Peter L. Bender

Zierler:

Okay. This is David Zierler, oral historian for the American Institute of Physics. It is July 31^st, 2020. I am so happy to be here with Dr. Peter L. Bender. Pete, thank you so much for joining me today.

Bender:

Fine. Thanks.

Zierler:

Alright. So, to get started, would you please tell me your title and institutional affiliation?

Bender:

I’m officially a Senior Research Associate at the University of Colorado, and I’ve been a Professor Adjunct in the Physics Department in the past. I’m still associated with them as a Senior Research Associate. I actually work in an institute called the Joint Institute for Laboratory Astrophysics. So, it’s a complicated situation. The Joint Institute, now called JILA, is joint between NIST (National Institute of Standards and Technology) and the University of Colorado. It was one of the very early such institutes that were formed. And I’ve been here ever since.

Zierler:

And of course, you’ve seen Boulder grow from a small town into a pretty big city today?

Bender:

Right. It wasn’t such a small town in ’62, when JILA was formed, but it’s grown.

Zierler:

Pete, let’s take it all the way back to the beginning. Tell me a little bit about your parents and where they’re from.

Bender:

Okay. My mother was raised in Seattle, and her father was somebody who was very heavily into science. He was George B. Rigg, and he was at the University of Washington for many years. I remember her saying that he went out to Seattle initially in 1905 from Iowa where he’d been before Seattle. He was a specialist in bogs and in things like the forests in the northwestern U.S. In addition, in the 1940s he was involved in things having to do with the ecology of the Puget Sound region and the western United States in general. And so, there is a background in the family of some science. G. B. Rigg was really very well known in the field.

Zierler:

Did you get to know him? Your grandfather?

Bender:

Yes. And after my mother got her B.A. degree from the University of Washington, she went to Sweden for one year. That probably had something to do with interest in the family later in visiting the Europe and collaborating with people there. My father was raised in Minneapolis and, after working several jobs, one of them as the newspaper reporter in Chicago, he ended up at City College in New York. There he taught mainly English courses for students, many of whom were not from the United States originally. Anyway, my family lived initially in New York City up near where the George Washington Bridge is now. They later moved to New Jersey, just across the river, and so my early days were in northern New Jersey. And for much of the time I was in junior high and high school the family lived in a town called Leonia, New Jersey. It was just two miles off the western side of the George Washington Bridge. So that’s the background in terms of where I came from.

Zierler:

So, although you were born in New York, you grew up mainly in New Jersey?

Bender:

We moved from New York, I think, when I was about five or something like that. We moved first to a town called Montvale in New Jersey, and then after a while to Palisades Park. After about 3 years, we moved again to the next town, Leonia, New Jersey in 1939. Leonia has some history to it. It was a place that was close to New York and easily enough accessible that there were some well-known people who lived there. This was mainly because of the closeness of Columbia University. For example, Professor Urey, who was a Nobel Prize winner in chemistry, lived there. And his daughters were in the same school where I was. And Enrico Fermi and his family lived there for a while.

Zierler:

Now, Pete, you went to public school throughout your childhood?

Bender:

Yes.

Zierler:

And at what point did you realize that you were interested in math and science?

Bender:

I’m not sure that there was a particular time, but I certainly had some interest in mathematics starting in high school. One of my memorable high school teachers was a woman in biology, but I think she had quite a bit to do with encouraging interest in science in general. There actually was somebody in math who I think I learned a lot from. He was doing things like running a math club, and he encouraged us to think about things like the possibility of doing computing in four dimensions. I think things like that helped a lot to get me interested in science.

Zierler:

Would you say looking back that your high school had a strong program in math and science?

Bender:

Yes. I think so, for the times anyway. Yes.

Zierler:

And when you were thinking about college, were you thinking specifically about physics or would that come later on?

Bender:

Related to that, I had an uncle, David Williams, who was the husband of my mother’s sister. Dave was in aeronautical engineering at the University of Michigan. Quite early, I thought some about going to college there. The contact with him, I think, had quite a bit of influence on what I did later. But then he moved to Columbus, Ohio. I thought a little bit about going to Ohio State, in Columbus, but I ended up deciding to go to Rutgers instead. This was partly because it was more local, and because I got a partial tuition scholarship there.

Zierler:

And what did you major in at Rutgers?

Bender:

I took courses in a combination of mathematics and physics, but I actually was in physics officially.

Zierler:

But you’ve always had an interest in pursuing mathematics as well?

Bender:

Yes. That’s right.

Zierler:

In what ways, Pete, looking back, did all of your formal education in mathematics serve you well in your career in physics?

Bender:

It helped a lot.

Zierler:

In what ways?

Bender:

There were some things in mathematics that were fairly abstract that didn’t help much with respect to experimental physics. I’ve never gotten into very esoteric parts of physics, since I was an experimentalist. Mathematics was a supporting thing. I actually had an assistantship one summer while I was in graduate school where I officially was working with a well-known faculty member in probability theory. But he actually just had me reading up on statistical processes. For a while I thought about actually going into mathematics ultimately. But I think it worked out very well, from my point of view, with my ending up in physics.

Zierler:

Now, even as an undergraduate did you know you wanted to focus on experimentation?

Bender:

No. I didn’t.

Zierler:

When did that come? Later on in graduate school?

Bender:

I guess I thought about physics as being mainly an experimental field, but I hadn’t thought about what I might do in it.

Zierler:

So, what were some of your favorite courses that you took at Rutgers?

Bender:

I think there were a number of the physics courses that were quite good. There was quite a strong emphasis on low temperature physics at Rutgers, and some of the people connected with that field gave courses I was interested in. But I also had some good math courses there. There was somebody named William Boone, who was connected with more esoteric parts of mathematics, who encouraged me really quite strongly. I think that connection may have had something to do with my ending up getting into graduate school at Princeton. Getting into Princeton really was the major payoff from the time I’d spent at Rutgers.

Zierler:

And so, your master’s degree at Princeton was actually in mathematics. It was not in physics.

Bender:

But that was later, after the PhD actually.

Zierler:

Right. It’s out of order. How did that happen?

Bender:

I hadn’t bothered to get an MA earlier, and at the time I was a physics major. However, I decided to get the MA in mathematics late, because I was thinking about applying for a post-doctoral position in Europe in mathematics.

Zierler:

Why did you choose Princeton? Was there a specific person you wanted to work with, or you were just attracted to the department?

Bender:

I knew they had very good physics people, but I didn’t know anything about the individuals at the time. So, it just was a case of, from Rutgers looking out, Princeton was certainly the closest of the very well-known places in physics. And I think that was the main reason for applying there.

Zierler:

But as you say, you really didn’t have a very well-formed idea of what you wanted to do prior to going to Princeton?

Bender:

Right.

Zierler:

And so how did you develop a specialty? What professors at Princeton did you become close with?

