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Oral History Transcript — Dr. Robert Cahn

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Interview with Dr. Robert Cahn
By Ursula Pavlish
At Lawrence Berkeley National Laboratory
August 2007

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Robert Cahn; August 2007

ABSTRACT: Cahn’s tenure and support of The Supernova Cosmology Project (SCP) as Director (1991-1996) of The Physics Division at Lawrence Berkeley Laboratory (LBL). Reviews of The SCP. Saul Perlmutter as building a new field of research in distant supernovae. Astrophysics in Berkeley. Style of research in The Physics Division at LBL. On discoveries as gradual and the importance of statistics and systematics. Pentaquark discovery as an example of error. Physical Review’s policy of what constitutes ‘evidence’ (three sigma) and what constitutes a ‘discovery’ (five sigma). Historical example of the discovery of the neutron. Historical example of the discovery of the Psi particle. Two milestones in the discovery of positive Lambda: the first distant supernova and then, finding batches of supernovae. Use of The Hubble Space Telescope by High-z Team and SCP. Controversy heated because of the possibility of winning The Nobel Prize.

Transcript

Pavlish:

I am here at Lawrence Berkeley National Laboratory to interview Professor Robert Cahn. Thank you for your time today, Professor Cahn. I would like to ask you what your role in The Supernova Cosmology project was.

Cahn:

Let us start by my first saying that I am not Professor Cahn. I am Dr. Cahn. I am a senior scientist at LBL. My primary connection with the project, certainly in its early years, was that from October 1991 until November 1996 I was the director of the Physics Division. The Physics Division, at that time, did primarily High Energy Physics but with some work in Cosmology, probing the Cosmic Microwave Background. I was pleased to see that effort recognized in a Nobel Prize. And we also did work on supernovae. So, I was not a scientist doing this work but I was pretty directly involved in it, directly involved in the issue of its funding.

Pavlish:

Did the team members give presentations to you?

Cahn:

Not really in that sense. I was the guy with the money, so people would come in and talk to me because they needed money, getting their projects supported. I was not getting any regular briefings, but I kept myself informed by talking to the people who were running the project. Initially, when I took over, the people really running it were Rich Muller and Carl Pennypacker. The program there was to automate a telescope to obtain nearby supernovae. This was done with success. It was a major project. There was also a lot of work on developing educational programs connected with Astronomy. Carl Pennypacker was involved with that. And Saul Perlmutter was part of this team, working on the nearby supernovae project. For the details of the history, about the people who were actually involved with it, I have to look back at my files to get the real details straight. There came a time when I was approached, I think initially by Gerson Goldhaber, who was one of the High Energy physicists who had moved into the area of supernova work. Gerson came in one day and said, “You ought to put Saul in charge of this,” even though Saul was quite junior to Rich and to Carl, but both of them had other interests. Carl was very involved in educational projects. Rich always had lots of other things going on. Saul was really leading the supernova search and this would just recognize that formally. So I consulted with them and both Rich and Carl agreed that we put Saul in charge. And so he took over. While I was still the Division Director, there was a discovery of a very distant supernova. That was quite important, because up until that time, there was only one distant supernova that had ever been found. It had been known for a long time that if you could find distant supernovae of Type 1A, you could do cosmology with them. But since no one ever found any, it just sort of languished. Now at the time, there was also a Center for Particle Astrophysics at Berkeley run by Bernard Sadoulet. That program was NSF sponsored. Bernard’s program supported a number of different efforts; particularly his own work on the direct detection of Dark Matter. And Saul’s program and a number of other programs as well. On the other hand, the Physics Division was partially supporting Bernard’s program quite significantly, as well as Saul’s. So, both Bernard and I were both supporting these projects. We did not always have complete agreement about how much to fund these two. Eventually, it became simpler for LBL to fund Saul and for Bernard to fund himself. That came about because I felt that Bernard was not sufficiently supportive of Saul’s efforts. It seemed more sensible for me to support Saul and for Bernard to support himself. There had been reviews of Saul’s project that had been critical.

