String theory on the way out?

[arthwollipot]

Was string theory ever on the way in, aside from in popular science magazines?

Yeah - it was huge in about 1986.

 

[arthwollipot]

"Replaced"? What do you mean? String theory is an alternate way to describe particles compared to the standard model, but doesn't predict anything on its own, no?

Yes, it does. The book cited in the OP The Elegant Universe, has a whole chapter on predictions that string theory makes. Of course, most of these predictions can't actually be tested at this time.

One big prediction of string theory is supersymmetry. String theory cannot work without it. The LHC may be capable of confirming supersymmetry when it ramps up to full power some time next year.

 

[arthwollipot]

I may be wrong, but I thought that was just one way of looking at anti-particles which may or may not be useful.

My understanding is that the mathematics is the same whether you're modelling an antiparticle travelling forward in time, or a particle travelling backward. Such equivalences are very important in M-theory.

Well, all the physicists studying string theory and "building" it aren't doing so simply because of popular science magazines. Clearly there's something that has drawn them to put their time and effort into creating this theory. It may not turn out to be fruitful, but of all the options available they considered it to be the most likely to bear fruit (or most interesting for some other reason, I suppose).

The main reason is that it is a quantum theory of gravity. The Elegant Universe takes the first six chapters or so explaining why this is important.

 

[paiute]

Lee Smollen seems to think that, although the string theorists have devoted many years and careers and money to their subject, the results have been of limited use. Of course, hindsight's great, but I wonder what the opinion of physicists here is. Are you pro or anti string/superstring theories? Do you think they will be the best answer in the long run? Or will the M theory, which seems to be the elusive theory of everything, win out?

String theory is pretty, but one criticism of it is that it makes no testable predictions. If so, then it might not be supplanted but merely bypassed. It will be a nice picture physics hangs on its wall. A more useful hypothesis may come along and be more akin to a hammer, which will go into the physics tool box.

By the way, when the elusive theory of everything is elucidated, that will be the end of this existence as we know it, at least according to S. A. Scoggin:

http://www.scribd.com/doc/19550880/GUT-The-Grand-Unified-Theory-A-oneact-play-with-seven-blackouts

 

[zooterkin]

Yes, it does. The book cited in the OP The Elegant Universe, has a whole chapter on predictions that string theory makes. Of course, most of these predictions can't actually be tested at this time.

One big prediction of string theory is supersymmetry. String theory cannot work without it. The LHC may be capable of confirming supersymmetry when it ramps up to full power some time next year.

So there's still a few loose ends to be tied up?

 

[sol invictus]

String theory created more hype than it should have, failed to live up to the impossible expectations that created, and is now suffering a backlash in the popular press.

But it's not "on the way out". It remains the only serious candidate for a quantum theory of gravity. Research into it had a massive impact on several branches of mathematics. And the fact that - if it is the correct description of nature - it is difficult to verify is not really a valid argument against it. Why should nature be convenient?

As for not making testable predictions, that is an oft-repeated falsehood. String theory makes an infinity of testable predictions, any one of which could be falsified tomorrow. The problem is that the easily falsifiable predictions it makes are shared with a large class of other theories. The predictions that are unique to string theory are probably out of reach of experiment for the foreseeable future.

 

[arthwollipot]

So there's still a few loose ends to be tied up?

Oh, definitely.

 

[Cuddles]

"Replaced"? What do you mean? String theory is an alternate way to describe particles compared to the standard model, but doesn't predict anything on its own, no?

As Sol says, this is not even close to correct. String theories make plenty of predictions and are entirely testable. The problem is simply that most of the predictions they make require investigating things at higher energies than is currently possible. This shouldn't really be particularly surprising, since obviously the things we can test right now have already been tested, and few people will bother with a theory that has already been proven wrong. Particle physics is no different from any other sciences in making predictions about things that can't necessarily be tested with current technology, it's just that the next step to make them testable now tends to require large, expensive machines that take time to design and build, so something that was predicted 20 years ago could end up being untestable for another 20 years.

The other problem, as Sol mentions, is that string theory is not actually a single theory, it's a whole class of theories that covers an essentially infinite range of predictions. Technically this isn't actually all that unusual either. Relativity, for example, has a huge number (presumably infinite?) number of possible space-time geometries that are all perfectly valid mathematically. The difference is that we already have a pretty good idea of what space-time is like and can easily rule out the majority with little effort, while string theories have the problems with testing noted above, as well as having had much less time for us to actually play around with them.

