There’s a new book out this week by Marcelo Gleiser, entitled A Tear at the Edge of Creation. Gleiser blogs at the NPR site 13.7, and that site also has a review of the book from his fellow blogger Adam Frank.
Gleiser started out his professional life as a string theorist, enchanted by the prospect of finding a unified theory, and for many years that motivated his research:
Fifteen years ago, I would never have guessed that one day I would be writing this book. A true believer in unification, I spent my Ph.D. years, and many more, searching for a theory of Nature that reflected the belief that all is one.
Over the years he began to become disillusioned with this quest, not only with string theory, but also with other closely associated ideas (e.g. GUTs and supersymmetry) about how unification is suppose to happen. Hopes that GUTs would give predictive theories of inflation or proton decay have fallen by the way-side, and about supersymmetry he is “very skeptical”:
The fact that the [lightest, stable superpartner] particle has so far eluded detection doesn’t bode well. To make things worse, results from the giant Super-Kamiokande detector in Japan and the Soudan 2 detector in the United States have ruled out supersymmetric GUT models, at least the simpler ones, based again on the proton lifetime. If SUSY is a symmetry of Nature, it is very well hidden.
About string theory itself, Gleiser refers to my book and Lee Smolin’s, and comments that:
Responses from notable string theorists were of course highly critical of the books and their writers. Some were even offensive. I find this sort of dueling pointless. People should be free to research whatever they want, although they should also reflect responsibly on whether their goals are realistic.
Ultimately, Gleiser came to the point of view that all hopes for a unified theory are a misguided fantasy, and explaining this point of view is the main goal of the book. In an interview on his publisher’s web-site, he says:
After years searching for a “final” answer, as a scientist and as a person, I realized that none exists. The force of this revelation was so intense and life transforming that I felt compelled to share it with others. It completely changed the way I think about Nature and our place in it.
In his book, he argues repeatedly against the fundamental nature of symmetries in our understanding of physics, seeing the failures of GUTs and supersymmetry as a failure of the idea of getting unification out of larger, more powerful symmetry laws. For him, symmetries are always just approximations, never exactly true principles. He claims to be more interested in asymmetries, in failures of symmetry laws, seeing in asymmetry a fundamental explanatory principle about the universe and humanity’s role in it.
Personally, I find myself in strong disagreement with him about this, and don’t see much evidence in the book that the abandonment of the search for symmetries that he advocates leads to any positive route to greater understanding of the universe. I agree with him about the failure of the most popular ideas about how to use symmetry to get beyond the Standard Model, but disagree with him about the implications of this.
The problem with both GUTs and supersymmetry is that one posits a new symmetry only to be faced immediately with the question of how to break it, with no good answer. To be successful, any new symmetry principle needs to come with a compelling explanation of how it is to be realized in fundamental physics. A repeated lesson of the development of the Standard Model was that major advances came not only from coming up with new symmetry groups, but through coming up with unexpected ways of realizing them (e.g. spontaneous symmetry breaking and confinement in gauge theory). I don’t believe that the gauge symmetries of the Standard Model are approximations, but rather that they are among our most powerful and fundamental physical principles, and that much work remains to be done to understand their full implications. The failures that have discouraged Gleiser have also discouraged many others, and a resulting abandonment of symmetry-based attempts to find a better, more unified, fundamental theory would be a shame.
there is another option, between the idea of Gleiser and yours: that the known gauge symmetries are exact, but that no higher symmetries exist. So far, experiments point more in this direction.
After all, Gleiser’s experience is not that unification is misguided, but only that supersymmetry-based unification (thus with this particular higher symmetry) does not work.
“Gleiser started out his professional life as a string theorist, enchanted by the prospect of finding a unified theory, and for many years that motivated his research.”
This does not appear to be a true statement, as can easily be checked by looking at Gleiser’s papers on SPIRES. It seems that he has worked in cosmology most of his 30 year career and did not start out as a string theorist as you have claimed. It would seem that you have simply asserted this in order to find yet another way to throw mud at string theory.
To quote from page 149 of the book:
“Together with my Ph.D. advisor, John G. Taylor, I even wrote one of the first papers on how superstrings could explain the Big Bang, back in 1985.”
I don’t think I’ve mischaracterized Gleiser’s attitude toward string theory at all. He’s quite critical of the whole general idea of trying to find a unified theory, including the specific approach of string-theory based unification, and that’s a subject he has worked on and thought about.
Off topic, but I noticed that your posting about the Paul Dirac biography isn’t listed in the book reviews section.
I only started using the “book review” tag relatively recently. Will add it to the Dirac bio posting, others as I notice them.
is that “tear” as in cry a tear? or a tear in my shirt?
tear as in my shirt is my guess. For one thing, the cover of the book features a torn photograph.
