-MARK TWAIN.

You can recognize a pioneer by the arrows in his back.

-BEVERLY RUBIK.

In Dan Brown"s book Angels and Demons, the bestselling predecessor to The Da Vinci Code, a small band of extremists, the Illuminati, have hatched a plot to blow up the Vatican using an antimatter bomb, stolen from CERN, the nuclear laboratory outside Geneva. The conspirators know that when matter and antimatter touch each other the result is a monumental explosion, many times more powerful than a hydrogen bomb. Although an antimatter bomb is pure fiction, antimatter is very real.

An atomic bomb, for all its awesome power, is only about 1 percent efficient. Only a tiny fraction of the uranium is turned into energy. But if an antimatter bomb could be constructed, it would convert 100 percent of its ma.s.s into energy, making it far more efficient than a nuclear bomb. (More precisely, about 50 percent of the matter in an antimatter bomb would be turned into usable explosive energy; the rest would be carried away in the form of undetectable particles called neutrinos.) Antimatter has long been the focus of intense speculation. Although an antimatter bomb does not exist, physicists have been able to use their powerful atom smashers to create minute quant.i.ties of antimatter for study.

PRODUCING ANTI-ATOMS AND ANTI-CHEMISTRY.

At the beginning of the twentieth century, physicists realized that the atom consisted of charged subatomic particles with electrons (with a negative charge) circulating around a tiny nucleus (with a positive charge). The nucleus, in turn, consisted of protons (which carried the positive charge) and neutrons (which were electrically neutral).

So it came as quite a shock in the 1930s when physicists realized that for every particle there is a twin, an antiparticle, but with an opposite charge. The first antiparticle to be discovered was the antielectron (called the positron), which has a positive charge. The positron is identical to the electron in every way, except that it carries the opposite charge. It was first discovered in photographs of cosmic rays taken in a cloud chamber. (Positron tracks are quite easy to see in a cloud chamber. When placed in a powerful magnetic field, they bend in the opposite direction from ordinary electrons. In fact, I photographed such antimatter tracks while I was in high school.) In 1955 the particle accelerator at the University of California at Berkeley, the Bevatron, produced the first antiproton. As expected, it is identical to the proton except that it has a negative charge. This means that, in principle, one can create anti-atoms (with positrons circulating around antiprotons). In fact, anti-elements, anti-chemistry, anti-people, anti-Earths, and even anti-universes are theoretically possible.

At present the giant particle accelerators at CERN and the Fermilab outside Chicago have been able to create minute quant.i.ties of antihydrogen. (This is done by blasting a beam of high-energy protons into a target using particle accelerators, thereby creating a shower of subatomic debris. Powerful magnets separate out the antiprotons, which are slowed down to very low velocities and then are exposed to the antielectrons that are naturally emitted from sodium-22. When the antielectrons...o...b..t around the antiprotons, they create antihydrogen, since hydrogen is made up of one proton and one electron.) In a pure vacuum, these anti-atoms might live forever. But because of impurities and collisions with the wall, these anti-atoms eventually strike ordinary atoms and they are annihilated, releasing energy.

In 1995 CERN made history when it announced that it had created nine antihydrogen atoms. Fermilab soon followed suit by producing one hundred atoms of antihydrogen. In principle, there is nothing to prevent us from creating higher anti-elements as well, except for the staggering cost. Producing even a few ounces of anti-atoms would bankrupt any nation. The current rate of production of antimatter is between one-billionth to ten-billionths of a gram per year. The yield might increase by a factor of three by the year 2020. The economics of antimatter are very poor. In 2004 it cost CERN $20 million to produce several trillionths of a gram of antimatter. At that rate, producing a single gram of antimatter would cost $100 quadrillion and the antimatter factory would need to run continuously for 100 billion years! This makes antimatter the most precious substance in the world.

"If we could a.s.semble all the anti-matter we"ve ever made at CERN and annihilate it with matter," reads a statement from CERN, "we would have enough energy to light a single electric light bulb for a few minutes."

Handling antimatter poses extraordinary problems, since any contact between matter and antimatter is explosive. Putting antimatter in an ordinary container would be suicide. When the antimatter touched the walls, it would explode. So how does one handle antimatter if it is so volatile? One way would be first to ionize the antimatter into a gas of ions, and then to safely confine it in a "magnetic bottle." The magnetic field would prevent the antimatter from touching the walls of the chamber.

To build an antimatter engine, a steady stream of antimatter would need to be fed into a reaction chamber, where it would be carefully combined with ordinary matter, creating a controlled explosion, similar to the explosion created by chemical rockets. The ions created by this explosion would then be shot out one end of the antimatter rocket, creating propulsion. Because of the antimatter engine"s efficiency in converting matter into energy, in theory it is one of the most appealing engine designs for future starships. In the Star Trek series, antimatter is the source of the Enterprise"s energy; its engines are energized by the controlled collision of matter and antimatter.

