Sep. 10, 2008 at 3:45 PM ET
|Theoretical physicist John Ellis writes out equations at|
Europe's CERN nuclear research center near Geneva.
John Ellis juggles the concepts of dark matter and dark energy, supersymmetry and black holes as if they were playthings. The British-born physicist chose his calling 50 years ago, when he was just 12 years old, and he's spent the past 35 years as a theoretician at Europe's CERN particle-physics center near Geneva.
So if you want to know what mysteries the world's largest atom-smasher could crack open, Ellis is your man.
It may sound as if Ellis is one of those long-haired physicists who never has to interface with the real world, but nothing could be further from the truth:
During my visit to CERN last year, I sat down in Ellis' office, looking around stacks of piled-up research papers, as the physicist gave me an entry-level explanation of what the Large Hadron Collider is all about. I've used some of Ellis' quotes in previous stories about the LHC, but here's a fuller version of the edited Q&A:
Cosmic Log: What are people looking for with the Large Hadron Collider?
Ellis: The LHC is the most powerful microscope that’s ever been built. It will be able to explore the inner structure of matter on a scale that is 10 times smaller than anyone’s been able to do before. Also, the LHC, I would say, is the most powerful telescope that’s ever been built, because we know that the way elementary particles interacted with each other controlled the very early history of the universe.
So with the LHC we are able to in some sense re-create the conditions that existed in the universe when it was just a fraction of a second old - the sort of thing that the optical telescopes and just can’t see. So, most powerful microscope … most powerful telescope.
Then, of course, there’s the question whether there’s any reason to expect, if you look on this particular very small scale inside matter, is there going to be anything there? And I think we have a couple of reasons for thinking there will be something there. We have this theory of elementary particles and the forces between them, the structure of matter. This model works extremely well, but we know it’s incomplete. And one of the reasons why it’s incomplete is, if you write down the basic theory, it looks like all the particles would be massless. That’s clearly not true. If you look in the universe today you see that some particles are heavy, some are not. So there has to be an explanation for that.
Now, in fact, this "Standard Model" of particles contains an explanation, but at the moment, it’s very much a theoretical explanation. It’s a hypothesis. We don’t know whether it’s correct or not.
This is an idea that was suggested by Peter Higgs and others way back in the 1960s. According to their idea, there should be a new particle which could be produced and observed at the LHC, called the Higgs boson. This is in some sense the holy grail of particle physics, to find this missing link in the standard model. So that’s one thing that we’re really looking forward to with the LHC. In fact, back when we persuaded the politicians to stump up the money to build the thing, that’s probably what we told them.
Now, the other reasons for thinking there are new physics … one of my personal research interests is dark matter. Astrophysicists tell us that something like 90 percent of the matter in the universe is some sort of invisible stuff, and nobody knows what it is. They can see that it attracts gravitationally visible particles, and presumably it’s not made of the same constituents as the visible matter. It could be something that we’re going to be able to produce with the LHC. There are various different ideas about what might be, but quite generally I think there are good reasons to think that these dark matter particles, if they exist, will be observable at the LHC.
Q: Just speaking about dark matter, is there thought that these are particles that exist homogeneously in all of reality? Are there these sorts of particles in this room, or do they only exist under special conditions that you would create at the LHC?
A: They exist everywhere in the universe, and not uniformly distributed. So if you go out in between the galaxies, there would be some low density of these things. But in fact galaxies were formed because these dark matter particles clumped together - and then the regular visible matter, you and me, were attracted by this dark matter and formed the visible galaxies and stars that astronomers look at. So the density in this room would actually be quite high, because we're sitting inside a galaxy.
What’s estimated is that if you took a liter bottle of mineral water, then on average this would contain something like one dark matter particle at any one time. However, this dark matter particle is traveling quite fast. It’s traveling at some fraction of the velocity of light, so it doesn’t stay inside the bottle. Also, this dark matter particle has extremely weak interactions. So most of the time, it would pass straight through the bottle without leaving any trace.
