Gary Godfrey

This is a transcript of the interview with Gary Godfrey of SLAC, as a part of the Oral Histories of Science Project of Foothill College.

Interviewer: Elliot Askarinam, June 2006

Askarinam: Okay, so first just start off with the basics. When were you born and where?

Godfrey: 1946 in San Francisco.

Askarinam: So is anyone in your family a scientist, and did this affect your decision to study science?

Godfrey: My father was a naval architect, so partly... Partly technical. Nobody directly in the sciences.

Askarinam: How about your mother?

Godfrey: My mother was an educator, taught elementary school.

Askarinam: Any siblings?

Godfrey: My brother, who is younger than I am, is now a molecular biologist.

Askarinam: Is that it?

Godfrey: That's it.

Askarinam: So did you ever feel in your career that science is too intense or too rigorous and decide to change majors or...

Godfrey: No. As a kid, probably starting around eight years old, I was very interested in electronics. And I had a neighbor who was a ham radio operator, who had a wonderful basement of old electronic parts dating back to the 1920s. So I was interested in building, this was before transistors, so lots of things with vacuum tubes. Early on I became a ham radio operator. I was 12 or 13, around there. I had a great deal of fun making receivers and transmitters. And generally not getting killed. [laughter]

Askarinam: So, did you ever consider studying any other fields?

Godfrey: Yes, I was originally going to be an electrical engineer. That's what I thought I wanted to be, I liked electronics. Then I went to Caltech as an undergraduate, and I got into freshman physics there. Feynman physics, it was the first or second year after he had given the lectures. I thought it was a lot of fun, I enjoyed it, and changed my major to be a physicist.

Askarinam: Is there a specific topic in physics that set it in stone that you wanted to do physics, or was it just the whole...

Godfrey: Probably more the quantum mechanics, the electromagnetism, not so much statistical mechanics, thermodynamics, not the classical fields, but the new stuff that was around was very interesting.

Askarinam: Once you started learning about quantum physics, did you know that was the career path you wanted to pursue in that field, or were you not really sure yet what you wanted to do?

Godfrey: As a freshman or sophomore, I certainly hadn't zeroed in on elementary particle physics, which became my specialty. But the courses you tended to take were ones involving nuclear physics, particle physics, there are experiments in undergraduate labs that involved things like looking at the decay lifetime of the muon. That kind of pointed in that direction.

I got some summer jobs working in a nuclear physics lab. Learned to program there. A lot of people in my generation, their usefulness came from learning to program, the generation before didn't learn about computers. I went to Berkeley for graduate school, took the standard round of physics courses there, particularly elementary particle physics courses. I joined a group at the Rad Lab, the Chamberlain group and was launched off in the particle physics. My thesis was on such a topic.

Askarinam: At that level in college did you ever imagine you'd be doing the kind of work that you're doing today?

Godfrey: No, actually, because today I am actually doing more astrophysics. [laughs] It's strange, but it turns out just because of some discoveries that have been made, many particle physicists have wandered over into astrophysics, and we can talk about that a little later. I have been trained, and the things I have done, you might call me an experimental particle physicist, not so much a theoretician who sits around and thinks about stuff, I build experiments.

Askarinam: More verifying the kinds of theories that are out there.

Godfrey: Hands-on building things, building instruments used to make measurements, and accelerators.

Askarinam: So what makes you different than someone who is an engineer, if you are doing the actual designing?

Godfrey: Aspects of what I do are engineering, but there are many experimental aspects to what I do. An engineer wouldn't figure out how to build a new wire proportional chamber. An engineer would have learned how to build something in school, and he'd apply variants of it and be able to calculate. Physics is a little more wide-ranging. There's things which nobody's built before. Based on principles and ideas you figure out how to make something. The wire proportional chamber for instance, it detects when a particle goes by. The particle leaves some ionization, and you measure the time to drift into a wire. This was not an engineering topic. This was "well, we have to detect particles, how can we do it?" So you figure out how to do it.

Askarinam: So there's more theory behind it.

Godfrey: More applying general ideas to build new stuff. And then you hear somebody else just had a great idea how to do something, and then you piggy back on it and use the idea yourself. CAT scanners were not developed by engineers, they were developed by high energy physicists who had an idea how to do these things.

Askarinam: Can you tell me about your degrees and what colleges who attended?

Godfrey: Just two, I went to Cal Tech for Bachelor of Science and the University of California Berkeley for the Ph.D.

Askarinam: And these are both physics?

Godfrey: The first was a BS in physics and the graduate degree was a doctorate in physics.

[Points to recording device] MP3?

Askarinam: It's still going right?

Godfrey: I just went to a concert and used one of these to record it. It's still going.

Askarinam: You talked a little bit about your internships, was this a constant thing you were doing in college time?

Godfrey: Yes, I think I started working in a lab at Caltech in my sophomore year. So I worked sophomore, junior and senior year. No not senior year. Sophomore and junior year.

Askarinam: Was this the nuclear...

Godfrey: It was called Keck. It was the Keck lab at Cal Tech. K E C K.

Askarinam: What exactly did you do there?

Godfrey: I worked for somebody who was modeling stars. A fellow named Willie Fowler, who was modeling how the nuclear reactions, how energy was generated in stars. Now, I didn't know anything about that, but I would receive little tasks to make a Fortran program. Or make a particular program work.

Askarinam: Like fusion reactions?

Godfrey: Yes. Many nuclear reactions inside. They were modeled on the computer. I would write little subroutines of one sort or another to contribute to that. That was one of the things there. Another was a small 12 MeV Van de Graff accelerator there. I would go down there and take shifts and analyze some data that would come out of that.

Askarinam: Was that the only internship that you did?

Godfrey: That was the only one at Cal Tech, Berkeley in summers... I worked a couple summers at Lawrence Livermore Lab. One summer I worked at the reactor out there. Another summer I worked at an accelerator analyzing data taken from a photon beam made by a high-energy LINAC. Let���s see, and then at Berkeley I was a teaching assistant for a couple of years and then I was a research assistant for the rest of my time there in the Chamberlain group.

Askarinam: So did you feel like these internships helped to hone in on your career path that you wanted to do, or you wanted to work, or was it a more just kind of fun?

Godfrey: It was fun and money. It was fun money. It was doing something that was interesting and getting paid for it. You supported yourself. Graduate school would be kind of difficult for people if they couldn't support themselves.

Askarinam: So what about family? Are you married? Any children?

Godfrey: Yes, wonderful wife of 25 years and two boys, 12 and 14.

Askarinam: So you said you're married 25 years. Is your wife also a scientist?

Godfrey: Nope.

Askarinam: Liberal arts?

Godfrey: Actually a hospital administrator, stayed away from marrying somebody in science.

Askarinam: [Laughter] Why's that?

Godfrey: I think you would choose it too. It's more interesting to have somebody with other interests. It's nice to be able to branch out a little bit.

Askarinam: So what about a hobbies? What do you like to do in your free time outside of work?

Godfrey: Hike, camp, build.

Askarinam: Build what?

Godfrey: House [laughs].

Askarinam: Build houses?

Godfrey: No my house.

Askarinam: Remodeling?

Godfrey: Nowadays, if you go out to try to buy a million-dollar house you need a big down payment and a large income. So you get a fixer-upper.

I enjoy music and play the banjo, what else? That's it.

Askarinam: For all of these projects, does everybody work in teams, or is it more secluded, and everyone's doing their own thing?

Godfrey: You mean like physics at SLAC?

Askarinam: Yes.

