Can we open portals through the wormhole?

Stephen Hawking made an amazing prophecy

lwpkommunikacio / FlickrLet's travel back to 2009: Stephen Hawking is sitting in a room decorated with balloons and waiting for his guests. The champagne is chilled and a large buffet is served. It is one of the most exclusive celebrations that has ever taken place - because only time travelers from the future are invited. But the astrophysicist waits in vain.

It was then that Stephen Hawking's attempt to prove that time travel was possible failed. Speaking at a symposium in 2012, he said, “I have experimental evidence that time travel is not possible. “I had a time traveler party, but I didn't send out the invitations until after the party. I sat there for a long time, but no one came. "

However, the astrophysicist did not rule out the possibility of time travel until his death in March 2018. In his last book, "Brief Answers to the Big Questions", he went back to the subject.

It is a "very serious question," he wrote - a question that unfortunately most people do not take seriously. "If someone applied for research funding to do research on time travel, the application would be rejected immediately."

Are time travel possible? Will we one day be able to build a machine to travel into the past and the future? In a guest post for the science portal “The Conversation”, researcher Peter Millington from the School of Physics and Astronomy at the University of Nottingham got to the bottom of these questions.

The speed of light plays a crucial role in time travel

Agarin / shutterstock“The ability to call our friends and family wherever they are in the world to know what they are just now we take for granted, ”writes Millington. We can never know exactly what the other person is doing at that moment, because the signals that transport the voice and images are transmitted extremely quickly - but not “right now”.

The highest speed at which a signal or - in physical terms - an electromagnetic wave can propagate is the so-called speed of light. It is exactly 299,792,458 meters per second. In the context of the special theory of relativity, Albert Einstein postulated that the speed of light is a universal constant, which means that light always travels at the same speed in a vacuum, regardless of the observer.

It is precisely this condition that plays a decisive role in the question of whether time travel is possible. So the law of causality follows from the fact that nothing can be faster than the speed of light. The law says that the effect of an action can only occur after the cause, which would make time travel into the past impossible. "If I went back in time and prevented my birth, the effect (I) would precede the cause (my birth)," Millington explains.

Time travel into the future: possible according to Einstein's special theory of relativity?

ShutterstockFrom the constancy of the speed of light, however, it also follows that space and time do not have to be absolute, but relative. A direct consequence of this is the so-called time dilation: This means that time passes at different speeds, depending on how fast objects are moving. A moving watch that is located in a car driving at constant speed, for example, ticks more slowly from the perspective of a stationary observer who is not in the car.

This situation can be compared with a journey into the future - even if the time difference between the moving driver and the stationary observer is only a billionth of a second.

Millington explains it with the following example: “If I were to fly in a spaceship at incredibly high speed and then return to Earth, it would have been less time for me than for anyone else I left [on Earth]. Everyone would come to the conclusion that my life moved like in slow motion - I would have aged slower than they - and I would find that hers moved faster. "

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

These questions are not easy to answer. Because, as Millington explains, the law of causality can no longer apply in such a case and we can no longer regard time as continuous or retrograde. In addition, the theory of relativity says that mass and energy are equivalent to each other. For all particles that have a so-called rest mass, this means that infinitely high energy is required to reach and exceed the speed of light. So far, no particles are known without a rest mass.

Time travel into the future through wormholes

As Stephen Hawking writes in his book, however, there could be a way out that makes time travel into the past possible: So-called wormholes that connect two distant places in the universe.

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

“The more mass we bring together in a region in space, the more space-time is distorted and clocks in the vicinity tick more and more slowly. If we pressed together enough mass, space-time would bend so that not even the light could evade the force of gravity and a black hole would form, ”Millington writes.

However, only the edge of this black hole is relevant for time travel. There time passes infinitely slowly relative to a distant observer. Physicists assume that wormholes can be formed from black holes. 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 stabilize such a tunnel, however, locations with a negative spatial curvature, i.e. a negative energy density, would be required. But can an energy density even become negative?

Most would surely say no to this question. However, the theories of quantum mechanics by no means rule out the existence of negative energy densities. With exotic particles it is possible to create points in the universe with negative and others with positive energy density, so that the total energy remains positive.

Also read: Sind are our days numbered? Stephen Hawking made a harrowing prediction

As Millington writes, however, there is as yet no theory that unites both Einstein's theory of gravity and quantum mechanics. Whether time travel into the past is actually possible must remain one of the many secrets of our universe for the time being.