If a spaceship left earth traveling at near or at light speeds (that being possible in this situation) would they not when coming back to earth, arrive many years in the future due to Einstein's theory of time travel and how could this be prevented while still maintaining these speeds. It is okay to draw off of science fiction novels or movies that offer an explanation on why they do not end up in the future.
Special relativity is the scientific explanation for the reasons why spaceship moving at or near-light-speed experience less time. The closer to light-speed the less time passes. To return to Earth after hundreds of years the spaceship has travel to somewhere that takes, at least, one hundred years to reach and make the return trip back again to Earth.
However, there are a reasonable number of destinations much closer to home. So traveling there won't mean our astronauts return home centuries later. For example, a trip to Sirius at 90% of light-speed will take ten years travel there and another ten years to return. The time dilation will be approximately two, so the astronauts will only be traveling for ten years for whole round trip. Plus however much time they spent exploring the Sirius system, so let's say five years. This means the astronauts will be away for twenty-five years, but they will only experience fifteen years.
An example from science-fiction, in Larry Niven's short story "Singularities Make Me Nervous" the astronaut travels at relativistic speed to a massive black hole where he takes advantage of the hypothetical property of black holes to shift things like spaceships back in time. He then makes the return journey and arrives back on Earth before he departed. If your astronauts discover a mechanism to shift them back in time they can, at least, return to Earth without centuries having gone by.
At the time Niven's story was written It was believed black holes could be used as time machines. The idea has been subsequently debunked scientifically, but the general principle still applies. Find a wormhole or a suitable space-time discontinuity that can act as a time machine and your astronauts will be home without too much passing.
Of course, if the future in which this relativistic travel takes place, has invented its own form of time travel, then they can manipulate time to avoid returning home centuries too late. Larry Niven's novel Rainbow Mars (1999) has a discussion about this possibility.
Not having access to the example for how this would work in Niven's book, here is my worked example of the process. A spaceship to Sirius at 90% light-speed gets there in ten years. They explore it for five years. before leaving Sirius, they activate a time machine which carries spaceship and astronauts slightly less than twenty-five years into the past. They fly home and arrive shortly after their original departure for Sirius.
If a spaceship traveled at exactly light-speed, absolutely zero time would pass on the spaceship. This means your astronauts can travel as far as they like, and no time will pass for them. This does have one drawback. If no time passes, how do know when to turn off your space-drive and stop traveling at light-speed. Fortunately, this should be even more impossible to do than traveling faster than light-speed.
If light-speed travel was possible, it would be extremely easy to travel and return not just centuries in the future, but millennia, and even millions of years.
There is a limit to far astronauts can travel and not centuries later, and that is travel no further than slightly less than fifty light years at relativistic speeds. At worst this means returning to Earth about one hundred years later, but the good thing is this won't be hundreds of years later.
ADDED TO THE ABOVE ON THE SAME DAY:
There is another less obvious but quite quirky way of avoiding astronauts returning centuries after their departure. This has been used in two science fiction novels. Greg Egan's Schild's Ladder (2002) where it is only mentioned in passing as part of the background to the story, and in Karl Schroeder's Lockstep (2014) where it is part of the plot driving the story. With Schroeder the concept is dealt thoroughly and is the focus of the story.
Basically people on a planet like Earth slow down their own time to match the passage of time experienced by astronauts travelling to somewhere in deep space. In Egan, the people are software entities, so they simply slow down the clock speed of the computers where their existence is implemented. In Schroeder, this is done using a combination of biosuspension (or, suspended animation) and a very regulated lifestyle. This combination is Lockstep.
With Schroeder's Lockstep if astronauts are travelling at an average relativistic velocity of 99% of lightspeed to a destination 693 light years distant. The trip time will be 700 years in the rest frame, but the astronauts will experience 70 years. The time dilation will be ten. On Earth people will enter biosuspension for nine days. On the tenth day they will all emerge in synchronization and do what they would normally do for one day. This would continue for the entire round trip of the spaceship.