Godfrey: Over the years, physics has become a much more collaborative field. For example, when I was at undergraduate school and worked at Keck Lab, you would see one professor with perhaps a graduate student and someone like me doing the experiment. That was it. And the experiment would not take very long. Maybe one or two runs on the accelerator to get your data, and analyze rapidly. At Berkeley, I worked with a wonderful person, Clyde Wiegand, and we were a very small group. It was he and I, and we measured x-rays from kaonic atoms. We were a small group at the time. At this time, already collaborations like the Mark I at Stanford was forming, which was perhaps... I think the Mark One was probably 50 people, maybe even 100 people. Today for instance, the experiment I'm on, GLAST, has maybe 300 collaborators. This is by no means the smallest experiment. Experiments are going on at the LHC at CERN, it's the Large Hadron Collider, that have a thousand plus people collaborating to build one monster experiment.

Askarinam: Is yours kind of the minimum amount of people or the average?

Godfrey: Certainly in high-energy physics, the experiments are very large. It's a combination of two factors, one is that the experiments cost a lot of money. One of the LHC detectors, ATLAS, is probably $1 billion. So that translates into needing many many people to build it. And also it's the only game in town. It's the only experiment, and it needs all these people to do the many tasks, to build it, to analyze the data.

It's a different psychology than in the past. It's been growing. It used to be the accelerators took the bulk of the people to build and the experiments were small. Now that's not true anymore, now everything's big. To look smaller you had to get bigger. You probably learned that courses right? The smaller you want to look, the higher the energy needed to see it. The uncertainty principle delta-p delta-x is greater than or equal to h-bar. So you need high momentums in order to probe small distances.

Askarinam: So what is the overall goal of SLAC?

Godfrey: Well it's just recently changed. It used to be strictly a high energy physics lab - let's say five years ago, we were strictly high energy physics. We had the highest energy electron accelerator in the world, at that time. And not only did we run the accelerator, we helped out in experiments. So many users would come here and the staff of SLAC would participate in these large experiments to measure various things. Then came the era of colliding beam rings in which you could get more energy in the center of mass by colliding heavy counter-rotating bunches of particles. Originally the beam would hit a fixed target. The result was a lot of junk would go spraying forward, because in order to conserve the momentum of the beam particle, something had to go forward. In the center of mass of all the particles, not nearly as much energy is available to make new things, as was in the beam originally. In a colliding beam machine, you can take an electron and an anti-electron and collide them. All the energy that was in the center of mass of those beams is available to make new stuff. When they hit each other, the whole mass of energy comes to rest, and new particles are made to come out. So to find new particles, it became the era of colliding beam rings. This started with SPEAR back in the 70s, mid-70s. It turned into PEP, which was the next larger machine here, and finally there was the SLC, which collided beams at the full energy of SLAC, which was just under 50 GeV per beam.

Now SLAC is no longer strictly... in the last few years SLAC is no longer strictly a high-energy physics lab. No longer is SLAC the highest energy machine. They're machines that are higher energy elsewhere. There's not enough land to build the monsters. The next monster that comes along won't be built here. So SLAC is diversified. The LINAC, which is the two-mile thing you see underneath the freeway is now being converted to a brilliant photon source called a coherent light source. The electron beam doesn't just get accelerated, there are magnets put along its path.. They're called wigglers and undulators, so the electron comes along and wiggles like this. And of course you may know that when you bend an electron beam, it radiates. It radiates light. So it radiates, and the next hump radiates, and this thing is all aligned such that the photon radiated with this wiggle arrives with the electron bunch at the top of the next wiggle. Photons are bosons; photons are even-spined particles. You may have picked this up in your quantum mechanics course.

Bosons like to be in exactly the same state other bosons are in. So when the top of this wiggle sees a photon that has some particular frequency and phase, instead of just radiating a random photon at some other frequency, it's much more likely to radiate at exactly the same frequency and wavelength. Likewise when these go down to the next hump it's even more likely, because they've got more photons. This is called a free electron laser. The LINAC is being turned into the world's largest and most brilliant free electron laser. It will have very very intense pulses of all the photons going the same direction, and very short pulses. The intent will be to do like x-ray crystallography, but you don't have to take an exposure all day on a crystal in order to get a nice photograph. You can do it in <10^-12 seconds. There are so many photons that you can do an entire experiment with one pulse. You'll scatter enough photons to take a picture of a chemical reaction as it happens. It will see what's going in 10^-12 seconds. It's something that's never been done. It will be interesting. It will certainly be practical.

It's called medium energy physics. It's no longer high energy physics. It's medium energy physics.

So that's one of the things SLAC has diversified into - photon physics. This has been going on over a period of time, because in the storage rings...

Am I giving you too much?

Askarinam: No, this is great.

Godfrey: ... In the storage rings. The first was the SPEAR ring in the 70s. This was a ring that was 700 feet in circumference. As the electron circulated around one way, and the positron around the other way, they are being bent and hence they emit light tangentially; there's a little pencil beam of light that just swings around with the particle. Early on, people used this light, much of it comes out in x-rays, and they'd do x-ray experiments. Once again, very brilliant, all the photons going in one direction, much more intense than anything you'd get out of an x-ray tube. So they would do many many experiments, crystallography experiments, finding out fluorescing materials, with very narrow bandwidth fluorescing materials. Lots of experiments, many users, hundreds of users coming from the outside to use SPEAR. PEP was the next stage. SPEAR is now completely dedicated to synchrotron light. PEP, when it was going there was some synchrotron light taken off that ring. And now the LINAC, the coherent free electron laser, is going to be the ultimate greatest in this. Just orders of magnitude beyond what has been done before.

Also the lab has diversified into astrophysics, and that's where I am right now. About 10 years ago our group began doing astrophysics. A new astrophysics building has recently been donated by Fred Kavli, which has been named the Kavli Institute.

Askarinam: So he's donated...

Godfrey: He's a private person, who decided to do something interesting with this money. A very motivated individual. And now many astrophysicists, new people, have come here to SLAC, whose specialty is astrophysics, studying active galactic nuclei, quasars, galaxies, you name it - general cosmology, things in astrophysics. The project I'm on now, GLAST, is a satellite. SLAC now has photon physics, particle astrophysics, and a group doing high energy physics. They're going to go off and work on ATLAS at the LHC. So the lab is diversified. And if the International Linear Collider gets funded, a large group of SLAC physicists will participate. This will be a monster. This will be a 20-mile-long machine.

Askarinam: It's in Europe?

Godfrey: It is not decided where it will be built. Right now the countries who've put major funds into it are Japan, the United States, and Europe. The decision where it's actually going to be built hasn't occurred yet. If it's built in the United States it will probably be built at Fermi Lab which is a high-energy physics lab outside Chicago. If it's built in Europe it will probably be somewhere around Hamburg. But there's a competition coming up. This will be a multibillion-dollar project, the cost estimate is not out yet, but it will be several billion dollars at least. And will be very technically challenging to make.

