Memory theories of personal identity and faster-than-light dependence

Consider this sequence of events:

• Tuesday: Alice’s memory is scanned and saved to a hard drive.
• Wednesday: Alice’s head is completely crushed in a car crash.
• Thursday: Alice’s scanned memories are put into a fresh brain.

It seems that on a memory theory of personal identity, we would say that fresh brain on Thursday is Alice.

But now suppose that on Thursday, Alice’s scanned memories are put into two fresh brains.

If one of the operations is in the absolute past—the backwards light-cone—of the other, it is easy to say that what happens is that Alice goes to the brain that gets the memories first.

Fine. But what if which brain got the memories first depends on the reference frame, i.e., the two operations are space-like separated? It’s plausible that this is a case of symmetric fission, and in symmetric fission Alice doesn’t survive.

But now here is an odd thing. Suppose the two operations are simultaneous in some frame, but one happens on earth and the other on a spaceship by alpha-Centauri. Then whether Alice comes into existence in a lab on earth depends on what happens in a spaceship that’s four light-years away, and it depends on it in a faster-than-light way. That seems problematic.

Killing coiled and straight snakes

Suppose a woman crushes the head of a very long serpent. If the snake all dies instantly when its head is crushed, then in some reference frame the tail of the snake dies before the woman crushes the head, which seems wrong. So it seems we should not say the snake dies instantly.

I am not talking about the fact that the tail can still wiggle a significant amount of time after the head is crushed, or so I assume. That’s not life. What makes a snake be alive is having a snake substantial form. Death is the departure of the form. If the tail of the headless snake wiggles, that’s just a chunk of matter wiggling without a snake form.

What’s going on? Presumably it’s that metaphysical death—the separation of form from body—propagates from the crushed head to the rest of the snake, and it propagates at most at the speed of light. After all, the separation is a genuine causal process, and we are supposed to think that genuine causal processes happen at the speed of light or less.

So we get a constraint: a part of the snake cannot be dead before light emitted from the head-crushing event could reach the part. But it is also plausible that as soon as the light can reach the part, the part is dead. For a headless snake is dead, and as soon as the light from the head-crushing event can reach a part, the head-crushing event is in the absolute past of the part, and so the part is a part of a headless snake in every reference frame. Thus the part is dead.

So death propagates to the snake exactly at the speed of light from the head-crushing, it seems. Moreover, it does this not along the snake but in the shortest distance—that’s what the argument of the previous paragraph suggests. That means that a snake that’s tightly coiled into a ball dies faster than one that is stretched out when the head is crushed. Moreover, if you have a snake that is rolled into the shape of the letter C, and the head is crushed, the tail dies before the middle of the snake dies. That’s counterintuitive, but we shouldn’t expect reality to always be intuitive.