# Time Travel Is Mathematically Possible With New Mind-Boggling Model

*Updated *| 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.

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 doppelgänger 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 to include comments from Marika Taylor.*