No sacred cows in physics

Interview with Sheldon Lee Glashow, Physics Nobel laureate.

Sheldon Lee Glashow.

WE continue here Frontlines interview with the 1979 Physics Nobel laureate Sheldon Lee Glashow, who has made seminal contributions to our present understanding of fundamental particles and forces of nature. The interview was conducted first on December 6 during Glashows visit to the Visvesvaraya Technological University (VTU) between December 5 and 7 as part of the Nobel Lecture Series launched in India by the global technology company Honeywells Indian arm, and later through e-mail exchanges.

Today, physicists describe the physical world at quantum level in terms of elementary particles and their forces of interaction essentially by the following picture. Of the four fundamental forces of nature that we know today, the weak nuclear force (which causes radioactivity such as beta decay) and the electromagnetic force (which causes electric bulbs to shine) can be described in a single unified mathematical framework called the Unified Gauge Theory of Weak and Electromagnetic Interactions. The strong nuclear force (which binds neutrons and protons within the nucleus) is a manifestation of a more fundamental force, called the colour force, with which quarks the elementary constituents that make up the strongly interacting particles called hadrons that include the familiar neutrons and protons interact. A separate mathematical framework describes this colour force. Though conceptually and structurally similar to the unified electroweak theory, the theory of the colour force, chromodynamics as it is called, is different from the electroweak theory in a significant way.

The most elementary particles of nature, as we know today, include six quarks and six leptons. These quarks and leptons seem to exist in three distinct families or flavours, as physicists call them. The six quarks have been named as up, down, strange, charm, top and bottom. The leptons include the familiar electron and its two heavier cousins, called the muon and the tau, and their three respective chargeless partners called neutrinos. The three neutrino types are distinct and interact only through the weak force. Such a picture (of quarks and leptons and the forces of interaction among them) has come to be called the Standard Model.

As mentioned in the first part of the interview, Glashow, along with Steven Weinberg and Abdus Salam, was awarded the Physics Nobel Prize in 1979 for their contributions to the theory of the unified weak and electromagnetic interaction between elementary particles, including, inter alia, the prediction of the weak neutral current. (In the theory of the weak force that existed before the unified picture emerged, the charge of a decaying particle, say the neutron in beta decay, changed by one unit, which was in conformity with the observed weak interaction processes until then. These are known as charged current processes. But the unified picture required the existence of decays in which the charge of the interacting particle does not change. These were the neutral current processes, which were subsequently discovered at CERN in 1973.) Glashows name is also associated with the Glashow-Iliopoulos-Maiani (GIM) Mechanism, which suppresses a certain kind of neutral current processes in conformity with observations.

In recent years Glashow has been engaged in the study of neutrinos, the highly elusive particles (because they interact very weakly with matter and are therefore very difficult to detect) which have today assumed centre stage in particle physics and astro-particle physics because of their highly enigmatic properties called oscillations, a certain quantum mechanical phenomenon in which one neutrino type could transform into another type discovered in the last ten years or so, which have posed more questions for physics than they solved. In fact, he delivered a colloquium on December 7 at the VTU on this subject, titled Neutrinos: How a Desperate Remedy Became a Profound Enigma.

On the other hand, physicists do not yet understand how gravity, the weakest of the four forces, acts at sub-nuclear dimensions and hence we do not yet have a quantum theory of gravity. Attempts at unifying the weak, electromagnetic and the strong forces into a single mathematical framework, though promising, have not been very successful. These include the Grand Unified Theories (GUTs) and Supersymmetric theories, the latter invoking a higher level of internal quantum symmetry among the particles beyond the Standard Model. A still higher level of unification, which would include gravitation as well, is even farther away, though a speculative mathematical framework called String Theory, requiring 10 space-time dimensions (instead of the familiar four), has been extensively investigated upon in recent years. Glashow has been a strong critic of string theory. It has not made one verifiable prediction, he said in his public lecture at the VTU.

This concluding part of the interview focusses on Glashows important research contributions to this continuously evolving picture. Excerpts:

Gary Feinberg and Steven Weinberg were your colleagues and friends at school and you went on to win the Nobel with Weinberg. How was it being with them in school and did it make a difference in your growing up with physics?

Yes... it was very important to have people like Weinberg and Feinberg as high school buddies. Another fellow, Dan Greenberger, taught me calculus in the school lunchroom. Gary, Steve and I would take the subway home together, where we would compare and compete with one another. Again, I had a marvellous set of peers in college [at Cornell]. I have always felt that I learned as much from my peers as from my teachers. There is a virtue in grouping together the best and the brightest: as at the Bronx High School of Science, which has produced seven Nobel laureates in physics!

