Hi,

I opened this bc the OP sent me a pm asking for input. In general it's a better idea to just post questions like this on a science or physics forum with experts keen on answering questions as a form of outreach. I don't post on those forums, not sure if i have the patience, and tbh I don't want to encourage people to pm this stuff, especially until I block email notifications of pms. i only signed up for this forum bc i saw some crank abusing a colleague and confusing others and I got annoyed, almost all my posts are me telling him off. If you are interested in particle physics / cosmology though, I suggest the best way thing to do would be to satisfy your curiosity by doing a physics (or maths) degree and then taking specialist courses at the end.

Anyway looks like you have already had some of the problems with your understanding pointed out by other posters.

Let me just clarify that the cosmological constant problem is a fine tuning issue, because I think you have misunderstood that point slightly. The problem is that we know some contributions give something of the order 10^120 and this must be cancelled by other seemingly unrelated contributions to leave something of order 1, which would be a pretty amazing coincidence unless we can find some underlying reason for this. Here is the proceedings from a talk on this by weinberg which explains the problem in a very simple way on the first page. note the numbers are different, partly because of units, which i didn't specify and because he is only assuming we understand the physics up to 100 GeV, which is what we have experimentally probed, whereas my numbers, which are more commonly quoted, assume an effective theory which holds up to the Planck scale, where new physics to describe gravity must enter.

With theories of extra dimensions (which one needs in string theory bit can also imagine indepently of that btw) one always has to explain how the world we observe is four dimensional. To do this either the unobserved extra dimensions are compactified in some way or we are localised to a four dimensional brane of the higher dimensional space.

I *really* want to make a joke about hairy balls...

Bewildered wrote:
Let me just clarify that the cosmological constant problem is a fine tuning issue, because I think you have misunderstood that point slightly.

So it may not have anything to do with the Cosmological Constant - so what would it imply?

So is compactification essentially finding a way to make extra-dimensions very small

Loke wrote:

Bewildered wrote:
Let me just clarify that the cosmological constant problem is a fine tuning issue, because I think you have misunderstood that point slightly.

So it may not have anything to do with the Cosmological Constant - so what would it imply?

I don't really see how it implies anything. For reasons explained above, it isn't really possible to deduce any hairy ball-style vanishing theorem for the total (matter+gravity) energy because:

1 The gravitational contribution to the total energy is non-local and there is no universally agreed definition of it
2 The energy just isn't a vector field quantity, and even if it was:
3 We don't know what the topology of spacetime is, so can't make deductions about vanishing of vector fields

Loke wrote:So is compactification essentially finding a way to make extra-dimensions very small

Yes. Compactification is essentially making a space of certain dimensions LOOK as if it's a lower dimensional space. In the case of string theory, the Calabi-Yau manifolds are placed at every point in spacetime. However, the Calabi-Yaus are of the Planck scale, so to large objects like humans and planets, the universe LOOKS 4 dimensional because the extra dimensions are too small for us to directly experience. However, a string is small enough to experience those small dimensions, so to a string, the universe is 10 dimensional.

EDIT: Here is a picture to help imagine it:



The grid would be the 3 spatial dimensions that we experience. The Calabi-Yau manifolds are the curled up dimensions that the strings experience.

Nautilidae wrote:

Loke wrote:So is compactification essentially finding a way to make extra-dimensions very small

Yes. Compactification is essentially making a space of certain dimensions LOOK as if it's a lower dimensional space. In the case of string theory, the Calabi-Yau manifolds are placed at every point in spacetime. However, the Calabi-Yaus are of the Planck scale, so to large objects like humans and planets, the universe LOOKS 4 dimensional because the extra dimensions are too small for us to directly experience. However, a string is small enough to experience those small dimensions, so to a string, the universe is 10 dimensional.

Loke wrote:

Nautilidae wrote:

Loke wrote:So is compactification essentially finding a way to make extra-dimensions very small

Yes. Compactification is essentially making a space of certain dimensions LOOK as if it's a lower dimensional space. In the case of string theory, the Calabi-Yau manifolds are placed at every point in spacetime. However, the Calabi-Yaus are of the Planck scale, so to large objects like humans and planets, the universe LOOKS 4 dimensional because the extra dimensions are too small for us to directly experience. However, a string is small enough to experience those small dimensions, so to a string, the universe is 10 dimensional.

Picture added.