RONALD MALLETT thinks he has found a practical way to make a time machine.
Mallett isn鈥檛 mad. None of the known laws of physics forbids time travel, and in
theory, shunting matter back and forth through time shouldn鈥檛 be that
difficult.
The catch usually comes when you try to make it work in practice. Remember
wormholes, those clever little tunnels in space and time that can supposedly be
used to travel from one moment to another? On paper, they鈥檙e a perfectly
respectable way to travel back in time. Trouble is, you need a supply of exotic
鈥渘egative energy鈥� matter to prise your wormhole open.
But Mallett, a professor of theoretical physics at Connecticut University,
believes he has found a route to the past that uses something much more down to
earth: light. Mallett has worked out that a circulating beam of light, slowed to
a snail鈥檚 pace, just might be the vital ingredient for time travel. Not only is
the technology within our grasp, Mallett has teamed up with other scientists at
Connecticut to work towards building it. 鈥淲ith this device,鈥� he says, 鈥渢ime
travel may become a practical possibility.鈥�
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It may be hard for us to climb into Mallett鈥檚 time machine, as slowing light
down requires temperatures close to absolute zero. But future, advanced
civilisations might work out a way to do it. And they might even come back to
tell us how. If it works in the way Mallett believes it might, his device would
provide time travellers from the future with their first gateway into our
history.
Mallett began his journey into the past when he was just ten years old. In
1955, his father died of a heart attack. 鈥淔or me, the sun rose and set on him.
It completely devastated me,鈥� Mallett says. But then he came across The Time
Machine by H. G. Wells. Even as a child, Mallett knew his father hadn鈥檛
taken care of himself. Drinking and heavy smoking took a toll on his weak heart,
and it gave out at the age of 33. 鈥淢y notion was that if I could build a time
machine, I might be able to warn him about what was going to happen,鈥� Mallett
says. 鈥淭hat became my guiding light.鈥�
What started as a childish notion grew into a passionate investigation of
everything ever written about time travel. When Mallett studied the work of
Einstein鈥攚ho died in the same year as his father鈥攈e realised that
Wells鈥檚 novel was right on track: time travel is, in theory at least,
achievable.
Einstein himself found the notion upsetting, but he had only himself to
blame. He showed that the effect we call gravity is a bending of space and time.
Anything that has mass or energy distorts the space and the passage of time in
its vicinity, a bit like the way the surface of a soft couch is distorted when
someone sits on it. Solving Einstein鈥檚 gravitational field equations tells you
just how space-time is distorted by mass and energy.
A lump of matter stretches space and time. So, for example, clocks run slower
in the gravitational field close to Earth than they do far out in space. And if
you set a massive lump spinning, it begins to whip space and time around after
it, like a rotating teaspoon dragging the foam on a cup of coffee. The denser
and faster-moving the matter, the more strongly it distorts space-time.
Take this idea far enough, and you find that time can be twisted so much that
instead of running in an infinite line from past to future, it is bent into a
ring. Follow this loop around, and you return to a particular moment, just as a
walk around the block brings you back to your front door.
Theoreticians have found some solutions to Einstein鈥檚 equations that include
these 鈥渃losed time-like loops鈥濃�攑hysicists鈥� jargon for a time machine. The
first to do so was the Austrian-born mathematician Kurt G枚del, in 1949, but
unfortunately his solution required the whole Universe to be
rotating鈥攚hich it鈥檚 not. Decades later Kip Thorne of Caltech came up with
the idea of using wormholes, which link different regions of warped space-time,
to provide such loops. Other loops can be made by infinitely long, spinning
cylinders鈥攕omewhat hard to come by鈥攐r fast-moving cosmic strings. In
the early Universe, these ultra-dense strands of matter may have been as common
as dirt, but alas, no longer.
Mallett鈥檚 idea of using light is much less outlandish. 鈥淧eople forget that
light, even though it has no mass, causes space to bend,鈥� he says. Light that
has been reflected or refracted to follow a circular path has particularly
strange effects. Last year, Mallett published a paper describing how a
circulating beam of laser light would create a vortex in space within its circle
(Physics Letters A, vol 269, p 214). Then he had a eureka moment. 鈥淚 realised
that time, as well as space, might be twisted by circulating light beams,鈥�
Mallett says.
To twist time into a loop, Mallett worked out that he would have to add a
second light beam, circulating in the opposite direction. Then if you increase
the intensity of the light enough, space and time swap roles: inside the
circulating light beam, time runs round and round, while what to an outsider
looks like time becomes like an ordinary dimension of space. A person walking
along in the right direction could actually be walking backwards in
time鈥攁s measured outside the circle. So after walking for a while, you
could leave the circle and meet yourself before you have entered it
(see Diagram).
The energy needed to twist time into a loop is enormous, however. Perhaps
this wouldn鈥檛 be a practical time machine after all? But when Mallett took
another look at his solutions, he saw that the effect of circulating light
depends on its velocity: the slower the light, the stronger the distortion in
space-time. Though it seems counter-intuitive, light gains inertia as it is
slowed down. 鈥淚ncreasing its inertia increases its energy, and this increases
the effect,鈥� Mallett says.
