There have been a number of stories in the past week about supposed time travellers, here to warn us of what lies in the future.
The Mirror reported on a 'British government worker' who claims to have visited the year 8973, where he found a utopian society populated by robot-human hybrids. So far, so good.
But another report told of a man from 2030 who warned us Donald Trump would not only win re-election in 2020, but change his name to Ilana Remikee.
It wouldn't be going out on a limb to say both men are talking complete nonsense, but time travel itself might not be. Technically, we're all travelling into the future right now.
But can we do it like they do in Star Trek? How would it work, and what would be the consequences?
How to do it
Travelling really, really fast
The closer you get to the speed of light, the slower through time you travel - at least relative to other things that aren't travelling very fast.
Cosmonaut Sergei Krikalev spent more than two years aboard space station Mir travelling at 27,360km/h. In that time, he aged 20 milliseconds less than anyone stuck on Earth.
Some have suggested by travelling at extreme speeds - many thousands of times faster than Mr Krikalev - we could effectively travel into the future. If you travelled to our neighbouring galaxy Andromeda at the speed of light, thanks to time dilation you'd get there instantly, from your perspective - even if 2.5 million years had passed on Earth. You'd effectively be travelling into the future.
This is the premise behind the movie Planet of the Apes - Charlton Heston's character spent 2000 years travelling near the speed of light, but to him, only 18 months had passed when he crashed in the year 3978.
In the 1979 Superman movie, the titular character is able to go back in time to rescue Lois Lane by flying faster than the speed of light. The problem with this is our current understanding of physics says anything that reaches the speed of light would have infinite mass and no length, which even the best physicists admit sounds impossible.
Gravity
Gravity bends what we call spacetime. This can be seen in GPS satellites, which have to account for the difference in gravity between where they are - 20,200km in the sky - and the surface of the Earth.
Gravity is much weaker up there, so time goes faster for GPS satellites than it does for us.
Shows like Star Trek have used a combination of the gravitational effects of black holes and stars to jump characters back and forward through the centuries.
Wormholes
Just like physical wormholes that could link one region of space to another (Star Trek: Deep Space Nine, Babylon 5, Interstellar, Doctor Who), it's proposed similar shortcuts through time could exist.
Physicists say there's nothing in Albert Einstein's equations which prevent them from existing, but doubts they could feasibly be made, at least in the near future.
What might happen if you go back in time
So, you've travelled back in time - but your time machine landed on your grandparents, instantly killing them and wiping yourself out of existence. D'oh! So, um, what happens next?
There are a few theories on how travelling back in time might affect the past, present and future.
Many-worlds interpretation
According to this theory, every time someone travels back in time a completely new timeline is created.
This means you haven't wiped out the timeline in which you jumped into a time machine, rendering the fact you just killed your own grandfather moot. He no longer exists in this timeline, and you won't be born, but it doesn't matter - this is now a completely new version of the world. You still go on to be born in the old timeline, just not this one.
The 2009 Star Trek film - spoiler alert - employs this version of time travel. Spock is sent back to the year 2258 where he sees his home planet of Vulcan destroyed, wiping out his entire race - which still exists in Star Trek TV series set more than 100 years later. In the later Star Trek movies however Vulcan remains destroyed, because they're set in the timeline created in the 2009 film - separate to the TV shows.
Single, changeble timeline
Many movies and Star Trek time travel plots use a single timeline however, in which actions in the past immediately change the future.
For example - spoiler alert - in Back to the Future, Marty McFly can literally see himself fade from existence in a 1980s photograph as he messes up his parents' blossoming teenage romance in the 1950s. Biff's acquisition of the sports almanac in 1955 instantly turns his 1980s equivalent into a proto-Donald Trump casino magnate.
This particular variant on time travel leads to numerous logical fallacies and plotholes, however - for example, if Marty stops his parents from getting together, he isn't born and can't travel back in time in the first place.
Whatever happened, happened
A third possibility is that going backwards is possible, but time travellers can't change anything because it's already happened - ie, they came from the timeline their past actions already affected.
- Michael Fassbender to fight Hitler in time travel movie
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It's a difficult premise to write into fiction, particularly television, as it requires early events in a series to line up with later episodes.
Lost largely pulled it off. In one memorable scene - spoiler alert - after going back in time 27 years, Sayid Jarrah tries to kill a 12-year-old Benjamin Linus, only to set off a chain of events which ends in Linus becoming the very man Sayid wants to kill. In another, characters from the future attempt to prevent a cataclysmic disaster known as 'the incident', but instead cause it to happen.
At no point in the series is the present day changed by any decisions made in the past by time-travelling characters because, as the show's physicist Daniel Faraday put it: whatever happened, happened.
The 'whatever happened, happened' format is easier to master in self-contained feature films, notable examples being Primer, Ethan Hawke sci-fi oddity Predestination and Spanish thriller Timecrimes.
Limited travel
Some physicists believe backward time travel could be possible, but only as far back as the exact moment the machine that makes it possible was built.
This would explain why as of yet, we haven't met any actual time travellers - we just haven't invented it yet. This also conveniently means we won't ever have to worry about accidentally killing our grandparents.
Our grandchildren, on the other hand…
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