Why is String Theory Considered to be a Scientific Theory?

[MTd2]

Dimensions with anyonic dimensions...

 

[negru]

But with SUSY you are not _forced_ to use a notation (superspace) involving dimensions. That's all I am saying.

But it's equivalent, so it's the same thing.

 

[marcus]

Funny how he doesn't like these extra dimensions but he's teaching a susy course at mit

Going from SUSY to extra dimensions is a big stretch for a lot of people.

Where, in the interview, does Wilczek say that he "doesn't like these extra dimensions"?

It seems to me that Wilczek is trying to speak accurately in this interview and to avoid expressing dislike, or any kind of biased emotional attitude.
The point is not to express a leaning "for" or "against". You trivialize things when you interpret everything in terms of attitudinal push and pull. There are facts and distinctions that need to be clarified.

We have to learn how to live with a certain confusion of language, which Inflector has pointed out: it is confusing to call it "string theory" because it is not a physics theory in the usual sense.

It does not demean or diminish string research activity or string math to say this. It does not need to evoke immediate defensiveness, excuses, obfuscation, distraction, testimonials to how hopeful and promising, paeans to greatness, attacks on rivals, etc. All that stuff is irrelevant to the main issue.

 

[inflector]

Thanks for the link to the bloggingheads.tv episode. I had seen that one but forgot that they addressed string theory specifically. The point I remembered was about 39 minutes towards the end of the section: "Light, matter—what’s the difference?" where they had this conversation after Wilczek started explaining about the Higgs field and Wright noted that is sounded like he was talking about aether:

WRIGHT: Weren't we taught in junior high school that the idea of the aether was discredited?

WILCZEK: The aether was never discredited. The aether is more dominant now in physics than ever. There were some very specific detailed models of what this medium—that we call empty space and see as emptiness in everyday life—is that had flaws and they were discredited.

But our modern physics is absolutely built from A to Z on the idea that space is a rich medium full of structure.

WRIGHT: So there is no empty space?

WILCZEK: There is no empty space in the sense of void. Space is full of fields that can vibrate that have spontaneous activity and really that's the primary ingredient of reality. Particles are just excitations of this medium, kind of bubbles or waves inside this medium which is always there and everywhere. It's really in modern physics the primary ingredient of reality. Particles or what could be considered ordinary matter are just secondary manifestations.

I found that to be an absolutely fascinating perspective and one that I hadn't seen put in those terms before.

My take on this is that it would help if people would try to speak precisely and acknowledge that it is NOT a scientific theory---that it is rather a body of mathematical theory with application in various areas but which has not, as yet, produced a theory of nature by accepted standards of Physics. I take Wilczek's word on this.

That would be my take as well. Doing otherwise actually hurts the cause of science in ways that those immersed in it cannot fully comprehend.

If you look at Robert Wright's puzzled look when he talks about extra dimensions and such, that's a good indication of the way that String Theory comes across to a non-believer. And Wright is a very smart guy, he's no dummy when it comes to science. But his eyes glaze over when he thinks about the extra dimensions of String Theory.

To a non-scientist, String Theory comes across as extremely complicated and very disconnected from everyday reality. Pictures of the Calabi-Yau shape (like http://members.wolfram.com/jeffb/visualization/calabi-grid.gif) might be pretty but they don't help make String Theory seem accessible or relevant to normal life. When the public at large doesn't understand science or even feel like that they could possibly ever do so, this can't help the overall budgets for science during times of economic pressure. Witness the pending budget cuts in the UK.

So it seems like a bad idea to spend a lot of time and effort publicizing a theory that isn't even actually a scientific theory yet. All the popular science books on String Theory haven't helped the cause of physics in the larger world, IMHO. The hype doesn't help over the long term.

The popular science web sites are even worse in this regard, every single paper related to quantum mechanics is one that "might possibly help scientists build quantum computers to solve every computer problem in microseconds" and every materials science paper is one that "might lead to radically new materials that will let us build bridges to the moon and elevators to space." After a while it all sounds like BS and that scientists are mostly full of BS. Even when it is not the scientists doing the hyping, in general.

Far better, to acknowledge the reality of the current state of affairs. It may not sell as many books and it may not get as many readers for your blog entry, but telling the truth won't hurt the credibility of science itself.

That way when the world needs to trust science, like say when we are warming up the planet and science points to human development as the cause of the problem, and science also predicts a future with enormous problems that we can avoid if we act in time, that way during these times of crisis, the public at large might actually believe what the scientific consensus says and act.

 

[inflector]

It does not demean or diminish string research activity or string math to say this. It does not need to evoke immediate defensiveness, excuses, obfuscation, distraction, testimonials to how hopeful and promising, paeans to greatness, attacks on rivals, etc. All that stuff is irrelevant to the main issue.

Yes, that's it exactly.

 

[atyy]

Compactification of the twisted heterotic string
Nair VP, Shapere A, Strominger A, Wilczek F
Nuclear Physics B, 1987

Resolution of cosmological singularities in string theory
Larsen F, Wilczek F
Physical Review D, 1997

An action for black hole membranes
Parikh MK, Wilczek F
Physical Review D, 1998
"Here we use the tree-level effective action obtained from string theory after compactification to four macroscopic dimensions."