It'll be 500 GeV in the center of mass to start off, 500 billion electron volts. Here at SLAC the machine got up to 95 GeV. There was a colliding beam ring at CERN, which got up to 200 GeV in the center of mass, and the ILC will start out at about 500 GeV in the center of mass and maybe it will double in the future. But what's happened is that you can't make bigger than what the CERN ring was. When the particle goes in a circle it radiates light, and hence energy. Circles are easy to make, because you can keep reusing the path the particle takes. For instance, you can put a little piece of pipe hanging in the vacuum. You make the pipe positive when the particle's approaching and negative when it's going away. Every time it goes through the piece of pipe it picks up energy. So you just have to have one little accelerator piece, or maybe several, and one ring of magnets. It's called a synchrotron. And so you gradually turn up the magnetic field to keep that higher-momentum particle, keeping the exact same path in a circle. Not bad, you don't need too many magnets, you just need a circle with magnets. The downside is the higher and higher you go in energy, the more light this thing is radiating around the circle, and eventually your electric power budget is breaking the bank. You can calculate the economics of it. You can imagine making the circle bigger, but it turns out the cheapest way to do it when you get above 100 or 200 GeV is now you make the thing in a straight line. It's one pass, and you have to have lots of little accelerator pipes in there. Actually they are cavities, like little coffee cans, you put in some microwaves. You make a standing wave. Inside the cavities the electric field goes this way, that way, this way, that way, back and forth billions of times a second. And then you time the whole thing, so that when a particle comes through the hole in the coffee can, the electric fields points that way, and the particle picks up a little energy. Then the electron leaves the hole and goes to the next cavity, which has been adjusted to be just right, so once again the electron gets accelerated. That's the way SLAC works, and that's the way the ILC will work as well. It will be a long straight machine. The gradients, SLAC's gradient is 20 million electron volts, so 20 MeV per meter. Let's see if I have that right. We're 3000 meters long. Yeah, so 20 times 3000 is about 60 GeV, so we have about 20 MeV per meter. What happens is if you try to put more microwave power? You can try to stuff more energy into the cavity and get a electric bigger field. Then you get in trouble. The electric field gets so high in the cavity that you rip electrons out of the surface of the metal. Then you get all these ���volunteer��� particles being accelerated at random times down the accelerator, and all the energy goes into making a low-energy blowtorch of particles coming out the end. These are not the high-energy particles you want, as well as destroying all the metal in the accelerator. So these machines have to get longer. Where SLAC is 2 miles, if you want to go up a factor of 10 in energy you have to make 20 miles long. The gradients won't be quite as large in the ILC because they're going to go superconducting to save power. It's not 10 times the energy for 20 miles long. It's only going to be five times the energy for being 20 miles long. Boy, you got the whole thing in a nutshell. [Laughs]

Askarinam: So what you like best about your job?

Godfrey: I enjoy building things, building pieces of apparatus, trying to design, and figure them out. Also enjoy being able to think about different things. It's a nice environment, people to talk to. There are many interesting things in physics going on right now. Very, perhaps, historical things. It's a grand puzzle. It's a spectacular puzzle that's been going on for many lifetimes, though it has accelerated in this century, the last century.

Askarinam: What do you like least about your job?

Godfrey: No answer...

Askarinam: So you're satisfied.

What's your career path, did you always work at SLAC, or where there other places you've worked?

Godfrey: After graduate school, I've been at SLAC my entire career.

Askarinam: Wow.

Godfrey: This is not the only path in physics. Some people work at universities and they postdoc at one place, they become an assistant professor somewhere else, a professor someone else. They're other ways to go about this, but early on, I joined the staff here at SLAC after being a post-doc here.

Askarinam: What's the biggest challenge you faced working here?

Godfrey: Biggest challenge... Hmmm... Lots of small challenges. There is quite a bit of pressure to have things built and working on time. Then whatever it is you're working on doesn't work, and it's like "why isn't it working" [laughs]. Now you have to get creative go off and figure out some tests, and learn a little bit more about the physics of the device you are working on. Try and figure it out and fix it. These are minor challenges along the way. Nothing not overcomable.

Askarinam: So what are your major responsibilities?

Godfrey: Well let's see, I've been part of this particular experiment GLAST. I was one of the people who thought experiment up. There were just a few of us back in 1991 or 92, who developed what the satellite would look like. I used instrumentation that we regularly used for accelerators, but was new to the astrophysics community. Then we got other people interested in this, and gradually it was funded by a partnership between NASA and the Department of Energy. They funded building the satellite and the launch is going to occur now in 2007, so it's been 16 years. Recently the part I played in this was part of integration and testing. The pieces of the apparatus were built all over the world at various institutions and then the pieces came together here at SLAC. The towers and calorimeters were assembled onto a grid, which was covered by an anti-coincidence detector.. The group I was in was doing particle testing. I got a small Van de Graff accelerator, which we made photons with to use to help to calibrate the instrument. We are going to take pieces of this instrument to an accelerator at CERN, later this summer, for further testing in a beam.

So that's part. And then I have some responsibilities for analyzing data that comes out. In particular, when the data starts coming down I'm going to be trying to be sure that the data's okay, some checks of that data as it comes down from the satellite, within a day. We'll know whether the data is okay, that there's no problem. After that I will be looking for gamma rays coming from the annihilation of dark matter. GLAST may or may not see them.

Askarinam: It's launching in 2007?

Godfrey: October of 2007.

Askarinam: From where?

Godfrey: Cape Canaveral in Florida. It will go up on a Delta Two Heavy. I've never seen one of these up close call before. I'm going to go down and watch it.

Askarinam: Oh great.

So do you face any business problems?

Godfrey: Business problems? At a research institution like this, the goal is little different than in industry. In industry, it's very important to put out a product that the public will buy, the consumers will buy, or your company will go out of business. So there's a lot of pressure to put out product. We produce a product, but it's a little different. There's an effort to disseminate to the public what goes on. So you'll see something on NOVA, or the SLAC website, or you'll see articles in Scientific American. Things go on to try to get people excited because this really is an exciting field. SLAC���s products are the discoveries of how the universe really works. You've read about black holes probably.

Askarinam: Yes. Fascinating.

Godfrey: You want know more [laughs]. That's part of it, because the money does come from Congress. It comes from your taxes, or your future taxes. There's an effort to convey these fundamental things that are being found. But the other consumers are just other physicists. There's a lot of internal review that goes on, talks at conferences, reviews within the lab of the project itself. Before the project even gets going, large groups of physicists from all over read the proposals and in various ways vote on them in a sense by various review committees.

Askarinam: So you're all well networked, and whenever there's a proposal you guys get together?

Godfrey: The field as a whole has pretty much agreed that GLAST was a good proposal, and that GLAST was one of the ways that money from NASA and the Department of Energy should be spent.

Askarinam: So what does it take to become successful in this field?

Godfrey: First of all, education-wise, math and science. You better eat it up. I mean, you better enjoy it and have an aptitude for it. I don't necessarily mean very abstract math, but calculus, all the standard math that gets used, a little bit of linear algebra. It should make some degree of sense to you. You need the foundation, and an enjoyment of science, the deductive process of it all. It is not random. [laughs] It is really a deductive field. Coming up with an idea of how something works, which is consistent with things that are measured in the past. Making new predictions, you see if your new picture... If the quarks, there really are quarks, well they didn't fly out across the room, well then they're different for some reason... Right now two big things are that it seems like something called dark matter is needed, and there's something called dark energy that seems to be needed. In order to make the equations which we're used to using, Newton's Laws and Einstein's field equations, in order to have them continue working the way that they work - they explain a lot of other stuff. The problem is this dark matter, nobody's detected it aside from its gravitational effects. No particle seems to have flown and interacted in a detector. No dark matter particle seems to have collided with another and annihilated, and made something you could actually see or work a detector. So there's something going on. There's a lot of people are out looking for dark matter. Trying to find the particle, to see if it has any interaction except gravitational. Dark energy seems to be this stuff that permeates all space, yet doesn't attract like normal stuff does. The Sun attracts the Earth. If you have a big gas cloud floating in space, it attracts stuff because it's got mass. But dark energy repels stuff. Space is filled with dark energy, which seems to repel stuff. It's being investigated how as the universe expands, does the dark energy just stay the same density, or does the density go down as space expands? So maybe these words are all wrong. Maybe there isn't a dark energy, maybe it's something else... perhaps the equations are wrong. It's all up in the air right now.

Askarinam: Like ether.

