# I Many Worlds Interpretation existence

1. Sep 21, 2017

### name123

I have a few questions about the Many Worlds Interpretation. I read the article https://plato.stanford.edu/entries/qm-manyworlds/ but was having trouble understanding what the "measure of existence" is supposed to represent in the theory, and why a believer in the idea should adhere to either the probability postulate or the behaviour principle.

I can illustrate my confusion through a consideration of a type of event where the probability of outcome A is thought to be 1/3 by non-MWI proponents, and the probability of outcome B is thought to be 2/3.

If the "measure of existence" for outcome A were thought to reflect the proportion of the amount of worlds in which outcome A would occur to the total amount of worlds created by an event, and likewise for outcome B, such that the probability could be thought to reflect the creation of 1 world in which outcome A occurs for every 2 worlds in which outcome B occurs, then I have no problem understanding.

But if the theory suggests that such an event will result in 1 world in which outcome A occurs and 1 world in which outcome B occurs, then I am not clear on why a person should not ignore the probability postulate and behaviour principle. To illustrate, I am not clear on why a person could not bet on a probability of 2/5 for an Event Type A outcome and accept bets on a probability of 3/5 for Event Type B and expect the majority of their descendants to have profited by repetition of such a strategy (a strategy of ignoring the probability postulate and behaviour principle)?

Also, is the MWI the only interpretation which does not have "spooky action at a distance"?

If so (if it does not have spooky action at a distance), how does it explain the non-zero probability amplitude of an electron appearing further than it could have travelled if it travelled at the speed of light?

2. Sep 21, 2017

### vanhees71

QT has no spooky actions at a distance, at least not relativistic local and microcausal QFT (by construction)! This is independent of interpretation!

3. Sep 21, 2017

### name123

How are the Bell tests explained?

4. Sep 21, 2017

### Demystifier

This is not independent of interpretation. All non-local hidden variable interpretations have some sort of action at a distance. However, signal locality is interpretation independent.

5. Sep 21, 2017

### Demystifier

No. See https://arxiv.org/abs/1703.08341 Sec. 5.3.

6. Sep 21, 2017

### Demystifier

Why do you think that there is non-zero probability amplitude of an electron appearing further than it could have traveled if it traveled at the speed of light? Reference?

7. Sep 21, 2017

### martinbn

Yes, but can you take those seriously.
And the only locality that deserves the name locality. Every other use of local (and nonlical) is just muddling the matters and pushing someone's agenda (of course I am referring to BM).

8. Sep 21, 2017

### name123

I do not have a reference but in another thread on this forum I had asked about the maximum distance an electron could tunnel during a time interval, and I came away with the impression that there was no maximum distance.

---
The proper framework for answering this question is not ordinary QM, but quantum field theory. In QFT, this question becomes: given that I have measured an electron to be at a particular position at a particular time, i.e., that there is an electron definitely present at some particular event in spacetime, is there a nonzero amplitude for an electron to be present at some other event that is spacelike separated from the first? It turns out, counterintuitively, that the answer to this question is yes. Informally, we might describe this as telling us that electrons have a nonzero amplitude to travel faster than light; but one of the things you quickly learn when studying QFT is not to trust such informal descriptions.
---

And while with QFT as I understand it, the two electrons cannot be said to be the same electron, I had assumed that would not be the case with MWI, and that with MWI the event would result in multiple worlds covering all the possible positional outcomes for the electron. Did I make an incorrect assumption there?

9. Sep 21, 2017

### vanhees71

By the correlations due to the preparation before the measurement, i.e., as usual, just taking Born's rule as a fundamental postulate of the theory and serious: The meaning of quantum states is probabilistic, and only probabilistic.

10. Sep 21, 2017

### Demystifier

Why not? And by the way, I don't remember that you ever said what do you think about this stuff? I mean, what is your favored interpretation of QM?

11. Sep 21, 2017

### name123

I do not know what you mean. Could you perhaps explain using an example of two entangled photons going to the two detectors, each of which can measure "spin" in one of three orientations? How for example is it explained that if they are measured in the same orientation close enough in time that a light signal would not have had time to have travelled from one detector to the other; that the spin will be the opposite?

Last edited: Sep 21, 2017
12. Sep 21, 2017

### Demystifier

I think that's wrong, but the issue is quite subtle and somewhat technical.

13. Sep 21, 2017

### vanhees71

You mean polarization. The two entangled photons are created long before you do the far-distant measurements of the single-photon polarization at "Alice's and Bob's place". The usual state prepared by parametric down-conversion is (I write only the polarization state)
$$|\Psi \rangle=\frac{1}{\sqrt{2}} (|HV \rangle-|VH \rangle).$$
Then Born's rule tells you that each of the single photons is perfectly unpolarized, i.e., their states are given by the statistical operator
$$\hat{\rho}_{\text{single photon}}=\frac{1}{2} \hat{1}=\frac{1}{2} (|H \rangle \langle H|+|V \rangle \langle V|),$$
but due to the entanglement implied by the pure state of the two-photon system, given by
$$\hat{\rho}_{\text{two-photon system}}=|\Psi \rangle \langle \Psi|,$$
whenever A finds a H-polarized photon, B must find a V-polarized one and vice versa.

This description, based on the minimal interpretation, clearly shows that the correlations, that are in the sense of Bell's inequality stronger than any local deterministic hidden-variable model can describe, are due to the preparation of the two-photon system in the entangled state and not due to some action-at-a-distance interaction of B's photon with A's far-distant measurement apparatus. This has been checked with astonishing accuracy, including closing several loopholes, discussed in the literature.