Bender:

Well, when I went to Princeton, I had an assistantship, and it was just blind luck that it was with Prof. Robert Dicke. He turned out to be an excellent person to work with. I actually deferred for one year before starting at Princeton, and I did worry about not being accepted again to go to Princeton at the end of the year. But the delay was in part to go to the Netherlands to do research there. I got a Fulbright grant to Leiden University, which is a very well-known older university that had a major role in low-temperature measurements over many years. I originally didn’t think I knew enough of any foreign language to apply anywhere except England. However, it turned out, after I’d applied, that I got a message back saying basically that it sounded like what I was interested in could be done in the Netherlands. And there were a lot fewer applicants there. So, I switched and that worked out very well. I learned a lot from working in Leiden and was fortunate to still be assigned to work with Prof. Dicke afterwards.

Zierler:

And what was Prof. Dicke working on at the time that you got to know him? What was his research?

Bender:

He was one of the very bright young people who had gone to the Radiation Lab at MIT during the war. He, along with Edward Purcell, Norman Ramsey, Robert Pound and Vernon Hughes had a very large role in understanding the opportunities for the use of electronics in the war effort. But then, after the war they started applying things that they knew from there in various different fields. But the research areas involved were close enough together so that there was a lot of overlap. Anyway, when Prof. Robert Dicke came to Princeton, he was quite interested initially in just plain making improvements in atomic physics measurements: precision measurements, including, for example, the hyperfine splitting in the hydrogen atom. Actually, he had a student, Jim Wittke, working on an improved measurement of that. And I worked the first year or two that I was at Princeton on a project connected with that measurement. The hyperfine splitting measurement was precision atomic physics, but Prof. Dicke was at the same time starting to take a very strong interest in precision measurements that went toward testing physical theories, such as, general relativity.

That really ended up directing much of what he did for the rest of his career. Just after I got my degree, he started a gravitational physics research group. And the people involved turned out to be an extremely capable group. Just a few years ago the laboratories at Princeton that his group had had were named one of historical sites in physics by the American Physical Society. He really got involved in a lot of important things in gravitational physics, and that had a strong influence on what I had opportunities to do later on.

Zierler:

Pete, what was it like to work under Dicke’s direction? Was he approachable? Was it easy to go to him if you had a problem? How did you work with him?

Bender:

It was easy to talk with him, but he was involved in so many things that I didn’t see him a great deal. So, it really was just occasionally checking in with him, when he was going around to the laboratories to see how the different assistants were doing.

Zierler:

And how did you go about creating a dissertation topic for yourself?

Bender:

I didn’t really. I think Prof. Dicke just suggested it. It was something called optical pumping of sodium vapor. And the optical pumping was something where, with the developments in electronics, people had realized that you can manipulate the states and substates of atoms by shining in the proper kinds of polarized light. Thus, the thesis I worked on was optical pumping in sodium vapor. He had the idea that, if you had an optically pumped vapor, one could do precision measurements of things like the hyperfine splittings between different levels in atoms.

It turned out later that Prof. Dicke didn’t really keep up with the literature very closely, and that Albert Kastler’s group in Paris had started work in this optical pumping field a few years earlier. And Kastler’s group really became the world leader in that field for many years. It turned out that some of what I had done on my thesis overlapped with things that were done over there also, so it wasn’t quite as novel in terms of results as it might have been. But it came out fairly well, and I did get my thesis out of it. The thesis had enough of the aspects of precision measurements so that I was encouraged to apply for a post-doctoral position at the National Bureau of Standards (NBS) in Washington.

Zierler:

Pete, what do you see as some of your principal conclusions and contributions from your dissertation work?

Bender:

In terms of contributions of it, the fact that it was a little behind what the French group was doing meant that it didn’t have any immediate impact. I later did some more things that were fairly straightforward extensions of that, mainly with Earl Beaty at NBS. We were doing optical pumping in cesium, and there was interest in using the optical pumping to be able to measure the splitting between two particular cesium hyperfine levels with high accuracy. The cesium hyperfine splitting was one of the leading candidates for a new international frequency standard, and it ended up for quite a few years being the official standard for frequencies. Our measurements didn’t directly lead to that, but they certainly were relevant.

We then did a lot of work on rubidium, another alkali atom. And there we made use of a technique that seemed fairly obvious to us for increasing the population difference between the two sub-levels that we wanted to look at a transition between. That approach actually was used in the rubidium standards that have flown for many years on Global Positioning System satellites. So, there was a strong connection between the optical pumping work at Princeton and a practical application later. However, I didn’t stay involved in that field.

Zierler:

Did you have any idea when you got to NBS that you would spend four decades working for NBS?

Bender:

No. I expected to be there for one or two years and then go on to somewhere else.

Zierler:

And so, it was initially a postdoc?

Bender:

Yes. It was.

Zierler:

And what was your initial work when you got down there?

Bender:

It turned out that the project that Lewis Branscomb arranged for me to work on was a joint effort with Raymond Driscoll, who was on the staff there. He already knew a lot about measurements of the proton gyromagnetic ratio. This is a number that can be determined by how fast polarized hydrogen atoms in water will precess in a magnetic field. So, it’s a fundamental constant and is relevant to the general question of definitions of fundamental constants in physics. I worked with Ray Driscoll for about four years, measuring the precession frequency of hydrogen. Basically, what we did was to measure the precession frequency of hydrogen atoms that were in a glass sample of water that was sitting in the middle of a solenoid. The solenoid was precision-wound by Ray Driscoll and colleagues and gave an extremely calculable magnetic field. So, they were able, from very careful measurements of the geometry of the solenoid, to calculate what the magnetic field was, based on the current that was being put through it. Thus, we were able to measure accurately the ratio of the proton precession frequency to the magnetic field strength, which is the gyromagnetic ratio. That was, I think, a very successful measurement. Not too many years later there was a group in England that made even better measurements of it, but at the time it was a quite useful measurement. So, working with Ray Driscoll on that turned out very well.

For the gyromagnetic ratio measurements, we were bothered by stray magnetic field variations in the Washington area. So, we ended up setting up our apparatus for a while at a magnetic observatory in Fredericksburg, Virginia. And it turned out that the interactions I had with people there helped to prepare me for getting into several types of geophysical measurements later.

Zierler:

Pete, in what ways did your graduate training prepare you specifically for the work that you were doing initially at NBS and in what ways were you learning on the fly because this was new science to you?

Bender:

The preparation was very good because Prof. Dicke’s group really emphasized understanding what the limitations in measurements were and figuring out ways to minimize them. So, I didn’t have the feeling it was something completely different. The thesis work got me started on an appreciation for precision measurements of various different kinds, and a large fraction of what I’ve been involved in since then has been related to that.

Zierler:

And Pete, was your sense at NBS that the research culture there encouraged basic science, or were there specific projects that were mission oriented that you were expected to slot yourself into?

Bender:

Oh, NBS definitely was mission-oriented. Allen Astin and Ernie Ambler were the heads of NBS for quite a few years. However, they and other people in charge recognized when there were scientific benefits and that such measurements were a goal also. So, I think there was strong encouragement for doing scientifically valuable things, although certainly a lot of attention was paid to what the major objectives of NIST were also.

Zierler:

And when did your work transition from post-doc to a regular position?

Bender:

I’m really not sure whether it was after one year or two years.

