# According to the MWI, does the universe always split with all possible outcomes or only splits for those particles that were observed?

Sorry in advance if this is a stupid question.

The Many World Interpretation (MWI) says that, at every point in time, the universe splits into a multitude of existences in which every possible outcome actually happens in parallel.

According to the MWI, does the universe really split at every point in time with all possible outcomes, or does it only split in relation to the objects that are observed?

As I understand (and I don't understand much TBH), according to the Copenhagen Interpretation, only one of the possible states "is picked" when you observe a particle (the waves collapse and so on). And so far, I thought that, according to the MWI, universe splits would only happen when particles were observed, similarly to the idea from the Copenhagen Interpretation. But by reading the quote above, I'm not sure anymore.

As an example: is the cat always dead and alive in different branches of the universe regardless of being observed or not, or does the universe only slip when someone looks at the cat?

• What do you mean by CI when you state: "...when particles were observed, similarly to the idea on CI..." ? Commented Jun 11, 2022 at 19:42
• @BrendanDarrer Copenhagen Interpretation. I've just edited it to make it more clear. Thanks for point it out. Commented Jun 11, 2022 at 22:02

The key to understanding the many-worlds interpretation is understanding that a measuring device ends up in a superposition state to an outside observer that is closed off from the device. See the Wigner's friend problem for a more detailed explanation of this.

To an observer outside the universe who has been isolated from it since the beginning of time, only unitary evolution has occurred, and therefore the universe is in a superposition of all possible outcomes. The many worlds interpretation is that each of these possible outcomes are real, and that if this observer were to check the universe -- he simply end up checking which of these branches he is in.

The Copenhagen interpretation, on the other hand, would say that all of those superposition states didn't really exist and didn't have meaning, and that the universe just becomes the possibility that that personal collapsed it to. This is a weakness of that interpretation, because you can only explain what happened in the perspective of the people who are in the superposition only retroactively after that outside observer has collapsed the state.

As an example: is the cat always dead and alive in different branches of the universe regardless of being observed or not, or does the universe only slip when someone looks at the cat?

Always alive and dead in both branches. The act of checking the outcome of the cat is simply figuring out which of the universes that observer is in.

• I'm not sure this answers the OPs question (at least as it's stated). For many quantum measurements there are a nearly infinite number of possible outcomes. The particle doesn't just go through one slit or the other. It has a virtually and maybe actually infinite range of places it can hit. So does the universe split infinitely or only split into the number of possible outcomes that your experiment was set up to detect? Commented Sep 4, 2022 at 21:21
• @PeterMoore, the OP awarded it as having answered the question, so I think it probably was answered. Commented Sep 4, 2022 at 22:11
• "The particle doesn't just go through one slit or the other." I think I understand your question, but I just want to point out that I think you are pointing out something that is different from what you are asking. In the double-slit experiment, we typically measure the outcome after it has spent some time propagating after it's gone through both slits. The interpretation of a "branching" in a person's perspective typically has to do with what happens in their perspective at the point of measurement. Commented Sep 4, 2022 at 22:15
• So if you're question is, "how does the universe branch when someone makes a measurment" the answer is what you say, there is a branch for every possible measurable outcome by the observer." But what we are really describing is just what we mean by a "branch", which what we mean are the universes where the observer measures a certain outcome. But in a single "branch" there are a cluster of uncountably infinite universes with every possible superposition of everything in the universe that is uncorrelated with you. What they all have in common though is that you made a particular measurement. Commented Sep 4, 2022 at 22:19
• If this is not sufficient enough of an explanation -- perhaps write a different question and I'll answer that one. Commented Sep 4, 2022 at 22:20