Evidence of 6 Dimensions or More?

shelflife writes "Nature.com is reporting that there may be evidence of 6 dimensions. Galaxies seem to behave as there were more matter in them than is actually visible. 'One explanation, they say, is that three extra dimensions, in addition to the three spatial ones to which we are accustomed, are altering the effects of gravity over very short distances of about a nanometre.'" Update by J : Like most of string theory, this is acknowledged by its authors to be "extremely speculative."

Comments

How dimensions wrap themselves up

[ReformedExCon]

The way I understood this phenomenon, as it was explained in Kaku's book, was that the extra dimensions were curled up on themselves so that they were smaller than could be detected.

The thought experiment was similar to the following. Imagine a sheet of paper with a line crossing from one edge to the opposite edge. You can see that the line exists when viewing the sheet in two dimensions. However, imagine if you rolled the sheet of paper up tightly with the line not directly aligned with the roll. Now you would have instead of a line a single dot or a series of evenly-spaced dots. The line hasn't gone anywhere, it has simply been rolled onto itself so that it seems to have become small and barely detectable.

Now extend that idea to multiple spatial dimensions beyond just two or three. Since we humans can only perceive three spatial dimensions, it is hard to imagine what multiple extra dimensions would be like. However, if we can take the extra dimensions and "roll" them into themselves, we can make a little more sense of the concept.

Re:So if we can't see it, it's in another dimensio

[Anonymous Coward]

It's simple: They plug what they observe into a mathematical model and see if they can come up with a model that matches observation. It's not simple blind guesswork.

Someone came up with a model called string theory that includes systems with multiple "hidden" dimensions.

The dark matter they're talking about in the article is behaving in a way predicted by one of the current string theory models, which doesn't fit the more traditional models, thus the assertion that it must be 6 dimensions at work.

Round and Round

[Quirk]

Resources

Greene's Elegant Universe

The Mechanical Universe

Last book I enjoyed, Three Roads to Quantum Gravity by L. Smolin... ya, ya, I know, nothing fits, is, isn't, yo momma... no yo momma... can, can't... I'm not touching you!

For the actual reference

[volsung]

The actual paper this article is about is here:
Observational Evidence for Extra Dimensions from Dark Matter

(It's actually a draft of a paper submitted to Physical Review Letters, not yet approved.)

It's a nice phenomenology paper without any heavy math that puts together a bunch of theoretical ideas floating around. Even better, it has testable hypotheses! (unlike many papers these days)

1. Gravity should deviate from the inverse-square law at the nanometer scale.
2. Dark matter should be composed of a particle with mass 3e-16 GeV/c^2. (For comparison, mass of electron is 5e-4 GeV/c^2.)
3. The large extra dimensions assumptions all this is based on would require us to see all sorts of quantum gravity interactions at the LHC.

Now short-range gravity experiments are just approaching the micron scale, so we're 3 orders of magnitude away from testing hypothesis #1. I doubt anyone has an idea how to close that gap right now.

Checking hypothesis #2 would require some independent way of determining the mass of dark matter particles. I don't know what the sensitivity range of the various dark matter experiments running or planned are. Maybe they would be able to see something this light.

#3 however is going to start running in 2 years, and then we'll get some good information either way.

Everyone abuses Occam's razor

[PsiPsiStar]

I'm starting to think that Occam's razor is abused more often than it is used correctly.

Parent asserted;
Occam's Razor, which is a basic tenent of modern scientific thought says that the simplest explanation is the best.

This is an abuse of the version of Occham's Razor used in modern scientific thought, though an oft repeated misinterpretation.

A better way of phrasing the desire for elegance in modern science is; "Given two identically predictive models, choose the one which requires the fewest assumptions." Reducing the number of assumptions is not always the same as 'simplifying' the problem.

Also, remember that the purpose of science is to generate predictive value. If one of those models is more complex but also more predictive, then it is ALWAYS the better model, no matter how complex.

The original version of Occam's Razor, as correctly expressed in the Wiki article, is "Entities should not be multiplied beyond necessity" where 'necessity' equates to generating the maximum level of predictive value.

Check out the following link, which gives a better summation of the role of Occham's razor in science than the wiki article does.

http://www.physics.adelaide.edu.au/~dkoks/Faq/Gene ral/occam.html