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

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

Rebooting the Cosmos

June 5, 2011

Over the last few days the World Science Festival has been running in New York and has featured some very interesting discussions amongst scientists on a variety of  popular science subjects such as the mutiverse and the search for life in space. I have been following it because it had been said that the results of the FQXi essay contest (Is Reality Digital or Analog) would be announced at the festival and I am interested to see which authors are going to beat me to all the juicy prizes. So far nothing seems to have been mentioned about that but last night there was an interesting discussion broadcast live with the title “Rebooting the Cosmos: Is the Universe the Ultimate Computer?” It could have been inspired by the essay contest.   Two of the four scientists (Ed Fredkin and Seth Lloyd) were cited in my essay for the contest and another (Fotini  Markopoulou) was cited in my earlier FQXi essay on the nature if time. The final member of the panel was computer scientist Jürgen Schmidhuber. So this was a discussion of great interest to me and I was not disappointed by it.


The main question up for discussion was, “Is the universe a giant computer?” There is an argument that says that in the future we will build computers so powerful that we will be able to simulate universes in great detail. Assuming that life continues for a long time and that a few people are interested in playing such games, there will be vast numbers of such simulations. In fact, within the multiverse there will be so many simulations going on inside computers that they will vastly outnumber all the “real” universes. The conclusion is that we are infinitely more likely to be in a simulation ourselves than in a “real” universe. Actually we really have to conclude that there are no real universes, just simulations within other simulations.

I don’t take this argument too seriously, but a growing number of physicists are taken by such ideas and it is at least worth thinking about on a philosophical level. Does this mean that we are vulnerable to the possibility that the being running our particular simulation might decide to pull the plug at any time and we would just cease to exist? Some people seem to think so! I take the view that if there is one simulation running that matches our universe then there must also be infinitely many of them, by the same argument. We have no way to distinguish which one we are in because we have no access to the outside “other” world as Ed Fredkin calls it. So what we should experience is the combined effect of all the simulations that matches ours at any given place and time. In that case it is safe to assume that the simulation will go on in most cases so we are in no real danger of being stopped, phew.

Although these ideas seem wacky and untestable, I think they are worth thinking about because they can give us some clues about how the universe could work is we accept that such possibilities can make sense. This is how the role of philosophy in physics should work. The broad concepts may not be scientifically falsifiable themselves but they can lead eventually to other ideas that are. Everyone understands this, right? I think it is not unreasonable to say that the universe runs like a quantum computer, rather than it is a quantum computer. This may become more apparent in the fundamental laws of physics when we come to understand them better.

As an independent physicist who sits at home linked into the internet I don’t get to meet real physicists very often. However, it happens that I have had the good fortune to meet Ed Fredkin and exchange some ideas with him. I even have a little anecdote so I am going to finish of with that. Fredkin is remarkably accomplished in a number of fields. He was an airforce pilot who became a professor of computer science at MIT despite a lack of academic background. As an architect of some of the early PDP computer systems he quietly invented a number of concept in operator systems that form the basis of all computers today. he also invented several technologies such as a desalination process that earned him considerable wealth. His interest in applying digital ideas to physics have also been very influential especially through his contact with Feynman, ‘t Hooft and others. You can hear about some of that in the program.

A few years ago I got to know Fredkin because of his interest in some things I wrote in my online book about “Event-Symmetric Space-Time”  the last time I met him was in Antibes where he keeps his yacht and we discussed his work on cellular automata. It is in a picturesque bay where they are used to celebrities passing through. One evening we were is a restaurant and he was telling me about his role in the  Sakharov affair, another unrecorded episode of his amazing life story. As we talked I noticed over his shoulder that the waiter who had served us was looking towards us curiously. Fredkin turned round and caught his eye, then he turned back to me and said “oh dear, I know what is going to happen next” Sure enough the waiter came back over to our table and spoke nervously. “Excuse me, but could I have your autograph?” he asked. “Of course” said Fredkin, “but who do you think I am?”, The waiter paused looking a little confused, then burst out, “Sir. I recognized you immediately. You are Steven Spielberg!”

4 Comments | Physics, Quantum Gravity | 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

Horizon: Before the Big Bang

October 15, 2010

This week the BBC showed a program in their long running “Horizon” series about the question “What came before the Big Bang?”  Here is the gist of the message: A few years back cosmologists accepted that time did not exist before the big bang, so the question did not make sense. The universe along with time itself just started to exist and has been evolving nicely ever since. But now cosmologists are forming all kinds of theories that do put something before the big bang to explain how and why it happened.

