Origins: Fourteen Billion Years of Cosmic Evolution (3 page)

Quarks are quirky. Unlike protons, which each have an electric charge of +1, and electrons, each with a charge of
–
1, quarks have fractional charges that come in units of 1/3. And except under the most extreme conditions, you’ll never catch a quark all by itself; it will always be clutching on to one or two other quarks. In fact, the force that keeps two (or more) of them together actually grows
stronger
as you separate them—as if some sort of subnuclear rubber band held them together. Separate the quarks sufficiently far, and the rubber band snaps. The energy stored in the stretched band now summons
E = mc
2
to create a new quark at each end, leaving you back where you started.

During the quark-lepton era in the cosmos’s first trillionth of a second, the universe had a density sufficient for the average separation between unattached quarks to rival the separation between attached quarks. Under those conditions, allegiances between adjacent quarks could not be established unambiguously, so they moved freely among themselves. The experimental detection of this state of matter, understandably named “quark soup,” was reported for the first time in 2002 by a team of physicists working at the Brookhaven National Laboratories on Long Island.

The combination of observation and theory suggests that an episode in the very early universe, perhaps during one of the splits between different types of force, endowed the cosmos with a remarkable asymmetry, in which particles of matter outnumbered particles of antimatter by only about one part in a billion—a difference that allows us to exist today. That tiny discrepancy in population could hardly have been noticed amid the continuous creation, annihilation, and recreation of quarks and antiquarks, electrons and anti-electrons (better known as positrons), and neutrinos and antineutrinos. During that era, the odd man out—the slight preponderance of matter over antimatter—had plenty of opportunities to find other particles with which to annihilate, and so did all the other particles.

But not for much longer. As the universe continued to expand and cool, its temperature fell rapidly below 1 trillion degrees Kelvin. A millionth of a second had now passed since the beginning, but this tepid universe no longer had a temperature or density sufficient to cook quarks. All the quarks quickly grabbed dance partners, creating a permanent new family of heavy particles called hadrons (from the Greek
hadros
, meaning “thick”). That quark-to-hadron transition quickly produced protons and neutrons as well as other, less familiar types of heavy particles, all composed of various combinations of quarks. The slight matter-antimatter asymmetry in the quark-lepton soup now passed to the hadrons, with extraordinary consequences.

As the universe cooled, the amount of energy available for the spontaneous creation of particles declined continuously. During the hadron era, photons could no longer invoke
E = mc
2
to manufacture quark-antiquark pairs: their
E
could not cover the pairs’
mc
2
. In addition, the photons that emerged from all the remaining annihilations continued to lose energy to the ever-expanding universe, so their energies eventually fell below the threshold required to create hadron-antihadron pairs. Every billion annihilations left a billion photons in their wake—and only a single hadron survived, mute testimony to the tiny excess of matter over antimatter in the early universe. Those lone hadrons would ultimately get to have all the fun that matter can enjoy: they would provide the source of galaxies, stars, planets, and people.

Without the imbalance of a billion and one to a mere billion between matter and antimatter particles, all the mass in the universe (except for the dark matter whose form remains unknown) would have annihilated before the universe’s first second had passed, leaving a cosmos in which we could see (if we had existed) photons
and nothing else—
the ultimate Let-there-be-light scenario.

By now, one second of time has passed.

At 1 billion degrees, the universe remains piping hot—still able to cook electrons, which, along with their positron (antimatter) counterparts, continue to pop in and out of existence. But within the ever-expanding, ever-cooling universe, their days (seconds, really) are numbered. What was formerly true for hadrons now comes true for electrons and positrons: they annihilate each other, and only one electron in a billion emerges, the lone survivor of the matter-antimatter suicide pact. The other electrons and positrons died to flood the universe with a greater sea of photons.

With the era of electron-positron annihilation over, the cosmos has “frozen” into existence one electron for every proton. As the cosmos continues to cool, with its temperature falling below 100 million degrees, its protons fuse with other protons and with neutrons, forming atomic nuclei and hatching a universe in which 90 percent of these nuclei are hydrogen and 10 percent are helium, along with relatively tiny numbers of deuterium, tritium, and lithium nuclei.

Two minutes have now passed since the beginning.

Not for another 380,000 years does much happen to our particle soup of hydrogen nuclei, helium nuclei, electrons, and photons. Throughout these hundreds of millennia, the cosmic temperature remains sufficiently hot for the electrons to roam free among the photons, batting them to and fro.

As we will shortly detail in Chapter 3, this freedom comes to an abrupt end when the temperature of the universe falls below 3,000 degrees Kelvin (about half the temperature of the Sun’s surface). Right about now, all the electrons acquire orbits around the nuclei, forming atoms. The marriage of electrons with nuclei leaves the newly formed atoms within a ubiquitous bath of visible light photons, completing the story of how particles and atoms formed in the primordial universe.

