How to Destroy the Universe (28 page)

Quantum theory may yet give the cryptographers the last laugh. Today, electronic communications are transmitted as pulses of electric current down a wire, but what if the quantum particles used to store data inside a quantum computer were also used to transmit it? The same sensitivity of quantum systems that gives rise to the phenomenon of decoherence means that an eavesdropper trying to listen in on a message transmitted as a beam of quantum particles must inevitably interfere with the message, altering some of its content. A transmitter and receiver could therefore check that no one has tried to intercept their communications by interspersing the actual message with a test signal—a
known sequence of bits. This could be used to distribute an RSA key to intended recipients only, rather than broadcasting it publicly. Any discrepancy in this test signal would mean that the key had been intercepted, in which case the transmitter simply sends a new one. Only once the receiver is in possession of a key that both parties are sure has not been intercepted will the transmitter use it to encrypt and send their actual message. This technology is already here. In 2007, Swiss company id Quantique used just such a protocol to transmit electronic ballot slips during the Geneva federal elections. It remains to be seen whether this will be the last word on cryptography or just the next escalation in the ongoing arms race between those who make codes and those who break them.

• Down force

• Quantum gravity

• The rocketmen

• The law of lifters

• Superconductivity

• Podkletnov's disc

Antigravity devices have long been staples of science fiction, blocking the gravitational force to enable you to float in the air as if you were in space. A working antigravity machine would mean safe air travel, pollution-free cars, ultra-efficient elevators and cheap, easy access to Earth's orbit. Various devices already use other fields of nature, mainly electromagnetism, to balance the force of gravity, allowing objects to hover in the air. But one maverick researcher thinks he knows how to switch off gravity itself.

Gravity is the force that keeps us stuck to the surface of planet Earth. It's the weakest of all the four forces of
nature, the others being electromagnetism and the strong and weak nuclear forces that operate inside the nuclei of atoms. Even so, with a whole planet's worth of mass to act as the source, there is still a considerable force pulling on the heels of anyone trying to escape its grasp. This fact is foremost in the minds of rocket engineers. Rockets expend a terrific amount of fuel to escape Earth's gravitational pull, making space travel an expensive and dangerous business.

Antigravity would have the power to change all that. With none of the planet's gravitational pull to overcome, the slightest upward shove would send a spacecraft climbing skywards like a helium balloon. However, as far as we know from observations—and from our best theory of gravity, Einstein's general theory of relativity—all positive concentrations of mass and energy generate attractive gravity. Generating repulsive gravity would demand negative mass or energy. This has been made in very tiny amounts in lab experiments but in order to counteract the gravity of a planet, you'd need a planet-sized blob of negative-mass material. And that's not something you find lying around. There is an alternative possibility, that one day we could try to block the gravitational interaction somehow. Just as shielding sensitive electronic equipment in a lead box can guard it against the potentially harmful effects of electromagnetism (see
How to cause a blackout
), it may also be possible to block gravitational fields.

Blocking gravity will require a detailed understanding of how the gravitational interaction is propagated. In general relativity, gravity is caused by the bending of space and time—and there is plenty of observational evidence to suggest that this theory works. Nevertheless, ultimately we will need a quantum theory of gravity: a version of general relativity that works down at the scale of subatomic particles. If you believe the Big Bang theory for the origin and evolution of our Universe—and most professional cosmologists today do—then space expanded from a superdense point. At and around this point, space itself was so small it existed in the quantum domain. And that means we need a quantum theory of gravity to explain its behavior if we are ever to comprehend how our Universe came to be.

Other theories have been quantized. For example, the “classical” theory of electromagnetism developed in the 1860s by James Clerk Maxwell and others was adapted a little under a century later to make a full-blown quantum theory of electric and magnetic fields. This new quantum theory was able to describe the interactions between charged subatomic particles in terms of the exchange of photons, particles of the electromagnetic field. The gravitational field, on the other hand, is mediated by particles called gravitons, which are yet to be discovered experimentally. Fathoming
their behavior enough to say how they can be blocked—if at all—will require a full mathematical theory of quantum gravity which, thus far, physicists have been unable to produce. Gravity and quantum theory, at least on paper, are a match made in hell. However, new particle physics models such as string theory and M-theory (see
How to visit the tenth dimension
) could show us the way forward.

If gravity can't be modified or blocked—at least not using the existing laws of physics—can we bring any other branches of physics to bear in order to counteract it? The answer, of course, is yes. Perhaps the simplest example would be vertical/short takeoff and landing (VSTOL) jet fighters. The first, and the most famous, of these is the British-built Harrier jump jet, which made its first flight in 1960.

