String theory in one sentence please

[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,

 

[MTd2]

If I switch almost all periods for commas, except for the last one, in both Polchinski's books and copy and paste them together, does it count as 1 sentence?

 

[suprised]

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

Kevin,

Well, I know many people who don't accept quantum mechanics, but also don't have the faintest clue about it: "I don't understand it, so it must be wrong". Same applies here, read some of the comments above and see what I mean...

 

[inflector]

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.

..[snip]...

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.

Well, the same could be said about the existence of God right? Can you prove that the theory of God is not provably wrong? For any test you might think to invoke, some theist somewhere will always be able to come up with a new place for God to hide.

Not currently provably wrong means that String theory is not currently a theory. At best, it's the beginning of an idea that might one day be a theory. Sure, we can't prove that it won't one day be a theory. I'll grant you that point. But again, you can say the same thing about God. Can you prove that we won't one day have a theory of God that is scientifically valid?

 

[suprised]

Of course string theory can be proven to be wrong. The very nature of stringyness is string resonances, an infinite tower of excitations. If one cannot find them, then string theory is disproven.

This is a matter of principle, whether one could actually do such experiments is a different question. But if people come up with this falsification idea, which is a matter of philosopy, then that's the answer at the same level of philosophy.

If people were smart enough to do a little bit of reading, and I mean serious reading, then this discussion woulnd't come up every couple of weeks.

 

[CHIKO-2010]

String theory is the only known consistent quantum-mechanical completion of gravity which also, at least in principle, is capable to describe known particle physics at ultra-high energies.

 

[suprised]

Well, the same could be said about the existence of God right? ..
..
Not currently provably wrong means that String theory is not currently a theory.

Sorry this is BS. It is not just a matter of randomly "declaring" God or String Theory or other ideas to underly things. Science works a bit different! What goes in string theory is an enormous amount of hard, highly non-trivial computational _results_. And these tell which things work or can work, and which not. This gives strong scientific reasons for it, unlike for God. At least I don't know of any computation that could be interpreted either in favor of or against the existence of a god.

 

[inflector]

Sorry this is BS.

Actually, it's a particular dialectic technique called reductio ad absurdum.

It is not just a matter of randomly "declaring" God or String Theory or other ideas to underly things. Science works a bit different! What goes in string theory is an enormous amount of hard, highly non-trivial computational _results_. And these tell which things work or can work, and which not. This gives strong scientific reasons for it, unlike for God. At least I don't know of any computation that could be interpreted either in favor of or against the existence of a god.

I was NOT proposing that belief in God is as scientific as belief in String Theory. What I was showing is that the specific test of whether or not a specific theory or idea is: "not provably wrong," is not useful as a discriminant because many other ideas which are not scientific or even useful share the trait of being "not provably wrong."

Therefore, I suggest that the proponents of String Theory not use this particular test as part of their argument. Its very use is the opposite of persuasive and smacks of desperation.

 

[Haelfix]

I would suggest staying away from this board then, b/c almost by definition, everything 'beyond the standard model' is not falsiable in *practise*. Quantum gravity (not just string theory) has always suffers from that fate.

Now, again if we had some sort of probe into the early universe or a galaxy wide particle accelerator, then sure we would look for stringy signatures and they would be highly apparent and the theory could be falsified rapidly. But we don't, and probably never will so pure theory work and plausibility arguments is the best you can do (without getting lucky)

 

[Kevin_Axion]

Perhaps the easiest approach would be to look for other intelligent beings and see what they have discovered.

 

[inflector]

I would suggest staying away from this board then, b/c almost by definition, everything 'beyond the standard model' is not falsiable in *practise*. Quantum gravity (not just string theory) has always suffers from that fate.

I am not proposing and have not proposed that we shouldn't consider and work on ideas which are not currently falsifiable. I am merely saying that this lack of provable falsifiability is not a point in their favor.

And I should further add that scientists, especially those who write popular books on physics, should STOP talking about these ideas as if they have been proven until they actually have some experimental verification.

Because, in the end, not provably falsifiable is exactly equivalent to not provably not even wrong which is also equivalent to not provably not not even wrong.

 

[MTd2]

I would suggest staying away from this board then, b/c almost by definition, everything 'beyond the standard model' is not falsiable in *practise*. Quantum gravity (not just string theory) has always suffers from that fate.

Not really... Not everything "beyond standard model" is quantum gravity. It's just that this board has its standards twisted.

 

[Kevin_Axion]

Michio Kaku always alludes to the idea of a single equation that would describe all physical processes in our universe which derives from Superstring Theory/String Field Theory (can someone explain the difference?). What form would this equations be in, a Lagrangian Density?

 

[atyy]

Michio Kaku always alludes to the idea of a single equation that would describe all physical processes in our universe which derives from Superstring Theory/String Field Theory (can someone explain the difference?). What form would this equations be in, a Lagrangian Density?

A complete formulation of string theory is unknown http://www.blau.itp.unibe.ch/stqg.pdf

 

[Kevin_Axion]

I know, I'm just asking if one were to be found what form would it be in? For instance the Standard Model can be formulated in a Lagrangian density.

 

[atyy]

I know, I'm just asking if one were to be found what form would it be in? For instance the Standard Model can be formulated in a Lagrangian density.

In AdS/CFT, the CFT is a standard field theory formulated with a Lagrangian.

However, AdS/CFT is probably not all of string theory, and suprised has mentioned something about derived, Fukaya categories in the past.

 

[tom.stoer]

I collected the statements we had over the last year; hopefully the quotes are complete and correct.

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.

According to string theory, elementary particles are not really pointlike, but have a shape of a short string, too short to be visible with present technology.

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.

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.

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.

String theory is the only known consistent quantum-mechanical completion of gravity which also, at least in principle, is capable to describe known particle physics at ultra-high energies.

 

[Entropee]

Thank you Tom, man its been a while since I wrote this essay though haha. Still seems to be a popular thread though.

 

[QGravity]

String Theory is an Idea that on mathematical side was & is on of the most fruitful Ideas that come to human mind (Just consider Geometric mirror symmetry, Seiberg-Witten theory (in comparison of donaldson theory for computing topological invariants , classification and other aspects of low-dimensional topology), string homology and ...) But on physical side (until now!) is not very desirable.
If LHC can see Ideas like extra dimension & supersymmetry, we can still believe in String Theory.