Saltatory relaxation of the cosmological constant
Feng JL, March-Russell J, Sethi S, Wilczek F
Nuclear Physics B, 2001
"...Microscopic considerations from string/M-theory suggest two major innovations in the framework. ... By requiring stability on the scale of the lifetime of the universe, rather than absolute stability, we derive a non-trivial relation between the supersymmetry breaking scale and the value of the cosmological term. It is plausibly, though not certainly, satisfied in Nature."

 

[atyy]

That way when the world needs to trust science, like say when we are warming up the planet and science points to human development as the cause of the problem, and science also predicts a future with enormous problems that we can avoid if we act in time, that way during these times of crisis, the public at large might actually believe what the scientific consensus says and act.

Yes, of course. Nonetheless, one should separate questions of science and sociology.

http://insti.physics.sunysb.edu/~siegel/vs.html "Don't blame the product for the advertisement. String theory has been grossly over-sold. It isn't even a "theory" yet, just a "model". It hasn't solved anything, much less everything. But just because it isn't everything doesn't mean it isn't anything. It has many interesting features, some of which have been reproduced in particle theory and proven useful, some of which haven't but would be desirable in a more realistic theory."

BTW, Warren Siegel has the other famous index complentary to Baez's http://insti.physics.sunysb.edu/~siegel/quack.html

Also, the guy who wrote the crackpot index you mention also writes in "Calabi-Yau Manifolds and the Standard Model" http://arxiv.org/abs/hep-th/0511086 "This is rather striking, because string theory focuses attention on spacetimes of this form, at least if we work in the Euclidean signature."

And notice Siegel does not say "string theory is not a theory", he says "string theory is not a "theory"".

He specifically says "At this point in time, there are no more-promising solutions to many of the problems of high-energy physics. Of course, alternatives exist, and you are welcome to try some (I do), but most of the complaints leveled against string theory can be applied to them even more strongly. (In particular, watch out for theories that claim to solve some problem simply because it is too difficult to even see if they have that problem.)"

 

[negru]

That way when the world needs to trust science, like say when we are warming up the planet and science points to human development as the cause of the problem, and science also predicts a future with enormous problems that we can avoid if we act in time, that way during these times of crisis, the public at large might actually believe what the scientific consensus says and act.

Not that it's related, but I personally don't want people to blindly follow science, nor scientists to tell people what they should do. Because this will necessarily lead to the corruption of science. Science hasn't worked so far because the scientists are perfectly moral and truth seeking creatures. It worked exactly because scientists only put forward their ideas, and whoever was interested bought them. If the science they bought was correct, they made use of it, and that ensured the success of correct science.

So if we want people to trust science, we'd best keep to presenting objective truth (as best as we can) with absolutely no "advice" attached.

 

[arivero]

One can write SUSY using superspace / fermionic extra dimensions. That's rather formal.

But if you look at the bosonic string in 26 dimensions, you will see that there is NO differenc between the first 4 and the the remaining 22 dimensions. That's what at least at this level there's a big differenc between the fermionic dimensions from SUSY and the bosonic dimensions in ST.

I think talking about these dimensions is misleading. What about the following: In QCD one has eight gluons, i.e. an additional index a=1..8 for the gauge field; do you think that the gauge field lives in 4*8 = 32 dimensions?

First, you are not doing the account properly. Each gluon is a generator of symmetries, associated to a real parametero, so there are 8 dimensions. You could always to do a group G living in GxG, so the answer naive is 16 dimensions for QCD.

But you can ask what is the minimum dimension where it can live as spacetime symmetry. The answer is that you must quotient by a maximal nontrivial subgroup, for QCD it is SU(3)/ SU(2)xU(1) (just a coincidence, as far as we know). So from the 8 gauges you must not multiply times 2, but substract 3 and 1, and then you get that SU(3) can live a an spatial symmetry of a 4 dimensional manifold. An explicit example is CP2.

 

[Haelfix]

The fog is getting thick once again in this thread.

Whether string theory is a 'theory' or not is nomenclature (you can call it a research direction or a program). It does however make falsifiable predictions (just at a scale that we will never measure), it is both consistent and unique as a mathematical structure, there are observations that would instantly favor it over others (for instance KK resonances at the LHC or cosmic strings ) and there are theories that presumably do live in the swampland (eg D=4 N=2 Supergravity)

The point being if any theory in quantum gravity deserves to be called a 'theory', then this is it.

 

[tom.stoer]

Can you comment on this?

The answer is that you must quotient by a maximal nontrivial subgroup, for QCD it is SU(3)/ SU(2)xU(1) (just a coincidence, as far as we know). So from the 8 gauges you must not multiply times 2, but substract 3 and 1, and then you get that SU(3) can live a an spatial symmetry of a 4 dimensional manifold. An explicit example is CP2.

 

[suprised]

The fog is getting thick once again in this thread.

Indeed, this fixation with extra dimensions, falsifiability... always amazing.

 

[inflector]

Indeed, this fixation with extra dimensions, falsifiability... always amazing.

Was the irony intended or accidental?

 

[mitchell porter]

My take on this is that it would help if people would try to speak precisely and acknowledge that it is NOT a scientific theory---that it is rather a body of mathematical theory with application in various areas but which has not, as yet, produced a theory of nature by accepted standards of Physics. I take Wilczek's word on this.