The challenge was to build a jet engine so powerful that its thrust alone could lift the weight of a fighter aircraft vertically upward, and to do so using no more fuel than a conventional jet. A team at Bristol Aero-Engines achieved this with their Pegasus engine. They used an ingenious water cooling system, which allowed the engine to be pushed way beyond its normal power rating for short periods. Four directable nozzles channeled its thrust downward for vertical takeoff and
landing, and could then be swiveled to point backward and generate the propulsion needed for forward flight. Along the way, this so-called “thrust vectoring” led to a new tactic in aerial combat, called VIFFing (“vectoring in forward flight”). Here, fighter aircraft use thrust vectoring to brake rapidly, dramatically improving their maneuverability. The technique has been perfected by Harrier pilots in the US Marine Corps. A similar system to VSTOL is used in jet packs, which use a directable rocket motor, most commonly running on hydrogen peroxide. Strapped to the back of an intrepid pilot, the jet pack enables them to hover in mid-air. However, the limitations on the amount of fuel that can be carried and the safety concerns—jet packs fly too low for a parachute to be of any use—mean that there are few of them in use today.

Roaring jet engines and rocket motors somehow don't seem to capture the spirit of what most of us probably think of at the mention of antigravity. These devices shouldn't be noisy (bar maybe a slight hum), they shouldn't rattle your fillings loose as you fly them, and they definitely shouldn't belch out clouds of hot exhaust gas that threaten to cook passers-by whenever you set down in a crowded street. There is, however, another kind of propulsion device that looks, feels and sounds more like the common conception of an
antigravity machine. It is called a lifter and it works on the principles of electromagnetism, taking advantage of a phenomenon known as an ion wind.

You'll occasionally see lifter models demonstrated at science fairs. They usually consist of a triangular balsa wood frame with a ribbon of aluminum foil stretched around the outer edge. Balsa wood posts protrude up from the frame, around which is stretched a length of fine-gauge wire, running a few centimeters above the foil. The lifter is then connected to a high-voltage source—typically around 30,000 V—with the positive terminal of the source clipped to the wire and the negative terminal to the foil.

When the current is switched on, the high voltage pulls negatively charged electron particles away from atoms of gas in the air and toward the positive wire. This leaves the atoms themselves positively charged—a process called ionization. The positive atoms are then repelled away from the wire and accelerated toward the negatively charged foil. As they move, they collide with other uncharged atoms, creating a breeze of air that travels downward. Newton's third law of motion says that for every action there is an equal and opposite reaction (this is the reason rockets fly)—and the reaction to the downdraft of air is an upward force that makes the lifter rise up and hover. Sadly the amount of payload that a lifter can raise is tiny—about a gram per
watt of power applied. This means that you have to make them from balsa wood and foil to be as light as possible. It also means that, given the hefty power supply needed, it's unlikely a lifter could ever fly under its own steam. Nevertheless, they are very cool gizmos indeed.

Other more sophisticated electromagnetic levitation effects have been discovered. One is called the Meissner effect and arises as a result of superconductivity. This is a property of certain materials whereby they assume a state with zero resistance to electrical current—usually when they are cooled close to absolute zero (−273°C/−459°F). A current in a loop of superconducting wire will, in principle, circulate forever. It has long been known that cooling a material down improves its electrical conductivity. Temperature is caused by vibrations at the atomic and molecular level, so a warm lattice of atoms and molecules has thermal motions that make it jiggle about, impeding the flow of electrons through it. In 1911, Dutch physicist Heike Kamerlingh Onnes demonstrated that this resistance can be made to vanish entirely. The theoretical explanation for the effect would follow in the 1950s. In a nutshell, electrons that are cooled sufficiently lock together to make so-called “Cooper pairs” that are able to slip freely through the lattice.

Superconductors are used to build powerful magnets for particle accelerators and medical imaging machines. Physicists are now trying to synthesize materials that can exhibit superconductivity at room temperature. The Meissner effect arises because superconductors expel any magnetic fields that try to pass through them. This happens because the magnetic field induces a current in a conductor by induction (see
How to cause a blackout
). In a superconductor, this current flows over the surface of the material, in turn setting up a magnetic field of its own—again by induction—which exactly cancels out the external field. This means that
a magnet placed above a superconductor will levitate, supported in mid-air by an equal and opposite magnetic field.

No round-up of antigravity research would be complete without a mention of the exploits of maverick Russian scientist Eugene Podkletnov. In 1996, Podkletnov, a physicist at Tampere University in Finland, claimed to have built a device that not only counteracts the force of gravity but actually blocks it. Like the Meissner effect, Podkletnov's work made use of superconductors. Specifically, he used a 30 cm (12 in) disc of superconducting yttrium-barium-copper oxide. He said that when this disc was cooled to −230°C (−382°F) and
spun up to 5,000 rpm, objects placed above it would lose about 2 percent of their weight. He was careful to stress that he had gone to considerable lengths to prevent air currents and other magnetic phenomena—such as the Meissner effect—from influencing the experiment.