As the universe continues to expand, its photons continue to lose energy. Today, in every direction astrophysicists look, they find a cosmic fingerprint of microwave photons at a temperature of 2.73 degrees, which represents a thousandfold decline in the photons’ energies since the time atoms first formed. The photons’ patterns on the sky—the exact amount of energy that arrives from different directions—retain a memory of the cosmic distribution of matter just before atoms formed. From these patterns, astrophysicists can obtain remarkable knowledge, including the age and shape of the universe. Even though atoms now form part of daily life in the universe, Einstein’s equation still has plenty of work to do—in particle accelerators, where matter-antimatter particle pairs are created routinely from energy fields; in the core of the Sun, where 4.4 million tons of matter are converted into energy every second; and in the cores of all other stars.

E = mc
2
also manages to apply itself near black holes, just outside their event horizons, where particle-antiparticle pairs can pop into existence at the expense of the black hole’s formidable gravitational energy. The British cosmologist Stephen Hawking first described the hijinks in 1975, showing that the entire mass of a black hole can slowly evaporate by this mechanism. In other words, black holes are not entirely black. The phenomenon is known as Hawking radiation, and serves as a reminder of the continued fertility of Einstein’s most famous equation.

But what happened
before
all this cosmic fury? What happened before the beginning?

Astrophysicists have no idea. Rather, our most creative ideas have little or no grounding in experimental science. Yet the religious faithful tend to assert, often with a tinge of smugness, that something must have started it all: a force greater than all others, a source from which everything issues. A prime mover. In the mind of such a person that something is, of course, God, whose nature varies from believer to believer but who always bears the responsibility for starting the ball rolling.

But what if the universe was always there, in a state or condition that we have yet to identify—a multiverse, for example, in which everything we call the universe amounts to only a tiny bubble in an ocean of suds? Or what if the universe, like its particles, just popped into existence from nothing we could see?

These rejoinders typically satisfy no one. Nonetheless, they remind us that informed ignorance provides the natural state of mind for research scientists at the ever-shifting frontiers of knowledge. People who believe themselves ignorant of nothing have neither looked for, nor stumbled upon, the boundary between what is known and unknown in the cosmos. And therein lies a fascinating dichotomy. “The universe always was,” gets no respect as a legitimate answer to “What was around before the beginning?” But for many religious people, the answer, “God always was,” is the obvious and pleasing answer to “What was around before God?”

No matter who you may be, engaging yourself in the quest to discover where and how everything began can induce emotional fervor—as if knowing our beginnings would bestow upon you some form of fellowship with, or perhaps governance over, all that comes later. So what is true for life itself is true for the universe: knowing where you came from is no less important than knowing where you are going.

CHAPTER 2

Antimatter Matters

P
article physicists have won the contest for the most peculiar, yet playful jargon of all the physical sciences. Where else could you find a neutral vector boson exchanged between a negative muon and a muon neutrino? Or a gluon exchange binding together a strange quark and a charmed quark? And where else can you meet squarks, photinos, and gravitinos? Alongside these seemingly countless particles with peculiar names, particle physicists must contend with a parallel universe of
anti
particles, collectively known as antimatter. Despite its persistent appearance in science fiction stories, antimatter is real. And as you might suppose, it does tend to annihilate upon contact with ordinary matter.

The universe reveals a peculiar romance between antiparticles and particles. They can be born together out of pure energy, and they can annihilate as they reconvert their combined mass back into energy. In 1932, the American physicist Carl David Anderson discovered the anti-electron, the positively charged, antimatter counterpart to the negatively charged electron. Since then, particle physicists have routinely made antiparticles of all varieties in the world’s particle accelerators, but only recently have they assembled antiparticles into whole atoms. Since 1996, an international group led by Walter Oelert of the Institute for Nuclear Physics Research in Jülich, Germany, has created atoms of antihydrogen, in which an anti-electron happily orbits an antiproton. To make these first anti-atoms, the physicists used the giant particle accelerator operated by the European Organization for Nuclear Research (better known by its French acronym CERN) in Geneva, Switzerland, where so many important contributions to particle physics have occurred.

The physicists use a simple creation method: make a bunch of anti-electrons and a bunch of antiprotons, bring them together at a suitable temperature and density, and wait for them to combine to form atoms. During their first round of experiments, Oelert’s team produced nine atoms of antihydrogen. But in a world dominated by ordinary matter, life as an antimatter atom can be precarious. The antihydrogen atoms survived for less than 40 nanoseconds (40 billionths of a second) before annihilating with ordinary atoms.

The discovery of the anti-electron was one of the great triumphs of theoretical physics, for its existence had been predicted just a few years earlier by the British-born physicist Paul A. M. Dirac.

To describe matter on the smallest size scales—those of atomic and subatomic particles—physicists developed a new branch of physics during the 1920s to explain the results of their experiments with these particles. Using newly established rules, now known as quantum theory, Dirac postulated from a second solution to his equation that a phantom electron from the “other side” might occasionally pop into the world as an ordinary electron, leaving behind a gap or hole in the sea of negative energies. Although Dirac hoped to explain protons in this way, other physicists suggested that this hole would reveal itself experimentally as a positively charged anti-electron, which had come to be known as a positron for its positive electric charge. The detection of actual positrons confirmed Dirac’s basic insight and established antimatter as worthy of as much respect as matter.

