Thread: What good is string theory?

Hi!

When quantum mechanics was established it allowed us to do things that we could not do before.

What would string theory allow us to do? Lets say that string theory was confirmed -- tests showed the projections string theory made were correct and now string theory was accepted. What would it allow us to do that we can't do now? What good would it do us?

Thanks,
Rusty

Originally Posted by rrw4rusty

Hi!

When quantum mechanics was established it allowed us to do things that we could not do before.

What would string theory allow us to do? Lets say that string theory was confirmed -- tests showed the projections string theory made were correct and now string theory was accepted. What would it allow us to do that we can't do now? What good would it do us?

Thanks,
Rusty

First, you need to understand that string theory has not yet even been clearly formulated. There is a lot of work left to do just to define what it really is. It also remains for string theory to produce a new testable prediction. That has not yet happened either.

That said, there is general problem in physics. There are three fundamental physical theories -- the electroweak theory, quantum chromodynamics and general relativity.

The electroweak theory and quantum chromodynamics are quantum field theories that separately address 1) a unified theory of the weak and electromagnetic interactions 2) the strong nuclear interaction and 3) gravitation. These theories, as they stand are not unified and in fact general relativity and the quantum field theories are not compatible.

One goal of research in theoretical physics is to develop a single unified theory that explains the electromagnetic, weak, strong and graviational forces. String theory is an attempt to do just that.

There are open questions in physics that such a theory would presumably help to answer. One is the information problem for black holes. Another is the physics of the universe near the time t=0 of the big bang. Another is the discrepancy between the cosmological constant predicted in terms of the zero point energy of the quantum vacuum and the observed expansion of the universe -- the prediction based on quantum electrodynamics is in erro by 120 orders of magnitude.

As with most physical theories we cannot say at this time what all future applications might be. Ordinary quantum mechanics as applied to solid state physics gave us the transisitor, integrated circuits, modern computers, and lasers. Who knows what deeper understanding could yield ? Applications generally come quite a bit after research scientists develop fundamental laws, since it is in the hands of engineers that new products are developed based on the understanding that results from the basic science.

String theory, like any avenue of research may or may not prove fruitful. It may pave the way to great things. Or it may prove to be just a dead end in terms of physics. The pursuit of string theory has produced some spectular new mathematics.

Thanks for replying. I already know everything you told me. I was just trying the waters at a dozen science forums to see if anything came up. If it did, that might lead to a 'prediction' we could test for.

r

Originally Posted by rrw4rusty

Thanks for replying. I already know everything you told me. I was just trying the waters at a dozen science forums to see if anything came up. If it did, that might lead to a 'prediction' we could test for.

r

So, you try to elicit from a public science forum something that the best professional minds in theoretical physics have tried but been unable to produce over the last 20 years or so and also claim to have a reasonable understanding of the subject ?

You have got to be kidding.

I'm a sci-fi writer (perfect for m-theory eh?). Watch all the videos made for laymen that you can find on the Internet plus a dozen of so audio lectures listened to on the tread mill and at night to put me to sleep and you'll know exactly what I know.

I got what I wanted from my post and a lot of people had a lot of fun answering. I didn't expect to find what the best minds in the field have looked for but I got some stuff I can use in my story.

I am sorry if my methods offended you.

r

Originally Posted by rrw4rusty

Thanks for replying. I already know everything you told me. I was just trying the waters at a dozen science forums to see if anything came up. If it did, that might lead to a 'prediction' we could test for.

r

Is the prediction that the phenomena GR and the Quantum field theories address will both come into alignment good enough? Or maybe Dark Matter will cease to be necessary as a means of explaining the orbits of distant stars around a galaxy's center.

There's the trouble. The things we're hoping to predict have already been observed. We'd have to come up with an observation nobody has ever thought to make in order to genuinely confirm a new theory. That's quite a feat. Probably we'll know what to look for once we have the theory in hand, but until then, I guess not.

Originally Posted by kojax

Originally Posted by rrw4rusty

Thanks for replying. I already know everything you told me. I was just trying the waters at a dozen science forums to see if anything came up. If it did, that might lead to a 'prediction' we could test for.

r

Is the prediction that the phenomena GR and the Quantum field theories address will both come into alignment good enough? Or maybe Dark Matter will cease to be necessary as a means of explaining the orbits of distant stars around a galaxy's center.

