Albert Einstein’s theory of general relativity suggests that time travel to the past is possible via rotating wormholes and/or black holes. The actual technical practicality of actually carrying out such journeys need not concern us since this essay is in the realm of the thought experiment. Now Stephen Hawking says time travel to the past is not possible because he proposes that there is such a thing as a yet undiscovered Chronology Protection Conjecture that prevents this and thus makes the world safe for historians. I’ve come up with a unified theory of time travel into the past that incorporates Einstein’s general theory of relativity; Hawking’s Chronology Protection Conjecture, along with other assorted bits like parallel universes that are thrown into the mix.
Time travel is a staple in sci-fi stories, novels, films and TV series. And, time travel is possible – in theory. We all know about journeying to the future which we do at the rate Black Travel of one second per second whether we like it or not. Apart from that, if one travels at close to light speeds relative to your place of origin then you can travel to the distant future (with respect to that place of origin) without aging an equivalent number of years (the twin paradox). Travel to the past is apparently allowed too, via the weird physics inherent in rotating worm holes and maybe Black Holes which is where Einstein’s general theory of relativity comes into play. The problem there is that relativity theory predicts worm holes, if they exist at all, will exist for nanoseconds and be very tiny to boot, and thus not very useful in the foreseeable future for the purposes of time travel. Because we don’t know exactly what the inside of a Black Hole is, and where it leads, if anywhere, current thinking suggests that jumping into Black Holes are a more useful means for committing suicide than for traveling to the past, but the jury is still out on that one.
Anyway, the fun bit about time travel is the various paradoxes that arise, the most famous one being the grandfather paradox. That is, what if you travel back in time and kill your grandfather before he sired your father (or mother). If you did that it means that you could never have been born, but if you were never born you couldn’t go back in time to kill your ancestor. This is the sort of stuff sci-fi authors (and philosophers) love – ditto physicists! My favorite time travel paradox however is the one where you get something for nothing. Say you have this edition of “Hamlet”, and you want Shakespeare to autograph it. So back you go in time to Shakespeare’s era. You knock on his door, but the housekeeper says he’s out for the day but if you leave the book he’ll autograph it and you can come by and collect it next morning. When Shakespeare comes home, he sees the book, reads it, and is so impressed he spends the night making a copy. You come back the next morning, collect your now autographed edition of “Hamlet”, and return to the present day with your now very valuable book. The question now becomes, where did the original “Hamlet” come from? You didn’t write it; but Shakespeare didn’t either as he plagiarized your copy which he then passed it off as his own work.
Another favorite is you meeting yourself. Say you’re 50 and not all that well off. You get the brilliant idea to travel back in time and convince your younger self to invest in some stocks you know will pay off big time later on down the track. And so it comes to pass that your younger self so invests, and becomes filthy rich, only, in leading such a high life, dies of a heart attack at the age of 45! Or you always regretted not proposing to the love of your life when you were young, and thus go back and convince your younger self to muster up the courage and do so. He does, but as they fly off on their honeymoon, the plane crashes with no survivors. Sometimes you don’t know when you’re well off.
Or if you can travel back in time, then of course others can to. Naturally there’s going to be lots of people interested in particular events, maybe even at the time, seemingly trivial events (yet which turn out in the long run to have had major impact(s)). And so you might have any number of people going back to particular historical focal points, each with their own particular agenda (most of which will be mutually exclusive), and ultimately causing havoc. I mean if person one goes back and influences an event producing a new outcome, then person two might go back and has a go at that result and things get altered again, which will then prompt person three to go back and influence things more to his liking, etc. In other words, history would never be fixed, rather always be fluid. The world is not safe for historians. Since we believe that history (or the past) is fixed, then that what’s written on your history book page today will not alter overnight. Thus, you have probably concluded that time travel cannot happen, will not happen, and has not happened, however much you yourself might wish to go back in time yourself and change something. (Don’t we all really wish some past something, personal and trivial, or perhaps something of major significance could be changed and you’d be that instrument of change?)
Its paradoxes and situations such as the above that prompted Stephen Hawking to postulate that there is as yet an undiscovered law or principle of physics which prohibits time travel to the past – he calls it his ‘Chronology Projection Conjecture’. Since we have never seen, according to Hawking, to the best of our knowledge at least, any time travelers – tourists or historians – from our future, he’s probably right.
So, putting it all together, here’s my theory of time travel: my unified theory of time travel, at least to the past.
Relativity theory has passed every experimental test thrown at it, so the theory isn’t in much doubt and one can have a high degree of confidence in what it predicts, even if that prediction is currently beyond any experimental test. Relativity theory allows for time travel into the past, but, IMHO, only to parallel universes (otherwise known as alternative or mirror or shadow universes) where no paradoxes can happen.
Why only parallel universes? The ways and means by which you can use relativity theory to time travel backwards involves rotating Black Holes or wormholes. There are serious reasons behind the speculation that what’s on the other side of a Black Hole and/or wormhole is another universe. So, therefore it’s relativity’s time travel allowance, but probably to another universe. The Black Hole or wormhole ‘exit’ isn’t within our Universe.
Whatever you do in that parallel universe is predetermined. It’s fate. It’s destiny – all because causality rules. Therefore, there are no unexpected ripple effects other than what was destined to happen. You were meant to be there and do what you do. Therefore, there will be no paradoxes arising.
Astrophysicist Stephen Hawking has proposed his Chronology Protection Conjecture that prohibits time travel to the past within your own universe because of the possible paradoxes that could arise. Why can’t you go back in time in your own universe? That would mean that at a specific time and place you both were not (originally) and were (as a result of going back) present. That’s a paradox. And if you were to travel back in time to a set of time and space coordinates you were actually originally at, then there would be two copies of you occupying the same space at the same time – also a paradox.