Godfrey: Yes. That was an idea which was around in the last part of the 1800s and it didn't work. It was good for some things, but it didn't work for other measurements and it got supplanted. So things are going to change.

Askarinam: So what is the overall job outlook in this field?

Godfrey: Not very good.

Askarinam: [Laughs] A very fast response.

Godfrey: This is not a growth industry. [Laughs] It was a growth industry during the Cold War. For our country's strategic interests, there was a push to educate people in the sciences. Particle physics rode on nuclear weapons for a long time. I never worked on nuclear weapons, but certainly as you went to higher energies and looked smaller and smaller, some people speculated something useful for weapons might be discovered again. The particle physicists of the 1940s were what we now call nuclear physicists. But at the end of the Cold War, that interest on the part of the country faded. Now there's a lot of talk about keeping up our technical competitiveness in the world. We should reinvest in the future of our science. What that means is funding post-docs, funding graduate students and research assistants. Having money so professors can get an NSF grant on a research topic that you are interested in. Funding has leveled off. There has been a promise now in the Bush administration that we will reinvigorate the sciences and put more money in, but let's see what happens.

Askarinam: Would you say it's a dying field?

Godfrey: Not dying, just constant right now, which in the long run means going downhill.

There are many of the aspects of science, but in high energy physics certainly the focus has shifted to Europe right now. Europe is coming on line within a year or two with the large hadron collider, which went in the old LEP tunnel. It was a colliding beam ring called LEP, large electron-positron collider, that was getting up to about 200 GEV in the center of mass. Europe replaced the magnets in that tunnel, since they had all the infrastructure, and it's now going to become a proton-antiproton collider. It���s going to go up to 10 TeV, trillion electron volts, in the center of mass. You couldn't get the electrons that high. The electrons would radiate away all their energy before getting anywhere near it. The protons don't radiate as much because they're more massive than electrons; they're 2000 times more massive. They eventually start radiating at some ultra high energy, but not until higher [than electrons]. You win because you use protons to go to 10 trillion electron volts in the center of mass. That's an enormous energy. That's about half of what the superconducting super collider was going to be in Texas. That was going to be 20 TEV in the center of mass, but then was cancelled. That was kind of the beginning of the end of high-energy physics. That was the future of this country. Then the focus switched to Europe. It's unclear what happens now with the ILC, the International Linear Collider. It may not go in the US. The US would have to make a pitch for it. It's unclear how much the government wants to support this thing, we'll see.

So the funding in this field has leveled. High Energy Physics is still intensely interesting, though if you're in it, you'll participate in a very large experiment. You may be even lost in a very large experiment. A thousand people experiment may not be you pursuing your individual research idea. [laughs]

Askarinam: Do you think that sometimes in working with so many people on a project that you kind of lose track of the overall mission?

Godfrey: Yes, absolutely. All these people have one little piece of it. If they try, they can keep their eye on the goal, but still the goal may only be one or two little things. For instance, here at SLAC, we have the last detector on PEP-Two. It is called BABAR. The main thing it is looking for is CP violation in B decays. You sit at a particular energy in this machine, at nine GeV in the center of mass, pardon, about 10 GEV in the center of mass, and make B B-Bar mesons, which then decay. You look at these decay products and measure these mixing angles. I'm not going to describe all what it is, but it's a few numbers. The SLD, which was the detector at the SLC, which was the previous colliding beam thing at SLAC, the big thing they were measuring was Sine Theta Weinberg. This particular weak angle, which they got to several decimal places better than anybody else has gotten. But it's one little thing that hundreds of people are focusing on and trying to measure. Maybe some mavericks will go off and will search for fractionally-charged quarks, but it's not the main thrust of it and they're probably not successful at finding anything.

Askarinam: What are entry-level positions in this field?

Godfrey: The typical way that somebody gets into this is that they are graduate student. They do their thesis on an astrophysics or high energy physics topic. Then they become a post-doc, usually at another institution, but perhaps the same one. And then they either go out into industry. Or perhaps they become an assistant professor. If they are outstanding, have done original research and presented this research, they become an assistant professor and perhaps work their way up to a faculty position from there.

It is not that someone sees an ad in the newspaper for assistant professor. It normally doesn't fly that way. Or somebody just might take a job at one of the labs as an accelerator physicist, and that would be their career.

Askarinam: So no one can come in here and just say "Hey I am very interested in this type of work and I would like to work here."

Godfrey: They could give it a try. A lot depends on your academic background and recommendations from people you might have worked with before. How else does somebody know about you... Someone comes in and says "Boy, I really like science", [laughs] "OK, here's a quiz on science."

Askarinam: It's more about getting involved with the field than people really pulling you in.

Godfrey: Yeah, you kind of come up through a long process. There's a lot of stuff, a lot of knowledge that people pick up.

Askarinam: So is everyone in here a physicist or do you have also engineers to help you design...

Godfrey: Oh, yes. Yes. Now one of these projects is not just a physicist or two who goes up and builds it. For instance, GLAST had an army of engineers and techs, quality control people and engineers of all disciplines: electrical, mechanical, quality control. Whereas in the old days, I would just go off, go to the machine shop and build something. Now I go and talk to an engineer and describe what it is, and he will design it, calculate the bending moments of all the pieces of metal. They'll do a professional design with drawings and submit it the to a machine shop. The scale of things are beyond a physicist going off and doing it himself.

Askarinam: So now it's for you guys are behind the theory aspects, and you tell them what you want to do, and they just go ahead and design.

Godfrey: Right, and you have meetings and you keep your eyes on what goes on. I basically understand what the engineers are doing. But these large experiments are engineered. Particularly the satellites going into space. You can't say "Oh, it didn't work, I think I'll fix it." Once the rocket takes off, any little thing can end it if it doesn't work. So devices are an extremely engineered and tested item.

Askarinam: So you guys have a crew of engineers that work with you guys?

Godfrey: It became a project. There is an overall engineer running the project. And then a hierarchy of various engineers hired going down in a pyramid management structure.

Askarinam: So what advice would you give someone seeking to learn more about career opportunities in this field?

Godfrey: Hmm... So for instance, you're a Junior? A Sophomore?

Askarinam: Sophomore, I'm transferring.

Godfrey: There's lots of areas of physics, there's laser physics, condensed matter physics, astrophysics, particle physics, device physics, tons of stuff. I would go and get a summer job working in a lab. Or work with a professor who needs help; work with him. They have some funds to help out. You'll get a small stipend working with them. That's the way it is at SLAC. We've got undergraduates working hard here. Then get into graduate school in the area that you're interested in. You might change your mind, but get in the area initially you're interested in. Join a group that's doing something you're interested in and go from there. The route is normally research assistant, graduate, then post-doc and so forth, or you get out of the academic track at some point and go into industry.

Askarinam: How is it for physicists to go into industry?

Godfrey: When you are younger, at your stage, you will be in demand. It used to be that physicists were very in demand because of their software skills. In the last fifteen years that's changed. Now there's lots of computer majors who can do software. You as a physicist, your value is you are a jack-of-many-trades. You are not the best person at designing something with transistors, or using a CAD program to lay out a device. However, you kind of know how to do it, and will pick it up pretty rapidly. But you also know more perhaps how a transistor works than the electrical engineer who knows all the parameters like the beta and the capacitive coupling. But you kind of know about band gaps and Fermi levels and if you put voltage on you can deplete it. You know many things about it. A mechanical engineer, he knows about Young's modulus, and where to go in the book to find the formula for how a beam deflects when it has a certain modulus. But you can actually sit down derive that. You're a little fuzzy to do it sitting with me here at the moment. But you know where to go and look in a book. You know something about moments. And you would actually go and do it with some calculus. You could derive the formula. Same with electronics. Many things, you are a jack-of-all-trades, and this ability is very very valuable.

