# Someone help me grok String Theory, please!

Aight… I get the fact that what we used to consider vanishingly small particles are actually (according to String Theory) tiny vibrating strings.

But I have a few questions:

1. What are the strings made of? (I have the feeling I’m not going to like the answer here)

2. Given the fact that the frequency of the string’s vibration gives rise to the “particle” mass: why? What is the mechanism whereby the vibrations impart mass (I’ll leave charge and spin out of it here, as I don’t get those at all! Mass I get…)? Is the mass an artifact of relavitistic effects caused by the physical movement of the vibrating portions of the string through space?

3. According to theory, the length of one of these strings is Planck’s length… that is, about 10[sup]-33[/sup] cm. Why? What is the tie-in, if any, with Planck’s Constant? (I’m not sure this can be answered with my current level of mathematics… I can’t count to 11 without either taking off my shoes or peeking down the front of my trousers…)

Anyone out there have a good grasp of String Theory? #2 is the one that has been really crinkling my forehead for the past few days.

Thanks!

I can highly recommend Brian Greene’s excellent book, “The Elegant Universe”. He explains all this stuff in layman’s terms, without being patronizing.

IIRC, the strings are much smaller than the planck length.

Try doing a search in GQ. There have been previous threads with some good links on string theory. I don’t have the time to look for them right now.

This looks like one for Chronos, but I think the strings are different sizes in different string theories.

The connection between Planck’s constant and the Planck length is:

λ[sub]P[/sup] = (Gh/2πc[sup]3[/sup])[sup]1/2[/sup]

where λ[sub]P[/sub] is the Planck length, G is the gravitational constant, h is the Planck constant and c is the speed of light in a vacuum. The Planck length is a dimesionless unit of length and it’s also thought to be the scale where quantum gravity predominates.

oops the equation should be:

λ[sub]P[/sub] = (Gh/2πc[sup]3[/sup])[sup]1/2/sup]

Where λ[sub]P[/sub] is the Planck length and π is pi

One last time using preview:

λ[sub]P[/sub] = (Gh/2πc[sup]3[/sup])[sup]1/2[/sup]

Try this.

1. Strings are “made of” spacetime. Like it? Nor me, but neither would I like gravity if I fell out of a window.

2. You are still thinking in macro (“everyday”) scale terms where mass is this real, solid stuff which hurts when you drop it on your foot. Mass and energy are equivalent, and string theory is an attempt to describe gravity in quantum terms (it has a long way to go yet, mind!).

3. The Planck length is the scale at which the strength of gravity becomes comparable to that of the other three fundamental forces, which implies that the forces are unified on this scale.

I cannot claim to have a “good grasp” of string theory (or more generally m-brane theory), but I’d swot up on Ist Year degree level physics (Uncertainty Principle, General Relativity) if you feel you are trying to run at string theory before you can walk.

(I should also state that the article above was written by a first-year physicist who writes a little confusingly at times, but gives a generally helpful overview with some nice pictures!)

These sites have all been recommended by people working in string theory on another board I frequent:

String theory resources:

Non-techincal overviews of the five different superstring theories:

http://www.sukidog.com/jpierre/strings/susy.htm

Also Sentient Meat, from what I gather the strings exist within space-time in the same manner as normal particles in peturbative string theory.

This isn’t considered a fact yet, is it? It’s still just a hypothesis, right?

“String theory” is a misnomer. It should be called “String model”. An idea doesn’t get the status of “theory” until it can pass tests, which string theory can’t, at least for the forseeable future. So no, it’s not considered fact.

The Planck scales (Planck length, Planck mass, etc.) are expected to show up naturally in any situation which involves both gravity and quantum mechanics. Whenever you’re doing QM, you’ve got hbars flying around all over the place, and whenever you’re doing gravity, you’ve got Gs all over the place. In both cases, you’ve got plenty of cs to go around, too. The upshot is that, whenever you’re doing GR and QM at the same time (as in String Theory), and you ask a question whose answer is a length, you expect that length to be expressed in terms of hbar, G, and c, which means that it’s expressed in terms of the Planck length. This does not mean that the length of the strings must be exactly sqrt(hbar*G/c[sup]3[/sup]): It would be wholly unsurprising if the answer turned out to be pi times that, or half that, or 1/n[sup]2[/sup] times that for integer values of n, or the like. But it’ll be something dimensionless times the Planck length, and most of the dimensionless somethings that show up in physics are in the vicinity of 1.

Okay, I can follow most of the equations, but what is "sup"i in the first two?

Oh, okay, my quote of your third equation reduced it to elements including “sup”. I think I can take it from there.

Gotta’ look into this preview tool. It look fantastic.

True Blue Jack

OK, thanks all! You’ve given me some things to ponder as well as read up on!

I am, actually, half-way through Brian Greene’s Elegant Universe (reading it for the third time now), and while he does a good job of explaining most things he does lose me from time to time.