The interview took place on March 21, 2005
Interviewed by: Eitan Crane and Chaim Shmueli, members of the Scientific American team
First we would like to congratulate you on the award.
Thank you.
The Israel Prize is an award that expresses appreciation for all of your scientific work. What do you think is your main achievement?
There are two answers to this. One is how people see me, and this is expressed in the reasons for receiving the prize given for my work on the thermodynamics of black holes. I started dealing with this in the 70s and it can be said that by the end of the 70s my part of this subject was finished, although there are always implications for later works. The second is from my point of view. Today I'm working on other things and I wouldn't want my whole career to end in the 70's. I think I'm still doing important things, but it takes time for the public, and even the important scientists, to catch on to what you've done. And so the award is actually given for the relatively distant past.
What are you researching today?
Today I am trying to change the theory of relativity, still within the framework of the principles established by Einstein, such as the principle of equivalence, or the theory's independence from different reference systems, to adapt to relatively new astronomical phenomena, such as the problem of dark mass or missing matter. When analyzing the motions within the galaxies with the help of Newton's theory, which for this purpose is a good approximation to the theory of relativity, it turns out that we do not see all the mass that causes the motion. Following this, the existence of "dark mass" was proposed, an explanation that is widely accepted today. But there are certain difficulties in this view. There is another way to try to explain the observations and that is to say that the accepted theory of gravity does not work well on the large scales of galaxies and above, and a revised theory is needed.
There are problems that are not well explained by the standard theory of gravity, such as the Tully-Fisher relation, according to which the energy supply in a galaxy in the visible field is proportional to the fourth power of the apparent rotation speed of the galaxy (a spiral galaxy rotates almost entirely at more or less the same linear speed, regardless of distance from the center, which is in itself quite strange.) The dependence of the optical power on the fourth power of the speed has no natural explanation when looking at dark mass, unless the dark mass is divided in an appropriate way. But this is not an explanation but an adjustment to the facts. On the other hand, if we accept Mordechai Milgrom's proposal from the Weizmann Institute, that the true theory deviates from Newton's theory at low accelerations, the Tully-Fisher law follows naturally, that is, directly from the equations
Today there are more and more features of galaxies that have a fairly simple explanation according to Milgrom's concept, and are more difficult to explain according to the concept of dark matter. And more and more people believe that this theory, which Milgrom calls MOND, works well. Only the question remains: "Why?" Because there is a prescription, MOND, that works well, but the reason why it works so well is not known. It can be said that MOND actually tells us what the real theory of gravity should look like. And that's what I want to achieve. Today I have a theory of relativity, which is supposed to replace general relativity, and its non-relativistic limit is not Newton's theory but MOND.
There is now a fashion, or an increased interest of physicists, who are engaged in testing the limits of general relativity.
True, deviations from general relativity are studied for all kinds of reasons. For example, the idea of branes (multidimensional membranes), according to which our universe is actually a subspace of the real universe. The real universe is multi-dimensional and we only see four dimensions, which is why the theory of gravity has deviations from the theory of relativity. My observation is empirical: MOND works, so I investigate how it can be obtained from relativistic theory. I think I succeeded in building such a theory, but it is not yet known if it is the only theory from which MOND can be derived. For many years MOND has been attacked on the grounds that, although the theory works, it cannot be correct, since it does not come from relativistic theory. Today this excuse is no longer valid.
Were there any predictions of the theory?
Yes, there were MOND predictions that were adjusted after a good few years. This is described in the literature. MOND is a theory from 1982 and at the end of the 80's it turned out that there is a prediction of MOND about the existence of low density galaxies, called Low surface brightness galaxies. They are very dim. And according to MOND such galaxies with such properties were expected, and they were not known at all at that time. Since then technology has developed and such galaxies have been found, and they have the properties predicted by MOND. That is, there were several MOND predictions that were confirmed. So the theory is not only a backward fit but also a true prediction. Now we try to see it as a fundamental theory of nature, and this is one of the things I am dealing with today.
