# Question about quantum parallel worlds

1. Nov 9, 2014

### bland

I would like some clarification about an aspect of this concept that has always bothered me. When the 'Many Worlds' or 'Parallel Universes' part of quantum physics is mentioned there is always what seems to me a very shallow and somewhat ludicrous example given like... a world where JFK was not shot, or a world where due to some coincidence I did or did not marry a particular person, and so on.

But, and this is what I would like clarified, is this or is this not the case that the many worlds interpretation means that a parallel world splits off at any quantum event, meaning that there is virtually an infinite split of events that occur from a single electron in an atom, never mind about some macro object doing or not doing a macro thing.

In short we could just look at the life of a bacteria or an ant and end up with limitless variations on it's life while the rest of the Universe remains unchanged.

In short, is this really science, or have I got this concept all wrong?

2. Nov 9, 2014

### phinds

I think you've got the idea. Personally, I find that whole thing silly but there are folks who know more physics than I'll ever know who take it seriously.

3. Nov 9, 2014

### bland

OK, thanks for that clarification. So every single subatomic particle has an infinite bunch of parallel worlds associated with it. And each one of those infinite parallel worlds that split off from that original subatomic particle, contain that same original subatomic particle which in turn have an infinite bunch of worlds splitting off from that, and so forth. Silly is not the right word. Finally I can retire that concept from further consideration.

4. Nov 9, 2014

### e.bar.goum

An Everettian Many Worlds Interpretation isn't actually that silly. Sure, it can sound silly, but it's not so crazy from the point of view of the formalism. Basically, the possibility of many worlds is automatically in the formalism of QM.

I encourage you to read this blog post from Sean Carrol about it. He obviously falls on the "MWI is reasonable" side of things, but it gives a good overview why it is not unreasonable to hold that view. He also has a couple of papers on the matter, but they're extremely technical.

There are legitimate problems with MWI - how do you get the Born rule? But "there are simply too many universes!" isn't one of them.

5. Nov 9, 2014

### VantagePoint72

e.bar.goum beat me to the punch—I was even going to link the same blog post!

I feel like most people miss the point of MWT. All it is, at its most basic, is quantum mechanics with an objective wavefunction and without collapse. "Worlds" are not some new thing tacked onto theory, they're just elements of some system's superposition state in an einselected basis. For something super simple like a spin-1/2 particle that starts in the state $|\psi\rangle = \frac{1}{\sqrt{2}}(|up\rangle + |down\rangle)$ and then evolves into a mixed state due to decoherence, the two "worlds" are the piece of the universe's wavefunction in which the the particle was up and the one in which it was down. The two "worlds" time evolve merrily along and can no longer interfere with each other due to entanglement with the surroundings. That's pretty much it.

6. Nov 10, 2014

### bland

I watched the Sean Carroll video which like all this stuff is well explained but nevertheless one surely wonders what is the place of the MWI in the scheme of things other than being an interpretation that can be subscribed to or not. I get that it is mathematically sound and therefore scientific but is it or will it ever be any more than that.

Off the record the MWI does make a particular kind of sense to me but only in a way that would get me thrown off the forum if I were to express it. Nevertheless I still must wonder (rhetorically only), 'is this science'.

7. Nov 10, 2014

### bhobba

Its mathematically very, maybe even breathtakingly, beautiful and elegant when you understand it. That appeals to a certain type, like me, although its not the one I hold to.

This stuff is simply philosophical since there is no way to experimentally tell the difference between any of them.

THE book to get on it is David Wallaces which I have a copy of:
http://users.ox.ac.uk/~mert0130/books.shtml [Broken]

Thanks
Bill

Last edited by a moderator: May 7, 2017
8. Nov 10, 2014

### e.bar.goum

That's exactly it. You must say that about every interpretation of quantum mechanics (copenhagen, de broglie-bohm etc. etc). None of them are testable, so which one you subscribe to is a matter of philosophy.

9. Nov 10, 2014

### tom.stoer

I think that in a certain sense MWI is different b/c it is not based on the Born rule as an axiom but tries to derive it as a theorem following from quantum dynamics. In this sense the MWI is more than an interpretation. It is - at least partially - a research program.

10. Nov 10, 2014

### bhobba

I know that's the claim.

I have gone through David's book, especially the section on answering objections.

I believe there is a tacit assumption in the approach of basis independence, ie non-contextuality, which means Gleason applies anyway. Still the idea of some kind of rational way of assigning a kind of confidence to what world will be experienced is very interesting to say the least.

Its a real strange beast - probabilities in a totally deterministic theory.

Thanks
Bill

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