String theory is very definitely a specific physical hypothesis about nature - a hypothesis about the fundamental degrees of freedom and their dynamics. See the analogy with general relativity in my comment #19 in this topic. The duality revolution tells us that all the different forms of string theory really are the same physical theory, playing out in different geometric backgrounds. So there are numerous possible string models of particle physics, just as there are numerous cosmological solutions to general relativity, but they all derive from the same fundamental theory. In fact, the cosmological analogy is more than an analogy, since it's "cosmological" attributes like topology and radius of the extra dimensions which distinguish one string model from another.

Wilczek is a physics Nobelist but he isn't a string theorist, and most actual string theorists would surely disagree with this statement. The theory has a long way to go - it might be likened to special relativity before Minkowski space, or QED before renormalization - but it is a physics theory.

 

[suprised]

Was the irony intended or accidental?

This is no irony. It is truly amazing how much effort goes all the time into trying to play down string theory, this must really itch a lot.

 

[MTd2]

This will just get worse down the road if no sign of SUSY is found on LHC. Much worse.

 

[suprised]

This will just get worse down the road if no sign of SUSY is found on LHC. Much worse.

True. As if low energy SUSY would have anything to do with string theory.

The true culprits are speculative beyond the standard model phenomenologists, incl. extra dimensional brane world builders and alike. A lot of their work is indeed below any standards. They vehemently don't want to be string physicists. But the fallout hits the string physicists, because the public can't discriminate.

 

[MTd2]

Wouldn't it be better to start answering threads like this?

https://www.physicsforums.com/showthread.php?t=437605

This is the kind of fundamental answer that is left in the air.

 

[suprised]

Well I would feel it more amusing to create another thread about "why is LQG still considered a theory by some?". I guess after all this string bashing, it's time to make clear where the hype, obfuscation etc really is. The following paper pretty much subsumes it:

http://www-spires.dur.ac.uk/cgi-bin/spiface/hep/www?eprint=arXiv:1009.4475

Here some excerpts, in classic marcus style:

Our main conclusions regarding the status of the two quantization approaches [LQG and SF] are the following:

• Although LQG can perfectly incorporate the full local Lorentz symmetry, we find some evidences that LQG might have problems to maintaining space-time diffeomorphism symmetry at the quantum level. Thus, we argue that it is an anomalous quantization of general relativity which is not physically acceptable.

• There is an alternative quantization following the same loop ideas, the so called Covari- ant LQG (CLQG), which has a potential to resolve the drawbacks of LQG. However, it is supplied with some serious technical obstacles (consisting mainly in finding a represen- tation of the algebra of connections) preventing yet the realization of this quantization program.

• The claim [21] that the recently introduced spin foam models [20, 21] have the same boundary states as the kinematical states of LQG cannot be formulated as such because they have completely different representations as functionals of connection.

• The new spin foam models in the presence of a finite Immirzi parameter represent quan- tizations which do not respect the standard Dirac rules and we argue that they are incompatible with a self-consistent canonical quantization. Moreover, any SF model derived by the usual strategy “first quantize, then constrain” (see section 3.1.2), in- cluding the models without the Immirzi parameter, does not implement consistently all constraints of general relativity and therefore cannot properly describe its quantum dynamics.

• A spin foam quantization consistent with the canonical one can be achieved by modifying the association of geometric bi-vectors to generators of the gauge algebra and by relaxing the closure constraint. The vertex amplitude should also be modified and in general is given by the integral formula (3.73) with a non-trivial measure which however remains still unknown.

Given these statements, we have to conclude that neither the canonical loop approach nor its spin foam cousin were able to provide so far a model which can be claimed to be free from inconsistencies and anomalies.

...


Besides, we do not consider some branches of LQG and SF such as, for example, Loop Quantum Cosmology (LQC) [23] and evaluation of the graviton propagator [24]. Since these branches are based on results and ideas of the two main approaches, they seem to have even less firm ground than those approaches themselves. Therefore, for example, if LQG in its present form fails to provide a consistent quantization of general relativity, it is highly unlikely that LQC can do better.


I am suprised that there matters have not caught attention here; perhaps because people are too obsessed playing down the only known consistent theory?

 

[MTd2]

Maybe it is not amusing because all QG fails to be scientific, as it pointed out by bcrowell?

 

[MTd2]

I am really interested in knowing the answer to that other thread

https://www.physicsforums.com/showthread.php?t=437605

because Eric Verilinde is trying to fix Matrix String theory, or whatever, with his holographic idea. So, I'd like to know what went wrong.

 

[marcus]

...
I am suprised that there matters have not caught attention here; perhaps because people are too obsessed playing down the only known consistent theory?

Noted and commented already, 23 September
https://www.physicsforums.com/showthread.php?p=2898140#post2898140

 

[Fra]

Well I would feel it more amusing to create another thread about "why is LQG still considered a theory by some?".

Maybe a good start would be if both ST and LQG could take two steps back and try to agree on something and then argue from a common point, why one speculation is more rational than the other and we can try to assess which is most plausible using arguments from the agreed perspective only?

Noone knows which will end up beating the other one, so all we can do is to enter a negotiation process for some some objectivity in where it's wisest to place our bets, using arguments only from the agreed perspective. Then maybe we won't find who is right, but at best which direction that's more rational. Or if both are flawed?

