Black holes don't just suck
If our eyes could observe the sky at radio or at X-ray wavelengths,we would see that some galaxies are straddled by vast balloons or lobes of plasma. This plasma contains charged particles that move at speeds close to the speed of light and radiate powerfully across a range ofwavelengths. The plasma lobes exhibited by some of these galaxies (examples of `active galaxies') are created by jets,travelling at speeds so fast that they are comparable with the speed of light, that are squirted out from the immediate surroundings of a black hole, outside its event horizon. Roger Penrose showed in general terms how extraction of the spin energy of a black hole from its ergosphere might be possible in principle. Roger Blandford and Roman Znajek have shown explicitly how the energy stored in a spinning black hole could actually be transferred into electric and magnetic fields and thereby provide the power to produce these relativistic jets of plasma. There are also other explanations for the mechanism by which jets are launched from near black holes. However, which of these is correct is the subject of active and exciting current research.
Whatever the mechanism(s) turn out to be, these jets are highly focused, collimated flows ejected from the vicinity of the black hole, but of course outside the event horizon. The regions in between galaxies are not, in fact, empty space. Instead they are filled with a very diffuse and dilute gas termed the intergalactic medium. When the jets impinge on the intergalacticmedium,shock waves form within which spectacular particle acceleration occurs, and the energized plasma which originated in a jet from near the black hole billows up and flows out of the immediate shock region. As the plasma expands, it imparts enormous quantities of energy to the intergalacticmedium. There are many instances of these plasma jets extending over millions of light-years. Thus black holes have tremendous cosmic influence,many light years beyond their event horizons. In this chapter, I will describe the influence and interactions of black holes on and with their surroundings.
As discussed in Chapter 6, at the centre of (probably) most galaxies is a black hole, on to which matter accretes, giving rise to emission of electromagnetic radiation. Such galaxies are called active galaxies. In some of these galaxies, the process of accretion is extremely effective and the resulting emission of radiation extremely luminous. Such galaxies are called quasars (a term which derives from their original identification as `quasi-stellar radio sources', vastly distant, highly luminous points of radio emission).We now understand that quasars are the sites of the most powerful sustained energy release known in the Universe. Quasars radiate energy across all of the electromagnetic spectrum, from long wavelength radio waves, through optical(visual) wavelengths, to X-rays and beyond. The radio lobes,mentioned above, can be especially dramatic because they extend across distances of over hundreds of thousands of light-years (see Figure 19). The energy radiated at radio wavelengths arises from those large lobes-reservoirs of ultra-hot magnetized plasma,powered by jets that transport energy over vast distances in space.Highly energetic electrons (highly energetic here meaning travelling extremely close to the speed of light) experience forces across their direction of travel from the ambient magnetic fields that pervade the plasma lobes within which they are travelling.
19. This is aradio imageofagiantquasar, spanningover onemillionlight-years in extent.
This acceleration causes them to emit photons of radiation (which may be radio, or in rare, highly energetic instances, at shorter wavelengths still, all the way up to X-rays) known as synchrotron radiation.
To give a sense of the scale of the power produced by quasars,consider the following values. The LEDs by whose light I am working have a power output of ten watts. They are illuminated by electricity frommy local power stationwhich produces a few billion watts (a billion watts is 109 watts or a gigawatt). The Sun outputs about 4 x 1026 watts, morethan ahundredmillion billion timesthat from this power station. Our Galaxy, theMilkyWay, containsmore than a hundred billion stars, and its power output is approaching1037 watts. But the power produced by a quasar can exceed even the Galactic power output by more than a factor of 100. Remember,this power is being emitted not by a galaxy of one hundred billion stars but by the processes going on around a single black hole. Such radiation could do considerable damage to the health of living creatures here on Earth, so it is just as well for us that there are no examples of such powerful quasars too near our Galaxy!
Jets in quasars are thought to persist for a billion years or less, an idea that comes from estimates of the speed at which these objects' jets grow and from measurements of the size they have grown out to. A simple relationship between distance and time and speed therefore gives a guide to the likely durations of jet activity in the quasars that are observed across the cosmos.