I think if this theory works in the end, then it's more important than black hole thermodynamics. It is an improvement of the gravitational theory. I wouldn't say that it came to replace it, because this theory has many ideas from what Einstein bequeathed to us. So we're not going to replace it, but we're going to improve it. As they always improve in science.
At a recent conference in California, the last Nobel laureate, David Gross, presented the 25 questions that, in his opinion, trouble physicists. What do you think are the most important questions in physics from your point of view?
Well, in astrophysics we have actually already touched on the main question, the topic of dark matter and the search for it, or the search for a theory that will eliminate the need for dark matter. That's really the number one issue. Since the cosmological theory is relatively simple, and there are also many observations, people have the feeling that they understand cosmology, and until some surprise comes, you can say that things "stick" together. We have a consistent picture, except for this point of dark matter: what is it? where is he? Does he even exist? It really is still a very prominent mystery.
And what about the observations that teach about the acceleration of the universe?
This is part of the dark matter problem. It is called "dark energy", but it is probably the same kind of problem, even if these are probably two different concepts. It turns out that we do not really understand the basic laws of the universe. The cosmology of the beginning of the universe - it is. But not the acceleration, and because we do not understand this acceleration, we cannot be sure that we understand the future, that is, what will happen in the universe during the next millions of years.
Elementary particles also have problems, but I am less experienced in this area. There the question is always "what is the fundamental thing?" Before they talked about elementary particles, today - about string theory. The problem with this theory is that it has a very loose contact with the experiments, mainly because very high energies are required to test the theory. The theory is so beautiful that they say it must be true, because it is so elegant. But without experimental testing, you don't accept a theory in physics just because it's beautiful. There were beautiful theories in the past that fell apart when they met the facts. Although it is said that Einstein was able to arrive at the correct theories without looking at the facts. But I think this is an almost unique case, maybe Dirac too. For most mortals elegance alone does not lead to the correct theory. And here the difficulty is how to bring the theory into good contact with the experimental measurements.
This links us to the next topic. Last week, findings from the accelerator in Brookhaven were reported in which the collision of gold atoms revealed hints of the formation of a micro black hole. Does this seem possible to you?
The idea comes from the theory of branes. If our four-dimensional world, space-time, sits on a kind of brane, which is part of a multi-dimensional space, then it is possible that the Planck scale, the highest energy scale we know, is actually not as high as we thought, it would seem to be high because we are not looking on the real universe, but on its projection. If this is true, it will be possible to create black holes in particle accelerators, because impossible energies are not required. If the results of the Brookhaven experiment turn out to be valid, it means that this idea of branes or something equivalent to it is true. I wouldn't say that it confirms the branes, because they might come up with another idea that gives the same results, but it means that we didn't have a correct picture of the physics of the space-time structure. The physics of spacetime must be something more complicated.
On the other hand, I am very careful about such news. There were already many stories that you did not adapt. All kinds of particles they didn't find: the magnetic monopoles, the quarks they thought they found and in the end it turned out they didn't (then they did find them experimentally). There was a similar story with Weber's gravitational waves - a story that you didn't match and still haven't measured the gravitational waves directly. So you should be careful and wait for them to measure and verify. We'll see.
If micro-black holes are indeed discovered, then according to you the main implications will be on the theoretical subject of branes?
Yes. The branes deal with space-time of more than four dimensions. Although the theorists speak of this as an accepted thing, physics as a whole still operates in four dimensions. Such a thing would cause a revolution. If the universe is four-dimensional, then it will not be so easy to create a black hole in the laboratory, because a simple calculation shows that the energy of the existing accelerators, and even the hadronic accelerator that is currently being built in Switzerland, will not have enough energy to create a black hole. So if they found a micro-black hole the story gets complicated: space-time is not four-dimensional, and physics is richer than we thought. But I am conservative in these matters and wait until everything becomes clear experimentally. I'm sure people are already writing articles and making all kinds of excuses out of it. But I always waited for the facts to align.
