For the Love of Physics

by Walter Lewin

  Of course, we still have massive amounts of fossil fuel on Earth, but we are using it up much, much faster than nature can create it. And the world population continues to grow, while energy-intensive development is proceeding at an extremely rapid clip in many of the largest growth countries, like China and India. So there really is no way around it. We have a very serious energy crisis. What should we do about it?

  Well, one important thing is to become more aware of just how much energy we use every day, and to use less. My own energy consumption is quite modest, I think, although since I live in the United States, I’m sure I also consume four or five times more than the average person in the world. I use electricity; I heat my house and water with gas, and I cook with gas. I use my car—not very much, but I do use some gasoline. When I add that all up, I think I consumed (in 2009) on average about 100 million joules (30 kilowatt-hours) per day, of which about half was electrical energy. This is the energy equivalent of having about two hundred slaves working for me like dogs twelve hours a day. Think about that. In ancient times only the richest royalty lived like this. What luxurious, incredible times we live in. Two hundred slaves are working for me every single day, twelve hours a day without stopping, all so that I can live the way I live. For 1 kilowatt-hour of electricity, which is 3.6 million joules, I pay a mere 25 cents. So my entire energy bill (I included gas and gasoline, as their price per unit energy is not very different) for those two hundred slaves was, on average, about $225 a month; that’s about $1 per slave per month! So a change of consciousness is vital. But that will only get us so far.

  Changing habits to use more energy-conserving devices, such as compact fluorescent lights (CFLs) instead of incandescent lights, can make a large difference. I got to see the change I could make in quite a dramatic fashion. My electric consumption at my home in Cambridge was 8,860 kilowatt-hours in 2005 and 8,317 kilowatt-hours in 2006. This was for lighting, air-conditioning, my washing machine, and the dryer (I use gas for hot water, cooking, and heating). In mid-December of 2006, my son, Chuck (who is the founder of New Generation Energy), gave me a wonderful present. He replaced all the incandescent lightbulbs (a total of seventy-five) in my house with fluorescent bulbs. My electricity consumption dropped dramatically in 2007 to 5,251 kilowatt-hours, 5,184 kilowatt-hours in 2008, and 5,226 kilowatt-hours in 2009. This 40 percent reduction in my electricity consumption lowered my yearly bill by about $850. Since lighting alone accounts for about 12 percent of U.S. residential electric energy use and 25 percent of commercial use, it’s clearly the way to go!

  Following a similar path, the Australian government started to make plans in 2007 to replace all incandescent lightbulbs in the country with fluorescent ones. This would not only substantially reduce Australia’s greenhouse gas emission, but it would also reduce energy bills in every household (as it did in mine). We still need to do more, though.

  I think the only way that we might survive while keeping anything like our current quality of life is by developing nuclear fusion as a reliable, serious energy source. Not fission—whereby uranium and plutonium nuclei break up into pieces and emit energy, which powers nuclear reactors—but fusion, in which hydrogen atoms merge together to create helium, releasing energy. Fusion is the process that powers stars—and thermonuclear bombs. Fusion is the most powerful energy-producing process per unit of mass we know of—except for matter and antimatter colliding (which has no potential for energy generation).

  For reasons that are quite complicated, only certain types of hydrogen (deuterium and tritium) are well suited for fusion reactors. Deuterium (whose nucleus contains one neutron as well as one proton) is readily available; about one in every six thousand hydrogen atoms on Earth is deuterium. Since we have about a billion cubic kilometers of water in our oceans, the supply of deuterium is pretty much unlimited. There is no naturally occurring tritium on Earth (it’s radioactive with a half life of about twelve years), but it is easily produced in nuclear reactors.

  The real problem is how to create a functioning, practical, controlled fusion reactor. It’s not at all clear that we will ever succeed in doing so. In order to get hydrogen nuclei to fuse, we need to create, here on Earth, temperatures in the 100-million-degree range, approximating the temperature at the core of stars.

