String theory in one sentence please

[suprised]

So here a single-sentence characterization:

String theory is a physical model based on tiny strings that incorporates both quantum mechanics and general relativity and attempts to unify gravity with particle physics.

 

[Bob_for_short]

We can use it for weak fields where pathologies are absent.

Only weak field effects are not sufficient to "prove" or prefer the entire GR.

Have you calculated g-2 in your theory? Have you also achieved an agreement with experiments up to 10 decimals (as QED has)?

Not yet. No one can fulfil the forth order QED calculations in one article.

By the way, agreement "up to 10 decimals" and even better can be obtained for any normal function developed in Taylor series up to the fourth order with the small parameter of about 0.001 ( ≈ α/2π ). There is too much exaggeration about predictive force of QED in this respect.

 

[Demystifier]

Only weak field effects are not sufficient to "prove" or prefer the entire GR.

With that I agree.

Not yet. No one can fulfil the forth order QED calculations in one article.

I wish you a good luck.

By the way, agreement "up to 10 decimals" and even better can be obtained for any normal function developed in Taylor series up to the fourth order with the small parameter of about 0.001 ( ≈ α/2π ). There is too much exaggeration about predictive force of QED in this respect.

But the problem is that in QED you do not know this "normal function" explicitly.

 

[Bob_for_short]

I wish you a good luck.

Thanks. You know, for some Lagrangians it is sufficient to prove renormalizability to start working with them seriously. In my case the theory is divergence-free which is much better than renormalizability.

...the problem is that in QED you do not know this "normal function" explicitly.

That's right. QED has tought us many wonderful things and I really hope it's not finished yet.

By the way, I encountered a case (not in QED) when the exact function is known explicitly but its Taylor series does not coincide with the perturbation theory expansion! See formula (A3.5) and page 16 in arXiv:0906.3504.

 

[Galteeth]

So here a single-sentence characterization:

String theory is a physical model based on tiny strings that incorporates both quantum mechanics and general relativity and attempts to unify gravity with particle physics.

String theory is an attempt to mathematically unify quantum mechanics and general relativity by using strings vibrating in multiple spatial dimensions as the fundamental building blocks of the universe.

 

[Demystifier]

The question remains: why strings? Screws are much better for unifying.

Screws do not predict gravity and gauge interactions.
Screws do not avoid UV singularities.

 

[Finbar]

String is a purely mathematical attempt to model reality. It is not provably wrong [and may never be], but, empirically unsatisfying thus far. It explains some observations, mostly at the quantum level, but otherwise has no compelling observational support.

Edit

String is a purely mathematical attempt to model reality. It is not provably wrong [and may never be], but, empirically unsatisfying thus far. It has no observational support.

 

[Entropee]

Thanks Marcus, It's an essay that nobody in my class will understand so i won't be graded down on it, but I am very interested in knowing more about the subject and where I went wrong. I was under the impression that when they smashed the particles together, there might be a loss in mass, and the mass had to go somewhere, so into another dimension? I am not sure I am only 17 so you guys obviously know way more about it than I do, any more info would be appreciated, and thanks again Tom for your summary. (Thanks to everyone else also contributing to the post too).

 

[tom.stoer]

Congratulations to you for keeping with us and surviving all these discussions.

I would still propose my explanation as a starting point

"String theory tries to construct a supersymmetric framework in 10 dimensions from which all known elementary particles and interactions including gravity should emerge from the modes of an one-dimensional, vibrating string."

but of course I agree with many aspects mentioned by others, especially (some changes from my side to achieve the one-sentence goal)

"It is a purely mathematical attempt to model reality which is not provably wrong [and may never be], but, empirically unsatisfying thus far and w/o any observational support."

I think those two statement could be the core of your talk or small paper or whatever. Then you have to explain a bit what it means ...

 

[suprised]

"String theory tries to construct a supersymmetric framework in 10 dimensions.."

In four dimensions as well, of course... ever heard about that ?

" purely mathematical attempt "

What means "purely" mathematical? Of course there are physical principles built in, like quantum mechanics, actions, scattering amplitudes etc... in particular it is based on the physical picture of a string, and the action is a manifestation of that. How can one call this pure mathematics? Are other theories of quantum gravity (I should say much less founded attempts) any less "mathematical"?