One of the challenges is how to verify that this thing exists. Basically, there are two approaches for this. At the LHC, what we do is we hope to produce heavier particles which decay into this invisible particle. So we wouldn’t actually see the dark matter particle directly, but we would see the heavier particles which decay into it. Then there are astrophysicists who plan to look directly for scattering of this dark matter particle in their detectors. Not in bottles of water, but detectors in deep underground experiments.
Q: Some people wonder what happens if these particles are not found. Perhaps there would be traces found in earlier experiments, such as the Large Electron Positron collider [LEP] or Fermilab? Is it just that they weren’t able to achieve the energies that could have found the traces of the Higgs boson, or supersymmetric particles, or other candidates for these dark matter constituents?
A: The LHC is far and away the most powerful particle accelerator that’s ever been built. These previous accelerators, like LEP or the Fermilab accelerator, explore some of the possible theories, but not all of them. Well, LEP didn’t find the Higgs boson. Fermilab has a chance. We’re breathing down Fermilab’s neck. It’s quite possible that Fermilab might be able to discover the Higgs boson before the LHC get started.
These other sorts of particles, the supersymmetric particles that are related to dark matter, there I think probably Fermilab just doesn’t have enough energy. It’s just not a powerful enough “microscope.” so I think that the LHC might have that particular field to itself, pretty much.
Q: Has it gotten to the point where people can talk about potential applications of the discoveries that might be made at the LHC? I’m sure when the politicians approved this money, they said, OK, you'll find the Higgs boson, but what is the payoff? Is it going to lead to limitless energy or other things that politicians like to talk about to their constituents?
A: We don’t justify CERN or other big particle accelerator laboratories on the basis of spinoffs or technology transfer, or anything like that. Of course, we do have programs for that. Personally, I believe that the most important knowledge transfer that we can make is by training young people who then maybe go off and do something else. I think that’s probably more important than some particular technological widget that we may develop.
I think the primary justification for this sort of science that we do is, fundamental human curiosity. I think people ever since the ancient Greeks and probably a long time before that have wanted to understand how matter is made up, how it behaves, where the universe comes from. And so we are responding, I think, to that continuing human urge.
It’s true, of course, that every previous generation that’s made some breakthrough in understanding nature has seen those discoveries translated into new technologies, new possibilities for the human race. That may well happen with the Higgs boson. Quite frankly, at the moment I don’t see how you can use the Higgs boson for anything useful. But maybe I’m wrong. It’s particularly difficult to predict technological applications decades away in the future.
Q: Anyone who’s tried to predict that a big discovery is not going to make a difference is generally proven wrong, going back to the Wright brothers. A century ago, people were asking whether there'd be any benefit to having airplanes. It's been that way with other innovations as well. You have that unpredictability, I suppose.
A: Right. Or consider Einstein’s theory of relativity, for example. This was pure knowledge. But now, the GPS systems that everybody has in their SUV have to incorporate Einstein’s theory of relativity in order to figure out where they are. Well, an SUV you don’t need to know so accurately. But airplanes, if they’re going to land without making a bump, OK, they have to incorporate both special and general relativity into their calculations of the signals from GPS satellites. So even apparently abstruse things like general relativity turn out to be relevant to the human race. Not back in 1915, when Einstein thought of it, but in 2007, yeah.
Q: I know you’ve spoken about what’s at stake in this search for the Higgs boson. Maybe you can elaborate a little bit about what happens if it’s not found, or what happens if it’s found but people don’t learn anything more about its properties other than that it exists. What does that do to the Standard Model?
A: I like to compare the situation in particle physics to our room, where there is a doorway. The Higgs is the door. So, it could be that when you go to the doorway and you open the door, there’s nothing outside. This seems very unlikely to me. So, I don’t think that the Higgs door, if you like, is just closing off the room and there is nothing beyond. I believe there’s going to be a lot more physics beyond. What it’s going to be, I don’t know. Maybe it’s supersymmetry. Maybe space has additional dimensions.
