Time travel is possible, but only if you have an object with an infinite mass

This photo provided by Universal Pictures Home Entertainment gives Christopher Lloyd, left, as Dr. Emmett Brown and Michael J. Fox as Marty McFly in the 1985 movie “Back to the Future.” (Universal Pictures Home Entertainment via AP)

The concept of time travel has always captured the imagination of physicists and laymen alike. But is it really possible? Of course it is. We are doing it now, aren’t we? We are all on a journey in the future a second at a time.

But that is not what you think. We can travel much further in the future? Absolutely. If we could travel close to the speed of light, or in the vicinity of a black hole, time will slow down, so we are able to travel arbitrarily far in the future. The really interesting question is whether we can travel back to the past.

I am a physics professor at the University of Massachusetts Dartmouth, and first heard about the idea of travel in time when I was 7, 1980 with an episode of Carl Sagan’s classic TV series “Cosmos.” I decided then that one day, I was going to aim for a deep study of the theory that underlies such a creative and unusual ideas: Einstein’s theory of relativity. Twenty years later, I came up with a Ph. D. in the area and are an active researcher in the theory ever since.

Now, one of my phd students has just published a paper in the journal Classical and Quantum Gravity that describes how to build a time machine with the help of a very simple construction.

Closed time-like curves

Einstein’s general theory of relativity allows for the possibility of the bending of time to such a high degree that it actually folds on itself, resulting in a time loop. Imagine that you are on a journey along this loop; that means that at a certain point, you might end up on a time in the past, and begin experiencing the same moments because everything is a bit of deja vu, except that you don’t even realize. Such constructions are often referred to as “closed time-like curves” or CTCs in the scientific literature, and popularly referred to as ” time machines.” Time machines are a by-product of effective faster-than-light travel arrangements, and understanding, can enhance our understanding of how the world works.

Over the past few decades well-known physicists such as Kip Thorne and Stephen Hawking, who produced groundbreaking work on models with respect to time machines.

The general conclusion that has emerged from previous research, including Thorne and Hawking’s, is that nature forbids time loops. This is perhaps best explained in Hawking’s “Chronology Protection Conjecture”, which essentially says that nature does not accommodate the changes of history, so spare us of the paradoxes that arise if time travel was possible.

Perhaps the most famous among these paradoxes that arise through time travel to the past is the so-called “grandfather paradox” in which a traveller goes back into the past and kills his own grandfather. This changes in the course of history in a way that there is a contradiction arises: The traveller was never born and therefore could not exist. There are a lot of movie and novel plots on the basis of the paradoxes that result from time travelling – perhaps some of the most popular are the “Back to the Future” films and “Groundhog Day.”

Exotic matter

Depending on the details, the different physical phenomena can act to prevent closed time-like curves of the evolution in physical systems. The most common is the requirement for a particular type of “exotic” matter that must be present in order for a time loop to exist. Loosely speaking, exotic matter is matter that has negative mass. The problem is the negative mass is not known in the nature.

Caroline Mallary, a doctoral student at the University of Massachusetts Dartmouth has published a new model for a time machine in the journal Classical & Quantum Gravity. This new model does not require any negative mass of exotic material and offers a very simple design.

Mallary the model consists of two super-long car ‘ s built of material that is not exotic, and have positive mass – parked in parallel. A car is going forward quickly, while the other parked. Mallary was able to show that in such an installation, a time loop can be found in the space between the cars.

So you can build this in your backyard?

If you suspect that there is a catch, you’re right. Mallary model requires that the middle of each car has an infinite density. That means that they contain objects called a singularity, with infinite density, temperature and pressure. Moreover, in contrast to the singularities that are present in the interior of black holes, making them totally inaccessible from the outside, the singularity in Mallary the model are completely bald and the observable, and therefore the true physical effects.

Physicists do not expect that such particular objects exist in nature. So, unfortunately, a time machine is not available anytime soon. However, this work shows that physicists may have to refine their ideas about why closed time-like curves are prohibited.

This story originally appeared in The Conversation.

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