Pavlish:

Were you involved in the reviewing process at all? That is something that is not clear to me. I heard that Robert Kirshner was on a review committee. Maybe Mike Turner was on one.

Cahn:

You are getting to the heart of it right there. Was Mike on it? There was a review committee for The Center for Particle Astrophysics itself. Kirshner was on that. Kirshner has been very critical. Kirshner is a real expert in supernovae as I am sure I do not have to tell you. I think he was quite skeptical that Saul would be able to do what he wanted to do. There were specific concerns that he had, K-corrections being the phrase that gets thrown around here. Kirshner doubted that the LBL group could deal with this and with a variety of other problems that needed to be overcome to turn discoveries of supernovae into cosmological measurements. So, Bernard, I think, wanted to reduce support. I have to go back to see if it is really true that Kirshner was the cause of that, but in my mind those are the connections. Eventually, we set up a specific review of Saul’s program. I should go back to my files on that. That review was a very positive review. On the basis of that we felt he should go ahead. (I will look at my files and find documents and so on.) Who was on that review committee? I think these are significant issues in understanding the history of this. In my mind, Saul played the key role in the whole thing, by simply establishing the field of distant supernovae. There was no such field before Saul. Saul showed that you could do this, after he had found half a dozen supernovae. And you could find them early on so that you could study them from before their peak brightness through their whole decay. Once you could do that and have large numbers of them, then this old idea of using them to do cosmology became practical. There were still difficulties to overcome. You will hear more about these, I am sure, from the people who are experts in supernovae, which I am not at all. The K-corrections is an example. There is dust. There are all kinds of things that could concern you, but the LBL team developed solutions for each of them. But certainly, demonstrating that you could get your hands on large numbers of supernovae was the key.

Pavlish:

The batches?

Cahn:

The batches, right. In my mind, Saul’s biggest achievement (in some sense) was to establish a whole new field of research. And then, what was remarkable was when they actually did these studies, they found the result was completely surprising. While Kirshner was skeptical and critical of the effort early on, he is a smart enough scientist to recognize an opportunity. He was very good at building a team that was quickly competitive, with outstanding people in it. I am not in a position to critically evaluate the papers. I am not a supernova scientist. I am an engaged outsider. But there is no question that Saul built this field.

Pavlish:

There is a story that Luis Alvarez first suggested a robotic supernova search.

Cahn:

That may be. That would not surprise me. Luis was the source of work in Astrophysics at Berkeley. He really was one of the great physicists of the 20th century. Certainly, he was one of the greats at Berkeley. He really got LBL involved in Astrophysics. Luis was a professor, but he was very, very closely tied to the lab. Astrophysics at Berkeley is very complicated because there are so many different parts to it. There is LBL. There is the Physics Department on campus. There is the Astronomy Department on campus. There is the Space Sciences Laboratory. And regrettably, they are all quite independent of each other. It is not as if we were one big single team. It is quite the contrary. Luis was a real genius. That is the only way to put it. If Rich says that Luis was the mind behind the automated supernova search then he probably was. Rich was Luis’ protégée and was involved in the early work with George Smoot on dipole anisotropy of the Cosmic Microwave Background. So Berkeley has a long history, LBL has a long history, in Cosmology. It was also a source of the Keck telescope.

Pavlish:

So there was not a question, when you were supporting The Supernova Cosmology Project, of whether they were really doing physics?

Cahn:

