[tt] Telegraph: The new theories that are killing time

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The new theories that are killing time
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Last Updated: 12:01am GMT 19/02/2008

Physicists are getting wound up in their attempts to find out what
makes the universe tick, says Roger Highfield
We all have a sense that time is flowing. We link it with change,
decay and death, and mark its passing with birthdays and
anniversaries. But now time could be running out - for the very
concept of time itself.

   Mankind 'shortening the universe's life'

   Parallel universe proof boosts time travel hopes

   Quantum theory and relativity explained

Traditionally, physicists have dismissed those who think about time
too much. Albert Einstein, impatient with philosophical
hand-wringing, once said that time is "what you measure with a
clock".


Changing face - New theories that are killing time

Traditionally, physicists have dismissed those who think about time
too much

But many scientists feel that an overhaul of what we mean by "time"
could lead to the next great leap forward in physics.
In the past few months some tantalising, and mind-boggling, ideas
have emerged: that there should be two dimensions of time, not one;
that time could grind to a halt in a few billion years; or, most
radical of all, that time does not even exist.
Despite this recent flurry of activity, the trouble with time dates
back much further, to 1905, when Einstein was an obscure patent
officer.

Before his extraordinary work, scientists such as Sir Isaac Newton
had viewed time as "absolute", meaning that clocks ticked at the
same rate throughout the universe.

That all changed when Einstein, then only 26, pondered how we know
what the time is and worked through the consequences of one subtle
thought: the information on the face of a clock cannot be
transferred into our brains faster than the speed of light.

To keep the universe tidy, he wanted to keep the laws of nature
constant, no matter where an observer was or how fast they were
going relative to a clock.
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But while exploring this, Einstein discovered a bizarre effect:
moving clocks tick more slowly than those at rest.

Once he had found this link between time and motion, he was able to
fuse time with three-dimensional space to envisage a
four-dimensional "spacetime", an idea physicists now use routinely.

Extending his work in his theory of general relativity, Einstein
revealed that the greater the gravitational pull, the more slowly a
clock will tick.

This led into some very strange areas. Time travel became possible
when Einstein's colleague Kurt Gödel showed that under general
relativity, you can, if you bend spacetime enough, create a loop and
end up before where you started.

Ever since Gödel unveiled this idea in 1949, physicists have argued
against time travel, because it undermines the idea of cause and
effect: a time traveller could go back to kill his grandfather,
meaning that he would never have been born in the first place.

But, six decades later, no killer reason to rule out time travel has
been found, even though Prof Stephen Hawking has tried to keep the
world safe for historians by devising a "chronology protection
conjecture", stating that quantum theory makes Gödel's loops
unstable.

However, there is a deeper problem with our understanding of time.
We can all feel that it flows from the past to the future, something
we can see vividly in thermodynamics - the theory of heat and work -
which says that disorder and chaos ("entropy") increase as time goes
by.

Our cup of coffee will always get colder, just as milk can be
stirred in but not out.

Yet both our theories about the very small (quantum mechanics) and
the very large (relativity) seem indifferent to the direction of
time.


They seem to allow the shards of a broken pot to come back together
again, or people to grow younger and wriggle into their mothers'
wombs. As Einstein put it when his great friend Michele Besso died:
"People like us, who believe in physics, know that the distinction
between past, present and future is only a stubbornly persistent
illusion."

Lee Smolin, of the Perimeter Institute for Theoretical Physics in
Canada, insists that "time is real" but says that reformulating it
could be central to finding the long-sought "theory of everything"
that unites all the forces and particles of the universe, notably
by merging relativity and quantum theory.

Spurred on by this idea, scientists have come up with some truly
bizarre temporal theories:
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TIME IS QUANTUM

Quantum theory says that energy comes in tiny, indivisible chunks,
called quanta. There is also a "quantum of time", the smallest
measurement of time with any meaning, equal to a second divided by
a huge number - one followed by 43 zeroes (this represents the
length of time a photon moving at light speed takes to travel the
smallest meaningful distance).

This could be the smallest unit of time possible, and thus of that
blend of space and time, spacetime. By this reasoning there is no
continuous flow of time, but rather spacetime moments running like
grains of sand through an hourglass.

Not all are convinced, however.

"Some theorists think that there are 'atoms of spacetime' but I
strongly think this is wrong," says Dr Nima Arkani-Hamed of the
Institute for Advanced Study in Princeton. "I think everyone would
agree that spacetime itself is on the chopping block, but it will
be replaced in my view by something infinitely more interesting
than 'atoms of spacetime'."

TIME WILL COME TO AN END

Einstein's equations describing spacetime break down when applied
to the extreme conditions at the birth of the cosmos in the Big
Bang, points out Prof Eva Silverstein of Stanford University.

"One goal of research into gravity is to understand how to describe
physics in these extreme conditions, including the problem of how
space and time emerge. We have made some steps in this direction
recently, but many difficult questions remain."

But the very fact that time was born has a fascinating corollary:
it may disappear. In billions of years time could cease to be,
according to Prof José Senovilla of the University of the Basque
Country, Bilbao, and his colleagues, who published their findings
in the journal Physical Review D.

To help explain away a cosmological mystery (an antigravity force
called dark energy), they argue that the dimension of time is
slowly turning into a new dimension of space. This radical theory
suggests the idea that time could eventually run out and everything
grind to a halt.

THERE IS MORE THAN ONE KIND OF TIME

If we use one idea of symmetry to create our "theory of
everything", a strange kind of "hypertime" emerges. Prof Itzhak
Bars of the University of Southern California in Los Angeles has
found a way to introduce two dimensions of time, to help create a
theory of quantum gravity (given the evocative title of M theory).

Under this idea, time is no longer a simple line from the past to
the future in a four-dimensional world consisting of three
dimensions of space and one of time. Instead - to simplify greatly
- he envisages the passage of history as curves embedded in six
dimensions, with four of space and two of time.

The good news, says Prof Bars, is that this approach overcomes
previous problems with hypertime and allows predictions that can be
tested. The bad news - at least for Doctor Who - is that his
proposal bans time travel. "The magic of this symmetry is to forbid
time travel as well other unphysical effects of hypertime," he
explains.

TIME DOES NOT EXIST

This is an equally outrageous way to solve our problems with time,
albeit at the smallest reaches of quantum theory. Prof Carlo
Rovelli, of the University of Marseille, reports in Physical Review
D that he has formulated a theory reminiscent of the old joke that
"time is what keeps everything from happening at once".

Time is a distance between events, just as space is a distance
between places. Prof Rovelli's approach in effect gets rid of time
by turning it into correlations between things in space.

So, rather than saying that you walked from Oxford Circus to
Victoria, passing first through Bond Street, then Hyde Park Corner,
you simply show the journey with no reference to time at all.
Reassuringly, when time is discarded in this way, quantum theory
still gives the same answers as before.

Why, then, can we look back at the past but not into the future?
Prof Rovelli, along with Alain Connes at the Collège de France in
Paris, have argued this flow of time is an illusion. They liken it
to the sensation of temperature.
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What we mean by temperature is molecular motion. A hot cup of tea
teems with more energetic water molecules than a cold one, but the
temperature we feel somehow gives us an average of all that
turmoil.

In a similar way, when our brains average what our senses detect,
what emerges is a sense of passing seconds, minutes and hours.
Time's flow is a measure of our ignorance.

This chimes with earlier work of Julian Barbour, an independent
thinker, who warned that when we unify general relativity and
quantum mechanics, time will be seen as a trick of the mind.

In other words, time's up.