How to Destroy the Universe (23 page)

Nearly 80 years later, in 1994, Mexican physicist Miguel Alcubierre published a theoretical paper detailing how general relativity could be used to build a “warp drive”—a way to travel faster than light by bending space and time, named in honor of the science fiction TV show
Star Trek
. Alcubierre imagined a hypothetical rocket ship. His idea was to arrange matter around the ship in just the right way that its gravity would cause space to rapidly expand behind it while the space in front would contract at the exact same rate. The effect would be to rapidly increase the distance between the ship and its starting point and at the same time shrink the distance between the ship and its destination—sweeping the piece of space containing the ship to the destination arbitrarily fast.

The only problem was that when Alcubierre solved Einstein's equations of general relativity to find out what properties the matter would need to have, he found that it was very bizarre indeed. He needed a kind of material that has negative pressure and mass, and is so weird that even physicists describe it as “exotic matter” (see
How to travel through time
). Tiny amounts of
exotic matter have been made experimentally. However, calculations have suggested that building a working warp drive would demand a quantity of exotic matter equal to a third of the mass of the Sun. So while scientists may have designed the definitive vehicle to break the light barrier, it seems that—just like the space rockets of today—the vast quantities of fuel required may ultimately turn out to be its downfall.

• Time dilation

• Back to the past

• Exotic matter

• Gateways to the past

• Paradox lost

• Time lock

All of us are shuffling forward through time at the sedentary rate of 60 seconds every minute. But could we ever supercharge our temporal travels and jump ahead into the future, or even reverse the flow of time to travel backward and explore the past? There is nothing in the laws of physics as they stand to rule time travel out. Scientists have devised a host of theoretical schemes by which it could happen, and many of them believe these ideas will one day become reality.

It is already possible to travel forward into the future. In 1905, Albert Einstein put forward his special theory of relativity, a new way of looking at the motion of
bodies moving at close to the speed of light. One of its central predictions was a phenomenon called time dilation, which essentially says that a moving clock ticks slower than a clock at rest. Close to light speed, time practically stands still. This applies to all clocks—mechanical, digital and biological. It means that an astronaut who climbs aboard a spacecraft and accelerates up to near the speed of light might only age by a second for every year that passes back on Earth as his clocks are slowed down. When he returns after a year, as measured in his own time, he finds that more than 31 million years have passed on his home planet. Time dilation is more than just a theory—it's a real effect that's been measured in experiments.

The trouble with setting off into the future like this is that at present no one quite knows how you would get back to your own time. Travel into the past is an altogether harder problem that has not yet been solved in practice. In theory, however, scientists have put forward a number of schemes that could earn them an awful lot in lottery winnings, and certainly enough money to buy tickets to see Chopin play live.

Most of these ideas revolve around Einstein's general theory of relativity, published in 1915. Whereas special relativity described space and time as “flat” and took no
account of the force of gravity, general relativity adds gravity into the mix by curving space and time. The crux of Einstein's new theory was a set of equations linking the curvature of space and time to the matter they contain. Just a year after Einstein published general relativity, an Austrian scientist called Ludwig Flamm came up with a mathematical solution of its equations describing what is known as a wormhole—a tunnel through the fabric of the Universe providing a shortcut linking regions vastly separated in space. The term “wormhole” wasn't actually coined until years later by the US physicist John Archibald Wheeler, who likened them to the hole munched through an apple by a worm. The distance from one side of the apple to the other via the wormhole is shorter than the distance around the apple's surface. That wormholes could be useful for time travel wasn't realized until much later. In 1986, a team of researchers led by US physicist Professor Kip Thorne calculated how Einstein's old idea of time dilation could be used to turn a wormhole into a tunnel not just through space, but back through time as well.

The basic idea was to have one mouth of the wormhole on Earth and to put the other on a spacecraft and fly it off at close to the speed of light for a year. Just like the fictional astronaut from earlier, the wormhole mouth on the spacecraft travels forward through time by 31 million years. The wormhole mouth itself only ages by
a year—because of time dilation—and, crucially, it remains connected to the other mouth on Earth, which has also aged by just a year. Here comes the clever bit. Anyone in the year 31 million
AD
who now jumps into the wormhole mouth on the spacecraft will emerge from the mouth that remained on Earth just a year after the spacecraft set off on its journey—they have traveled 31 million years into the past.

Traveling back through time using a wormhole looks great on paper. The trouble starts when you begin working out the engineering details needed to turn the theory into reality. Einstein's equations of general relativity reveal the kind of matter that's needed to create and hold open a wormhole big enough for a person to squeeze through. It is the same weird stuff that we encountered in the last chapter, so-called exotic matter. Exotic matter has negative pressure inside it. Try to blow up a balloon with the stuff and the balloon actually deflates. The energy associated with this negative pressure generates a kind of negative, or repulsive gravity, and this is what holds the wormhole tunnel open. Exotic matter isn't the kind of material that you typically find lying around in large quantities. But minute amounts of it have been observed in a phenomenon known as the Casimir effect. This effect causes two metal plates placed just a few billionths of a meter
apart in a vacuum to feel a force pulling them together. The force is caused by the negative pressure of exotic matter being created between the plates.

