# CI vs MWI - contradictory?

Tags:
1. Feb 4, 2016

### entropy1

Suppose we fire a photon P at a polarisation filter F1, and it passes the filter, thus forcing the polarisation of P in an eigenstate M1. Subsequently, the photon falls through a polarisation filter F2, forcing P in eigenstate M2.

Now, if I understand correctly, the probability of P passing F2, thus becoming in eigenstate M2 when in known eigenstate M1, can be exactly determined by the relative settings of F1 and F2.

So, we have P passing F2 with a known probability. Now, In CI the probability represents the factual state of P (M2 or the perpendicular to M2), while MWI allows for both M2 and perpendicular to M2. So, it seems as if in MWI both eigenstates (M2 and the perpendicular) both have a probability of 100% of occuring, only in two different worlds. So, it seems as if MWI contradicts the probabilistic nature of QM?

What is the explanation for this?

2. Feb 4, 2016

### phyzguy

What do you mean by "occurring"? Suppose I flip a coin and it comes up heads. Do you think that the fact that MWI says that it came up tails in another "world" which is completely unobservable to us means that the event of coming up tails actually "occurred"?

3. Feb 4, 2016

### entropy1

Yes, at least, that's the way I understood the interpretation of MWI.

4. Feb 4, 2016

### StevieTNZ

100% probability of occurring if you take all -four- "worlds" into account. Same probability of occurring in only one "world", 50% for each filter.

QM probabilistic predictions only apply to one "world".

5. Feb 4, 2016

### stevendaryl

Staff Emeritus
Here's an analogy: You buy a lottery ticket. You have a one in 50,000,000 chance of winning the big prize. But on the other hand, you know that it is certain that somebody wins it.

Probability is consistent with absolute certainty, if you have lots of observers involved.

6. Feb 5, 2016

### Demystifier

Lottery is not a good analogy for MWI, because in lottery most of players do not win at all.

A much better analogy is biological twins. They are created by a cell splitting, after which two beings exist independently. They are almost identical, but differ in some minor details. For instance, one of them is slightly fatter than the other one. Then, if you have a twin brother, you may ask yourself what is the probability that you are the fatter one.