Why is String Theory Considered to be a Scientific Theory?

[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?

 

[marcus]

... It appears to me that string theorists are very defensive about the actual state of things. Why is this?...

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 failure if not found. ...

... I also have no particular reason to believe that another approach like LQG or CDT is better...

You are posing a challenge. It's clear and makes sense, at least to me. It wouldn't be satisfactory for people to copout by saying "Well nobody else can either!"

For comparison's sake I will give some clue as to LQG falsifiability---it has no extra spatial dimensions and people talk about LHC seeing evidence of extra spatial dimensions. That would falsify LQG.

Here's another indicator of how near to testability LQG is, or how far from testability it is. You have to judge how near or far:
http://arxiv.org/find/grp_physics/1/au:+Barrau_A/0/1/0/all/0/1

==quote arxiv list of most recent papers by a prominent phenom'ist at U.Grenoble==
Showing results 1 through 25 (of 59 total) for au:Barrau_A

1. arXiv:1009.5532 [pdf, other]
Baryonic acoustic oscillations simulations for the Large Synoptic Survey Telescope (LSST)
A.Gorecki (1), A.Abate (2), R.Ansari (2), A.Barrau (1), S.Baumont (1), M.Moniez (2), ((1) LPSC, Grenoble, (2) LAL, Orsay)
Comments: 4 pages, 2 figures, 10th Rencontres de Blois proceeding
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

2. arXiv:1009.4769 [pdf, ps, other]
Investigating The Uncertainty On The BAO Scale Measured From Future Photometric And Spectroscopic Surveys
Alexandra Abate (1), Alexia Gorecki (2), Reza Ansari (1), Aurelien Barrau (2), Sylvain Baumont (2), Laurent Derome (2), Marc Moniez (1) ((1) LAL, Orsay, (2) LPSC, Grenoble)
Comments: 5 pages, 3 figures, Proceedings of the conference "45th Rencontres de Moriond" - Cosmology Session, La Thuile, Val d'Aosta, Italy, March 13 - 20, 2010
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO)

3. arXiv:1003.4660 [pdf, ps, other]
Inflation in loop quantum cosmology: Dynamics and spectrum of gravitational waves
Jakub Mielczarek, Thomas Cailleteau, Julien Grain, Aurelien Barrau
Comments: 11 pages, 14 figures. Matches version published in Phys. Rev. D
Journal-ref: Phys.Rev.D81:104049,2010
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Theory (hep-th)

4. arXiv:1001.2973 [pdf, ps, other]
Very high energy gamma-rays and the Hubble parameter
A. Gorecki, A. Barrau, J. Grain, E. Memola
Comments: Proc. of the 12th Marcel Grossmann meeting on general relativity. 3 pages, 1 figure
Subjects: Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Astrophysical Phenomena (astro-ph.HE)

5. arXiv:0911.3745 [pdf, ps, other]
Loop quantum gravity and the CMB: toward pre-Big Bounce cosmology
Aurelien Barrau
Comments: Proceedings of the 12th Marcel Grossman Meeting on General Relativity. 3 pages, no figure
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

6. arXiv:0910.2892 [pdf, ps, other]
Fully Loop-Quantum-Cosmology-corrected propagation of gravitational waves during slow-roll inflation
J. Grain, T. Cailleteau, A. Barrau, A. Gorecki
Comments: 9 pages, no figure, minor corrections
Journal-ref: Phys.Rev.D81:024040,2010
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

7. arXiv:0902.4810 [pdf, ps, other]
Internal structure of a Maxwell-Gauss-Bonnet black hole
S. Alexeyev, A. Barrau, K.A. Rannu
Comments: 5 pages, 5 figures, published version with minor changes
Journal-ref: Phys.Rev.D79:067503,2009
Subjects: General Relativity and Quantum Cosmology (gr-qc)

8. arXiv:0902.3605 [pdf, ps, other]
Inverse volume corrections from loop quantum gravity and the primordial tensor power spectrum in slow-roll inflation
J. Grain, A. Barrau, A. Gorecki
Comments: 15 pages, 5 figures, published version with minor modifications, results unchanged
Journal-ref: Phys.Rev.D79:084015,2009
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

9. arXiv:0902.0145 [pdf, ps, other]
Cosmological footprints of loop quantum gravity
J. Grain, A. Barrau
Comments: Accepted by Phys. Rev. Lett., 7 pages, 2 figures
Journal-ref: Phys.Rev.Lett.102:081301,2009
Subjects: General Relativity and Quantum Cosmology (gr-qc); Cosmology and Extragalactic Astrophysics (astro-ph.CO); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th); Quantum Physics (quant-ph)

10. arXiv:0805.0356 [pdf, ps, other]
Holonomy corrections to the cosmological primordial tensor power spectrum
A. Barrau, J. Grain
Comments: 5 pages, Proc. of the 43rd Rencontres de Moriond "Cosmology 2008"
Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph); High Energy Physics - Phenomenology (hep-ph); High Energy Physics - Theory (hep-th)

==endquote==

Barrau gave a talk in July at the Paris ICHEP (international HEP conference). Abstract and slides here:
https://www.physicsforums.com/showthread.php?p=2812649#post2812649
Loop quantum gravity and the early universe
Content: Loop quantum gravity is, together with string theory, one on the major candidate approach to quantize gravity. It provides a framework which allows for a non-perturbative and background-independant canonical quantization of general relativity. In this talk, I will briefly go through the basic conceptual groundings of the theory and switch to the latests developments associated with its implementation in the cosmological context. I will show that the Big Bang is replaced by a Big Bounce (therefore solving the initial singularity problem) and that inflation unavoidably occurs. Furthermore, the primordial tensor power spectrum should exhibit some characteristic features that could lead to experimental tests of this "Planck-scale" physics.

