# What is string theory

#### The_Z_Factor

I was talking to a friend of mine the other day and he mentioned string theory to me..I had no clue what it was so I looked it up but didnt exactly understand it. My friend tells me it's a theory of parallel universes? He said something like there are different dimensions with different time zones, and there's more than one you. I dont quite follow..if that's true, then how has this been 'eligible' to be a theory. Im not saying I dont believe it, but I mean, how could it be proven or how was it thought up?

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#### Hernik

I'll give it a shot - and if I'm wrong I hope I'm corrected quickly and punished accordingly!

String theory is an attempt to find a mathematical model which can unite the physics of quantum mechanics and general relativity. That is incorporate a description of gravity into the physical model of particles. If string theory succeeds in this we will have one physical model which can be used to derive all other aspects of nature. This has been a problem up til now because Einsteins relativity cannot be used to calculate the electromagnetic behaveour of particles or describe radiation or how particles hold together. And quantum mechanics on the other hand is useless in predicting the mass of a particle.

I once heard an explaination of this problem (And it might have ended up in a tv-programme which I then am simply quoting) that it is like trying to understand what a city is from the map of the routes of the city buses and the map of the metro. You can use either map to get around town. And sometimes you can get to the same place with both maps but at other times you can't. And you don't get it because there must be an reason for it you should think. So the idea is it must be possible to make a better theory - a better map - of physics - so that you with one description - one set of rules can explain all the behavour of nature. String theory tries to do that. The main idea is that what appear to us as particles is the result of energy vibrating like grotesquely small strings. Working from this concept mathematically it seems to be possible to unite the forces of nature in one single description. But at a cost: There has to be many more dimensions than the ones we experience. 10 or 11 in some of the theories and more in other editions. One prediction from string theory is that there are more than one spacetime. As far as I know there has been no test of this idea nor the idea that energy actually behave like strings. So maybe it is more fair to call it "string hypothesis"?

#### The_Z_Factor

Hmm...but why does there have to be more than one dimension? Because otherwise it just couldnt be possible? How do other dimensions make it possible?

#### Fredrik

Staff Emeritus
Gold Member
Z factor, I suggest you check out the book "The Elegant Universe" by Brian Greene. You might want to start with the TV documentary based on it.

Also, please be careful with how you use words like "dimension" in a forum full of physicists. Those words don't mean the same thing here as in "Buffy: The Vampire Slayer".

#### shoehorn

Hmm...but why does there have to be more than one dimension? Because otherwise it just couldnt be possible? How do other dimensions make it possible?
The friend who told you about these things seems as confused about them as you are. Presumably, when he was talking about parallel universes, he was making a stab at describing the many-worlds interpretation of quantum theory. However, this has little, if anything, to do with string theory.

#### Fredrik

Staff Emeritus
Gold Member
Presumably, when he was talking about parallel universes, he was making a stab at describing the many-worlds interpretation of quantum theory.
I don't think so. He was probably talking about chapter 4 of hour 3 of the Nova series (Link in my previous post).

#### Demystifier

2018 Award
Basically, string theory is an assumption that elementary particles are not really pointlike dots, but very short 1-dimensional objects - strings. Such an assumption looks rather innocent. However, if one further requires that these strings should be quantum (not classical) objects, then truly remarkable consequences emerge: gravity, states qualitatively resembling the states of the Standard Model, removal of UV divergences, extra dimensions, a huge number of solutions corresponding to a huge number of alternatives to the standard model, ...
The fact that such a large number of physically interesting properties emerges from such a simple assumption makes many physicists believe that string theory could be the "theory of everything". Unfortunately, nobody has yet succeeded to prove that it really is, which makes even more physicists highly skeptical about it.

#### AlphaNumeric2

Hmm...but why does there have to be more than one dimension? Because otherwise it just couldnt be possible? How do other dimensions make it possible?
You're confusing the sci-fi notion of 'extra dimensions' with the physics notion of 'extra dimensions'.

In sci-fi, when they say "The 5th dimension" or "Another dimension", they generally mean a parallel universe. In physics, this isn't the same. Extra dimensions are basically 'extra directions'. On the scale of the very very very small, objects can move up/down, left/right, backwards/forewards AND in other directions (6 or 7 more!). That is what extra dimensions means to particle physicists.

Our universe, at large, at 3 space dimensions ( move up/down, left/right, backwards/forewards ) and a time dimension. String theory works (usually) in 9 spacial dimensions and a time dimension.

#### NerfMonkey

Just elaborating on what others have said, my understanding of the extra-dimensional part of string theory is that it arises because the different ways in which the strings could vibrate to produce certain types of particles would require a higher number of dimensions. If the strings were confined to only three spatial dimensions they would be incapable of producing the physical particles that make up the universe (or at least some of them).

Definitely check out The Elegant Universe as Fredrik suggested. It took me three months to get through and (almost) fully comprehend, but it's definitely worth it.

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