Question: String theories are very difficult to prove as strings are so small for the LHC to detect. But, is it that the LHC should detect a single string? Can we make a situation which produces a group of millions of strings large enough for the LHC to detect? I have read that particle colliders detect the traces of emission of those particles that they find and not the particle itself. So, is there way for us to figure out the effects of a group of strings and look for the effects at LHC?

David Freeborn answered on 21 Jun 2013:

Hi rajathjackson,

Good question! No, the LHC can’t even come close to actually detecting strings. We’ve got pretty good at detecting quarks, but strings are much, much smaller- about the size of the Planck Scale. To put that into some perspective, if an atom were magnified to the size of the solar system, a string would be the size of a tree.

I really like this awesome website called “Scale of the Universe” by the way- zoom right in and you can see how small strings are relative to quarks!
http://htwins.net/scale2/

In string theory, each string corresponds to a single fundamental particle, and the particles are different quantum excitations of these strings. But the string is much, much smaller than the particle it corresponds to, because the particle’s size is really an “interaction size”, i.e. it tells us how close another string needs to be to interact with it. That means that it’s almost impossible for us to ever deal with the strings themselves, rather than the particles that they correspond to, and this is why it’s almost impossible to experimentally test string theory.

That means, bringing a bunch of strings together doesn’t really help us. All we’d see is a bunch of particles interacting. It wouldn’t make it any easier for us to “see” the strings, because the only way to “see” a string is to fire a particle (like a photon of light) at it, and hoping it will interact. But the photon will interact at the particle-interaction distance, and never get close to the actual “string” itself.

You’re right that particle detectors usually detect the traces of particles and not the particles themselves. This is basically because most subatomic particles are very unstable, and decay in something like 10^(-20) (10 to the power of minus 20) seconds or much less. So usually we detect the particles that are given off in the decay, not the actual particles themselves. But this isn’t going to help us look for strings, because each string corresponds to a particular particle anyway.

One way we *could* test string theory is to look at MUCH higher energies. When we get to higher energies, the interaction size of particles shrinks, and if we could get up to the Planck Energy- about 10^14 times bigger than anything accessible at the LHC, then the particle-interaction size would become comparable to the string size.
The sad news is those energies probably won’t be accessible to human beings for centuries. That’s *way* beyond current technologies.

Our only other hope is to try and detect signatures of the extra dimensions predicted by string theory. If extra dimensions exist, sometimes a photon or other particles may get “lost” down these extra dimensions. This would correspond to some of the forces of nature being very slightly weaker than expected.

I hope that helps! Sorry if it wasn’t clear- feel free to ask more questions if it’s not!