Why I Still Like String Theory

May 16, 2013

There is a new book coming up by Richard Dawid “String Theory and the Scientific Method. It has been reviewed by Peter Woit and Lubos Motl who give their expected opposing views. Apparently Woit gets it through a university library subscription. I can’t really review the book because at £60 it is a bit too expensive. Compare this with the recent book by Lee Smolin which I did review after paying £12.80 for it. These two books would have exactly the same set of potential readers but Smolin is just better known which puts his work into a different category where a different type of publisher accepts it. I dont really understand why any author would choose to allow publication at a £60 price-tag. They will sell very few copies and get very little back in royalties, especially if most universities have free access. Why not publish a print-on-demand version which would be cheaper? Even the Kindle version of this book is £42 but you can easily self publish on Kindle for much less and keep 70% of profits through Amazon.

My view is equally predictable as anyone elses since I have previously explained why I like String Theory. Of the four reasons I gave previously the main one is that it solves the problem of how quantum theory looks in the perturbative limit about a flat space-time with gravitons interacting with matter. This limit really should exist for any theory of quantum gravity and it is the realm that is most like familiar physics so it is very significant that string theory works there when no other theory does. OK, so perturbative string theory is not fully sewn up but it works better than anything else. The next best thing is supergravity which is just an effective theory for superstrings.

My second like is that String Theory supports a holographic principle that is also required for quantum gravity. This is a much weaker reason because (a) it is in less well known territory of physics and requires a longer series of assumptions and deductions to get there (b) It is not so obvious that other theories wont also support the holographic principle.

Reason number three has not fared so well. I said I liked string theory because it would match well with TeV scale SUSY, but the LHC has now all but ruled that out. It is possible that SUSY will appear in LHC run 2 at 13 TeV or later, or that it is just out of reach, but already we know that the Higgs mass in the standard model is fine-tuned. There is no stop or Higgsino where they would be needed to control the Higgs mass. The only question now is how much fine-tuning is there?

Which brings me to my fourth reason for liking string theory. It predicts a multiverse of vacua in the right quantities required to explain anthropic reasoning for an unnatural fine-tuned particle theory. So my last two reasons were really a hedge. The more evidence there is against SUSY, the more evidence there is in favour of the multiverse and the string theory landscape.

Although I dont have the book I know from Woit and Motl that Dawid provides three main reasons for supporting string theory that he gathered from string theorists. None of my four reasons are included. His first reason is “The No Alternatives Argument”, apparently we do string theory because despite its shortcomings there is nothing else that works. As Lee Smolin pointed out over at NEW, there are alternatives. LQG may succeed but to do so it must give a low energy perturbation theory with gravitons or explain why things work differently. Other alternatives mentioned by Smolin are more like toy models but I would add higher spin gravity as another idea that may be more interesting. Really though I dont see these as alternatives. The “alternatives theory view” is a social construct that came out of in-fighting between physicists. There is only one right theory of quantum gravity and if more than one idea seems to have good features without them meeting at a point where they can be shown to be irreconcilable then the best view is that they might all be telling us something important about the final answer. For those who have not seen it I still stand by my satirical video on this subject:

A Double Take on the String Wars

Dawid’s second reason is “The Unexpected Explanatory Coherence Argument.” This means that the maths of string theory works surprisingly well and matches physical requirements in places where it could easily have fallen down. It is a good argument but I would prefer to cite specific cases such as holography.

The third and final reason Dawid gives is  ”The Meta-Inductive Argument”. I think what he is pointing out here is that the standard model succeeded because it was based on consistency arguments such as renormalisability which reduced the possible models to just one basic idea that worked. The same is true for string theory so we are on firm ground. Again I think this is more of a meta-argument and I prefer to cite specific instances of consistency.

The biggest area of contention centres on the role of the multiverse. I see it as a positive reason to like string theory. Woit argues that it cannot be used to make predictions so it is unscientific which means string theory has failed. I think Motl is (like many string theorists) reluctant to accept the multiverse and prefers that the standard model will fall out of string theory in a unique way. I would also have preferred that 15 years ago but I think the evidence is increasingly favouring high levels of fine-tuning so the multiverse is a necessity. We have to accept what appears to be right, not what we prefer. I have been learning to love it.

