Monday, February 12, 2007

A Prediction Falsified but Ignored?

OK, I may get into trouble with this, but hey, we only live once! :)

Back in 1998, Nima Arkani-Hamed et al. published a paper in PRL titled "The Hierarchy Problem and New Dimensions at a Millimeter"[1]. In it, they essentially predicted that the "extra dimension" in which gravity might "leak out" into could be detected at the millimeter scale. This, of course, created quite a buzz, since now, something like this could actually be tested. It means that at that scale, there should be a deviation of the Newton's gravitational law.

Well folks, it is close to 10 years now since that paper was published, an in the mean time, several tests have been done. Starting with Eric Adelberger's group at the U. of Washington, the Newtonian gravity has been tested several times, and up to the micrometer scale, and no deviation has been observed![2]

Now, I am sure that Arkani-Hamed is a fine physicist that has made several other contributions to this particular field of physics. However, in many of the interviews that I've read and reports on him, how come no one has ever addressed this fact that one of his major predictions that got him a lot of publicity has been thoroughly falsified? I mean, if I'm interviewing someone and one of the major point that that person is making has clearly not come true, do I try to ignore it like a 20,000 lb gorilla in the room? I simply can't. Even Barbara Walters would not have missed such an opportunity, and we all know how soft Barbara can be!

Maybe he has been confronted with this fact at various conferences, etc., I don't know. I don't move in the same circles. So if anyone knows anything about this, can you pass it on to me? I just find it incredulous that no one has called up on this.

Zz.

[1] N. Arkani-Hamed et al. Phys. Lett. B 429, 263 (1998).
[2] http://www.aip.org/pnu/2000/split/pnu483-1.htm ; http://www.aip.org/pnu/2005/split/734-2.html ;

1 comment:

GS said...

Dear ZapperZ,

The paper of Arkani-Hamed didn't predict a particular number of extra dimensions. It said that -if- the number of extra dimensions is 2 (if I remember the number correctly), then their size would be a fraction of a millimeter. However, he doesn't claim that the number -must- be 2. The possibility of more than 2 extra dimensions (which implies that their size is smaller) hasn't been falsified in the experiments you mention.

Another thing to note, is that the theory is a -suggestion- for physics beyond the Standard Model. It can't be derived just from the physics we know. This is just one out of many possibilities. Even Arkani-Hamed himself suggested other completely different alternative scenarios. All these theories are consistent with what we know and satisfy basic mathematical consistency with principles that are believed to be true (e.g., Lorentz invariance, quantum mechanics). Only experiment can tell which of the possibilities (if any) was chosen by Nature. (Alternatively, string theory might be able to tell us the whole story one day.)