14. Sep 21, 2017

### martinbn

Action at a distance is very unappealing. I am not sure if it even makes sense.

I don't have a preferred interpretation. I struggle with QM, probably because my mind is to classical. I like relativity, anything else seems boring in comparison. Anything that hints that relativity might be wrong is too hard for me to swallow. Anyway I think the QM is incomplete and needs to be reworked in a major way. Something along the lines form QM to relativistic QM and QFT, but using general relativity instead.

15. Sep 21, 2017

### Demystifier

So basically no present interpretation of QM makes much sense to you, am I right? Your position looks similar to that of late Einstein. He believed that somehow a unified classical field theory will explain the quantum phenomena.

And what do you mean by the idea that "action at a distance does not make sense"? This idea can be defined precisely in a mathematical sense, some theories of that kind (like Newton gravity) are partially successful even in a physical sense, so are you saying that it looks like a non-sense in a philosophical sense?

Last edited: Sep 21, 2017
16. Sep 21, 2017

### name123

So there are the two entangled photons, and that their measurements will be correlated is given by the preparation of the two-photon system in an entangled state. Let us assume the measurement at Alice's detector takes place very slightly before the measurement at Bob's detector, but not a big enough time delay that light could have travelled from one detector to the other. And let us assume that Alice's detector measures an "up" spin. How is the other photon which was entangled influenced by the measurement at Alice's detector? I presume you do not think that the other photon already had a "down" spin before the measurement at Alice's detector.

17. Sep 21, 2017

### vanhees71

Well, the only way out of classical thinking is to do a lot of applications of QT to real experiments. The best interpretation in this state of feelings is the shutup-and-calculate interpretation (which is very close to the minimal interpretation, if not identical with it anyway). You really get trained to neglect the all too natural feeling classical intuitions.

Another way on looking at it, is also the historical perspective: The physicists at the end of the 19th century thought, all there is remaining to be figured out is to measure some fundamental constants of nature to better and better precision, and everything is understood by classical physics (or even classical, Newtonian mechanics, but the idiosyncrazy of the socalled (a)ether was pretty obvious already then). So some professor, the young Planck attended to to get an idea what study at the university (I forgot his name) told him that he shouldn't waste his talent to physics, which is boring for a brilliant mind as his. So he should rather consider to study ancient Latin and Greek ;-)).

However, there were some "clouds" on the horizon, showing that maybe this classical world view is not the theory of everything as the physicists thought at the time. One thing, which was not solved was the black-body spectrum (which was finally solved by Planck in introducing quantum theory for the first time, however not in it's modern form). Another one were the deviations of some thermodynamic quantities like the specific heat at low temperatures.

The final death of classical physics, I think was Rutherford's discovery of the basic structure of matter as consisting of atoms with a small positively charged center (now called the atomic nucleus) and surrounding negatively-charged electrons. In the classical picture this could only be understood that the electrons run around the nucleus in analogy to the planets running around the sun (with the em. force substituting the gravitational force a la Newton), but this would imply that matter couldn't be stable since the electrons would radiate off their energy very quickly and crash into the nucleus, but hat obviously doesn't happen.

This brought the idea that classical physics must be utterly flawed, at least if it comes to the atomic and subatomic scale, and thus the physicists started to create new theories. In the case of the atom it was Bohr just finding the ad-hoc solution that there are certain orbits of the electrons, given by some funny quantization condition, later better understood by Sommerfeld in terms of action-angle variables of analytical mechanics. Unfortunately this model only worked for the hydrogen atom. As we know today, that's just due to the very high symmetry of the $1/r$ Coulomb potential, i.e., almost an accident. For all other atoms, using the precise data on their spectra, the physicists had to make new ad-hoc assumptions, leading finally to the conclusion that this kind of model (now called "the old quantum theory") is still too classical, and the so far final word on it is modern quantum theory discovered by Heisenberg, Born, Jordan and Schrödinger and Dirac (i.e., in independently three different forms, but almost immediately proven to be in fact the same theory). So the physicists, being conservative by nature, i.e., not giving up well-established models/theories easily, were finally forced to quantum theory as a quite radical new way of modeling nature. Compared to QT relativity was not too revolutionary, although of course one had to get used to the idea of 4D spacetime and the invalidity of Newton's absolute space and time, but as long as one stays within classical physics, relativistic classical field theory is quite analogous to classical mechanics (at least mathematically).

18. Sep 21, 2017

### vanhees71

According to standard QED Bob's photon isn't influenced by the measurement of Alice's photon at all! According to standard QED the interaction of the photon with A's detector is local and cannot have any superluminal influence on B's photon and/or detector. As I said, before the measurement of either photon its polarization (NOT spin!) was maximally indetermined!

19. Sep 21, 2017

### name123

The preparation would determine the correlation between the two, but not the polarisation of B's photon.
Prior to measurement of A's photon, the polarisation of B's photon would be undetermined.
Upon measurement of the polarisation of A's photon the polarisation of B's photon would no longer be undetermined.

So how did the measurement at A's detector determine the polarisation of B's photon ?

Last edited: Sep 21, 2017
20. Sep 21, 2017

### martinbn

Yes, something like that. Except that I don't think it will be classical field theory. I want to see QT modified in such a way so that it makes sense in GR, without changing GR. Then see what the consequences are and what the interpretation will be.
Yes, mathematically it is perfectly fine, it may be that the world is like that. But the ontology is very strange, unsatisfying, probably even non sensible.