I dont know how Dawid defines the scientific method. It goes back many centuries and has been refined in different ways by different philosophers. It is clear that if a theory is shown to be inconsistent, either because it has a logical fault or because it makes a prediciton that is wrong, then the theory has to be thrown out. What happens if a theory is eventually found to be uniquely consistent with all known observations but its characteristic predictions are all beyond technical means. Is that theory wrong or right? Mach said that the theory of atoms was wrong because we could never observe them. It turned out that we could observe them but what if we couldn’t for practical reasons? It seems to me that there are useful things a philosopher could say about such questions and to be fair to Dawid he has articles freely available on line that address this question, e.g. here, so even if the book is out-of-reach there is some useful material to look through. Unfortunately my head hits the desk whenever I read the words “structural realism”, my bad.

update: see also this video interview with Nima Arkani-Hamed for a view I can happily agree with

 https://www.youtube.com/watch?v=rKvflWg95hs

71 Comments | String Theory, Supersymmetry | Permalink
Posted by Philip Gibbs

We need to find the Theory of Everything

January 27, 2013

Each week the New Scientist runs a one minute interview with a scientist and last week it was Lisa Randall who told us that we shouldn’t be obsessed with finding a theory of everything. It is certainly true that there is a lot more to physics than this goal, but it is an important one and I think more effort should be made to get the right people together to solve this problem now. It is highly unlikely that NS will ever feature me in their column but there is nothing to stop me answering questions put to others so here are the answers I would give to the questions asked of Lisa Randall which also touch on the recent discovery of the Higgs(-very-like) Boson.

Doesn’t every physicist dream of one neat theory of everything?

Most physicists work on completely different things but ever since Einstein’s attempts at a unified field theory (and probably well before) many physicists at the leading edge of theoretical physics have indeed had this dream. In recent years scientific goals have been dictated more by funding agencies who want realistic proposals for projects. They have also noticed that all previous hopes that we were close to a final theory have been dashed by further discoveries that were not foreseen at the time. So physicists have drifted away from such lofty dreams.

So is a theory of everything a myth?

No. Although the so-called final theory wont explain everything in physics it is still the most important milestone we have to reach. Yes it is a challenging journey and we don’t know how far away it is but it could be just round the corner. We must always try to keep moving in the right direction. Finding it is crucial to making observable predictions based on quantum aspects of gravity.  Instead people are trying to do quantum gravity phenomenology based on very incomplete theories and it is just not working out.

But isn’t beautiful mathematics supposed to lead us to the truth?

Beauty and simplicity have played their part in the work of individual physicists such as Einstein and Dirac but what really counts in consistency. By that I mean consistency with experiment and mathematical self-consistency. Gauge theories were used in the standard model, not really because they embody the beauty of symmetry, but because gauge theories are the only renormalisable theories for vector bosons that were seen to exist. It was only when the standard model was shown to be renormalisable that it become popular and replaced other approaches. Only renormalisable theories in particle physics can lead to finite calculations that predict the outcome of experiments, but there are still many renormalisable theories and only consistency with experiment can complete the picture. Consistency is also the guide that takes us into theories beyond the standard model such as string theory that is needed for quantum gravity to be consistent at the perturbative level and the holographic principle that is needed for a consistent theory of black hole thermodynamics.

Is it a problem, then, that our best theories of particle physics and cosmology are so messy?

Relatively speaking they are mot messy at all. A few short equations are enough to account for almost everything we can observe over an enormous range of scales from particle physics to cosmology. The driving force now is the need to combine gravity and other forces in a form that is consistent non-perturbatively and to explain the few observational facts that the standard models don’t account for such as dark matter and inflation. This may lead to a final theory that is more unified but some aspects of physics may be determined by historical events not determined by the final theory, in which case particle physics could always be just as messy and complicated as biology. Even aside from those aspects, the final theory itself is unlikely to be simple in the sense that you could describe it fully to a non-expert.

Did the discovery of the Higgs boson – the “missing ingredient” of particle physics – take you by surprise last July?