I suspect "purely mathematical" simply needs to be translated "incomprehensible to me".

Why people who have no idea about this subject confuse the discussion with second hand opinions and misleading, if not outright incorrect statements. No wonder why any expert in this field is driven out from this forum here. Good bye.

 

[Fredrik]

Tom, I think your sentence would be much better, at least for "normal" people, if you just replaced the word "supersymmetric" with "mathematical".

I'm not crazy about Finbar's sentence. I think it's pretty misleading actually.

a) it's misleading to call it "purely mathematical"
b) words like "unsatisfying" are very subjective shouldn't be included in a brief explanation of what the theory says
c) the fact that you're not floating away from the chair you're sitting on is experimental support for string theory, so you should find a better way to express your thoughts about the experimental situation.

A few comments about a): There's no such thing as a purely mathematical theory, and a mathematical model is always purely mathematical. A theory consists of a mathematical model and a set of axioms that tells us how to interpret the mathematics as predictions of results of experiments. It's OK to say that string theorists are trying to find an appropriate mathematical model, and that their work at this time is therefore "purely mathematical", but if they don't ever do anything more than that, it won't ever be a theory. So it's wrong, or at least misleading, to talk about string theory as a "purely mathematical attempt". What they're attempting to do is certainly more ambitious than to just find some cool mathematics.

 

[Bob_for_short]

The problem is that the UV divergence of QG is managed with some cut-off (string size) and a great deal of physical meaning is given to it without experimental evidence whereas a cut-off is just an awkward stop-gap.

 

[tom.stoer]

"In four dimensions as well, of course... ever heard about that ?

The space in which strings move is 10-dim. Everything else is refinement, solving the equations, constructing vacua etc. The starting point and one of the core problems is that it's 10-dim. One does not know how to select the "correct" vacuum = our universe in 4 dim. And one does not even know why 6 dims are compactified; why not less or more?

What means "purely" mathematical?

It's quite simple. w/o suport from experiments it is a mathematical excercise, not phsics. It will become a physical theory if it either post-dicts some already know particles, masses, coupling constants etc., or if it makes at least one falsifiable prediction.

Are other theories of quantum gravity (I should say much less founded attempts) any less "mathematical"?

As long as physical predictions atre missing they suffer from the same problems.

No wonder why any expert in this field is driven out from this forum here. Good bye.

If they are not willing to participate it's not my problem. The problem is neither that we are talking about weaknesses of the theory nor that the theory has weaknesses. The problem is that the experts are partially not willing to even accept that these issues exist. Best example is the finiteness of the theory: there's the claim that ST is finite in all orders PT. The problem is that beyond 2 loops there is not even a definition of the amplitude!

 

[Coin]

I think the problem with describing string theory in one sentence is there isn't exactly just one "string theory". 1990s string theory is somewhat different from modern M-theory is different from AdS/CFT. I'd attempt something like:

"String theory" refers to a group of related research programs, which share the starting point of suggesting things that appear to be particles can be thought of as a side-effect emerging from the vibrations of one-dimensional strings.

And then explain what this has to do with the LHC.

Researchers believe that if string theory is real and not just a quirk of mathematics, particles must be "supersymmetric": this means every type of particle has a rare "superpartner" which has so far never been seen, but which the LHC would probably be able to find.

That sound about reasonable?

 

[tom.stoer]

I agree with the first sentence; its focus is slightly different, so if it could me merged with mine it would be OK.

Regrading the LHC there's a problem: if the LHC finds SUSY this is a hint for a support of strings, NOT a proof (SUSY / SUGRA could exist even if ST does not); if the LHC does not find SUSY it simply may be there at even higher energies. So either way ST could survive by making the right turn - and that's the core roblem with it: anything goes ...

 

[Entropee]

Now that I am done with this retarded essay that I got a B on (the only coment on the entire essay said "too technical") quite a few months ago, maybe we can continue to talk about this? If anyone has anything else to say that is...

 

[Younger Dryas]

Your teacher has given you a trick question; since the sad truth is that string theory
has never been stated in one sentance or in a thousand sentaces! You will learn
nothing about physics in it, but you can learn much about the social and psychological
forces at work in modern day university science.