Zierler:

Did you give serious consideration to moving on from NBS, or at what point did you realize you were quite happy, and you wanted to say onboard?

[ed. The name of NBS was changed a few years later to National Institute of Standards and Technology (NIST).]

Bender:

Well, I had gotten involved while I was there on some measurements of a number of things. One was measurement of magnetic fields, trying to make simpler and more accurate measurements of the magnetic field for applications in various areas. For example, one of the projects later on was trying to see if there were magnetic precursors to earthquakes. In principle if you monitored the magnetic field locally, there might be some change because of the changes of stress in the ground in a fault zone which would give you some preliminary information. We actually later worked on that for quite a while. Things like that led me to have quite a few contacts at Varian Associates in Palo Alto, California, and I thought about possibly working there. I actually interviewed at Iowa State University about the possibility of an assistantship there after I had been at NIST for maybe two years. But there were still interesting things going on where I was, and I ended up not ever getting an official invitation to go anywhere else.

Zierler:

Now, when did you actually move out to Boulder?

Bender:

That wasn’t until 1962. There were interests by several people at NIST in Boulder in areas in astrophysics. Even though they were at NIST, they had connections with the university, and they were active in the astrophysical community. There was a very active astrophysics community in Boulder. And Lewis Branscomb had interests also in the astrophysical direction. There were possible applications of atomic physics type measurements in fields like that and discussions started on the possibility of setting up a Joint Institute, either between the NIST Atomic Physics group and University of Colorado or with some other group.

This was a time when there was considerable interest in the possibility of moving some government activities out of the Washington area. It was around 1960 when there were strong international tensions, such as Cold War type things, so that concentrating too much scientific activity in the Washington area didn’t seem like a good idea. The group in physics at the University of Colorado was enthusiastic about our Joint Institute proposal, and they encouraged it strongly. Wes Brittain, who was chairman of the Physics Department at the time, was one of those in Boulder who was enthusiastic for the idea. However, there also was strong interest from a number of astrophysics groups in Boulder.

The Physics Department here had some activity going on having to do with rocket measurements. One of the people here was fairly heavily involved in some of the measurements that were being done with the V-2 rockets that had been brought over to this country or with other rocket platforms. And they were interested in the connections between the atomic physics and the astrophysics communities, and that helped a lot to provide sort of the basis for the formation of this joint institute. It ended up with there being an agreement in the summer of 1962 that the Atomic Physics section at NIST in Washington would move out to Boulder to form the Joint Institute for Laboratory of Astrophysics at the University of Colorado. So that’s the history.

Zierler:

Pete, what’s your sense of why Colorado? I mean, was NIST looking nationwide? Do you have a clear understanding of why they settled on the University of Colorado?

Bender:

I think the strong interest in astrophysics in Boulder had a strong influence. But also, Ed Condon, who earlier had been the head of NBS, was from Colorado. My impression is that he knew about the levels of astrophysics and physics activity in Boulder, and he encouraged the choice of Colorado. However really it was the agreement between the different people at NIST and the research opportunities in Boulder that led to forming the Institute.

When we moved to Boulder in the summer of 1962, my family consisted of myself, my wife Bernice, our daughter Carol, and our sons Paul and Alan. Bernice had some allergy problems living in the Washington area, and they were much less in Colorado. We liked living in Boulder and never regretted the move.

Zierler:

In what ways did moving out to Colorado change what you were doing, and in what ways did what you were doing continue on projects that you had brought with you?

Bender:

Initially, I stayed involved in some things related to frequency standards and fundamental constants. However, what happened in the next decade or more was influenced strongly by the arrival in April 1964 of a post-doc named Jim Faller, who had been one of the students in Bob Dicke’s group at Princeton. There and later, he developed methods for making extremely accurate measurements of the local acceleration of gravity, little g, i.e., how fast something will drop in the Earth's gravity field. He really made substantial improvements in methods for doing that, and over a number of years since then people working with him or associated with him have made great progress in that. And the instruments that they produced are, I think, the best you can find anywhere in that field. But that’s a different story.

When Jim arrived in Boulder, he had with him a draft of a paper suggesting putting optical reflectors on the moon. This was an idea that had resulted from discussions at Princeton in Bob Dicke’s group after I had left there. In those discussions, they were talking a lot about things like satellite measurements to check on various parts of gravitational theory. And making very accurate laser measurements to a satellite was one of the things that was discussed. Jim thought about such things and decided that putting something on the moon was really the right way to go.

In 1964, it was not that many years before the planned Apollo 11 launch in 1969, and so it was reasonable to think about putting something on the moon. However, several of us, including Bob Dicke and me, looked at the proposal Jim had made, and we didn’t think the lasers at the time were up to it. We thought that the laser pulses had too long a duration, and it would be hard to determine which received photons came from the reflectors on the moon. But two years later, in 1966, the Q-Switch lasers came along, and that changed the situation completely. It became clear that there really was a very reasonable possibility of measuring the distance to a reflector package on the moon using Q-switched lasers. Thus, a group of us put together a paper that was submitted to the Journal of Geophysical Research proposing this. It initially was turned down, but Bob Dicke had enough experience with things in the academic world that he knew we shouldn’t just quit at that point. So, he sent a message back to the editor and made the arguments for why the paper was worth publishing, and it got published. This really was what led to the whole program of laser distance measurements to the moon.

What happened after 1964 was that we tried to get money from NASA in order to study the possibilities of what could be done. Jim Faller’s original idea was to make use of one of the unmanned landers that were going to the moon, just because that would be the cheapest thing that could be done. But the lunar program had gone on long enough by then so that NASA was talking almost completely about the manned program. We next tried to get money to study a proposal for putting reflectors on during one of the early manned Apollo Program flights, but were told that they had already selected all the lunar surface experiments through Apollo 21. Thus, we’d have to wait until Apollo 22 had an announcement of opportunity to be able to do anything more.

We were able to get a little bit of funding for supporting some studies, but not very much. Thus, it didn’t seem like the project was going anywhere in the near future. However, Jim and some others did some studies of how you would design this reflector package in order for it to work well, if we should have a chance to fly it.

And then, in September of 1967, we were having a discussion meeting of our group at NASA Headquarters. Bob Dicke was there, Dave Wilkinson from Princeton, who was very heavily involved in precision tests of astrophysical principles and so forth and, of course, Jim Faller. While we were having our meeting, a NASA meeting was going on also. We heard the NASA people discussing the fact that they were worried about the Apollo 11 astronauts getting too tired working against the joints in the space suits. This was because other astronauts were having that problem in flights around the Earth and in training. So, NASA had just decided that on Apollo 11 they’d only allow the astronauts out on the surface for two hours. But the official Apollo 11 lunar surface experiment package that had been developed over a number of years would take four hours to set up. Thus, the Apollo 11 astronauts wouldn’t have time to carry out the planned measurements. The NASA people were sort of looking puzzled and we asked, “Well, could our reflector package be taken, because you would only have to set it up and orient it roughly in the right direction?” But they said, “No, because it’s not an approved lunar surface experiments package.” But then we heard a guy name Johannes Geiss, from Switzerland, talking about something where he was preparing a metal foil that would be put on a pole and then that would be placed on the lunar surface during the explorations during the two hour period on the surface. The astronauts would bring it back and it would be analyzed for high energy particles. Then Robert Dicke asked, “How come that’s still a possibility?” And they said, “Well, it's just a contingency experiment. That means it would be done only if the astronauts have time.” So we asked, "Could our experiment be carried as a contingency experiment?” And they sort of looked puzzled and said, “Well, maybe.” And 21 months later we had a retroreflector package on the surface of the moon.