AN ANTIMATTER ROCKET.

One of the main proponents of the antimatter rocket is physicist Gerald Smith of Pennsylvania State University. He believes that in the short term as little as 4 milligrams of positrons would be sufficient to take an antimatter rocket to Mars in just several weeks. He notes that the energy packed into antimatter is about a billion times greater than the energy packed into ordinary rocket fuel.

The first step in creating this fuel would be to create beams of antiprotons, via a particle accelerator, and then store them in a "Penning trap," which Smith is constructing. When built, the Penning trap would weigh 220 pounds (much of it being liquid nitrogen and liquid helium) and would store about a trillion antiprotons in a magnetic field. (At very low temperatures, the wavelength of the antiprotons is several times longer than the wavelength of the atoms in the container walls, so the antiprotons would mainly reflect off the walls without annihilating themselves.) He states that this Penning trap should be able to store the antiprotons for about five days (until they finally are annihilated when mixed with ordinary atoms). His Penning trap should be able to store about a billionth of a gram of antiprotons. His goal is to create a Penning trap that can store up to a microgram of antiprotons.

Although antimatter is the most precious substance on Earth, its cost keeps dropping dramatically every year (a gram would cost about $62.5 trillion at today"s prices). A new particle injector being built at Fermilab outside Chicago should be able to increase the production of antimatter by a factor of ten, from 1.5 to 15 nanograms per year, which should drive down prices. However, Harold Gerrish of NASA believes that with further improvements the cost could realistically go down to $5,000 per microgram. Dr. Steven Howe, of Synergistics Technologies in Los Alamos, New Mexico, states, "Our goal is to remove antimatter from the far-out realm of science fiction into the commercially exploitable realm for transportation and medical applications."

So far, particle accelerators that can produce antiprotons are not specifically designed to do so, so they are quite inefficient. Such particle accelerators are designed primarily to be research tools, not factories for antimatter. That is why Smith envisions building a new particle accelerator that will be specifically designed to produce copious quant.i.ties of antiprotons to drive down the cost.

If prices for antimatter can be lowered even further by technical improvements and ma.s.s production, Smith envisions a time when the antimatter rocket could become a workhorse for interplanetary and possibly interstellar travel. Until then, however, antimatter rockets will remain on the drawing boards.

NATURALLY OCCURRING ANTIMATTER.

If antimatter is so difficult to create on Earth, might one find antimatter more easily in outer s.p.a.ce? Unfortunately, searches for antimatter in the universe have turned up very little, which is rather surprising to physicists. The fact that our universe is made up mainly of matter, rather than antimatter, is difficult to explain. One might naively have a.s.sumed that at the beginning of the universe, there were equal, symmetrical quant.i.ties of matter and antimatter. So the lack of antimatter is puzzling.

The most likely solution was first proposed by Andrei Sakharov, the man who designed the hydrogen bomb for the Soviet Union in the 1950s. Sakharov theorized that at the beginning of the universe there was a slight asymmetry in the amount of matter and antimatter in the big bang. This tiny symmetry breaking is called "CP violation." This phenomenon is currently the center of much vigorous research. In effect, Sakharov theorized that all the atoms in the universe today are left over from a near perfect cancellation between matter and antimatter; the big bang caused a cosmic cancellation between the two. The tiny leftover matter created a residue that forms the visible universe of today. All the atoms in our bodies are leftovers from this t.i.tanic collision of matter and antimatter.

This theory leaves open the possibility that small amounts of antimatter may occur naturally. If so, discovering that source would drastically reduce the cost of producing antimatter for use in antimatter engines. In principle, deposits of naturally occurring antimatter should be easy to detect. When an electron and an antielectron meet, they annihilate into gamma rays at an energy of 1.02 million electron volts or more. Thus by scanning the universe for gamma rays at this energy, one could find the "fingerprint" for naturally occurring antimatter.

In fact, "fountains" of antimatter have been found in the Milky Way galaxy, not far from the galactic center, by Dr. William Purcell of Northwestern University. Apparently a stream of antimatter exists that creates this characteristic gamma radiation at 1.02 million electron volts as it collides with ordinary hydrogen gas. If this plume of antimatter exists naturally, then it might be possible that other pockets of antimatter exist in the universe that were not destroyed in the big bang.

To look for naturally occurring antimatter more systematically, the PAMELA (Payload for Antimatter-Matter Exploration and Light-Nuclei Astrophysics) satellite was launched into orbit in 2006. It is a collaborative effort between Russia, Italy, Germany, and Sweden, designed to search for pockets of antimatter. Previous missions searching for antimatter were carried out using high-alt.i.tude balloons and the s.p.a.ce Shuttle, so the data was collected for no more than a week or so. PAMELA, by contrast, will stay in orbit for at least three years. "It is the best detector ever constructed and we will use it for a long period," declares team member Piergiorgio Picozza of the University of Rome.