Well, it is really physics. The Physics Division is a misnomer. It had meant the High Energy Physics Division. But in fact, there was a long tradition of doing other things. For example, back in the 1970s, the dipole anisotropy in the CMB. The Physics Division is supported by the Department of Energy and in particular, the Office of High Energy Physics. Our work on cosmology didn’t fit into their program, but they tolerated it. It was thanks to the freedom they allowed us that we were able to pursue work in cosmic microwave background and in supernovae. When I took over, from Pierre Oddone who was the previous Division Director who is now the director at Fermilab, there were already, in addition to George Smoot’s work on the CMB, there were already efforts in finding supernovae that were in place when I took over. It was not started under my term, but at least under Pierre’s. Was it part of the program? Well, I do think that the Physics Division has had a tradition of allowing people to go off in their own directions when they have new ideas, and it has been extremely successful. These are not the only examples. When you are just looking at nearby supernovae, of course, it was pretty far from High Energy Physics. Once you get into distant supernovae, then you have the connection with Cosmology. And that is pretty close. Rich Muller was always a free spirit and has always had lots of ideas that were unconventional. He always had new things to look for and to study. Moon rocks, or whatever. Something different. Something that other people had not thought of. It is part of our tradition to give people like that the freedom to explore. It is harder and harder to do it now. We have resources that are less able to cover what we are doing. We do not have money around to spare with which to do exploratory work. We are trying. That stuff often pays off. Here is a great example of it.

Pavlish:

I have to admit that I am not experienced interviewing people with administrative scopscientists as well as directors. I do not really have the sophisticated questions I would like. I would be interested to know more details about those reviews.

Cahn:

In can go back and see if I can actually find, for example, the reports, which I may have. But it is a critical thing in my mind (this is on the record but it is also off the record, in the sense that I get to edit it). You are doing your work at Harvard. Bob Kirshner is a very prominent person in this field. He is a superb speaker, and an all-around outstanding scientist. He has strong views on where credit needs to be given. I am not particularly sympathetic to those views. It is not unusual within the scientific community for there to be a lot of squabbling about credit for major discoveries. We will get to this later, probably, with some of your philosophical questions. Often discoveries are not so simple. Occasionally they are. Something everybody recognizes and says: oh, here is a new fact; let us change our view of everything. More typically, things come gradually. I think this is an example where there were several levels of discovery and each one increased people’s confidence in the result. Bob Kirshner would like to say: oh, the result was submitted to the journal on such and such a date. That is the discovery and all the other stuff is irrelevant. I think it is very hard to credit that. It would be very hard to maintain. There are very subtle things, I think, that are involved in this. It is nice to imagine that there is something simple called a discovery. I would speculate that you may find in your studies that that is actually not correct; that the understanding of things is often gradual. And not just the understanding. Here, the understanding was not the issue. Everybody knew what these things meant. The question was: How well have we established it? The public has, I think, an extremely naďve view of things. Real scientists know that establishing something is something that almost always has to come by degrees. Most results are statistical in nature. That means that there is some time when statistically they are not significant. You have to go from not knowing it, through a period where you see some sign of it, and it has some statistical significance. Then, over time, you have more and more statistics. And if it is a real discovery, well, yes, the statistical significance goes forever. I will give you an example of something recent that got an enormous amount of attention, and was flat out wrong.

Pavlish:

Cold Fusion?

Cahn:

No. This one is slightly more obscure than cold fusion and a better example than cold fusion. Have you read Frank Close’s book about cold fusion? Have you read much about cold fusion? It is quite interesting. It is more dramatic than the example I am going to give, and more sensational. But the one that I am going to give is I think more pertinent. That is, something about Pentaquarks. Have you heard about Pentaquarks?

Pavlish:

No.

Cahn:

