How long can string theory be considered a subject of physics?

[Ivan Seeking]

This accusation is made by one physicist in the streaming video links provided by Integral [Thanks Integral! I had missed these shows when they aired and I was POed!]. I have heard or read similar statements elsewhere. As I understand the state of the theory, whatever its latest evolution is properly called [M Theory still?, or is it N Theory now?], no one has conceived of any experiment that could actually be done to test this theory. If none are known to be possible, then how long can this considered to be a subject of physics? In this sense, Physics without a testable hypothesis is Philosophy.

I have a vague understanding of ST, and I appreciate to some extent why the theory is so attractive, but I don't understand the acceptance as a viable theory if in fact it may be "forever safe". Based on what I have heard and read, acceptance of this as science strikes me as a very treacherous position for physicists to take.

Enlightenment or comments? Again, I don't claim to be an expert here; this is only what I’ve heard and read. My read on this so far is that until a testable hypothesis is at least suggested, ST is philosophy.

 

[fando]

When was ST ever called Physics? Is it official or something? And wasn't the testable part supposed to be the existence of supersymmetric particles? What happened to that?

 

[Mike2]

Originally posted by Ivan Seeking
This accusation is made by one physicist in the streaming video links provided by Integral [Thanks Integral! I had missed these shows when they aired and I was POed!]. I have heard or read similar statements elsewhere. As I understand the state of the theory, whatever its latest evolution is properly called [M Theory still?, or is it N Theory now?], no one has conceived of any experiment that could actually be done to test this theory. If none are known to be possible, then how long can this considered to be a subject of physics? In this sense, Physics without a testable hypothesis is Philosophy.

String theory or M-Theory may turn out to be like the study of logical deduction. It works for reality as well as for fiction. But then who would say that logical deduction is of no practical use in physics?

 

[selfAdjoint]

I'm sure others will provide better explanations than mine, but here's a start. The entire construct of stringy physics (which is huge and varied now) is "contingently" without experimental support. I say contingently because that doesn't happen by construction, by the inner nature of the theory, but just because the theory describes things far beyond the current (or future expected) level of experimental energy. But it is not the case that there is no hope of ever testing stringy physics. There are at least two possibilities that are being explored today:

- Astrophysics. Light traveling over the huge distances of the universe would accumulate even the tiniest disturbances to its path. There is hope of detecting a signal from the quasars' light which would confirm or falsify some stringy predictions.

- Low energy limit. Any theory that proposes to be true in our world has to reproduce our known physics as the energy is reduced from the Planck level to our own level. Stringy physics can already "do" this, if you squint and cross your eyes. The ability to do it for real is a genuine falsification test of stringy physics.

Finally there is a special case, the Hawking-Beckenstein thermodynamics of a black hole. This is a striking and counterintuitive result that was originally achieved by semiclassical results. That is, bits of two theories were put together without really unifying the theories. But stringy physics has succeeded in deriving these results - in a special but significant case - by the proper method of counting states. This was a great success for stringy physics and should count for a lot against the jibes of the unfalsifiable crowd.

 

[Jimmy]

There is a lot of criticism from other scientists who believe that without any experimental means to test string theory, it is doomed.
But there is at least one thing that is testable according to Ed Witten:

NOVA: It seems like the standard criticism of string theory is that it isn't testable. How do you respond to that criticism?

Witten: One very important aspect of string theory is definitely testable. That was the prediction of supersymmetry, which emerged from string theory in the early '70s. Experimentalists are still trying to test it. It hasn't been proved that supersymmetry is right. But there is a very precise relationship among the interaction rates of different kinds of particles which follows from supersymmetry and which has been tested successfully. Because of that and a variety of other clues, many physicists do suspect that our present decade is the decade when supersymmetry will be discovered. Supersymmetry is a very big prediction; it would be interesting to delve into history and try to see any theory that ever made as big a prediction as that.

http://www.mu6.com/stringtheory.html

 

[marcus]

Originally posted by fando
When was ST ever called Physics? Is it official or something? And wasn't the testable part supposed to be the existence of supersymmetric particles? What happened to that?

This might be a good place to gather links to some of the critiques and evaluations that have appeared.

Here is one I thought was very even-handed and optimistic about both main approaches. It appeared in 2003 and was by the well-known string theorist, Enrique Alvarez
"Loops versus Strings"
http://arxiv.org/gr-qc/0307090
(invited talk to a conference of string and other particle theorists in July 2003)

A more negative evaluation of string theory was presented by Peter Woit, a mathematical physicist at Columbia whose specialty is Quantum Field Theory and allied topics
"String Theory: An Evaluation"
http://arxiv.org/physics/0102051

A prominent string theorist, Tom Banks, one of the inventors of M-theory, has provided a lengthy critique called "A Critique of Pure String Theory" (echoing the title of a philosophy book about Pure Reason), dated June 2003
http://arxiv.org/hep-th/0306074

Indeed there seems to be an ongoing controversy around issues like
lack of empirical content, testability, falsifiability, scarcity of predictions, supersymmetry not turning up as expected, whatever you wish to characterize it.

