FAMED astrophysicist Stephen Hawking believes humans are capable of time travel.

Claiming he is not as concerned about being labelled crazy as he once was, Hawking has publicly aired his second startling theory in two weeks, after last week claiming it was "entirely reasonable" to assume aliens existed.

Preparing for the debut of his Discovery documentary, Stephen Hawking's Universe, which screens next week, Hawking said he believed humans could travel millions of years into the future and repopulate their devastated planet.

Hawking said once spaceships were built that could fly faster than the speed of light, a day on board would be equivalent to a year on Earth.

That's because - according to Einstein - as objects accelerate through space, time slows down around them.

Which also means that Hawking's theory only applies to moving forwards through time.

Moving backwards is impossible, Hawking says, because it "violates a fundamental rule that cause comes before effect".

If moving backwards through time was possible, a person could shoot their former selves.

“I believe things cannot make themselves impossible,” Hawking said.

However, once spaceships approached the speed of light, their crew would start skipping through Earth years on a daily basis, giving the human race a chance to start again.

“It would take six years at full power just to reach these speeds," Hawking said.

"After the first two years, it would reach half light speed and be far outside the solar system.

"After another two years, it would be traveling at 90 per cent of the speed of light.

“After another two years of full thrust, the ship would reach full speed, 98 per cent of the speed of light, and each day on the ship would be a year on Earth.

"At such speeds, a trip to the edge of the galaxy would take just 80 years for those on board.”

Manchester University professor Brian Cox told The Times that Hawking's theory had already found some basis in experiments carried out by the Large Hadron Collider in Geneva.

"When we accelerate tiny particles to 99.99 per cent of the sped of light in the Large Hadron Collider at Cern in Geneva, the time they experience passes at one-seventhousandth of the rate it does for us," Prof Cox said.

Hawking admits he is obsessed with time travel - he told the Daily Mail if he could go backwards he'd visit Marilyn Monroe in her prime or drop in on Galileo - but said as he got older, he cared less about what people thought of his theories.

“Time travel was once considered scientific heresy, and I used to avoid talking about it for fear of being labelled a crank," he said in Stephen Hawking's Universe.

"These days I’m not so cautious.”