  Scientists have been working hard on fusion for many years—and I think they are working harder on it now that more and more governments seem genuinely convinced that the energy crisis is real. It’s a big problem, for sure. But I’m an optimist. After all, in my professional lifetime I’ve seen changes in my field that have been absolutely mind-blowing, turning our notions of the universe upside down. Cosmology, for instance, which used to be mostly speculation and a little bit of science, has now become a genuine experimental science, and we know an enormous amount about the origins of our universe. In fact, we now live in what many call the golden age of cosmology.

  When I began to do research in X-ray astronomy, we knew of about a dozen X-ray sources in deep space. Now we know of many tens of thousands. Fifty years ago the computing capacity in your four-pound laptop would have taken up most of the building at MIT where I have my office. Fifty years ago astronomers relied on ground-based optical and radio telescopes—that was it! Now we not only have the Hubble Space Telecope, we’ve had a string of X-ray satellite observatories, gamma ray observatories, and we’re using and building new neutrino observatories! Fifty years ago even the likelihood of the big bang was not a settled issue. Now we not only think we know what the universe looked like in the first one-millionth of a second after the big bang—we confidently study astronomical objects more than 13 billion years old, objects formed in the first 500 million years after the explosion that created our universe. Against the backdrop of these immense discoveries and transformations, how can I not think scientists will solve the problem of controlled fusion? I don’t want to trivialize the difficulties, or the importance of doing so soon, but I do believe it’s only a question of time.

  CHAPTER 10

  X-rays from Outer Space!

  The heavens have always provided a daily and nightly challenge to human beings seeking to understand the world around us, which is one reason physicists have always been entranced by astronomy. “What is the Sun?” we wonder. “Why does it move?” And what about the Moon, the planets, and the stars? Think about what it took for our ancestors to figure out that the planets were different from the stars; that they orbited the Sun; and that those orbits could be observed, charted, explained, and predicted. Many of the greatest scientific minds of the sixteenth and seventeenth centuries—among them Nicolaus Copernicus, Galileo Galilei, Tycho Brahe, Johannes Kepler, Isaac Newton—were compelled to turn their gaze to the heavens to unlock these nightly mysteries. Imagine how exciting it must have been for Galileo when he turned his telescope toward Jupiter, barely more than a point of light, and discovered four little moons in orbit around it! And, at the very same time, how frustrating it must have been to them to know so little about the stars that came out night after night. Remarkably, the ancient Greek Democritus as well as the sixteenth-century astronomer Giordano Bruno proposed that the stars are like our own Sun, but there was no evidence to prove them right. What could they be? What held them in the sky? How far away were they? Why were some brighter than others? Why did they have different colors? And what was that wide band of light reaching from one horizon to the other on a clear night?

  The story of astronomy and astrophysics since those days has been the quest to answer those questions, and the additional questions that arose when we started to come up with some answers. For the last four hundred years or so, what astronomers have been able to see, of course, has depended on the power and sensitivity of their telescopes. The great exception was Tycho Brahe, who made very detailed observations with the naked eye, using very simple equipment, that allowed Kepler to arrive at three major discoveries, now known as Kepler’s laws.

  For most of that time all we had were optical te
lescopes. I know that sounds odd to a non-astronomer. When you hear “telescope,” you think, automatically, “tube with lenses and mirrors that you peer into,” right? How could a telescope not be optical? When President Obama hosted an astronomy night in October 2009, there were a bunch of telescopes set up on the White House lawn, and every single one of them was an optical telescope.

  But ever since the 1930s, when Karl Jansky discovered radio waves coming from the Milky Way, astronomers have been seeking to broaden the range of electromagnetic radiation through which they observe the universe. They have hunted for (and discovered) microwave radiation (high-frequency radio waves), infrared and ultraviolet radiation (with frequencies just below and just above those of visible light), X-rays, and gamma rays. In order to detect this radiation, we’ve developed a host of specially designed telescopes—some of them X-ray and gamma ray satellites—enabling us to see more deeply and broadly into the universe. There are even neutrino telescopes underground, including one being built right now at the South Pole, called, appropriately enough, IceCube.