And perhaps as a mind game: if they create a black hole in the accelerator, will it be possible to create a mini big bang in the accelerator?
The question is what is a big bang. Because a black hole is not a model of a big bang. He is the exact opposite.
But both constitute some kind of singularity?!
Can be. A black hole emits radiation - this is Hawking's evaporation. And at the end, when it completely evaporates, the question arises as to what happens. It is not known if it evaporates completely or if a singularity remains. If a singularity remains, then there is something that somehow approaches the beginning of the universe. It's not exactly the same singularity, but something of the same family. So maybe we can learn something. And in general - in all these collision accelerators, as well as in Brookhaven, they try to create conditions similar to the conditions of the Big Bang - that is, the quark soup or the quark plasma - which is one of the characteristics of very early ages in the universe. It's not quite a singularity but it's something that was, or should have been at the beginning.
You mentioned Hawking radiation. Is there experimental evidence in accelerators or in astronomy for this radiation?
Not as far as I know. Certainly not in astronomy. And the reason is that in the usual view of four-dimensional space-time the temperature of black holes is too high. Hawking radiation above all naturally occurring radiation can only be seen in the case of very small black holes. This is because in black holes the temperature increases as the black hole gets smaller. And apparently there are no such small black holes around us. Maybe they weren't created at all, but in any case there aren't, so I'm not aware of any experimental evidence. I know of an experiment called Milgro, where they look for the flashes originating from such little black holes, but I don't know that they found anything in it.
Does this have anything to do with intense gamma bursts?
There are gamma-ray bursts, but they have a much more mundane explanation, or several explanations that don't involve black holes and their vapors. The explanations may include black holes of the large, astrophysical type, which does not belong at all to the topic of Hawking evaporation. But there was an experiment in the accelerator that was supposed to show a phenomenon similar to Hawking radiation. When you look at electrons that are accelerated in a particle accelerator, there is a phenomenon that has been predicted to exist, called Unruh radiation. And as we understand it, its physics is not that far from the physics of Hawking radiation. An accelerating observer, even in a vacuum, should see thermal radiation called Unro radiation. This is a theoretical prediction from the 70s made right after Hawking's prediction. And this radiation is not very different from Hawking radiation, only the situation is different. The physics needed to understand it is very close to the physics needed to understand Hawking radiation. Then there was a claim that some data obtained from the acceleration of particles in the accelerator at CERN saw something that could be interpreted as the result of Unro radiation. There is a dispute among the researchers about the correctness of the claim, but the claim was made. For example, the famous John Bell believed that it was Unro radiation, but I'm not sure about that. This is the only thing related to the subject of Hawking radiation and any experimental evidence that can be interpreted as relevant, to the best of my recollection.
The main contribution you had on the topic of black holes was that for the first time you and Hawking touched on the connection between thermodynamics, quanta and relativity. It unites fields that seem completely different from each other. Do you see in this a union that may have a deep meaning like the union of fields?
Unification of fields in the sense of unified field theory is not here. Although there is a grouping of these three topics, in fact there is a use of a simple general relationship here without any complications. And the fields are normal fields. In other words, there was no intention to create a unified theory. What's more, we find there some connection between topics that are usually separate in physics. And I want to say in a framed article that I really like physics that has connections between things that don't seem connected, it always fascinates me and I'm always looking for problems of this kind. So there really is thermodynamics there that has something to do with gravitation on a quantum level. There is no dispute that such a connection exists, it's just that we still don't understand the exact nature of this connection. For example, one of the questions is the entropy of a black hole, which is the theoretically outstanding thing in this whole subject. People would like to calculate this entropy like they calculate the entropy of a gas, or of a crystal. In statistical mechanics, entropy is the logarithm of the number of quantum states. There have been attempts to do this for a black hole, and string theorists brag that they do know how to calculate this entropy by counting states, but this is only possible in very limited situations where the entropy of a black hole can be calculated from basic principles of its physics, but not in general. This remains a mystery. What this entropy is, which states of a black hole it counts, and whether it counts states at all - we do not clearly know.
