Consider the following scenario, (the “grandfather paradox”). I decide that I will try to build a time machine which will allow me to travel back in time in order to kill my grandfather, and thus prevent my birth. In a single universe this creates a serious paradox, but the multiverse appears to offer us a way out of this by creating a new universe in which I was never born. However, we should look more closely at this situation. My decision to try to build a time machine creates a range of paths through different universes, all of which become “real”. The range must include paths in which I do, and do not actually make this decision.
The paths in which I do make the decision must include paths in which I do, and do not succeed in building the time machine.
The paths in which I build the machine must include paths in which the machine is, and is not successful.
The paths in which the machine is successful must include paths in which I do, and do not travel back to the appropriate point in time.
The paths in which I travel back to the appropriate point in time must include paths in which I do, and do not kill my grandfather.
All these paths are created by my initial choice. They must all become real, whatever my first decision happens to be. Even if I decide not to try to build the time machine, this decision will mean that another version of me, in another universe will decide to build the machine; so all the ramifying possibilities mentioned above, and perhaps lots more, will become “real”. So, if time travel to the past is, or ever will be, possible, and if the multiverse theory is right, and if I even think about building a time machine, then all these possibilities become “realities”, whatever decision I make. The point I seem to have reached here is that I have to accept that whatever choice I make in this, or any other matter, makes absolutely no difference in the context of the multiverse, because any choice I make will be countered by the choices made by other versions of me in other universes.
Does it make any difference if we assume that the universes are not created by the making of choices, but already exist? The answer must be “no”, because even if the universes have always been there, the paths through them, taken by the various versions of me, will be governed by the decisions made by each of those versions of me as we (I?) make our ways through our different worlds. Throughout the multiverse, all possible paths will always be taken, or so we are told. Of course, any decision I make will be of importance to the version of me that I recognise in the universe I experience as real, but the multiverse theory says that the other versions of me are just as real, and just as really “me” as the one I experience. It seems that, in the multiverse, I have no monopoly on “me-ness”. It also seems that if I want to travel back in time, all I have to do is decide not to build a time machine and use it to go back and kill my grandfather, and this will ensure that I will do just that in another universe. Who needs to build time machines?
What have I just done?