Did you have occasions to work with them in your research years? Your Nobel Prize winning work itself was, of course, done independently and about eight years earlier than Weinberg, isnt that right?

Weinberg and I collaborated on a number of papers. I never collaborated with Feinberg, but we were in close touch, both socially and scientifically for many years. My Nobel work was done in Copenhagen in 1960/61, whereas Steves was done at MIT [Massachusetts Institute of Technology] in 1967. Earlier, we were colleagues at Berkeley; later we were colleagues at Harvard.

You have mentioned in your autobiographical essay posted on the Nobel website that Julian Schwinger, your thesis superviser, was the first to think of unification of weak and electromagnetic interactions and, in fact, you two had worked out a theory but the draft was lost. Can you recall some of that work? How does it relate to the present formalism?

Schwinger and I spoke of writing a paper together, but nothing came of it. His notion was a theory based on [the mathematical group] SU(2), but as I realised later in Copenhagen, such a theory was no good. There was no lost draft. The Prize-winning research I did in 1961 was not taken seriously at the time: neither by me nor by others. Only later, when appropriate experiments and further theoretical work were done, was it recognised.

But you did state in that essay that one of you lost the first draft of the manuscript, and that was the end of it. What did this statement refer to?

I dont remember. Perhaps there was a draft, but it hardly matters. Neither of us had gotten beyond the notion of an SU(2) theory involving just the photon and a charged intermediary... and such a model just doesnt work.

In that essay you have also regretted that even though you were quite involved with the internal symmetry structure of hadrons as proposed by Gell-Mann [which classified particles as different multiplets] you missed the structure of the Cabibbo Current [which suppresses certain kind of charged current weak processes] and the Gell-Mann Okubo Mass formula [relationship between masses of particles within a multiplet from internal symmetry considerations alone].

In the early 1960s, I collaborated a great deal with my good friend Sidney Coleman (who passed away recently). For example, we deduced the so-called Coleman-Glashow mass formula. But indeed, we missed the Gell-Mann-Okubo formula, which we should have thought of first. So it goes.

But discovering the GIM mechanism must have pleased you immensely, especially when charm was discovered. How did that come about?

Good question. I first came up with [the idea of a new quantum attribute of elementary particles called] charm in 1964 in a paper with James Bjorken. Six years later, with [John] Iliopoulos and [Luciano] Maiani, we recognised the essential need for charm as a mechanism to suppress unseen strangeness-changing neutral currents. Its surprising to me that no one came up with this idea in the intervening years. It was not until November 1974, with the discovery of the J/Psi particle, that our ideas were proven to be correct. It felt wonderful!

Of late you have been working extensively on neutrinos. You also gave a talk on the subject to the students here. How is it that what were once seen as innocuous particles have assumed the centre place in physics and cosmology in the last couple of decades? Do you see these as playing an even greater role in the physics of the future like photons?

The fact that neutrinos [hitherto assumed to be massless] have mass I told you earlier about the Japanese experiments at SuperKamiokande that detected oscillating neutrinos [see Part I of the interview] is an issue that goes beyond the Standard Model. So the study of neutrino oscillations is a window into physics beyond the Standard Model. There are a number of important outstanding questions, which can and will be addressed by neutrino experiments over the next decade.

The Standard Model does not naturally accommodate a neutrino with mass. You yourself have proposed some solutions to this.

Various people have.What is your current view on this?

It was realised from the beginning of 1979 that it is quite straightforward to supplement the standard theory with heavy neutrinos in such a way as to produce neutrino masses naturally. Many people, including me, came up with the so-called seesaw mechanism for neutrino mass generation. This scheme involves the existence of three very heavy sterile neutrino states. These would be very unstable particles, so they have no immediate observable consequence. This idea is very simple and is very attractive and it works. It doesnt answer all of our questions by any means but it gives a sort of perfectly logical framework for producing the parameters that are responsible for neutrinos [having masses and] oscillating.

However, some theorists invoke them to realise [Andrei] Sakharovs explanation for the particle/anti-particle asymmetry of the universe. [In 1967 Sakharov proposed three necessary conditions for matter-antimatter asymmetry to arise in the early universe.] In this context, a most important question is whether or not lepton number is absolutely conserved. Ongoing searches for neutrinoless double beta decay which can occur when the neutrino and the anti-neutrino are the same particle will shed light on this issue.

But dont experiments rule out such heavy neutrinos?