 

[SusanB-M1]

I thought I might try and respond to individual posts, but I don't know anywhere near enough of course to do that properly, but I am reading every one with much interest and appreciation. The part of Lee Smollen's book that I have been listening to this afternoon mentions that the visionaries in this field should be given the time and space to think freely. Seems like a good idea.

ETA MattTheTubaGuy: That sounds excellent - all the very best of luck. From what Lee Smollen says, it seems that everyone was making an assumption that certain basic facts had been proved, but that in fact they hadn't.

 

[SusanB-M1]

It died 20 seconds after Laird Scranton claimed it as evidence that the Dogon tribe of west mali had been in contact with Aliens

Oh dear! Sounds fairly typical!

***

There's one thing I've learnt - and that's how to rattle off the phrase Calabi-Yao shapes, but haven't yet had an opportunity to bring it up in conversation.

 

[ZirconBlue]

Personally, I'm hoping we'll eventually move on to G-string Theory.

 

[Tim Thompson]

String Theory

In my opinion, the focal point of Lee Smolin's book comes down to this:

The issue is not whether string theory is worth doing or should be supported, but why
string theory, in spite of a dearth of experimental predictions, has monopolized the
resources available to advance fundamental physics, thus choking off the investigation
of equally promising alternative approaches.
The Trouble With Physics: The Rise of String Theory, the Fall of Science, and What Comes next
Lee Smolin, Houghton Mifflin 2006, pages 267-268.
​

I don't trust Smolin's judgement. For one thing, he is himself the principle author of one of the "equally promising alternative approaches" which is apparently suffering from the monopolization of resources by string theory. John Schwarz gave a talk on string theory to the Los Angeles Astronomical Society a year or so ago, and he made the observation that "scientists vote with their feet". They work on what interests them, like anybody else, and string theory is a fascinating topic to work on for theoretical physicists.

There is another book of similar ilk by Peter Woit.

Much theoretical activity by scientists is speculative, in the sense that it consists of
asking questions of the kind, “What if I assume X is true? Could I then construct a
real theory using this assumption?” This is certainly the kind of thing that scientists
spend a lot of time doing, and one presumably doesn’t want to label it unscientific.
Superstring theory is very much a speculative endeavor of this kind. Theorists involved
in this area are considering a very speculative assumption: that one should replace the
notion of elementary particles with strings or more exotic objects, and trying to see
whether a scientific theory capable of making falsifiable predictions can be built on this
assumption. Generalizing the notion of “scientific” to include speculation of this kind
would definitely make superstring theory a science. But does one really want to say
that all such speculative activity is scientific?
Not Even Wrong: The Failure of String Theory & the Continuing Challenge to Unify the Laws of Physics
Peter Woit, Basic Books 2006, page 208
​

Woit concludes that the answer to the last question here is no. But I would ask instead, “But does one really want to say that any such speculative activity is science”? Replace “all” with “any”. I see no reason why speculative activity that is integral to the process of scientific discovery must necessarily be labeled unscientific.

Furthermore, both Smolin & Woit take the opinion that because the problems of string theory are not already solved now, then we should stop trying altogether. My response is to note that Isaac Newton published his theory of gravity in 1687 (Philosophiae Naturalis Principia Mathematica), at which time he was already aware of the basic problem of general relativity, for which Albert Einstein published the (or at least a) solution in 1915, a mere 228 years later. String theory was conceived from the dual resonance model published by Veneziano in 1968, and born in 1970 from the combined efforts of Yoichiro Nambu, Leonard Susskind & Holger Bech Neilsen (Nambu shared the 2008 Nobel Prize in Physics, though not for string theory). So we have had about 40 years to figure out string theory, which is arguably a harder problem to solve than was general relativity. We have 188 years left, if we follow the "228 year rule", that I just made up. I have to say I am unimpressed by the observation that the problem of string theory is not yet solved.

Also, as Sol Invictus has already noted, there are testable predictions made based on string theory, but one should also note that some of those predictions can be and in fact are being tested, even now. One prediction is the presence of spatially compact extra dimensions (covered in some detail in Elegant Universe). Those dimensions can reveal themselves in high energy accelerator experiments if they are spatially very small, or in short range experiments on the inverse square law of gravity. Supersymmetry is a prediction of string theory, and laboratory searches for supersymmetric partners of known particles are something we can do now, even if the results may be hard to interpret.