I think it is interesting just because it is an example of how results can go away. You were a physics major, so you know that Baryons are made out of three Quarks. Mesons are made out of a Quark and an Anti-Quark. So people found some states, they thought, that had to be made out of four Quarks and an Anti-Quark, and could not be made just out of three Quarks because of the various quantum numbers. There were ten different experiments that found evidence for this. Some experiments were claiming seven sigma. It is just wrong. It is just flat out wrong. Now, what happened? Well, that is going to be very hard to say. I would say there was some pretty bad science. There were people who misled themselves for one reason or another. And then people who, under the influence of one paper, thought they saw something. But it all fell apart. This is, in some sense, a much more interesting example of the History of Science than Dark Energy. Dark Energy is a much bigger thing. But this is a perfect example of how science sometimes works. One of the first groups to really have important evidence went back and redid the experiment with ten times the statistics and it was just absolutely not there. So, what that means, is that their claims previously of having five sigma, seven sigma, are just wrong. Just plain wrong, bad science. This was not completely insignificant. Physics Today declared this discovery; in whichever year they said they made it, to be the discovery of the year in particle physics. Well, it just happened not to have been a discovery. So, more typically you might have an effect in an experiment that is two sigma. And then, you take more data and it goes away eventually. Almost always a discovery is statistical. And if it is statistical, then it necessarily goes through a period where it is a one Sigma effect. When is it a discovery? The journals have to worry about this issue. Physical Review has a particularly rigid policy, which I think is misguided because it is too rigid. But, it is an example. They say, five sigma in an observation, it is a discovery. And three sigma is evidence for it. If you want to write a paper, you have discovered some new bump, and you want to call it a particle, if you have three sigma you can call it evidence — you cannot call it an observation. If you have five Sigma, that is their rule. Now, it all sounds sort of sensible unless you are looking at what three sigma and five sigma mean. You can look it up. The odds that it is wrong if it is three sigma are tiny. And five sigma, it is incredibly tiny. So why do they require three sigma and five sigma? Well, it is because they know full well that people underestimate their errors. In particular they underestimate their systematic errors. And so they found out, sort of empirically, that if you make people have five sigma before they can call it an observation, you are not going to go wrong. Well, with Pentaquarks, they did. But, that was unusual. All this is to say that it is not simple to decide when something has been discovered. And if you set that threshold too low, well then you make discoveries that go away. Every experimenter knows this as well. And so if they have something that is a two Sigma effect, statistically, that means it is extremely likely to be true. But they also know that in practice, it does not work that way. So almost no one would publish a result that is two sigma. Certainly in Particle Physics, where we are very statistics-driven. Anyway…

Pavlish:

That is a better answer than I could have even asked for.

Cahn:

My point is then, how does it apply here? Well, so Saul had a result and it has some level of confidence. Bob Kirshner had a result that has some level of confidence. How do we evaluate things? Well, it is complicated. Because when you do an experiment, there are two aspects to errors, to uncertainties. There is a statistical one, which is in principle straightforward. You say, “Okay, I have so many events. What is the likelihood that I would have found this result simply by chance?” So, that is a statistical issue. But then there are systematic issues. “Maybe I did something wrong in my measurements. Maybe I have overlooked some effect which has nothing to do with my statistics?” Often systematic errors do not go away. Sometimes they do. Each experiment has a responsibility not only for finding statistical error but for estimating systematic error. Now, what I just said to you before, was that results that are three sigma often go away, even though, statistically they should not. So, what is happening is that typically people are underestimating their systematic errors. That is not too surprising because generally it means that there is something going on that they did not know.

Pavlish:

Unknown unknowns.

Cahn:

Exactly. How do you know what your unknown unknowns are? You do not know them. If you do not know that there is a problem then you cannot estimate its effect. And this is something that everybody knows. Different groups will come to different conclusions as how to treat their systematic errors. So, you can say: Why not be conservative? We will calculate our systematic errors and then we will double them. Well, the first thing is that means that your results will seem less important. They will have less significance. But you can also tell that people have faked their systematic errors when there is not enough fluctuation in their independent measurements. There has to be fluctuation in the results. And so, sometimes you can tell that people are being too conservative because the Chi-squared is too small. Do you understand what I mean by Chi-squared? Sorry if I am making this too technical. To form Chi-squared, you just take the sum of some measurements and compare them with a prediction and divide by the square of the expected error. If you overestimate the errors, put a bigger number here, this will come out too small. You can tell when people have faked it.

Pavlish:

You mean they faked it by making the systematic error too big?

Cahn:

Right. And that also can be bad. If you make them too small, you make your result look too significant. If you make them too big, you are giving people the wrong answer. It is not an easy thing to deal with. Different collaborations will have different approaches. Typically people would say that it is better to be conservative and overestimate the errors a little bit. You do not want to underestimate them because then you will find discoveries that are wrong. People would prefer not to do that. Each experiment has its own sense of how it is going to deal with systematic errors. It is not like statistics where everyone agrees on “here is how you do it.’ Systematic errors are much more subjective. And, being subjective, it depends on personalities. Some people will be very conservative, some people will not be. I think this is quite relevant.