As these radio-emitting lobes expand, their magnetic fields weaken as do the `internal' energies of the individual electrons in the lobes. These two effects serve to diminish the intensity of the radiation with time and with distance from the black hole; how dramatically this intensity falls off depends on how many highly energized electrons there are compared with how many less energetic ones there are. It's a property of synchrotron radiation that the lower the magnetic field strength is, the more energetic the electrons need to be to produce the radiation at the wavelength that your radio telescope is tuned to receive at. This compounds the diminishing of the synchrotron radiation as the plasma lobes expand into outer space. Not only do the electrons lose energy as the plasma expands, but because the magnetic field strength is weakening, only increasingly energetic electrons are relevant to what is observed by your telescope and, very often, there are vastly fewer of these than there are of the lower energy electrons anyway.As far as radio lobes of quasars are concerned, the lights can go out really quite rapidly.
The show isn't over, but the spectacle does move over to a different waveband. Something rather remarkable happens: the lobes light up in X-rays. This happens via a scattering process known as inverse Compton scattering. In the presence of a sufficiently large magnetic field, electrons can emit synchrotron radiation and thereby lose energy. Another mechanism of losing energy that is relevant to our discussion here happens via the interaction of these electrons with photons that comprise the Cosmic Microwave Background (CMB), the radiation that is left over from the Big Bang and which currently bathes the Universe in a cool microwave glow. It is possible for such an electron to collide with a photon from the CMB so that the photon ends up with a lot more energy than it had before the collision and the electron ends up with a lot less energy than it had before the collision (energy is conserved overall, remember). Of particular interest is that when the energies of the rapidly moving electrons reduce to a mere one thousand times the energy of an electron at rest (having previously been a hundred or a thousand times higher than this) their energies are perfectly matched so that they will upscatter CMB photons into the X-ray photons. The interaction of an energetic electron with a low-energy photon to yield a high-energy photon is somewhat analogous to the situation in snooker where the white cue ball (imagine this is an electron) collides with one of the red snooker balls (for the purposes of this illustration please overlook the fact that this ball isn'tmoving at the speed of light!) and the red ball gains a lot of energy at the expense of the cue ball.Whereas (hopefully) the red ball ends up in one of the pockets on the snooker table, the photon (which originally had a wavelength of about a millimetre) acquires about one million times as much energy as it had before the collision so that its wavelength becomes a million times shorter.
The Chandra satellite, launched by NASA in 1999, is sensitive to X-ray wavelengths and in fact can detect pairs of dumb-bell lobes in the X-rays just as a radio telescope can detect these double structures at cm-wavelengths. Figures 20 and 21 show in contour form double structures observed at radio wavelengths and in greyscale form the double structures in X-rays.
In fact if we were able to monitor the life cycle of one of these quasars throughout all these evolutionary stages (analogously to how a biologist might observe the life cycle of the frog from frogspawn, to tadpoles, to tadpoles with little legs, to little frogs with stumpy tails, to larger frogs to dead frogs) we would observe a cross-over fromthe double structures being radiant at radio wavelengths to becoming increasingly dominant in the X-ray region. First the radio structures would fade beyond detectability then the X-ray structures would fade beyond detectability. Of course, if the jets were to re-start, for example if the black hole were to get more fuel, then the jets would fuel new radio-emitting double lobes and then X-ray emitting lobes again. As we have seen in Figures 20 and 21, in some quasars we can see both the radio and the X-ray double structures at the same time but in others,only one or the other (Figure 22). In a couple of remarkable cases we see the X-ray double structure corresponding to a previous incarnation of jet activity, but also some new radio activity,at a different angle because the direction along which the oppositely-directed jets are launched has swung round, i.e. it has precessed; an example of this phenomenon is seen in Figure 21.
20. This giant quasar is half amillion light-years in extent, and has adouble-lobe structure at both radio (shown as contour lines) and X-ray(shown in greyscale) wavelengths.