It was sheer luck. If we hadn’t been at NASA Headquarters at the right time, I think we wouldn’t have heard about the contingency experiment as a possibility until too late to have done anything. It still took a lot of effort by Jim Faller and others during the in-between period to do the final detailed design of what the package would look like. But that worked out well, and the package was carried on Apollo 11. And after the astronauts had talked with the President and done their main tasks, they did still have time to set up our package. The astronauts actually were quite interested in seeing what happened as a result of this. So that was how we got a reflector package on Apollo 11. It turned out that our group, which we called the Lunar Ranging Team, was able to get retroreflector packages carried on Apollo 14 and 15 also.

For quite a while the University of Maryland served as the base institution for our group, and some of the funding was handled through it. Jim Faller was one of the two leaders of one of the two groups that tried to get reflections from the reflector package very soon after the landing. They actually were the group that was successful in obtaining return pulses, and that started out the lunar ranging program, which is continuing right up till the present. We were very lucky that the Lick Observatory was able to make available time for the initial measurements.

Fortunately, there was a fellow named Harlan Smith at the University of Texas who was the director of the McDonald Observatory in West Texas, and he was willing to allow time on their 107-inch telescope there and to plan on making continual lunar ranging measurements over an extended period of time. Measurements still are being made from there, and from other countries. In addition, roughly ten years ago, Tom Murphy and his group from the University of California, San Diego started up a new lunar ranging program. They got funding from both NSF and NASA to set up a new station, and I think they did everything right. This ranging station is at the Apache Point Observatory in New Mexico. They get time, I think, about three times a week on the main telescope there for an hour or so in order to make range measurements to the different reflector packages on the moon. There are the three from the Apollo Program, and the Russian-French collaboration provided two more on the lunar surface. And the San Diego group has done things so well that they’re now down to literally three or four millimeters in terms of the precision of the measurements from the laser transmitter on the surface of the Earth to a given reflector package on the moon. It is really amazing to me that they’ve been able to do that so well.

Jim Faller’s suggestion of putting reflectors on the moon and the success of the lunar ranging program were what got me interested in relativity tests and other precision measurements in space. This really, again, was through Bob Dicke. After the lunar reflectors were actually operating, we learned about a proposal that there was a test that could be done of what’s called the strong equivalence principle and that a fellow named Ken Nordtvedt at Montana State University had developed the theory for how you would do that. And it immediately became clear that that was going to be one of the important things that would come out from the lunar ranging. Over the years, the lunar range measurements and other measurements improved enough so that that test now has high accuracy. I don’t know the latest numbers, but it’s one of the most precise tests of gravitational theory that’s come out from the solar system. And we don’t think it would have happened without that lucky coincidence of getting on Apollo 11.

Zierler:

Pete, I’m curious if you followed the research that was done by Irwin Shapiro at Harvard?

Bender:

Very closely. We benefited a great deal from what he and his group did. As soon as it was clear that the lunar ranging was going ahead, they started collaborating directly with the lunar ranging team. And some of their people attended our meetings. At one point our group thought we were just about to the point of being able to say something about the strong equivalence principle test, and a paper was being prepared by the group. We didn’t see the effect at that time but were still worried about the possibility that something was going wrong with our handling of the data. Then, at one of our meetings, Irwin Shapiro’s group let us know that they actually were seeing a positive effect. And that probably kept us from publishing something completely wrong. When our people went back over the data processing, they found that there was a certain effect that had been ruled out as being too small. But that had been an error, and it had affected the results at the frequency we were interested in. So, the results concerning the strong equivalence principle were finally published as a pair of papers in Physical Review Letters, one of them by the Harvard Smithsonian group and then the other by our group. So, Irwin and his group had a lot to do with the analysis of the lunar ranging data. And he really has been one of the main leaders in the whole field of gravitational physics tests in space. That’s still an active area, and I think it will continue to be for some time.

Zierler:

You’ve done a tremendous amount of advisory work for the National Academy of Sciences and the NRC. How did you get involved in that capacity?

Bender:

Really most of my connection with advising NASA concerning precise astrophysical measurements in space came because Bob Dicke decided not to go to a meeting having to do with relativity tests. This was a meeting at JPL where he was invited to give a talk. The LLR strong equivalence principle test was intended to be the subject. However, he decided not to go, and he suggested I go instead. I went, and that really got me involved in discussions about different kinds of relativity tests and led to opportunities to do a lot of other things because of my being involved in those discussions. I ended up getting put on an advisory committee for gravitational physics and relativity. Nancy Roman set up that group in order to give advice having to do with things like gravitational waves and so forth. Rai Weiss from MIT was the chair of the committee.

There was a meeting of the committee in 1974 in Cambridge where a discussion about the possibility of gravitational physics measurements in space came up. This is something that I had heard vague mention of, but nothing more. But Rai said he had had an inquiry from somebody at Huntsville about the possibility of a gravitational wave experiment in space. This is just, I think, two years after Rai had written a report at MIT about the possibility of ground-based gravitational wave detectors. And the proposed measurements in his report were what ultimately led to LIGO and Virgo, and thus to the very successful last few years of ground-based GW measurements.

Anyway, I was on the committee, and after Rai mentioned the space measurement suggestion, it was discussed quite a bit over dinner that evening. Basically, Rai described what had been proposed. This was at a time when there was a lot of discussion about big manufacturing projects in space doing things like producing large structures made of aluminum tubing. Basically, what Rai had been asked about was building two one-kilometer-long aluminum truss structures, mounting them at right angles to each other, and hanging mirrors from the ends. Then the whole structure would be mounted on a satellite. Short pulsed changes in the mirror separations would be looked for in order to search for gravitational waves.

I forget exactly who was at the dinner, but there were several people who were quite active in gravitational theory there as well some experimentalists. Several of us said immediately that it didn’t seem to us that that was the way to do it: that you really ought to do it by hanging the retroreflectors on separate satellites. This was partly because we were familiar when the proposals for putting satellites in Earth’s orbit and measuring their orbits using laser pulses from the ground. That had come up in a number of discussions of various different things that some of us had had. We started out discussing separations of a few thousand kilometers between a triangular array of satellites. But Ron Drever from Cal Tech said, “Well, why stop there?” And very soon we were talking about going up to a million-kilometer separation between the satellites. This made it a very different sort of mission, and discussions of how it would be done continued through the end of the decade.

Zierler:

When did it become LISA, Pete?