We knew that it would be discovered or ruled out by the end of 2012 in the worst case. In the end it was found a little sooner. This was partly because it was not quite at the hardest place to find on the mass range which would have been around 118 GeV. Another factor was that the diphoton excess was about 70% bigger than expected. If it had been as predicted they would have required three times as much data to get it from the diphoton excess but the ZZ channel would have helped. This over-excess could be just the luck of the statistics or due to theoretical underestimates, but it could also be a sign of new physics beyond the standard model. Another factor that helped them push towards the finish line in June was that it became clear that a CMS+ATLAS combination was going to be sufficient for discovery. If they could not reach the 5-sigma goal for at least one of the individual experiments then they would have to face the embarrassment of an unofficial discovery announced on this blog and elsewhere. This drove them to use the harder multivariate analysis methods and include everything that bolstered the diphoton channel so that in the end they both got the discovery in July and not a few weeks later when an official combination could have been prepared.

toeAre you worried that the Higgs is the only discovery so far at the LHC?

It is a pity that nothing else has been found so far because the discovery of any new particles beyond the standard model would immediately lead to a new blast of theoretical work that could take us up to the next scale. If nothing else is found at the LHC after all its future upgrades it could be the end of accelerator driven physics until they invent a way of reaching much higher energies. However, negative results are not completely null. They have already ruled out whole classes of theories that could have been correct and even if there is nothing else to be seen at the electroweak scale it will force us to some surprising conclusions. It could mean that physics is fine tuned at the electroweak scale just as it is at the atomic scale. This would not be a popular outcome but you can’t argue with experiment and accepting it would enable us to move forward. Further discoveries would have to come from cosmology where inflation and dark matter remain unexplained. If accelerators have had their day then other experiments that look to the skies will take over and physics will still progress, just not quite as fast as we had hoped.

What would an extra dimension look like?

They would show up as the existence of heavy particles that are otherwise similar to known particles, plus perhaps even black holes and massive gravitons at the LHC. But the theory of large extra dimensions was always an outsider with just a few supporters. Theories with extra dimensions such as string theory probably only show these features at much higher energy scales that are inaccessible to any collider.

What if we don’t see one? Some argue that seeing nothing else at the LHC would be best, as it would motivate new ideas.

I think you are making that up. I never heard anyone say that finding nothing beyond the Higgs would be the best result. I did hear some people say that finding no Higgs would be the best result because it would have been so unexpected and would have forced us to find the alternative correct theory that would have been there. The truth of course is that this was a completely hypothetical situation. The reason we did not have a good alternative theory to the Higgs mechanism is because there isn’t one and the Higgs boson is in fact the correct answer.

Update: Motl has a followup with similar views and some additional points here

238 Comments | Higgs Hunting, Large Hadron Collider, Quantum Gravity, String Theory | Permalink
Posted by Philip Gibbs

String Theorists get biggest new science prize

July 31, 2012

Yuri Milner is a Russian hi-tech investor who dropped out of physics classes as a student. He must have done quite well with his investments because he has just given away $27,000,000 in prizes to nine physicists in $3,000,000 chunks. He plans to do the same every year making his the biggest recurring science prize of them all. Recipients of the prize this year which is given in fundamental physics are Ed Witten, Alan Guth, Nima Arkani-Hamed, Jaun Maldacena, Nathan Seiberg, Maxim Kontsevich, Ashoke Sen, Alexei Y. Kitaev and Andre Linde. Congratulations to them all.

Past winners will select future winners so we can expect to see a lot of rich people in string theory and cosmology in the coming years.

44 Comments | Prizes, String Theory | Permalink
Posted by Philip Gibbs

String Theory returns to symmetry

July 31, 2012

The strings 2012 conference has finished and it is great to see that all the talks are online as slides and videos. Despite what you hear from some quarters, string theory is alive and progressing with many of the brightest young people in physics still wanting to do strings. Incredibly the next three strings conferences in Korea, US and India are already being organised. How many conference series have that many groups keen to organise them?

It has become a tradition for David Gross to give some kind of outlook talk at these conferences and this time he said there were three questions he would like to see answered in his lifetime

  • How do the forces of nature unify?
  • How did the universe begin and how will it end?
  • What is string theory?

