Suppose that time travel were possible. Then you could go back in time and kill your own grandfather. But that’s impossible, because then you would never be born, hence you would never able to go back in time to kill your grandfather in the first place. This is the Grandfather paradox.

The Multiverse thesis says that when you travel back in time, you arrive at a different universe. The Multiverse thesis provides a solution to the Grandfather paradox. If you travel back in time and kill your grandfather, you will succeed in killing your grandfather in the new universe, but your grandfather in your old universe will live to father your father, who will in turn father you. No paradox.

The Multiverse thesis gives rise to an interesting question. What happens if two people choose to travel to different times in the past? Suppose you choose to travel to Ancient Rome, whereas I choose to travel to 1780’s America. According to the Multiverse thesis, we will both arrive at a different universe from our current universe. But do we arrive at the same universe, or do we arrive at separate universes? Well, here’s a reason to think each of us gets their own universe.

Suppose that we both arrive at the same universe. And, suppose that you give the Ancient Romans some modern technologies, which in turn leads to further technological advances. Then the 1780’s come, and I arrive expecting to see people riding horses and buggies, but to my surprise, I see people riding cars and flying airplanes.

In this scenario, did I really travel back in time? I’m inclined to say no. I traveled to an alternate universe alright, but that’s not time travel. So, if it’s possible for more than one thing to travel back in time, each time traveler must get their own universe. More generally:

Different times, different universes: if A and B travel to different times in the past t1 and t2, then A and B arrive at different universes.

This looks like a sensible thing to say. As Nikk Effingham (2012) has argued, however, this thesis entails that we cannot survive time travel—at least not given our current scientific understanding of time travel.

Our current scientific understanding is that the only way to travel back in time is to go through a special region of spacetime—a “gateway”, so to speak—that connects a region in the future with a region in the past. A feature of these gateways is that they send you back by a set period of time. If an object goes through at 10:05 pm and arrives at 10:00 pm, for instance, a second object going through at 10:10 pm should arrive at 10:05 pm.

To see why you cannot survive going through a gateway, imagine a string of particles a, b, c… going through the gateway at a constant rate. Suppose that the string moves slowly, so that each particle goes through a full minute after the previous one. Since the gateway sends back each particle by a set period, each particle is heading to a different time in the past. If a is heading to 10:00pm, for instance, then b is heading to 10:01 pm, c is heading to 10:02 pm, and so on. Suppose the process continues until the string is gone.

Now, what happened to the string? Where is it gone? Well, since a and b were headed to different times in the past, Different times, different universes entails that a and b are now in different universes. And, since c was headed to a different time still, c is also in a different universe. In short, each particle went to its own universe. The string is now spread throughout the Multiverse.

In this example, the string was supposed to move really slowly, but the speed doesn’t matter. No matter how fast the string moves, there is always going to be some interval between the moments each particle goes through. Thus, the particles are always going to be headed to different times in the past, therefore they will always end in different universes.

Now, if you are a string, you do not want to go through a gateway. The same is true of ordinary human beings. That’s because we are also composed of tiny particles. If I push you through a gateway, your particles will be entering the gateway at different times. Consequently, they will be headed to different times in the past. And, according to Different times, different universes, that means your particles will end up in different universes. And, you may be a tough guy or gal, but I very much doubt you can survive that.

Moral: if you push someone through a gateway, don’t expect the person (the whole person) to come out the other end.

To sum up:

If the Multiverse thesis is true, then Different times, different universes is true. If Different times, different universes is true, we cannot survive time travel. Therefore, if the Multiverse thesis is true, we cannot survive time travel.