For the Love of Physics (30 page)

However, only 0.7 percent of natural uranium consists of uranium 235 (99.3 percent is uranium-238). Therefore, nuclear power plants use
enriched
uranium; the degree of enrichment varies, but a typical number is 5 percent. This means that instead of 0.7 percent uranium-235, their uranium fuel rods contain 5 percent uranium-235. Thus a 1,000-megawatt nuclear reactor will consume about 8,000 kilograms of uranium per year, of which about 400 kilograms is uranium-235. In comparison, a 1,000-megawatt fossil-fuel power plant will consume about 5 billion kilograms of coal per year.

The enrichment of uranium is costly; it’s done with thousands of centrifuges. Weapons-grade uranium is enriched to at least 85 percent uranium-235. Perhaps you now understand why the world is very worried about countries that enrich uranium to an unspecified degree that cannot be verified!

In nuclear power plants, the heat produced by the controlled chain reactions turns water into steam, which then drives a steam turbine, producing electricity. A nuclear power plant’s efficiency converting nuclear energy into electricity is about 35 percent. If you read that a nuclear power plant produces 1,000 megawatts, you do not know whether it is 1,000 megawatts total power (of which 1/3 is converted to electrical energy and of which 2/3 is lost as heat), or whether it’s all electric power in which case the total plant’s power is about 3,000 megawatts. It makes a big difference! I read yesterday in the news that Iran is shortly going to
put on line a nuclear power plant that will produce 1,000 megawatts of electricity (that’s clear language!).

As concern about global warming has increased dramatically in the past few years, the nuclear energy option is coming back into fashion—unlike power plants burning fossil fuels, nuclear plants don’t emit much in the way of greenhouse gases. There are already more than a hundred nuclear power plants in the United States, producing about 20 percent of the energy we consume. In France this number is about 75 percent. Worldwide, about 15 percent of the total electric energy consumed is produced in nuclear plants. Different countries have different policies regarding nuclear power, but building more plants will require a great deal of political persuasion due to the fear generated by the infamous nuclear accidents at Three Mile Island and Chernobyl. The plants are also
very
expensive: estimates range from $5 to $10 billion per plant in the United States, and around $2 billion in China. Finally, storing the radioactive waste from nuclear plants remains an enormous technological and political problem.

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!

T
he 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 telescopes. 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).