The Casmir effect happens because empty space isn't really empty. It is actually a bubbling mass of so-called virtual particles—subatomic particles of matter that pop in and out of existence on extremely short time-scales in accordance with the uncertainty principle of quantum theory (see
How to be everywhere at once
). Another aspect of quantum theory called wave–particle
duality (see
How to harness starlight
) says that these particles can equally well be thought of as waves. Between the plates, these waves are a bit like vibrating guitar strings—where the only vibrations allowed are those for which the length of the string is a whole number of half wavelengths.

In the Casimir effect, this means that only waves for which the gap between the plates is a whole number of half wavelengths can exist. Outside the plates, however, all waves are allowed. Converting back to particle language, this means that there are fewer particles rattling about between the plates than there are outside them. In other words, the pressure between the plates is lower. If the space outside is a zero-pressure vacuum, then the pressure inside must be less than zero—there is negative pressure.

The Casimir effect is named after Dutch scientist Hendrik Casimir, who first predicted it in 1948. It was verified experimentally in 1997 by physicist Steve Lamoreaux in New Mexico. The amount of exotic matter produced in the Casimir effect is tiny, around a billion-billion-billionth of a gram. By comparison, sustaining a man-sized wormhole tunnel demands a quantity of the stuff roughly equal to the mass of Jupiter.

The other drawback to using wormholes to visit the past is that it's impossible to go back to an era before the time machine was created. If your spaceship with a wormhole in the cargo hold left Earth traveling at near light speed today, you could never hope to use the resulting time machine to visit the Cretaceous Period, the D-Day landings, or even yesterday afternoon.

As well as the technical difficulties in building a time machine, some physicists and logicians have raised objections on the grounds of the causal inconsistencies that it might give rise to. For example, the “granny paradox” asks what would happen if you went back in time and killed your maternal grandmother before she gave birth to your mother. In that case, you would never have been born, and so could never have gone back in time and killed her, in which case you would have been born, and so on. Another is the “free lunch paradox,” where a generous time traveler takes copies of all seven Harry Potter novels back in time and makes a gift of them to the young, impoverished J.K. Rowling, who promptly copies them, publishes them and never looks back. In this scenario, where exactly did the creative spark for the precocious young wizard originate?

Time travel paradoxes seem like show stoppers, yet canny scientists have realized that there is a way to travel into the past without disrupting the order of cause and effect. One possible way out of time travel paradoxes could come from the many worlds interpretation of quantum physics (see
How to live forever
). In a nutshell, this says that our universe is just one of many in a sprawling structure known as the multiverse—and that every time our universe is confronted by multiple possibilities it splits into a number of new universes, where each possibility is actually played out. If you believe the many worlds interpretation then time travel is automatically paradox free. That's because traveling back in time takes you into the past of a different universe from the one that you've come from. Assassinating your grandmother in this new universe has no bearing on the version of her in the universe where you were born. Similarly sending all your Harry Potter novels back in time will only place them in the hands of another J.K. Rowling living in a parallel universe. The version in your home universe will still have to earn her fame and fortune the hard way. Another possibility is an idea known as self-consistency. This says that if someone or something travels back through time there will always be at least one self-consistent sequence of events—and that nature favors such an outcome.

Nevertheless, some physicists remain staunchly averse to the idea of journeying back through time. At the forefront of these temporal skeptics is British physicist and mathematician Stephen Hawking. He finds the notion so abhorrent that he's put forward what he calls the “chronology protection conjecture”—a hypothetical mechanism to rule out time travel, either by destroying a time machine as it tries to form, or by destroying anyone or anything that tries to use one. Hawking has yet to find a solid mechanism within the laws of physics by which the conjecture could be implemented, though the virtual particles responsible for the Casimir effect offer one possibility, becoming magnified to destructive energies as they loop through the time machine over and over again.

If Hawking is right then time travel in our Universe will never be possible. Then again, if he's wrong (and he has been before), we're simply waiting for technology to catch up with the science. It wouldn't be the first time. Five hundred years ago, Leonardo da Vinci produced a design for a glider. It was never built, but modern reconstructions of the model have confirmed that had he possessed the advanced materials and construction techniques available to aeronautical engineers today, it would have flown. How ironic that time travel may also be a case of the right idea, but at the wrong time.

• Are we alone?

• Where to look?

• How many ETIs?

• From the ashes

• DIY SETI

• The “Wow!” signal

• Mr. President …

Are we alone in the Universe? Few areas of science capture the imagination in quite the same way as the search for extraterrestrial intelligence, also known as SETI, a project by astronomers to detect signals broadcast by alien civilizations. A new network of radio telescopes under construction in northern California could mean that we're about to find it.