As luck would have it, slowing light down has just become a practical
possibility. Lene Hau of Harvard University has slowed light from the usual
300,000 kilometres per second to just a few metres
per second鈥攁nd even to a standstill
(快猫短视频, 27 January, p 4).
鈥淧rior to this, I wouldn鈥檛 have thought time travel
this way was a practical possibility,鈥� Mallett
says. 鈥淏ut the slow light opens up a domain we just haven鈥檛 had before.鈥�
To slow light down, Hau uses an ultra-cold bath of atoms known as a
Bose-Einstein condensate. 鈥淎ll you need is to have the light circulate in one of
these media,鈥� Mallett says. 鈥淚t鈥檚 a technological problem. I鈥檓 not saying it鈥檚
easy, but we鈥檙e not talking about exotic technology here; we鈥檙e not talking
about creating wormholes in space.鈥�
Mallett has already caught the interest of his head of department, William
Stwalley, who leads a group of cold-atom researchers. Their first experiment
will be designed only to observe the twisting of space, by looking for its
effect on the spin of a particle trapped in the light circle. If they can then
add a second beam, Mallett believes evidence of time travel will eventually
appear.
He鈥檚 not sure how time travel would manifest itself. Perhaps what starts out
as a single trapped particle would acquire a partner鈥攖he particle visiting
itself from the future.
Stwalley is more interested in the practical challenges of the experiment,
and remains sceptical about possibilities of time travel. 鈥淎 time machine
certainly seems like a distant improbability at best,鈥� he says.
Last month, Mallett gave his first talk on the idea at the University of
Michigan at the invitation of astrophysicist Fred Adams, who accepts that the
theoretical side of Mallett鈥檚 work stands up to scrutiny. 鈥淭he reception was
cautious and sceptical,鈥� Adams admits. 鈥淏ut there were no holes punched in it,
either. The solution is probably valid.鈥�
But even Adams isn鈥檛 convinced that the experiment will work. That鈥檚 hardly
surprising, as time travel raises disturbing questions. Could you go back and
murder your grandparents, making your birth impossible? There may be ways out of
this problem (see 鈥淧aradox lost鈥�), but most physicists think that any attempt to
mess with history should be impossible. The Cambridge astrophysicist Stephen
Hawking calls this the 鈥渃hronology protection conjecture鈥�.
The general theory of relativity, which Mallett used to work out his theory
of time travel, does not take account of quantum mechanics. Could this be the
crucial omission that means time machines won鈥檛 work in the real Universe?
Hawking and Thorne say that any time machine would magnify quantum fluctuations
in the electromagnetic field, and destroy itself with a beam of intense
radiation. But to know for sure, we need a theory of quantum gravity鈥攁
theory that merges quantum theory with relativity.
Even Mallett doesn鈥檛 claim that time travel is definitely within reach.
鈥淲hether it will do what I predict is something that one will only know by
performing the actual experiment,鈥� he says. Then there鈥檚 the problem of getting
on and off the loop of time without destroying it鈥攐r yourself. 鈥淚 really
don鈥檛 know whether you could use this in the sense of H. G. Wells鈥檚 time
machine,鈥� says Mallett.
But who knows? In a few years, we may have entered an era when time travel is
possible, and all kinds of strange people, things and situations from the future
might come to visit. One thing seems certain, though. Even if the Connecticut
time machine works, it won鈥檛 be taking any Yankees back to the court of King
Arthur. Mallett鈥檚 circle of light won鈥檛 allow anyone to travel back beyond the
point where time first formed a closed loop. So it will be impossible to go back
to a time before it was set up. 鈥淎 later person could only travel back to the
time when the machine is turned on,鈥� Mallett says. This may explain why we have
never been overrun by visitors from the future.
It also means that although Mallett might change the Universe, he won鈥檛 ever
achieve his childhood dream. Mallet鈥檚 father will remain forever beyond his
reach.
Time travel is littered with paradoxes. The most notorious is the idea of
travelling back to the time before your parents were born and killing your
grandparents, making it impossible that you would ever exist. And if you didn鈥檛
exist, you wouldn鈥檛 be able to travel back, so you wouldn鈥檛 kill your
grandparents, so you would be born after all . . . Any influence on the past can
lead to self-contradictory logical loops like this.
People have dreamed up ways to try to break out of the loop. One is the
鈥渃onsistent histories鈥� approach, which says that you must be somehow forbidden
from doing anything that would change the past. However hard you try, something
will stop your killing spree. But this is uncomfortably deterministic. In a
universe with time travel, should everything be predetermined?
Another way out is the 鈥渁lternative histories鈥� hypothesis. In this idea, you
go back to a different history from the one you left. You are free to do
anything in this alternate version of history鈥攌illing your grandparents
included. It won鈥檛 change anything in the history where you originated.
This has parallels in the 鈥渕any worlds鈥� interpretation of quantum mechanics,
an explanation of how the bizarre quantum laws allow unobserved particles such
as atoms and electrons to be in two places at once. Every time an observation
forces them to choose one position or another, a new universe is
created鈥攐ne where they took one position, one where they took the other.
So perhaps a time machine would take you into a parallel universe.