You overlap into many fields. I find that I can sit down and talk to essentially anybody in any technical field and I know enough to get several layers deep in the discussion with them before we diverge.

Askarinam: So you have an overall picture of everything. You know enough about it to have an intellectual conversation with anybody.

Godfrey: Yes, and if something new comes along, that the engineer didn't learn how to do in school... Physicists are very useful because... You also pick up a technique of how to attack problems. You go out and ask stupid questions. You have absolutely no hesitancy about asking the stupidest question from somebody. And then you ask the next question, and the next question, and the next question. You'll pick up the technique if you haven't already in your labs, of just kind of grinding forward and solving something, or developing a new gizmo. Understand this technique, no bullshit. Understand it.

Askarinam: How are we doing on time?

Godfrey: We're doing fine. There's a 4:30 talk I want to go to.

Askarinam: What do you see yourself doing five or ten years from now?

Godfrey: Ten years from now, probably retiring. I'll be in my 70s by then. Or just seventy. Maybe, maybe not. I'm interested in looking at the data that comes out of this experiment, GLAST. There are some interesting questions, scientific questions, which we've been wanting to find the answers to. I don't know what the next experiment I'll work on after this is. I know there are things I like thinking about. I'm looking to do a different experiment in the future. There's an experiment here at SLAC called LSST, which isn't funded yet. Large Synaptic Survey Telescope. It's going to be a telescope that will have a meter diameter focal plane. It will take a very large angular picture of the sky. The focal plane will be covered with CCD detectors. The main thing it will study is weak lensing. Which is once again, looking at the dark matter that's around the universe. If there is a galaxy far away, and a galaxy in between you and that thing far away, the nearby galaxy will bend the light that's coming from the far away galaxy. It will act like a lens. There are strong lenses, in which you can actually get multiple images of the galaxy that's far away. The nearby galaxy is so close to your line of sight of the far away galaxy that light rays could come around to the left of it, and you could look to the left a little bit and see a perfectly good galaxy, or you could look to the right a little bit and see another picture of the galaxy. That's strong lensing. Then there's weak lensing, in which the mass in between isn't big enough or quite lined up well enough to completely make separate images of the far away galaxy. But it just kind of distorts the far away galaxy, it might change a circle into an ellipse. That's called weak lensing. So the primary mission of the LSST is to look at a gazillion background galaxies far away and measure their distortions and deduce from that all the matter that is in between, much of which isn't seen, because it's dark matter. And try to get the distribution of dark matter in the universe.

Askarinam: LST?

Godfrey: LSST, Large Synaptic, S-Y-N-A-P-T-I-C, Survey Telescope. Actually, I'm not quite sure what synaptic means. But the idea of what it's supposed to do is to take a very large angular area of the sky and take a picture of it all at once. That's not typical, and means it will have a very short focal length. Most telescopes get just a small, arc-minutes, angular bite of sky. You need to move the telescope all over the place and take many many pictures. It takes forever to do it. LSST is going to be able to see lots of sky quickly.

Askarinam: Will it have the same depth...

Godfrey: It will have a large area mirror. A usual telescope has a big mirror, so it gets a lot of light, but it will focus right on axis, at its focal point. So what a telescope does is it coverts angle, rays coming in at a slightly different angles, it converts each angle to a slightly different position. If the rays come directly on the axis, the focal point is right here on the axis. If the rays come in a little bit off-axis, the focal point moves over a little bit. But the problem with the telescopes is that the focal point of the off-axis rays is not on a plane where it is in focus. If you put a flat piece of silicon detector there, or a CCD, or a piece of film, the focal point may be a little bit in front of it or a little bit behind it. Only a little area in the middle is going to be in focus. The LSST is going to be designed so that over the whole square meter, everything is in focus. It represents a wide range of angles in the sky. Each different position on the plate is a different angle in the sky

Askarinam: So if you could go back and do everything all over again, would you do anything differently?

Godfrey: Oh, I don't know. I think for me this has been an enjoyable experience. There are still many things I stay awake at night thinking about, trying to figure out. Other paths would have been interesting, I can imagine doing many other things. This path has been satisfying and certainly it's been intellectually exciting. It's been fine for me. You better like science though.

Askarinam: What's your greatest research discovery?

Godfrey: Hmmm.... God, from the ridiculous to the sublime. Great research discovery? Me personally, nothing earth-shattering that's changed the course of science. You're not talking about Einstein here [laughs]. Things that I have found interesting, or that I have thought were accomplishments... Early on, an experiment called the Crystal Ball... Let's say even before that. I thought my thesis is interesting. We measured x-rays from kaons going about nuclei... A kaon is a type of particle. It doesn't live very long, only lives about 10 to the minus eight seconds, but it's long enough. If you take a negative one of these, it will come to rest in matter, and make a little atom with the nuclei in the material. It's a lot heavier than an electron so makes a very small atom, and the kaon jumps down through its Bohr orbits, just like an electron jumps down through its Bohr orbits. When it jumps between the different states, it emits lights, just like electrons do. The light is more energetic, because the kaon is closer. What we measured, the energy of these x-rays for lots of different elements, and verified that the Klein-Gordon equation really predicts the energy levels, and some other effects like vacuum polarization. So I thought that was kind of cool. In the Crystal Ball experiment, my first experiment here at SLAC, we collided electrons and positrons. It had just been discovered that a particular energy, 3.1 GeV, lots of things started happening.

Askarinam: When you collide to get 100% energy, right?

Godfrey: Yes. 100% energy, It's sitting still in the lab.

Just a year before, at a time called the November 1974 revolution, people were colliding electron-positrons at different energies. They would see a few events come off, until they hit this one particular energy, and all of a sudden, a thousand times more events per second started coming off. They went a little higher energy, and it stopped. And so there was a resonance at 3.1 GeV in the center for mass. It was called the Psi. This caused much consternation, because this resonance was very very narrow. It was less than an MeV wide. It turned out to be only 87 kilovolts wide. This was very surprising to people because all the high energy resonances we'd seen up to that time were much wider.

Askarinam: A resonance means?

Godfrey: A resonance means a bump. On the lower axis would be the energy, and the vertical axis would be the number of events per second you see. So you expect to see a bump. By the uncertainty principle, the wider the bump is, the shorter the time that the particle lived for. It's the delta-E delta-t business. When an electron and positron annihilite, it makes a quark anti-quark. The quark anti-quark stuck together and made some state. The state lived for a little bit of time and then it fell apart, and you see all this junk fly out. At this energy it was thought, "this is so much greater in energy than the mass of the particles that it can fall apart into, that it would fall apart rapidly." It would live for a very short time, and then fall apart into some 100 MeV pions. Three GeV falling apart into few 100 MeV pions? It's a lot more energy than you need to make a few hundred MeV. So these things should immediately fall apart. It should happen in ten to the minus twenty-two seconds. A very small amount of time. The bump should be a hundred MeV wide. Hundreds, that what all had been seen. Instead this was a very narrow bump.

Askarinam: So they expected a hundred MEV wide?

Godfrey: They expected hundreds of MEV wide. It should be really broad, if it was falling apart into normal quarks. Instead there was this unexpected little narrow thing. I was a graduate student at the time. "What in the world is this thing?" It turned out that a theorist, had predicted maybe there is another quark. So this thing could've been a charm and a charm-bar quark. It was not far above threshold, in fact it was below threshold. The charms were stuck together. The reason it was so narrow was that you had to wait for the charm quark to decay weakly, to turn into another sort of quark. Quarks don't turn into other quarks rapidly. They like to stay themselves. They're building blocks. And so it was gradually realized that this thing could be described as being a C C-bar quark. Well, if it is a C C-bar quark, it has energy levels. There should not just be two of them this close. A little farther apart there should be a little higher energy level, and a little higher. And it turned out as you raised the energy of the machine, not only do you see the Psi, but you see the Psi-prime and the Psi-double-prime and other higher energy states.