So here is a list of the scientists that featured and the theory they adhere to:

  • Andrei Linde: Multiverse inspired eternal inflation
  • Param Singh: Big Bounce due to repulsive gravity at small distances
  • Lee Smolin: Black Holes spawning baby universes
  • Michio Kaku: Vacuum fluctuation from empty space
  • Neil Turok: Colliding Branes
  • Roger Penrose: The future is empty expanding space = a new big bang
  • Laura Mersini Houghton: String cosmology

Each of these ideas has been around for some time and has been worked on by several people. The individuals mentioned here are not necessarily the ones who invented them. The Penrose theory is an exception in that it is a new idea that features in his next book.

In the program each of these scientists was interviewed while they tried to solve one of those  wooden puzzles

The obvious conclusion to draw is that there are a lot of viable theories out there which cannot all be right. Each of the scientists seemed to have quite a strong belief in the theory they supported, but they would all acknowledge that more experimental input is needed to resolve the question. All of them are driven by a philosophical argument that temporal causality must hold absolute so some prior cause of the big bang is needed.

Along with all the theorising and philosophising, a couple of experiments were mentioned which they think might help test these different hypothesis. The first was LOFAR, a low-frequency radio telescope array that may detect background remnants from the big bang. The standard prediction is that it will be white noise, but anything else could be a clue that separates different theories, prepare your predictions in advance please. The second experiment was the more familiar LIGO and its space bound successors LISA. These may be able to detect a gravitational wave remnant from the big bang that could also have a distinctive signature. It is hoped that either of these experiments may see past the wall of last scattering from which the cosmic microwave background emerged to provide information from an earlier time.

Personally, I don’t accept the philosophical need for something before the big bang and I don’t particularly like any of the theories mentioned. I think it is more likely that there was no space or time prior to big bang singularity which itself is a high temperature and density phase with no fixed topology or geometry for spacetime. I am not alone in preferring theories that do not require time to extend before the big bang, but the program has selected those that do. Where was Hawking’s view for example?

I think that explaining the universe requires us to look at ontological causality rather than temporal causality and the big bang is just one feature of the universe, not the reason for its existence. Although the experiments mentioned and others may throw some light on the nature of the big bang, we first need a better understanding of quantum gravity. There is still scope for theoretical developments that may help even before the experiments bear fruit. Even if you favour the string theory/M-theory route to quantum gravity (as I do), a better understanding of their foundations is required before we can hope to answer these questions about cosmology.

Despite that, I don’t think it is wrong to explore a wide range of cosmological ideas of this kind provided they have some good mathematics behind them. It is time for science to start trying to answer such questions. They will have to be looked at from all angles, philosphical, mathematical and experimental if we want to get the right understanding.

For the record I thought this was a good Horizon program, some of their physics/cosmology episodes lately have been a bit empty and ill-conceived. The position was too one-sided, but well researched. I’m glad they did not make the mistake of mentioning the LHC as if it was likely to resolve these questions, but did mention some other experiments that stand a better chance.  

If you missed the program or it is has not yet aired in your country, I dare say you will find it on the web using Google video search. I wont provide any links because I don’t know which if any are legal copies, or how long they will remain available, or whether the same links will work everywhere.

60 Comments | LIGO/VIRGO, Physics, Quantum Gravity, Review, String Theory | Permalink
Posted by Philip Gibbs

Duff, String Theory, Entanglement and Hyperdeterminants

September 2, 2010

Mike Duff has been back in the science news with the publication of one of his papers and a suitably hyped press release from Imperial College.  The research does not actually propose a test of string theory, it merely uses some mathematical ideas in a way inspired by string theory to analyse the entanglement of qubits. Even so, the work is still pretty exciting because of this connection.

Duff’s work in this area began when he noticed that hyperdeterminants come up in the theory of entanglement and also as U-duality invariants defining the entropy of black holes in string theory. At the time, not many applications of hyperdeterminants were well-known, so their appearance in two parts of physics at the same time was taken as a sign that there may be some connection, Duff and his collaborators have been exploring this idea ever since.

The hyperdeterminant is a generalization of determinants to multi-dimensional matrices, or hypermatrices. For a 2x2x2 hypermatrix the hyperdeterminant is a homogeneous degree four polynomial in the 8 components of the hypermatrix, and is known as Cayley’s hyperdeterminant. It can be an invariant characterising the entanglement of three qubits, or an invariant of U-duality for a black hole.