This is what I try to do for myself, and I have objections to both ST and LQG. So I don't ask which one, I ask what else. That said, perhaps one or them or both, will converge to sometihng better, that's still possible since ideally everyone can learn, and be wrong.

/Fredrik

 

[tom.stoer]

Well I would feel it more amusing to create another thread about "why is LQG still considered a theory by some?". I guess after all this string bashing, it's time to make clear where the hype, obfuscation etc really is. The following paper pretty much subsumes it:

http://www-spires.dur.ac.uk/cgi-bin/spiface/hep/www?eprint=arXiv:1009.4475

...

suprised,

we had a couple of very fruitfull discussions here, so I wonder what you want to tell the PF community here; I don't think it's helpful to cite not widely respected (!) papers in order to point a finger at conjectured (!) weaknesses of an (in the string theory context) unrelated theory (LQG).

You can't strengthen string theory by weakening LQG.

If you think that this thread is for the birds, then we should stop it instead of focussing at sideshows.

 

[marcus]

I should qualify this a bit to avoid misunderstanding

Noted and commented already, 23 September
https://www.physicsforums.com/showthread.php?p=2898140#post2898140

I have high regard for both Philippe Roche and Sergei Alexandrov. And have been watching both their work since about 2003.

IMHO Alexandrov has contributed significant value to the LQG program by his constructive criticisms--which he has offered repeatedly and which have repeatedly been answered. He is also the inventor (around 2003) and main proponent of CLQG (where roughly speaking you do canonical quantization on the 4d connections instead of those of a 3d slice). Unfortunately CLQG has not caught on and even more awkwardly there is a conflict of terminology because now one refers to the spinfoam approach as "covariant LQG".

Roche is an important senior figure who has contributed significantly to several QG programs including LQG. He was co-organizer with Rovelli of the Loops 04 conference.

If history is any guide, this recent Alexandrov Roche paper will serve as a valuable constructive criticism. It may be slightly out of date. But any points in the paper which apply to the current formulation(s) of LQG will (if past performance is any guide) be answered.

In my September comment on the paper, to Atyy, I meant to suggest that it should not alarm or distract us, as observers, and that we can take a "wait-and-see" attitude.

This is really off-topic! The thread is about string, not LQG! But I want to make clear the respect due especially to Philippe Roche. And besides, who knows? perhaps "suprised" and he are acquainted.

 

[inflector]

Indeed, this fixation with extra dimensions, falsifiability... always amazing.

Was the irony intended or accidental?

This is no irony. It is truly amazing how much effort goes all the time into trying to play down string theory, this must really itch a lot.

Your statement surprises me.

You point out two separate items for special treatment:

1) fixation with extra dimensions

2) fixation with falsifiability

The reason I found your statement ironic, is that it seems to me that the heart of scientific skepticism is to take a position of doubt with respect to:

1) Claims for which there has been zero scientific evidence. Normally this is applied to things like God, alien visitors or abductions and such. But it technically applies to the idea of extra dimensions as well. This is a weakness of string theory. It may indeed be reality that there are extra dimensions but it is still a weakness of string theory that it requires them because we have not seen them or any evidence thereof.

2) Claims which cannot be experimentally falsified. Again, this is normally applied to quackery and crackpot ideas intended to replace general relativity and the deep contemplations of serious science with the drug-stupor-induced hallucinations of a weekend armchair theorist. But at the very minimum, the lack of falsifiability is a weakness of string theory. It does not mean it is not true. But it is still a weakness.

So, it seems to me that it should not be "always amazing" that some people consistently apply the principles of scientific skepticism to serious scientific ideas as well as frivolous ones. Isn't that what we should expect of the scientific mindset: consistent application of a rigorous way of thinking? Isn't that what separates science from religion? Or belief in evolution from belief in God?

I am quite sure that if anyone eventually fixes these two problems with current string theory, there will be celebration all over the world of the string. The reason is that they are weaknesses. Experimental evidence of additional dimensions would be a Nobel worthy discovery. A Nobel would probably go to experimental physicists who are able to run a test of string theory which it passes or fails. It would be big science, on the order of Michelson-Morley. Our descendants will likely remember it 100 years from now.

So I am surprised and amazed that you are "truly amazed," surprised.

 

[Haelfix]

Look, you keep harping on these trivialities, but understand its the same thing that quantum gravity people have been listening to for 30 years. In fact not so long ago, (say before Penzias and Wilson) the same criticisms were applied to classical cosmology.

Unfortunately it is also ridiculously unhelpful, b/c like it or not, gravity exists, quantum mechanics exists ergo there is a system by which the two must join in some way and it is a scientists job to figure it out.

It is possible that QG or String theory will be a purely theoretical undertaking, with no experimental support in our lifetimes or ever. It is up to you if you think that's worthless and akin to philosophy. Personally, I think the math does matter and that it will lead us eventually to the right answer, one way or the other. Ultimately the fact of the matter is that there are certain things that we often simply 'know' exist, without experimental support. In the 60s and 70s, theorists working on the standard model more or less knew certain particles existed before they were discovered (hence a ton of Nobel prizes that ensued from that golden age).

Today, I can safely say that the Higgs Boson (or more precisely something responsible for ElectroWeak symmetry breaking) exists and will be discovered at the LHC. I say that with enormous 'god' or 'religous' like confidence, b/c I believe in the theoretical and mathematical structure of quantum mechanics.