I see no reason to think that string theory is on the way out, or anything close to that. And so I will close with an interesting quote from Stephen Weinberg ...

String theory is very demanding; few of the theorists who work on other problems have
the background to understand technical articles on string theory, and few of the string
theorists have time to keep up with anything else in physics, least of all with high energy
experiments. Some of my colleagues have reacted to this unhappy predicament with
hostility to string theory. I do not share this feeling. String theory provides our only
present source of candidates for a final theory - how could anyone expect that many of
the brightest young theorists would not work on it? It is a pity that it has not been more
successful, but string theorists like everyone else are trying to make the best in a very
difficult moment in the history of physics. We simply have to hope that string theory will
become more successful, or that new experiments will open up progress in new
directions
Dreams of a Final Theory: The Scientists Search for the Ultimate Laws of Nature
Steven Weinberg, Pantheon Books 1992, pages 218-219
​

 

[SusanB-M1]

Tim Thompson

That was a very interesting post - , which I shall read again.
Thank you.

 

[ponderingturtle]

Also, as Sol Invictus has already noted, there are testable predictions made based on string theory, but one should also note that some of those predictions can be and in fact are being tested, even now. One prediction is the presence of spatially compact extra dimensions (covered in some detail in Elegant Universe). Those dimensions can reveal themselves in high energy accelerator experiments if they are spatially very small, or in short range experiments on the inverse square law of gravity. Supersymmetry is a prediction of string theory, and laboratory searches for supersymmetric partners of known particles are something we can do now, even if the results may be hard to interpret.

I thought that the LHC was supposed to be a hard test of supersymmetry.

 

[Tim Thompson]

String Theory II

I thought that the LHC was supposed to be a hard test of supersymmetry.

I think that is more a function of LHC press releases, or oversimplified popular expositions. It is certainly true that the LHC will provide interpretable evidence for the presence of supersymmetry in the energy range it can probe, but that energy range is necessarily limited by the technology of the device. At this point I think that the range of possible supersymmetric particle masses is more likely to be constrained by observation then by theoretical prediction, since the theory remains fairly undetermined in many ways.

 

[Complexity]

I, too, have been under the impression that Lee Smolin's opposition to superstring/M-theory research is more self-serving than honest.

 

[ponderingturtle]

I think that is more a function of LHC press releases, or oversimplified popular expositions. It is certainly true that the LHC will provide interpretable evidence for the presence of supersymmetry in the energy range it can probe, but that energy range is necessarily limited by the technology of the device. At this point I think that the range of possible supersymmetric particle masses is more likely to be constrained by observation then by theoretical prediction, since the theory remains fairly undetermined in many ways.

This was not from a press release by a friend who is a theoretical high energy physicist.

 

[arthwollipot]

I thought that the LHC was supposed to be a hard test of supersymmetry.

The LHC, when fully powered up, will operate at energy levels where heavy supersymmetric particles may be detected. Supersymmetric particles such as the "selectron" are theorised to have masses that are well above what the previous generation of colliders (excepting perhaps the Tevatron) are capable of detecting. If they are detected, it will be hard confirmation of supersymmetry, and strong, but highly circumstantial, evidence in favour of string theory.

 

[ponderingturtle]

The LHC, when fully powered up, will operate at energy levels where heavy supersymmetric particles may be detected. Supersymmetric particles such as the "selectron" are theorised to have masses that are well above what the previous generation of colliders (excepting perhaps the Tevatron) are capable of detecting. If they are detected, it will be hard confirmation of supersymmetry, and strong, but highly circumstantial, evidence in favour of string theory.

I thought that the Tevatron hit some of the lower possible energies needed for supersymmetric particles, and that the LHC hit the higher ones. Basicly that if they are not found it then the predictions of the theory are not valid. Now they might just find a way to push them off to still higher energies.

 

[arthwollipot]

I thought that the Tevatron hit some of the lower possible energies needed for supersymmetric particles, and that the LHC hit the higher ones. Basicly that if they are not found it then the predictions of the theory are not valid. Now they might just find a way to push them off to still higher energies.

If they are not found, then supersymmetry is not confirmed to be true. You can't prove something false by not finding evidence of it. But yeah, you're basically right, as I understand it.