# Strings can be seen? Surely not!

1. Jun 13, 2015

### james gander

I have just read this in a Brian Greene book, what does he mean by "they APEAR pointlike even when examined by the most powerful equipment" Seems very misleading, can soemone explain what led people to the theory of the strings existence. Surely they cant bee seen.

here is the quote:
"The strings of string theory are so small—on average they are about as long as the Planck length—that they
appear pointlike even when examined with our most powerful equipment."

Thanks

2. Jun 13, 2015

### Staff: Mentor

They appear pointlike in the way stars do (unless you are very close or have a very good telescope). They are not points, but the resolution is not sufficient to see any structure so they appear pointlike.

3. Jun 15, 2015

### james gander

So
That would meen we would of split quarks then wouldnt it? How do they we see point-like strings if they are what makes up quarks. We cant split quarks. can we?

4. Jun 15, 2015

### Staff: Mentor

I don't understand that question.
We don't know, but it is not necessary: strings would lead to quark interactions that look different at the Planck energy, compared to other theories.

5. Jun 16, 2015

### ChrisVer

I don't understand your confusion. First of all, nobody has seen a string interaction and so you cannot say for sure they are point-like particles of that they have a length equal to the Planck length. The last is just the natural choice of lengths in a gravity contained theory, just like the natural length for the nuclei is obtained to be the fm. For strings you can't even say they exist. But even if they do, then your equipment will still see them as pointlike objects because your resolution is nowhere near to what would be an internal structure of strings.

Pointlike particles are those that have no internal structure. The proton for example has an internal structure of quarks and gluons, and at large enough energies/small enough distances you can see them (by interacting with the proton's partons), as for example was done with the deep inelastic scattering experiments.

In string theory quarks or electrons etc, are not point like particles, but they are associated with the oscillation modes of the superstrings.

6. Jun 17, 2015

### james gander

Thanks for the reply but my confusion was with part of Brian Greene's book - The elegant universe. In it he says.

"The strings of string theory are so small—on average they are about as long as the Planck length, they appear pointlike even when examined with our most powerful equipment.

I am just a science enthusiast i have never studied or worked in science so i thought i would check on this site just in case i was wrong and he actually has seen strings. I thought strings are what makes up quarks and i new we have not split a quark (if that is wrong please let me know) so how could he say stuff like "strings appear point-like"

So i think you would agree that it is a dubious sentence.
Thanks

7. Jun 17, 2015

### ChrisVer

Exactly because the string theory characteristic length is the Planck length... of course that doesn't mean that string have to be that small, but oh well it can give you some order of magnitude. That length is the alternative to saying the Planck energy (since energy-distances are inverse related in QFTs: smaller distances=higher energies).The Planck mass energy is around $10^{19}GeV$ . Compared to our best reaches (around the $10^3GeV$ scale) we are still several orders of magnitude away from seeing a string.So it has to appear point-like (just as mfb explained how distant stars appear pointlike). Or just like to mechanics an atom can be seen as a point like particle (its inner structure becomes clearer for larger energies/smaller distances)...So to look into an atom and find how it is composed with, you have to shoot at it a beam of certain energy of photons or other particles. The Hydrogen atom for a photon of wavelength (distance) larger than some Bohr radii appears pointlike.

If now quarks etc are composed of strings, well that question does not accept an answer since string theory is an unverified theory. If you want to believe they do, believe it, if you don't want to, then don't believe it, but there is no ground to support your belief on.

8. Jun 17, 2015

### Staff: Mentor

In string theory, and only there. We don't know if string theory is a good model.
That is exactly the point: quarks appear as pointlike because we do not see a substructure. If quarks correspond to excitations of strings, then those strings appear pointlike to us as well because we do not see their substructure.

9. Jun 17, 2015

### Obliv

We should define 'powerful equipment' and 'appear' because many people can make the mistake of assuming this means a very powerful microscope or something else. Strings, along with quarks, are too small to reflect light and will not be seen regardless, no?