 

[Finbar]

Today, I can safely say that the Higgs Boson (or more precisely something responsible for ElectroWeak symmetry breaking) exists and will be discovered at the LHC. I say that with enormous 'god' or 'religous' like confidence, b/c I believe in the theoretical and mathematical structure of quantum mechanics.

I don't know if quantum mechanics is directly responsible for spontaneous symmetry breaking? Doesn't it come just from the classical action with the mexican hat potential?

Seems to me that it should be your confidence in gauge theory not quantum mechanics that gives you the faith in the Higgs?

 

[Haelfix]

It comes from my confidence in not violating partial wave unitarity bounds arising from say WW scattering for instance. This is decidedly quantum mechanics.

This is often called something like a 'LHC no lose theorem'.

See
http://www.science20.com/quantum_diaries_survivor/altarelli_approximately_impossible_lhc_fails

 

[Finbar]

It comes from my confidence in not violating partial wave unitarity bounds arising from say WW scattering for instance. This is decidedly quantum mechanics.

This is often called something like a 'LHC no lose theorem'.

See
http://www.science20.com/quantum_diaries_survivor/altarelli_approximately_impossible_lhc_fails

Ah ok. Thanks. Yes I recall here about this now. So QM tells us that a theory which explicitly breaks gauge invariance is an ill theory as it violates unitarity bounds.



I think something like this is what string theorists would like for quantum gravity. Maybe a proof that if we don't see stringy like objects at or before the Planck scale unitarity bounds break down. Or a proof that AS in gravity violates unitarity bounds.

 

[chrispb]

Sure, there are string theorists that insist that the string scale is at the Planck scale. But what about models with large extra dimensions (ADD), or RS/RS-like models where the KK modes are hiding at a TeV instead of the Planck scale? There's an example of physicists working hard to come up with concrete models with concrete predictions for the LHC. Granted, it's far from the opinion shared by every other physicist out there, but it's worth considering, I'd think.

 

[inflector]

Look, you keep harping on these trivialities, but understand its the same thing that quantum gravity people have been listening to for 30 years. In fact not so long ago, (say before Penzias and Wilson) the same criticisms were applied to classical cosmology.

I understand that you consider falsifiability to be a triviality. That's one perspective. I only ask that you consider that many smart people don't share this view. That doesn't make them enemies of string theory.

Unfortunately it is also ridiculously unhelpful, b/c like it or not, gravity exists, quantum mechanics exists ergo there is a system by which the two must join in some way and it is a scientists job to figure it out.

It may not be helpful but it is not the job of scientific principles to be helpful. They are there to serve as measuring sticks by which we can compare different ideas. All else being equal, a falsifiable theory that passes tests and one that predicts new phenomena which are then experimentally confirmed is far preferable to one that cannot be falsified or tested. The preference for falsifiability and for ideas with some experimental evidence may be a simple point, but it is not a triviality. I don't see how any scientist could argue this.

It is possible that QG or String theory will be a purely theoretical undertaking, with no experimental support in our lifetimes or ever. It is up to you if you think that's worthless and akin to philosophy.

This is a straw-man argument. I never said that QG or string theory was worthless and akin to philosophy.

Personally, I think the math does matter and that it will lead us eventually to the right answer, one way or the other..

This is a straw-man argument too. I never said that math doesn't matter either.

Ultimately the fact of the matter is that there are certain things that we often simply 'know' exist, without experimental support. In the 60s and 70s, theorists working on the standard model more or less knew certain particles existed before they were discovered (hence a ton of Nobel prizes that ensued from that golden age).

The way this seems to work is that many physicists predict lots of different phenomena and some of them are right. The ones that are win Nobel prizes and the ones that aren't don't and are forgotten by history.

After all, in the 19th century physicists had been predicting an aether because they just "knew" it had to exist. Then through experiment, it was eventually proven that the idea they had was wrong. Subsequently, it has been shown that they were partially correct, but not in the way that they thought. The empty vacuum is not empty but is filled with fields and warped by the presence of mass and energy, and has other traits.

I believe that most of what physicists "know" today but don't have proof for might end up like like this. Many of these ideas will turn out to be partially true and partially false when we eventually come up with a more complete theory.

Today, I can safely say that the Higgs Boson (or more precisely something responsible for ElectroWeak symmetry breaking) exists and will be discovered at the LHC. I say that with enormous 'god' or 'religous' like confidence, b/c I believe in the theoretical and mathematical structure of quantum mechanics.

Here we have a philosophical difference. I don't hold any ideas with that level of confidence unless I have seen the proof myself and understand it. I'm not saying your position is irrational. I think that it is good for science, in general, for there to be a mixture of different perspectives.

What I find puzzling is the seeming intolerance on the part of the "true believers" of string theory for the idea that others hold different opinions and have different standards for their beliefs. Luboš Motl typifies this attitude. I also find puzzling the lack of respect for the position that is possible to hold an idea in high regard while still being open to the idea that it might be flawed or incomplete in some fundamental way.

I have not found that same level of intolerance for dissent or doubt among the proponents of alternative QG ideas in my personal experience.

 

[tom.stoer]

The problem with this is discussion is that all what has been could be right; only time will tell.

If you believe in string theory (w/o having experimental support) it makes sense to study this theory; if you don't believe in string theory then you should do something different.