PAMELA is designed to detect cosmic rays from ordinary sources, such as supernovae, but also from unusual ones, such as stars made entirely of antimatter. Specifically, PAMELA will look for the signature of anti-helium, which might be produced in the interiors of anti-stars. Although most physicists today believe that the big bang resulted in a near perfect cancellation between matter and antimatter, as Sakharov believed, PAMELA is based on a different a.s.sumption-that whole regions of antimatter universe did not undergo that cancellation and hence exist today in the form of anti-stars.

If antimatter exists in minute quant.i.ties in deep s.p.a.ce, then it might be possible to "harvest" some of that antimatter to use to propel a starship. NASA"s Inst.i.tute for Advanced Concepts takes the idea of harvesting antimatter in s.p.a.ce seriously enough that it recently funded a pilot program to study this concept. "Basically, what you want to do is generate a net, just like you"re fishing," says Gerald Jackson of Hbar Technologies, one of the organizations spearheading the project.

The antimatter harvester is based on three concentric spheres, each made out of a lattice wire network. The outermost sphere would be 16 kilometers across and would be positively charged, so that it would repel any protons, which are positively charged, but attract antiprotons, which are negatively charged. The antiprotons would be collected by the outer sphere, then slow down as they pa.s.sed through the second sphere and would finally stop when they reached the innermost sphere, which would be 100 meters across. The antiprotons would then be captured in a magnetic bottle and combined with antielectrons to make antihydrogen.

Jackson estimates that controlled matter-antimatter reactions inside a s.p.a.cecraft could fuel a solar sail to Pluto using just 30 milligrams of antimatter. Seventeen grams of antimatter, says Jackson, would be enough to fuel a starship to Alpha Centauri. Jackson claims that there might be 80 grams of antimatter between the orbits of Venus and Mars that might be harvested by the s.p.a.ce probe. Given the complexities and cost of launching this huge antimatter collector, however, it probably won"t be realized until the end of this century, or beyond.

Some scientists have dreamed about harvesting antimatter from a meteor floating in outer s.p.a.ce. (The Flash Gordon comic strip once featured a rogue antimatter meteor drifting in s.p.a.ce, which could create a terrifying explosion if it came in contact with any planet.) If naturally occurring antimatter is not found in s.p.a.ce, we will have to wait decades or even centuries before we can produce significantly large quant.i.ties of antimatter on the Earth. But a.s.suming that the technical problems of producing antimatter can be solved, this leaves open the possibility that one day antimatter rockets may take us to the stars.

Given what we know of antimatter today, and the foreseeable evolution of this technology, I would cla.s.sify an antimatter rocket ship as a Cla.s.s I impossibility.

FOUNDER OF ANTIMATTER.

What is antimatter? It seems strange that nature would double the number of subatomic particles in the universe for no good reason. Nature is usually quite sparing, but now that we know about antimatter, nature seems to be supremely redundant and wasteful. And if antimatter exists, can anti-universes also exist?

To answer these questions, one has to investigate the origin of antimatter itself. The discovery of antimatter actually dates back to 1928, with the pioneering work of Paul Dirac, one of the most brilliant physicists of the twentieth century. He held the Lucasian Chair at Cambridge University, the same chair held by Newton, and the chair currently held by Stephen Hawking. Dirac, born in 1902, was a tall, wiry man who was in his early twenties when the quantum revolution broke open in 1925. Although he was studying electrical engineering at that time, he was suddenly swept up in the tidal wave of interest unleashed by the quantum theory.

The quantum theory was based on the idea that particles like electrons could be described not as pointlike particles but as a wave of some sort, described by Schrodinger"s celebrated wave equation. (The wave represents the probability of finding the particle at that point.) But Dirac realized that there was a defect with Schrodinger"s equation. It described only electrons moving at low velocities. At higher velocities, the equation failed because it did not obey the laws of objects moving at high velocities, that is, the laws of relativity found by Albert Einstein.