Maybe it’s something that we haven’t thought of yet. I certainly hope it’s something that we haven’t thought of yet. It would be great to come across a real surprise. When we look deeper inside matter, we shouldn’t be overconfident that we know everything that’s possible down there.
In fact, one of the most interesting possibilities of all is that there is no door there at all. That there is no Higgs boson. Well, this might be a little bit difficult to explain to our politicians, that here they gave us 10 billion of whatever, your favorite currency unit, and we didn’t find the Higgs boson. But in some ways, theoretically, that would be the most interesting possibility, because it would really mean that we had to tear up our notebooks of the last 45 years and start more or less from scratch.
Of course, if there’s no door in your room, then you can get outside very easily. Then I think there has to be something outside. Probably the most likely option then might be extra dimensions. And there are some ideas where if you have some additional dimensions of space, you could somehow do the job that the Higgs does in the standard model.
Q: What would you look for if you were looking for extra dimensional physics of some sort?
A: I think the first thing that you would look for is the "dog that did not bark," to quote Sherlock Holmes. Remember the short story where this was the most important clue, the dog that did not bark? So you would first look for the Higgs in all the standard ways that we expect it to show up. The first very important statement to be able to make would be, "The Higgs does not exist. We have looked for the Standard Model Higgs, and it is not there."
Making such a statement would be an incredibly difficult thing to fit to do, because it involves looking for the Higgs in very many different ways. The detectors have to be understood very well, the accelerator has to work very well. It’s not going to be something that one says immediately. One might say we haven’t found the Higgs yet, or we’re getting more and more concerned that Mr. Higgs has disappeared and so on. It might take a while to say, actually, Mr. Higgs doesn’t exist at all.
Then, of course, there’s the question of "what is there if there isn’t a Higgs?" There are particular types of events that you could look for in the LHC where you could find evidence for these extradimensional theories that replace the Higgs. So, I guess at the same time as some team is trying to find the Higgs, or maybe prove that it doesn’t exist, there would be some other team looking for these distinctive features of these extra dimensions.
Q: Are some theorists already starting on non-Higgs theories for how things work?
A: For sure. There have been ideas about so-called "Higgsless" theories around for some time. Most of these theories don’t work for some reason or another. They run into some problem with the data taken with previous accelerators. There are perhaps one or two variants of those ideas which are not in serious conflict with previous data. It’s actually pretty difficult to come up with a theory that doesn’t have a Higgs in it.
Q: We’ve slipped into this concept of supersymmetry – I wonder if you have a stock explanation for how supersymmetry figures into the frontier of particle physics.
A: Supersymmetry is an idea according to which in parallel to the particles that were made of, the electron for example, that goes around an atom, or the photon, which causes light, in parallel to those particles there would be other particles, call it the selectron, call it the photino, which have identical interactions. So, the selectron, for example, would have an electric charge, and it would couple to the photino in the same way that the electron couples to the photon – and that’s the way that electromagnetic radiation works, and TV and computers and so on.
These would have the same forces has regular particles, so what’s the difference? The difference is in the way in which these particles spin. This is maybe somewhat of a different concept to get across, because it doesn’t have any obvious metaphor, but you have to think of elementary particles as being like ballet dancers. Ballet dancers can spin. And different types of elementary particles spin at different rates. So, for example, the photon spins twice as fast as an electron. The Higgs boson would be the one particle in the standard model that doesn’t spin at all.
Now, according to supersymmetry, all the known ballet dancers have partners, and these partners of the ballet dancers spin at a different rate. So, for example, the supersymmetric partner of the photon, the photino, this would spin at half the rate of the photon. The supersymmetric particle of the electron, the selectron, wouldn’t spin it all, just like the Higgs boson.
So why on earth would you postulate such a thing? When I say all this, this all sounds very complicated and arbitrary, and I can just imagine people saying, "These guys are completely crazy." The reason why people like this idea is … well, there are many reasons. One very concrete reason is that supersymmetry would help the Higgs to its job. You can write down a theory where there is just a simple Higgs boson that gives masses to the other elementary particles, end of story. But such a theory is a very unstable theory.