  For the last forty-five years—my life in astrophysics—I have been working in the field of X-ray astronomy: discovering new X-ray sources and developing explanations for the many different phenomena we observe. As I wrote earlier, the beginning of my career coincided with the heady and exciting early years of the field, and I was in the thick of things for the next four decades. X-ray astronomy changed my life, but more important, it changed the face of astronomy itself. This chapter and the four that follow will take you on a tour of the X-ray universe, from the standpoint of someone who’s worked and lived in that universe for his entire scientific career. Let’s start with X-rays themselves.

  What Are X-rays?

  X-rays have an exotic-sounding name, which they received because they were “unknown” (like the x in an equation), but they are simply photons—electromagnetic radiation—making up the portion of the electromagnetic spectrum that we cannot see between ultraviolet light and gamma rays. In Dutch and in German they are not called X-rays; instead they are named after the German physicist, Wilhelm Röntgen, who discovered them in 1895. We distinguish them the same way we identify other inhabitants of that spectrum, in three different but connected ways: by frequency (the number of cycles per second, expressed in hertz), by wavelength (the length of an individual wave, in meters, in this case nanometers), or by energy (in electron volts, eV, or thousands of electron volts, keV).

  Here are some quick points of comparison. Green light has a wavelength of about 500 billionths of a meter, or 500 nanometers, and an energy of about 2.5 electron volts. The lowest-energy X-ray photon is about 100 eV, forty times the energy of a photon of green light, with a wavelength of about 12 nanometers. The highest-energy X-rays are about 100 keV, with wavelengths of about 0.012 nanometers. (Your dentist uses X-rays up to about 50 keV.) At the other end of the electromagnetic spectrum, in the United States, radio stations broadcast in the AM band between 520 kilohertz (wavelength 577 meters—about a third of a mile) and 1,710 kilohertz (wavelength 175 meters—nearly twice the length of a football field). Their energy is a billion times less than green light, and a trillion times less than X-rays.

  Nature creates X-rays in a number of different ways. Most radioactive atoms emit them naturally during nuclear decay. What happens is that electrons jump down from a higher energy state to a lower one; the difference in energy can be emitted as an X-ray photon. These photons have very discrete energies as the energy levels of the electrons are quantized. Or, when electrons pass by atomic nuclei at high speeds, they change direction and emit some of their energy in the form of X-rays. We call this kind of X-ray emission, which is very common in astronomy as well as in any medical or dental X-ray machine, a difficult German name, bremsstrahlung, which literally means “braking radiation.” There are some helpful animated versions of bremsstrahlung X-ray production here: www.youtube.com/watch?v=3fe6rHnhkuY. X-rays of discrete energies can also be produced in some medical X-ray machines, but in general the bremsstrahlung (which produces a continuous X-ray spectrum) dominates. When high-energy electrons spiral around magnetic field lines, the direction of their speed changes all the time and they will therefore also radiate some of their energy in the form of X-rays; we call this synchrotron radiation, but it’s also called magnetic bremsstrahlung (this is what is happening in the Crab Nebula—see below).

  Nature also creates X-rays when it heats dense matter to very, very high temperatures, millions of degrees kelvin. We call this blackbody radiation (see chapter 14). Matter only gets this hot in pretty extreme circumstances, such as supernova explosions—the spectacular death explosions of some massive stars—or when gas falls at very high speeds toward a black hole or neutron star (more on that in chapter 13, promise!). The Sun, for instance, with a temperature of about 6,000 kelvin at its surface, radiates a little less than half its energy (46 percent) in visible light. Most of the rest is in infrared (49 percent) and ultraviolet (5 percent) radiation. It’s nowhere near hot enough to emit X-rays. The Sun does emit some X-rays, the physics of which is not fully understood, but the energy emitted in X-rays is only about one-millionth of the total energy it emits. Your own body emits infrared radiation (see chapter 9); it’s not hot enough to emit visible light.

