Monday, February 13, 2006

Life in a 5-d hole

The current New Scientist has one of those intriguing articles on speculative cosmology, with US astrophysicist Paul Wesson asking - what if the universe was a five-dimensional black hole? (The full feature is sadly subscription-only online.)

The answer to what it would be like is that it would be remarkably similar to what we see. These 5-d ones aren't like the familar four-dimensional (that's three of space, and one of time) black holes - and by 'familiar' I mean that even if you don't have much of a grasp on general relativity, you'll probably get the idea from if you get too close to one, it'll suck you in, rip you apart, and not even your constituent atoms will be seen again.

These 5-d ones are much friendlier places to hang around. And handily, there's plenty of circumstantial evidence that suggests that the universe has at least five dimensions, otherwise it just shouldn't work in the way that it apparently does. A 5-d universe just wouldn't look any different to what we see as a 4-d one.

The most persuasive clue found by Wesson is that the universe (as it would be in a variant of the big bang model dubbed the 'big bounce', which seems to demand five dimensions) looks remarkably like a hypothetical five-dimensional black hole, if you look at a ballpark measure of their physical characteristics called the Kretschman Scalar. This measure basically measures the gravitational field at some point in space, and takes a variety of dimensional forms for different models - but in these two cases, it comes out very similar. As Wesson says:
It is conceivable that a bouncing universe and a 5D black hole could have identical Kretschmann scalars by coincidence alone, but given the number of parameters involved, it is very unlikely. Much more likely is that there is some kind of similarity between the two objects.

Intriguing stuff, that demands further investigation. Wesson throws in a coda illustrating the possible ramifications, albeit in a way that risks drawing in the astral-travelling crowd:
We are led to consider the idea of a set of "Russian doll universes", with each world embedded in another world of higher dimensions. There is as yet no way to know what we will discover when we probe these higher-dimensional universes.



Anonymous simon said...

Hi Tim

I happened on your post while looking for info on Tim Wyatt's new tailoring business but got sucked in (pardon the pun - ha, ha!) to your post on 5D black holes. Now, I'm no expert but I think I get the gist of what you're saying, but what is the 5th dimension? Did you mention that? I would intrigued to know.

1:30 pm, February 15, 2006  
Blogger Tim Chapman said...

Extra dimensions are fairly common in theoretical physics - some versions of superstring theory need 26 or more dimensions, including both extra dimensions of space and extra dimensions of time.

As I understand it, the five-dimensional universe model in this theory features one extra dimension of space. The obvious question is why we can't perceive this dimension in the same way as we do the familiar three. The usual explanation is that this extra dimension is very tightly curved in on itself. It's like if you took a sheet of paper and rolled it up very tightly - the two-dimensional sheet would, if you don't look too closely, look like a one-dimensional line.

That's a bit of a rough analogy, but that's the way I understand it. To find if the universe really does contain these extra dimensions, you basically need to examine its behaviour on a very small scale, which usually involves looking at how the most basic sub-atomic particles behave at very high energies. That's part of what places like CERN look at.

1:45 pm, February 15, 2006  

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