Some of these points surfaced in an interesting interview with Leonard Susskind, a founder of string theory, which took place recently and is online. Here is a link to a PF thread about the Susskind interview.

https://www.physicsforums.com/showthread.php?s=&threadid=11355

Here is a link to remarks by Lee Smolin and others conversing with Susskind, during that interview.
http://www.edge.org/discourse/landscape.html

Some of the points made by Tom Banks in his "Critique of Pure String Theory" were:
*String theory is not background independent.
Page 6 "...the old dream of background independence in string theory is a chimera."

*String field theory does not do what Banks thinks it should do.
Page 30 "...String filed theory does not give us a non-perturbative definition of a quantum theory."

*String theory seems unable to work without supersymmetry.
Page 30 "...There are no known asymptotically flat string vacua with broken SUSY. Kutasov and Seiberg have given a very general argument
for why this is so..."

*There are way too many string theories and seemingly no effective way to choose. See page 22 and also a further paper by Banks "Is There a String Theory Landscape?"
http://arxiv.org./hep-th/0309170
that also appeared in 2003.

As I see it, it is the critiques by string theorists themselves that are potentially the most informative, which is why the links to Enrique Alvarez and Tom Banks and the interview with Leonard Susskind are among the first listed. For that matter, Lee Smolin has published a fair amount of research in string theory (although his name is more associated with Loop Gravity) so he probably qualifies as having at least in part an insider's perspective.

For a side-by-side comparison of progress and open questions in Loop and String, with additional comment on other theories, see Smolin's 2003 paper:

"How far are we from the theory of quantum gravity?"
http://arxiv.org/hep-th/0303185

Smolin's paper is long and technical, going into considerable detail about prospects of testing quantum gravity theories. For a lighter, untechnical, comparison of Loop and String, there is an imagined conversation between two theorists, which I am told has now been published in October 2003 issue of the International Journal of Modern Physics (IJMPD vol 12 no 9):

Carlo Rovelli
"A Dialog on Quantum Gravity"
http://arxiv.org/hep-th/0310077
dated October 2003

 

[alexsok]

For a lighter, untechnical, comparison of Loop and String, there is an imagined conversation between two theorists, which I am told has now been published in the International Journal of Theoretical Physics

It is most certainly not for the layman though and it is technical, just without the equations, which isn't to say that it couldn't perplex a potential reader with the mindblowing amount of ways these guys go around a problem.

 

[marcus]

Originally posted by alexsok
... it is technical, just without the equations,...

alexok, you are right. I found the Dialog fresh and entertaining---just a personal reaction---and written in an informal style. But indeed it is technical--merely without equations. I should have said "less" technical, or "un-mathematical".

A propos the infamous richness, Michael Douglas at Rutgers seems to be a world authority on Statistics of String Theory Vacua and he just posted a paper

http://arxiv.org/hep-ph/0401004

called, appropriately enough, "Statistics of String Vacua"

Tom Banks and E. Witten are among those he thanks for collaboration and discussions in his Acknowledgements. The paper was presented at a 2003 conference on String Theory Phenomenology. In it he derives numbers like 10¹⁰⁰ possible string theories and discusses the logical connotations of this. Wasnt 10¹⁰⁰ the number that someone christened a "Google" (before it became a search-engine)?

 

[selfAdjoint]

A mathematician, I think his name may have been Kastner, was teaching math concepts to a Kindergarten class. This was about 1940, when enthusiasm for spreading math ideas to the broader community was high, part of the communitarian ethos that arose in response to the depression, and that has now, alas, passed away.

Anyway, Kaster was explaining the decimal system and he wrote a 1 on the blackboard and started adding zeros after it. He asked the class how many zeros he should write and got the answer "a hundred!" So he put a hundred zeroes after the one. Then he said to the class "This big number is bigger than a million, but it has no name. You can give it a name today! What name will you give it?"

Than class came up with the name Google. There was a comic strip in those days called Barney Google, and the title character was a dapper little gent drawn with big round eyes. There was even a song about him: "Barney Google, with the goo-goo-googly eyes". So the class, thinking all those zeros looked like eyes, said Google.