The last of these questions is one he has been asking for quite a few years now. We know string theory only as a small set of perturbative formulations linked together by non-perturbative dualities. There has to be an underlying theory based on some unifying principle and it is important to find it if we are to understand how string theory works at the all-important Planck scale. This time Gross told us that he has heard of something that may answer the question. Firstly he now thinks the correct question to ask is “What are the underlying symmetries of string theory?” and he thinks that work on higher spin symmetries could lead to the answer. What is this about?

For about 16 years it has been known that an important element of quantum gravity is the holographic principle. This says that in order to avoid information loss is black holes, the amount of information in any volume of space must be bounded by the area of a surface that surrounds it in Planck units. This might mean that the theory in the bulk of spacetime is equivalent to a different theory on the boundary. How can that happen? How can it be that all the field variables in the volume of spacetime only carry an amount of information that can be contained on the surface. We can reason that measurement below the Planck length is not possible, but even then there should be at least a few valid field parameters for each plank volume of space. If the holographic principle is right there must be a huge amount of redundancy in this volumetric description of field theory. Redundancy can be taken to imply symmetry. Each degree of symmetry or dimension of the group Lie algebra tells us that one field variable is redundant and can be taken out by gauge fixing it. In gauge theories we get one set of redundant parameters for each point in spacetime but if the holographic principle is correct there must be a redundancy for almost every field variable in the bulk of spacetime and we will need it to be supersymmetry to deal with the fermions. I call this complete symmetry and I’ve no idea if anyone else appreciates its significance. It means that the fields of the theory are given by a single adjoint representation of the symmetry. This does not happen in normal gauge theories or in general relativity or even supergravity, but it does happen in Chern-Simons theory in 3D which can be reduced to a 2D WZW model on the boundary, so perhaps something is possible. Some people think that the redundancy aspect of symmetry means that it is uninportant. They think that the field theory can be reformulated in a different way without the symmetry at all. This is incorrect. The redundant nature of the local symmetry hides the fact that it has global characteristics that are not redundant. In holographic theories you can remove all the local degrees of freedom over a volume of space but you are left with a meaningful theory om the boundary.

If there is symmetry for every degree of freedom in the bulk then the generators of the symmetries must match the spin characteristics of the fields. Supergravity only has symmetries corresponding to spin half and spin one fields but it has fields from spin zero scalars up to spin two. String theory goes even further with higher excitations of the string providing an infinite sequence of possible states with unlimited spin. This may be why the idea of higher spin symmetries is now seen as a possible solution to the problem.

Surprisingly the idea of higher spin symmetry as a theory of quantum gravity is far from new. It goes back to the 1980s when it was founded by Vasiliev and Fradkin. It is a difficult and messy idea but recent progress means that it is now becoming popular both in its own right and as a possible new understanding of string theory.

There is one other line of development that could lead to a new understanding of the subject, namely the work on supersymmetry scattering amplitudes. Motl has been following this line of research which he calls the twistor mini-revolution for some time and has a nice summary of the conference talk on the subject by Nima Arkani-Hamed. It evolved partly out of the need to calculate scattering amplitudes for the LHC where people noticed that the long pages of solutions could be simplified to some very short expressions. After much thought these expressions seem to be about permutations and Grassmanians with things like infinite dimensional Yangian symmetry playing a big role. Arkani-Hamed believes that this is also applicable to string theory and could explain the holographic principle. The Grassmanians also link nicely to algebraic geometry and possibly work on hyperdeterminants and qubits.

I have to confess that as an undergraduate at Cambridge University in the late 1970s I was completely brainwashed into the idea that symmetry is the route to the underlying principles of nature. At the time the peak of this idea was supergravity and Stephen Hawking - who had just been inaugurated into the Lucasian chair at Trinity college – was one of its greatest advocate. When string theory took over shortly after, people looked for symmetry principles there too but without convincing success. It is true that there are plenty of symmetries in string theory including supersymmetry of course, but different sectors of string theory have different symmetry, so symmetry seems more emergent than an underlying principle. I think the generations of undergraduates after mine were given a much more prosaic view of the role of symmetry and they stopped looking out for it as a source of deep principles.

