Perhaps Chris can delete the above anonymous clone of my following post...

As a "starter", I can recommend reading ziggy's and my post in

http://www.shatters.net/forum/viewtopic.php?t=2809
Apparently, my "Help! My Unicorn run away" thread has meanwhile managed to

**run away** both from CelestiaUsers and the Purgatory;-). Let's see where it will end...

Since, after

**redemption of the 'Unicorn' from Purgatory**, the above angel turned out to have obviously male gender, I preferred to rank a little plant around his waist in order not to endanger our many very young readers;-)

**Contemplations on Extra Dimensions**
============================

Let us get back to this most basic question of extra dimensions, but forget about the specific framework within string theory that I touched upon further up in this thread...

---------------------------------------------------------------------------------------------

There exists a "lore" that convinces us that we live in

3 space + 1 time = 4 dimensions.

We also believe that these 4 dimensions are "non-compact".

[

**glossary**: While an

**infinite** line is a simple "non-compact"

manifold, we may simply arrive at its

**compactified** "brother" by

**identifying** its +infinity and -infinity ends such as to make a

large

**circle** of it! So the simplest "role models" for compact

spaces are always n-dimensional spheres, starting with the circle]

Once we are ready to contemplate about more than 4 spacetime

dimensions, we should be immediately

**overwhelmed by some most basic,**

existential questions:

Why on earth, are we then living in 4=3+1 and not in

5, 6 or 8 dimensions?? Is perhaps our very existence intrinsically

coupled to the number of dimensions being exactly 4? Why does there

only seem to be

**one time** and not perhaps two or more? [ In relativity,

the time dimension is treated on exactly the same footing as space dimensions]

The answers to all those most fundamental questions are still pending,

but the quest for them makes people work very hard indeed...Recently

we might have seen some "light at the end of tunnel" though...

Normal matter that we describe within the so-called Standard Model of

strong, electromagnetic and weak interactions, cannot propagate a

large distance in possible extra dimensions without conflict with

observations. It has been recently recognized, however, that this

conflict can be avoided if the Standard Model is

**confined** by

some mechanism to act only in a (3+1)-dimensional

**subspace**, a

so-called

**"3-brane"**, embedded in a higher dimensional

environment (the so-called "bulk")!

[

**glossary**:

**p-branes** are extended p-dimensional objects

that derive their name from being higher-dimensional generalizations

of "membranes"; we are thus said to live on "brane-worlds" in the

respective literature]

However, this 3-brane trick for the Standard Model forces will not

work for

**gravity** which

**necessarily propagates in all**

existing dimensions, since it is the dynamics of spacetime itself!

The outstanding experimental success of Newton's 1/r^2 law (=>

**Celestia!**) and general relativity would therefore seem to imply

four non-compact dimensions. n extra compact dimensions of size R would modify

Newton's law into a 1/r^(2+n) behaviour at distances r<<R. At large

distances r>>R the gravitational flux lines cannot penetrate anymore

in the extra dimensions and the familiar 1/r^2 law is always recovered!

About 5 years ago it was realized by some theorists from Stanford and

Trieste that tests of Newton's law did not exist at distances

**smaller than about 1mm!**. So in view of the above said, a huge

new 'fashion' took off, since suddenly there was the exciting possibility

that

**large** extra dimensions of order ~1mm were "just next door"

and could be hunted for via deviations from Newton's law!

Let me stop [or break] here in order not to overdo;-)

Bye Fridger