Black holes are an irreversible phenomenon. Is this a fundamental property that may explain the irreversibility that exists in the world?
There is a question in physics, where does the second law of thermodynamics come from. Because microscopic physics is reversible. Today we know a little more about where the irreversibility comes from, but there was a time when quite serious people thought that maybe the black holes determined the arrow of time. Today we don't really need that. Even without black holes we understand where there is a direction of increasing entropy. It has to do with starting conditions, and stuff like that. And the black hole just reflects that.
In classical physics what are perceived as the fundamental laws are the properties of material bodies: the equations of state of bodies, the position of particles, their velocities and so on. Today there are more and more clues to the importance of information in physics: Shannon's theory, Wolfram's radical concept and of course also your contribution of black hole entropy. This is also the case in biology whose basis is the genetic information. What do you have to say about that?
Of course, I think so. Information is the basic concept in science. But we don't know how to do physics only on the basis of information alone, but we need the usual differential equations and the information stands a little to the side, and we don't know how to combine it and formulate the basic questions in the sense of information. I think sometime in the future we will know. Now we see only hints of it. For example in the theory of black holes, since entropy is the main thing there, and entropy is one facet of information, the matter of information appears there. There is also the holographic principle of Toft ('t Hooft) and Susskind which says that the way to correctly understand gravity is through information belonging to physics that is located precisely at the edge of the universe. As you said, in biology a long time ago they came to the conclusion that the main avenue is the genetic information. I have a slightly funny story. After I wrote the article in Scientific American in English, someone wrote me an e-mail, and introduced himself as a biologist, and since both fields deal with information, he asked me to explain to him exactly what it was about. I tried to explain to him, and he read the article and repeated that there was no connection (laughing...) very categorical.
These are differences between schools of thought.
There is a problem of different culture. In biology they think very differently and it will take a long time before we unite them with physics. Everyone wants to unify everything, but a lot more work is needed.
Modern physics is moving further and further away from intuition. The public wants to know and understand, while the scientists want legitimacy for the amounts of money they request for experiments. And they have difficulty explaining and justifying their demands. What do you think should be done?
It is a real problem how to explain what is done in physics to those who need to understand, and to the audience that pays taxes, and actually pays our salary and the price of the experiments. And the problem is that physics is getting further and further away from the world perceived by the eyes and senses, and it will get worse. I don't think we'll ever get back to intuition. The point is that our senses are tuned to give us a chance to operate with some degree of safety in the day-to-day world. This does not concern the real structure of nature, which is probably completely different. When physics began to develop, people worked close to what we see with our senses, and the concepts were very close to what we understand with our senses. But as we understand more and more, and dig deeper and deeper, we move away from the world of the senses - and it will get worse and worse. And the whole story how to explain it will get worse and worse. The only possible answer in my opinion is that the audience needs to acquire a culture of a little more abstract thinking. It will no longer go with intuitions of what is happening in the bathtub or of rolling balls - it will not be enough to understand. But the general public today is more cultured than the public of a century ago. They know more - both mathematics and a bit of physics - so the level can be raised, I think. The one who should be responsible for this is mainly the school. They also need to move forward. And we scientists are also responsible for this. We are not doing a good enough job. The people at the university are not good at explaining to the general public what we do. People in the humanities and social sciences have an easier time explaining their work. Biology is easier to explain than physics, but this changed with the increasing importance of molecular biology. So in all the natural sciences the problem of spreading knowledge to the general public will get worse and worse.
Is it considered today in the academy?
Yes, but I think not enough. Of course there are people who recognize the problem, and some who explain better than others. There are good popular science books on physics, but not enough.