These quarks rotate. They each spin, and there are different ways they can spin around each other. Thus the quarks make the Chi state. It's a little different energy. We went out and we saw with the Crystal Ball, we saw transitions from Psi-prime to the Chi state down to Psi. We saw the gamma-rays that came off. It was predicted that such gamma-rays should come off, theoretically, if they acted like little atoms of C C-bar quarks. We actually went off and saw them, I thought that was really cool. They were about the energy and intensity that was predicted. That was the great success of the Crystal Ball, was seeing these. In terms of my curiosity I thought that was neat.

Askarinam: When you said they were expected that they would be 100MeV wide they turned out to be?

Godfrey: I think it turned out to be a few MeV. Very narrow. So it meant that it lived a long time. The narrower it is, the longer it lived. How could it live this long, if it was made of regular quarks? They would have fallen apart really quickly and made much lighter mass particles. They would have instantaneously fallen apart.

Askarinam: And it's because it take a while for the quarks to change?

Godfrey: That's right, you have to wait for the charmed quark to actually turn into an up or down quark. It's called weak decays. This was beginning of... People had known about at this time... Do you want all this stuff? I can ramble on forever.

Askarinam: We haven't gotten to up and down yet, I know a little bit about it...

Godfrey: You want me to ramble on?

Askarinam: Yeah.

Godfrey: Protons and neutrons... Well, a long time ago, back in the 30s, there were protons and neutrons and electrons. And then Anderson saw positrons and muons come out of the sky. People figured out there were some pions and muons. We're in the 40s now. Then in the 60s people thought that maybe there was a sub-structure to these things. This was Gell-Man, he said maybe there are things called quarks, mathematically described by SU3. There's an up quark, a down quark and a strange quark. He showed how you can put these things together, and make the things that you see. The protons and the neutrons...

Askarinam: So there are four different types of quarks?

Godfrey: Not at this time. Actually there are six now. But at that time only three were known... All the stuff we have around us is made of two, ups and downs. Gell-Man had this SU3, there's three, up, down and strange. For instance a proton is up-up-down. All the protons and neutrons are made of three quarks. These things called mesons, for instance pions, are made of a quark and an anti-quark. So people knew about mesons. Gradually in the 60s this zoo of mesons and baryons, baryons are things you can build with three quarks, came to be found. And then there were bubble chambers, where strange particles were found. Strange meaning that they tended to take a long time to decay. That was something changing; we now know that it is a strange quark changing its name. And his model predicted you could build all the things seen out of three quarks. All the things that he predicted you saw, and you didn't see anything that he didn't predict. It was very successful and that's called the quark model. It was SU3 and later...

Askarinam: This was Gambol?

Godfrey: This was Gell-Mann. G E L L M A N N. Two words, capital M. He got a Nobel prize for this. This was the world. Three quarks. Then in November of 1974, all of the sudden this narrow thing popped up, and all of the sudden there was another quark. And it was given the whimsical name the charm quark. Glashow had predicted that maybe there's a fourth quark and here are some things it would do.

Askarinam: Can you spell that?

Godfrey: Glashow. G L A S H O W. He predicted lots of things. At the same time the Psi was found at SPEAR, by the way, the Psi was simultaneously found at Brookhaven, by Ting. T I N G. He did it a different way, not in a colliding beam, but in a fixed target experiment. Ting named the new particle "J". So essentially simultaneously it was found here at SPEAR and it was found at Brookhaven. And Ting and Richter, Richter was the guy who got the credit here, R I C H T E R, Richter and Ting shared the Nobel prize of the discover of charm... J/Psi.

Askarinam: So Glashow predicted it, but Ting and Richter found it?

Godfrey: They weren't looking for Glashow's prediction, Glashow's prediction was buried in a blizzard of theoretical papers [laughs]. But it came to be realized that he had it right. He got a Nobel prize for that. So now the picture is... Shortly after this a lepton was discovered. Another thing occurred in the the colliding beam ring spectrum. You collide electron-positrons. You see the peak at the Psi, at a little higher energy it goes back down. And then you see another little peak which is the psi-prime. You go higher energy, and then all of the sudden it didn't go up and come down. It went up and stayed up. [laughs] I remember at the time being a grad student and thinking "God, what in the world is going on?" It turned out that charm had become free. You've got enough energy that the charmed quarks no longer stuck together. They can now go flying off, and they cloak themselves, you can't have single quarks. They pull other quark pairs out of the vacuum. This pulls an anti-U, and this one pulls out a U. They turn into regular baryons and mesons. But, in order to explain the rise, it also took a new lepton. Up until this time there was an electron and its neutrino, a muon and its neutrino. And all of the sudden now there was another lepton, tau and its neutrino. The Standard Model, as we now know it, has taken shape. The Standard Model is that there is an up and a down quark, and it for some reason it is associated with an electron and its neutrino

Askarinam: What do you mean associated with?

Godfrey: Well...

Askarinam: Electrons are not made out of quarks.

Godfrey: They're not. Forget the word associated. I'll just do the quarks first. There's an up and a down quark. There's a charm and a strange quark. At this point that's where things stand. There's an up and a down and a charm and a strange. And there's three little doublets of leptons. Doublets mean two. There's an electron and his neutrino, there's a muon and his neutrino, and there's a tau and his neutrino. All of these neutrinos are different. The electron, the muon and the tau, these are different things; they are different masses, Half-MeV, 100 MeV, 1800 MeV. A lot of different masses. So they are different things, but there are three little doublets of them, and two little doublets of quarks. And so the guess was "there should be another doublet of quarks." So that's what I mean by associated. For each doublet of quarks there is a doublet of leptons.

Askarinam: So leptons are just these three sets of pairs?

Godfrey: The leptons are the electron, the muon, tau and the neutrinos. They're different than quarks. Quarks have this strong interaction. For instance, they bind tenaciously to other quarks. It's called the color force. That's what holds them together, but that's something else. Quarks also never fly across the room, you never see a free quark. Because it turns out it is a very peculiar force between quarks. When they are very close to each other, they don't very much interact at all. It's called asymptotic freedom. This is weird. They're kind of free. But as you start pulling them a part, there's a spring between them, and the spring pulls harder and harder and harder. So by the time you get a little bit apart, you've stored so much energy in the spring, that the spring snaps. And the energy that was in the spring is used to create a quark anti-quark pair. And so you end up with a quark anti-quark here and a quark anti-quark here, and they look like just mesons. So when you try to bust quarks free, what happens is that the spring stores up energy, boop! It turns into quark anti-quarks, cloaks the quarks and makes them look like the particles you see every day. And it took a long time to realize this process.

Askarinam: So the actual energy that it takes to pull them apart creates new quarks?

Godfrey: Yes. The energy gets stored in the spring. This is very different than a plus and a minus charge. You take a plus and a minus charge, boy, they're really pulling together when they're close, but as you pull them apart the force gets less and less and there's nothing when they are far apart. Gravity's also that way. But this is a really weird force; this is the color force. The color force is just different than all your normal experiences. Not only does the spring pull back harder and harder as you pull it apart, which means its storing energy in the spring, but at some point you store up so much energy that it essentially snaps and turns into quark anti-quark pairs. You get enough energy to make new quark anti-quark pairs.

