Science Matters (20 page)

There are four major accelerator centers you’re likely to read about:

A unified field theory is one in which two forces, seemingly very different from each other, are shown to be basically identical. In
a sense both Newtonian gravity and Maxwell’s equations represent unified field theories. The first showed that heavenly and earthly gravity were identical, the second that electricity and magnetism were really the same thing. Today the term is used to refer to new theories in which two or more of the four fundamental forces are seen to be the same.

You can visualize how seemingly different forces might be identical by thinking about the analogy of the ice skaters and the bucket of water. Suppose you had two sets of skaters—one set with a bucket of water frozen to ice, the other with a bucket of antifreeze—and suppose that the temperature in the rink was below freezing. The exchange that leads to the force might look very different—in one case it would involve a solid block of ice, in the other a fluid. We might argue that there were two different forces operating in the rink. If we raised the temperature, however, the ice would melt and we would see that the two forces were fundamentally the same—the similarity had been masked by the original low temperature.

In the same way, physicists argue that we have four forces today only because temperatures are low. If we allow particles to collide at very high speeds, the temperature at the point of collision will go up and we should see forces unify. The theories that predict how this unification will take place are the modern unified field theories.

The theories say that unification will take place in stages as the energy and temperature go up—two forces unifying, then a third joining those two, and finally the fourth coming in. The first unification, the one that brings together the weak and electromagnetic forces, has already been seen in the world’s great accelerators. The theories that describe this unification did quite a good job of predicting things like the masses and production
rates of the W and Z particles, so we have a great deal of confidence in them.

The next unification, in which the strong and electroweak forces come together, is described by a theory with the somewhat prosaic name of the standard model. It has been tested experimentally and come through with flying colors, so physicists have some confidence in it. The last unification, in which gravity joins the others in a single unified force, remains at the frontier of theoretical development, and will be discussed in the next section.

Gravity is fundamentally different from the strong, electromagnetic, and weak forces. For one thing, it is much weaker than they are. This may seem like a strange thing to say, since gravity plays such a large role in our lives, but think about this: we know that a small magnet, one that can fit in the palm of your hand, can hold up a nail through the electromagnetic force, even though the entire Earth is pulling down on it with the force of gravity.

More important, as we will see in Chapter 12, our best theory of gravity, called general relativity, describes the gravitational force in geometrical terms—it is seen as the result of the warping of space and time by the presence of matter. The other three forces, however, are described in terms of the exchange of gauge particles, as we saw in the previous section. Trying to reconcile these two different viewpoints turns out to be very difficult, but it is a task that has to be tackled if we are to unify all the forces. This final unification can be thought of as the current frontier in
the millennia-old quest to understand the fundamental nature of matter. Here are some terms you may read about as new data and ideas come in:

T
HE SKY IS WONDROUS
on a clear, cold, moonless night far from city lights. We marvel at the majestic sea of stars—thousands of stars, navigated by a half-dozen planets and the occasional brief meteor.

Scientists are no different in their sense of awe and wonder, and they turn to the stars in their search for answers to questions about the meaning of it all. To our unaided eyes all the stars look like brilliant points of light, some a little brighter and others fainter, some colored with a hint of red or blue. But when we focus our telescopes skyward we see many different kinds of stars. Some are hot and dense, burning their fuel at an incredible rate. Others are cool, consuming fuel much more slowly We see stars in their infancy and stars growing old. And once in a great while we catch a glimpse of a star in its final cataclysmic hours, wracked by a massive fatal explosion. All this variety of stars tells a story:

The life of any star is a constant battle against the force of gravity, which tries to pull the star in on itself. Against this unremitting force, stars deploy a variety of countervailing strategies. Some of these strategies allow stars to stave off collapse temporarily and some strategies allow them to stave it off forever. But nothing can protect the largest stars from eventual collapse into a black hole, the ultimate victory of gravity over matter.

All stars begin as diffuse clouds of dust in deep space. Somewhere in the cloud is a place where matter has gathered by chance more thickly than elsewhere, and the force of gravity exerted by the clump pulls in neighboring materials. This makes the clump more massive and increases its gravitational attraction, so even more material is pulled in. It’s not hard to guess the outcome of this process—the cloud starts to collapse around the original concentration of matter.

As the contraction progresses, the pressure and temperature at the center increases. First, electrons are torn off their parent atoms, creating a plasma. Then as the contraction continues, the nuclei in the plasma start moving faster and faster until, at last, nuclei approaching each other are moving so fast that they can overcome the electrical repulsion that exists between their protons. The nuclei come together and nuclear fusion begins—the nuclear fires ignite. Energy from fusion pours out from the core, setting up a pressure in the surrounding gas that balances the inward pull of gravity. When the energy reaches the outer layers, it moves off into space in the form of electromagnetic radiation and the stabilized cloud begins to shine. A star has been born.