Could you explain the reason why you believe that gauge symmetry is perfect?
My understanding is that the very nature of the Big Bang itself forces us to search for a unification scheme above a certain energy.
Regarding symmetries Susskind says pretty much the same with Gleiser in this interview:
But when he talks about symmetries he is referring to global and not gauge symmetries. He doesn’t consider gauge as real symmetries in the strict sense. At least this is what I understood.
I don’t get it. Why is it called 13.7 ? I thought all NPR radio stations in the US are by convention near 90FM. 1/137 ?
rrtucci: 13.7 as in 13.7 billion years, estimated age of the universe.
I don’t see how the Big Bang has anything to do with it.
Adding gauge-non-invariant terms to the Standard Model action is believed to ruin fundamental consistency properties of the theory such as unitarity. There’s also neither any experimental evidence for such terms, nor any theories of them viable enough to motivate people to even bother to look for such effects.
Peter, what do you mean when you say that the Big Bang has nothing to do with the unification of forces?
This is pretty much conventional wisdom. To see what I mean check this article in Wikipedia for example regarding the evolution of forces during the Big Bang.
Or maybe you mean something else?
The era of “shut up and calculate” is long gone. Now it seems to be
“Don’t compute, just blather on”
What we can observe about the Big Bang just doesn’t give us any useful information about unification and Beyond Standard Model physics. What gets a lot of attention is that people have been hopeful that we might observe things like monopoles or cosmic strings that were remnants of BSM physics produced at early stages of the Big Bang, but nothing like this has ever been seen. All our observations of the Big Bang are completely consistent with the SM. Nothing we see about the Big Bang is helpful for understanding LHC energy physics, much less higher energies.
As an empirical matter, I take an observed symmetry as a starting point: to say that a real object is circular or square starts a discussion in terms of a particular mathematical object, after which we can introduce more subtle measures of how close the real object is to being an ideal circle or square. If we start with a real object that is close to being a circle, it’s relatively natural to use representations of the rotation group to express how close the object is to being a circle. When we later discover that an object is close to being an ellipse, it may become relatively natural to introduce other, different measures of how close the object is to being an ellipse.
In the more abstract arena of correlations between observables at time-like separation, which have traditionally been discussed from a starting point of a Hamiltonian or Lagrangian presentation of a free quantum field dynamics, it is relatively natural, and quite empirically effective, to introduce gauge-invariant deformations of that type of presentation, while keeping the CCRs undeformed. That is by no means true if we adopt a different mathematical starting point for discussing correlations at time-like separations, or if we were to decide, more radically, that it is worthwhile to discuss something other than correlations, expected values, and other statistics as a universal representation of empirical data.
The idea of representations of a symmetry group that act (linearly or non-linearly) on a representation space is sufficiently general as a way to organize calculation that it seems likely to be part of at least the next generation of mathematics that is used to describe what mathematical object the world is like, and how close the likeness is, just as a matter of correspondence. But in the more removed future we might have to say that some more abstract mathematical concept has taken over from “symmetry” as the label du jour.
Gleiser, I suppose, has to some extent given up the quest to find some mathematical object that is more like the way the world is, in a tractable and useful way, than what we currently have.
All our observations of the Big Bang are completely consistent with the SM. Nothing we see about the Big Bang is helpful for understanding LHC energy physics, much less higher energies.
One of the most important observations from the Big Bang is inflation, and the hypothetical particle which causes this, the inflaton, is not a part of the Standard Model.
What about the evidence widely believed to support the inflationary cosmology? I don’t think inflation can be explained using only Standard Model physics.
So is the search for symmetry and unification a good approach to doing physics, or not? In some cases, the search for them has led to profound discoveries. In other cases, they have proved a wild goose chase and wasted productive years by great scientists. In still other cases, symmetry and unification were the result but never the motive.
Peter, classical symmetries rely on the classical concept of spacetime. Do you not think there is a good chance that the non local formulation of the SM (which must exist according to the evidence from twistor theory) might employ distinct, albeit necessarily deeper, principles?
there is *no* observational evidence for inflation. As explained in the wikipedia article you cite, there is evidence for flatness, homogeneity, a specific fluctuation spectrum, etc., etc.. But many cosmological models gives those same results, also models without inflation. (This is not mentioned in the wikipedia article, but any cosmologist will confirm this.)
To claim that cosmology provides any evidence beyond the standard model is wishful thinking!
Bill K and John,
My comment about SM and the Big Bang was perhaps slightly too strong. The problem with inflation is that it gives us very little extra to go on, basically something like “there’s some kind of other physics, which can be crudely modeled by an inflaton with an effective potential we have some very minimal information about”. The original hope was that this inflaton would correspond to a particle that fit into the GUT scenario, and that then inflation would be experimental evidence for unification, but this has not worked out.
