When
the new ideas of quantum mechanics spread through science like wildfire
in the first half of the 20th century, one of the first things
physicists did was to apply them to gravity and general relativity. The
result were not pretty.
It immediately became clear
that these two foundations of modern physics were entirely incompatible.
When physicists attempted to meld the approaches, the resulting
equations were bedeviled with infinities making it impossible to make
sense of the results.
Then in the mid-1960s, there was
a breakthrough. The physicists John Wheeler and Bryce DeWitt
successfully combined the previously incompatible ideas in a key result
that has since become known as the Wheeler-DeWitt equation. This is
important because it avoids the troublesome infinites—a huge advance.
But
it didn’t take physicists long to realise that while the Wheeler-DeWitt
equation solved one significant problem, it introduced another. The new
problem was that time played no role in this equation. In effect, it
says that nothing ever happens in the universe, a prediction that is
clearly at odds with the observational evidence.
This
conundrum, which physicists call ‘the problem of time’, has proved to be
thorn in flesh of modern physicists, who have tried to ignore it but
with little success.
Then in 1983, the theorists Don
Page and William Wooters came up with a novel solution based on the
quantum phenomenon of entanglement. This is the exotic property in which
two quantum particles share the same existence, even though they are
physically separated.
Entanglement is a deep and
powerful link and Page and Wooters showed how it can be used to measure
time. Their idea was that the way a pair of entangled particles evolve
is a kind of clock that can be used to measure change.
But
the results depend on how the observation is made. One way to do this
is to compare the change in the entangled particles with an external
clock that is entirely independent of the universe. This is equivalent
to god-like observer outside the universe measuring the evolution of the
particles using an external clock.
In this case, Page
and Wooters showed that the particles would appear entirely
unchanging—that time would not exist in this scenario.
But
there is another way to do it that gives a different result. This is
for an observer inside the universe to compare the evolution of the
particles with the rest of the universe. In this case, the internal
observer would see a change and this difference in the evolution of
entangled particles compared with everything else is an important a
measure of time.
This is an elegant and powerful idea.
It suggests that time is an emergent phenomenon that comes about
because of the nature of entanglement. And it exists only for observers
inside the universe. Any god-like observer outside sees a static,
unchanging universe, just as the Wheeler-DeWitt equations predict.
Of
course, without experimental verification, Page and Wooter’s ideas are
little more than a philosophical curiosity. And since it is never
possible to have an observer outside the universe, there seemed little
chance of ever testing the idea.
Until
now. Today, Ekaterina Moreva at the Istituto Nazionale di Ricerca
Metrologica (INRIM) in Turin, Italy, and a few pals have performed the
first experimental test of Page Wooters ideas. And they confirm that
time is indeed an emergent phenomenon for ‘internal’ observers but
absent for external ones.
The experiment involves the
creation of a toy universe consisting of a pair of entangled photons and
an observer that can measure their state in one of two ways. In the
first, the observer measures the evolution of the system by becoming
entangled with it. In the second, a god-like observer measures the
evolution against an external clock which is entirely independent of the
toy universe.
The experimental details are
straightforward. The entangled photons each have a polarisation which
can be changed by passing it through a birefringent plate. In the first
set up, the observer measures the polarisation of one photon, thereby
becoming entangled with it. He or she then compares this with the
polarisation of the second photon. The difference is a measure of time.
In
the second set up, the photons again both pass through the birefringent
plates which change their polarisations. However, in this case, the
observer only measures the global properties of both photons by
comparing them against an independent clock.
In this
case, the observer cannot detect any difference between the photons
without becoming entangled with one or the other. And if there is no
difference, the system appears static. In other words, time does not
emerge.
“Although extremely simple, our model captures
the two, seemingly contradictory, properties of the Page-Wooters
mechanism,” say Moreva and co.
That’s an impressive
experiment. Emergence is a popular idea in science. In particular,
physicists have recently become excited about the idea that gravity is
an emergent phenomenon. So it’s a relatively small step to think that
time may emerge in a similar way.
What emergent
gravity has lacked, of course, is an experimental demonstration that
shows how it works in in practice. That’s why Moreva and co’s work is
significant. It places an abstract and exotic idea on firm experimental
footing for the first time.
Perhaps most significant
of all is the implication that quantum mechanics and general relativity
are not so incompatible after all. When viewed through the lens of
entanglement, the famous ‘problem of time’ just melts away.
The
next step will to extend the idea further, particularly to the
macroscopic scale. It’s one thing to show how time emerges for photons,
it’s quite another to show how it emerges for larger things such as
humans and train timetables.
And therein lies another challenge.
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