Science & Discovery
From the Archives:
An Interview with Stephen W. Hawking
In a series of articles published in Mercury during the 1970s, the ASP interviewed some of the most-known researchers in astronomy. This is one of those interviews.
By William J. Kaufmann
NASA/Kim Shiflett
NASA
Editor's note: This interview is a reprint, and originally appeared in Mercury's first decade. Thank you to Vandana Kaushik for the archival help.
Some of the most profound and far-reaching discoveries in relativistic astrophysics during the past five years have been made by Dr. Stephen W. Hawking. Dr. Hawking was born in 1942, and his interest in general relativity was stimulated by his early association with Dr. Dennis Sciama. Along with Dr. R. Penrose, Dr. Hawking proved some basic theorems dealing with singularities and cosmology. Most recently, Dr. Hawking has suggested that there might be numerous tiny black holes scattered throughout our universe which would decay quantum mechanically. He is one of the youngest scientists ever elected to fellowship in the Royal Society and is the author of a brilliant book "The Large Scale Structure of Space-time" with Dr. G. F. R. Ellis. As a Sherman Fairchild Distinguished Scholar, he spent 1974–75 in the Relativity Group at Caltech, on leave from the Department of Applied Mathematics and Theoretical Physics at the University of Cambridge in England. Dr. Hawking has authored numerous papers in scientific journals.
William J. Kaufmann: Stephen, anyone working in black hole physics these days is aware that many of the most important developments in general relativity center about your discoveries. We are honored that you have taken time from your busy schedule to do this interview for Mercury. Perhaps we could begin then by having you tell us briefly: what is a black hole?
Stephen W. Hawking: A black hole is a region of space where the gravitational field is so strong that no light or anything else can escape to infinity.
Kaufmann: By "nothing escaping" you are of course referring to what is now called classical black holes. How do you conceive of such classical black holes [usually] being formed in the universe?
Hawking: There seem to be two ways in which they could be formed. The first is that when a star of more than twice the mass of the sun burns up all its nuclear fuel it will not be able to support itself by thermal pressure against gravity and so the dying star will begin to fall in on itself. Now so far as we know, there is no force in physics that can prevent the star falling in on itself. It will therefore collapse right down to zero volume and infinite density. This results in a so-called "singularity" of space-time, a region where space-time becomes so twisted up that it just comes to an end and all the known laws of physics break down. Now this singularity will probably not be visible to someone on the outside because when the star gets very small, say a few miles across, the gravitational field become so strong that all the light from the star is dragged back and cannot escape to infinity. Thus we are left with a black hole, a region of space from which no light can escape. That is one way in which black holes can arise.
Kaufmann: But you mentioned there was another way?
Hawking: Yes, well I should say first of all, we have fairly good observational evidence that black holes of this first kind occur in binary star systems like Cygnus X-1. The other way black holes might arise is more speculative. We know that the early universe must have been very hot and dense. We also know that it cannot have been completely uniform because if it had been completely uniform the universe would be uniform today.
Kaufmann: Then there wouldn't be stars or people or anything?
Hawking: Or galaxies. We know that the universe is not uniform today so it cannot have been completely uniform at early times. Now in such a situation, one might imagine that some regions of the early universe got so compressed that they were able to overcome pressure forces and they collapsed to form small black holes. By small black holes I mean black holes from say about 10s grams upwards. They would be very small in spatial extent. A black hole of about 1 gram mass would be about 10–28 centimeters across and a black hole of about 1015 grams mass, which is about the mass of a mountain, would be about the size of an elementary particle about 10–13 centimeters across. There is no observational evidence as yet that these small black holes exist, but if they do, it would be extremely interesting.
Kaufmann: In other words, while we see large black holes in binary x-ray stars because gasses fall down them and emit x-rays, what would one of these very tiny black holes look like? What unusual properties would they have. say, different from the larger classical black holes?
Hawking: Well, if they were just like large black holes they would be very difficult to detect at all because they are so small. One of them could pass right through this room and we might not notice it. However, there appears to be a quantum mechanical effect which is very important for these small black holes but is not really significant for large black holes. This quantum mechanical effect makes the black holes decay by emitting radiation with a certain characteristic temperature which is inversely proportional to the mass of the black hole; for example for a black hole of about 1015 grams the temperature is about 1011 degrees Kelvin.
In 2008, Stephen Hawking presented a lecture titled "Why we should go into space" as part of NASA's 50th anniversary.
[NASA/Paul E. Alers]
Kaufmann: I have a graph I have prepared to show that.
Hawking: Such black holes would emit hard gamma rays and also electrons, positrons, gravitons and neutrinos.
Kaufmann: This is a very surprising result, namely that a black hole the size of an elementary particle could be emitting so much energy. Usually people think of black holes as something from which nothing can escape -not even light. Perhaps you could briefly describe in a little more detail, this mechanism which you have discovered.
Hawking: Yes, a black hole is a region from which classically nothing can escape. This is because the gravitational field provides a "potential barrier" around the black hole through which particles cannot tunnel classically. But we know from quantum mechanics that when you have a potential barrier, particles can always tunnel through it quantum mechanically even if they cannot cross it classically. That is really what is happening here. You get particles tunneling out through the potential barrier around the black hole..
Kaufmann: In other words, in the case of a classical black hole the gravitational field is so strong that it acts like a huge brick wall. But in the case of a tiny black hole, that wall is as thin as tissue paper and things can get through.