There's the trouble. The things we're hoping to predict have already been observed. We'd have to come up with an observation nobody has ever thought to make in order to genuinely confirm a new theory. That's quite a feat. Probably we'll know what to look for once we have the theory in hand, but until then, I guess not.

Dark matter is not a driver. The issues that are the impetus for the dark matter hypothesis are evident in models based on Newtonian gravity. You don't even need general relativity. Quantum mechanics is essentially irrelevant.

You run into problems in a couple of areas. First, just on general principles quantum mechanics and general relativity are fundamentally incompatible. QM is a stochastic theory, and predicts only probabilities. General relativity, on the other hand, is a completely deterministic theory.

There is also the matter of the singularities that one finds predicted by general relativity for black holes and the big bang. There are approaches that eliminate these singularities, Einstein-Cartan theory for instance, but that does not really answer the important physical questions. The problem is that whether you use Einstein-Cartan theory or standard general relativity, you are force to deal with conditions that include both very large gravitational effects and very high densities in which quantum effects would be expected to be important. GR cannot handle the quantum effects and quantum theory cannot handle the high gravitational effect. What is needed is a theory that incorporates both in a compatible manner. The Black Hole Information problem also requires that one simultaneously address high gravitational fields and quantum effects. A unified theory is needed to address such questions.

As mentioned earlier, there is a big disconnect between the observed cosmological constant and that which is predicted by our understanding of the zero point energy of the vacuum and its implications for the stress-energy tensor of general relativity. Something is clearly amiss there.

We don't in fact, even have a unified quantum theory that incorporates both the electro-weak and strong interactions. There is research in progress to try to develop such a theory, and there has been for many years. A successful theory of this type would be called a Grand Unified Theory or GUT. One that would also include gravity has been called a Theory of Everything (TOE). Unfortunately we have neither a GUT nor a TOE.

Finally, there is simply the issue of intellectual curiosity and the desire to understand how nature works. We know that gravity is important. We know that quantum effects are important. We strongly desire a theory that can handle both. Nature clearly does it. We want to understand how.

None of this is likely to make an immediate big difference in technology. Quantum electrodynamics for ordinary phenomena is pretty good. Good enough for everyday things like electronics and chemistry. But it is clear that there are some major gaps in our understanding of nature.

There are several "testable" aspects of string theory that are not present in the standard model. For example, many people in my field (including myself) have searched for the possibility that the fundamental constants are changing in time. This is allowed by string theory, but not by the standard model.

The problem is, however, that these "exceptions" are allowed by other theories as well - and string theory doesn't "predict" them, it just "allows" them. The thing that really sets string theory apart from other theories is that it is pretty - in a mathematical symmetry sense. But pretty doesn't count for much to an experimentalist.

Take a look at http://www.sqcomic.com/archive.html?20091101 for the full story.

Originally Posted by ingenio

There are several "testable" aspects of string theory that are not present in the standard model. For example, many people in my field (including myself) have searched for the possibility that the fundamental constants are changing in time. This is allowed by string theory, but not by the standard model.

The problem is, however, that these "exceptions" are allowed by other theories as well - and string theory doesn't "predict" them, it just "allows" them. The thing that really sets string theory apart from other theories is that it is pretty - in a mathematical symmetry sense. But pretty doesn't count for much to an experimentalist.

Take a look at http://www.sqcomic.com/archive.html?20091101 for the full story.

It might help if someone, anyone, could rigorously define string theory, or indeed any of the may string theories.

If there is an experiment that could confirm or deny time dependence of fundamental constants they I sugges that some experimentalist do them. I suspect that success comes with a ticket to Stockholm.

Susskind claims that there are something like 10^500 string theories, based on allowable physical laws. I don't doubt that someone could cook up a version in which the defining constants change in time. That strikes me as a liability rather than a virtue.

Heck, there doesn't even seem to be agreement as to whether supersymmetry is required for string theory. I think most string theorists are of the opinion that it is, but Susskind has written differently.

Will the real string theory please stand up ?

Hi,

I love cosmology, quantum mechanics, relativity, string theory and especially M-Theory and I've followed all of these in 'layman' (i.e. no math) books, videos, lectures, forums and the news since '76 or in M-Theory's case since there was material available. Despite all this I'm sure I know very little. That said...

Dark matter is not a driver. The issues that are the impetus for the dark matter hypothesis are evident in models based on Newtonian gravity. You don't even need general relativity. Quantum mechanics is essentially irrelevant.