Equations with double solutions are not unusual. One of the simplest examples answers the question, What number times itself equals nine? Is it 3 or
–
3? Of course, the answer is both, because 3 x 3 = 9 and
–
3 x
–
3 = 9. Physicists cannot guarantee that all the solutions of an equation correspond to events in the real world, but if a mathematical model of a physical phenomenon is correct, manipulating its equations can be as useful as (and somewhat easier than) manipulating the entire universe. As with Dirac and antimatter, such steps often lead to verifiable predictions. If the predictions prove incorrect, then the theory is discarded. But regardless of the physical outcome, a mathematical model ensures that the conclusions you may draw from it are both logical and internally consistent.

Subatomic particles have many measurable features, of which mass and electric charge rank among the most important. Except for the particle’s mass, which is always the same for a particle and its antiparticle, the specific properties of each type of antiparticle will always be precisely opposite to those of the particle for which it provides the “anti.” For example, the positron has the same mass as the electron, but the positron has one unit of positive charge while the electron has one unit of negative charge. Similarly, the antiproton provides the oppositely charged antiparticle of the proton.

Believe it or not, the chargeless neutron also has an antiparticle. It’s called—you guessed it—the antineutron. An antineutron has an opposite zero charge with respect to the ordinary neutron. This arithmetical magic derives from the particular triplet of fractionally charged particles (the quarks) that form neutrons. The three quarks that compose a neutron have charges of –
, –
, and +
, while those in the antineutron have charges of
,
, and –
. Each set of three quark charges adds up to zero net charge, yet the corresponding components do have opposite charges.

Antimatter can pop into existence out of thin air. If gamma-ray photons have sufficiently high energy, they can transform themselves into electron-positron pairs, thus converting all of their seriously large energy into a small amount of matter, in a process whose energy side fulfills Einstein’s famous equation
E = mc
2
.

In the language of Dirac’s original interpretation, the gamma-ray photon kicked an electron out of the domain of negative energies, creating an ordinary electron and an electron hole. The reverse process can also occur. If a particle and an antiparticle collide, they will annihilate by refilling the hole and emitting gamma rays. Gamma rays are the sort of radiation you should avoid.

If you somehow manage to manufacture a blob of antiparticles at home, you have a wolf by the ears. Storage would immediately become a challenge, because your antiparticles would annihilate with any conventional sack or grocery bag (either paper or plastic) in which you chose to confine or carry them. A cleverer storage mechanism involves trapping the charged antiparticles within the confines of a strong magnetic field, where they are repelled by invisible but highly effective magnetic “walls.” If you embed the magnetic field in a vacuum, you can render the antiparticles safe from annihilation with ordinary matter. This magnetic equivalent of a bottle will also be the bag of choice whenever you must handle other container-hostile materials, such as the 100-million-degree glowing gases involved in (controlled) nuclear fusion experiments. The greatest storage problem arises after you have created entire anti-atoms, because anti-atoms, like atoms, do not normally rebound from a magnetic wall. You would be wise to keep your positrons and antiprotons in separate magnetic bottles until you must bring them together.

To generate antimatter requires at least as much energy as you can recover when it annihilates with matter to become energy again. Unless you had a full tank of antimatter fuel before launch, a self-generating antimatter engine would slowly suck energy from your starship. Perhaps the original
Star Trek
television and film series embodied this fact, but if memory serves, Captain Kirk continually asked for “more power” from the matter-antimatter drives, and Scotty invariably replied in his Scottish accent that “the engines canna take it.”

Although physicists expect hydrogen and antihydrogen atoms to behave identically, they have not yet verified this prediction experimentally, mainly because of the difficulty they face in keeping antihydrogen atoms in existence, rather than having them annihilate almost immediately with protons and electrons. They would like to verify that the detailed behavior of a positron bound to an antiproton in an antihydrogen atom obeys all the laws of quantum theory, and that an anti-atom’s gravity behaves precisely as we expect of ordinary atoms. Could an anti-atom produce antigravity (repulsive) instead of ordinary gravity (attractive)? All theory points toward the latter, but the former, if it should prove correct, would offer amazing new insights into nature. On atomic-size scales, the force of gravity between any two particles is immeasurably small. Instead of gravity, electromagnetic and nuclear forces dominate the behavior of these tiny particles, because both forces are much, much stronger than gravity. To test for antigravity, you would need enough anti-atoms to make ordinary-sized objects, so that you can measure their bulk properties and compare them to ordinary matter. If a set of billiard balls (and, of course, the billiard table and the cue sticks) were made of antimatter, would a game of anti-pool be indistinguishable from a game of pool? Would an anti–eight ball fall into the corner pocket in exactly the same way as an ordinary eight ball? Would anti-planets orbit an anti-star the way that ordinary planets orbit ordinary stars?