 

[MTd2]

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?

Do you even know the level? This is not a trivial issue. Suprised said low energy SUSY models are not string theory. That means things are not accessible for the average professor level physicist, for example, working with fundamental experimental high energy physics on CERN. Even if said person developed detectors and statistical analysis tools like Tommaso Dorigo.

To tell you the truth, it is possible to have a glimpse of the current level by a fast reading of string theory papers, specially those involving top down constructions from heterotic and f-theory compactification GUT models. But, most details that are understandable to a wider audiance are in the beginning and end of the paper. The critical advances requires very specialized details that are lost in the middle of several papers and whose comprehension would require a lot of reading.

So, for most non genius, one:

1. Either work on the theory,
2. Do some other research.

If one criticise, the specialist can always run into intricacies and call the accusing party "arm chair physicist". Which is quite correct, but, it ends up looking like trying to criticize biblical scholars on the matter of scatological issues.

 

[inflector]

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?

I am currently an advanced amateur, I'd say.

I have a very high aptitude for math and science. I attended one of the top two public high-schools in Massachusetts and when I was on our math team as a freshman we won the state math competition. I was the best in my class at math and science. I got a perfect score on the math SATs and 99.7th percentile on the English before they dumbed down the test in the 90s.

Unfortunately, I stopped taking math in college after differential equations a long time ago because I dropped out of college to trade commodities in the mid-80s. This was after my second year at Worcester Polytechnic Institute (a decent school but no MIT or Caltech or Harvard). I left high-school a year early because I was bored. I dropped out of college because it started to be more like high-school and wasn't challenging enough. I was learning more on my job by myself than at school.

I took first-year physics in college but not chemistry or quantum mechanics.

I taught myself programming computers and got a job before I ever owned a computer and ended up programming algorithmic commodity trading systems on the Apple IIe in 1981 through 1983 while I was in high-school and commuting from college.

I learned enough to get a job trading for one of the best traders in the world in Chicago when I was 19 years old. I worked with some very smart people in a small group that became famous for our trading success. I personally turned $2 million of this trader's money into more than $33 million in just over four years. I quit trading after just over four years because I was bored and had enough money I could do anything I wanted. Several of the guys I traded with run hundreds of millions or billions in hedge funds now.

I taught myself business, started an Inc. 500 computer board repair company and several software companies, one of which I took public and ran for a while. I consulted to startups in Silicon Valley in the late 90s at the height of the internet craze helping with marketing and software architecture.

I am also an author, my first book which discussed my trading experience, sold over 70,000 copies and is printed in 10 languages.

I have never held a conventional job. I have always done something new because I hate boredom and I love to learn. I especially love challenges and anything that others consider to be impossible. I have made more mistakes than anyone I know because I have never been satisfied doing what I already know. I am always trying something new and difficult, and once I master any given subject, I move on to something more challenging.

I have been looking for a big challenge and a way to make a big difference in the world and I came to physics a few years back after I started looking at transportation systems. It was clear to me that we needed electric vehicles of various sorts and that let me to investigate fusion power and the reasons why we had not yet figured out how to do practical fusion. From those investigations, it became clear to me that science still did not have a clear picture of the nucleus and factors which affected nuclear binding energy. This surprised me.

I subsequently found that there are many open questions in physics which I had always assumed we had an answer for. What is mass? How does mass bend spacetime? What is time? What is responsible for the phenomena we attribute to dark matter and dark energy? and many more as I'm sure you all know.

So I feel like these are challenges that I can really dig into and ones that are supremely difficult. I like this.

So I've been working on some ideas and trying to learn enough physics so I can contribute something useful to science.

I am trying to figure out what directions to take with my personal research and have settled on the quantum gravity problem in particular as the most interesting challenge.

Now I represent an oddity here, perhaps. You won't find many people who have been as successful (and failed spectacularly at times) and who have done so many other things in life, or who are as old as I am digging into a field as difficult as physics, and especially quantum gravity. You also won't find very many people with the aptitude for science and math that I have who took as little actual math in college as I have yet who are still going to jump into physics for the first time in their mid 40s. I think my perspective from other fields and disciplines and skills will be useful. I also think it is possible I might perhaps be able to see something new. Of course, I know that a lot of scientists think this at some level.