There are good reasons to believe in string theory, and there are good reasons to be sceptical. It's a matter of taste and especially weighing and personal assessment. That's why "right" or "wrong" are (up to now) the wrong categories.

The question was if string theory is a scientific theory. The central problem I see is falsification. One must distinguish between falsifiable in pinciple and falsifiable in practice. String theory can definately be falsified in principle, but (today) it fails to be falsifiable in practice. But not to be falsifiable in practice is not really a problem of string theory, but of experimental physics (a few centuries ago the theory that "the Earth is round" had some support, was falsifiable in principle but not falsifiable in practice; this changed with seafaring; today the theory that "black hole exists" is falsifiable in principle but not in practice; nevertheless nobody would say that theories regarding black holes are non-scientific). Unfortunately not being falsifiable in practice is common to all theories of quantum gravity. Therefore taking this argument too seriously would mean to stop all attempts towards quantum gravity.

But I think we had these discussions here a couple of times; we went though it and we found interesting topics beyond these "trivialities".

 

[suprised]

1) Claims for which there has been zero scientific evidence. Normally this is applied to things like God, alien visitors or abductions and such. But it technically applies to the idea of extra dimensions as well. This is a weakness of string theory. It may indeed be reality that there are extra dimensions but it is still a weakness of string theory that it requires them because we have not seen them or any evidence thereof.

Why don't you spend some time in reading what has been posted around here in other threads, regarding the meaning of extra dimensions. Then you'd understand what I mean with fixation or obsession with extra dimensions: it's a red herring!

See eg. the lower part of:
https://www.physicsforums.com/showpost.php?p=2822816&postcount=14

and:
https://www.physicsforums.com/showpost.php?p=2917134&postcount=13

 

[suprised]

...One must distinguish between falsifiable in pinciple and falsifiable in practice. String theory can definately be falsified in principle, but (today) it fails to be falsifiable in practice. But not to be falsifiable in practice is not really a problem of string theory, but of experimental physics (a few centuries ago the theory that "the Earth is round" had some support, was falsifiable in principle but not falsifiable in practice; this changed with seafaring; today the theory that "black hole exists" is falsifiable in principle but not in practice; nevertheless nobody would say that theories regarding black holes are non-scientific). Unfortunately not being falsifiable in practice is common to all theories of quantum gravity. Therefore taking this argument too seriously would mean to stop all attempts towards quantum gravity..

This reflects the opinion of the people working in the field. This issue is being rised again and again. Unfortunately this is tied to the way a forum works. Perhaps one could make it sticky, somehow? Some kind of thread "myths and facts about string theory". Just to fight the desinformation.

 

[tom.stoer]

Yeah, I support this to become a sticky note

 

[inflector]

Why don't you spend some time in reading what has been posted around here in other threads, regarding the meaning of extra dimensions. Then you'd understand what I mean with fixation or obsession with extra dimensions: it's a red herring!

See eg. the lower part of:
https://www.physicsforums.com/showpost.php?p=2822816&postcount=14

and:
https://www.physicsforums.com/showpost.php?p=2917134&postcount=13

I understand the posts you've linked to here and happened to have read them when you first wrote them as I followed both those threads and have been following most of the threads in "Beyond the Standard Model" for two years or so.

Nevertheless, I still believe that extra dimensions are a difficulty that most string theorists would say has not been resolved. The primary reason is that I haven't seen the perspective, that extra dimensions are just a helpful mathematical tool and not necessarily physical, as one that is widely held among the most vocal proponents of string theory, even among the pioneers like Susskind, Witten, etc. Perhaps string theory needs more variety in the presentation made in more accessible books. Perhaps people like Brian Greene haven't really helped promote string theory because of their illustrations of how extra dimensions can be compactified if they aren't real dimensions but only mathematical degrees of freedom. If this is a red herring, it's one that was first brought up by the early string theorists themselves so it is a "mistake" in perspective that needs active correction by the proponents.

I personally happen to believe that the theory that we end up having 100 years from now will have elements that we will be able to point to and say: "here are the extra dimensions that the mathematics of string theory was alluding to." So I think there is something there. I share your belief that the mathematics is indeed telling us something useful and interesting.

 

[inflector]

The question was if string theory is a scientific theory. The central problem I see is falsification. One must distinguish between falsifiable in pinciple and falsifiable in practice. String theory can definately be falsified in principle, but (today) it fails to be falsifiable in practice. But not to be falsifiable in practice is not really a problem of string theory, but of experimental physics (a few centuries ago the theory that "the Earth is round" had some support, was falsifiable in principle but not falsifiable in practice; this changed with seafaring; today the theory that "black hole exists" is falsifiable in principle but not in practice; nevertheless nobody would say that theories regarding black holes are non-scientific). Unfortunately not being falsifiable in practice is common to all theories of quantum gravity. Therefore taking this argument too seriously would mean to stop all attempts towards quantum gravity.

I agree with this point. I would further add that is important to distinguish between the two sub-categories of "falsifiable in principle":

1) Those theories which make specific predictions that we cannot currently test. The Earth is round is a specific statement that we could define a test for even when we lacked the ability to verify the test. Black holes exist is another theory like this: we know enough about what the theory implies that we can formulate many tests that we can run to verify if we have seen a black hole. There are also specific ideas related to black hole theory for which have very specific predictions like Hawking Radiation.