Twistor theory is a very good example of a different way of realizing symmetries, one that could turn out to have deep significance for unification.
Cosmology very strongly points towards the existence of dark matter which cannot be explained by the SM.
The main evidence for dark matter comes not from cosmology but from observations of relatively nearby galaxies. As with inflation, what we know about the Big Bang gives us only very minimal information about dark matter and what sort (if any.. ) of non-SM physics it might come from.
My point remains that, despite a lot of hype promoting the idea that the way to get information about unification is to study cosmology, this hasn’t really worked out.
The rotation curve of galaxies is an important piece of confirming evidence for dark matter but by itself it does not show that the dark matter is not a SM particle.
To do that you need Cosmic microwave background data or a combination of galaxy survey data and big bang nuclear synthesis data.
I’m pretty ignorant on these matters, but nucleosynthesis falls squarely within the SM of particle physics. In fact, it is pre-SM physics, since it was first developed before the SM was even formulated.
‘After years searching for a “final” answer, as a scientist and as a person, I realized that none exists.’
This is classic sour grapes. He is saying that he failed to find such a theory and since he is so smart, no one else can either.
Conservation laws are the foundation of physics and conservation laws follow from continuous groups, therefore groups form the foundations of physics.
Personally I don’t see symmetry as fundamental to unification. Symmetry is an important property but it is just an abstraction forming only part of a proper description of any object.
SM is mostly based on guesswork and since it is much easier to guess a symmetry then to guess all the other details of a description symmetry has played an important role. But now that we know all the relevant symmetries the task is to elucidate all the other details of description of fundamental physics.
a highly esteemed colleague once said that all of particle physics is mislead by the higgs mechanism. most people expect the symmetry to increase by going to higher energies, where actually experience tells us that it is exactly opposite. in solid state physics you can plainly see this. and also in high energy physics it is to be expected: the modern view of renormalizability has told us that no matter what happens at the high scale, at low energies we will end up with the SM lagrangean with only the few renormalizable terms that it contains.
it would be very strange in my opinion, if this symmetry generating mechanism, that is already discovered, would be unused by nature and instead of a proliferation of terms we would see them thinning out at high energies. to me, this scenario seems strongly disfavoured by the renormalizability of the SM.
big bang nuclear synthesis data indicates that baryons only make up about 5% of the energy density needed to close the Universe. Redshift surveys indicate that non-relativistic matter makes up about 30%, so 25% of this is inferred to be non-baryonic matter.
Independently, the CMB can constrain both quantities. Some assumptions are needed to do this in each case, as with all science. If you combine more data sets you can then reduce the number of assumptions.
I agree with you that Big Bang nucleosynthesis predicts an amount of baryonic matter that, when compared with the matter needed to explain astronomical observations of gravitational effects, turns out to be just about 5% of the mass/energy density of the universe.
But nucleosynthesis is a collection of purely SM processes. In fact, since the synthesis of nuclei occurs at typical energy scales of tens of MeV, it is pretty insensitive to the high energy structure of the SM, or its extensions. For example, nuclear synthesis occurs in the Sun all the time, and it’s quite independent of the precise value of the top mass.
the SM has a larger symmetry group than any of its lower-energy effective theories. At a scale of, say, 1MeV, particle physics looks like QED + Fermi model + nuclear physics. At the electroweak scale, it’s all just a single gauge theory (with a product group, so not unified yet). So, at least in this case, higher energy leads to more symmetry. Which comes to show that the RG-flow point of view you are taking does not prohibit that from happening.
By the same token, it might well happen that at 1TeV particle physics looks like the SM + a bunch of effective interactions, which are all derived from a single theory with an even larger symmetry group at some higher scale. Or it might well be that something else happens…
I took a look through this book earlier this evening. It consists of a lot of short chapters about stuff unrelated to particles, cosmology, strings, etc …
Overall it reads a bit like a rambling monologue, with some nuggets about his change of heart about unification and the “eureka moment” which led to it.
A moment spent googling ‘big bang nucleosynthesis’ will help clear up your confusion. Yes, it’s true that the nuclear reactions are low energy. However the net yield of light isotopes produced (particularly deuterium) is sensitive to the temperature curve of the cooling big bang, and this in turn is influenced by whether you use the standard model or one of its extensions. So the amount of primordial deuterium that one observes has a direct bearing on particle models.
Bill, there’s no confusion at all. Element abundances in the Universe depend on nucleosynthesis rates and on the thermal history of the Universe. Nucleosynthesis data do not disagree with the SM. The thermal history of the Universe is another story, to which I made no reference in my posts.
I never claimed BBN was not explained by SM processes.
An interview with Marcelo Gleiser is here.
He may have a case to make, but judging from this interview it isn’t much of one.