Due to my brainwashing I have never been able to get over the idea that symmetry will play a huge role in the final theory. I think that all the visible symmetries in string theory are remnants of a much larger hidden symmetry so that only different residual parts of it are seen in different sectors.  In the 1990s I developed my own idea of how infinite dimensional symmetries from necklace algebras could describe string theory in a pregeometric phase. The permutation group played a central role in those ideas and was extended to larger string inspired groups with the algebra of string creation operators generating also the Lie algebra of the symmetry. Now that I know about the importance of complete symmetry and higher spin symmetry I recognise that these aspects of the theory could also be significant. Perhaps it is just a matter of time now before string theorists finally catch up with what I did nearly twenty years ago :)

In any case it is good to see that there is now some real hope that the very hard problem of understanding string theory from the bottom up may finally have some hope of a solution. It will be very interesting to see how these ideas mature over the next few strings conferences.

16 Comments | Conference, String Theory | Permalink
Posted by Philip Gibbs

Bayes and String Theory

June 12, 2012

If Supersymmetry is found or excluded at the Large hadron Collider, how will it affect your opinion on string theory as unification of gravity and particle physics? This is a hard question and opinions differ widely across the range of theorists, but at the least any answer should be consistent with the laws of probability including Bayes Law. What can we really say?

A staunch string theorist might want to respond as follows:

“I am confident about the relevance of superstring theory to the unification of gravity and the forces of elementary particles because it provides a unique way to accomplish this that is consistent in the perturbative limits (Amongst other reasons.) Unfortunately it does not have a unique solution for the vacuum and we have not yet found a principle for selecting the solution that applies to our universe. Because of this we cannot predict the low energy effective physics and we cannot even know if supersymmetry is an observable feature of physics at energy scales currently accessible. Therefore if supersymmetry is not observed at the TeV scale even after the LHC has explored all channels up to 14 TeV with high integrated luminosities, there is no reason for that to make me doubt string theory. On the other hand, if supersymmetry is observed I will be enormously encouraged. This is because there are good reasons to think that supersymmetry will be restored as an exact gauge symmetry at some higher scale, and gauged sypersymmetry inevitably includes gravity within some version of supergravity. There are further good reasons why supergravity is not likely to be fully consistent on its own and would necessarily be completed only as a limit of superstring theory. Therefore if supersymmetry is discovered by the LHC my confidence in string theory will be greatly improved.” 

On hearing this a string theory skeptic would surely be seen shaking his head vigorously. He would say:

“You cannot have it both ways! If you believe that the discovery of supersymmetry will confirm string theory then you must also accept that failure to discover it falsify string theory. Any link between the two must work equally in both directions. You are free to say that supersymmetry at the electro-weak scale is a theory completely Independent of string theory if you wish. In that case you are safe if suppersymmetry is not found but by the same rule the discovery of supersymmetry cannot be used to claim that superstring theory is right. If you prefer you can claim that superstring theory predicts supersymmetry (some string theorists do) but if that is your position you must also accept that excluding supersymmetry at the LHC will mean that string theory has failed. You can take a position in between but it must work equally in both directions.”

  The Tetrahedron of Possibilities

What does probability theory tell us about the range of possibilities that a theorist can consider for answers to this problem? Prior to the experimental result he will have some estimate for the probability that string theory is a correct theory of quantum gravity and for the probability that supersymmetry will be observed at the LHC. In my case I assign a probability of PST = 0.9 to the idea that string theory is correct and PSUSY = 0.7 to the probability that SUSY will be seen at the LHC. These are my prior probabilities based on my knowledge and reasoning. You can have different values for your estimates because you know different things, but you can’t argue with mine. There are no absolutely correct global values for these probabilities, they are a relative concept.

However, these two probabilities do not describe everything I need to know. There are four logical outcomes I need to consider altogether:

  • P1 = the probability that both string theory is correct and SUSY will be found
  • P2 = the probability that string theory is correct and SUSY will not be found
  • P3 = the probability that string theory is wrong and SUSY will be found
  • P4 = the probability that string theory is wrong and SUSY will not be found

You might try to tell me that there are other possibilities, such as that SUSY exists at higher energies or that string theory is somehow partly right, but I could define my conditions for correctness of string theory and for discovery of SUSY so that they are unambiguous. I will assume that has been done. This means that the four possible outcomes are mutually exclusive and exhaustive. We can conclude that P1 + P2 + P3 + P4 = 1. Of course the four probabilities must also be between 0 and 1. These conditions map out a three-dimensional tetrahedron in the four-dimensional space of the four probability variables with the four logical outcomes at each vertex. This is the tetrahedron of possible prior probabilities and any theorists prior assessment of the situation must be described by a single point within this tetrahedron.