2) Those theories which are in principle falsifiable in the future but for which there exist not enough specific definition for one to make predictions that can be falsified. The problem with these types of theories is not in the reach of our experiments but in the state of the theory itself. I think that all quantum gravity theories are still in this state.

For string theory, in particular, you have some of the pillars making statements like "we don't even know what string theory is," yet. For example, when http://www.pbs.org/wgbh/nova/elegant/view-gross.html" :

One of the strangest aspects of where we are in string theory after 35 years is that we don't really know what string theory is. There are all these people working on string theory and doing wonderful things, sometimes answering old problems, sometimes coming up with new scenarios. But if you really ask them, "What is string theory?" they'll give you a glib remark, a glib description, and describe certain of its aspects. If you ask them again, "What is string theory?" if they're honest they'll say, "Well, we don't know." We have this incredibly powerful set of tools and methods that describe this intellectual structure, and yet we really don't know what lies at the core of that, what the unifying principles are, what the theory actually is that has all of these different aspects that we can partially describe.

This leads me to believe that string theory falls mostly into the second category of falsifiable in principle. There is not yet enough definition to even specify an experiment that all the proponents could point to and say: if that experiment fails, then we know that string theory is false.

String theory itself may have aspects of both of these sub-categories but I have never seen a major theorist make specific predictions that say: "If we don't find X when we run experiment Y, then string theory is false." Perhaps these exist, but it seems to me that string theory is such a broad discipline that there are way too many different aspects of the theory for anyone test to falsify them all.

 

[suprised]

If this is a red herring, it's one that was first brought up by the early string theorists themselves so it is a "mistake" in perspective that needs active correction by the proponents.

That's what many colleagues and myself do. We can't be held responsible for what others have been writing in the past. In fact there is nothing really wrong in using the term extra dimensions, as compared to just "internal degrees of freedom". Few people would even care about this, because nomenclature is not important. The issue just comes up by the self-declared critics who get obsessed with fighting "all the fancy mathematics that has nothing to do with nature" and tout this all over the internet. Pointless!

if that experiment fails, then we know that string theory is false.
.

Again, for the n-th time: the very nature of strings are stringy resonances. If they couldn't be observed, would prove string theory to be wrong. (Obviously this is a matter of principle, as we humans cannot build such accelerators, at least as far as we can imagine).

String theory is presented here often as a random theory with arbitrary predictions. In reality it is extremely constrained and rather the opposite of an arbitrary theory. This seems hard to grasp, tho.

 

[inflector]

Again, for the n-th time: the very nature of strings are stringy resonances. If they couldn't be observed, would prove string theory to be wrong. (Obviously this is a matter of principle, as we humans cannot build such accelerators, at least as far as we can imagine).

Look, I know this may seem obvious to you, but you must be able to see that statements like the one from David Gross above make it hard to believe that string theory is well defined.

Further, if this "very nature" of "stringy resonances" can be mathematically defined then why aren't there very specific statements of what this means one should expect to see in some future experiment. Why don't you propose a specific experiment or set of experiments that would disprove string theory? Why don't you get other theorists to agree on these specific tests? This might give clever experimentalists a chance to come up with a novel way of testing string theory that the theorists might not imagine is possible but that actually is.

String theory is presented here often as a random theory with arbitrary predictions. In reality it is extremely constrained and rather the opposite of an arbitrary theory. This seems hard to grasp, tho.

I certainly never said it was random or arbitrary, and I certainly don't think that.

The best way to settle the matter is to propose very specific tests and then get other theorists to agree that the tests are valid tests of string theory, even if they are not currently achievable due to technology and energy limitations. If you can do this, that says something.

Perhaps this has been done, but I haven't seen it and though I am a novice, I've been paying attention and looking for any such proposed tests.

 

[MTd2]

Again, for the n-th time: the very nature of strings are stringy resonances. If they couldn't be observed, would prove string theory to be wrong. (Obviously this is a matter of principle, as we humans cannot build such accelerators, at least as far as we can imagine).

String theory is presented here often as a random theory with arbitrary predictions. In reality it is extremely constrained and rather the opposite of an arbitrary theory. This seems hard to grasp, tho.

No, that is easy to grasp. The point is, if a theory whose unique predictions are completely outside human feasibility for the foreseeable future are untainable, is it scientific? Sure it is in principle, but science is of experimental nature. So, it sounds like the answer is yes/no at the same time. This is really uncomfortable.

 

[suprised]

So, it sounds like the answer is yes/no at the same time. This is really uncomfortable.

Indeed. But what are the options? We are not engineers who can redesign a machine if the customer doesn't like it. The natural constants, or scales, are what they are and any theory of quantum gravity has to cope with this. Those armchair critics who cry after "alternative theories" just should go on and try to do better.

 

[tom.stoer]

The point is, if a theory whose unique predictions are completely outside human feasibility for the foreseeable future are untainable, is it scientific? Sure it is in principle, but science is of experimental nature. So, it sounds like the answer is yes/no at the same time. This is really uncomfortable.

Again: string theory and other theories of quantum gravity are constructed in order to explain quantum gravity - for which we have zero experimental support! So it's the first time that one has to construct a theory (mostly based on mathematical considerations) for a domain which is (at least today) not testable in practice.