The steadiness of the jet axis of many quasars and radio galaxies is a pointer to the steadiness of the spin of the supermassive black hole, acting like a gyroscope. Why some of these jet axes should precess but not others will be answered when we can discover what controls the angular momentum ofthejets at the launch point near the black hole. Whether this is the spin axis ofthe black hole itself, or whether it is the angular momentum vector ofthe inner part of the accretion disc, compounded no doubt by the Lense-Thirring or Bardeen-Petterson effects I mentioned in Chapters 3 and 7 respectively, is not yet clear and more data are required to fully elucidate the observed behaviour. But, there are clues from smaller objects closer to home that may suggest that the precession of jet axes is everything to do with the accretion disc's angular momentum.
21. The double-lobe structure observed in this quasar at radiowavelengths [contours] showing themore recent activity to bedifferently oriented from that showing at X-ray energies [greyscale](the relic emission revealed by inverse Compton scattering of CMBphotons) suggesting that the jet axismay have precessed as the jet axesinmicroquasars do.
22. This is an X-rayimage andshowsthe double-lobestructurestraddlingthis galaxywhichis onlydetectableat X-raywavelengths.
Microquasars
The quasars we have been discussing so far are all supermassive black holes that lie at the centres of active galaxies. However,it turns out that there is another class of objects that behave very similarly but are on a much, much smaller scale. These lower mass black holes can be observed rather closer to home,indeed located within our own Milky Way Galaxy, and they are called `microquasars'. Although the difference in scale size is vast,microquasars in our Galaxy and extragalactic quasars at the centres of other galaxies are both sources of plasma jets with analogous physical properties. Both of these are thought to be powered by the gravitational infall of matter onto a black hole. In the case of a microquasar, the black hole has a mass comparable with that of the Sun. In the case of a powerful extragalactic quasar, the mass of its black hole can be a hundred million times larger than the mass of our Sun. As far as the astrophysicist is concerned, an important advantage of the local examples is that being less massive,they evolve much more rapidly, on timescales of days rather than millions of years in the case of quasars. Nonetheless, as in the case of quasars, the jets which are squirted out from near the centre of all the activity are launched from outside the event horizon,and very likely from the innermost edge of the accretion disc.
Complex mechanisms are at play, and there isn't a simple relationship between the speed at which a jet is launched and the mass of the black hole with which it is associated. In the course of monitoring the jets in the black hole microquasar called Cygnus X-3 there are occasions when the speeds at which the jet plasma moves away from the black hole are found to vary. This has been measured by time-lapse astronomical measurements in which observations at successive times allow us to determine how fast the jet plasma is hurtling away from the vicinity of the black hole.Such measurements have shown on one occasion the jet speed to be 81% of the speed of light whereas four years later to be 67%of that speed. There is no suggestion that the jet speed is merely reducing with time, since fast and slower jets speeds in this microquasar appear to have been witnessed on a number of occasions since its discovery. Varying jet speeds seem to characterize another well known microquasar in our Galaxy, called SS433, that I shall describe in more detail below. The jet speed in this microquasar seems to change quite a bit as well, indeed it can be anywhere between 20 and 30% of the speed of light over just a few days.
The beauty of symmetry
Figure 23 shows a radio image of SS433, a microquasar in the Galaxy, which is a mere 18,000 light-years distant from us. The striking zigzag/corkscrew pattern is the structure of the plasma jets as they appear to us on the plane of the sky. The individual bolides of plasma that make up the jets are moving at tremendous speeds that vary between 20 and 30% of the speed of light. The directions along which the bolides are moving varies with time in a very persistently periodicway. In fact the axis alongwhich the jets are launched precesses in much the same way as does the paddle of a kayakist, in the frame of reference of the kayak, except on a timescale of six months rather than several seconds. This same behaviour is apparently taking place in at least some quasars(see Figure 21) albeit in that case in such slow motion that we are unable to appropriately time-sample the changes taking place.
