# Hint needed on srting theory

1. Apr 14, 2008

### Asaba E

Well I'm just very curious about the giant experiment which is to be tested this month about the one superstring .I have studied quantum mech. but not the string theory so I dont seem to understand what its all about so could some one just brief me on string theory

2. Apr 15, 2008

### chris_183

string theory is a theory of quantum gravity in which subatomic particles are modelled as being one dimensional strings of energy vibrating in multidimensional spacetime (11 spacetime dimensions in M-theory, which is suppose to bring together all current string theories). basically, it's trying to unite quantum mechanics with general relativity. the problem is that there are many difficulties with doing this such as the so-called problem of time. another alternative theory is called loop quantum gravity and in my opinion has been far more successful than string theory.

3. Apr 15, 2008

### f-h

Ehhh.....
The LHC experiment, which I assume you are referring to, will not switch on this month, and it will take a considerable amount of time before we have results and data from it.
It also can not test String Theory or LQG. At best ideas inspired by them, but even that looks extremely unlikely.
String Theory has been mathematically much more successful then LQG with a wealth of highly non trivial mathematics emerging as well as a considerable sharpening of the theoretical physics communities understanding of gauge theories and QFT.
LQG is simpler in scope and ambition, more thorough in the conceptual issues arising from GR (though not necessarily the ones from QFT), pursued by an order of magnitude fewer people, etc.
Their focus is somewhat different then, but by any measure its hard to claim that LQG is more successful then ST.

4. Apr 16, 2008

### Demystifier

Except that LQG is mathematically more rigorous than ST. Otherwise I fully agree with you.

5. Apr 16, 2008

### chris_183

okay perhaps claiming that LQG has been more successful than string theory in every way was somewhat misstated, but i do think that LQG will prove to have more to teach us in the long run. as well as this i just find it impossible to accept what string theory has to say about reality (most obviously, that the universe actually has many more spatial dimensions than are commonly observed). LQG's simplicity is what sets it apart for me and as einstein famously said "Everything should be made as simple as possible..."

6. Apr 17, 2008

### Demystifier

Given the fact that Einstein also searched for the unifying theory of "everything", I think that simplicity of LQG is not exactly what Einstein meant by "simplicity".

7. Apr 17, 2008

### chris_183

i was using the quote to prove a point, of course i wasn't claiming that einstein was referring to LQG (especially seeing as LQG was only developed in the 1990s and einstein died in 1955!).

8. Apr 18, 2008

### Demystifier

My point was that if Einstein WERE familiar with LQG, he would probably not consider it simple.

9. Apr 21, 2008

### Riogho

String theory cannot be tested. The anthropic principle cannot be used as scientific fact. With string theory there are only three possibilities:

1) String theory is right, and the random multiverse is right so to accommodate them we must change the rules that govern science.

2) some way may be invented to test the result, and

3) String theory is not the best theory of nature. Nature is best described by another, which leads to genuine predictions, which experiments will eventually confirm.

String theory is worse then wrong. Not only does it offer no predictions but it is actually a step backward for science, due to the fact that there are actually an infinite number of string theories that may be adjusted to fit any experiment. The problem with that is the theory does not give any results, because we cannot know which theory is accurate until an experiment is performed and then there are still an infinite numbers of theories out there that all predict the same thing. “ Q: Doesn’t String theory predict particles they might discover with the LHC when it comes online in 2007? PETER WOIT: The problem is no matter what they see they can go out and find a string theory that will match the results. There isn’t a prediction at all, about what they’re going to see.”

10. Apr 22, 2008

### masudr

Thank you for your hunch. I'm sure it'll prove useless in the long run.

Can we get over Einstein please? He made several incredibly important contributions to physics about 90 years ago. These contributions were full of insight and also arose from remarkably simple observations about the universe. OK, I've done my bit.

His word isn't gospel (and it never was). Let's not assume his quotes and stuff somehow define the way the universe works.

11. Apr 22, 2008

### masudr

That's very interesting. Can you do some calculations which show this, please?

Did you know that Newton's equation of motion $f=dp/dt$ works for an infinite number of functions $f$, and that $f$ may be adjusted to fit any experiment?

12. Apr 22, 2008

### masudr

You're going to need more than a hint. String theory is a very large field of study and uses highly abstract mathematical structures. If you want to learn string theory, you need to start reading.

If you've done QM but no quantum field theory, then you need to study that first and then understand how the standard model arises as a particular quantum field theory (to wit: the particle content arises from irreducible representations of the Poincaré algebra, and the interactions arise from SU(3)xSU(2)xU(1) gauge theory).

Then there are several books, including Witten's 2 volumes, or Polchinski, or even newer stuff like Zwiebach (though he only covers the bosonic string), and Kiritsis (a nice book, benefiting from being a couple of decades away from the conception of String theory).

Once you have mastered string theory, try reading Becker & Becker's book on M-theory to understand how all that fits together.

Be warned: you will need to learn (and be good at) a lot of maths, even for the first step.

13. Apr 22, 2008

### Riogho

You don't need to do calculations to show that. Any credible string theorist will admit that there are an infinite amount of possible string theories. With at least 10^500 of them with a positive cosmological constant.

Stop being a douschebag.

14. Apr 22, 2008

### masudr

No, there are 10^500 vacua, that's slightly different from there being 10^500 theories.

Also, if you are going to call someone a douchebag, learn to spell it right.

15. Apr 22, 2008

### nrqed

And any credible physicist will admit that string theory is still in its infancy. There are still much we don't understand about the theory and there is no way to tell if a deeper understanding of the theory will not radically change the situation regarding the possible vacua.

16. Apr 22, 2008

### humanino

Name one you have spoken to.

Reading one book and imagining you got the point does not give you authority to call people names.

17. Apr 22, 2008

### nrqed

I am curious..what is your background in physics?

In some very objective sense, work on string theory has already led to more useful scientific results in science than LQG. Witten did not receive the Field medals for nothing. String theory had led to new mathematical results!! So as a scientific theory it has already done more than LQG (if we include maths as part of science). In addition it has led to powerful calculational tools for computations in point particle QFT. So even if string theory would happen not to be a description of Nature, it would still be a useful tool to do physics.
As far as a description of Nature, has LQG made testable predictions? So why is it better than string theory?

LQG incorporates only gravity, not the other forces of Nature. String theory does. IN LQG, gravity has to be put in by hand from the start. String theory automatically predicts gravity. In LQG the number of spacetime dimensions has to be put by hand. In string theory, this number is fixed by the theory. And so on.

I could say that LQG has an infinite number of theories...one LQG theory in 4 spacetime dimensions, one in 5, 6 ,7 .......

On the other hand, string theory can only exist in a specfic number of spacetime dimensions.

Therefore string theory is more predictive!

18. Apr 22, 2008

### humanino

Just to make this point very clear, because it is important to people in my field. From our point of view, string theory originated as an effective model for strongly interacting systems. String theorie(s) have actually completed this design goal to a great extent today, by providing many fresh ideas in non-perturbative QCD, namely AdS/QCD. I am aware that this is a narrow aspect in the broad field of string theory research, but at least it is one where, whithout any doubt, you have experimental predictions, testable models, potential money, already now.

Effective vs fundamental, who knows exactly where one ends and the other begins...

19. Apr 26, 2008

### Riogho

I didn't say anything about LQG.