First collisions at the LHC with unprecedented Energy! (Ask a particle physisicist)

[Alex K]

Francis,

Great question because it raises important points and a misunderstanding.

First, concerning string theory. String theory is in trouble mostly because the size of strings by default is expected to be near the planck scale. This is because closed strings are the gravitons which communicate gravity and the strength of gravity is roughly set by their size parameter, the string length. If this is true, we can't directly resolve strings in experiments cause they are too small, because gravity is so weak - and so the question arose how to test string theory at all. A different hope would be that string theory makes some hard predictions about new particles and their masses, but even here it turns out that there is still so much freedom in constructing string models that it is difficult to find unique predictions. So the trouble is not so much that string theory gets falsified at the LHC, but that it CAN'T be falsified there. What is the connection to black holes - If the string length is as small as expected, one can't make mini black holes at the LHC afaik.

So you might ask, is there a loophole that might us allow to test it directly after all? And there is one, that I alluded to earlier: if some of the 6 extra space dimensions are unusually large (up to micron level), the weakness of gravity would not be due to the smallness of strings, but because it gets diluted in the extra dimensions. In that case, strings could be large and be resolved at the LHC. This scenario would possibly also allow the creation of mini black holes, because gravity becomes strong at LHC energies. This is how I see the two issues related...
Mini black holes would reveal an unexpected chance to study quantum gravity in the lab, and string theory is one example. If they are not made at the LHC, this by itself cannot rule out ST. It just is an.indicator that we might have no direct avenue to test it in the lab.
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Second question(s):

I am completely at a loss what you even mean by space without a universe or beyond the universe. If there is space and one can move towards it, to me it is by definition part of our universe. Maybe you can elaborate!

I'd say it is not clear at all that time and space started with the big bang. Afaik we simply don't understand what was going on when you go back in time far enough.Time and space could be part of something much more complicated. A clean timeline as the smooth linear thing we like to imagine possibly doesn't exist microscopically.

The size of the observable universe very probably isn't even near its full size, if it is finite in size at all. IF it is finite, it doesn't have to have a boundary either, it could lead back onto itself like the surface of a sphere or a donut. If it has a boundary, that boundary could behave in different ways. It might behave like a perfect reflector or it might absorb everything and convert its energy to whatever. I'm not exactly an expert on that kind of scenario.