To the young Dirac, the challenge was to reformulate the Schrodinger equation to accommodate the theory of relativity. In 1928 Dirac proposed a radical modification of the Schrodinger equation that fully obeyed Einstein"s relativity theory. The world of physics was stunned. Dirac found his famous relativistic equation for the electron purely by manipulating higher mathematical objects, called spinors. A mathematical curiosity was suddenly becoming a centerpiece for the entire universe. (Unlike many physicists before him, who insisted that great breakthroughs in physics be firmly grounded in experimental data, Dirac took the opposite strategy. To him pure mathematics, if it was beautiful enough, was the sure guide to great breakthroughs. He wrote, "It is more important to have beauty in one"s equations than to have them fit experiments...It seems that if one is working from the point of view of getting beauty in one"s equations, and if one has a really sound insight, one is on a sure line of progress.") In developing his new equation for the electron, Dirac realized that Einstein"s celebrated equation, E = mc2, was not quite right. Although it is splattered over Madison Avenue ads, children"s T-shirts, cartoons, and even the costumes of superheroes, Einstein"s equation is only partially correct. The correct equation is actually E = mc2. (This minus sign arises because we have to take the square root of a certain quant.i.ty. Taking the square root of a quant.i.ty always introduces a plus or minus ambiguity.) But physicists abhor negative energy. There is an axiom of physics that states that objects always tend to the state of lowest energy (this is the reason that water always seeks the lowest level, sea level). Since matter always drops down to its lowest energy state, the prospect of negative energy was potentially disastrous. It meant that all electrons would eventually tumble down to infinite negative energy, hence Dirac"s theory would be unstable. So Dirac invented the concept of the "Dirac sea." He envisioned that all negative energy states were already filled up, and hence an electron could not tumble down into negative energy. Hence the universe was stable. Also a gamma ray might occasionally collide with an electron sitting in a negative energy state and kick it up into a state of positive energy. We would then see the gamma ray turn into an electron and a "hole" develop in the Dirac sea. This hole would act like a bubble in the vacuum; that is, it would have a positive charge and the same ma.s.s as the original electron. In other words, the hole would behave like an antielectron. So in this picture antimatter consists of "bubbles" in the Dirac sea.

Just a few years after Dirac made this astounding prediction, Carl Anderson actually discovered the antielectron (for which Dirac won the n.o.bel Prize in 1933).

In other words, antimatter exists because the Dirac equation has two types of solutions, one for matter, and one for antimatter. (And this in turn is the outcome of special relativity.) Not only did the Dirac equation predict the existence of antimatter; it also predicted the "spin" of the electron. Subatomic particles can spin, much like a spinning top. The spin of the electron, in turn, is crucial to understanding the flow of electrons in transistors and semiconductors, which form the basis of modern electronics.

Stephen Hawking regrets that Dirac did not patent his equation. He writes, "Dirac would have made a fortune if he had patented the Dirac equation. He would have had a royalty on every television, Walkman, video game and computer."

Today Dirac"s celebrated equation is etched in the stone of Westminster Abbey, not far from the tomb of Isaac Newton. In the entire world, it is perhaps the only equation given this distinctive honor.

DIRAC AND NEWTON.

Historians of science seeking to understand the origins of how Dirac came up with his revolutionary equation and the concept of antimatter have often compared him to Newton. Strangely, Newton and Dirac share a number of similarities. Both were in their twenties when they did their seminal work at Cambridge University, both were masters of mathematics, and both shared another stark characteristic: a total lack of social skills, to the point of pathology. Both were notorious for their inability to engage in small talk and simple social graces. Painfully shy, Dirac would never say anything unless asked directly, and then he would reply "yes," or "no," or "I don"t know."

Dirac was also extremely modest and detested publicity. When he was awarded the n.o.bel Prize in Physics, he seriously considered turning it down because of the notoriety and trouble that it would generate. But when it was pointed out to him that rejecting the n.o.bel Prize would generate even more publicity he decided to accept it.

Volumes have been written about Newton"s peculiar personality, with hypotheses ranging from mercury poisoning to mental illness. But recently a new theory has been proposed by Cambridge psychologist Simon Baron-Cohen that might explain both Newton"s and Dirac"s strange personalities. Baron-Cohen claims that they both probably suffered from Asperger"s syndrome, which is akin to autism, like the idiot savant in the movie Rain Man. Individuals suffering from Asperger"s are notoriously reticent, socially awkward, and sometimes blessed with enormous calculational ability, but unlike autistic individuals they are functional in society and can hold productive jobs. If this theory is true, then perhaps the miraculous calculational power of Newton and Dirac came at a price, being socially apart from the rest of humanity.

ANTIGRAVITY AND ANTI-UNIVERSES.

Using Dirac"s theory, we can now answer a host of questions: What is the antimatter counterpart of gravity? Do anti-universes exist?

As we discussed, antiparticles have the opposite charge of ordinary matter. But particles that have no charge at all (such as the photon, a particle of light, or the graviton, which is a particle of gravity) can be their own antiparticle. We see that gravitation is its own antimatter; in other words, gravity and antigravity are the same thing. Hence antimatter should fall down under gravity, not up. (This is universally believed by physicists, but it has actually never been demonstrated in the laboratory.) Dirac"s theory also answers the deep questions: Why does nature allow for antimatter? Does that mean anti-universes exist?