Bender:

Let me put in some background information before I get to that. There was initially support from NIST for looking at this because the Director thought it would help in developing precision measurement techniques that would be helpful in other research areas. And then, in the late 1980s, NASA decided they needed to think some about a future program expansion, and they called it the Astrotech 21 Program. It was supposed to go through the year 2021 or something like that, and for a while there was money available for studies. And there was a workshop organized with participation by all of us who had been involved in gravitational wave space studies plus quite a few people from the Jet Propulsion Laboratory. Ron Hellings from JPL had gotten very much interested in gravitational wave studies, and he organized the workshop. The proceedings document from that workshop was published in 1991, and it really had all of the basics there for later studies. There was a lot of discussion of just what you could assume in the way of laser stability, and other things like needing to make the individual test masses in the space craft extremely free of disturbing forces. So, a number of those things were considered in a lot of detail.

But then NASA had some setbacks in funding. They stopped funding studies like this, and they cut off other funding for research going in this general direction, so that things at that point didn’t look good. However, a number of people in Europe had gotten interested in this whole idea, and there were people from Hanover in Germany, Glasgow, and a few other places who were very much interested. Thus, they put in a proposal to the European Space Agency for studies of a gravitational wave mission. It was under the medium-sized mission category of proposals, and ESA decided to go ahead with a preliminary study of this. But, although the initial proposal was for a joint study with the U.S., ESA at the time was unhappy about NASA having backed out of something that NASA had been planning to participate in. And so, they decided to have just the European groups participate in the study. However, three U.S. observers were invited to attend the study meetings. They were Tuck Stebbins from Goddard, who had been involved in the gravitational wave studies for many years, William Folkner from JPL, and myself.

In the first part of the study, the LISA proposal was compared with an alternate GW mission design called Sagittarius that a number of European scientists had proposed, along with Ron Hellings and a few others from the U.S. It was a geocentric mission, and this was expected to cut down on the initial cost. There was quite a long discussion of both approaches, but it was decided that the initial LISA design would be the subject for the rest of the study.

In the course of talking about making the proposal to ESA initially, there were discussions particularly with the people from Hanover and from Glasgow of just how to proceed. And during the discussion, I think that Jim Hough from Glasgow was the one who actually proposed the name Laser Interferometer Space Antenna (LISA) for the mission.

Unfortunately, the result of the ESA study was that the cost of LISA would be too much for a medium-sized mission, so the medium-sized mission proposal was turned down.

Zierler:

And Pete, what’s a medium-sized mission? What does that look like?

Bender:

There was talk about numbers like 500 million euros. Some people said it’s more than that, but I don’t know just how high their estimates went.

However, an opportunity to propose for a large mission then came up. I think that was early in ’94, and it was decided to go ahead with proposing LISA. The resulting study was very detailed, and it ended up with LISA being one of the three missions approved for the future ESA program. Additional studies of LISA were started, assuming that it would be pursued mainly as an ESA program. But at some point, when things were going slowly, the possibility of getting NASA involved officially in the mission came up. So, in about ’97 it was proposed that NASA be a joint partner with ESA, and that ended up being approved. From then up until about 2011, the mission was studied as a fully joint mission between ESA and NASA. However, at that point NASA announced that they would have to back out from LISA because of cost overruns it had had in other parts of its program. The delays on the James Webb Space Telescope were one of the factors.

After that LISA was pursued for a while as an exclusively ESA mission. But later the proposal was made that it be joint again to some extent. That’s been accepted by NASA, but the level of NASA funding is still uncertain.

Zierler:

Pete, can you talk a little bit about your work in geophysics and Earth sciences?

Bender:

Okay. That’s the other major area that I’ve been interested in, and it’s really what I’ve mainly been involved in the last few years. The studies on LISA are going ahead, but the majority of the studies are being done in Europe. ESA has an arrangement where, for studies that they’re strongly interested in, they arrange for two contractors to go over roughly the same ground so that they have two different opinions about how to proceed. And they went into this procedure for LISA for what they call the phase zero studies. Because of this competition between the companies, they don’t allow information about what details of the studies are to be made available. So, I haven’t really been in a position to follow closely what’s going on. There are some people from the U.S. who are involved closely enough so that they can take part in discussions about those studies, but it just hasn’t been feasible for me to do that.

So, recently I’ve mainly been thinking about things related to the Earth’s gravitational field. I got interested quite early in things connected with earthquake prediction. One of the things was the possibility of using laser measurements between places a few kilometers apart across fault zones in order to look for precursory displacements in the ground. That was something that a number of us in Boulder worked on quite a bit. Judah Levine at NIST and JILA took a major role in that. He also took over running a 30-meter interferometer that we had built in a tunnel in a mine, and used it as a geophysical instrument for many years in order to look for changes in that baseline. In addition to seismic signals and Earth tides, he was able to see things like strain pulses from nuclear weapons tests underground.

In trying to understand the local Earth tides, the 30-meter interferometer was really quite useful. The results were compared with those from a one-kilometer long strain meter that was operated in California. So, Earth strain was one area of geophysical interest.

Magnetic field studies also were interesting. We had a JILA post-doc named Randolph Ware who made measurements of the Earth’s field which had a high precision. Because of the possible use of such measurements in seismic zones, we got some support from the Geological Survey for that work. But then there was a magnitude five Earthquake in California where there were magnetometers set up close by, and they not only didn’t see any precursory signal, they didn’t even see any co-seismic signal. That was discouraging enough so that the Geological Survey backed off temporarily on magnetometer measurements. I think they’re still doing some magnetic field measurements under their earthquake study program, but this is mainly in regions where very large earthquakes may be expected.

[ed. The following section in brackets was written and inserted by the interviewee.]

[However, the main geophysical application that I got involved in was satellite measurements of short time variations in the Earth's mass distribution. In 1993, Ben Chao and Oscar Colombo proposed a particular design for a dual satellite mission where changes in the 500 kilometer separation of the satellites could be measured extremely accurately by laser interferometry. The changes in the separation would directly determine the variations in the geopotential along the orbit. From the measurements at different times, the changes in the Earth's mass distribution would be determined. Shortly after this, Ben Chao, Oscar Colombo, and I wrote a paper describing in more detail the accuracy that could be achieved.

In 1995 a proposal for such a mission was made, with Mike Watkins from the University of Texas as the P.I. The proposal was accepted, and the detailed design of the mission, called GRACE (Gravity Recovery and Climate Experiment), was started. There was strong pressure for an early launch, and there unfortunately was not time to space qualify a laser interferometry system to use on it. Thus, a considerably lower accuracy microwave interferometry system was used on the GRACE mission.

GRACE was launched in 2002 and immediately began obtaining useful results. The 30 day averages of the results started being used by many groups to keep track of changes in the mass distribution, including those in polar areas where changes due to the motion or breaking off of ice sheets was of particular interest.

I was also interested, along with Art Stolz and Doug Larden from the University of New South Wales, in the use of the Global Positioning System (GPS) satellites for determining crustal movements in seismic zones. This led to my collaboration with Clyde Goad and John Bossler from the National Geodetic Survey on a paper where we said that 1 centimeter relative displacement measurements over 100 kilometer baselines in seismic zones appeared to be feasible. In hindsight, that result was considerably too pessimistic. People are now getting measurement accuracies of 0.1 or 0.2 centimeters.

