# A Brane New World

1. Sep 19, 2010

### PhanthomJay

Before I post my question, I'd first like to remind everyone (who don't already know) that Hawking's new book, "The Grand Design", is available through Amazon, and may also already be in the the bookstores. If it's as good as his last 2, it should be a beauty!

Now for my question:
A recent post in this forum on "Planck Length" got me thinking. In Hawking's prior book, "The Universe in a Nutshell", he mentioned the possibilty of other universes (Branes) that could exist outside of our own. We would never be able to observe such Universes, since light is confined to 'our' Brane Universe (although he noted that short wave gravity waves might be able to penetrate through the Brane into our Universe). Anyway, he noted that if such universes exist, they could be as close as a "few millimeters" away from ours. Now I was wondering, why a few millimeters? Wouldn't a "Planck's Length" away from ours be more 'logical'? What's the significance of a ' silly few millimeters"? The Planck length is very significant..a few millimeters is not, in my opinion. Agree?

2. Sep 19, 2010

### jswu1996

I agree, although hawking probably mean "a few millimeters away" as a metaphor for very close, the thing is, it would be impossible to figure out the distance it is away from yourself anyways because it is another universe, in another dimension, that may not have the spacial dimensions we do.

3. Sep 19, 2010

### Kevin_Axion

The General Public appeals to terms like millimeters, systems their accustomed to. There is no use in using a concept like Planck Length when it serves no relevance or purpose to the layman reader.

4. Sep 19, 2010

### PhanthomJay

If that's the case, Hawking could have said 'less than a millimeter'. I think he didn't know. Gee whiz, does anyone?

5. Sep 20, 2010

### Chalnoth

Talking about distance in these terms is actually sensible. The extra dimensions in string theory are, after all, exactly like the three spatial dimensions we are familiar with. So it makes perfect sense to use the same units of length as a measure.

The difficulty is that we can't visualize such things very well, because this would be one millimeter in a direction orthogonal to all three spatial dimensions we are familiar with! What we usually do, then, is visualize our own three spatial dimensions in just two dimensions, seeing it as a membrane stretching through a higher-dimensional space. There could be another brane right next to it, and we'd be hard pressed to notice.

However, this doesn't mean that all interaction with another brane is impossible. Gravity cannot be confined to a brane like the other forces can, and thus we may potentially be able to detect the existence of higher dimensions (or of other branes!) through very detailed tests of gravity, especially at short distances.

To date, we have seen no evidence suggesting the existence of additional dimensions through gravity, and sadly there is a huge range of parameter space that is likely completely unmeasurable in practice. But who knows? Maybe we'll get lucky...

6. Sep 20, 2010

### PhanthomJay

I missed that 'orthogonal 'part. Thanks!

7. Sep 20, 2010

### jswu1996

Doesn't this also depend on how one would see the dimensions? For example, I see that while we are aware of the first and second dimensions, we do not truly live in them. We only live in the third and fourth dimension. Or the "Brane" with the third and fourth dimensions. Although we only imagine time and not phisically see it, i do not think it is a completely different dimension than the first three. Why does time have to be a completely different dimension? What is so special about it? I just see it as another dimension, therefore, how would one be able to use the units of measurement that we do in our third dimension to measure those of the fourth or higher ones?

8. Sep 20, 2010

### RLutz

We live in all three of those dimensions. I can move up, down, left, right, and to, and fro. (3 independent axes). I have length, width, and height.

As far as time being just another dimension, I always thought that was a decent way of thinking about it. Pretend you have a car, and that car always goes 50 miles per hour and can only go exactly 50 mph. That means you can go 50 mph east, or 50 mph north, or, and this is the important part, you can go 50 mph in a northeast direction (such that your northward speed would be say 30 mph and your eastward speed would be 40 mph).

Likewise, we are all constantly moving at the speed of light, it's just that in our day to day experiences we point pretty much straight east (on the time axis). We use all of our "light speed" to travel forward through time. If we speed up our velocity through space, then we use less of our "light speed" on traveling through time, and more of it traveling through space. This is why as one begins to travel through space at velocities approaching the speed of light, time slows down for them, since more of their "velocity" is being used to travel through space, and less of it is being used to travel through time, just like when you pointed your car northeast. You were still going 50 mph, but only 30 mph east and 40 mph north. You are still traveling at the speed of light, just less "east" and more "north" (that is, less through time and more through space).

9. Sep 20, 2010

### Chalnoth

Huh? We live in all four dimensions. If we only lived in the "third" dimension and time, we would be one-dimensional beings that experience time. You need all three spatial dimensions plus time to describe our experiences.

Yes, time and space are but two different aspects of the same thing. The main difference with time is its directionality. You can move left or right, forward or backward, up or down. But you can only move forward in time. This is a pretty major difference.

Yes, we absolutely can. The unit conversion factor is the speed of light, so one millimeter = $3.3 \times 10^{-12}$ seconds.

10. Oct 8, 2010

### ninjanik18

Apparently light is leaking into our universe/brane from the 7th dimension?

11. Oct 8, 2010

### Chalnoth

Er, no.

12. Oct 8, 2010

### Kevin_Axion

Photons are open strings in the context of superstring theory and therefore they must propagate along the brane.