Pavlish:

This would naturally lead into, or relate to the question of how experiments end, which is another way of asking, when are discoveries made, when is consensus formed around a result?

Cahn:

I have not read Peter Galison’s book, but I recognize the title. I am not sure I think it is a great title. In most of these things, the experiments do not end in the sense that Okay, Saul’s group made a measurement, the High-z Team made a measurement. Now SNLS… they make measurements. Will people keep making measurements? Of course they will. The experiment is not something that is simple to define, because there may be new things to discover. First of all it is just simply the question of refining the measurements. That is already extremely interesting here. Who knows, we may make another discovery. You can write down a formula that says: here is what The Hubble Diagram should look like. Here are the only parameters. Maybe it will not fit any of these. Systematic errors are quite relevant to this issue. You were saying — when do experiments end? Well, the experiments keep going. There are more things to learn. But, if the question is when is a discovery made? I do not think that there is any instant when it is made, usually. When was the neutron discovered? There were two groups before Chadwick who had the neutron.

Pavlish:

That is one of the few dates I know — 1932.

Cahn:

Right. Yes, that is the right date. But of course Chadwick had seen experiments both by Boethe and Becker and by Joliot-Curie (Irene Curie and Frederick Joliot). They had both seen the neutron but did not understand that that is what it was. Chadwick said, “Hey, they have discovered the neutron.” He went and did a series of experiments that showed that this indeed was the neutron and not a Gamma ray, for example, by looking at recoils and so on. This is an anecdote that I read somewhere. So, Chadwick gets the Nobel Prize for discovering the neutron. The story is that somebody said to Rutherford (I guess Rutherford had enough influence to decide the Nobel Prize) somebody said, “Of course you are going to see that Joliot and Curie also get the prize.” Supposedly, Rutherford said, “Oh, no. They will do something else.” And so they did. They discovered artificial radioactivity and they won the Nobel Prize. But the point is — so, who made the discovery? In that case, everybody says Chadwick because he understood it. And that is, maybe, sensible. Here, I do not think there is going to be any sensible conclusion other than that both groups made the discovery. I think there are great advantages in knowing that the other team has found it. There has always been a lot of controversy about the discovery of the Psi — as to whether the people at Stanford knew that Sam Ting had this thing in his data. Of course, since I have a SLAC background, I believe that is completely wrong and that I know the story. If you know that somebody has something, this is a very powerful piece of information. It makes you not worry so much about an incredible result. As much as some people would like to think that they could not have possibly been influenced by this team announcing that Lambda was there, of course they were. They would have been crazy not to have been influenced. I mean, they would have been skeptical. They would still want to make sure they got it right. There is a tremendously powerful piece of information. And so people who say, “Oh well, the group did not say the universe is accelerating. They did not use this word, they did not use that word.” It is a lot easier if you already know that the other group has got it. Alex Filippenko has written a very biased account of the discovery of dark energy in which he minimizes the role of the SCP (LBL) group and maximizes the role of its competitor, the Hi-Z Team. Nonetheless he reveals that at the time of the January 1998 meeting of the American Astronomical Society, Adam Riess “did not yet feel ready to announce the possible discovery of cosmic acceleration, since various checks were still being made…” At that meeting, Saul Perlmutter did present evidence for accelerating expansion. Filippenko continues, “But, of course, members of the HZT did not fail to notice that the SCP’s result pointed to the same conclusion that Adam had made from the HZT data.” That conclusion was the one they were not ready to present at the time the SCP result was made public. Again, so you make a presentation. Is that the same as publishing it? Well, not exactly. Until it is published, you are not so sure that you should believe it. Because, maybe the collaboration will not publish it ever, if they find something is wrong with it. What you say publicly is not exactly the same as publishing. But, on the on the other hand, to be the first ones to come out and say, “Looks like Lambda is positive.” Is quite significant. The High-z Team says, “Well, we published first.” Well, yes, they did publish first. But they knew full well what Berkeley had. I think that to say that one group found it without the other group ridiculous. It does not make any sense. The community would never accept the idea that one group discovered it and the other did not.

