[jules]

Note that the multiverse that this article is talking about is very different than the many worlds multiverse. The many worlds multiverse is just that the whole universe is in a quantum state. The multiverse that this article is talking about is different. Due to the expansion of space we can only see some finite region around us. If you go far enough away, then the expansion of space between that point and the earth is faster than the speed of light, so we can never reach that point, and nothing from that point can reach us. Effectively we are in a different universe than that point. The multiverse theory is that some things that appear constant in our part of the universe, such as the fine structure constant, may well be not exactly constant. Then it could be the case that the fine structure constant in that far away part of space that we can never reach is different than our fine structure constant. Then because the universe continues to expand, regions that were connected become disconnected. So one region with fine structure constant x may split into two regions one with fine structure constant x + 0.0002 and another with fine structure constant x - 0.0001. That process creates an infinite tree of regions. If a region happens to have physical "constants" that create deflation rather than inflation, that branch of the tree dies. By the anthropic principle we live in a region with properties that produce humans.

By the way, I don't get the obsession with quantum mechanics interpretations. Copenhagen and many worlds give exactly the same predictions, so if one is correct then so is the other. Who cares how you choose to interpret it. By analogy to probability theory, Copenhagen is like conditioning on an observation, many worlds is like looking at the whole probability distribution. These are just two ways of looking at the same thing. It's as silly as arguing whether the Hamiltonian or Lagrangian formulation of classical mechanics is correct. Both are.

[loup-vaillant]

> By the way, I don't get the obsession with quantum mechanics interpretations. Copenhagen and many worlds give exactly the same predictions, so if one is correct then so is the other.

Not quite. While they predict the same observations, they certainly don't predict the same universe. Under the Copenhagen interpretation, there is only one cat, who is either dead or alive. The possibility you don't see doesn't even exist, the collapse has seen to that.

We can make an analogy with the expansion of space being faster than light. Let's say you send a life ship far away into deep space to do some colonisation. Let that ship travel beyond our observable bubble (it's a very high tech ship).

So, once your ship is so out of reach that it can't even send any signal back (not even in theory), does it still exist? If you take the current laws of physics at face value, it's still out there. The colonists are on their own, but they should be fine. On the other hand, if there is some kind of "collapse" where anything that goes beyond our observable bubble just disappears, then you have sent the colonists to their death. Oops.

For the record, I must say I am very uncomfortable about having the fundamental constants of the universe change as we go beyond our observable bubble. That sounds like an additional assumption, and I don't like it at all. I'd sooner believe in a Tegmark level IV multiverse.

[jules]

It's not entirely clear what happens when a spaceship goes outside our observable bubble due to issues with conservation of information (similar to how it's not entirely clear what happens when you drop a spaceship in a black hole). Let's leave that aside and assume that the spaceship and all its information is truly lost when it leaves our bubble. Then the question of whether that spaceship still exists is not a sensible question in physics, because there is no experiment that can confirm or deny it. It's a metaphysical/philosophical question. My point is that BOTH the claim that it disappears AND the claim that it does not disappear aren't sensible. Many worlds says that if you observe the alive cat, there still exists some other version of you that observes the dead cat. That is a metaphysical claim. Copenhagen says that there is no other version of you that observes the dead cat. This is also a metaphysical claim. I say that both claims are silly if you truly believe in the predictions of quantum mechanics. Whether the other version of the cat still exists is not a question worth worrying about, like it's not worth worrying about whether an invisible god exists or not. The simplest theory is to make no claim either way.

By the way, many worlds by itself is not actually a complete theory. It simply says that the wavefunction of the entire universe evolves according to the rules of quantum mechanics. To actually get predictions out of it you have to say something extra about observers within that universe. We certainly don't experience multiple simultaneous versions of ourselves, but we do experience multiple simultaneous versions of the things around us (e.g. double slit experiment). You need additional rules about what an observer in the universe will see and with which probabilities.

[loup-vaillant]

> It's not entirely clear what happens when a spaceship goes outside our observable bubble due to issues with conservation of information (similar to how it's not entirely clear what happens when you drop a spaceship in a black hole).

Crap. Okay, I'll keep that in mind.

> Then the question of whether that spaceship still exists is not a sensible question in physics, because there is no experiment that can confirm or deny it.

We could say it's not a sensible question in science (no experimental difference), but I think this is still a very important ethical question: I would still care about whether the colonists live or die.

Long term, this could be a very practical question: how should we expand? Must we stay within reach, or can we safely go as far away as possible? At this point, I don't really care if it's a metaphysical question. From the look of it, there's a definite answer, and one which will influence expansion policy a great deal.

Though to be fair, the point is kinda moot until we have a theory of everything.

[jules]

It gets very weird. Some models say that going outside of the observable bubble is the same as passing the event horizon of a black hole. When you are in a spaceship and you pass the event horizon of a black hole, nothing happens to you yet. The event horizon is not some kind of physical barrier; you can pass it without even noticing that you passed it, though of course when you are past it there is no turning back. From the outside perspective however, there is a problem because information is conserved. According to some theories that information will be radiated out. So from the outside perspective the spaceship has been completely destroyed and radiated out. From the inside perspective the spaceship is happily orbiting. Something similar may happen when a spaceship leaves our bubble. Of course these models may be wrong, but it shows that the question of whether the spaceship still exists is not very clear, and may depend on who you ask. So is it ethical or not? I'd say yes because from their perspective nothing happens, well, from their perspective we die and burn. Note that in QM it's clearer: if you are the one that sees the dead cat, then for all practical purposes it's truly irrelevant whether the one that sees the alive cat exists -- there is no information being radiated back or something else that indicates that that version exists or not. The predictions of MW and copenhagen are identical, whereas with black holes we could observe whether something is being radiated out or not, and we could drop ourselves into a black hole to see what happens.