Time travel has been a mainstay in science fiction for decades now but it could be actual science in the future as the knowledge and know-how of travelling to the future already exists, while the real challenge is building a time machine.

The concept of time machine is normally depicted as being some bizarre contraption that rips through space. Scientists clarify that a real time machine, should one be built, will just be a powerful rocket and it will involve space travel.

It means moving really fast could actually make time slow down relative to those who are not moving as fast. Consider astronauts on the space station who spend months in space, travelling constantly at speeds over 8 km a second, they are moving at speeds that might not be achievable on Earth.

That means humans up in the space station are experiencing time differently than those on earth and would technically age a bit less. When astronaut Scott Kelly returned after a year in space, the gap between him and his older, identical twin, Mark Kelly, actually widened by a tiny bit, say scientists.

At any given point, everyone is travelling through time, albeit, in one direction and one second at a time. Time travel would mean moving into time, or into the future at a rate higher than that, and it is possible, say experts. "Indeed, we can jump forward into the future as much as we want. It's only a matter of going really, really fast," Paul Sutter, an astrophysicist at Ohio State University, told Space.com via email.

Einstein's theory on special relativity speaks of time being relative to the speed in which an object is moving. "The faster you move through space, the slower you move through time. We've been able to measure this with ultra-precise atomic clocks in jet airplanes, and the precision offered by the GPS system needs to take this into account. Sci-fi always seems to require complicated contraptions to jump in time, when all you need is a very large rocket," Sutter explains.

What would happen if a craft capable of much higher speeds were to be built? The magic number is 299,792,458m per second, or approximately 300,000 km per second, denoted as "c". That is not possible to achieve and nothing can travel faster than light. Having said that, a report by Popular Mechanics speaks of how time travel has actually been achieved, but at sub-atomic levels.

"On a subatomic level, it's been done," explains Ronald Mallett, of the University of Connecticut. The Large Hadron Collider (LHC)- a particle accelerator, routinely sends particles to the future by taking them up to near light speed. Protons reach 99.999999 percent the speed of light inside the LHC, a speed at which its relative time when compared to humans making the observations is moving about 6,900 times slower.

So, how long would it take for someone to travel thousand years into the future? J. Richard Gott, Princeton physicist and author of the book "Time Travel in Einstein's Universe" says, "If you want to visit Earth in the year 3000, all you have to do is to get on a spaceship and go 99.995 percent the speed of light."

In this case, let us assume there is a human crew that is able to reach 99.995 percent the speed of light and they are sent on a trip into the cosmos to a nearby exoplanet that is about 500 light years away. At nearly the speed of light, it would take the crew about 500 years to get there and another 500 years to get back, so they have travelled 1,000 years in all.

To be precise, they would be returning to Earth in the year 3018. But the crew moving so fast would not feel the same as it did for the rest of those on Earth. In fact, the time-travellers' internal clocks would have completely slowed down.

Their clock will be ticking at 1/100th of the rate of the clocks on Earth and they are only going to age about 10 years, explained Gott. A whole millennium would pass by for humans on Earth, but for the time-travelling crew, it would just be one decade.

"If we [on Earth] were watching through the window, they would be eating breakfast veeeerrry slooooowly," says Gott, "But to [them], everything would be normal."

The challenge, however, is not the science of it, but the engineering feat of building a ship that can reach such speeds. The Parker Solar Probe is reportedly the fastest ever spaceship built by humans ever but its top speed is a paltry .00067 percent the speed of light.

Humans need to accelerate to reach near-light speed, but light itself does not accelerate, it just remains at that speed.

This theoretical space craft that will travel to the year 3018 would actually take a lot longer than 10 years to reach that far into the future, notes the report. Gott calculates that steady acceleration up to near light speed would actually increase ageing of the crew to 24 years, "but you would still get to visit Earth in the year 3000," says Gott.

Time travel
The science on time travel is strong, but the technology to achieve it is not here yet- Representational ImageCreative Commons