Pavlish:

But in terms of important milestones?

Cahn:

Oh, sure. The discovery of the first distant supernova by the group here. And then, secondly, finding a whole group of them. It suddenly made it possible to do this program. If it was not for the Berkeley group, a lot of the ideas in Cosmology about supernovae, about distant supernovae, would not be as well-known. Only one supernova had ever been found and that was after peak. So, the group here showed that you could find large numbers of distant supernovae. That established a new field. I consider that breakthrough number one. There are two parts: When they found one and when they found a group. They showed that there was such a thing as a field of Distant Supernova Cosmology. Another important thing about which again there were disputes, was the use of The Hubble Space Telescope. That was very valuable and that was originally proposed by Saul. You should dig into that story. Ask Kirshner about it. When I talked to him about it, years ago, he was extremely candid in his description of it to me. Because Saul had proposed it. The person who was in charge of The Hubble Space Telescope then turned around and told Kirshner he should do it, too. Kirshner told me that initially, he thought it was not such a good idea. Of course, it was a good idea. Kirshner, for all that I am critical of him, can also be quite candid. You might find it interesting to ask him how it came about that they did the HST work. That was important, because it enabled the group to get the data on High-z supernovae, which were particularly critical to the HZT since they had a much smaller sample of supernovae. These things are incremental. You cannot say: It is all right here; this is the turning point. From my perspective, the turning point was when the group here showed that Lambda was probably positive. From Bob Kirshner’s perspective, it was when his group showed data that he thought were more convincing. Were they more convincing? I do not know. Bob Kirshner would like to have that decided by what people wrote about it, I think. The High-z Team was very good at publicizing it and finding catchy phrases. If you look at what the data were, you might come to a different conclusion. I do not think it is so simple, and I think trying to split it, and saying this group deserves more credit than that group… I find it distasteful when it is clear that both groups did important work. In my mind, Saul’s group deserves special credit for being pioneers. There is no question about that. Kirshner is on record for saying, “This stuff will not work.” Then, he is on record, recognizing it would work. So that is to his credit. A fundamental contribution in recognizing, that you could separate out Lambda, that was done by Saul and Ariel Goobar. Originally, the supernova technique was viewed as a way of measuring a single parameter, the deceleration parameter (at the time everyone knew the universe was decelerating!). Saul and Ariel showed how a more complete analysis would actual give a determination of Lambda — the cosmological constant. There were a lot of fundamental contributions along the way. The most important was to show that you could do it, make these measurements, and correct for all the problems that seemed to make using supernovae for cosmology impossible. Saul’s original measurements were consistent with not having a cosmological constant. That is statistics. Results can change. Sometimes effects go away. That effect, the effect of not having Dark Energy, went away. Dark Energy appeared in its place. We do not know until we have enough statistics. But it is not just statistics. It is also systematics. I think that the Berkeley group has tended to be very conservative. Well, one way to be definitive is to underestimate your systematic errors, and you can get much stronger statements. Occasionally you will be wrong. As I said for example, with our Pentaquark friends.

Pavlish:

Is that written up, the Pentaquark story?

Cahn:

Well, a great expert is on the sixth floor — George Trilling. As a narrative, I do not know whether it is written up. This is The Review of Particle Physics; it is put out by the PDG, the Particle Data Group. If you want to write another paper, on how people get things wrong, this one is really interesting. It is so interesting to see how many of the experiments go back and try to understand how they screwed up. Because there are still results out there that are not directly contradicted which most of us believe are wrong. The statistical significance claimed by people for Pentaquarks was probably way beyond what we have even now, even now for Dark Energy. But the Pentaquarks were wrong. Is there a chance that Dark Energy will go away? I am not enough of an expert to know. But the fact is, that these results fit in nicely with results that are completely independent of them.

Pavlish:

The Pentaquarks did not have that?

Cahn:

The Pentaquarks did not have that. And in fact, George Trilling and I wrote an article, which, basically showed that they were inconsistent with everything that we knew; that their properties were much more bizarre than people had realized; that they made it look extremely improbable that it was correct.