Anyway, so you have three doublets of quarks, and there's three doublets of leptons. And leptons are very different than the quarks. An electron flies across the room, we know all about it. We see them. We see muons, we see taus. They fly around. Not like quarks. Both these particles have the similarities, the quarks and the leptons, that they are building blocks. What does that mean? That means that no matter how close you look at them with a microscope or an accelerator, making very high energy particles to look at them, they are not fuzzy balls. They are not like a little distribution of charge. If we look real closely it starts looking like... How do you tell if a thing is a fuzzy ball or not? You can imagine taking an electron and scattering off it. The scattered electron gets so close, and then the charge repels it. You can't get closer than this. But now you get a higher energy beam. That gets even closer, then gets knocked away. The symptom is that you can get some events at wide angles. If I can come real close to this thing, I can get a lot of kick, and I can bend this beam at a wide angle. Eventually if it's a fuzzy ball, I'll actually penetrate the fuzzy ball, and now I don't get as big a kick because it's only the charge that's inside the radius that I'm at that is what gives me a kick. The other stuff doesn't matter. So I get inside I get a soft kick. If it's a fuzzy ball, I keep going up in energy, and eventually I don't get these wide kicks anymore. I only get these little kicks. Protons it was discovered, protons are fuzzy balls, except for these little seeds, the quarks inside them. This is how it was discovered that protons have structure, we scattered electrons on them. But quarks and electrons, at the highest energies looked at, are still giving kicks at wide angles. They are not fuzzy balls, they are building blocks.

They seem to have no sub-structure, there's no sub-structure to quarks and leptons. That's the similarity they have, they're building blocks. Not that people don't have theories, there are some theories around that maybe something makes up quarks or makes up leptons. But no experimental measurement has given any indication of that yet.

Askarinam: Quarks and electrons or quarks and leptons?

Godfrey: An electron is a lepton. Leptons is the class of things: neutrinos, electrons, muons, taus. Quarks are the class of things: up, down, charm, strange, top, bottom.

Askarinam: So electrons are a subset of lepton?

Godfrey: Yes, a subset of lepton. And leptons weakly interact with one other. It's a much weaker interaction they have than the strong interaction, the color force. The color force doesn't work on leptons. You're going to take a course and you'll get all this stuff in the course.

Askarinam: One thing I never understood is that electrons have zero volume?

Godfrey: As far as anybody knows they are point particles. As far as anybody experimentally knows... Why great theorists can wax poetic. You can try to make electrons up of little things. But no experiment has been at a high enough energy to probe... At the highest energy used leptons and quarks still look like point particles.

Askarinam: So is that why you are doing projects like GLAST? Because these high energies are not available here so you look at...

Godfrey: Not specifically GLAST. GLAST is not going to tell us if electrons have sub-structure or not. But that's one reason for the ILC. We're going to collide electrons-positrons at really high energy and see what happens. What gets made? There are many ideas - things might get made. There's things called super-symmetric particles; there's axions. Theorists have fertile imaginations as to what... There's various reasons for hypothesizing these things. Mathematical reasons. Maybe some new set of particles will come out. Super-symmetry, for instance, says the idea is that every particle has a super-symmetric buddy. If I said electron to you, there's a selectron. Stick an s in front of it. The leptons all have buddies called sleptons.

Askarinam: So how is... wait...

Godfrey: So what's different about sleptons?

Askarinam: What about a positron?

Godfrey: That would be a slositron [laughs], you stick an s in front of it... spositron? [laughs] And the idea is that where an electron is a spin-1/2 particle, known as a fermion, its super-symmetric buddy will be a boson. It will be a spin-1 particle. And of course it will be some different mass. And the theorists have some reason for this, which at the moment I don't have in my head, why all the buddies need super-symmetric partners. But the thought is that maybe they'll exist in this 100GeV realm. They'll be 100s of GeV in mass.

Askarinam: What's the difference between these symmetry particles?

Godfrey: How is a super-symmetric particle different than a regular particle?

Askarinam: Yes.

Godfrey: If the regular particle is a fermion, the super-symmetric particle is going to be a boson, and it will be a different mass. And it is something which is conserved. Super-symmetric particles only decay into super-symmetric particles. There's some conservation law involved. Now a super-symmetric particle and an anti-super-symmetric particle could annihilate, and at some rate they are predicted to go into regular particles, but it's a very small rate.

In fact, one of the things were are going to look for with GLAST is... One thought is that dark matter consists of the lowest mass super-symmetric particle. It has a name... neutralino. It's the lowest super-symmetric mass particle. If there ever were any of these things made, they gradually decay and you would be left with the lowest mass ones around. The thought is that these permeate the universe. This is the dark matter which is the majority of the mass of our galaxy. Ninety percent of the mass in our galaxy is in dark matter. There's this stuff which is missing that you need to make Newton's Laws work for how stars circulate in galaxies. Maybe it's these neutralinos. Particle theorists say neutralinos. People have been looking for dark matter in the lab. They go down at the bottom of a mine, and make a big detector and hope that a neutralino comes in, very very rare, comes in and knocks a nucleus apart in their experiment. They'll see a flash in their experiment. These have been going on for years, with enormous volume, big detectors. You got to go down in mines because all the cosmic rays will make your detector flash all the time if you don't get away from them. They haven't found any.

In GLAST, we're going to look at the center of the galaxy and other places where there should be lots of this dark matter. And maybe occasionally neutralinos bump into one other. And if they do, the theorists have calculated there's some rate at which it would turn into regular particles. Pions. Or might even turn into two gamma rays. Well GLAST is an instrument which looks at gamma rays. Maybe we look at the center of the galaxy and we see a certain energy gamma coming out. If we see a 100 GeV gamma, we see a line, this might indicate dark matter annihilation.

Askarinam: What is this going to tell you? Why are you interested?

Godfrey: If we look over there and see a 100 GeV gamma ray line, a bump in the energy spectrum, we've discovered dark matter. We've discovered it is annihilating, making two photons, that's the gamma ray line, we've discovered dark matter. And maybe super-symmetry is even right.

Askarinam: So basically it is the collision of two particles...

Godfrey: Two things that in order to go away need to collide with each other.

Askarinam: How do you know that it isn't some nuclear reaction from a star?

Godfrey: Because nuclear reactions are MeV. There will never be a 100 GeV line from nuclear burning in a star. If neutralinos are around, they are going to be up in the 100 GeV realm.

Askarinam: So basically you've done your process of elimination and said that the only way you can get 100 MeV.

Godfrey: 100 GeV

Askarinam: 100 GeV is by these particles that are colliding.

Godfrey: Well, if it's not that, somebody else will figure it out. But we've certainly got something interesting [laughs].

Askarinam: You have to find the source of these things.

Godfrey: The odds of seeing this are... Who knows? It might be pretty low. I personally think it's pretty low. But you got to look. There are many ideas. You have to look and see. How did we get onto this? How did we get onto super-symmetry?

Askarinam: I have no idea. [Laughs] We were talking about the leptons...

Godfrey: Oh that's right. Leptons. So these are the building blocks. There are ideas for other things around, maybe substructure, maybe super-symmetric particles, maybe a thing called the axion, another neutral particle that some theorists like. Gosh, maybe there's more quarks. It's unlikely there are more quarks, it turns out.

Askarinam: Why's that?

Godfrey: Because an experiment was done. It was done here both at SLAC and at LEP at CERN. When you collide an electron-positron it turns out there's a big resonance at 92 GeV. It's called a Z boson. I told you about the resonances down at 3.1 GeV. That was the quark-quark state. But there's this other thing up at 92 GeV. It's called the Z boson. It's one of the carriers of the weak force. All forces are carried by a particle. Electromagnetism is carried by the photon. The reason that you see something is because an electron jigged over there, a photon went over and jiggled an electron in your eye, and caused chemical things to happen.

