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Physicists have come up with what they claim
is a mathematical model of a theoretical "time machine" - a box that
can move backwards and forwards through time and space.

The trick, they say, is to use the curvature
of space-time in the Universe to bend time into a circle for hypothetical
passengers sitting in the box, and that circle allows them to skip into the
future and the past.

"People think of time travel as something
as fiction. And we tend to think it's not possible because we don't actually do
it," said theoretical physicist and mathematician, Ben Tippett, from the
University of British Columbia in Canada.

"But, mathematically, it is
possible."

Together with David Tsang, an astrophysicist
at the University of Maryland, Tippett used Einstein's theory of general
relativity to come up with a mathematical model of what they're calling a
Traversable Acausal Retrograde Domain in Space-time (yep, the acronym is
literally TARDIS).

But before we get into the madness of legit
time travel, let's put this into perspective real quick - the researchers
aren't claiming to have a blueprint for a Doctor Who-style time machine that
can be built tomorrow. They say the materials we'd need to build this thing are
so exotic, we haven't even discovered them yet... but we'll get to that in a
minute. Firstly, let's talk about what Tippett and Tsang are actually
proposing.

The model is based on the idea that instead of
looking at the Universe in three spatial dimensions, with the fourth dimension
(time) separated, we should be imagining those four dimensions simultaneously.
That allows us to consider the possibility of a space-time continuum, where different
directions in space and time are all connected within the curved fabric of the
Universe.

Einstein's theory of relativity links
gravitational effects in the Universe to a curvature of space-time - the
phenomenon thought to be behind the elliptical orbits of planets and stars. If
space-time were 'flat' or uncurved, planets would move in straight lines. But
according to relativity, the geometry of space-time becomes curved in the
vicinity of high-mass objects, which causes planets to bend their paths and
rotate around their star instead.

What Tippett and Tsang argue is that it's not
just physical space that can be bent and twisted in the Universe - time itself
can also be curved in the vicinity of high-mass objects.

"The time direction of the space-time
surface also shows curvature. There is evidence showing the closer to a black
hole we get, time moves slower," said Tippett. "My model of a time
machine uses the curved space-time to bend time into a circle for the
passengers, not in a straight line. That circle takes us back in time."

In order to harness this theoretical property,
the physicists propose creating a kind of 'bubble' of space-time geometry,
which carries whatever's inside it through space and time along a large
circular path. If this bubble can hit
speeds greater than the speed of light - something the pair says is
mathematically possible - this would allow it to move backwards in time.

"It is a box which travels 'forwards' and
then 'backwards' in time along a circular path through spacetime," the
researchers explained in their 2017 paper. "Delighted external observers
would be able to watch the time travellers within the box evolving backwards in
time: un-breaking eggs and separating cream from their coffee."

You can see the basic idea in the image below,
with a passenger inside the bubble/time machine (person A), and an external
observer standing beside it (person B). The arrow of time - which under normal
circumstances (in our Universe, at least) always points forward, making the past
become the present - is represented by the black arrows:

Both person A and person B will experience
time in dramatically different ways, the researchers explained:

"Within the bubble, A will see the B's
events periodically evolve, and then reverse. Outside the bubble, observer B
will see two versions of A emerge from the same location: one's clock hands
will turn clockwise, the other counterclockwise."

In other words, the external observer would
see two versions of the objects inside the time machine: one version evolving
forwards in time, the other backwards. While Tippett and Tsang say the maths is
sound, the problem now is we don't actually have the right materials to build
what they're proposing.

"While is it mathematically feasible, it
is not yet possible to build a space-time machine because we need materials -
which we call exotic matter - to bend space-time in these impossible ways, but
they have yet to be discovered," said Tippett.

Their idea recalls another theoretical time
machine - the Alcubierre drive, which would also use a shell of exotic matter
to transport passengers through time and space (hypothetically). Both ideas
can't go very far without some idea of how to actually produce these
space-time-bending materials, but as Tippett points out, we're never going to
stop wondering about the possibilities of time travel, and this is just one
more direction we can take this mind-bending physics.

"Studying space-time is both fascinating
and problematic," he said. "Experts in my field have been exploring
the possibility of mathematical time machines since 1949, and my research
presents a new method for doing it."

A version of this article was first published
in April 2017.

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