Separately from measurements to individual satellites from the ground, there was a proposal in 1969 by Milo Wolff of MIT to make much more accurate measurements of changes in the separation of two satellites in nearly the same low-altitude orbit. The idea was to use microwave interferometry measurements between the satellites to measure the Earth's gravitational field with high spatial resolution and to monitor changes in it due to motions of mass on the Earth. There was a Committee on Geodesy that convened a workshop in 1977 on the scientific importance of understanding such changes in the Earth's mass distribution. As a result of that workshop, the first recommendation for a "satellite geodesy mission" was made. The recommendation was mainly based on making it possible to follow changes in the water distribution around the Earth.

During the 1980s there were lots of studies done of possible satellite geodesy missions. One that was studied at length was for a quite elaborate mission called Gravsat. However, a much more modest study that was very useful was done by a group at the Johns Hopkins Applied Physics Laboratory. It was on how well you could do if you made K-band microwave phase measurements between two satellites. They concluded that changes in the satellite separation could be measured down to the micron level in this way.

Despite all these studies, no decision was made for some time. Then, in 1992, a proposal for a quite low-cost mission was put in to NASA by groups from the University of Texas and JPL. This was in response to a request from NASA for proposals for an Earth Explorer mission. The lead author was Mike Watkins from the University of Texas, and the proposal led directly to a detailed description of the GRACE mission. However, there was strong competition for this mission from other areas of Earth Science. Fortunately, Bill Kaula, who was one of the leaders in the field of geodesy, was on the review committee. And I think he did a lot to make clear the strong scientific justifications for the GRACE mission. After some delay so that a group from Germany could obtain authorization to participate, the GRACE mission was approved. There were of course studies of the detailed design of the mission. One of the open questions was whether to use microwaves or lasers for measuring changes in the satellite separation. A number of groups had suggested using laser interferometry, including first Ben Chao and Oscar Colombo, and then four of us from JILA jointly with Bill Klipstein from JPL. However, no laser interferometry between satellites had been done at the time, so there would be extra costs for the detailed design and testing of the laser systems. The people who were working on the GRACE studies tried very hard to get the laser interferometry included, but the cost cap on the mission and the strong desire for an early launch date didn't permit that.

When GRACE was launched in 2002, the results for 30-day or longer period variations in the Earth's mass distribution were quite valuable. The accuracy of the results was limited roughly equally by the noise of the KA band microwave ranging system and the noise in the accelerometers that corrected for spurious accelerations. But the results for medium and short wavelength variations in the mass distribution were better than had been possible with other approaches.]

When it became clear that the early GRACE results were quite valuable, Mike Watkins and others pushed through rapid approval for a GRACE Follow-On (GFO) Mission. The main justification for designing and flying GFO quickly was to avoid having a large gap between the GRACE and GFO measurements of the global changes in the geopotential. High-accuracy laser interferometry between the satellites was included, but improved accelerometers to correct much better for spurious accelerations of the end mirrors were not. This was because it was urgent to launch GFO as soon as possible. It finally was decided that, although the laser interferometry system would be included on GFO, it wouldn't be the primary system for measuring changes in the spacecraft separation. The primary system was still the KA band microwave ranging system.

Soon after GFO was launched, laser interferometry measurements were made which showed very quickly that the system could achieve very high precision for measurements of changes in the distance between the space craft. The precision was about two orders of magnitude better than for the microwave system. The laser system has continued to operate with just a few brief down times since then, so that they’ve gotten laser interferometry measurements between the satellites most of the time. A few papers are starting to come out now giving results from these measurements.

Unfortunately, the scientific results with the laser interferometry system were not nearly as much of an improvement as was expected. The results were limited strongly by the accelerometer noise for the accelerometers aboard the individual satellites, as mentioned earlier. However, it was recognized as early as 2015 by John Conklin at the University of Florida that a much simplified version of the extremely low noise gravitational response sensors (GRSs) that were being developed for the LISA mission could be designed and tested quite rapidly. The development of these simplified GRSs was started soon afterwards. Such simplified GRSs appear to be able to reduce the spurious acceleration noise in a third GRACE mission to the level of laser interferometer measurement noise.

Zierler:

What do you see, Pete, as some of the primary accomplishments of LISA and what work remains to be done?

Bender:

Although LISA is not in the cards until about 2034, the LISA studies have convinced people that very high precision laser interferometry measurements in space are quite feasible. But there still are some choices to be made in the final detailed spacecraft design.

Zierler:

When did you start to get involved in massive black holes and what’s the state of research today on massive black holes?

Bender:

When Jim Faller and I initially got interested in gravitational waves, it really was based on what the Rai Weiss group at MIT had proposed for ground-based GW detectors, and then on studies by various different groups that came out after that. But quasars had been observed earlier, where one extremely bright object near the galactic center could outshine the whole rest of the galaxy. A proposed explanation for those extremely bright objects was that they were where galactic material was falling into extremely massive black holes. However, nobody knew how you could grow black holes that were that big. It was only careful studies by many different groups over a few years that led to people believing very massive black holes really were the explanation. There were puzzling aspects observed in supernova explosions that led to suggestions that maybe a black hole was being produced there. I wasn’t following the details of what went on at that time, but the case was made for there being something like few solar mass or larger black holes being produced by normal mass growth processes. However, for quasars, there really wasn’t any explanation other than galactic material being pulled into super massive black holes that seemed to work.

So, the question came to be how could you grow something that was extremely massive at early times. Some of the quasars were very far away. At present, the situation is that there really isn’t a single solution. A number of groups have talked about growing the black holes just by successive collisions starting out from maybe four or five solar mass black holes, with enough of them merging together to form intermediate sized black holes, and eventually very massive ones. But there’s always been serious difficulty with making plausible arguments for how this could be done quickly enough.

When a 10⁹ solar mass black hole was discovered at a redshift of seven, that convinced people that growth from solar mass black holes was not the only possible explanation. Mitch Begelman at JILA has been heavily involved since the early quasar studies, since he was an author on what I think was the first paper about black holes being responsible for the quasars. In recent years, he and some colleagues have looked a lot at the possibility that, instead of building things up from stellar mass black holes, maybe there was something that would slow down the evolution of galaxies enough so that they would reach high density before the main star formation took place. And with this delay in star formation, you could end up with intermediate mass black holes forming early in the centers of galaxies that had just recently started to form stars. I think at the moment that this is an equal contender for how you produce the super massive black holes at early times.

Zierler:

Pete, can you talk a little bit about the connection between time variations in the Earth’s gravity field and hydrology? How do all of these things fit together and what’s your research in these areas?