So far I have only given two values that describe my own assessment so to pinpoint my complete position within the three-dimensional range I must give one more value. If I thought that string theory and SUSY at the weak scale were completely independent theories I could just multiply as follows

P1 = PST .PSUSY = 0.63
P2 = PST .(1 – PSUSY) = 0.27
P3 = (1 – PST) .PSUSY = 0.07
P4 = (1 – PST) .(1 – PSUSY) = 0.03

The condition that the two theories are independent fall on a surface given by the equation P1 . P4 = P2 . P3 that neatly divides the tetrahedron in two.

As I already explained I do not think these two things are independent. I think that SUSY would strongly imply string theory. In other words I think that the probability of SUSY being found and string theory being wrong is much lower than the value of 0.07 for P3 . In fact I estimate it to be something like P3 = 0.01. I must still keep the other probabilities fixed so P1 + P2 = PST = 0.9 and P1 + P3 = PSUSY = 0.7. This means that all my probabilities are now known

P1 = 0.69
P2 = 0.21
P3 = 0.01
P4 = 0.09

Notice that I did not get to fix P1 separately from P3. If I know how much the discovery of SUSY is going to affect my confidence in string theory then I also know how much the non-discovery of SUSY will affect it. It is starting to sound like the string theory skeptic could be right, but wait. Let’s see what happens after the LHC has finished looking.

Suppose SUSY is now discovered, how does this affect my confidence? My posterior probabilities P’2 and P’4 both become zero and by the rules of conditional probabilities P’ST = P1/PSUSY = 0.69/0.7 = 0.986. In other words my confidence in string theory will have jumped from 90% to 98.6%, quite a significant increase. But what happens if SUSY is found to be inaccessible to the LHC? In that case we end up with P’ST = P2/(1-PSUSY) = 0.21/0.3 = 0.7 . This means that my confidence in string theory will indeed be dented, but it is far from falsified. I should still consider string theory to have much better than level odds. So the skeptic is not right. The string theorist can argue that finding SUSY will be a good boost to string theory without it being falsified if SUSY is excluded, but the string theorists has to make a small concession too. His confidence in string theory has to be less if SUSY is not found.

Remember, I am not claiming that these probabilities are universally correct. They represent my assessment and I am not a fully fledged string theorist. Someone who has studied it more deeply may have a higher prior confidence in which case excluding SUSY will not make much difference at all to him even if he believes SUSY would strongly imply string theory.

33 Comments | Large Hadron Collider, String Theory, Supersymmetry | Permalink
Posted by Philip Gibbs

Witten and Knots

November 16, 2011

If you are at all interested in mathematical physics you will want to watch Ed Witten’s recent talk on his work in knot theory that he gave at the IAS. Witten gives a general overview of how he discovered that the Jones polynomial used to classify knots turns out to be “explained” as a path integral using Cherns-Simon theory in 3D. More recently the Jones Polynomial was generalised to Khovanov homology which describes a knotted membrane in 4D and Witten wanted to find a similar explanation. He was stuck until some work he did on Geometric Langlands gave him the tools to solve (or partially solve) the riddle.

Geometric Langlands was devised as a simpler variation on the original Langlands program that is a wide-ranging set of ideas trying to unify concepts in number theory. Witten makes some interesting comments during the question time. He says that one of the main reasons that physicists (such as himself) are able to use string theory to answer questions in mathematics is that string theory is not properly understood. If it was then the mathematicians would be able to use it in this way themselves, he says. Referring to the deeper relationship between string theory and Langlands he said.

“I had in mind something a little bit more ambitious like whether physics could affect number theory at a really serious structural level like shedding light on the Langlands program. I’m only going to give you a physicists answer but personally I think it is unlikely that it is an accident that Geometric Langlands has a natural description in terms of quantum physics, and I am confident that that description is natural even though I think it mught take a long time for the math world to properly understand it. So I think there is a very large gap between these fields of maths and physics. I think if anything the gap is larger than most people appreciate and therefore I think that the pieces we actually see are only fragments of a much bigger totality.”

