There’s an interesting exchange today between Sean Carroll and George Johnson on bloggingheads.tv. They cover a range of topics, including the controversy over string theory and the role of blogs. Johnson describes his time last fall at the KITP, telling about drinking very expensive Scotch with Steve Shenker, and many discussions with the string theorists there who were concerned that they were getting bad publicity and didn’t know what to do about it. Some were concerned that Lubos Motl was not exactly representing them well.
Sean gives a fairly standard defense of the landscape (“maybe it really is the way the world works”), with no discussion of the main problem with the landscape, that it shows no signs of making falsifiable predictions that would make it legitimate science. Some other points of the discussion include speculation about “what if Feynman and Gell-Mann had blogs?”, and Sean’s analogy of the string/LQG debate with mainstream/”heterodox” academic economics.
Update: Sean has his own posting about this.
Regarding the calculation of the mass of the electron, please see hep-th/0703280.
I agree, string theory has not reached the level of maturation of QFT with the Standard Model. Keep in mind that this is only because the Standard Model is operative at low energy where there is a lot of experimental data.
What is needed is a string theory model which completely describes the low-energy physics of the Standard Model and can make detailed predictions for physics beyond the Standard Model. Even if some parameters need to be fitted by hand to get agreement with the Standard Model, once this is done all of the other (new) physics should be fixed as well. The problem would be if there are many string theory models which reproduce the standard model exactly, yet have completely different predictions for new physics. I don’t expect this to happen.
Again, I would like to make the point that it is possible for string theory to be falsified by theoretical criteria alone, although this hasn’t happened. Peter’s main argument that it isn’t falsifiable is wrong for this reason alone. In addition, it is always possible that new, unexpected phenomena will be discovered that are not compatible with string theory.
Eric, do you really think the true theory is that complicated? You put in a lot of fitting parameters and are still off by a factor of 6.5 on the electron mass; wouldn’t that seem to say that your model cannot fit the SM?
“In addition, it is always possible that new, unexpected phenomena will be discovered that are not compatible with string theory.”
Please describe these hypothetical phenomena.
“Please describe these hypothetical phenomena.”
I believe that violation of unitarity is incompatible with ST.
“Does quantum field theory by itself offer any falsifiable predictions?” Yes, certainly it does: the existence of antiparticles, with the same mass and opposite charges as known particles, is a direct consequence of the axioms of special relativistic quantum field theory. So are the relation between spin and statistics and the CPT theorem. The existence of antiparticles and CPT were predicted before they were tested and they and the spin-statistics connection are confirmed by many experiment and to a large number of decimal places.
One can add to this the classic tests of QED, the magnetic moments of the electron and muon and the Lamb shift, which hold robustly to many decimal places, independently of the strong and weak interactions, higher generations etc. The fact is one can make extremely successful predictions without knowing very much at all about what the gauge groups and mass spectra are, using only the well confirmed fact that there is an unbroken U(1) gauge symmetry coupled to light fermions.
This gives us confidence that the axioms of QFT are reliable over a wide range of scales. There is not a single comparable success in string theory or any post standard model theory.
Sure, antiparticles are a necessary ingredient of any QFT. However, can you predict the specific particles that will be present in your theory from first principles? Absolutely not. You have to assume a gauge group, from which it will follow that the matter content will be representations of this gauge group, with the only constraint that all anomalies must cancel. You cannot determine from first principles what the matter content, masses, or charges of your QFT will be. The Standard Model is just that, a model which happens to be a QFT, and in this regard it is not unique.
I believe that violation of unitarity is incompatible with ST.
Violation of unitarity is incompatible with common sense. Total probability for all alternatives simply cannot be greater (or less) than 1. Quantum mechanics + preservation of probabilities = unitarity (Wigner theorem).
They are only “necessary” if you require a charge conjugation symmetry such as CPT.
“What is needed is a string theory model which completely describes the low-energy physics of the Standard Model and can make detailed predictions for physics beyond the Standard Model. Even if some parameters need to be fitted by hand to get agreement with the Standard Model, once this is done all of the other (new) physics should be fixed as well. The problem would be if there are many string theory models which reproduce the standard model exactly, yet have completely different predictions for new physics. I don’t expect this to happen.”