Bender:

Okay. I talked some about the time variations earlier, but there is a lot more to say about the connections to hydrology. The basic problem is understanding how the charges in the mass distribution are related to the geopotential variations. As an example, in studies that have been done in hydrology, they start from all the other information that is available about changes in the rainfall coming down, transpiration of water from leaves, rain sinking down into the upper layers of the ground where the ground is fairly porous, how it gets from there down into deeper layers in the Earth, etc. Information on stream and river runoff also are included. But all of these things aren’t understood well, and there are questions like how the water drains out from the surface water layers or these deep layers into streams, how the streams combine into rivers, how the water flows out through those into the oceans, etc. All of those steps are things that people understand something about in principle, but there aren’t enough detailed and careful measurements at enough locations to be able to really understand what’s going on. The hydrology studies that have been done indicate that things like how long it takes for water to go from the ground level into streams and into the ocean can be uncertain by substantial factors. However, the results from different approaches to estimating the changes in the geopotential can be compared by looking at the satellite geopotential variation results, if the accuracy and the spatial resolution are sufficient.

What we and a few other groups have been looking at recently is whether you can use the geopotential changes along individual arcs, to compare the results from different geophysical approaches. We think that some of the studies that have been done, particularly by a group from the University of Newcastle in Australia, have been very useful in demonstrating more rigorously how you can go directly from measurements of changes in the satellite separation to changes in the geopotential along the satellite orbit. They’ve shown some cases where you can look at the short wavelength variations along track and measure features that you just can’t get from any of the longer-term average studies that are being done. So, we think that will help substantially in terms of justifying higher accuracy measurements, where you replace the accelerometers with something about 2 orders of magnitude more accurate.

Zierler:

Pete, what are some of the technical challenges associated with putting optical clocks in space and what’s the value of completing this kind of a project? How does it move the science forward?

Bender:

Neil Ashby at NIST and I have written up some things on this, and a number of groups are strongly interested in it. The problem is that it is fairly expensive because the surroundings need to be very carefully controlled for the present optical clocks. If there are breakthroughs made on how simple the clocks can be made, that would help a great deal. But I don’t know how long it is going to take for that to happen. Even though a number of groups are trying to push for something in that direction, in most cases, they’re just trying to get clocks in space in moderate altitude Earth orbits. That will be much simpler than putting a high-accuracy optical clock on a mission where the line of sight would go past the sun, in order to measure the Shapiro time delay still more accurately.

Zierler:

And what are the overall goals? What can be accomplished by having a viable program of having optical clocks in space?

Bender:

The justifications may rest mostly on practical applications, like the Global Positioning System. From the scientific point of view, the problem is that there are a number of the main issues people worry about where you don’t have any measurement you can point to and say, "if we measure this with such and such an accuracy, it’ll solve that problem." Dark matter is one of the major theoretical concerns at present, plus anything having to do with the question of interactions between quantum mechanics and gravitational theory. For dark matter, the problem is everybody agrees that understanding that should have a high priority, but each of the possible candidates that have been suggested so far is quite uncertain. You just don’t know what level you’d have to make the measurements to in order to find out something interesting. So, it’s a little hard to predict ahead of time which things should go ahead the most rapidly.

Zierler:

What is the BepiColombo Mission and how will it move the field forward in relativity research?

Bender:

Neil Ashby and I have been very much interested in that. This is partly because we at one point had a Visiting Fellow at JILA named Stan Peale. Stan had a great deal to do with studies in planetology, and he was the one who predicted shortly before one of the missions to the neighborhood of Jupiter that its moon Io might possibly have volcanic activity on it. This was because he’d done studies of the interiors of planets, and of the question of at what point you get a molten core. And Mercury was one of the main planets he was interested in.

It turned out that he came out with the report about possible volcanoes on Io just before a mission flew by there and saw them. So that increased the interest in this general field a lot. For Mercury, there really were arguments, pro and con, about whether it would have a liquid core or not. There were some things that seemed very convincing about there being a liquid core. But other arguments having to do with the elemental composition it was likely to have had when it was first formed went against that. Stan had looked into this issue, and when we started talking about measuring things like distances to other planets, he was very much interested in what we could do with respect to Mercury. So, we started looking into that, and we wrote a few papers about putting something down on the surface of Mercury, to which you could make microwave [distance] measurements with high accuracy. But it turned out that it was going to be a long time in the future before there was a lander, and orbiters started being proposed. At one point there was an opportunity for proposals to ESA for a Mercury orbiting mission, and there were several proposals made, including one from JPL that I got slightly involved in. But the winner was the proposal for BepiColombo, where the satellite that was put into low orbit would be down low enough so that you really could learn things about the gravity field of Mercury very well from it.

Zierler:

Well, Pete, now that we’ve worked our way up to your current research, I’d like to ask you some broadly retrospective questions about your career. Sort of big basic questions, and the first is we haven’t really talked about the value or the role of the growing computational power over the course of your career. In what ways have computers enhanced or aided your research or have even provided opportunities that might not have existed otherwise?

Bender:

I think computer development provided a lot of opportunities for people I collaborated with, but not for me personally. So, I benefited from it certainly, but indirectly along the way. I’ve never been good at doing computations myself. I haven’t done any large number crunching type activities.

Zierler:

What have been some other major technological advances beyond computers that have been fundamental for your research and career?

Bender:

Well, the lasers have been a large part of it, along with the development of the high-precision microwave measurements to distant spacecraft, and things like that.

Zierler:

And in what ways have lasers advanced specifically?

Bender:

Well, going to Q-Switch lasers was a huge advantage, and the things like the optical frequency chains that Jan Hall at JILA was one of the two proposers and developers of. The development of optical clocks in space also is an important possibility from my point of view. But I don’t have anything else really particularly in mind.

Zierler:

Looking back on all of your work in gravity what do you think are the long-term prospects for incorporating gravity into a grand unified theory?

Bender:

I really think that there will be progress in that direction, but I just have no idea about how long it’s likely to take. I suspect this is still a long way in the future.

Zierler:

But do you think that it’s possible that it will happen, or perhaps it’s never going to happen, or it’s just beyond human understanding?

Bender:

Well, I don’t think it’s beyond human understanding, but I’m really not enough of a theoretician to give you a sensible answer to that. Actually, in terms of what’s been most important to me, it’s that I’ve ended up being able to interact with a very large number of people doing very interesting research. Jim Faller has been a major part of that, and Jan Hall also. There was a guy named John Wahr in geophysics who I had a lot of interaction with and who was very influential in the field, and Bruno Bertotti, a theoretician from Italy doing gravitational theory. Bruno was a Visiting Fellow here and we had a lot of common interests. Dave Wilkinson from Princeton also was a Visiting Fellow at JILA and had the office next to mine for a while. He’s best known for what he did concerning the cosmic microwave background. The Wilkinson Mission is one of the orbiting missions that’s determining variations of the cosmic background. So, there have been just a huge number of people I've benefitted from working with. When I started thinking about people I've been heavily involved with, there probably are 50 or more people I've benefitted strongly from working with. There was really a lot of luck involved in who I've had a chance to work with, starting out with Robert Dicke and going on through many other people like Lewis Branscomb who was the initial head of JILA, and so forth.

Zierler:

Pete, if you look at your body of research in geophysics and you look at your body of research in astrophysics, one quite literally is in this world and one is out of this world, right?

Bender:

Yes.

Zierler:

In what ways are you asking very different questions and setting up experiments in very different ways in each of these fields, and in what ways are you doing research and asking questions that bridge that divide?