Pavlish:

That was while people still believed it?

Cahn:

Yes, that is while people still believed it. You know about the width of particles? Width is related to lifetime. The longer the lifetime, the smaller the width. It is just an uncertainty relationship. People thought these states were quite narrow. 15 MeV. George and I showed that they were much narrower than that even. Improbably narrow — so that they were quite unlikely. They were inconsistent, indirectly, with previous data. I think it is quite unlikely that Dark Energy is going to go away. But again there is no single moment, usually, when something is believed. Occasionally, with something like the Psi, where you look at it and it is a million sigma or something, there is just no question in your mind. There is no way it could go away. But that is not the way things generally happen There were people, and maybe people continue today, who try to show that we are wrong. That Dark Energy is not there because we have not properly taken into account dust, for example. Those are legitimate issues. There are always legitimate issues, and you have to convince yourself that that is not what is going on. Because when a result is as surprising as this was at the time, you have to worry. Now it is sort of accepted. It no longer seems surprising. Well — of course — it makes everything fit. It is just right. We should have known it all along. That is not the way it seemed like at the time.

Pavlish:

Well, that is a whole lot of new information and new knowledge about the topic.

Cahn:

My word of caution here is: things are not simple. You probably do not need these words of caution. Yes, discoveries get made, but they are probably not made in an instant. It is well known that it takes a long time for discoveries to sink in. Using our example of the neutron; it took a year for people to understand that because the neutron exists, it did not make any sense anymore to think that there are electrons in the nucleus. There are neutrons and protons. It took a whole year before that really became established. There is a great book by Abraham Pais. It is called Inward Bound. It is a history of particle physics. The first half of it he writes as a historian. The second half is more of what he lived through — that part is not as good. The part he wrote as a historian is terrific — really super. It is at the right level for the public without undergraduate degrees in Physics. It is really marvelous. At one place, you can read about how long it took for the neutron to be understood as what is really in the nucleus. There are two things. One is that it takes a long time for people to really understand what a discovery is. But it also often takes a long time for people to understand that a discovery has been made. When the Psi was discovered, there were debates for a year as to what it was. I can teach students that Psi is Charm and Anti-Charm bar. Everybody understands that. For a year people were resisting it because they insisted on real proof. That real proof came. Yes, it is nice to say that on November 11, 1974, the Charm quark was discovered. Yes, sort of. Sort of. I think discovery is…

Pavlish:

…a problematic term…

Cahn:

It is problematic. It is interesting. I think that what you will find, is that in some cases, discovery is something that happens over a period. It is important to recognize that some discoveries go away. That is why people have to be careful. I think the Pentaquark is not the best example because it is so egregious. You should not get five Sigma and seven Sigma effects that go away, unless you have really done something very, very bad. But three Sigma effects that go away? That happens. That is why Phys. Rev. says it is evidence; it is not an observation. Even though I do not like the policy, it still makes the point. It does not happen in an instant. When Bob Kirshner says, “You did not say that the universe was accelerating.” Well, they said what they said: Here is the evidence; here is where we are. That is where it was. When Bob Kirshner had his stuff, he already knew that Saul had these things and it would make him a lot more confident. He might deny that: We had our data already; we were not influenced at all. But his collaborator, Alex Filippenko, made it clear. “HZT did not fail to notice that the SCP’s result pointed in the same direction.” If he had his data all along why did he not present it in January? Why was their presentation in February? Why were they sitting on it if they already knew the answer? If they did not know the answer, how is it that they were not influenced by Saul? I think, in the end, the squabbling here would probably occur no matter what. But, there is no question that the intensity of it is due entirely to the Nobel Prize. That is what it is all about. I do not know who it is that Bob Kirshner thinks should get the Prize. I do not think one can seriously consider it without Saul getting it. Who gets it with Saul? You can watch the pattern of who gets the prize with Saul, whatever prize it is. Saul has won three big prizes. Who is the one who was named with him? The real interest here is in the science.