Travelling forward in time is no big deal. We all do it all the time at a speed of one second per second: sometimes awake, sometimes asleep! Every second of every minute of every hour we go from present to future.
Travelling backward in time, however, is thought to be impossible. There's the so-called "Grandfather Paradox": If you could travel backward in time, you could murder your grandfather, though why you would want to is never explained.But if you did murder your grandfather before the birth of your father then you would never be born, and therefore you could never go back in time and murder your grandfather. The paradox seems fierce enough, but there is a way around it. First though let's take a closer look at what travelling forward in time entails.
In science fiction people travel for thousands of years to distant galaxies by virtue of deliberately induced coma, deep hibernation, cryogenic freezing or similar means. Shut down consciousness for the journey, and it doesn't matter how long it takes to get there.
Whether it's a hundred or a thousand years, if you're in a coma your mind is not gathering new data, but the rest of the Universe is. When you come out of the coma, when your consciousness is restored, the world has changed but you have not. "You" have moved in time from A to B without "you" changing, with the essential "you" intact, (if you define "you" to be your mind/consciousness -- a dubious proposition in my view).
In contrast, Going back in time is believed by some* to be impossible: that to go back in time would prevent going back in time from happening, in the first and last place, per the Grandfather Paradox.
(DIGRESSION ALERT: BTW I don't understand why it has to be the Grandfather Paradox. Why not simply the Father Paradox? Killing your father before your birth prevents you from being born just as effectively as killing your grandfather before your father's birth. End Digression.)
Nor does there have to be any killing for the paradox to exist: just to be where/when you don't belong introduces a change to the present and therefore to the future. So even if you could go back in time and very kindly refrain from killing your grandfather, just by being when/where you shouldn't be means you couldn't possibly be when/where you are, were, will be...
But in my view there is a resolution to the Grandfarther Paradox, if the Everretian “many worlds”/Multiverse interpretation of quantum physics holds true.
In the “many worlds” interpretation, Reality is continuously splitting into different “slices”, which in turn are themselves continuously splitting into new reality pathways or worldlines, in an infinitely iterative process of quasi-random, self-similar fractalisation (nah, just kidding -- made that last bit up!).
In one slice, I smoke a cigarette while writing this post. In another slice I do not. In another slice, I light a cigarette but stub it out immediately. In another slice, the cigarette smokes this post while typing me.
The point is that in an Everettian Multiverse, there could be a slice of reality – a worldline -- in which your grandfather exists but neither your nor your father exist, another slice in which all three of you exist, another slice in which none of you exist, etc. The existence of mutually contradictory worldlines is permissible only as long as there is no connection of any kind, causal or otherwise, between the various slices. They are all quarantined from each other for all time.
If you were to travel back in time and murder your grandfather before the birth of your father, you would create a new slice of reality: one in which neither you nor your father exist. This would be in addition to the slice of reality in which both you and your father exist. But there can never be any connection between the two slices: they are mutually exclusive. Still, the fact that there can’t be and is not a connection between two contradictory things doesn't mean that they can’t or don't both exist. They can and do, just as long as they have nothing to do with each other.
In contrast to the “many worlds interpretation” is the standard model, the "Copenhagen Interpretation".
- The Copenhagen Interpretation holds that there are many possible/potential worlds and worldlines, but only ever ONE is actually actualised (via the collapse of the wave function (a mathematical expression used to calculate the probability that a particle will be in a given location or state of motion at a given moment in time (which doesn’t exist))).
- According to the Many Worlds Interpretation, there are many possible/potential worlds and worldlines, and they are ALL actualised: they all exist.
* There's nothing in principle preventing the Laws of Physics running backwards in time: all known physical processes and events are reversible in theory and some believe have been reversed in practice at another phase / in other phases of the Universe's lifecycle. For example, there is nothing in the Laws of Physics preventing or outlawing a process whereby a shattered mirror falls up from the floor and reconstitutes itself whole upon the bathroom wall. The so-called "arrow of time" is a great mystery. Does time have an arrow, or is that an illusion? If it is, it's a real illusion: the illusion really exists.
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