Science Talk - April 30, 2008 Plasma Physics: From Black Holes to Radio Reception Plasma plays a big role from the ionosphere to black holes. Stanford physicist Roger Blandford explains plasma and its connection to black holes in a conversation with Scientific American's JR Minkel. Plus, we'll test your knowledge of some recent science in the news. Web sites mentioned on this episode include www.snipurl.com/26dun-sciam1; www.snipurl.com/26dv2-sciam2; www.nybg.org/darwin Plasma plays a big role from the ionosphere to black holes. Stanford physicist Roger Blandford explains plasma and its connection to black holes in a conversation with Scientific American's JR Minkel. Plus, we'll test your knowledge of some recent science in the news. Web sites mentioned on this episode include www.snipurl.com/26dunsciam1; www.snipurl.com/26dv2-sciam2; www.nybg.org/darwin Podcast Transcription: Steve: Welcome to Science Talk, the weekly podcast of Scientific American for the seven days starting April 30th, 2008. I'm Steve Mirsky. This week on the podcast, we'll enter the fascinating world of plasma—not the blood kind, the physics kind—with Stanford University physicist Roger Blandford. Plus, we'll test your knowledge about some recent science in the news. Roger Blandford is the coauthor of the Blandford-Znajek Process, the leading explanation for how black holes produce jets of plasma traveling at near light speed, but what's plasma? Well, he'll explain that. He's the director of the Kavli Institute for Particle Astrophysics and Cosmology at Stanford. He's also a professor at the Stanford Linear Accelerator Center. Blandford's research interests range from highenergy astrophysics and cosmology to general relativity and gravitational lensing. On April 12th, he gave a plenary lecture at the Annual Meeting of the American Physical Society in St. Louis. Scientific American's JR Minkel was at the meeting and he spoke to Blandford after his talk. Minkel: I wonder, could you start by telling our listeners what plasma is? Blandford: Oh! Plasma is an ionized gas—it's one where the electrons are separated from the nuclei, usually formed at high temperatures; and most of the baryonic matter in the universe is in the form of plasma. Minkel: Now what's baryonic matter, for those who don't know? Blandford: This is just regular matter like you and I, and we just use that phrase to distinguish it from the mysterious dark matter, which actually has a high average density in the universe, as we now know. Minkel: Is plasma dangerous? If I stuck my hand into it, what would happen? Blandford: Well it depends how tenuous it is, but if it were dense of the sort that you could make in a laboratory, you would be subject to burns and in many circumstances radiation exposure. So it's a good thing to do remote experiments on it—and as astrophysicists we can do remote experiments. Minkel: So, where in the universe do we find plasma? Blandford: Well, if we just go outside of the surface of the Earth, the first place we find it is in the ionosphere, and one of the reasons that we can bounce radio waves off the ionosphere is because there is plasma there. [As] we go farther away, we find the Earth's magnetosphere, which is the magnetic wave that's tied to the North and South poles that also contains a lot of plasma, the so-called Van Allen Belts and so on, and then extending back beyond the Earth to the so-called magnetotail—just this sort of lamb's tail that extends back beyond the Earth—that's full of plasma. If we go out into the solar wind, which is the gas that emanates from the surface of the sun and blows past the Earth and the other planets, that also is full of plasma. We go out into the interstellar medium, this is the gas between the stars like the sun, that too is mostly plasma—not all of it, some of it is in the form of neutral gas, but a large fraction of it is in the form of plasma—and then if we go outside the galaxy itself, into the space between the galaxies, the so-called intergalactic space, then again, that is mostly plasma. Closer to home, I suppose I left out the sun, which of course, itself is mostly plasma, because [the] high-temperature center of the sun is 15 million degrees, and so that is plenty hot enough to separate the electrons and the protons and to make sure that they move around freely inside the center of the sun. Minkel: So, it sounds like there is a lot of plasma out there. What fraction of the universe is plasma? Blandford: Well! We don't know for sure, but of the, what I call, baryonic matter, which is 5 percent of the total mass energy density of the universe, one would guess about 90 or 95 percent of it, is in the form of ionized gas called plasma. Minkel: So, there is plasma coming out of black holes, is that correct? Blandford: Well, we think there is plasma around black holes. The black holes that we can observe directly through their radiant emission are mostly in a configuration where gas swirls around the black hole in the form of an accretion disk and that accretion disk— most of the mass is going to be in an ionized form, and then some of that gas gets expelled from the environment around the black hole, while it is still outside the black hole, it gets squirted out in the form of an outflow, a wind like the solar wind and then [a] much faster, collimated outflow called a jet. But there are two jets—one that goes up and one that goes down—and these are associated with