String theory is now over 30 years old, and has yet to make a single testible prediction.
Articles in the popular press about LHC testing string theory are hype. Piles of books, and reams of video documentaries boil down to jibberish. Even with a high school education
you can watch "The Elegant Universe" and see the lack of any real science content.

Those that so despirately promote the theory are usually trying to defend their
job position.

But the real importance of string theory is the lasting damage it has done to physics and
science in general. The accepted rules of the scientific method have been thrown out
the window.

To see how absurd things have gotten, read up on the "Anthropic" principle.

Also take a look at the books written by Peter Woit and Lee Smolin, and see how
violently they have been attacked by the string community!

Woit's blog is a good place for a reality check:

http://www.math.columbia.edu/~woit/wordpress/

 

[Entropee]

I agree that the strong and weak anthropic principles are one of the most rediculous things I have ever read about but I'll have to check out Woit's blog.

 

[tom.stoer]

Again: here's my proposal:

String theory tries to construct a supersymmetric framework in 10 (11) dimensions from which all known elementary particles and interactions including gravity emerge (uniquely) from the modes of an one-dimensional, vibrating string.

Questions:
- what was your porposal?
- what was your teacher's proposal?

 

[Entropee]

It was just an analysis of the LHC and I wanted to include a part about how it was a possible test for string theory but she told me I had to define SR in one sentence or it would be too technical...

 

[darkside00]

String theory is ridiculous.

Hardly a sentence

 

[rai linga]

I will offer a simpler, and somewhat philosophical explication of what, I believe, Superstring theory represents. It also causes me to be convinced that the basic idea of the theory pretty much has to be right!

At base, I think it represents the resolution of a confusion about the relationship of mathematics to physics. Physics requires some physical extension in "reality"; whereas math is pure theory. In math, one commonly utilizes the idea of dimensionless points. Physics, which is so used to importing ideas from math, seems to have forgotten the philosophical difference between the 2 disciplines. It began utilizing the idea of "point-particles" ---which were conceived of as "dimensionless"; i.e. without physical extension. To my mind, with Superstring theory physicists (through extremely difficult mathematics) finally "came to their senses".

Physics is a discipline, unlike mathematics, where some physical extension is absolutely necessary. Thus, it was never possible that there could have been such thing as "dimensionless" "point" particles. Though subatomic "particles" are conceived to be the smallest possible "things", they still need to have some physical extension in order to "exist" in "reality". Superstring Theory, philosophically, recognizes that the smallest possible physical extension must be a one-dimensional entity (once you accept that "dimensionless" cannot be a physics construct, there must be at least one physical dimension in play). Yet, of course, the most basic physical entity must also be the "smallest possible" one-dimensional entity.

Now, most people would believe that there is no such thing as the smallest possible thing. One would logically assume that "you can always cut it in half" and get something smaller. This logical assumption is very similar to the belief that, however fast something is moving, it can always go a little faster. Einstein proved, at the time shockingly, that the latter assumption was wrong. You cannot always go a little faster. The universe has a "speed limit", and nothing can ever accelerate to the speed of light. Yet few people realize that, just like the false assumption about speed, so our assumption that there is no limit to smallness is also wrong.

All physicists realize that the is a limit to smallness ---it is called Planck Length. So, at last, we can reasonably talk about "least possible physical extension" as a meaningful physics construct replacing the construct of "dimensionless" mistakenly imported from mathematics. The simplest physics entity that could ever "exist" would be a one-dimensional entity of Planck Length. Is it, then, surprising that this turns out to be the very definition of a Superstring?

Lo and behold, when the universe is viewed in terms of this simplest of all physical construct, we see the very term "particle" becomes replaced by the more accurate metaphor "string". Then we suddenly see that the vast "zoo" of physics entities formerly called "particles" are understood as Superstrings vibrating differently in 6 dimensions (within a larger 11 dimensional hyperspace).

As has been said often, each different string vibration is like a different musical note. Like one guitar string can create a B flat, an F#, or a myriad of other notes; so each superstring can vibrate to create of create a myriad of subatomic structures ---photons, electrons, neutrinos, gravitons, etc.

Anyways, that's the simple way I think of Superstring Theory such that it seems to me so obviously right.

 

[robheus]

String theory is another failed attempt to describe everything without any experimental background.

