# String Theory

## Main Question or Discussion Point

Hi, so i've just recently like really found out about string theory. I mean, i've seen posts around here on it, but i've never really followed them well. I was talk with my physics professor about some crazy thought i had and he was like "oh, string theory." heh... right, and here i thought i was some creative genius who had though up the whole idea...
ANYWAYS- someone want to sort of fill me in on like the basics? Like, the basic basics. Or at least direct me to where i can find basic info. Like, easy to understand stuff. I just want some really rudimentary info to decide whether i'd like to pursue the subject or no.

Thanks yall
~gale~

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ahrkron
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Hi Gale17,

A good place to start would be http://www.superstringtheory.com/

The very basics?

If, instead of point particles, you use "strings" as the basic objects, many interesting things happen, especially in the interface between QM and GR, which has a very special place in the heart of every theoretical physicist.

In order to choose the right details to go into, it would be useful to know what sort of physics and math you are acquainted with, even if you would like a purely qualitative description.

Staff Emeritus
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Well, I am sure there are better folk than I on these boards to explain strings, but I'll try. BTW, the all time best book on string theory for the public is The Elegant Universe, by Brian Greene. He really explains it.

First of all, a string is just what it says, the same shape as a piece of string lying on the floor, but small - very small. String size is part way between the size of a particle like a proton and the planck length, which is a calculated length that's about 10 to the minus thirtythird power meters. That's decimal point, thirty two zeros and a one. String size is more like 10 to the minus eighteen or twenty.

Don't ask what a string is made of. Anyone who has an answer is guessing or lying. The question never comes up in string theory textbooks.

So the strings can vibrate, and the vibrations are actually what we see as particles. An important case is when the ends of the string join and it becomes a loop. Then you have vibrations travelling in both directions around the loop, and they interfere and make special nodes, and these are associated with some key particles. The nodes are classified by the momentum and particle mass they generate. The first (least momentum) state is the tachyon, it has its mass-squared less than zero, so its mass would be an imaginary number. In relativity (this is all relativistic) that tells you it moves faster than light. Tachyons are bad news in quantum physics because they collapse the vacuum. Bye Bye physics?

The string theorists tried for years to get rid of tachyons, but now they sort of accept them and ask "Is there life after vacuum collapse?" and the answer is Yes.

Now the next up state on the closed string is a biggy. These particles have zero mass, and one of them is a graviton. It acts just like a graviton should, and even simulates Einstein's equations of general relativity. Who needs GR? This is one of the (legitimate) boasts of string theory.

The heavier particles further up the state ladder are not seen, they are supposed to be too massive to ever show up in our low energy world.

Finally, about extra dimensions. The equations they develop in describing these strings have the dimension of spacetime in them. And after a while you can actually derive an equation: D = 26. In superstring theory which I mention below, the equation comes out D = 10. All that about compacted dimensions is just because they have to have 26 (or 10) dimensions, and since we don't see them they have to be hidden some way, and the easiest way is to assume thay are balled up way tiny and below the scale that our physics can see.

Now this is all about the simple strings. The next advance was to link string theory with supersymmetry, another theory that had developed about the same time string theory did. When those theories were merged you got superstring theory, and I am gong to let someone else take that.

like selfAdjoint explained, once you merge string theory with supersymmetry you get superstring theory, and result that the more convenient is that the universe described by superstring theories have 10 dimensions. It was discovered that there are only 5 consistent superstring theories with 10 dimensions, that are type I,type IIA, type IIB, heterotic SO(32),and heterotic E8XE8
Also, is necessary to point that around 1985, was discovered that strings were not all the story, but only a special case of objects called p-branes. So, a string (that is a one dimensional object), is a 1-brane, a membrane is a 2-brane, but there are 3-branes, 4-branes, and so on. Then there are a special case of p-branes called D-branes, that can serve like anchorage to open strings. In fact, the brane world theory, says that particles with spin no equal to 2(like photons, quarks and electrons) are anchoraged to the brane, and this 3-brane is our universe, while particles with spin=2 (like the graviton) are not anchoraged to the D-brane and can even scape of it!
Now, in 1995, Edward Witten (who is sometimes referred like the most intelligent person alive) was able to demonstrate that the 5 superstring theories are just a special case of a more general theory, called M-theory, that also include 11-dimensional supergravity. He did it relating these 5 theories through something called dualities, the more famous of these dualities are called T-duality and S-duality (and both pertain to a more general class called U-duality)
M-theory is actually a intense object of research, because is the more promising candidate for a TOE (theory of everything)
M-theory is also a theory of quantum gravity, but not the only, its more fierce opponent is loop quantum gravity (LQG), but there also exist twistor theory, euclidean quantum gravity, Sorkin posets, simplicial quantum gravity, etc.