Everybody working in QG knows about this problem. The alternative is not to construct a different theory as it makes QG not better testable. The alternative is not to ask for experiments as they are in principle not feasible (at least not directly; maybe some better hints com from cosmological data). And the alternative is not to construct no new theory at all b/c we know that quantum physics and gravity as of today are inconsistent when combined.

I agree that this situation is uncomfortable, but there is no easy way out.

 

[inflector]

Again: string theory and other theories of quantum gravity are constructed in order to explain quantum gravity - for which we have zero experimental support! So it's the first time that one has to construct a theory (mostly based on mathematical considerations) for a domain which is (at least today) not testable in practice.

Everybody working in QG knows about this problem. The alternative is not to construct a different theory as it makes QG not better testable. The alternative is not to ask for experiments as they are in principle not feasible (at least not directly; maybe some better hints com from cosmological data). And the alternative is not to construct no new theory at all b/c we know that quantum physics and gravity as of today are inconsistent when combined.

I agree that this situation is uncomfortable, but there is no easy way out.

The same exact situation existed for quantum mechanics with respect to realism before EPR. The existence of EPR prompted John Bell to come up with his inequalities and eventually these were found testable in practice. String theorists should come up with a test or set of tests. Even if it takes 200 years to get the technology to run the test it will still be a good exercise.

Just throwing up your hands and saying we can't do it now seems like a copout. You can't run the tests now but why can't you come up with the idea for the experiment?

 

[marcus]

...So it's the first time that one has to construct a theory (mostly based on mathematical considerations) for a domain which is (at least today) not testable in practice...

Maybe one should mention the CMB at this point. An empirical test can be either experimental or observational. The CMB is, after all, the enormously magnified map of a small region of space, which bears traces from a time when the same region was even much smaller.

...Just throwing up your hands and saying we can't do it now seems like a copout. You can't run the tests now but why can't you come up with the idea for the experiment?

I think the traditional discipline of empiricism is to accept the idea that one should not propose untestable theories. There is a kind of traditional "faith" that with enough ingenuity it is not *necessary* to propose untestable theories.

Have we seen convincing evidence that we have now to break with that 400-year old Baconian tradition?

I'm not convinced. I think that QC is the "business end" of QG---cosmology is where the truth comes out.

Have a look:
http://arxiv.org/find/grp_physics/1/au:+barrau_A/0/1/0/all/0/1

 

[negru]

Just throwing up your hands and saying we can't do it now seems like a copout. You can't run the tests now but why can't you come up with the idea for the experiment?

What do you mean can't come up with an idea? We just need to build a big accelerator.

 

[suprised]

Just throwing up your hands and saying we can't do it now seems like a copout. You can't run the tests now but why can't you come up with the idea for the experiment?

You think this is a necessary advice? Guess what string physicists work on! Do they need to be told? They sit down working hard to further developing their theories and understand them better.

It's the armchair critics who made this up, fabricate a "controversy" where there was none, invent "string wars", appear in the media crying "failure" and demand "you must stop now". Even here this has created veritable obsessions. In reality there are no such string wars and most of this discussion is pointless; and actually quite unimportant to the general public. At any rate, while those keep on shouting and writing books and blogs, the physicists continue with their hard and difficult work.

 

[inflector]

What do you mean can't come up with an idea? We just need to build a big accelerator.

And what will you find whenever we can build one? What predictions does string theory make?

That we'll find stringy stuff isn't a very well defined experiment.

 

[inflector]

You think this is a necessary advice? Guess what string physicists work on! Do they need to be told? They sit down working hard to further developing their theories and understand them better.

I won't dispute that string theorists work hard. I've just been trying to get to the bottom of the actual state of the theory without having to spend the 10 years it might require to understand it at the level of an expert.

I'm asking questions to figure out where things currently stand. It appears to me that string theorists are very defensive about the actual state of things. Why is this? Einstein couldn't come up with a theory of quantum gravity, neither could Wheeler, lots of smart people are working on this. So one can't fault string theorists for not solving a problem that no one else can solve yet either. I get that.

I proposed a simple discrimination above: the two different ways in which a theory can be falsifiable in principle, and showed that there were two different criteria:

1) Can we run the test now?

2) Do we even know what tests we could run and what they would show?

I never got an answer to this question. So I'll ask you straight: I know that string theory fails to be falsifiable on the first account, but does it fail on the second?

From what I've seen the answer must be yes, because I've never seen a proposed experiment or prediction of new behavior that would categorically disprove string theory if not found. Not even one that would require galactic-scale accelerators to test.

So does such a prediction and proposed experiment exist? That's all I want to know.

If the answer is no, I'm not proposing that string theory is a dead end or a failure.

I'm in favor of radical freedom in research. I think that any scientist should be able to fund any research they want and we should split the money evenly. If you want to do big research then you'd have to get a bunch of people together to pool funding. I don't think politicians or bureaucrats should divide up funding dollars. I think the individual scientists should do it.

So I have no problem with string theory or the people continuing to work on it. I also have no particular reason to believe that another approach like LQG or CDT is better.

I just want to know what the actual state of the research is.

 

[dx]

I just want to know what the actual state of the research is.

To know what the state of string theory research is requires already quite an advanced level of knowledge about theoretical physics. If I may ask, what is your current level of preparation in physics?