Exploring the Impact of High-Speed Particle Collisions at the LHC

[jscottcc]

I am aware that the LHC accelerates subatomic particles to just under the speed of light then smashes them together. However, at the time of collision, would this mean that the combined speed of the impacting particles is almost two times the speed of light? Or are the particles individually moved to half the speed of light, and their combined speed at the time of the collision close the speed of light?

A little while ago I remember reading something along the lines of "Scientists surprised that LHC collisions do not produce mini black-hole." First off, if this is inaccurate, stop reading this paragraph. But if not, then I have some questions. I am aware that gravity is both the driving force behind black holes but also by far the weakest of the four fundamental forces. Therefore the amount of gravitational force needed to cause a black-hole would be colossal, and while i don't know the math or equations, I just don't see two subatomic particles as ever having a possibility of creating enough gravitational force to cause a black hole even if the occupied the exact same location. So where does the idea of these collisions creating black holes come from?

And lastly, does Physics Forum have an IRC channel?

 

[Kracatoan]

Technically, black holes can be as small as you want, just as long as you've got the matter close enough that its density prevents light from escaping the area which you've compressed the stuff into. That said, it is incredibly difficult to do this, hence why black holes as we know them are extremely massive. With regards to the LHC, only a few theories of physics predicted that it would produce black holes, seeing as in order to produce black holes with the LHC, more dimensions than we see are required. I'm not entirely sure why extra dimensions are required, but apparently they are. There's also the fact that the miniature black holes would evaporate extremely quickly, making them hard to detect.

The first question is pretty much answered here - https://www.physicsforums.com/showthread.php?t=452235&page=2

 

[jtbell]

I am aware that the LHC accelerates subatomic particles to just under the speed of light then smashes them together. However, at the time of collision, would this mean that the combined speed of the impacting particles is almost two times the speed of light?

From the point of view of an observer standing still in the laboratory, the distance between two particles (one in each beam) decreases at a rate equal to nearly twice the speed of light. There is nothing wrong with this according to relativity, which says that the speed of any single particle, according to any unaccelerated observer, must be less than the speed of light.

If you were able to "ride along" on one of the two colliding particles, from this point of view the other particle would be coming towards you at a whisker less than the speed of light.

 

[Kracatoan]

As for the IRC channel thing, were you looking for something like this -

https://www.physicsforums.com/chat/flashchat.php

 

[bcrowell]

With regards to the LHC, only a few theories of physics predicted that it would produce black holes, seeing as in order to produce black holes with the LHC, more dimensions than we see are required. I'm not entirely sure why extra dimensions are required, but apparently they are.

The 1/r² nature of the gravitational force is because we (probably) live in three spatial dimensions. As field lines spread out from a source in three dimensions, the surface area of a sphere increases as r², so the density of flux goes like 1/r².

If there are actually, say, 5 spatial dimensions (but 2 are rolled up and invisible on a normal scale), then gravity would really be a 1/r⁴ force at short range. This would make gravity much stronger at small distances, and therefore it would be much easier than expected to form microscopic black holes.

There's also the fact that the miniature black holes would evaporate extremely quickly, making them hard to detect.

Rapid evaporation is actually what would make them *easy* to detect. If they didn't evaporate, they'd simply fly off at more than escape velocity, and we'd never know they'd been there. Evaporation gives them a clearly defined experimental signature, which is that all the known conservation laws of particle physics (e.g., baryon number) would be violated.