Hawking: Yes, they can get through in the case of a large black hole as well, but in that case the wall is so thick that only a very small amount gets through. But in the case of a small black hole the wall is thin enough that quite a significant amount can get out.
Kaufmann: In other words, an ordinary black hole such as we have discovered in the sky. might have a temperature of less than one millionth of a degree. In the case of these tiny [holes]. as you go to smaller and smaller masses the temperature gets higher and higher. But doesn't this mean that the black holes will evaporate?
Hawking: Yes it does. As these particles tunnel out, they carry away some of the mass of the black hole. So the black hole loses mass and it gets smaller and as it gets smaller it radiates faster so eventually it radiates itself away. For a black hole of 1015 grams, the time to evaporate completely is about the age of the universe. If such black holes were formed in the early universe they would be just about completely evaporated by now. On the other hand, a large black hole like the one in Cygnus X-1 would take a very long time to evaporate. I think about 1064 years.
Kaufmann: And by the same token, black holes which are less than 1015 grams would have already evaporated?
Hawking: Yes.
"I think it is very important for astronomers to look for these small black holes and the best way to do it is probably to look for gamma rays coming from the explosions at the end of their lifetime." — Stephen W. Hawking
[communicate science]
Kaufmann: What do you think astronomers should look for in order to perhaps discover these evaporating black holes? What would they look like if one of them went off in the sky tonight?
Hawking: Well I hope one does not go off very nearby because the final stages of the evaporation are very rapid indeed and at the end you would get a large burst or explosion in the region maybe of a million megatons. Now that would not be at all comfortable to have occurring near the earth. However, I don't think we need worry because we can place an upper limit on the density of these objects in the universe. That upper limit indicates that the nearest small black hole is not likely to be nearer than the orbit of Pluto.
Kaufmann: The typical density we might expect in the universe is one per solar system volume or something of the sort?
Hawking: Observations of the gamma ray background around 100 MeV indicate that about one per solar system is an upper limit to the densities of these small black holes. I think it is very important for astronomers to look for these small black holes and the best way to do it is probably to look for gamma rays coming from the explosions at the end of their lifetime. Don Page and I have estimated that there might be about one such explosion per month within the volume of about one parsec from us.
Kaufmann: That's roughly about the same frequency that astronomers have been observing gamma ray bursts. Do you think that they have already detected these things?
Hawking: Well, no. I think that the gamma ray bursts that have been observed in the last few years arise from some different cause than small black holes. These are soft gamma ray bursts around 150 KeV whereas the bursts that we would get from small black holes are hard gamma rays around 100 MeV so they are different. No one has been looking for bursts in that region so far. To do that would require rather large area gamma ray detectors and these may have to wait until the space shuttle is in operation.
Kaufmann: From what you've said about small black holes it sounds as though they look a lot like small white holes. In fact. in a recent paper you showed that small black holes are indistinguishable to an outside observer from small white holes. Could you elaborate a little bit on this?
Hawking: Yes. In the classical theory, black holes are objects which matter or radiation can fall into but from which nothing can escape, whereas white holes are really the time reverse of black holes and are objects into which nothing can fall but from which matter or radiation can escape. The discovery that black holes emit radiation quantum mechanically has really removed the distinction between black holes and white holes because black holes can now both absorb and emit particles. Similarly white holes can absorb particles by a quantum mechanical annihilation process and they can also emit them to an external observer. There is no way we can know whether a given object is a black hole or a white hole.
In 2018, Stephen Hawking’s ashes were interred at Westminster Abbey in London. This memorial stone marks the site, near other science greats like Sir Isaac Newton.
[Wikimedia commons/ JRennocks]
Kaufmann: I wonder — we know that classical black holes could be forming today. Is it at all possible. since it's a long time since the big bang, that tiny black holes could be forming today?
Hawking: I think it is unlikely because to form a small tiny black hole requires enormous densities and pressures and these are not likely to occur at the present day. Of course it is interesting to speculate on whether one might be able to make small black holes in the laboratory. However I think Professor Wheeler has a calculation which says that if you wanted to form a 1015 gram black hole you would need to take all the deuterium from all the oceans of the world and make it into one enormous hydrogen bomb. If you did that it would implode the matter in the center sufficiently violently that it would indeed form a small black hole. But I doubt very much whether that experiment will ever be carried out.
Kaufmann: That does seem a little bit impractical. Perhaps you could speculate what it might be like if we did discover a 1015 or 1016 gram black hole nearby. Could it be used?
Hawking: Such a black hole would be radiating at the rate of approximately a million megawatts. It would be a very attractive energy source if we could harness it. The difficulty would be that we certainly could not have it on the earth because it would just fall through the floor and to the center of the earth, but there might possibly be some way of putting it in orbit around the earth and then channeling the radiation from it to some useful purpose.
Kaufmann: The idea that something smaller than the size of an atom can emit millions of megawatts of energy is certainly among one of the most exciting discoveries in theoretical physics today. ✰
(Originally published in the November / December 1975 issue)
William J. Kaufmann (1942–1994) was on the Board of Directors of the Astronomical Society of the Pacific. He spent most of the 1974-75 academic year in Caltech's Relativity Group along with Dr. Hawking.
Mercury is distributed by the Astronomical Society of the Pacific, a nonprofit organization dedicated to astronomy literacy, education, and outreach.
Mercury is an advertisement-free publication. If you are interested in supporting Mercury, please email us.