Effectively one hundred and forty years go by for the astronauts and the people on Earth during the time of the astronauts' voyage. Yes, this is the future, so we can assume they have achieved extreme longevity. This means that interstellar expeditions lasting two centuries will be the equivalent of several years for us more short lived humans.
This can be scaled back for more realistic interstellar voyages to nearby stars in the vicinity of the solar system. However, if the folks back were prepared slow themselves down so that no more passed for people on Earth than for astronauts travelling across interstellar space, then even if strictly speaking many centuries had passed for both the Earth and the astronauts they will experience the same amount of time. There won't be a time discrepancy of centuries for the returning astronauts.
Note: Karl Schroeder devised his fictional Lockstep system as a way of getting around the problems of travelling astronomical distances without faster-than-light travel
It depends on how far they travel.
If they travel hundreds of light years while traveling near the speed of light then yes, it will be hundreds of years in the future. If they travel 1 light year near the speed of light, then it will be one year in the future. The way the time dilation works is that the travelers on the ship will experience less time than has actually passed.
Say that the ship is traveling at 99.99% the speed of light. They travel to a star 70 light years away. While it will still take the ship 70 years to get there, at that particular speed the passengers on board the ship will have only experienced around a single year.
So, in a way you could say its a way to travel into the future, but once you look at it it's not really.
The way I see it, there are two ways to prevent it:
Travel slow enough that the time dilation stops being so much of an issue (it will still be one, it's up to you how much is tolerable).
Violate causality so that people traveling don't actually travel, but instead, merely arrive at their destination (some kind of warp drive or wormhole).
If you still want them to actually travel, and have them travel at relativistic speeds, then I don't believe it is possible according to our current understanding of science.
Depending on the physics of your setting, superluminal speeds might be a workaround to this problem, but that's outside the scope of science-based.
Other answers have nicely summed up how travelleing faster slows time for the traveller (but leaves it as it was for everyone else). Thus, hurtling around the galaxy at light speed might be fun, but by the time you get home your friends and relatives will all be long since gone and so you can't tell them about it.
Our current understanding of physics doesn't have a solution to this problem (or is it a feature?) of physical travel. Science fiction has had a few ideas:
- Warp drive
- 'folding space'
- Send a robot probe, and then just use VR to pretend you went there
- Make the rest of the universe's time go slower while you're flying so your time moves at the same speed as theirs (or make yours go faster, I guess)
My personal favourite is that you make a relativistic tube in space, inside which Eistein's law doesn't apply (is that 'hyperspace'?). You then fly down that tube at light speed. However, to go even faster, you can make a second tube around the first. You fly down the inside of the first tube and it flies down the inside of the second tube. Thus, you're actually travelling 2x light speed, yet feeling none of the effects of it. I'm still figuring out how to construct a working demonstration of this though ;-)
If they are making use of an Alcubierre Drive to travel near light speed, they can remain in Earth's inertial reference frame for their entire trip. It doesn't cause them to move through space per se. It merely warps space around them much like dark energy is causing distant galaxies to recede away from us. This way they could remain in the present all the time.
But Alcubierre drives could also theoretically be used to go faster than light. And if they were used for that, our space travelers could set out for Sirius, spend 5 years there, and then return to Earth just a few minutes after they left. Or they could return before they left. But something tells me the space government would make returning before you leave highly illegal. Not because they're worried about the fate of the universe, just because thinking about it makes their heads hurt.
You can counteract Velocity Time Dilation with Gravitational Time Dilation. A large gravitating mass could produce enough time dilation to counteract the effects of traveling close to the speed of light. At close to the speed of light, the gravitating mass needed would need to be equal to the mass of a large planet or star crammed into a small compartment on the ship. The ship's passengers could be spared the deadly effects of extreme gravity by accelerating the ship at an incredible rate in the direction of the mass.
The problems are: The faster the ship's velocity, the more gravitating mass is required to counteract Velocity Time Dilation. The more gravitating mass is used, the more acceleration is required to counteract the crushing gravity.