According to this view...What you do when you have these heavy neutrinos? They are very heavy. So we are not going to see them. So what you do is to integrate them out. We dont talk about them. The result is that the theory is described by a totality of ten observable parameters. Those are the [three] charged lepton masses, the [three] neutrino masses and the parameters that measure oscillations [among the three neutrino types]. So here are ten numbers connected with the leptons...and there are similar ten numbers connected with the quarks. So we have in all twenty numbers. Most of these numbers have been measured...not all of them but most of them. And the ones that havent been measured have been constrained. So with all these hints...with twenty hints you ought to be able to formulate a framework...to go further...to get a deeper understanding. But nobody has yet succeeded.

One of your more recent ideas has been to violate Lorentz invariance [that is integral to Einsteins special relativity] to generate neutrino masses.

Thats another direction...

Why do you see it necessary to give up a sacrosanct principle such as Lorentz invariance and reformulate special relativity as you have done, which you have called Very Special Relativity (VSR)?

Well, there are no sacred cows in physics. Laws of physics such as conservation of energy, or whatever, are made to be tested. There is no guarantee that Lorentz invariance is an exact symmetry of nature. That special relativity is precisely true. The attributes of special relativity that have been tested best are the universality of the speed of light that is, the speed of light is the same in all directions and it is the same to all uniformly moving observers and that the speed of light coincides with the maximum attainable velocity of particles. Well, the VSR... the Very Special Relativity (which I developed with Andrew Cohen) ... makes the hypothesis that is a little bit weaker than Einsteins hypothesis. The hypothesis that we make is simply that the velocity of light is the same for all uniformly moving coordinate systems in all directions. That is not the same as Lorentz invariance. In fact, it demands a smaller four-parameter [mathematical] group instead of the six-parameter Lorentz Group, which is quite less restrictive.

And the question is this. Is the nature consistent with being invariant under this smaller group and not the larger group? Now it turns out that the nature is approximately invariant. And that is something also that this theory dictates because if there were no CP violation...[that is,] if CP were exact, then VSR transforms into ordinary special relativity. Since CP is [only] almost exact, then the departures from special relativity will be very small.

The question is how small? Well, we dont know how small. But we know that there are some mysterious and small parameters in the Standard Model, namely the neutrino masses. So it is not entirely stupid to identify the scale of neutrino masses with the scale of [Lorentz invariance violation in] VSR. So we have done that. In doing that we have made some predictions, which have not yet been tested.

Like what?

Well, the one that is most interesting is what I call the transverse electric dipole moment, which means that if you have the electron spinning one way and you put an electric field perpendicular to the spin [direction or axis] and the energy of the electron will depend on which way the electric field is facing. And it turns out that it is difficult to measure that. And nobody has reported having measured it. Experimenters have their own plans and nobody has decided to do this particular experiment. We also predicted an effect with the neutrinos involving the effect of the neutrino masses on [the energy] spectrum [of the electron] in the tritium [beta decay] experiment. [In tritium beta decay a tritium nucleus decays into a helium-3 nucleus with the emission of an electron and an electron-type anti-neutrino.]

The end point of the spectrum gets changed [in a specific manner]. And whether there is an effect of that kind or not depends on whether the effect is observable in the first place. Nobody has yet seen evidence of neutrino masses in tritium decay. May be neutrino masses are too small to make an observable effect there. We dont know. The situation is very much time-dependent. If we were to believe the cosmologists and astrophysicists, they claim that they have shown that the sum of neutrino masses is less than one electron-volt! If thats true then it would be impossible to see the effect of neutrino masses in tritium decay.

Isnt this astrophysical limit on neutrino masses from considerations of neutrinos contributing to dark matter? I thought that the limit was much higher.

You are quite right. Say ten years ago one would say that if neutrino mass is to explain the dark matter, then it has to have a mass of 20 electron-volts or something like that. Or conversely, it could not be higher than that. Now remember that we are learning a great deal cosmologically from studies of the Cosmic Background Radiation such as done in the very recent past and recent studies of galaxy surveys. Astrophysics is moving very quickly. So today, the limits, depending on who you talk to, range from 0.2 to 2 electron-volts. It depends on what day you make the survey and how many prior hypotheses you make. But there is a limit and the limit is getting better.

How significant is the connection that is currently emerging between particle physics and astrophysics and cosmology, even though one could argue that cosmic observations have large uncertainties?

Cosmology, astrophysics and particle physics have been closely linked since Newtons time. In the 19th century, the development of spectroscopy made it possible to learn the chemical composition of the stars. Nuclear physics of the early 20th century let us understand how stars shine. More recently, cosmologists first determined that there are not more than three light neutrino species, and today, as I already mentioned, they have deduced the best upper limits on neutrino masses. What this represents is a remarkable unification of the studies of largest and smallest structures in the universe.