To my knowledge, this is the best description of string theory.

 

[analmux]

I did some graduate student research on superstring theory, but I lost my enthousiasm for doing any more research in this topic totally.

I also do mathematics, but a lot of mathematics, used in many of the articles I had to study, isn't always used in a clear way. It took ages to realize it's not my fault I didn't understand some parts.

 

[nrqed]

"It is a purely mathematical attempt to model reality which is not provably wrong [and may never be], but, empirically unsatisfying thus far and w/o any observational support."

This is of course an exaggeration and a gross misrepresentation of the situation. Has anybody proved that string theory is not provably wrong? Of course not. The fact that people haven't been able so far to make testable predictions does not prove that it will never happen. I think it is wrong to mislead the OP this way.

 

[tom.stoer]

This sentence (which goes back to Chronos; refer to post #16) refers to the status as of today. Of course this situation could change in the future - nobody knows.

String theory either produces low-energy effective theories which are not unique and therefore do not provide experimentally testable predictions (a) or it produces high-energy / Planck-scale predictions which are out of reach (b). Then there are some predictions (c) which can always be pushed to higher energies making them unvisible, such as supersymmetry.

Regarding (a) there are candidates which are rather interesting but which string theory can't derive so far (e.g. standard model particle spectrum and coupling constants).
Rearding (b) I am not sure if there are predictions which can be tested in principle (scattering crosss sections), however as of today they cannot be tested in practice.
Regarding (c) one should just check what we expect for the LHC: all predictions to be tested at LHC come from SM or MSSM or some variations); large extradimensions / black holes do not originate from string theory - strictlyspeaking. They may be there or they me be not there; it doesn't matter for string theory.

It is not up to somebody else toprove that string theory is not provable wrong. It's just an observation of the current status. That's why there is the small remark [and may never by].

Let's discuss "It is ... not provably wrong ... and w/o any observational support."

Can you tell us one prediction which is
- unique to string theory (i.e. not derivable via e.g. the MSSM and/or SUGRA only)
- makes a unique prediction for string theory (not for a specific solution)
- and which has the potential to kill once experimentally falsified?

 

[nrqed]

Let's discuss "It is ... not provably wrong ... and w/o any observational support."

Yes, let's discuss this precise statement (and nothing else).
Can you prove that string theory (the *theory*, not our actual knowledge of some solutions) is not provably wrong? The answer is a simple Yes or No. If the answer is no, then the statement was false. If the answer is yes, then the statement is true. That's all that I am saying.

Can you tell us one prediction which is
- unique to string theory (i.e. not derivable via e.g. the MSSM and/or SUGRA only)
- makes a unique prediction for string theory (not for a specific solution)
- and which has the potential to kill once experimentally falsified?

No but it does not prove that the theory is not provably wrong. Only that we haven't found a way yet to falsify it. That's a completely different statement. That's all I am saying.

 

[tom.stoer]

Please read carefully what we have said so far: "... is not provably wrong [and may never be] ..." says that the theory currently is not provably wrong and may never be (the latter one is a guess, of course, indicated by "may"); nobody claimed that there is proof that this holds in principle and forever; it's a description of the status quo.

Your "No but ..." confirmes this assessment of the status quo (at least from your perspective) as you cannot tell us any such experimental prediction.

 

[analmux]

To my mind, with Superstring theory physicists (through extremely difficult mathematics) finally "came to their senses".

Question: What's so extremely difficult about the mathematics used in superstring theory?

Physics is a discipline, unlike mathematics, where some physical extension is absolutely necessary. Thus, it was never possible that there could have been such thing as "dimensionless" "point" particles.

I think saying an object is dimensionless differs from saying it can be regarded as dimensionless.

> Superstring Theory, philosophically, recognizes that the smallest possible physical extension must be a one-dimensional entity (once you accept that "dimensionless" cannot be a physics construct, there must be at least one physical dimension in play).

Question: What's so philosophical about realizing that this physical extension must be one-dimensional?

Now, most people would believe that there is no such thing as the smallest possible thing. One would logically assume that "you can always cut it in half" and get something smaller. This logical assumption is very similar to the belief that, however fast something is moving, it can always go a little faster. Einstein proved, at the time shockingly, that the latter assumption was wrong. You cannot always go a little faster. The universe has a "speed limit", and nothing can ever accelerate to the speed of light.