Obviously, even during Newton's time, physics seemed complicated, and intuition was slowly built up. Would it be possible, as a mental game, to teach relationships in first grade?
I think such a day will come and it is getting closer and closer. When my son started studying physics at the university, the theory of relativity appeared already in the first semester of the first year. In my time, such a thing would never have been thought of at all. And he understands. and why? Because if you don't "confuse" the brain, and pardon the expression, with all the old concepts of mechanics, you can understand relativity. The math there is quite simple. It's just not particularly difficult algebra. It is possible to understand this and create the right intuitions. It should be remembered that in Newton's time, the problem to understand was mathematical, he had to invent the infinitesimal calculus for gravitation and differential equations. But the physical ideas then were quite basic: spheres, moving particles, things very close to the senses. Today we are in a completely different situation. The mathematics has indeed become more complicated, but the main problem is that the basic physical ideas no longer derive from the senses. These are abstract things that you don't think about when you look at nature. It will be necessary to educate the general public to think more abstractly. I don't see why not. There will always be those who cannot, but there are people, including people who are not from our circles, who can be explained to and they understand. I am now having a rambling discussion with someone from a medical school in California. He deals with brain research and is interested in quantum issues and he poses me completely difficult questions. I have to think a lot before I answer him. So it's a fact, there are people who come from the field of biology and can think abstractly. I think the wider audience can be educated to think abstractly, and I think it should be done.
Instead of trying to illustrate, you suggest improving the ability to abstract?
No, illustrations are always needed. You must start with something familiar. But it's a trick. It is such a deception designed to attract the listener until he understands the more abstract things. But it is allowed to use all kinds of such tricks. Good writers also did it, great philosophers too. The main thing is that we reach a minimum level of understanding.
Do you see a change in the level of students who arrive at the university? And do the problems of the education system affect the higher studies?
There is a problem in Israel with physics students, there are ups and downs in interest. But these are changes that reflect what is happening in the US with a delay of several years. That is, if there is a burst of interest in physics in the US, after a few years it happens in Israel as well. And if there is a low in the number of students there, after a few years there is a low here too. This is a known and recognized phenomenon. The reason for this is not only the resources allocated to education, but a more general reason: sometimes there is enthusiasm for scientific breakthroughs, and sometimes there is anger at the damage that science has caused humanity. For example, following the proliferation of nuclear weapons and the proliferation of experiments, there was a decline in the number of physics students in the US, and what happens in the US in the fields but reaches us sooner or later. Such phenomena affect more than the problems of the education system. It is true that the state of education in Israel is not promising and needs to be improved, otherwise we will end up in a bad situation. But this is not the main reason.
Regarding the quality, there are always very good students, in all periods. But because the level of education here is not reasonable, even though smart and original people will always come to the universities, their background will be weaker and they will have to study longer before they can do research. This can be saved if they are exposed to certain branches of mathematics and physics earlier - as was the case in the good old days in Israel. There were better days - when I arrived in Israel, I remember being impressed that they knew so much infinitesimal calculus. At that time, people knew more when they entered university.
Jacob D. Bekenstein
Contributed to the foundation of the thermodynamics of black holes and other aspects of the relationship between information and gravity.
Today he is engaged in the development of an alternative theory of relativistic gravity to general relativity that will make it possible to understand the universe without needing dark matter.
He holds the Polk Chair in Theoretical Physics at the Hebrew University of Jerusalem, a member of the Israel National Academy of Sciences and a Rothschild Prize laureate.
Beckenstein is a fourth generation student of Ludwig Boltzmann.
Bekenstein's doctoral dissertation supervisor is John Archibald Wheeler, Wheeler's doctoral supervisor, Karl Herzfeld, was a student of Friedrich Hazenorl, Boltzmann's student.
Prof. Jacob Bekenstein from the Hebrew University - laureate of the Israel Prize in Physics
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Of blessed memory