  One of the most interesting—and useful—aspects of X-rays is that certain kinds of matter, like bones, absorb X-rays more than others, like soft tissue, which explains why an X-ray image of your mouth or hand shows light and dark areas. If you’ve had an X-ray, you’ve also had the experience of being draped with a lead apron to protect the rest of your body, since exposure to X-rays can also increase your risk of getting cancer. Which is why it’s mostly a good thing that our atmosphere is such a good absorber of X-rays. At sea level about 99 percent of low-energy X-rays (at 1 keV) are absorbed by just 1 centimeter of air. For X-rays at 5 keV, it takes about 80 centimeters of air, nearly three feet, to absorb 99 percent of the X-rays. For high-energy X-rays at 25 keV, it takes about 80 meters of air to absorb the same proportion.

  The Birth of X-ray Astronomy

  Now you understand why, back in 1959, when Bruno Rossi had the idea to go looking for X-rays from outer space, he proposed using a rocket that could get completely outside the atmosphere. But his idea about looking for X-rays was wild. There really were no sound theoretical reasons to think there were X-rays coming from outside the solar system. But Rossi was Rossi, and he convinced his former student Martin Annis at American Science and Engineering (AS&E) and one member of his staff, Riccardo Giacconi, that the idea was worth pursuing.

  Giacconi and his co-worker Frank Paolini developed special Geiger-Müller tubes that could detect X-rays and fit into the nose cone of a rocket. In fact, they put three of them in one rocket. They called them large-area detectors, but large in those days meant the size of a credit card. The AS&E guys went looking for funding to underwrite this experiment, and NASA turned their proposal down.

  Giacconi then changed the proposal by including the Moon as a target and resubmitted it to the Air Force Cambridge Research Laboratories (AFCRL). The argument was that the solar X-rays should produce so-called fluorescent emission from the lunar surface and that this would facilitate chemical analysis of the lunar surface. They also expected bremsstrahlung from the lunar surface due to the impact of electrons present in the solar wind. Since the Moon is so close, X-rays might be detectable. This was a very smart move, as AS&E had already received support from the Air Force for several other projects (some of which were classified), and they may have known that the agency was interested in the Moon. In any event, this time the proposal was approved.

  After two rocket failures in 1960 and 1961, the launch one minute before midnight on June 18, 1962, had the stated mission of trying to detect X-rays from the Moon and to search for X-ray sources beyond the solar system. The rocket spent just six minutes above the 80-kilometer mark (over 250,000 feet up), where the Geiger-Müller tubes could detect X-rays in the range
from about 1.5–6 keV without atmospheric interference. That’s the way you observed in space with rockets in those days. You sent the rockets out of the atmosphere, where they scanned the skies for only five or six minutes, then they came back down.

  The truly amazing thing is that right away they found X-rays—not from the Moon, but from someplace outside the solar system.

  X-rays from deep space? Why? No one understood the finding. Before that flight we had known of exactly one star that emitted X-rays, our own Sun. And if the Sun had been 10 light-years away, say, which is really just around the corner in astronomical terms, the equipment in that historic flight was a million times too insensitive to detect its X-rays. Everyone knew this. So wherever this source was located, it had to emit at least a million times more X-rays than the Sun—and that was only if it was really close by. Astronomical objects that produced (at least) a million or a billion times more X-rays than the Sun were literally unheard of. And there was no physics to describe such an object. In other words, it had to be a brand new kind of phenomenon in the heavens.

  A whole new field of science was born the night of June 18–19, 1962: X-ray astronomy.

  Astrophysicists began sending up lots of rockets fitted with detectors to figure out precisely where the source was located and whether there were any others. There is always uncertainty in measuring the position of objects in the heavens, so astronomers talk about an “error box,” an imaginary box pasted on the dome of the sky whose sides are measured in degrees, or arc minutes, or arc seconds. They make the box big enough so there is a 90 percent chance that the object is really inside it. Astronomers obsess about error boxes, for obvious reasons; the smaller the box, the more accurate the position of the object. This is especially important in X-ray astronomy, where the smaller the box, the more likely it is that you will be able to find the source’s optical counterpart. So making the box really, really small is a major achievement.