Albert Einstein’s theory of general relativity suggests that time travel to the past is possible via rotating wormholes and/or black holes 寵物移民加拿大. 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 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.