The primary fuel for the fusion reaction is hydrogen. Two protons (the nuclei of hydrogen atoms) come together to form deuterium
(an isotope of hydrogen consisting of one proton and one neutron) and some other particles. Subsequent collisions of the deuterium with other protons eventually produce helium-4, a nucleus consisting of two protons and two neutrons. In symbolic form, the nuclear reaction can be written as:

As in nuclear reactions, the conversion of mass (in this case some of the mass of the four initial protons) supplies the energy.

While the star is contracting and stabilizing itself, some interesting events are taking place out in its periphery. The original cloud will, in general, have some small rotation. As the contraction starts, the rotation speed increases. The cloud is like an ice skater who, when she pulls her arms in while spinning, spins faster. If nothing counters it, contraction will increase the spin until the star is torn apart. There are two ways for the nascent star to avoid this fate: it can split into two, forming a double star system, or it can form planets. In both cases, the spin is transferred from the body of the star to the revolution of the stars or planets around each other. Most stars seem to take the double star route—at least two thirds of those you see in the sky are multiple star systems. The search for planetary systems around other stars has revealed several stars with multiple planets, and astronomers are searching for a system like our own.

The appetite of stars for hydrogen is truly prodigious. The sun, for example, consumes some 700 million tons each second, with about 5 million tons being converted into energy (primarily in the form of gamma rays). Yet so large is our star that it has burned its
hydrogen at this rate for 4.6 billion years and will continue to do so for more than five billion years before running out of fuel.

How long will a star live? That of course depends on how much hydrogen it has and how fast it is consumed. Oddly enough, the larger a star is, the shorter its lifetime. The reason for this seeming paradox is simple: the bigger a star is, the greater is the gravitational force trying to make it collapse and the more hydrogen has to be burned to keep the star stable. The sun, a quite ordinary star, has enough fuel to keep gravity at bay for ten billion years, but a star thirty times as massive as the sun must burn its fuel in such a profligate way that it will shine for only a few million years. A star much smaller than the sun, on the other hand, will live for tens of billions of years—longer than the age of the universe. The star just pokes along, doling out miserly bits of energy into space as it husbands its hydrogen throughout a long and frugal life.

Profligate or miserly, every star must eventually burn up all of its hydrogen, filling the core with helium ash. When the hydrogen is gone, the outward force generated by the nuclear reactions disappears and gravity resumes its inevitable inward march. The inner parts of the star start to contract and warm up. For a star like the sun, the interior heating temporarily produces more energy as hydrogen burns just outside the core and the outer regions of the star are pushed farther outward, creating what astronomers call a red giant. Five billion years from now the body of the sun will extend out past the present orbits of Mercury and Venus and will scorch Earth’s lifeless surface.

The core continues to contract even as the outer layers puff up, and soon the core becomes so hot that helium, the ash of the
hydrogen fire, itself starts to fuse. In a series of reactions, three helium nuclei come together to form nuclei of carbon. Once the helium is consumed (a process that may take only a few minutes in a star like the sun), the collapse starts again in earnest. The bloated outer layers are blown off, while the inner region continues to contract. There is no more fuel to burn, so something else has to stop the collapse. That “something else,” for the sun and stars like it, is related to the behavior of electrons. Electrons in the star cannot overlap—they need elbow room. Only so many can be crammed together in a given volume. When the core has collapsed down to the size of the Earth, its electrons will have reached the point where they cannot be further compressed and the star will be stabilized forever, with gravity pushing in and the electrons pushing out. A star held up by pressure from its electrons is called a white dwarf. It generates no internal energy, having used up all its fuel, but continues to glow for a long time as it cools off.

Currently, theorists believe that stars with masses up to eight times that of the sun will end up as white dwarves. Made almost entirely of carbon nuclei, such a star is truly, as our childhood rhyme told us, “like a diamond in the sky.”

If the star is very massive, however, its death is much more spectacular. It burns through its hydrogen quickly. Then, after a short collapse, it starts burning helium to make carbon. When the helium is exhausted and the inevitable collapse starts again, the temperatures at the center of the star get so high that even the carbon starts to fuse. This pattern continues, the ashes of each fire serving as fuel for the next as the star desperately tries to stave off the inevitable. In the final stages of nuclear burning, iron starts to be produced. Iron is the ultimate nuclear ash. It is impossible to get energy from iron by allowing it to fuse with another nucleus, and it is impossible to get energy from it by fission. As
the star’s core clogs up with iron, there is no way for the star to generate more energy. Again the collapse starts, but this time the force exerted by the electrons is not enough to overcome gravity Electrons are forced inside the protons in the core, neutrons are produced, and the core shrinks quickly to a sphere of neutrons—a neutron star—about ten miles across. The force of gravity and the pressure of neutrons against each other balance and, providing the force of gravity is not too strong, the core stabilizes.