I've heard of string theory, and I know that it's baisically the belief that multiple universes exist in parallell and only gravitons can travel between them. My question is: Do you believe in String Theroy, and what do you know about it that you could teach me?

Nope, I see no reason why only gravitons can travel between them. Do you have some references about that ?
I personally think that, if gravitons can travel across them, then also neutrinos can travel, and then, potentially, any wave or material particle can cross.
Whether it's easily realizable or not is another topic, but I see no reason for restricting to gravitons.
Typically, Newton's apple is gravitons-sensitive. So, the apple could also travel. Of course, the apple is probably simultaneously present on all plans, simply being eaten in some of them and still on the tree in the other ones. But, then, we touch the Heisenberg uncertainty field, which leads us to the bridge of te General Unified Theory of the Universe. Simply remember the Heisenberg's cat paradox, and you will see what I mean.
Regards
Yordan

Ah, how I wish I'd taken up the Nuclear Physic options instead of Cosmology for my Physics A Level, it looks far more entertaining. Still, hindsight is a wonderful thing, isn't it? Anyway, good old quantum mechanics and whatnot still gives me an insight in to Heisenberg's cat paradox (as well as an almost unhealthy number of physic-y programs on television), and even that worries my mind just a little bit.

Still, on the plus side, I've managed to read textbooks over the shoulder of those lucky individuals who ended up with an option that isn't (in my opinion) pretty dull (and the textbook even has prettier pictures!) so I do have some kind of rough idea of what the whole thing's all about, but I wouldn't overly fancy having any kind of debate about whether it's "real" or not. Personally, I find the whole Xth dimension aspect a lot more interesting, with dimensions layering up to give those multiple universes. How string theory relates to that, I'm not entirely sure, but I'm certain there's some link in there somewhere.

Anyway, that's my (mildly ignorant) two pence on the matter.

I have read Elegant Universe by Bryan Green. It had a lot of storys about string theory. Basically, String theory explain

Universe by view of wave theory. And It is trying to combine 4 major force.

Actually I didn't understand it well. So If u want to know about that subject, I recommend U to read the book.

Have Fun

Seems, any theory is be built over some axioms. We believe in the theory only if we believe in these axioms!

For my life experience, the item (which exist in 3 dimensional space and in the time) is more understandable and more obviously visible than string theory properties.

Nope, I absolutely do not agree. A theory is based on verified facts. This theory explains where these verified facts are coming from, and your theory explains the structure of the universe. This theory must also lead to facts thact can be verified. If the verification is OK, your theory is OK. If the verification fails, your theory is false or my verification is false.
If you start from axioms, you build an hypothesis. This hypothesis is valuable as long as no fact can refute it. However it still remains an hypothesis.

One small example to illustrate my words:

Euclid of Alexandria created his plane geometry, which has been successfully used by people for thousands years, in assumption of small set of axioms and he provided many other propositions from those axioms!
So, to be involved in this theory we need to accept all it's fundamental "bricks" - axiomatic approaches.
Everybody knows that there are only 5 main postulates of this theory.
But how we can check, for example, parallel postulate having our life experience? We can only propose that this postulate is true and to look how Euclidean geometry describes the environment where we live!
And we can be happy having this knowledge... or probably unhappy, if we see some facts that cannot be explained by Euclidean geometry!...
If we are unhappy with Euclidean geometry, we can accept, for example, an alternative (non-Euclidean) systems of geometry where parallel postulate is replaced by a conflicting axiom.

I only want to say that such practice of accepting fundamental "bricks" (axiomatic approaches), that are not always can be verifyed with our life experience, is is valid for any theory in any area!

Euclide's relies on a postulate. And we now, it's a postulate, something we cannot verify. From that postulate we deduce things we know that are correct, so probably his assumptions are exact. However, it remains a postulate.
The generalized Relativity is a theory. It's based on mathematical calculations, rather complicated but affordable. By the way, do you know that the original calculations were made by Einstein's wife ? Einstein was a real genius, the thought "we should be able to demonstrate this", his wife said "Yes I can" and she demonstrated the things.
So, this is a real theory. It relies on facts, and demonstrates things which again can be measured (binding a light beam when too near from a black hole for instance).
That's the difference I make between postulates and theories.
Another funny thing is squarizing a circle : a postulate says it's impossible, and effectively nobody was able to do it. However nobody was able to prove that it's impossible (that's the nice thing in real life : if I can do something, I prove that this thing is possible ; if I cannot do something, maybe it's not possible but also maybe I am stupid ).
Sorry being so dogmatic, but in a previous life my job was physics. And I had to take care not to go the wrong way when choosing between assumptions and theory. A theory has to be demonstrated. An assumption is an act of faith : other people may believe it or not.

Sometimes, one can make up an axiom or a postulate, and see what follows from it. This is how much of abstract math has been developed. The result may prove to be of benefit in physics or engineering, if it seems to make a connection with physical processes.

It is said that Albert Einstein wasn't at all interested in manifold geometry until someone explained to him that it was used to describe curved space. He then realized that it could mathematically describe the behavior of gravity. The result was his General Theory of Relativity.

This also happens with general observations. Some engineers in Australia, I believe, were trying to get rid of background noise in long-distance microwave transmissions. They noticed that it peaked at a certain time of day, and the time of that peak came 4 minutes earlier each day. Mundane observation, right? Well, it turns out that they were the first observers of the "black-body" remnant of the Big Bang. I believe they got the Nobel Prize for it. Just an observation looking for a theory.

Similarly, people have been observing sunspots since not long after Gallileo. They noticed that the "Little Ice Age" coincided with the "Maunder Minimum" in sunspots. Theory: if no sunspots, then it gets cold. Recently, a Scandanavian astrophysicist has offered an explanation in terms of the solar wind (which is present with sunspots) reducing the cosmic ray flux to the Earth, causing fewer clouds, and thus letting the Earth warm. Guess what? The sun has been unusually quiet (1 sunspot per month) recently. It's too early to be certain, but we could be in for another really cold spell. Would that be enough to that embarrass all those "Anthropogenic Global Warming" folks?

I agree. My previous post seemed somehow mad, but I simply wanted everybody to be clear concerning terminology.
Just say "I am talking about an axiom", I will agree with you.
Don't say "I will proove my theory" if you don't want to perform a scientific demonstration.
Last time I saw a guy saying "I will calculate the distance", and I was really surprised seeing him measuring the distance. A calculation should be predictively deduced from a theory, and a measurement is evidence of a fact.
Sorry being so dogmatic, an old teacher is always waking up inside of myself.