Askarinam: And gravity is a graviton?

Godfrey: Gravity is a graviton. The weak force is carried by W's and Z's. W plus-minus and Z. They are called the weak bosons.

Askarinam: So those are being exchanged...

Godfrey: They are being exchanged, just like photons are exchanged to make things you are more familiar with.

Askarinam: What does W and Z stand for? Do they stand for anything?

Godfrey: That's just letters. The Z is 92 GeV and the W is 180 GeV.

Askarinam: Why did you say plus-minus?

Godfrey: Because they have charge.

Askarinam: Oh, they are charged particles.

Godfrey: Yes, they have charge as it turns out. Weird, we're used to having neutral. Anyway, there's a W-plus, W-minus and Z, Z is neutral. So you can collide electron-positron, make this neutral thing Z, and Z has the property that it can go to any quark anti-quark. It can go to an up up-bar, and it goes with the same rate it goes to a down down-bar, a strange strange-bar, a charm charm-bar. You can go to a top top-bar, and a bottom bottom-bar. It goes to all these things. Let's see, what's the top mass? Pardon me, the top mass, it doesn't go to the top. Two tops are more massive than the Z, so it doesn't go to two free tops. But it goes to up, down, charm, strange and bottom. It goes to all of those.

Askarinam: But wasn't there six?

Godfrey: There are six, but top is the sixth one.

Askarinam: What do you mean?

Godfrey: Up, down, charm, strange, top, bottom. There are six quarks, but Z can only go to five of them because the top is too massive. It can also go to neutrinos, pairs of neutrinos. It can go to electron-neutrino, anti-electron-neutrino, muon/anti-muon neutrino, tau/anti-tau neutrino.

Askarinam: If the electron and the positron are colliding, and we get 100% energy, how are we getting these other particles?

Godfrey: At just the right energy, at 92 GeV, it makes a Z. And there's a Z particle sitting there now.

Askarinam: For picoseconds?

Godfrey: Its width is 2.5 GeV. It's 2.5 GeV wide. It's very wide, so it decays very quickly.

Askarinam: So they are just unstable?

Godfrey: It's unstable. The Z lifetime is very short. Immediately it decays into a pair of quarks or pair of leptons or pair of neutrinos. Well, what's been measured is that there's only three generations of neutrinos: the electron neutrino, the muon and the tau. Those are the only things that the Z decays into. If there was a fourth neutrino that the Z could decay to, then the Z would be wider than measured.

There is no fourth neutrino. How does this mean the number of quarks? People have in their head, because of this pattern that seems to be there, for every pair of leptons, thing and its neutrino, for every little doublet, there's also a doublet of quarks. The measurement is that there is no fourth neutrino, therefore there is no little fourth doublet of leptons. Therefore, by this pretty picture, there is no fourth doublet of quarks. Do you see the picture? That's the idea. So people are not expecting a fourth generation of quarks. There's no super-tau, there's no fourth generation...

Askarinam: Because...

Godfrey: There's no fourth neutrino because Z didn't make them.

Askarinam: From experiment?

Godfrey: From experiment. By the way, the other fact, neutrinos are very small mass. Neutrinos, in fact, at that time, people thought neutrinos were zero mass. It's now known that they do have a little mass, but it's less than, or of the order of one eV. One electron-volt, Z is 92 billion. So the Z can certainly make a pair of one-electron-volt things. So it makes electron neutrino, anti-electron neutrino, it makes mu neutrino, anti-mu neutrino, it makes tau neutrino, anti-tau neutrino. And that's all. There's no other neutrinos it's decaying to.

Askarinam: Now these are all probabilities, they don't know what they are going to get each time?

Godfrey: It's not that this Z is going to go to neutrinos. You make a bunch of Z's and you measure the fraction which go each way. In fact this neutrino result, where it comes from is... you don't actually see neutrinos, they are very weakly interacting... the Earth doesn't stop one of these guys. So you don't see these. What you actually see is you measure the width of the Z. If it could decay to one more neutrino, Z would decay a little quicker, it would have one more channel to go into. So the width of the Z would be a little bigger. So by very carefully measuring the width of the Z, you can tell, calculate, that there's not another neutrino, there's only three.

Askarinam: But you have the anti-somethings.

Godfrey: It goes to a thing-antithing. It goes to a neutrino-antineutrino. That's its decay. But there's not a fourth neutrino. If there's not a fourth neutrino, there's not a fourth lepton doublet. There's no new neutrino and its buddy. And therefore there's no quark doublet. So no more quarks. There's a way around this. It could be that whereas all these other neutrinos are one eV or less, maybe the new one is 100 GeV. Well, but then a 92 GeV Z could not decay into two 100 GeV things, by conservation of energy. Ninety-two can't turn into two hundred. That could be a way out. We don't know that yet.

Askarinam: Quarks aren't the fundamental particles of leptons, right?

Godfrey: Correct. Quarks are not inside leptons, and leptons are not inside quarks.

Askarinam: We said leptons are neutrinos, electrons and muons?

Godfrey: Electrons, muons and taus, those are charged goodies. And then there is a neutral goody that goes with each one of those, which is the neutrino. There's a neutrino that goes with the electron, another neutrino that goes with the muon, and another neutrino that goes with the tau. So there's three different neutrinos that go with the three different charged leptons. They are little doublets, little pairs of two things.

Askarinam: And that's why you said they associate.

Godfrey: And each one of those doublets, for instance, the electron and his neutrino are associated with the up and the down quark. Pictorially associated. The charmed and strange quarks are associated with the muon and his neutrino. The top and the bottom quark are associated with the tau and it's neutrino. And that's where the picture stands so far. And nobody knows why there are the same number of leptons as there are quarks. Nobody knows why this pattern holds. Nobody knows why it should stop where it does, at three generations. Why does it stop at three generations? Why isn't there a fourth generation? Nobody knows. No explanation that has been connected to other things, connected to some mathematics, connected to some other measurements... It's just the way it is.

Askarinam: Has anyone started doing any math on any of this stuff?

Godfrey: There's no math around that even hits it, why there's three generations. At least not that I know.

You've reached the bottom of your list.

Askarinam: Yes.

Godfrey: You've certainly got enough to write a few pages.

Askarinam: Yes. Could we recap a little bit on the GLAST project?

Godfrey: We're hitting my clock.

Askarinam: I'm sorry.

Godfrey: It's all right, let's do five more minutes and wrap it up.

Askarinam: On the GLAST project we wanted to study the gamma rays because...

Godfrey: We're going to look at gamma rays which come from far away in space. We didn't even talk about astrophysics, we got stuck in particle physics. There are things out there that put out high-energy gamma rays, far above visible light. Visible light is like a couple of electron volts. But we're going to measure gamma rays between 20 million electron volts, and 300 billion electron volts. 20 MeV to 300 GeV. It's known that these come out of the cosmos. There seem to be sources of them. There are things called active galactic nuclei, they are sources of some of these, gamma rays. Black holes are thought to power many of these sources. There are solar-size mass black holes in our own galaxy, they can put out gamma rays. And then there are ones which are galactic-size black holes, 10^8 solar mass sort of things. There's some very science-fiction energy sources in the universe which tend to radiate the bulk of their energy in high-energy gamma rays. We're going to look at them, measure their time variation, just catalog them. Where are all of these things? Are they associated with optical sources? What is the shape of the energy spectrum? How fast does it fall as you increase the energy? People have some ideas of how these things... well, does it fit this fellow's ideas? He predicts a certain fall in the energy spectrum, a certain pulsation maybe...

Askarinam: I was only planning on an hour...

Godfrey: We missed out on astrophysics, but got lots of particle physics.