system
Given the following post, respond as if you're commenting on Reddit:
human
How many dimensions are out there? 10 or 24 or more?

I have read about 10 but is 24 just a theoretical number?
gpt
Observations show that there are 4 dimensions.

3 Space like dimensions.
1 time dimension.

The easiest way to show this is via the propagation of the electromagnetic force, and of gravitation in Maxwell and Newtonian mechanics.  They both fall off via 1/r^2.  This only works if there are 3 space like dimensions.

The 4 dimensional tensor of general relativity also only gives realistic predictions about the universe if there are only 4 dimensions.  Here is more complicated as the dimensions in General and Special Relativity are mixed depending on the observer.  Quantum Field Theory is also embedded into a 4 dimensional Minkowski space.  Changing it to have 5 dimensions disagrees with experiment. 


If this is so clear why is there a question about the number of dimensions?  Because there is an interesting relationship mathematically between the forces if we allow the number of dimensions to be larger than 4.  This was first shown in the 20s by Kaluza–Klein theory which showed that if there were 5 dimensions, with one hidden by being cured up, you could reproduce Maxwell's EM theory as well as general relativity.  It was later extended to include Quantum EM.

So why don't we say the universe is 5 dimensional? Or 11 (M-Theory) or 26 Dimensional (Bosonic String Theory)?

Because these dimensional theory's produce no correct independent predictions that can be measured.  No evidence of any extra dimension currently exists.  There are a ton of interesting Mathematical reasons to look at or be interested in Extra dimensions, however ideas are tested by experiment.  No experiment has shown to be only compatible with extra dimensions.  No independent prediction of having extra dimensions has been shown to be correct.

How would we know if extra dimensions are correct?  If Lorenz violations are found.  If resonances in some particle are found.  If suddenly at a specific distance scale a force was found to change in strength.   None of these things have been found so far.  This doesn't show that the idea is wrong, simply that it isn't yet useful in making predictions about the universe we live in.