James asked:

Is time travel possible?

Answer by Craig Skinner

In brief:

* relativistic time travel into the future is well-established.

* time travel into the past is logically possible but may not be physically possible.

* time travel into the past may be possible only as far as the point when the time machine was invented but no farther back.

Space is so accommodating: three dimensions to travel in, in a direction and speed of our choosing. Time is stingy: only one dimension, one-way travel along it, and at one speed, crawling into the future at one hour per hour.

Could we time travel more freely?

First, we (probably) couldn’t travel to the future or to the past if these don’t exist. For those who think only the present is real (‘presentists’), there is no future or past to go to or from (the no-destination problem). Similarly, for those who think the past and present are real, but not the future (the Growing Block view of time), there is no future to go to: also we can’t expect to get travellers from a nonexistent future, and this is true for all past times, so there is never a departure point for travel into the past. In short (leaving aside some dodgy arguments to the contrary) we need the Static Block View in which all times co-exist, linked by earlier-than and later-than relations (‘eternalism’, the B-series view). This matches the familiar spacetime manifold view of modern science. This view will be assumed in what follows (but I am not denying that, for us, there is such a time as the present).

It’s a well-established feature of Special Relativity that measure of elapsed time depends on the frame of reference. An astronaut accelerating away from Earth to reach almost light speed, returning twenty years older, could arrive back to find Earth a million years older. However, she can’t go back in time to join her long-dead loved ones. This ‘sliding’ rather than ‘jumping’ time travel, due to ‘time dilatation’, has been empirically confirmed using atomic clocks and measurement of lifetimes of short-lived particles moving at high speed.

What about travel to the past?

I’ll deal with logical puzzles, then with the physics.

Some say time travel yields logical contradiction and is thus impossible. Two paradoxes illustrate this.

1. The grandfather paradox.

My grandfather died peacefully in bed aged 85 years. But I travel back to when he was a lad and kill him. So, I can’t exist. But I do exist. Contradiction. I don’t think there is a problem here. The fallacy is thinking that there could be more than one VERSION of the past ie that I could go back and CHANGE the past. But the past can’t be changed. I could go back and AFFECT the past, but if so I was there (as a traveller from the future) when the past events happened all those years ago – the actions of any and all travellers from the future are already built in to the past. So, since my grandfather survived to reproduce, I didn’t kill him. Therefore if I travel to the past intending to kill him, I won’t succeed. Of course if I try repeatedly, there will be a series of flukes and coincidences that beggar belief (the gun jams on my first attempt, next try I fire but miss, next I was given blanks by mistake, fourth attempt I kill the wrong person, and so on). But this is just what we must expect in the unusual circumstances of time travel.

I’ll just mention parallel- and branching- universes. I could travel to the past of a universe just like ours up to the time of my arrival but having a different future in that I kill the lad who would have been my grandfather, so that I never get born in that universe, but am there as a visitor to do the killing (so no paradox). Alternatively, I go back in our own universe, do manage to kill my grandfather, and this causes the universe to branch in two (one where I don’t have a future existence, and the other where I came from). However, I feel we can deal adequately with the puzzles of time travel without invoking branching/parallel universes.

2. The free knowledge problem.

Looking for a sock in a drawer, I find a notebook giving detailed instructions on how to build a time machine. I labour for twenty years, build it, then travel back twenty years to leave the instructions in the drawer.

I am a physicist, fed up with failed attempts to find a theory of quantum gravity. I travel two hundred years into the future, look up the accepted quantum gravity theory (aha!), write it all down, travel back, submit it for publication and it becomes the (Nobel-prize winning) accepted theory.

In these strange-loop scenarios, we can’t say where the knowledge came from in the first place. Strange indeed, but knowledge is knowledge, do we have to know it’s origin. I can imagine a distant future when we know how to make universes with specified sets of laws of nature, one of us then travels back 14 billion years and sets off the Big Bang which began our universe.

As for the physics, the possibilities include a high speed rotating cylinder, and worm-holes. But the cylinder would need to be more massive than all the matter in all the galaxies in the known universe, not to mention the colossal speed of rotation. And the worm holes would be picosecond-lasting and with diameters less than proton-sized unless enlarged and stabilized by unthinkable quantities of exotic matter. So don’t hold your breath. Importantly, most physicists think time travel could go no farther back than the point when the time machine was first built. Which explains why, so far, no visitors from the future have been received. For if, in the future, travel to ANY past time were possible, tourists would have ‘must see’ trips. Christ’s crucifixion, say, might be enduringly popular. In which case contemporary accounts of the event would have recorded the mysterious presence of huge numbers of oddly dressed strangers in the crowd, and no such accounts exist.