Time travel is the concept of movement between certain points in time, relative to movement at any given point in space. It is uncertain if time travel to the past is physically possible, but forward time travel, outside the usual sense of the perception of time, is an extensively-observed phenomenon and well-understood within the framework of Albert Einstein’s theory of special relativity and general relativity. The 20th-century scientist developed the idea that space and time are really aspects of the same thing—space-time.
There is a speed limit of 300,000km per second (or 186,000 miles per second) for anything that travels through space-time, and light always travels the speed limit through empty space.
Special Relativity says that something unique happens when you move through space-time, especially when your speed relative to other objects is close to the speed of light.
Time goes slower for you than for the people you left behind, so you won’t notice this effect until you return to those stationary people.
Professor Brian Cox demonstrated this idea during a BBC Doctor Who special, using a member of the audience to help – Professor Jim Al-Khalili.
He set up the Professor in a moving chair across the stage, with a beam of light moving up and down.
He said in 2015: “A beam of light bouncing between mirrors could be used to build a very accurate clock.
“Then Einstein imagined what that clock would look like if it was moving relative to us.
“So Jim is going to be moved along the stage while moving the clock and we will dim the lights so we can see what that looks like from our perspective – stationary relative to Jim.
“We’ve placed a head camera on Jim so you can see the clock in exactly the way we pictured it when it was stationary – relative to us – and the light beam is bouncing up and down between the mirrors.
“But if you look, we’ve got a little video effect on there so you can see the trail of the light we see is tracing out a triangular pattern across the stage.”
The experiment then showed how the audience had a different perspective to the light than the Professor.
Dr Cox added: “But what we saw there from the audience perspective was: as Jim moved the light took a triangular path as it bounced across the stage.
“Here is what Einstein’s suggestion that the speed of light is constant for all observers implies.
“See this path (moving) is obviously longer that this path (stationary), but if we all agree on the speed of light, then it is obvious that it must take the light longer for the moving clock than it does for the stationary.
“Moving clocks run slower – this is true – time really did pass at a different rate for Jim than it did for you.
“There is no sleight of hand here, Jim really is a time-traveller.”
Dr Cox went on to reveal how the experiment proved Einstein’s theory and may mean real time travel is possible in the future.
He continued: “Our time is personal to us – this is what Einstein had discovered.
“There is no such thing as absolute time, but we don’t notice this in everyday life because the amount by which time slowed down for Jim was minuscule because of the speed he was travelling at.
“But if we sent Jim off in a rocket into space at 99.5 percent of the speed of light for five years according to his watch and then tell him to turn around and come back.
“For him, the journey would take 10 years, but for us, to our watches, 29 years would have passed.”