the region very close to the black hole and those jets contain plasmas that are moving at relativistic speeds, that is to say, speeds close to that of light. Minkel: And how hard is to get something to produce jets moving at nearly the speed of light? Blandford: Well, nature doesn't seem to be very challenged in this regard because it makes jets under many different environments. Even protostars—these are young stars that are just forming and making their own planetary disks and so on—they make very powerful outflows called, the same sort of jets obviously moving at slower speeds, but they are full of plasma, that is flowing out at high speed; white dwarfs, neutron stars, black holes big and small, they seem able to do this task, it really seems to be a very common phenomenon. Nature is able to do it at will. We have a harder time understanding in detail how these jets are formed, but I think that we are getting confused on a higher plane now, let me put it that way and a lot of the sort of ideas that were possibilities in the past have now really been excluded and we do have a much more sophisticated understanding of some of the general principles, but I think not all of them. Minkel: So, what is it that we've come to understand lately about plasma astrophysics? Blandford: Oh! About plasma astrophysics I would say the first thing is we understand that magnetic fields are very, very important in accretion disks and the region around black holes and neutron stars and those magnetic fields are almost certainly integrally important in forming the jets and the outflows. So, I would say that's the first thing that we understand. And we understand that on the basis of direct observations, which have become very much better over the past five or 10 years and also as a result of theoretical investigations, particularly those involving sophisticated numerical computations; and here we are able to do the sort of experiments, with the computer if you like, that were not possible 10 or 15 years ago. Now, we can do those experiments and understand how the laws of physics behave in these environments. So, that's the first thing we've understood. I think the second thing that's very exciting is understanding how the highenergy particles are accelerated. Nature is able to accelerate particles like protons to energies that are as large as say that of a well-hit baseball, and it's been a puzzle for a long while to know how it does that. We know that for energies of modest to intermediate energy, the culprit or the source of the acceleration appears to be the shock front that surrounds a [an] expanding supernova blast wave; that is to say, we have a star that undergoes a massive cosmic explosion [and] drives a strong shock wave out into the surrounding interstellar medium, and the gas around the shock wave, and all the magnetic fields associated with it are capable of accelerating particles to very high energies; and also incidentally magnifying and amplifying the magnetic field associated with that shock front and giving a lot of x-ray emission and radio emission and so on, and so we've understood that. I think we have now a much better understanding from an observational perspective and again theoretical modeling is becoming much more sophisticated, and although there is [are] still lots of puzzles involved and lots of, you know, healthy scientific debates, which what makes the subject very interesting at this time. There are some things that people are no longer debating, which they would have been doing so five or 10 years ago. Minkel: And these accelerated particles, those are what you call cosmic rays. Blandford: Cosmic rays is [are] historically the particles that hit the Earth, they were discovered in the early part of the 20th century and mostly that's what people think of as cosmic rays, but relativistic particles exist again throughout the universe and they don't actually have to hit the Earth for their effects to be observed and for them to pose, you know, interesting astrophysical problems for us to try and solve. Minkel: So, it sounds impressive for a particle to have the kinetic energy of a struck fastball. What does that mean exactly—if one of them hit my head, would it hurt me? Blandford: No. That's a very interesting physics question. Let me say, we haven't found one yet with the energy of a home run, so I shouldn't boast too much—my experimental colleagues are looking for a home run, if you like, but it's a bloop single would be about the right energy you have. In fact if it hit you on the head, what it would do, it would just go straight through and one of the reasons is this, is the difference between momentum and energy. It has the energy of a baseball but the momentum of a snail. So that wouldn't be so bad, if you stopped it into your head, you wouldn't actually feel it, but in practice any cosmic ray wouldn't get as far as your head, because that energy would be stopped in the upper atmosphere. Minkel: So, the jets that you said were sort of a generic feature coming out of, I think, you said proto-planetary disks and as well as around black holes— so, what's the mystery with those, are they, especially powerful or impressive in some way? Blandford: Some of them are. In some active galactic nuclei, you have a black hole and accretion disk and the majority of the power is associated with these outflowing jets, far more than is associated with the radiant energy