The detailed appearance of the zigzag/corkscrew pattern on the sky depends directly on the physical motions of the bolides, as well as the time when the observation is made. One of the remarkable features of the jets is their symmetry: the physical motions of the components in the eastern jet are equal and opposite to those in the western jet: when one bolide of plasma is at 28% of the speed of light, so too is its counterpart in the oppositely-directed jet; for a different bolide of plasma moving at 22%of the speed of light, so too will its counterpart in the oppositely-directed jet. The fact that one jet appears to have a zigzag structure while the other appears to have a rather different corkscrew pattern is a consequence of the jet plasma always moving at speeds comparable with the speed of light, and well-known relativistic aberrations that occur under such circumstances. The power radiated by this microquasar is rather modest relative to that of an extragalactic quasar but it is still vast in comparison to the power of the Sun which seems somewhat puny, having a total luminosity of only 4 x 1026 watts, a factor of a hundred thousand smaller than that radiated from the microquasar in Figure 23.
23. The jets of themicroquasar SS433 as they appear at radiowavelengths.
Jet launch
The Virgo Cluster is a cluster of well over a thousand galaxies just over fifty million light-years distant from theMilkyWay. At its heart is a giant galaxy called M87 (an abbreviation of Messier 87,listed in the catalogue produced by the French astronomer Charles Messier). And, at its heart, is a supermassive black hole whose mass is three billion times that of our Sun. Emanating from this is a strong straight jet, as shown in Figure 24.
24. A jet of plasma squirted out at speeds close to that of light, fromthe supermassive black hole at the heart of the M87 galaxy.
This jet is readily visible at optical wavelengths, at radio wavelengths, and at X-ray wavelengths. It is thought that the infallingmatter accretes at a rate of two to three Sun's worth of mass per year, onto the very central nucleus where an accretion disc of the sort described in Chapter 6 is thought to be at work.The speed at which this jet propagates away from its launch point,likely at the innermost region of the accretion disc, is very close to the speed of light, and so we refer to it as a relativistic jet. Jet speeds close to the speed of light are revealed by successive monitoring with the VLBA instrument that I introduced in Chapter 7, and the Hubble Space Telescope and Chandra X-ray satellites which are each above Earth's atmosphere and thus attain higher sensitivity than if they were on the ground. At 50 million light-years from Earth an object moving at the speed of light would move across the sky at four milli-arcseconds per year.When we consider that an arcsecond is 1/3600 of a degree, then four-thousandths of this may sound like a tiny angle to measure,but such separations are easily resolvable with an instrument like the VLBA. The VLBA has already imaged the base of this jet to within less than about thirty Schwarzschild radii of its supermassive black hole.
Figure 25 shows an example of the lobes and plumes of radio emitting plasma fed by the relativistic jets from the supermassive black hole in M87.
By way of further illustration that expansive lobes are associated with relativistic jets, Figure 26 shows an example that extends6 degrees across the sky, and is shown to give a sense of scale with respect to the telescope array used to make the observation. The telescope, used by Ilana Feain and her colleagues, was the Australia Telescope Compact Array.
The mechanisms by which relativistic jets are launched from the vicinity of a black hole remain much closer to conjecture than to acceptance beyond all reasonable doubt. Nonetheless, various independent lines of research by entirely independent teams based in different countries around the world seem to be implying that the preponderance of evidence is that the basic emerging details are correct. Beyond the broad picture, however, the mechanisms and their detailed functioning are conjectural, but being patiently tested amid insufficient photons and selection effects. Proof doesn't belong in science but evidence very much does.We are hindered because even the most advanced imaging techniques deployed today cannot separate and resolve the smallest regions where most of the energy is released, but this is where numerical simulations on powerful computers can transcend the limitations of current technology. Indeed results from simulations of jet launch from accretion discs that fully account for general relativity effects are just being published.These simulations, with known input ingredients and axioms,allow jets and discs to evolve to size-scales where their properties can be confronted against state-of-the-art observations.
25. The radio-emitting lobes that are fed by the relativistic jetemanating out of the supermassive black hole at the centre of theM87 galaxy.
So what do we now know about the masses of black holes in the Universe? It seems that they fall into two main classes. First,those that have masses similar to those of stars. These stellar mass black holes come in between around three to thirty times the mass of our Sun and come from stars that have burned all their fuel.