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Monique
Staff Emeritus
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Hi Gale, I would read the book by Brian Greene, if you have some spare time that is :) I really liked it, the only problem is that you first have to go through relativity and stuff to get to the good part.

I was planning on asking you guys what the main developments have been in the last few years? I mean, I read the Elegant Universe three years ago, any exiting experiments happened since then?

Monique
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I once saw a documentary with Edward Witten and in another episode with Brian Greene, these people are soo cool!! A one hour one to one conversation (of course one hour out of a whole day). It was about their view of the world, lots of philosophical stuff, how they grew up, what their passions are.. I think it was a Dutch production, I am not sure though.. anyone knows about this?

Monique
Staff Emeritus
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A quick google search for Edward Witten and VPRO (dutch channel).. it WAS a Dutch production.. but I guess the conversations were in English so IF anyone is interested.. at the time the videos were for sale.

"Van de Schoonheid en de Troost" Wim Kayzer
Translated: of Beauty and Consolation
What makes life worth living?, the presentor asked these people:

artist: Karel Appel (1921, Nederland),
pianist: Vladimir Ashkenazy (1937, Rusland), *
soprano: Catherine Bott (1952, Engeland),
writer: John Coetzee (1940, Zuid-Afrika),
dirigent: Richard Dufallo (1933, VS- † 1999 - kort na de opname), *
physicist: Freeman Dyson (1923, Engeland),
museumdirector: Rudi Fuchs (1942, Nederland),
ethology-writer Jane Goodall (1934, Engeland),
zoölogist en paleontologist Jay Gould (1941, VS), *
writer: Germaine Greer (1939, Australië),
writer: György Konrád (1933, Hongarije),
poeter and psychiatrist Rutger Kopland (1943, Nederland),
experimental physicist Leon Lederman (1922, VS),
psychologist Elizabeth Loftus (1944, VS),
neurofysiologist Gary Lynch (1943, VS),
violist ann dirigent Yehudi Menuhin (1916, VS - †1999), *
filosofer Martha Nussbaum (1947, VS),
filosofer Richard Rorty (VS, 1931),
historicus Simon Schama (1945, Engeland),
filosofer Roger Scruton (1944, Engeland),
writer Wole Soyinka (1934, Nigeria),
writer-filosofer George Steiner (1929, geb. in Frankrijk),
writer Tatjana Tolstaja (1951, Rusland),
writer Dubravka Ugrešiæ (1949, Kroatië),
writer Steven Weinberg (1933, VS),
scientist and mathematician Edward Witten (1951, VS),

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Staff Emeritus
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Writer Steven Weinberg?

sol1
Originally posted by Gale17
Hi, so i've just recently like really found out about string theory. I mean, i've seen posts around here on it, but i've never really followed them well. I was talk with my physics professor about some crazy thought i had and he was like "oh, string theory." heh... right, and here i thought i was some creative genius who had though up the whole idea...
ANYWAYS- someone want to sort of fill me in on like the basics? Like, the basic basics. Or at least direct me to where i can find basic info. Like, easy to understand stuff. I just want some really rudimentary info to decide whether i'd like to pursue the subject or no.

Thanks yall
~gale~
The Holographical Principle and M Theory

Now having gone through some points of view here, where has stringtheory lead us and we come to some interesting information in that link.

I was planning on asking you guys what the main developments have been in the last few years?
I haven't read The elegant universe (shame on me, shame on me), but given that the book was written in 2000, it sure don't include a idea of P.Steinhardt that he casted in 2001: the Ekpyrotic universe. It tries to describe the Big Bag this way: our universe is a 3-brane immersed in a 5-dimensional spacetime, and there's another 3-brane that is approaching to our 3-brane, til the two 3-branes collide and merge, and this collison heats up our 3-brane. According to Steinhardt, this collision is the Big Bang

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Hmm... well i follwed most everything until meteor opened his mouth...
branes? what? hmm.... well i only skimmed those links, so i'll read them in full later, and i'll check out that book soon as i find my walltet, since it has all my library cards...

I'm in calc and physics now, college level.. but i never took the high school levels. Thats why i was like 'basic basics' i don't understand too much, i just haven't learned enough. So lets pretend you're trying to explain string theory to a third grader... a wicked incredibly smart third grader... but still a third grader... how would you do it?

Originally posted by Gale17
So lets pretend you're trying to explain string theory to a third grader... a wicked incredibly smart third grader... but still a third grader... how would you do it?
Heh. That reminds me of an anecdote about the famous physicist Leo Szilard. He was trying to get a biologist to explain something to him about biology, and the biologist asked him at what level he should pitch his explanation. Szilard's response: "Assume infinite intelligence and zero prior knowledge."

