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Many have
dreamed of figuring out how to travel in time—and dismissed it as impossible.
Now, researchers have proposed a mathematical model that makes time travel
possible, using concepts of Einstein’s theory of general relativity coupled
with the hypothesis that time is not a separate dimension. Which is kind of
Exciting!

Traditionally,
we think of the universe as being made up of three spatial dimensions, and a
fourth dimension representing time. But mathematician Ben Tippett at the
University of British Columbia, Canada, says this is wrong. He believes time
should not be separated from other three spatial dimensions—instead all four
run together, simultaneously.

Working with
David Tsang, an astrophysicist from the University of Maryland, he has worked
out a way to use this principle to make time travel possible. Their findings
have now been published in the journal Classical and Quantum Gravity.

“People
think of time travel as something fictional,” Tippett said in a statement. “And
we tend to think it’s not possible because we don’t actually do it. But,
mathematically, it is possible.”

In an email
interview with Newsweek , he explained how the time machine—Traversable Acausal
Retrograde Domain in Spacetime, or TARDIS—would work. In general relativity,
the curvature of spacetime causes gravity by exerting a force on objects
passing them. These curves cause planets to orbit stars—if spacetime was not
curved, all the planets and stars would travel along straight lines. So if
spacetime is curved, and we run time along it simultaneously, then
theoretically the bend can be turned into a loop, making time travel possible.

“Since the
1950s, there have been many other proposals for spacetimes which allow people
to travel backward in time,” he says. “My work was to model a ‘time machine,’
where passengers inside of a box of limited size could travel along a circle
through space and time, returning to their own pasts.

“The shape
of spacetime was used to turn the direction of the arrow of time inside of the
box in space and time. I then used Einstein’s theory to analyze this strange
spacetime, and determine what would be required to build such a thing.”

Tippet and
Tsang’s time machine model creates a spacetime curvature that is bent into a
circle. Anything—a box with someone inside, for example—moving along this
curvature would be anchored to this version of time and would move backward.
Someone watching from the outside would be able to see events running in
reverse.

“It is
because time and space are attached together that the time machine to behave in
this way,” Tippet says. “In the simplest way, the orientation of the arrow of
time inside the box is not anchored to the orientation of the arrow of time
outside the box.

“Initially,
they are pointing in the same direction; and then the direction of the arrow of
time in the box turns so that ‘forward in time’ inside the box corresponds to
the ‘sideways’ spatial direction outside of the box. And then the arrow of time
inside the box continues to rotate in space and time until it returns to its
original orientation.”

If you were
inside the time machine making breakfast, the hands of your wristwatch would be
moving forward and you would feel a “persistent acceleration,” Tippet explains.
But if you were to look outside, things would get very strange.

“You would
see two strange things: First you would see a second version of you standing in
an identical copy of your box, but timeshifted (so, at a previous time), and
also, time would be running in reverse. Your doppelgĂ¤nger would be un-frying
eggs, and putting them back in their shells; and un-stirring the cream from
their coffee. The hands on the clocktower outside would behave erratically,
first moving clockwise, then counterclockwise, according to which part of the
bubble’s journey you were currently sitting through.

“The fun
thing is that the outside viewer would see two version of you: One where time
was moving forward in time (cracking and frying eggs) and the other moving
backward in time (un-stirring the cream in their coffee).”

But will
such a machine ever exist? Tippet says no. “Our paper included a careful
analysis of this geometry, and the problems it would have in being built,” he
says. “Generally speaking, backward time travel usually causes singularities
(places where there are holes in the universe) or instabilities which would
cause them to collapse into a black hole if they get poked the wrong way. So
unfortunately, I don’t foresee this as being feasible."

Marika
Taylor, professor of theoretical physics at the University of Southampton,
commented on the study. She tells Newsweek over email: “Mathematical models for
time travel all use the idea of creating shortcuts in a spacetime. In the study
the authors explore a version of this idea, bubbles in a spacetime.

“However the
main problems in all these models are that quantum effects [effects that cannot
be explained by classical physics] often destroy the spacetime shortcuts and
that exotic forms of matter are required to create the shortcuts.”

Exotic
matter refers to a class of material yet to be discovered. Unlike ordinary
matter, exotic matter causes space and time to expand and gravity to be
repulsive. In the study, the researchers note that time can only be bent into a
circle by using exotic matter.

Taylor
continues: “The authors of this study are open about the fact that they have
these problems too: their bubbles have to be supported by exotic forms of
matter (matter that has never been found in Nature!), and there are
‘singularities’ in their spacetimes (which mean quantum physics effects are
very important and may likely cause their bubbles to be unstable and collapse).

“So, in
summary, while their work is interesting and adds to the existing literature,
it doesn't really show that time travel is possible in our Universe. It is not
clear that such exotic forms of matter actually can exist in our Universe (it's
considered very unlikely).”

This piece
has been updated.

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