In some science fiction tales, the protagonist discovers a new Earth-like planet in outer s.p.a.ce. In fact, the new planet seems identical to Earth in every way, except everything is made of antimatter. We have antimatter twins on this planet, with anti-children, who live in anti-cities. Since the laws of anti-chemistry are the same as the laws of chemistry, except charges are reversed, people living in such a world would never know they were made of antimatter. (Physicists call this the charge-reversed or C-reversed universe, since all charges are reversed in this anti-universe, but everything else remains the same.) In other science fiction stories scientists discover a twin of the Earth in outer s.p.a.ce, except that it is a Looking Gla.s.s universe, where everything is left-right reversed. Everyone"s heart is on the right side and most people are left-handed. They live out their lives never knowing that they live in a left-right reversed Looking Gla.s.s universe. (Physicists call such a Looking Gla.s.s universe a parity-reversed or P-reversed universe.) Can such antimatter and parity-reversed universes really exist? Physicists take questions about twin universes very seriously, since Newton"s and Einstein"s equations remain the same when we simply flip the charges on all our subatomic particles or reverse the left-right orientation. Hence, C-reversed and P-reversed universes are in principle possible.

n.o.bel laureate Richard Feynman posed an interesting question about these universes. Suppose one day we make radio contact with aliens on a distant planet but cannot see them. Can we explain to them the difference between "left" and "right" by radio? he asked. If the laws of physics allow for a P-reversed universe, then it should be impossible to convey these concepts.

Certain things, he reasoned, are easy to communicate, such as the shape of our bodies and the number of our fingers, arms, and legs. We can even explain to the aliens the laws of chemistry and biology. But if we try to explain to them the concept of "left" and "right" (or "clockwise" and "counterclockwise"), we would fail each time. We would never be able to explain to them that our heart is on the left side of our body, in which direction the Earth rotates, or the way a DNA molecule spirals.

So it came as a shock when C. N. Yang and T. D. Lee, both at Columbia University at the time, disproved this cherished theorem. By examining the nature of subatomic particles they showed that the Looking Gla.s.s, P-reversed universe cannot exist. One physicist, learning of this revolutionary result, said, "G.o.d must have made a mistake." For this earthshaking result, called the "overthrow of parity," Yang and Lee won the n.o.bel Prize in Physics in 1957.

To Feynman, this conclusion meant that if you are talking to aliens on a radio, it is possible to set up an experiment that could enable you to tell the difference between left- and right-handed universes by radio alone. (For example, electrons emitted from radioactive cobalt-60 do not spin in equal numbers in a clockwise or counterclockwise fashion, but actually spin in a preferred direction, thereby breaking parity.) Feynman then envisioned that a historic meeting finally takes place between the aliens and humanity. We tell the aliens to stick out their right hand when we first meet, and we will shake hands. If the aliens actually stick out their right hand, then we know that we have successfully communicated to them the concept of "left-right" and "clockwise-counterclockwise."

But Feynman then raised an unsettling thought. What happens if the aliens stick out their left hand instead? This means that we have made a fatal mistake, that we have failed to communicate the concept of "left" and "right." Worse, it means that the alien is actually made of antimatter, and that he performed all the experiments backward, and hence got "left" and "right" mixed up. It means when we shake hands, we will explode!

That was our understanding until the 1960s. It was impossible to tell the difference between our universe and a universe in which everything was made of antimatter and was parity-reversed. If you flipped both the parity and the charge, the resulting universe would obey the laws of physics. Parity by itself was overthrown, but charge and parity was still a good symmetry of the universe. So a CP-reversed universe was still possible.

This meant that if we were talking to aliens on the phone, we could not tell the difference between an ordinary universe and one that was both parity- and charge-reversed (i.e., left and right are interchanged, and all matter is turned into antimatter).

Then in 1964 physicists received a second shock: the CP-reversed universe cannot exist. By a.n.a.lyzing the properties of subatomic particles, it is still possible to tell the difference between left-right, clockwise-counterclockwise if you are talking by radio to another CP-reversed universe. For this result, James Cronin and Val Fitch won the n.o.bel Prize in 1980.

(Although many physicists were upset when the CP-reversed universe was shown to be inconsistent with the laws of physics, in hindsight the discovery was a good thing, as we discussed earlier. If the CP-reversed universe were possible, then the original big bang would have involved precisely the same amount of matter and antimatter, and hence 100 percent annihilation would have taken place, and our atoms would not have been possible! The fact that we exist as a leftover from the annihilation of unequal amounts of matter and antimatter is proof of CP violation.) Are any reversed anti-universes possible? The answer is yes. Even if parity-reversed and charge-reversed universes are not possible, an anti-universe is still possible, but it would be a strange one. If we reversed the charges, the parity, and the march of time, then the resulting universe would obey all the laws of physics. The CPT-reversed universe is allowed.

