We can only guess what they look like on the inside.
Share via Print 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. Welcome to Science Talk, the weekly podcast of Scientific American for the seven days starting April 30th, 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 also a professor at the Stanford Linear Accelerator Center. Blandford's research interests range from high-energy astrophysics and cosmology to general relativity and gravitational lensing.
I wonder, could you start by telling our listeners what plasma is? 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.
Now what's baryonic matter, for those who don't know? 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.
If I stuck my hand into it, what would happen? 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. So, where in the universe do we find plasma?
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. 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.
So, it sounds like there is a lot of plasma out there. What fraction of the universe is plasma? 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.
So, there is plasma coming out of black holes, is that correct? 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.
And how hard is to get something to produce jets moving at nearly the speed of light? 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.
So, what is it that we've come to understand lately about plasma astrophysics? 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 high-energy 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.Giant Space Explosion That's Never Been Witnessed Before Challenges What We Know About Stars and Black Holes.
and mysterious. better understanding of what's occurring in this class of. Astronomers have constructed the largest-ever three-dimensional map of massive galaxies and distant black holes, which will help the investigation of the mysterious “dark matter” and “dark energy” that make up 96 percent of the universe.
The study, which uses data from NASA’s Nuclear Spectroscopic Telescope Array and the ESA’s XMM-Newton telescope, also gave scientists a better understanding of the behavior of quasar winds. The presence of this potential warp, the extent of the disk, and the spin of the black hole are all pieces of the puzzle that will help us better understand the behavior of stellar-mass black holes .
Giant Space Explosion That's Never Been Witnessed Before Challenges What We Know About Stars and Black Holes. and mysterious. better understanding of what's occurring in this class of. I am talking about black holes, and their mysterious ways.
Finding out more about black holes will allow us a better understanding of space time and maybe could even give us an idea of distinguishing between science fiction and fact. But first, to understand black holes, you have to .