that is emitted by the accretion disk and the hot gas surrounding it. So, that is a, you know, an observational statement and a very interesting one. So these are not sort of small players, these are major parts of the energy budget of an accreting black hole and by extension, they have an important impact on their environment; and the jets associated with accreting black holes and nuclei galaxies inflate giant lobes of plasma outside the galaxy and these heat the surrounding gas, they affect the fuel supply, they stimulate star formation, they in fact stimulate galaxy formation. So, black holes as well as being sort of agencies of doom and destruction in the end of time and allegories of halo and all the rest of it, are also bringers of life. So, they in fact can be very much part of the regenerative part of an ecological cycle, if you like, for the universe. Minkel: So, how large are these jets? If there are spawning galaxies they must be pretty big? Blandford: The biggest jets are megaparsecs, which means, many millions of light years in size. So, yes they go way outside the galaxy. Minkel: And in your talk, you showed some rather pretty simulations of some of these jets—what have they told us about the jets? Blandford: Well, analyzing the radio, optical and x-ray and now gamma ray images of jets and data from jets have helped us to understand that they are moving at relativistic speed. They probably contain electrons and positrons, at least in their earliest stages, although that is not clear, that's all the way along the jet. They live for hundreds of thousands of years, millions of years probably, and they probably fire up many times during the lifetime of a galaxy. They have a major impact on their surroundings. They can inflate giant bubbles of plasma, which will float away from the source galaxy, you know—in the gravitational field these giant bubbles will just float away and they again can be responsible for heating the gas that surrounds the galaxy. Minkel: And the simulations tell us all that? Blandford: No, these are observations that have really told us that. Some of the simulations and theoretical work has anticipated the observations, some of it has actually followed the observations. That's the normal process. In science sometimes you get things right ahead of time, sometimes you produce the explanation after you see the result of the experiment or the observation. Minkel: So, the simulations tell us we know the underlying physics behind the observations. Blandford: The simulations are in some cases, able to rationalize what we see. I think there is still quite a lot that we are not agreed upon in modeling of these jets and accretion disks and so on. So, there is still quite a lot that are genuine healthy areas of debate, but I think there is [are] so many other areas where indeed the very existence of massive black holes themselves in the nuclei of galaxies was a contentious matter; as recently as 15 years ago, there were people who still had alternative view points. I think one doesn't hear of them anymore now—everyone accepts that every galaxy worth the name has a massive black hole in its nucleus and when it is accreting that gas forms a disk around it. I think that is no longer debated, and so that's just one of many examples of what was originally a theory or hypothesis becoming an established scientific fact. Minkel: So, if I can give you the opportunity for self emotion [promotion], what has been your biggest contribution to this field? Blandford: Oh gosh! I think I've done a lot of things in collaboration with people. I think the work that, I think, [I'm] probably best known for all was a collaboration I did with a colleague called Roman Znajek, where we proposed a particular mechanism for extracting, using electromagnetic fields, the spin energy of a black hole. It is still in some sense a bit of a conjecture, and I would say it has not reached the status of established fact, but for Roman and myself at that time, it was fascinating physics. I am still fascinated by it and certainly it's something that I very much enjoyed thinking about and working on. This is quite a long time ago, so I would say that's probably the thing that I am most associated with and certainly something that I still find very fascinating. Minkel: Extracting the spin energy of a black hole that's a mechanism for producing a jet? Blandford: Yes, in fact, I would argue that in fact, this is where the power for the big relativistic jets that we see actually comes from. It comes from the spinning space-time around the black hole and in fact it is not very well known, but that energy is there for the taking—up to 29 percent of the so-called rest mass energy of a spinning black hole is extractable—and original conjecture, which is not, as I say [said], yet established fact, but certainly taken much more seriously than it was at that time—10 or 15 percent of the rest mass energy of the black hole, about half of the spin energy, is in practice according to our conjecture, is in fact, the power source for these relativistically moving jets. Minkel: Very cool. Thank you very much for talking to us. Blandford: My pleasure. Steve: Check our JR Minkel's recent article on plasma jets at http://www.snipurl.com/26dun-sciam1 and to see some nifty Plasma sims that Blandford used in his talk at the American Physical Society meeting, see JR's blog item at http://www.snipurl.com/26dv2-sciam2