Time reversal is a bizarre symmetry. In a T-reversed universe, fried eggs jump off the dinner plate, reform on the frying pan, and then jump back into the egg, sealing the cracks. Corpses rise from the dead, get younger, turn into babies, and then jump into their mother"s womb.

Common sense tells us that the T-reversed universe is not possible. But the mathematical equations of subatomic particles tell us otherwise. Newton"s laws run perfectly well backward or forward. Imagine videotaping a billiard game. Each collision of the b.a.l.l.s obeys Newton"s laws of motion; running such a videotape would make for a bizarre game, but it is allowed by the laws of Newton.

In the quantum theory things are more complicated. T-reversal by itself violates the laws of quantum mechanics, but the full CPT-reversed universe is allowed. This means that a universe in which left and right are reversed, matter turns into antimatter, and time runs backward is a fully acceptable universe obeying the laws of physics!

(Ironically, we cannot communicate with such a CPT-reversed world. If time runs backward on their planet, it means that everything we tell them by radio will be part of their future, so they would forget everything we told them as soon as we spoke to them. So even though the CPT-reversed universe is allowed under the laws of physics, we cannot talk to any CPT-reversed alien by radio.) In summary, antimatter engines may give us a realistic possibility for fueling a starship in the distant future, if enough antimatter could be made on Earth, or found in outer s.p.a.ce. There is a slight imbalance between matter and antimatter because of CP violation, and this in turn may mean that pockets of antimatter still exist and can be harvested.

But because of the technical difficulties involved in antimatter engines, it may take a century or more to develop this technology, making it a Cla.s.s I impossibility.

But let"s tackle another question: Will faster-than-light starships be possible thousands of years in the future? Are there loopholes to Einstein"s famous dictum that "nothing can go faster than light"? The answer, surprisingly, is yes.

11: FASTER THAN LIGHT.

It"s quite conceivable that [life] will eventually spread through the galaxy and beyond. So life may not forever be an unimportant trace contaminant of the universe, even though it now is. In fact, I find it a rather appealing view.

-ASTRONOMER ROYAL SIR MARTIN REES.

It is impossible to travel faster than the speed of light, and certainly not desirable, as one"s hat keeps blowing off.

-WOODY ALLEN.

In Star Wars, as the Millennium Falcon blasts off the desert planet Tatooine, carrying our heroes Luke Skywalker and Han Solo, the ship encounters a squadron of menacing Imperial battleships...o...b..ting the planet. The Empire"s battleships fire a punishing barrage of laser blasts at our heroes" ship that steadily break through its force fields. The Millennium Falcon is outgunned. Buckling under this withering laser fire, Han Solo yells that their only hope is to make the jump into "hypers.p.a.ce." In the nick of time the hyperdrive engines spring to life. All the stars around them suddenly implode toward the center of their view screen in converging, blinding streaks of light. A hole opens up, which the Millennium Falcon blasts through, reaching hypers.p.a.ce and freedom.

Science fiction? Undoubtedly. But could it be based on scientific fact? Perhaps. Faster-than-light travel has always been a staple of science fiction, but recently physicists have given serious thought to this possibility.

According to Einstein, the speed of light is the ultimate speed limit in the universe. Even our most powerful atom smashers, which can create energies found only at the center of exploding stars or the big bang itself, cannot hurl subatomic particles at a rate faster than the speed of light. Apparently the speed of light is the ultimate traffic cop in the universe. If so, any hope of our reaching the distant galaxies seems to be dashed.

Or maybe not...

EINSTEIN THE FAILURE.

In 1902 it was far from obvious that the young physicist Albert Einstein would be hailed as the greatest physicist since Isaac Newton. In fact, that year represented the lowest point in his life. A newly minted Ph.D. student, he was rejected for a teaching job by every university he applied to. (He later found out that his professor Heinrich Weber had written horrible letters of recommendation for him, perhaps in revenge for Einstein"s having cut so many of his cla.s.ses.) Furthermore, Einstein"s mother was violently opposed to his girlfriend, Mileva Maric, who was carrying his child. Their first daughter, Lieserl, would be born illegitimate. Young Albert was also a failure at the odd jobs he took. Even his lowly tutoring job abruptly ended when he was fired. In his depressing letters he contemplated becoming a salesman to earn a living. He even wrote to his family that perhaps it would have been better had he never been born, since he was such a burden to his family and lacked any prospects for success in life. When his father died, he felt ashamed that his father had died thinking that his son was a total failure.

Yet later that year Einstein"s luck would turn. A friend arranged for him to get a job as a clerk in the Swiss Patent Office. From that lowly position Einstein would launch the greatest revolution in modern history. He would quickly a.n.a.lyze the patents on his desk and then spend hours contemplating problems in physics that had puzzled him since he was a child.