For an outside observer this might be true, but we should also realize there is no real "kinetic energy limit". When traveling nearly at light speed my proper time will be slower with respect to an outside observer. This means that it won't take me at least 10 years to reach a star at a 10 lightyears distance. If I have enough energy I can reach the star within the time I desire. In reality this is out of the question, because there's always an energy limit, and to accelerate to such speed the only way is to use the rocket technique of shooting away mass. So, even if nuclear fusion energy engines were possible, there's still a problem.

All physicists realize that the is a limit to smallness ---it is called Planck Length. So, at last, we can reasonably talk about "least possible physical extension" as a meaningful physics construct replacing the construct of "dimensionless" mistakenly imported from mathematics.

As a comment on you saying "mistakenly imported": I think we should realize that, during centuries of development of mathematics and physics, many topics of basic mathematics are inspired by human intuition of understanding how the world turns. The concept of 'dimension' is clearly inspired by doing ordinary (physical) measurements in the real world. Saying we live in a 3-dimensional space is nothing more than saying we need to use a ruler 3 times to find the position of an object. Now, what mathematics does is simply making up an imaginary ideal case scenario, say Plato's imaginary world of perfect circles, triangles etc.

The simplest physics entity that could ever "exist" would be a one-dimensional entity of Planck Length.

A 1-dimensional object, or string, embedded in our space is defined to be a thing with finite length, but with zero width or height. Thus a string is still "dimensionless" in the "transverse" direction. So, I wonder what's the conceptual difference between dimensionless things, like points, and 1-dimensional things.

 

[rai linga]

Very interesting response. Again, I stand corrected on may of the points you make. I'm not a mathematician or a physicist, so I know my limits. I would just suggest by your last comment that mathematics is different in kind than physics ---one doesn't seamlessly blend into the other ----like chemistry blends into biology, for example. When you say mathematics sets up an "imaginary ideal case" ---that necessarily differs greatly from Physics which (while utilizing these mathematical idealizations) is totally devoted to "the real case". And when you say, there is nothing wrong with saying something --a subatomic particle, for example-- "can be regarded as dimensionless", that's true for some limited purposes. But, ultimately, it leads to the problem of the singularity which then, I am told, leads to nonsensical results. The "Planck length" limit built into string theory prevents all dimensionality from evaporating into a dimensionless singularity at the limit. So the dimensionless point works fine in math, but not in the real world described by physics.

 

[rai linga]

Very interesting response. Again, I stand corrected on may of the points you make. I'm not a mathematician or a physicist, so I know my limits. I would just suggest by your last comment that mathematics is different in kind than physics ---one doesn't seamlessly blend into the other ----like chemistry blends into biology, for example. When you say mathematics sets up an "imaginary ideal case" ---that necessarily differs greatly from Physics which (while utilizing these mathematical idealizations) is totally devoted to "the real case". And when you say, there is nothing wrong with saying something --a subatomic particle, for example-- "can be regarded as dimensionless", that's true for some limited purposes. But, ultimately, it leads to the problem of the singularity which then, I am told, leads to nonsensical results. The "Planck length" limit built into string theory prevents all dimensionality from evaporating into a dimensionless singularity at the limit. So the dimensionless point works fine in math, but not in the real world described by physics.

 

[Xyooj]

String is a purely mathematical attempt to model reality.

sounds about rite for me :)

 

[Kevin_Axion]

Why do you people reduce Superstring Theory to such nothingness? Because it doesn't provide experimental falsifiability? Many things being studied in Theoretical Physics use concepts developed by Superstring Theory there is no use in saying it is useless if you don't understand it or find it "unnatural". We have to use the ideas we have and sometimes nature is more than it appears, just get over trying to say Superstring Theory is useless when it was and still is one of the most productive areas of research in Theoretical Physics.

Kevin,

 

[suprised]

Kevin,

many can't, and so don't want to understand, so it is pointless in arguing.

 

[Kevin_Axion]

Kevin,

many can't, and so don't want to understand, so it is pointless in arguing.

That is true, but no one can understand Quantum Mechanics yet they accept it, it comes down to falsifiability most likely.

Kevin,