We aim to find out. Today, Lost in Science launched the World Time Machine Project, designed to solve the dilemma of why we’ve never encountered visitors from the future, by actively seeking them out.
There are of course other problems with time travel, not the least of which is the grandfather paradox. This is a hypothetical scenario where you go back in time and kill your grandfather before your parents were born, thereby preventing your own birth and making it impossible for you to have gone back in time in the first place.
This horribly specific paradox can be generalised to any situation where you travel back in time and prevent yourself travelling back in time.
(It also reveals another problem with time travel, which is the generation of mind-bendingly convoluted sentences.)
One potential solution to this disastrous collapse of causality is Stephen Hawking’s Chronology Protection Conjecture, which basically says that whatever idea for a time machine you come up with, there will be some other law of physics that stops it working.
But although this is in its own way quite intriguing – suggesting some deep, hidden principle unifying the way physics works – it’s also a bit boring, because it means we can’t have time machines.
A more appealing option (to me, at least) is the grandly titled Novikov Self-consistency Principle, which says that even with a time machine there always has to be a way of running events without generating a paradox, and only these “self-consistent” sequences of events are possible.
Igor Novikov and his colleagues presented this idea in a paper that modelled a time machine made out of wormholes (Friedman J, Morris MS, Novikov ID, Echeverria F, Klinkhammer G, Thorne KS & Yurtsever U 1990, “Cauchy problem in spacetimes with closed timelike curves”, Physical Review D, vol. 42, no. 6, pp. 1915-1930).
Wormholes are tunnels, or short cuts through space and time. You can see this in the diagram above: it’s much quicker to follow the path marked ℓ than to go the long way, around the folded over bits of spacetime.
Through various manipulations of special and/or general relativity, it’s possible to create a big time difference between the two mouths of the wormhole, as seen in normal space (the flat bits in the diagram above). Then by taking the short cut, you can easily travel forwards or backwards between the two times.
A simple type of grandfather paradox is created if you send a billiard ball through the wormhole on such an angle that, when it comes out at the earlier end, it collides with its former self and stops it going in the wormhole in the first place. See diagram (a) in the figure below: α is the original trajectory of the billiard ball, β is its later self emerging from the wormhole and γ is the altered path of the earlier ball after the collision.
But the self-consistency principle tells us that there’ll always be an option (b), where the angle that the second ball strikes the first is not enough to stop it going in the wormhole.
Of course, another self-consistent option would be for the ball to be on a trajectory that completely misses the wormhole in the first place. You can see a good example of that in the diagram below: both possibilities are self-consistent and they produce identical outcomes. One is just a bit more complicated than the other.
Either way, as long as the solution doesn’t have paradoxes, it’s allowed by the laws of physics.
So does this mean it’s not possible to change the past? In one way, yes, as any attempt to create a paradox leads to an impossible series of events that, by definition, can’t happen.
But in another way of looking at it, the billiard ball does affect its past. Could that be considered change?
What would you experience if you tried it? Would you be aware of history changing around you, like Marty McFly? Or would there be a sort of feedback loop, where you bounced back and forth until you discovered a self-consistent way to do things? Or is it more subtle than that?
Consider the grandfather paradox again: maybe, instead of killing your grandfather and wiping yourself out of existence, you travel back in time and remove the reason you wanted to kill him in the first place. That would be self-consistent, but you’d still have to travel back in time to make it happen. So where does that leave free will?
Perhaps the only way to find out is to ask someone who’s done it. Which brings us back to the World Time Machine Project.
Go to the page, read the conditions, respond if you’re eligible. It’s your chance to make history – just make sure it’s a self-consistent one.