What was the secret of his genius? Perhaps one clue to his genius was his ability to think in terms of physical pictures (e.g., moving trains, accelerating clocks, stretched fabrics) rather than pure mathematics. Einstein once said that unless a theory can be explained to a child, the theory was probably useless; that is, the essence of a theory has to be captured by a physical picture. So many physicists get lost in a thicket of mathematics that lead nowhere. But like Newton before him, Einstein was obsessed by the physical picture; the mathematics would come later. For Newton the physical picture was the falling apple and the moon. Were the forces that made an apple fall identical to the forces that guided the moon in its...o...b..t? When Newton decided that the answer was yes, he created a mathematical architecture for the universe that suddenly unveiled the greatest secret of the heavens, the motion of celestial bodies themselves.

EINSTEIN AND RELATIVITY.

Albert Einstein proposed his celebrated special theory of relativity in 1905. At the heart of his theory was a picture that even children can understand. His theory was the culmination of a dream he had had since the age of sixteen, when he asked the fateful question: what happens if you outrace a light beam? As a youth, he knew that Newtonian mechanics described the motion of objects on the Earth and in the heavens, and that Maxwell"s theory described light. These were the two pillars of physics.

The essence of Einstein"s genius was that he recognized that these two pillars were in contradiction. One of them must fall.

According to Newton, you could always outrace a light beam, since there was nothing special about the speed of light. This meant that the light beam must remain stationary as you raced alongside. But as a youth Einstein realized that no one had ever seen a light wave that was totally stationary, that is, like a frozen wave. Hence Newton"s theory did not make sense.

Finally, as a college student in Zurich studying Maxwell"s theory, Einstein found the answer. He discovered something that even Maxwell did not know: that the speed of light was a constant, no matter how fast you moved. If you raced toward or away from a light beam, it still traveled at the same velocity, but this trait violates common sense. Einstein had found the answer to his childhood question: you can never race alongside a light beam, since it always moves away from you at a constant speed, no matter how fast you move.

But Newtonian mechanics was a tightly constrained system: like pulling on a loose thread, the entire theory could unravel if you made the smallest change in its a.s.sumptions. In Newton"s theory the pa.s.sage of time was uniform throughout the universe. One second on the Earth was identical to one second on Venus or Mars. Similarly, meter sticks placed on the Earth had the same length as meter sticks on Pluto. But if the speed of light was always constant no matter how fast you moved, there would need to be a major shakeup in our understanding of s.p.a.ce and time. Profound distortions of s.p.a.ce and time would have to occur to preserve the constancy of the speed of light.

According to Einstein, if you were in a speeding rocket ship, the pa.s.sage of time inside that rocket would have to slow down with respect to someone on Earth. Time beats at different rates, depending on how fast you move. Furthermore, the s.p.a.ce within that rocket ship would get compressed, so that meter sticks could change in length, depending on your speed. And the ma.s.s of the rocket would increase as well. If we were to peer into the rocket with our telescopes, we would see clocks inside the rocket running slowly, people moving in slow motion, and the people would appear flattened.

In fact, if the rocket were traveling at the speed of light, time would apparently stop inside the rocket, the rocket would be compressed to nothing, and the ma.s.s of the rocket would be infinite. Since none of these observations make any sense, Einstein stated that nothing can break the light barrier. (Because an object gets heavier the faster it moves, this means that the energy motion is being converted to ma.s.s. The precise amount of energy that turns into ma.s.s is easy to calculate, and we arrive at the celebrated equation E = mc2 in just a few lines.) Since Einstein derived his famous equation, literally millions of experiments have confirmed his revolutionary ideas. For example, the GPS system, which can locate your position on the Earth to within a few feet, would fail unless one added in corrections due to relativity. (Since the military depends on the GPS system, even Pentagon generals have to be briefed by physicists concerning Einstein"s theory of relativity.) The clocks on the GPS actually change as they speed above the Earth, as Einstein predicted.

The most graphic ill.u.s.tration of this concept is found in atom smashers, in which scientists accelerate particles to nearly the speed of light. At the gigantic CERN accelerator, the Large Hadron Collider, outside Geneva, Switzerland, protons are accelerated to trillions of electron volts, and they move very close to the speed of light.

To a rocket scientist, the light barrier is not much of a problem yet, since rockets can barely travel beyond a few tens of thousands of miles per hour. But within a century or two, when rocket scientists seriously contemplate sending probes to the nearest star (located over 4 light-years from Earth), the light barrier could gradually become a problem.

LOOPHOLES IN EINSTEIN"S THEORY Over the decades, physicists have tried to find loopholes in Einstein"s famous dictum. Some loopholes have been found, but most are not very useful. For example, if one sweeps a flashlight across the heavens, in principle the image of the light beam can exceed the speed of light. In a few seconds, the image of the flashlight moves from one point on the horizon to the opposite point, over a distance that can stretch over hundreds of light-years. But this is of no importance, since no information can be transmitted faster than light in this fashion. The image of the light beam has exceeded the speed of light, but the image carries no energy or information.

