading a science magazine in my dentist's waiting room, and there was a brief article on new research into gravity. I read it rather quickly, and I'm not sure if I remember correctly, but it was something like this (I'm not a scientist, so please correct if I'm gettting this wrong):

The force of gravity is measurable and consistent, except at a microscopic level, where anomalies are observed. One hypothesis is that there exists a particle called a fat graviton that causes these anomalies. The particle had not been discovered as of the date of the article, but conclusive proof of its existence or lack thereof was anticipated this year.

I'm not sure if I'm remembering the size of this correctly, but it seemed to me at the time that the size was very, very large, which made me wonder why it had never been observed before this.

Since then, I haven't heard anything else about this. Was this disproved?

11-17-2005, 11:54 AM
Here's what I think you're asking about; let me know if I'm completely off-base:

General relativity (or hell, even Newton's theory) tells us that the force due to gravity should be inversely proportional to the square of the distance. However, the experiments that have lead us to believe this all involve very large distances, like the size of the Solar System or the Earth. Further, some versions of string theory predict that the force of gravity should deviate from this inverse-square behaviour at small scales -- on the order of tens of microns. These deviations can be viewed as coming from the "fat gravitons" that you mentioned.

As for whether they've been proven or disproven, I believe that the Eot-Wash group is the main group studying this kind of thing. (They're at the University of Washington, IIRC.) As of a year and a half ago they had observed no deviation from inverse-square behaviour (http://arxiv.org/abs/hep-ph/0405262) down to a scale of 100 microns or so; but all that these theories tell us is that these "fat gravitons" should start to affect things above a scale of 20 microns, so that's not conclusive. The Eot-Wash group is still running experiments, though, so who knows what will happen?

Of course, my info might be out of date, too; other people who have a better idea what's going on in the field (Chronos?) might want to chime in here.

11-17-2005, 12:22 PM
April's Physics World had a sober article (http://physicsweb.org/articles/world/18/4/6/1) on the experimental situation and some of the theoretical predictions of deviations.

Luboš Motl's string-theory blog shortly thereafter reported a rumour (http://motls.blogspot.com/2005/06/deviations-from-newtons-law-seen.html) that the Eot-Wash group have seen a deviation. That's still merely gossip at this stage.

The area's definitely worth keeping an eye on, but I haven't seen anything more recent than that.

11-17-2005, 12:49 PM
Thanks, folks.

I really don't understand a lot of this intuitively, but it fascinates me and eventually some of it sinks in.

I have a feeling I'll be spending a bit of my work day on the Physics Web site. :)

Thanks again,

11-17-2005, 05:38 PM
To start with, no deviations from inverse-square behaviour have yet been detected. At the very smallest scales, it's exceedingly difficult to detect gravity at all, much less measure it. It is, however, sometimes hypothesised that there might be deviations at some scales, and experiments are sometimes designed to detect these deviations (only sometimes, because sometimes the theoretically-predicted deviations are far beyond the capabilities of current experiments).

The most commonly-discussed models which predict deviations from inverse-square behaviour at small distances are those involving extra dimensions. In such models, space has extra dimensions, which wrap in on themselves at scales possibly as large as a tenth of a millimeter. However, only gravity can "feel" these extra dimensions, not any of the other particles or forces. The effect is that at small scales (smaller than the size of the extra dimensions), gravity would spread out in 3+n spatial dimensions, and therefore fall off as 1/rn+2. At larger scales, however, due to these extra dimensions being rolled up, the gravitational force would appear as our normal 1/r2 (it's already spread out as much as it can in the extra dimensions). And anything other than gravity would always behave as if there were three spatial dimensions. This model does not, however, say much of interest about the gravitons themselves, just about the spacetime they propagate through.

There is another set of models where gravity might deviate from inverse-square behaviour, if gravitons are not massless as currently believed, but rather have a small but finite mass. I suppose one might fairly call such gravitons "fat". But in this case, the deviations would show up not on small scales, but on very large scales. The basic problem is that massive particles can decay, and generally do, after a time related to their mass. So if you have gravitons travelling between two very distant masses, not all of them will make it all the way (this is a vast oversimplification, but gets the idea across). So the gravitational force at very large distances will be somewhat less than inverse-square. But at short distances, most of the gravitons won't have a chance to decay, so the effect won't be noticeable.

And I, too, have heard rumors about the Eöt-Wash group detecting a deviation, but so far, that's all they are, rumors. They haven't yet said anything of the sort in their publications. The experiments they're performing are very delicate, and it's quite possible that the effect they're seeing (if any; again, this is only rumor) are just experimental error. Naturally, if they do see a deviation, they'll want to be dead certain that their results are solid before publishing. It would be exceedingly cool if there really were a deviation, but let's not get our hopes up yet.

11-18-2005, 08:41 AM
I'm not sure if I'm remembering the size of this correctly, but it seemed to me at the time that the size was very, very large, which made me wonder why it had never been observed before this.
A particle being larger isn't generally helpful when it comes to detecting it. The thing to remember is that -- as Chronos mentioned -- more massive particles typically decay more quickly. So if you want to see one, you'll probably have to produce it yourself in an accelerator and then recognize that you produced it by observing its decay products. But the more massive particles are harder to make, and furthermore even when you do make one you might not recognize it for what it is, since two different particles may produce similar decay products.

11-18-2005, 08:58 AM
Here's a blog entry (http://motls.blogspot.com/2005/06/deviations-from-newtons-law-seen.html) that says a bit about deviations from the inverse-square law, and fat gravitons in particular.

Apparently, the Washington physicists like to think about the theoretical concept of a "fat graviton" being relevant for their current observations. Only virtual particles with the wavelength longer than 100 microns (small enough momenta) should contribute to the vacuum energy because the modes of the graviton do not exist at shorter distances. This also means that gravity becomes weaker at shorter distances.
Perhaps someone more knowledgeable than me can comment on whether or not this makes sense.

11-18-2005, 03:01 PM
Hm, this'll teach me to pay closer attention to the rumors. I hadn't heard that the alleged deviation is weaker than expected. This is not what's predicted by the extra dimension models. I'll have to dig deeper into this to say what's going on.