See also NEW

39 Comments | Number Theory, String Theory | Permalink
Posted by Philip Gibbs

Dirac Medal for Chris Isham

July 1, 2011

Chris Isham has been awarded this years Dirac Medal of the Institute of Physics for his work on quantum gravity. For information about his many contributions to the field you can just look at the IOP page about the award.

In addition to the Dirac Medal the IOP has just announced a whole slew of other medals named after British Physicists. The Newton Medal this year goes to Leo Kadanoff who noticed the important role of scale invariance and universality in critical systems. The Faraday Medal is taken by Alan Watson for leadership of the Pierre Auger Observatory that studies ultra high energy cosmic rays. The Chadwick Medal is won by Terry Wyatt for work on Hadron Colliders. Another Imperial College prof being honoured is Arkady Tseytlin for string theory research who got the Rayleigh Medal.

There are a load more which you can read about here. Congratulations to them all.

6 Comments | Physics, Prizes, Quantum Gravity, String Theory | Permalink
Posted by Philip Gibbs

Strings 2011

June 27, 2011

The strings 2011 conference has opend today in Sweden. You can watch videos of the talks live or recorded straight after, starting with the introduction by David Gross.

Gross asked the usual questions starting with the number one “What is String Theory?” and ending with number 11 “What will we learn from the LHC?”

He is a bit disappointed that the LHC has not found anything surprising yet, but he still holds high hopes for SUSY.

There have been promising new results in trying to solve large-N SUSY gauge theory which has beautiful mathematics: twistors, polytopes in the grasmanian for example. Since this theory is dual to string theory Gross thinks these discoveries could tell us about the fundamentals of string theory.

He goes on to mention entropic gravity which he said was also promising but he had a little smirk on his face when he said it and also implied that it is ambitious. There will be a talk from Verlinde later in the conference.

Apparently it is unfortunate that we seem to live in De Sitter space. The theories work much better in anti- De Sitter space. There are lots of questions but the most important product of knowledge is ignorance, then again it would be nice to have some answers he said at the end.

 

 

 

 

14 Comments | Conference, Large Hadron Collider, String Theory | Permalink
Posted by Philip Gibbs

Mike Duff on M-Theory in New Scientist

June 2, 2011

This weeks New Scientist features four articles by Mike Duff on M-Theory in which he explains the motivations behind it and answers his critics. It is worthy that New Scientist has allowed him to attack some earlier articles in the magazine that attempted to compare cosmic strings with pseudoscience, and M-Theory with religion. My impression is that more people are beginning to realize that there are good reasons why many of the best theorists are not giving up on string theory just because a few people use such rhetoric to try to discredit its successes.

M-Theory came to prominence in 1995 when Ed Witten started to take the idea of supermemberane theories in 11 dimensions seriously, but its history goes back to at least 1987 when Mike Duff and others classified the possibilities for membrane theories in various dimensions. They showed that the recently discovered superstring theories might emerge from dimensional reductions with the membranes wrapped round to form the strings. Physicists still don’t have a full description of the dynamics of these membranes but a partial solution is provided by Matrix Models.

In his New Scientist article, Mike Duff explains how M-Theory came about. It is important to appreciate that it is not just a wild idea that someone came up with at random. It follows from a need to bring together the standard model of particle physics with general relativity in a way free of the infinities that plague some approaches. The five Superstring theories in 10 dimensions are the only obvious solutions to this problem and they can all be unified into a unique framework using M-Theory. No other approach answers the same questions.

But M-theory is not without its problems. There is an embarrassment of choice when you look at  ways to reduce it to 4 spacetime dimensions in order to match it to physics accessible to experiment. It is hoped that the Large Hadron Collider will discover supersymmetry bringing some hope that a connection between string theories and physics at reachable energies is possible. The trouble is that string theory does not make a definitive prediction that supersymmetry will be observed, and conversely the existence of supersymmetry does not necessarily imply string theory. At best we can say is that there is a correlation between these two ideas so the discovery or not of supersymmetry in the Higgs sector will have a strong influence on the acceptability of string theory.