If you look at one of his early papers on this you may notice that he actually cites one of my number theory papers, so you can see that I have some personal interest in this subject. He only cited me because he liked my formula for the hyperdeterminant in terms of Levi-Civita symbol which is

|A|   = -\frac{1}{2} \epsilon^{ab} \epsilon^{cd} \epsilon^{ij} \epsilon^{kl} \epsilon^{rs} \epsilon^{tu}   a_{air} a_{bjt} a_{cks} a_{dlu}

In fact the connection is much more interesting than that because my paper makes a link between hyperdeterminants and elliptic curves. Further work on this has shown that the next hyperdeterminant up for a 2x2x2x2 hypermatrix is related to the j-invariant and of course the j-invariant has been connected to the entropy of black holes too. This larger hyperdeterminant is a polynomial of degree 24, and here the number 24 is connected to the well-known importance of this number in string theory.  I’d be happy to explain this to Duff over a pint if he wants to get in touch :)

Duff has taken his knowledge of these invariants in black hole entropy and applied it to count the number of possible states of entanglement for 4 qubits. That is what the latest paper is about. I think the real excitement is the idea that there may be some connection that is more than just a similarity of the mathematics. The question is, can the work be extended to much larger numbers of qubits in a way that makes string theory look like a theory of entanglement with the qubits playing the role of quantized information at a fundamental level? I don’t know if that is what Duff is thinking of, or if he has some deeper reason to expect something like that to be true, but it is much more interesting than the non-idea that this work provides a test for string theory.

Update: Here are some other articles worth linking to: Kea is covering hyperdeterminants as M Theory lessons 345, 346, 347, 348, 349, 350 and of course she has written earlier stuff that you can search for. Lubos has of course mentioned this topic before and even used my formula for the hyperdeterminant here. Another reasonable report on the latest findings can be found at Universe Today.

39 Comments | Hyperdeterminants, Quantum Gravity, String Theory | Permalink
Posted by Philip Gibbs

Suzy at Last?

July 30, 2010

The first time I went to a lecture on supersymmetry the auditorium was so packed that many people could not get in. I was pleased I had anticipated the high demand and arrived very early. In his talk entitled “Is the End in Sight for Theoretical Physics?” The speaker explained to us that supersymmetry was the greatest hope for theoretical physics because it offered the possibility to unify the gauge theories of particle physics with a quantum theory of gravity in a way that might avoid the infinities of quantum field theory.

The speaker was of course Stephen Hawking and the occasion was his inauguration as Lucasian Professor in Cambridge. The version of supersymmetry that had him so excited was N=8 Supergravity in 4 dimensions. Cautiously he predicted that a complete theory of particle physics could be worked out in 20 years time using this new superunified theory.

30 years have passed and we know that things did not work out quite as Hawking has hoped. He thought that N=8 supergravity might be a unique candidate for a fully unified theory of physics, although the particles we now know as fundamental would have to be composite. He did not consider higher dimensional theories because he thought that details such as the number of spacetime dimensions could be explained by anthropomorphic arguments.

A few years later, supergravity was replaced by superstring theories and higher dimensions became mandatory. The underlying theory still possesses a similar uniqueness but now anthropomorphic arguments are needed to  select the real world vacuum from a vast landscape of possibilities that superstring theory offers. Hawking has now retired as Lucasian professor to be replaced by one of superstrings’ pioneers,  Michael Green. Supersymmetry and superstrings face a skeptical backlash from a large section of the younger generation who are disillusioned by its failure to provide clear predictions for particle physics or cosmology after so much time.

Now the table may be turning full circle and this time support for supersymmetry comes not just from theory, but from experiment too. The version of supersymmetry that has come to the fore is the Minimal Supersymmetric Standard Model – an extension of the well established Standard Model of particle physics that includes an additional broken supersymmetry. This leads to one superpartner for every familiar particle that we know already, plus an alternative Higgs sector with fives Higgs particles, two of them charged.

The MSSM first appeared just a year after Hawking’s lecture. Since its early days it has been understood that it improves the naturalness of low energy particle physics due to anomaly cancellations that help keep the Higgs sector light. With the addition of supersymmetry the three running coupling constants converge at one energy point, suggesting a dessert of new physics up to a more complete unification at the GUT scale. The model also provides a natural R-parity symmetry that would make its lightest particle stable. This offers a unique candidate for dark matter whose stability would otherwise be very hard to explain.

For the last decade or perhaps more, theorists have been anticipating the imminent discovery of supersymmetry in the world’s highest energy particle accelerators. Fermilab was thought to have a chance of discovery with the Tevatron and there were even some false starts that faded away as the statistics grew. Now their hopes turn to the Large Hadron Collider but the Tevatron is not finished yet. In recent months we have seen some tantalising results reported by Fermilab that support the MSSM.  Nothing is conclusive yet, but the combined evidence all seems to point in the right direction.