About your last sentence: Until there is an actual string theory that makes actual predictions, it is hard to know what can be expected.
You also said:
“Again, I would like to make the point that it is possible for string theory to be falsified by theoretical criteria alone, although this hasn’t happened. Peter’s main argument that it isn’t falsifiable is wrong for this reason alone. In addition, it is always possible that new, unexpected phenomena will be discovered that are not compatible with string theory.”
Yes, agreed, any theory can be falsified if it is logically inconsistent, and this does not require any experimental input. However, the point about falsifiability that has repeatedly been made by a number of people is that because no predictions have yet been made by string theory, it is impossible for any experimental result to falsify it. This is what I take the “not even wrong” label to mean.
All the best,
I’d like to emphasize that it may very well be possible to make definitive predictions in the near future and there very well may be experimental results in the future that may be able to confirm or falsify it. In my opinion, the claim that string theory is not falsifiable is itself ‘not even wrong’ because we really don’t know what new developments or discoveries my be found in the future. Saying we shouldn’t study string theory because we do know enough about it yet to make predicitions is rather dumb. How are we ever going to understand it well enough to make predictions if we don’t study it? Why not give up on experimental high-energy physics as well since we’ll never be able reach Planck-scale energies?
I think one reason why antiparticles are necessary in QFT comes from the desire to preserve causality. Look at Peskin and Schroeder.
“In my opinion, the claim that string theory is not falsifiable is itself ‘not even wrong’ because we really don’t know what new developments or discoveries my be found in the future.”
You seem to have a problems with the present tense. Right now, at this very moment, string theory is ‘not even wrong’ because it makes no predictions of any value. Saying that it ‘may very well be possible in the near future’ to make predictions doesn’t obviate the fact that — right now — it does not.
The FSM ‘may very well be possible in the near future’ to falsify. However, the FSM, as it stands now is ‘not even wrong.’
This is the nature of research in progress. I’m really getting tired of this abuse of Pauli’s phrase ‘not even wrong’. He certainly didn’t mean it in the strict, limited way that you and Peter use it. Essentially, he meant that a theory was ‘half-baked’, i.e. not consistent. The inability to test a theory due to current practical limitations is not a valid reason for dismissing a theory.
As an example, consider the theory that intelligent life exist elsewhere in the universe. This idea is almost certainly right, however I can’t think any way to disprove the idea. Is this idea ‘not even wrong’?
Lots of perfectly reasonable theories will be “not even wrong” in the sense advocated in this comment section. Your theory about intelligent life and the scalar-tensor theory I mentioned earlier are examples. As Peter points out, string theory has more tunable parameters than scalar-tensor gravity. However, string theory also has stronger theoretical motivation: the desire to find a consistent quantum theory of gravity.
Like much of what is discussed here it seems to me to be really a question of personal prejudice. Does the potential future promise of string theory justify the effort that’s currently being put into it. Many people think “yes”. Some people think “no”. This is fine because everybody is allowed to choose her own research programme. I’m don’t see what all the fuss is about.
I agree with you completely.
‘… everybody is allowed to choose her own research programme. I’m don’t see what all the fuss is about.’ – anon2
Silly me! I thought the whole problem was that string theory is hyped to be the ‘only consistent theory of quantum gravity’.
I was evidently deluded, since I thought that such marketing of currently non-falsifiable string theory made life hard for those who are pursuing other research programmes. Of course, I see the light now! Everyone is free in spirit to choose their own research programme, they just aren’t generally free in practice to actually pursue it, unless by chance they have chosen string theory…
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I have no argument with your last comment addressed to me, particularly the parts of it that are directed at others. (For example, I have never suggested that individuals should study or not study any particular subject.)