But has the particle physics-cosmology convergence thrown any light so far on matter-antimatter asymmetry in the universe?

Yes and no. There are several ingenious explanations for the matter-antimatter asymmetry, but we dont yet know which is correct nor how to compute the (known) size of the asymmetry. In this connection, one of my favourite papers (with de Rujula and Cohen) proves that the universe as a whole cannot be matter-antimatter symmetric.

Is it possible for you to elaborate a little more on this work of yours on the fundamental nature of matter-antimatter asymmetry in simple terms?

We asked whether the observable universe could consist of random patches of matter and antimatter, so that there could be overall matter-antimatter symmetry. We computed the observable effects of particle-antiparticle annihilation at the boundaries of these patches.

The smaller the patches, the greater are the effects. We showed that cosmic ray data in hand (especially soft gamma rays) constrains the mean patch size to be of order of the size of the observable universe. Thus the notion of overall matter/antimatter symmetry is excluded, and the likelihood of the AMS [Alpha Magnetic Spectrometer] experiment to observe, say, anti-alpha particles from distant sources is very small. Of course, that experiment too is moot... its launch having been recently cancelled by NASA [National Aeronautics and Space Administration] owing to financial constraints.

Your other big effort was the grand unified theory of strong, electromagnetic and weak interactions, called the SU(5) model. What is your present view on such Grand Unified Theories (GUTs) and where does the approach stand today?

The SU(5) model that I proposed with [Howard] Georgi is very attractive. I was sad that it does not conform to experimentthe fact proton does not decay. However, most theorists still believe in the unification of the electroweak and chromodynamic forces... perhaps in a supersymmetric context. The basic idea is too gorgeous to be wrong.

What are the chief hurdles in unifying all forces, gravity included, that particle physics faces today?

Does there have to be a quantum theory of gravity? Not unless it makes some testable predictions. The Large Hadron Colider (LHC) experiments [at CERN] may tell us something about gravity, if the extra dimensions notion [as required by string theory] turns out to be right. I am more focussed on more modest questions, such as an explanation for the curious spectrum of quark and lepton masses.

Your remark does there have to be a quantum theory of gravity? is somewhat surprising because quantum theory tells us that if there is a force field there must be a quantised manifestationof the same. Isnt that so?

Probably yes... but the quantum aspects of gravity are likely to be unobservable. We have not yet observed gravitational waves... although we may do so in the near future, There is simply no way to observe the particle aspects of gravitons [the quantum of gravity analogous to the photon of electromagnetism], such as the gravito-electric effect, because of gravitys weakness.

Of course, this may change if ideas of extra dimensions prove to be fruitful.... we shall see.

String theory, with its need for ten dimensions, has been one approach towards this but you have been a strong critic of it. This was also evident in your talk here. What are your key objections? Elsewhere you have even criticised the use of the phrase Theory of Everything?

I dont like the phrase theory of everything because I dont quite know what it means. String theory will become a part of science only when it becomes subject to empirical tests. At present, string theory is not falsifiable, it is devoid of predictions, it is not yet a well-defined discipline and, significantly, does not recapitulate the successful Standard Model of particle physics.

In one of your earlier interviews you had made several strong remarks about string theorists. You had said, String theory appears to be consistent and is very beautiful, very complex, and I dont understand itThe theory is safe, permanently safe. I ask you, is that a theory of physics or a philosophy?

Yes, I stand by those statementsIn fact, today there is not just one string theory; there is a class of string theories. So which is the correct theory? But string theorists have redefined their paradigm that within the framework of string theory it is not possible to answer this question. This is surrender.

You had also expressed concern over the increasing disconnect between such theorists and experimentalists these days. You had even remarked: There are physicists, and there are string theoristsWe dont listen to them, and they dont listen to us. We cant understand them, and what we do is not of any direct interest to them.

Yes. As I said, I stand by that too. Google my piece with [Paul] Ginsparg, Desperately Seeking Superstrings.

One of the cornerstones of the Salam-Weinberg-Glashow Standard Model and the holy grail of particle physics is the hypothetical Higgs boson. What if the LHC does not show up the Higgs particle or the other supersymmetric particles? Where does it leave the unified picture?

There are three possibilities at LHC: (a) no new physics whatsoever; (b) a Higgs and nothing new but a Higgs, and (c) Lots of new physics. I would bet on c.