For those of us who grow up with the idea that supersymmetry is the final move in a game of unification that leads inevitably to a complete theory, these reports are too hard to dismiss. After the ICHEP conference we drool over the results that should have been shown, but weren’t. Plots which show inconclusive signals of less than 3-sigmas statistical significance are quick and easy to approve for publication. They don’t lead to big headlines. Anything above three sigmas would count as an observation and that puts it in a different league of results. With some history of failed observations from the past, Fermilab are likely to put off publication until the next round of data is seen to add rather than subtract from the result. For us the outsiders, the mere absence of certain plots starts to look like a sign to get excited about.

For the supersymmetry skeptics the conclusions to be drawn are different. Any signal below 3 sigma is to be dismissed as noise. They can even dismiss the exclusion of the Higgs mass range that now strongly supports a light Higgs sector as predicted by supersymmetry. It is indirect and still inconclusive.

If supersymmetry is indeed just below the surface, what will happen next? The Tevatron will continue to analyse the data they have while collecting some more until about 2013. The signal will grow until it is clear that something new has been seen. The LHC will not have the luminosity to see the low mass Higgs sector before the Tevatron, but supersymmetry will offer other new particles of higher mass. The LHC might pick out some of those very quickly and start to study their properties. Very soon the parameter space of supersymmetric models will be narrowed down. There will be a huge spurt of activity amongst theorists as they figure out how particle physics works at this scale. If there really is a desert of new physics beyond supersymmetry it may be possible to work out a convincing scenario for physics right up to the GUT scale. Possibly the next generation of accelerators will be needed to pin down most of the coupling constants. If they are clever enough, there may be enough information to figure out the mechanism for breaking supersymmetry at the GUT scale. That could reveal a perfectly supersymmetric world at higher energies with far fewer free parameters.

It will not stop there. If supersymmetry is part of gauge field unfication then its unbroken gauge form will include supergravity. The experimenters will have had their day again as theory pushes into higher energies with renewed confidence. How far it will run is hard to say but the connection between supersymmetry and quantum gravity is hard to pull apart. Knowing the details of supersymmetry at the electroweak scale could be enough to lead us to the end of theoretical particle physics in the sense that Hawking predicted 30 years ago. Perhaps even superstrings will suddenly look right again. Until we have the next results from experiment we cannot be sure, but that is what makes the current situation so exciting. In just a few years - perhaps even just months - a renaissance of  particle physics merging experiment and theory might be well underway. It might pan out in a less predictable way than I have suggested here, but it is sure to be revealing, if it happens at all.

Update: see also the discussion on Lubos blog, and of course his many detailed pages extolling the virtues of supersymmetry.

26 Comments | ICHEP, Large Hadron Collider, Physics, Quantum Gravity, Supersymmetry, Tevatron | Permalink
Posted by Philip Gibbs

QCD Phases on the Lattice and Quantum Gravity

July 24, 2010

Yesterday there were some sessions on Lattice techniques aimed at non-specialists attending the ICHEP conference. Apparently the attendance was disappointing. That is not very surprising given the competition from other parallel sessions where new physics could be announced. Lattice theory has been around for a long time and mostly looks at QCD which is far from new.

As an ex-lattice gauge theorist myself I think there are some aspects of it that people working on more sexy subjects such as quantum gravity would benefit from understanding better. In particular they should understand how the phase diagram of QCD at high temperature and density is being charted using these non-perturbative methods. The reason they need to know this is that a similar phase structure should exist in quantum gravity and there is likely to be a strong (but approximate) correspondence through AdS/CFT duality that relates quantum gravity to a QCD-like theory.

In the QCD theory of the strong interactions there is believed to be a temperature known as the Hadgdorn temperature above which nuclear matter breaks down into a quark gluon plasma. This happens at around 10 billion degrees Kelvin. In quantum gravity according to string theory (if you don’t like string theory dont switch off, this is just a short diversion) there is another Hagdorn temperature at around the Planck scale. That’s about 1032 degrees Kelvin. What happens there?

According to string theory the length of strings becomes very large and effectively the concept of the string breaks down. Sometimes string theorists call this the topological phase of string theory because they think that spacetime loses its geometry in the hotter phase. The truth is that not much is known about what really happens because most of string theory is based on perturbative calculations and phase transitions are very non-perturbative. What might happen is that not only geometry of space-time is lost but topology too. In that case it should be called the non-topological phase, or pregeometric phase. To put it another way, spacetime evaporates. Even if you don’t believe in string theory you might still consider this possibility. Some non-string theorists talk about geometrogenesis which is the process of cooling from the high temperature pregeometric phase to the more familiar geometric phase at the start of the big bang.

For now we can get some feel for the phase structure of quantum gravity by looking at the phase structure of QCD which brings me to one of the ICHEP talks from yesterday. However I’ll do that in a separate post in case people get confused and think it was about quantum gravity.

10 Comments | ICHEP, Lattice Theory, Quantum Gravity | Permalink
Posted by Philip Gibbs

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