It’s a little bit like if you had a hill and you put a ball on the top. You give it a little tap, and it runs away. In the same way, this Higgs theory is very unstable. So what you want to do is to make it, in some sense, as if it was not sitting at the top of the hill but at the bottom of the valley. In some sense - and my physicist friends will raise their hands in horror at what I’m saying - but in some sense, the job of supersymmetry is to put the Higgs at the bottom of a nice, stable valley so it doesn’t roll away. This trick works just simply because these supersymmetric particles have the same interactions as regular particles, and because they have this different rate of spin.
Q: You mentioned that the LHC would be a telescope as well as a microscope. Could you explain that a little bit more? You’re looking back, I suppose, at the phenomena that existed at the very beginning of the universe. Is that how you use it as a telescope?
A: Yes, we know that the universe is expanding. This was discovered by Edwin Hubble in the 1920s. So what this means is that if you go back to earlier and earlier times in the history of the universe, all the stuff that we see in the universe today would have had to be much closer together, and would be much denser.
There are various pieces of evidence from astronomers and astrophysicists that indeed that happened. For example, the universe is full of radiation that was sent out when the universe was a few hundred thousand years old. We believe the abundances of light elements in the universe like helium were made when the whole universe had a temperature about the same as the center of our sun. That’s what you need in order to get nuclear reactions to take place. So we have some confidence in this picture of a universe expanding from some initial very small, very hot, very dense state. This is the right way to go. This is the big bang.
With the collisions at the LHC, we are able to re-create the energies that particles would have had, the density of material that the universe would have had when it was something like a millionth of a millionth of a second old. So this is what I mean, that it’s the most powerful telescope ever built, because we can see things that happened way before the production of this radiation that fills the universe.
Q: Some folks have been worried about the LHC and wonder whether this would destroy the universe. Maybe you could explain why those people can be reassured that the universe would not be destroyed.
A: There’s no danger from LHC collisions. The collisions that we produce have an energy which is much less than some of the cosmic rays make.
The experiments that we will do with the LHC have been done billions of times by cosmic rays hitting the earth, they’re being done continuously by cosmic rays hitting our astronomical bodies, like the moon, like the sun, like Jupiter and so on and so forth. And the earth’s still here, the sun’s still here, the moon’s still here. LHC collisions are not going to destroy the planet.
That being said, indeed it will be extremely exciting if the LHC did produce black holes. OK, so some people are going to say, “Black holes … those big things eat up stars?” No. These are microscopic, tiny little black holes. And they’re extremely unstable. They would disappear almost as soon as they were produced. The theories which predicts such things are theories with extra dimensions of space, and you can calculate in these theories how these things would be produced and how they would decay.
Actually, this would be very exciting, because it would be a way of testing our quantum theories of gravity in a way that nobody could imagine doing any other way in the laboratory. So, if the LHC were to make microscopic black holes, it would be tremendously exciting, and no danger.
Q: Are there any specific experiments, if you were to say, OK, we will be looking for these miniature, microscopic black holes, and we would expect to see that in ALICE, LHCb, or ATLAS, or CMS? Are there particular experiments that would provide the evidence for that?
A: There are two big discovery experiments which are going to be looking for things like the Higgs boson, dark matter particles and black holes. Those are ATLAS and CMS. I would say that both of them are equally good. They look at these Higgs bosons, and supersymmetric particles and black holes in somewhat different ways, but the basic ideas are rather similar. They use different technologies, but the physics objectives are very similar.
We believe that either of the experiments could, within maybe a couple of years, start to provide us definitive answers about whether the Higgs exists and whether supersymmetric particles exist. These definitive answers won’t come as soon as you flip the switch. It won’t be on day one or even day two that you learn about whether the Higgs boson exists. It will take a while. But I think that either or both of ATLAS and CMS should be coming up with answers after a year or two.