So... what is it you want to know, beyond what selfAdjoint and others have already explained? The idea of string theory is pretty simple: instead of zero-dimensional point particles, like in ordinary quantum field theory, postulate that everything is made up of one-dimensional strings. Well, that was the original idea. Then it was extended to include two-dimensional surfaces -- "membranes", and higher, p-dimensional surfaces (where p <= the dimensionality of space), called p-branes. (Pun probably intended.) These strings are very small, and there is basically just one kind of string (well, not quite, depending on what you mean by that, but it will do for now) -- all the different particles we see are supposed to be just different vibrational modes of a string.

Incidentally, the question of "what are strings made of", which selfAdjoint says never appears in string theory books, doesn't appear for a reason: in string theory, strings aren't made up of anything else, they are fundamental. They just have length, and tension, and that's about it in terms of physical properties.

(Well, you can attach "charges" called Chan-Paton factors on the ends of strings that have ends, as opposed to the closed strings, and conformal fields that live on the string worldsheets, but let's not get into details.. this is third grade after all.)

sol1
Originally posted by Gale17
Hmm... well i follwed most everything until meteor opened his mouth...
branes? what? hmm.... well i only skimmed those links, so i'll read them in full later, and i'll check out that book soon as i find my walltet, since it has all my library cards...

I'm in calc and physics now, college level.. but i never took the high school levels. Thats why i was like 'basic basics' i don't understand too much, i just haven't learned enough. So lets pretend you're trying to explain string theory to a third grader... a wicked incredibly smart third grader... but still a third grader... how would you do it?
Stringtheory was lead through certain evolutions and today it is called M theory.

What was mentioned in regard to Steinhart and the Colliding branes gives some insight into the question of what existed before the big bang. Here is a article for undertanding that helps to ignite the mind into the developement that has been steadily been moving forward and this is where Steinhart comes in.

Colliding Branes(click on movie to view)

Enjoy

sol

Oy.. i can't spell wallet.. and i had to read it spelled wrong three times cause you guys had to quote me... err, dumb me...

So... what is it you want to know, beyond what selfAdjoint and others have already explained? The idea of string theory is pretty simple: instead of zero-dimensional point particles, like in ordinary quantum field theory, postulate that everything is made up of one-dimensional strings. Well, that was the original idea. Then it was extended to include two-dimensional surfaces -- "membranes", and higher, p-dimensional surfaces (where p <= the dimensionality of space), called p-branes. (Pun probably intended.) These strings are very small, and there is basically just one kind of string (well, not quite, depending on what you mean by that, but it will do for now) -- all the different particles we see are supposed to be just different vibrational modes of a string.
Ok well... i don't know anything about ordinary quantum theory... i've never really learned about any of this stuff and i've only just now even really found out about it. So would it be better if i knew something about quantum feild theory first and then tried understanding string theory? heh... that was a really good quote by the way... that's exactly the kind of thing i was trying to say.

and is there no difference between m theory and string theory? i could just look up either for the same info?

And rrg... i hate work... i haven't enough free time to really dig into those links...

Originally posted by Gale17
Oy.. i can't spell walltet..
So it seems..

So would it be better if i knew something about quantum feild theory first and then tried understanding string theory?

If you want to actually learn the guts of string theory, like how to calculate things, then you should learn quantum field theory first. But you can understand many of the concepts in string theory without knowing quantum field theory, or even quantum mechanics.

(In fact, string theory is conceptually simpler than QFT in some ways, more like ordinary QM: you do first quantization instead of second quantization, unless you're doing string field theory.. but never mind..)

My advice, from years of self-study: don't sweat the prerequisites. No matter what you study, there will be subjects that would help if you already knew them; don't let that be an obstacle. If you wait until you have all the prerequisites for something, it will be forever until you know anything, and you'll miss out on all the interesting things in life. After you have read about a subject, it will usually become obvious when a lack of prerequisites is starting to become a problem, and you need to go back and learn more. In the meantime, absorb what you can, even if you don't understand most of what is being said.

and is there no difference between m theory and string theory? i could just look up either for the same info?
M-theory grew out of string theory. Technically, "string theory" is just a special case of M-theory, but the terms are mostly interchangeable. There were several different string theories, but it was found that they were all unified into one theory, and that theory included not just strings but membranes and higher-dimensional objects -- that is what is known as M-theory.

And rrg... i hate work... i haven't enough free time to really dig into those links...
Such is life... you have to study one bit at a time.. (unless you're a grad student or professor or something, in which case you're paid to study)