Later Kaster wrote a book about math concepts for the popular market and he told this story in it. So that's how the name Google got attached to the number [tex]10^{100}[/tex].

 

[marcus]

Originally posted by selfAdjoint
...to the broader community was high, part of the communitarian ethos that arose in response to the depression, and that has now, alas, passed away...

... [tex]10^{100}[/tex].

yes! I had forgotten about that and am glad you refreshed my
memory---I mean the connection of communitarian ethos with
Great Depression of Thirties (and probably with the broadly shared
trauma of WW2). Perhaps explosive demographic shifts, plus changes in the patterns of immigration had something to do with the demise of that easier cohesion, one might even say that "Jimmy Stewart" communitarian assumption. Aieee! how times change!

I had heard of Barney Google and his eyes, but never made the connection.
I had never heard the story of the hundred zeros on the blackboard and the schoolkids.

 

[selfAdjoint]

"Bliss was it in that time to be alive, but to be young was very heaven." Porbably misquoted from memory.

You could respond to both Picasso and Norman Rockwell, without any tension or irony. The intellectual community rose eagerly to the hopes raised by the new deal, and yes, the "Good War" as it was later ironically called, did continue that excitement.

It all petered out in the A-bomb, McCarthyism, and the crash of the "New Math".

 

[jeff]

Originally posted by marcus
Indeed there seems to be an ongoing controversy around issues like
lack of empirical content, testability, falsifiability, scarcity of predictions, supersymmetry not turning up as expected, whatever you wish to characterize it.

What do you mean by "there seems to be an ongoing controversy", and in particular, what do you mean by "supersymmetry not turning up as expected"? Do you mean to somehow leave members here with the impression that LQG is without controversy? Does LQG stand in relation to the above issues of "controversy" much differently than strings or any other QGT? Would you mind putting this into a bit better perspective for us?

Originally posted by marcus
As I see it, it is the critiques by string theorists themselves that are potentially the most informative

Well, I'm sure we all understand why you'd prefer people agreed with you on this marcus, but really, unless you're an expert, starting from such papers isn't such a good idea. Btw, are there LQG analogs of these papers, because if there are, I'd find it interesting that you've never mentioned them, and if there aren't, well that would be kind of interesting too!

 

[Ivan Seeking]

Thanks for the great answers.

How significant is this here? Does this only pose a problem for a few specific interpretations of LQG, or could this mean big problems for the entire LQG crowd?

http://xxx.lanl.gov/abs/astro-ph/0308214

"Some models with large extra dimensions are ruled out by the existence of absorption in the very high energy spectra of nearby BL Lac objects. The fact that more distant brighter sources are not seen can also be taken as indirect evidence of intergalactic absorption by pair production interactions. The constraints based on analysis of the Crab Nebula ã-ray spectrum, discussed in the previous section, imply that the quantum gravity scale is orders of magnitude above the Planck mass scale. This indicates that the class of models considered here with linear Planck scale suppressed terms in the dispersion relations cannot be reflective of physics at the Planck scale. Models such as loop quantum gravity with a preferred inertial frame are ruled out by this line of reasoning. Alternative models to consider might be models with quartic momentum terms with supression in the dispersion relations, Lorentz invariant quantum gravity models, or really new Planck scale physics such as string theory, which preserves Lorentz invariance."

https://www.physicsforums.com/showthread.php?s=&threadid=12204

 

[marcus]

Originally posted by Ivan Seeking
Thanks for the great answers.

How significant is this here? Does this only pose a problem for a few specific interpretations of LQG, or could this mean big problems for the entire LQG crowd?

http://xxx.lanl.gov/abs/astro-ph/0308214

"...Models such as loop quantum gravity with a preferred inertial frame are ruled out by this line of reasoning..."

https://www.physicsforums.com/showthread.php?s=&threadid=12204

Floyd Stecker's line is high energy cosmic ray observation, not quantum gravity. In the paper you mentioned, he has a mistaken idea about LQG---that it necessarily involves a "preferred frame".

Smolin dealt with this misconception in a widely read and commented on April 2003 paper. "How far are we from the quantum theory of gravity?"
http://arxiv.org/hep-th/0303185
See for example page 18. Floyd Stecker cited Smolin's paper but was evidently still in the dark in August 2003 when he wrote his.

Stecker was referring in August to the same Crab Nebula observations that Smolin discussed in April (and said appeared to rule out any "preferred frame" version of quantum gravity). Both refer to
the same research, by Ted Jacobson, D. Mattingly, S. Liberati. See
http://arxiv.org/astro-ph/0212190. The difference is that Stecker mistakenly THOUGHT it ruled Loop Gravity out, and Smolin indicates that it does not.