Some years ago, Stephen Hawking stated that he had "experimental evidence that time travel is not possible".

However, in his final book "Brief Answers to the Big Questions" he also stated that the notion of time travel was a "very serious question".

Researcher Peter Millington from the University of Nottingham wrote a guest post for The Conversation, in an effort to get to the bottom of some of these questions.



Stephen Hawking made a pretty big statement in his final book, one that should serve as food for thought.

Let's travel back to 2009 — Stephen Hawking is sat in a room decorated with balloons, awaiting his guests. The champagne is chilled, a large buffet is out.

It's one of the most exclusive celebrations ever: only time travelers from the future are invited — but the astrophysicist waited in vain.

At that time, Stephen Hawking's attempt to prove that time travel was possible failed. According to a report by IFLScience, he spoke at a symposium in 2012 and said, "I have experimental evidence that time travel is not possible." He explained that he had organised a party for time-travelers, but had sent out the invitations after the party. "I sat there a long time, but no one came."

However, the astrophysicist didn't exactly rule out time travel on his death in March 2018. In his posthumous book "Brief Answers to the Big Questions", he came back to the topic again.

He wrote that it was a "very serious question", also adding, however, that "if one made a research grant application to work on time travel it would be dismissed immediately".

Is time travel possible? Will we one day be able to build a machine to travel to the past as well as the future? In a guest post for The Conversation, researcher Peter Millington of the School of Physics and Astronomy at the University of Nottingham tried to get to the bottom of some of these questions.

The speed of light plays a decisive role in time travel

"We take for granted the ability to call our friends and family wherever they are in the world to find out what they are up to right now," wrote Millington. "But this is something we can never actually know. The signals carrying their voices and images travel incomprehensibly fast, but it still takes a finite time for those signals to reach us."

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The highest speed at which a signal or — physically speaking — an electromagnetic wave can propagate is what is known as the speed of light. It is exactly 299,792,458 meters per second. Albert Einstein postulated within the framework of his theory of relativity that the speed of light is a universal constant, i.e. that light always moves at the same speed in a vacuum — and independently of the observer.

It is precisely this condition that plays a decisive role in the question of whether time travel is possible. The law of causality follows from the fact that nothing can be faster than the speed of light. The law states that the effect of an action can only occur after the cause, which would make time travel into the past impossible. "For me to travel back in time and set in motion events that prevent my birth is to put the effect (me) before the cause (my birth)," explained Millington.

Is time-travel into the future possible according to Einstein's theory of relativity?

Albert Einstein postulated within the framework of his theory of relativity that light always moves at the same speed in a vacuum. Hulton Archive/Getty Images

From the constancy of the speed of light it follows, however, that space and time must not be absolute, but relative. A direct consequence of this is that time passes at different speeds depending on how fast objects move. For example, a moving clock in a car moving at a constant speed ticks more slowly from the point of view of a resting observer who is not in that car.

This is comparable to a journey into the future — even if the time difference between the moving driver and the resting observer is only a billionth of a second.

Millington explained the whole thing with the following example: "If I were to fly off at incredible speed in a spaceship and return to Earth, less time would pass for me than it would for everyone I left behind. Everyone I returned to would conclude that my life had run as if in slow motion — I would have aged more slowly than them — and I would conclude that theirs had run as if in fast forward."

Read more: 15 of the most remarkable and memorable things Stephen Hawking ever said

And what would happen if, contrary to Einstein's theory of relativity, we could actually move faster than light? Would it then be possible for us to travel back in time?

The answers to these questions aren't straightforward. As Millington explained, the law of causality could no longer apply in such a case and we could no longer regard time as forward or backward. Moreover, the theory of relativity states that mass and energy are one and the same. For all particles that have a "rest mass", this means that an infinitely high energy is required to reach and exceed the speed of light. So far, there are no known particles without a rest mass.

Time travel into the future through wormholes

However, as Stephen Hawking writes in his book, there could be a way time travel into the past may be possible: wormholes that connect two distant places in the universe.

An illustration of an Einstein-Rosen bridge, or wormhole in the fabric of space. Shutterstock

In Einstein's general theory of relativity, gravity is a consequence of the way in which mass warps space and time — mass distorts space-time and this in turn influences the movement of mass. In physics, spacetime refers to the joint representation of three-dimensional space and one-dimensional time in a four-dimensional mathematical structure.

"The more mass we squeeze into a region of space, the more spacetime is warped and the slower nearby clocks tick. If we squeeze in enough mass, spacetime becomes so warped that even light cannot escape its gravitational pull and a black hole is formed," wrote Millington.

However, only the edge of this black hole is pertinent when it comes to time travel: there, time passes infinitely slowly relative to a distant observer: your clock would tick infinitely slowly relative to those far away from it. Physicists assume that wormholes can be formed from black holes.

A 1998 illustration of a spacecraft using negative energy to warp spacetime and travel faster than light-speed. Les Bossinas/NASA

Wormholes are a kind of tubes in space-time that make it possible to get from A to B at the speed of light. In order to stabilise such a tunnel, however, locations with a negative spatial curvature, i.e. a negative energy density, would be required. But can an energy density become negative at all?

Most people would answer this question with a resounding "no", if basing their answer off the classical physics of the 19th century. The modern theory of quantum mechanics, however, doesn't exclude the existence of negative energy densities: empty space is not empty, according to quantum mechanics.

Read more: The US military released a study on warp drives and faster-than-light travel. Here's what a theoretical physicist thinks of it.

Instead, it's filled with pairs of particles popping in and out of existence. A region in which fewer pairs were allowed to pop in and out than everywhere else would have negative energy density.

However, as Millington writes, there is still no theory that marries Einstein's theory of gravity with quantum mechanics — whether time travel to the past is possible or not will remain one of the many secrets of our universe.