26. Compositepicture showingan opticalimage ofthemoon andtheAustralia Telescope CompactArray, andaradio image ofCentaurusA.
Then there are the supermassive black holes which go all the way up to about ten billion solar masses. As we have discussed, these are found in the centres of galaxies including our own and are responsible for the extraordinary phenomena of active galaxies and quasars.
We have talked about things falling into a black hole, but what happens when a black hole falls into a black hole? This is not an abstract question, since it is known that black hole binaries can exist. In such objects two black holes are in orbit around each other. It is thought that, because of the emission of gravitational radiation, the black holes in a binary will begin to lose energy and spiral into each other. In the final stages of this spiralling, general relativity is pushed to breaking point and the black holes suddenly coalesce into a single black hole with a common event horizon.The energy released in the merger of two supermassive black holes in a binary system is staggering, potentially more than all the light in all the stars in the visible Universe.Most of this energy is dumped into gravitational waves, ripples in the curvature of spacetime, which propagate across the Universe at the speed of light. The hunt is on for evidence of these waves. The idea is that as a gravitational wave passes by a material object, like a long rod,its length will fluctuate up and down as the ripples in spacetime curvature flow through it. If you can measure these tiny length changes, using a technique such as laser interferometry, then you have got a method to detect gravitational waves produced elsewhere in the Universe. Both ground- and space-based gravitational wave detectors, examples of which have been built and more of which are planned, have the potential to pick up signals from black hole mergers. In fact, gravitational waves are so difficult to detect that you need a very strong source to have any chance of such experiments working, and a black hole merger is high on the list of candidates for such strong sources. At the time of writing, gravitational waves have not yet been directly detected,but the experiments are ongoing.
Our best theory of gravity, which comes from Einstein's general theory of relativity, has survived countless tests since its discovery in 1915. It has been shown to give far better agreement to experiment than Newton's theory which it supplanted. However, if general relativity is ever going to be tested up to its limits, you can confidently expect that black holes will prove to be the ultimate testing ground of this cornerstone of modern physics. Where gravity is the most intense in the smallest region of space, so that quantum effects should be important, is exactly where general relativity might break down. However, it might also be that general relativity breaks down on large scales in the Universe.Of course, a hot topic at present is the completeness of general relativity to explain the accelerated expansion of the Universe on the largest scales. Possible deviations away from general relativity are being discussed in connection with accelerated expansion and dark energy. If gravitational waves are detected from the mergers of black holes, or if observations extend our understanding of the fundamental physics which occurs in the vicinity of these fascinating objects, then there's a good chance that we will be able to see how well Einstein's theory holds up or whether it needs to be replaced by something new.
Why do we study black holes?
There are a number of reasons for investigating black holes and one is that they open up the exploration of physics parameter space that is otherwise simply inaccessble to the budgets of even an international consortium. Black hole systems represent the most extreme environments that we can explore, and as such probe the extremes of physics. They bring together both general relativity and quantum physics whose unification has not yet been achieved and remains very much a frontier of physics. A second reason is that trying to understand black hole phenomena arouses fascination in scientists and many thoughtful lay people, and provides a route by which many people can be stimulated by science and motivated to learn about the almighty magnificence of the Universe around us. A third and perhaps surprising reason is Earthly spin-offs. How could black hole research conceivably change our lives? The answer is that it has already done so. As I type these final sentences of this little book into my laptop, it simultaneously backs up my work onto my University server via the 802.11 WiFi protocol. This intricate and clever technology emerged directly out of a search for a particular signature, at radio wavelengths, of exploding black holes led by Ron Ekers to test a model suggested by (now Astronomer Royal, Lord) Martin Rees.Ingenious radio engineers in Australia, led by John O'Sullivan, in the course of devising interference suppression algorithms for the tricky business of detecting subtle signals from distant space realized that these could be applied to transform communication here on Earth. Black holes therefore have the power to rewrite physics, reinvigorate our imagination and even revolutionize our technology. There are many spin-offs from black holes-way beyond their event horizons.