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:

B. K.
Tippett et. al.

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."

The research
was published in Classical and Quantum Gravity.

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

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