One ought to be painstakingly honest about the state of one’s field of study, for the benefit of scientists outside the field, incoming graduate students, and also lay people. (And perhaps for our own benefit most of all.) I have met many lay people and students in the past decade with distorted views about string theory. Advocates of string theory could largely disarm many of its critics simply by being honest and direct about the state of the subject.
Scientific theories make predictions about the results of future experiments, then the experiments are carried out. If the experimental results match the predictions, within acceptable limits, then fine. If not, then the theory is discarded or modified. That is how the game is played.
You have not responded to my requests for predictions of string theory, although you imply that there are some, just at energies that are too high to access now. Are there any? If there aren’t any, then fine, but let’s be honest about that. Let us say, OK, it’s not a scientific theory yet, it’s in development, Eric Mayes finds it an interesting subject to work on, no problem. Best wishes to Eric Mayes, we hope he comes up with some good ideas that turn this into a proper scientific theory. Repeat, I have no quarrel with your research choices, Eric, and I sincerely wish you well.
But in the meantime, let’s not delude ourselves into thinking that the next experiment can produce results that will either support or falsify string “theory”, when there is no theory yet.
Ok, here are my predictions. Supersymmetry and the Higgs will be discovered at LHC with the Higgs mass between 116-121 GeV.
Is your prediction for the Higgs mass based on a string-theory calculation? And your statement that supersymmetry will be found at the LHC does not qualify as a prediction unless it is quantitative.
All the best,
ST predicts the existence of superpartners but it does not make a unique prediction about the SUSY breaking scale and mass spectrum.
It’s clear and simple – the existence of superpartners is a generic prediction of ST.
Their mass spectrum depends on how SUSY is broken but that’s a second question for which there is no unique ST prediction yet.
So, what! In the SM you need 19 parameters as input. Once the scale of m_3/2 is fixed, lots of SUSY models will be ruled out just based on this one input parameter.
In my biased opinion, SUSY is the best candidate to explain the dark matter so based only on this assumption the LHC has a pretty good chance to discover superpartners. There are of coures many other well known reasons (stabilizing the hierarchy, radiative EWSB, heavy top, precision gauge coulpling unification, etc) to expect that TeV scale SUSY is around the corner.
Old news, but what about this?
Which SUSY models does this screw up?
“Which SUSY models does this screw up?”
“The MSSM does not lead to any significant deviation from the SM expectation for CP violating phenomena such as de
N, εK, ε/ε and CP violation in B physics; the only exception to this statement concerns a small portion of the MSSM parameter space where a very light ˜t (m˜t
“The MSSM does not lead to any
significant deviation from the SM expectation for CP violating phenomena such as d^e_N, ε_K, ε’/ε and CP violation in B physics; the only exception to this statement concerns a small portion of the MSSM parameter space where a very light ˜t (m˜t less than 100 GeV) and χ+ (mχ ~ 90 GeV) are present.”
Flavour and CP violation in supersymmetry
Antonio Masiero and Oscar Vives
‘Source’, not ‘Sourse’.
“ST predicts the existence of superpartners but it does not make a unique prediction about the SUSY breaking scale and mass spectrum.”
The existence of superpartners is an assumption of ST, not a prediction. And it would be more accurate to say that ST makes no prediction about the SUSY breaking scale and mass spectrum.
The word “prediction” has a precise meaning, and ought to be used with care.
All the best,
Re: antiparticles, I don’t agree. Causality, or at least spacelike commutativity is connected with the spin statistics theorem, but has nothing to do with the existence of antiparticles. Yes, naive quantization of the Dirac equation leads to particle/antiparticle pairs of the same mass, but one could with equal validity just 2nd quantize Majorana spinors which have no such property.
I just deleted a sequence of comments by Santo and Eric, which, on Eric’s part, mainly consisted of the sort of insults about incompetence and stupidity that some string theorists seem to feel is the best way to defend their point of view. Please stop this.
This discussion has pretty much degenerated, and I’ll cut off any more comments that aren’t well-informed and to the point. Before posting a comment here that carries on a pointless argument, ask yourself if you’re writing something that a sensible person would find informative and worth spending their time reading. If the answer is negative, restrain yourself.
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