I am trying to take advantage of my fresh perspective so I have spent a lot of the last two years coming up to speed on the historical development of physics and the reasons why we have our current beliefs. I believe that there are clues in the history that may hint at possible assumptions that science is making that won't hold in the end, and that perhaps some of the answers are difficult to find because everyone holds the same assumptions. So I am very skeptical of anything that I read unless I personally understand the logic behind it. I take nothing at face value and consider no one to be an unimpeachable expert on every subject.

I believe I can learn from almost anyone, yes, even the cranks and quacks at times, but I take no one's opinions as my own without having investigated the reasons for their beliefs myself. I am trying to build a giant tree in my head of all the assumptions of physics and how they interrelate so that I can understand what we actually know and what we just mostly think must be true but haven't really proven. I distinguish very carefully between the specific results of experiments and their specific interpretations.

I am currently reading Feinstein's Lectures on Physics, Mary Boaz's Mathematical Methods in the Physical Sciences, and Penrose's The Road to Reality to bone up on the basics. I'm trying to figure out what next steps to take which is why I am interested in finding out the state of String Theory. I want to know if I should explore it further or work on something else.

That's probably way more information that you wanted or required but some of that may be relevant to your answer.

 

[marcus]

...
If one criticise, the specialist can always run into intricacies and call the accusing party "arm chair physicist". Which is quite correct, but, it ends up looking like trying to criticize biblical scholars on the matter of scatological issues.

MTd2, your command of English would be excellent even were you a native speaker. But be careful to distinguish between eschatological and scatological. Two separate meanings, one Bible-related and the other slightly off-color.

I am currently an advanced amateur, I'd say.

I have a very high aptitude for math and science...

...I am also an author, my first book which discussed my trading experience, sold over 70,000 copies and is printed in 10 languages.

...So I've been working on some ideas and trying to learn enough physics so I can contribute something useful to science.

...I am trying to figure out what directions to take with my personal research and have settled on the quantum gravity problem in particular as the most interesting challenge...

... I am trying to build a giant tree in my head of all the assumptions of physics and how they interrelate so that I can understand what we actually know and what we just mostly think must be true but haven't really proven. I distinguish very carefully between the specific results of experiments and their specific interpretations...

I am currently reading Feinstein's Lectures on Physics, Mary Boaz's Mathematical Methods in the Physical Sciences, and Penrose's The Road to Reality to bone up on the basics. I'm trying to figure out what next steps to take which is why I am interested in finding out the state of String Theory. I want to know if I should explore it further or work on something else.
...

I see how you might be able to "contribute something useful to science". Write a book about the revolutions going on in physics, from a layman's perspective.

Your assets are that you think clearly and write well, and have high aptitude, and have had an interesting life (presumably born around 1965), which means that your own story as an adventurous character going around in search of understanding, talking to various physicists, visiting various observatories, accelerators, institutes, etc would likely make a good narrative.

The public tends to like these books telling the story of an individual's science odyssey. It makes it easier for the reader to put himself into it and get engaged with the subject matter. Both Smolin and Susskind have used this device. You get to include photographs.

Go visit IceCube the neutrino telescope in Antarctica, and interview somebody. Go visit the MAGIC air cherenkov imaging telescope on San Juan island in the Canaries. Visit that "Auger" large area cosmic ray detector spread in Argentina. Visit the ESA (euro space agency) and talk to the people handling the Planck spacecraft mission---mapping the cosmic microwave background. Talk to people at LISA gravity wave detector.
You'd learn a lot and have a successful book and teach other people a lot by seeing frontier science through an intelligent layman eyes.
Just a thought. No time to edit. Sorry if too pretentious sounding.

 

[dx]

Thats very interesting inflector.

Well, I'm no expert in string theory, but here's a brief outline of its starting point.

In ordinary physics, we have a background spacetime M, and particles/fields propagating on that space in a relativistic way. This picture of particles/fields propagating in space, interpreted quantum mechanically, is the basis of quantum field theory. For example, we can have a field $\varphi : M \rightarrow R$, called a scalar field, where R is the set of real numbers. From the field, we construct a 'Lagrangian', such as $g(d\varphi,d\varphi)$, where g is the 'metric tensor' which encodes the metric properties of spacetime. This object determines the equations of motion of the field. Quantum field theory is the basis for the comprehension of a large amount of experimental data, but on the logical level, there are many contradictions and difficulties when one tries to extrapolate this to extreme situations.

In string theory, instead of a background spacetime, the basic space is an auxiliary space called the worldsheet (the string analog of the world-line of a particle). There is no classical spacetime in the ordinary sense. The coordinates of spacetime become fields on the worldsheet. Spacetime is supposed to arise out of quantum fields propagating on the worldsheet. Research in string theory has mainly been a mathematical exploration of this idea, and its consequences for physics. Gauge theory, general relativity etc. (the other main ingredients of ordinary physics) are not put into string theory, but seem to arise out of it. The only input that string theory takes are special relativity and quantum mechanics. Many of the leaders of string theory research, like Witten, think that the precise fomulation of string theory will involve a radical generalization of the notion of geometry, and finding this is one of the themes of current efforts.