A second unresolved problem with M-Theory is the absence of a full non-perturbative formulation that is required to make possible any analysis of its phenomenology at the Planck scale. These shortcomings have been explored in a paper on the arXiv last week by Steve Giddings. Mike Duff has identified some relationships between string theories and the information theory of qubits that might just be the first signs of where to look for such a formulation. In work with Borsten, Dahanayake, Ebrahim Marrani and Rubens, Duff has explored a subtle relationship between the classification of STU black holes and 4 qubit entanglement. He takes pains to stress that for the moment at least they “are only claiming that it is useful, not deep.”

The idea that the laws of physics emerges from the dynamics of information has been around for some time and has been boosted in recent years by the theoretical success of the holographic principle and entropic gravity. Whether or not this is a way to understand the fundamentals of M-theory is unclear. It’s a hard problem but not without hope.

Having been lucky enough to meet Mike Duff and some of his students, I know that he remains committed to his work on M-theory and the search for a deeper understanding of its principles. He is unusually open to new ideas but is quick to get to the mathematical details and dismiss anything that simply does not work out. It is not so hard to invent ideas using some persuasive numerology that sound good through the written word, but nature prefers the sound logic of equations.

10 Comments | Physics, Quantum Gravity, String Theory | Permalink
Posted by Philip Gibbs

Octonions in String Theory

April 29, 2011

John Baez and his student John Huerta have an article in Scientific American this month about octonions in string theory and M-theory. Peter Woit has given it a bit of a cynical review describing it as hype. The defence from John in the comments is worth reading. Here is a bit of what he says

“So, don’t try to make it sound like an obscure yawn-inducing technicality about “some supersymmetry algebra working out a certain way in a certain dimension”. It’s a shocking and bizarre fact, which hits you in the face as soon as you start trying to learn about superstrings. It’s a fact that I’d been curious about for years. So when John Huerta finally made it really clear, it seemed worth explaining — in detail in some math papers, and in a popularized way in Scientific American.”

The article entitled “The Strangest Numbers in String Theory” is about an early observation from the study of superstring theories that the four division algebras are related to four classical formulations of superstring theory in 3, 4, 6 and 10 dimensions. The four division algebras are the reals, complex numbers, quaternions and octonions with dimensions 1,2,4 and 8 and the dimensions of the superstring theories are 2 dimensions higher in each case.

Many of our readers will be familiar with the internet writings of John Baez where he has described these things so I wont attempt to cover any details. His student John Huerta has just finished off his thesis in which he clarifies these observations using higher dimensional lie algebras. The results extend to one dimension higher when strings are replaced by membranes. In the quantum theory only the highest dimensional versions related to the octonions hold up consistently giving us the 10 dimensional superstring theories and M-theory in 11 dimensions. Of course this is not complete since we still don’t know what the full formulation for M-theory is. Even these higher dimensional observations are not new, see for example Mike Duffs brane-scan from 1987 where the relationships were already plotted out. This new work clarifies these results using the concept of 3-lie-algebras from n-category theory.

The Scientific American version does not go into great detail but is a very well written introduction to the ideas. If you don’t have access to it don’t worry, John says he will be allowed to post an online version after a month or so. You can also explore what has been posted already starting here which is more advanced than the article but still very pedagogical.

Personally I find these algebraic ideas for M-theory very enticing. It is a major goal to formulate a complete non-perturbative version of string theory that encompasses all its forms and I think the purely algebraic approach is the best line of attack. It is especially intriguing that the octonions have such a direct relationship to the dimensions in which these theories work, but ultimately the algebraic structures we need to understand it fully are probably much more complex.

The work of Mike Duff and his collaborators which brings in the algebra of hyperdeterminants and qubits to understand a slightly different role of the octonions in string theory is one of the areas to follow. This work brings in the duality algebras found in string theory black holes. I know that several of our regular commenters are very familiar with this already so I need not give more details. Indeed it is the fact that the same algebraic structures keep appearing in different contexts that is so intriguing, yet so confusing, as if we are missing some principle of unification that relates these things.

12 Comments | Octonions, String Theory | Permalink
Posted by Philip Gibbs

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