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

[Alex K]

So when are we going to have things that are dimensionally transcendental?

That depends entirely on what Mother Nature has in store for us. We are but humble recipients of Her favors.

[nihilistcat]

I have a question. My understanding is researchers at CERN will be looking for (among many other things) mini-black holes (of the type that was created during the big bang i.e. primordial black holes). Apparently, if these mini-black holes are discovered, it would lend support to string theory, and so if they're not discovered, presumably it creates problems for string theory.

My question is ... if these primordial mini-black holes cannot be found at this high energy level, then would it debunk string theory altogether, only rule out some aspects of string theory, or would it not do much to impact the validity of string theory one way or the other?

I have another question, or maybe this can be considered a grievance with the theoretical physics community. We always hear that its nonsensical to describe anything that might lie beyond the boundaries of our universe as comprising time or space. So the maxim "universe from nothing" has become popularized.

I understand space-time (as we know it) came into existence at the big bang. However, to say that the idea of "space" necessarily relies on the existence of our universe, is I think using the term in way that's inconsistent with the way many if not most people conceptualize space. And as far as I can tell, there's no real scientific reason for this view of space. If I were to ask a theoretical physicist what would happen to a chunk of matter if I were able to toss it out of our universe, I'm pretty sure the answer would be, I don't know. Well, if deep space beyond our universe could accommodate matter (in other words, if the outer barrier of our universe is not the end of everything in existence), then we have something that could be reasonably defined as space (and in all likelihood, it would be at least close to a perfect vacuum, no activity beyond maybe some virtual particles emerging then destroying each other, and while any matter may crystallize because of the temperature, unless we can say that if we tried ejecting a piece of matter from our universe, it would essentially hit a wall and bounce back, then how do we say something like space only makes sense in the context of our universe)?  

To me, I don't know means I don't know, it doesn't mean replace my ignorance with semantics about how space or time can only exist within our universe. So I'm interested in hearing thoughts from a physicist on this question?

[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.

[Longhorn]

Do you have more pretty gifs of the thingy and will you show us

[Alex K]

The CMS detector, opened.



It weighs as much as the Eiffel tower.
This is a schematic cut through CMS, and where it detects different types of particles




And this is what the reconstruction of an actual collision event it it looks like

[Longhorn]

Oooh, aaaaah

Pretty sciency thingies, me likey

[Pyrrho]

Alex K, you write about the universe being hypothetically finite (post 33).  That suggests that it might be infinite.  (Or course, it might not be.  "Might" gives one quite a lot of wiggle room.)


What do you believe the universe is like, and why?  I want a broad view of things, so you don't need to go into too much detail, but enough detail to get the general idea.  But, of course, you should give as much detail as you wish to include.

And I would like to know which bits of the story are known, and which bits are better described as being your best guess.

I realize this is just a little question, just asking you to explain the universe to us.

[Alex K]

Pyrrho,

This is a question one has to remain agnostic about for the time being. All we know from observations of the microwave backgroubd is that it is at least a few times the size of the observable universe. My guess, which is more like a matter of taste, would be that it is finite and wrapped up, and that our visible universe is a super tiny patch of the whole. I would think that bc the observable universe looks geometrically flat, and that could be because we only observe a miniscule part of a curved whole.

[Pyrrho]

Pyrrho,

This is a question one has to remain agnostic about for the time being. All we know from observations of the microwave backgroubd is that it is at least a few times the size of the observable universe. My guess, which is more like a matter of taste, would be that it is finite and wrapped up, and that our visible universe is a super tiny patch of the whole. I would think that bc the observable universe looks geometrically flat, and that could be because we only observe a miniscule part of a curved whole.

Okay, I like the idea of an infinite universe.  If it is a matter of taste, why not go big?

If you look again at the way I asked the question, you will see that I fully expected the answer to be a "best guess" (my words in my previous post) rather than something known.

It is good to know about the background radiation indicating that our universe is much larger than just what we can see.

Do you expect the revamped toy that is the subject of this thread to find anything that will tell us more about the size of the universe?

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If it makes you feel better, I was listening to Bach while asking these questions, not Mozart.

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[Anomalocaris]

Why is it necessary for the entire universe to be at least a few times the size of the observable universe?