Cremation is bad for you

I recently heard of somebody who thought that cremation was okay for you. This person claimed that the laws of physics say that it is possible to undo any change. There is a rather limited sense in which that is kinda true, but it’s irrelevant.

The laws of motion of physical systems usually give later states of a system in terms of earlier states. But for many of those laws, the same trick also works in reverse: you could work out earlier states from later ones. So then all you have to do to get the earlier state is reverse the motion of the system. Heck, you don’t even have to do that. If you know the later state you can just simulate the reversal. And having done that, you could set up a new copy of the earlier version.

So if you get cremated, you can just ask somebody at some future time to reverse the evolution of your ashes and you’ll be fine, right?

Not so much. If you get burned then lots of information about you leaks into the environment. Some of it goes into vibrations in the air that other people hear. Some of it goes into vibrations in the body of the oven. Some of it goes into light that may change the state of atoms in the oven or be seen by people outside the oven if it has a transparent front. And there may be other stuff like that. That leaked information would influence the evolution of all those systems, and everything they interact with, e.g. – the people at your funeral, crematorium workers etc. all those people would interact with others and pass on that information. This information would indirectly spread to the future people who will supposedly create the knowledge required to uncremate you. In order to uncremate you, all those interactions would have to be undone. That would include undoing the creation of the knowledge required to uncremate you.

But you might think you can just measure the current state of the world and simulate the reversal. That won’t work either. The world is governed by quantum mechanics. In order to get the full quantum state of a system, you need to have multiple copies of it prepared identically (1). But there aren’t multiple copies of the whole of human civilisation, so that’s impossible.

If you want to avoid death, you have to think about what is required to make your scheme work to see if it is possible. Anything else is just wishful thinking.

(1) You may have heard that quantum mechanics implies the existence of multiple versions of every object around you, with different versions in other universes so that you can’t interact with them. This idea is true, see “The Fabric of Reality” and “The Beginning of Infinity” by David Deutsch. But here I am referring to multiple copies in the same universe.