It seems to me that 'Dark Matter' is really 'Dark Gravity' for this is what we're talking about and 'Dark Matter' assumes you need matter for the mysterious gravity we see.

On one had we have the mystery of gravity's weak strength (compared to other forces) or you could say 'missing gravity'.

On the other hand we have Dark Matter which is... 'extra gravity'.

Lets see... missing gravity... extra gravity... hm... no connection?

M-Theory suggests one with gravitons that drift between branes which I've spelled out in other posts and I won't do it here. Has no one considered this? Is it stupid and naive? If you added the force of gravity to that supposedly produced by dark matter would you have as much as the other forces?

Just dust from the armchair.
Rusty

Originally Posted by DrRocket

Susskind claims that there are something like 10^500 string theories, based on allowable physical laws. I don't doubt that someone could cook up a version in which the defining constants change in time. That strikes me as a liability rather than a virtue.

Heck, there doesn't even seem to be agreement as to whether supersymmetry is required for string theory. I think most string theorists are of the opinion that it is, but Susskind has written differently.

Will the real string theory please stand up ?

It doesn't matter how many string theories there are as long as they agree where observable phenomena are concerned.

For example, think of electromagnetic potential. The actual value of the electromagnetic potential V(x) is not in itself a physical quantity. What's physical is the gradient. So you have a situation in which there are an uncountable number of ways of describing a physical system, but each of these ways predicts the same physics.

To give a more interesting example, if you have a bosonic string propagating on a closed circle of radius R, then the physics is essentially unchanged if you replace R with 1/R. The physically meaningful quantity, the partition function of the system, is in fact invariant under this transformation. This observation, called T-duality, has led to the discovery of much more profound dualities in string theory, such as mirror symmetry.

So having more than one string theory is not a bad thing so long as the underlying physics is the same.

As far as supersymmetry goes, my sense of things is that string theory is not much of a subject without it. I think actually that I'd be more surprised if supersymmetry was false than if string theory were false. One of the things I find compelling about supersymmetry is that a lot of beautiful and interesting mathematics (example, Hodge theory, index theory, Morse theory) just kind of falls naturally into your lap via the supersymmetry point of view.

Originally Posted by salsaonline

Originally Posted by DrRocket

Susskind claims that there are something like 10^500 string theories, based on allowable physical laws. I don't doubt that someone could cook up a version in which the defining constants change in time. That strikes me as a liability rather than a virtue.

Heck, there doesn't even seem to be agreement as to whether supersymmetry is required for string theory. I think most string theorists are of the opinion that it is, but Susskind has written differently.

Will the real string theory please stand up ?

It doesn't matter how many string theories there are as long as they agree where observable phenomena are concerned.

For example, think of electromagnetic potential. The actual value of the electromagnetic potential V(x) is not in itself a physical quantity. What's physical is the gradient. So you have a situation in which there are an uncountable number of ways of describing a physical system, but each of these ways predicts the same physics.

To give a more interesting example, if you have a bosonic string propagating on a closed circle of radius R, then the physics is essentially unchanged if you replace R with 1/R. The physically meaningful quantity, the partition function of the system, is in fact invariant under this transformation. This observation, called T-duality, has led to the discovery of much more profound dualities in string theory, such as mirror symmetry.

So having more than one string theory is not a bad thing so long as the underlying physics is the same.

As far as supersymmetry goes, my sense of things is that string theory is not much of a subject without it. I think actually that I'd be more surprised if supersymmetry was false than if string theory were false. One of the things I find compelling about supersymmetry is that a lot of beautiful and interesting mathematics (example, Hodge theory, index theory, Morse theory) just kind of falls naturally into your lap via the supersymmetry point of view.

I don't disagree.

However there seem to be a couple of problems: The conjecture that M-theory unites 5 competing string theories is still a conjecture.

According to Susskind there are something like 10^500 string theories that produce different physics. That is the basis for his "landscape" of universes.

I think I agree with you on supersymmetry. But I remain a bit confused as to the precise logical role of supersymmetry in string theories. I think that most, if not all of them, are dependent on supersymmetry. But I have seem some writings to the contrary -- particularly some popularized stuff from Susskine.

It is good to see that you have not disappeared entirely.

I find this claim about 10^500 string theories producing different physics both interesting and a little suspicious. Can you provide a reference so I can see for myself what the context is?

Originally Posted by salsaonline

I find this claim about 10^500 string theories producing different physics both interesting and a little suspicious. Can you provide a reference so I can see for myself what the context is?