Bender:

Well, there really is a lot of overlap between the LISA mission and future Earth gravity missions. But the overlap between them is in terms of the experimental measurement techniques.

Zierler:

And what does that overlap tell us more broadly about physics?

Bender:

Well, I don’t know. You make use of things that you can produce in the laboratory in order to investigate scientific questions. You send out space missions, if necessary, in order to get closer to what you want to look at. One of the areas for the future where I just don’t know what’s going to happen has to do with looking for indications of extraterrestrial life on other planets. There, my feeling is that that certainly is an important and significant scientific question, and the measurements that are now underway are very reasonable. I think measurements for signs of life in terms of oxygen lines and lines from a few other elements from other stellar systems certainly are worth looking for. The only thing I see on the negative side is that if there isn’t anything significant found within some number of light years, I just think at some point there’s just little chance of being able to find any signs of life in a reasonable length of time. So, I think that there are strong grounds for not neglecting other areas of science compared with looking for extraterrestrial life. The search for locations suitable for life certainly is an important and publicly very popular subject now. But it’s not clear to me that it will continue to be as important in 30 years or so.

Zierler:

Pete, in what ways has your affiliation with NIST and JILA allowed you to do things that you might not have been able to do had your career trajectory been more traditionally academically oriented in a physics department?

Bender:

I think it’s been very helpful to have connections with both NIST and the University of Colorado. The NIST side of things has provided a great deal of support, the Physics Department has certainly bloomed over the years and is now a world leader in quantum optics, and astrophysics in Boulder has been very strong since the early days.

Zierler:

What are you most excited about for the future, both near term and long term, in terms of where the research that you’ve been involved in is headed?

Bender:

Long term, I'm strongly interested in both astrophysical measurements in space and Earth sciences. However, in the short term I'm mainly interested in working in the Earth science area. This is because I think that there are chances of a fair amount of progress in a fairly short period of time.

Zierler:

Why is that? Why would there be that prospect?

Bender:

If groups in the United States and in Europe are able to propose either a joint GRACE-3 mission or complementary missions, there’s a possibility for something very good being done. Actually, one of the things that’s sort of amusing is that I was a coauthor quite early on a paper with David Weitz and Steven Nerem from Colorado where we talked about future measurements of the Earth’s gravity field. It would be very valuable scientifically, we said, if you had two pairs of space craft, with one in a polar orbit and the other in an orbit inclined by about 70 degrees with respect to the ecliptic. This is because, with the polar orbit, you get information about changes along the orbit, but it’s always in the north-south direction. Whereas, if you had a second pair at an inclination of about 70 degrees, that would help a lot to give important information about geopotential variations in the east-west direction also. And that approach has been studied a lot in Europe, and I think it’s also included in some of the U.S. studies. However, my own feeling at the moment is that getting the acceleration noise on a GRACE 3 spacecraft down to a lower level is an even higher priority than that.

Zierler:

Why is it a higher priority?

Bender:

It’s just that if you went to two pairs, you’re stuck probably with the old-style accelerometers for the first one, and you lose a lot in the accuracy of the results. Thus, it really would affect what can be done over not just the next 10 years, but probably the next decade after that also. So, I think that just going ahead without reducing the acceleration noise limitation to two pairs probably is not the right order in which to do things. Thus, I would put the higher priority on the lower acceleration noise. But two pairs is basically a good idea. No question. We just happened to publish something on it before somebody else. This was because there was a conference I was giving a talk at something like that.

Zierler:

Do you think NASA is well positioned into the future to continue doing fundamental research?

Bender:

I think there are pressures on it to go in somewhat different directions, which is unfortunate.

Zierler:

What kind of pressures and what different directions are you referring to?

Bender:

Well, something where you can get a number of congressmen from different places to support your proposal will do better.

Zierler:

[laughs]

Bender:

The scientific community gets listened to a lot, and I think the NASA heads have tried really hard to take advantage of that. Thus, I think that NASA’s reputation has been a lot better than it could have been, given the outside pressures that they sometimes have.

Zierler:

What do you think about the influence of privately funded space missions? Are they important for fundamental science or are they more of a distraction?

Bender:

I am worried about things like the very large numbers of communication satellites making it much harder to do astronomy measurements from the Earth’s surface. There are proposals for putting up thousands of spacecraft at high enough altitude so that they would be continuously illuminated by the sun and therefore interfere with astronomical observations even at night-time on the Earth. I think that regulation is going to be needed in terms of what gets put into space. Ultimately, mining operations on asteroids or something like that certainly may be a possibility for being important, but I think that conserving the resources on the Earth’s surface really ought to have a much higher priority. I suspect that it will be a long time before asteroid mining operations really become economically desirable.

Zierler:

What do you see as the prospect for international space collaborations? Do you see as the wave of the future large-scale space missions not being so nationalistically defined?

Bender:

I think there will be some missions where several spacecrafts are put up by different countries to collaborate. However, I think there really are difficulties in getting a number of countries to make their contributions on the same timescale and to make joint decisions. This is because the operating procedures are different. We’ve seen a little of that between ESA and NASA. ESA has different procedures from NASA for how to pursue things, and some of them are very good. However, there are some aspects of the procedures that really cause problems.

Zierler:

There’s only limited resources for the future, time, money, interest, collaborations. For you personally, of all of the things you’re involved with, what do you see as most important in terms of driving basic research forward?

Bender:

That’s really a bigger question than I have much feeling about. I think the limitations on what’s going to be possible in future space research are going to come from what the conditions are on the Earth, in many different ways. It’s hard to say which are the Earth’s worst problems, but certainly wars and inequality of resources in different countries are at the top of the list. However, I think overpopulation also is a strong contender. If population growth gets under control, I think then the prospects for space research will be a lot better.

Zierler:

You’re referring to all of the problems that can continue to happen and get worse because of overpopulation and the problems that that would create for science continuing to advance?

Bender:

Yes. Everything really depends on the overall economic conditions on the Earth and on the relationships between people in different countries. In terms of what’s really important, I would put things having to do with equal opportunity and with race relations a lot higher than the research opportunities in science.

Zierler:

Perhaps what’s going on currently is making you think along these lines? This is a very strong reminder that science is fundamentally a human endeavor. It’s not separated from all of the other problems and challenges that we face collectively.

Bender:

Right. As an example from my own family, my sister Mary Ellen Bender was a student at Bennett College in Greensboro, North Carolina in April, 1960, when students were trying to find a way to protest against racial and other discrimination. Four students were the first to sit in at a Woolworth in Greensboro, and Mary Ellen then became the first woman and the first non-minority person to sit in. I've never done anything like that myself, but this was an example of the efforts by many people that are now going on to protect the rights of minorities and to recognize the need for eliminating the welfare differences that now exist between different parts of the world.

Science is an interesting thing, but it is not the most fundamental thing.

Zierler:

Well, Pete, on that note it’s been an absolute pleasure spending this time with you. I want to thank you so much for doing this with me.

Bender:

Fine. I appreciate your patience.

Zierler:

My pleasure.