Similarly, if we have a pair of scissors, the point at which the blades cross each other moves faster the farther you are from the joining point. If we imagine scissors that are a light-year long, then by closing the blades the crossing point can travel faster than light. (Again, this is not important since the crossing point carries no energy or information.) Likewise, as I mentioned in Chapter Four, the EPR experiment enables one to send information at speeds faster than the speed of light. (In this experiment, we recall, two electrons are vibrating in unison and then are sent speeding in opposite directions. Because these electrons are coherent, information can be sent between them at speeds faster than the speed of light, but this information is random and hence is useless. EPR machines, hence, cannot be used to send probes to the distant stars.) To a physicist, the most important loophole came from Einstein himself, who created the general theory of relativity in 1915, a theory that is more powerful than the special theory of relativity. The seeds of general relativity were planted when Einstein considered a children"s merry-go-round. As we saw earlier, objects shrink as they approach the speed of light. The faster you move, the more you are squeezed. But in a spinning disk, the outer circ.u.mference moves faster than the center. (The center, in fact, is almost stationary.) This means that a ruler stick placed on the rim must shrink, while a ruler placed at the center remains nearly the same, so the surface of the merry-go-round is no longer flat, but is curved. Thus acceleration has the effect of curving s.p.a.ce and time on the merry-go-round.

In the general theory of relativity, s.p.a.ce-time is a fabric that can stretch and shrink. Under certain circ.u.mstances the fabric may stretch faster than the speed of light. Think of the big bang, for example, when the universe was born in a cosmic explosion 13.7 billion years ago. One can calculate that the universe originally expanded faster than the speed of light. (This action does not violate special relativity, since it was empty s.p.a.ce-the s.p.a.ce between stars-that was expanding, not the stars themselves. Expanding s.p.a.ce does not carry any information.) The important point is that special relativity applies only locally, that is, in your nearby vicinity. In your local neighborhood (e.g., the solar system), special relativity holds, as we confirm with our s.p.a.ce probes. But globally (e.g., on cosmological scales involving the universe) we must use general relativity instead. In general relativity, s.p.a.ce-time becomes a fabric, and this fabric can stretch faster than light. It can also allow for "holes in s.p.a.ce" in which one can take shortcuts through s.p.a.ce and time.

Given these caveats, perhaps one way to travel faster than light is to invoke general relativity. There are two ways in which this might be done.

1. Stretching s.p.a.ce. If you were to stretch the s.p.a.ce behind you and contact the s.p.a.ce in front of you, then you would have the illusion of having moved faster than light. In fact, you would not have moved at all. But since s.p.a.ce has been deformed, it means you can reach the distant stars in a twinkling of an eye.

2. Ripping s.p.a.ce. In 1935 Einstein introduced the concept of a wormhole. Imagine the Looking Gla.s.s of Alice, a magical device that connects the countryside of Oxford to Wonderland. The wormhole is a device that can connect two universes. When we were in grade school, we learned that the shortest distance between two points is a straight line. But this is not necessarily true, because if we curled a sheet of paper until two points touched, then we would see that the shortest distance between two points is actually a wormhole.

As physicist Matt Visser of Washington University says, "The relativity community has started to think about what would be necessary to take something like warp drive or wormholes out of the realm of science fiction."

Sir Martin Rees, Royal Astronomer of Great Britain, even says, "Wormholes, extra dimensions, and quantum computers open up speculative scenarios that could transform our entire universe eventually into a "living cosmos.""

THE ALCUBIERRE DRIVE AND NEGATIVE ENERGY.

The best example of stretching s.p.a.ce is the Alcubierre drive, proposed by physicist Miguel Alcubierre in 1994 using Einstein"s theory of gravity. It is quite similar to the propulsion system seen in Star Trek. The pilot of such a starship would be seated inside a bubble (called a "warp bubble") in which everything seemed to appear normal, even as the s.p.a.cecraft broke the light barrier. In fact, the pilot would think that he was at rest. Yet outside the warp bubble extreme distortions of s.p.a.ce-time would occur as the s.p.a.ce in front of the warp bubble was compressed. There would be no time dilation, so time would pa.s.s normally inside the warp bubble.

Alcubierre admits that Star Trek may have had a role to play in his finding this solution. "People in Star Trek kept talking about warp drive, the concept that you"re warping s.p.a.ce," he says. "We already had a theory about how s.p.a.ce can or cannot be distorted, and that is the general theory of relativity. I thought there should be a way of using these concepts to see how a warp drive would work." This is probably the first time that a TV show helped to inspire a solution to one of Einstein"s equations.