It comes out in Leonard Susskind's book The Cosmic Landscape

I think quite a few things in that book are a little suspicious.

Originally Posted by DrRocket

Originally Posted by salsaonline

I find this claim about 10^500 string theories producing different physics both interesting and a little suspicious. Can you provide a reference so I can see for myself what the context is?

It comes out in Leonard Susskind's book The Cosmic Landscape

I think quite a few things in that book are a little suspicious.

The 10^500 figure sounds like he's counting Calabi-Yau 3-folds. And, okay, to the extent that these 3-folds may have different topologies, they may give rise to different physics. But given the impact of mirror symmetry (which started from the observation that two different Calabi-Yau 3-folds can produce the same physics), this objection seems a little short-sighted.

The Calabi-Yau condition on a 3-fold, while not enough to nail down the structure, is still pretty strong. For example, birational Calabi-Yau n-folds have the same Hodge numbers and the same elliptic genus (the second of these is a partition function in (2,2) super-conformal field theory).

As much as we can sit here and say what it could possibly be used for the fact is that most uses for proven theories are not shown until AFTER they have been formally accepted or proven and have actually had confirmed equations out, for all we know the confirmation of String Theory could allow us to walk through walls, however you will never actually know what effects it could have until solid proof/evidence confirms it and creates credible equations

Originally Posted by salsaonline

Originally Posted by DrRocket

Originally Posted by salsaonline

I find this claim about 10^500 string theories producing different physics both interesting and a little suspicious. Can you provide a reference so I can see for myself what the context is?

It comes out in Leonard Susskind's book The Cosmic Landscape

I think quite a few things in that book are a little suspicious.

The 10^500 figure sounds like he's counting Calabi-Yau 3-folds. And, okay, to the extent that these 3-folds may have different topologies, they may give rise to different physics. But given the impact of mirror symmetry (which started from the observation that two different Calabi-Yau 3-folds can produce the same physics), this objection seems a little short-sighted.

The Calabi-Yau condition on a 3-fold, while not enough to nail down the structure, is still pretty strong. For example, birational Calabi-Yau n-folds have the same Hodge numbers and the same elliptic genus (the second of these is a partition function in (2,2) super-conformal field theory).

If you have the time and inclination to read it I would be very interested in hearing your opinion of that book. It should be available at most libraries, and is pretty inexpensive on line (a couple of bucks used). My personal opinion is that it is interesting but should be read with more than one grain of salt.

Hey guys, I really found this thread interesting I don't know if because I'm medicated with marijuana but this tripped me out. However, you guys use these terms, I don't know where I have been at. I'm 19 and going to CSUN am i behind for not knowing this info?

Sorry correction i turned 20 on November 7th.

Originally Posted by NewbieAlert

Sorry correction i turned 20 on November 7th.

But I, like, totally forgot, dude.

Yes I accidentally forgot. Anyways, I like the whole idea about there being space within solids or something along those lines. Sorry but I don't know the big words.

Originally Posted by NewbieAlert

Sorry but I don't know the big words.

Don't worry, you'll learn. Or they will come back to you in a couple of days.

Originally Posted by rrw4rusty

Hi!

When quantum mechanics was established it allowed us to do things that we could not do before.

What would string theory allow us to do? Lets say that string theory was confirmed -- tests showed the projections string theory made were correct and now string theory was accepted. What would it allow us to do that we can't do now? What good would it do us?

Thanks,
Rusty

One of the major goals of sting theory was to unite quantum mechanics with General Relativity and it was thought that eventually a so-called mathematical theory of everything could be developed. String theories, called M-theory in their collective, started out with several big handicaps. It had to assume many dimensions of reality that had never been observed, and it assumed that the foundations of reality were string-like rather than particle like, which also had never been observed. In the course of time many strictly mathematical problems with string theory continued to surface. Originally high hopes were given to string theory but today it receives only a small portion of grant funding that it once did. Accordingly if it had validity, conceivable all other equations in physics might somehow be derived from it, and maybe new applicable equations might have been derivable. But string theory in general has made few predictions, and none of these few predictions have been observed to date. Many or most outside the field may now consider string theory a dead end.

String theory also assumed that both Quantum Mechanics and General Relativity are valid theories themselves, but today not all in science believe this is an indisputable assumption.

I personally believe that conventional string theory proposes extra dimensional complications that are not needed to explain reality, therefore historically I expect M-theory to be noted as a curiosity related to the Woo Woo theoretical physics of our times.