The fatal flaw in the Many Worlds Interpretation (MWI)?

[scuddyx]

In the double slit experiment, an interference pattern is created by individual photons travelling through both slits and interfering with themselves. The MWI proposes that the photon goes through both slits but as branches in different universes. These universes are separate and do not interact with each other. If these universes can’t interact with each other how can an interference pattern be created?

[Halc]

MWI (DeWitt) posits splitting of worlds at time of measurement, not photons going through different slits in different worlds.

RSF (Everett) does not posit any ontological split at all.

I do believe that there is a fatal flaw in the DeWitt version, but it isn't that.

[swansont]

2 hours ago, scuddyx said:

In the double slit experiment, an interference pattern is created by individual photons travelling through both slits and interfering with themselves. The MWI proposes that the photon goes through both slits but as branches in different universes. These universes are separate and do not interact with each other. If these universes can’t interact with each other how can an interference pattern be created?

You don’t get an interference pattern from one photon. (one photon is not a pattern)

The MWI explanation would be that if you sent 100 photons (individually) you would have multiple universes, each with their own distribution of where the photons went. Some universes would end up with same distribution, of course.

For example, let’s pretend there’s two options - diffract left or right. One photon gives you two universes - one where the photon went left, one right. After another, you have 4 - one for LL, one for RR, and LR and RL, so two with one in each direction. And so on, and so on.

[scuddyx]

2 hours ago, swansont said:

You don’t get an interference pattern from one photon. (one photon is not a pattern)

The MWI explanation would be that if you sent 100 photons (individually) you would have multiple universes, each with their own distribution of where the photons went. Some universes would end up with same distribution, of course.

For example, let’s pretend there’s two options - diffract left or right. One photon gives you two universes - one where the photon went left, one right. After another, you have 4 - one for LL, one for RR, and LR and RL, so two with one in each direction. And so on, and so on.

You are correct in saying that an individual photon can't create an interference pattern - but 100 photons sent individually would eventually build up an 'interference' type pattern.

If there was no interference, each photon taking the left or right slit would be just pile up on the screen behind the two slits, with no pattern.

As the photons are sent individually the pattern can only be created by the photon interfering with itself.

The MWI interpretation says the universes can't interact.  The wave-function of the photon passes through both slits - not individual photons in separate universes. 

The MWI  interpretation fails to explain the double slit experiment. 

[swansont]

44 minutes ago, scuddyx said:

You are correct in saying that an individual photon can't create an interference pattern - but 100 photons sent individually would eventually build up an 'interference' type pattern.

If there was no interference, each photon taking the left or right slit would be just pile up on the screen behind the two slits, with no pattern.

I didn’t say no interference. I said no pattern from one photon. I explicitly pointed this out, hoping to avoid confusion. Oh well.

The rest of my post explains the pattern with the MWI

44 minutes ago, scuddyx said:

As the photons are sent individually the pattern can only be created by the photon interfering with itself.

The MWI interpretation says the universes can't interact.  The wave-function of the photon passes through both slits - not individual photons in separate universes. 

The MWI  interpretation fails to explain the double slit experiment. 

No, you just failed to read/understand the explanation.

[Halc]

1 hour ago, scuddyx said:

As the photons are sent individually the pattern can only be created by the photon interfering with itself.

The MWI interpretation says the universes can't interact. 

Read my post again as well.  MWI does not claim that the worlds split before the photon goes through the slits.  So still one world, with interference. The split happens when it is measured: when the dot appears on the target.  One world for each possible location for the dot, which is a lot more than 2 worlds.

Quote

The wave-function of the photon passes through both slits - not individual photons in separate universes.

MWI does not claim that it passes through one slit in each of 2 worlds.  Still the same world at that point.

Edited June 13, 2020 by Halc

[joigus]

As everyone has explained it's not about photons interacting with each other. My way of wording it would be that it's about photons being affected by their common 'wave function.'

The real problems with the MWI, I think, are the lack of falsifiability and a disastrous non-compliance with the principle of parsimony.

[scuddyx]

On 6/13/2020 at 6:38 PM, scuddyx said:

In the double slit experiment, an interference pattern is created by individual photons travelling through both slits and interfering with themselves. The MWI proposes that the photon goes through both slits but as branches in different universes. These universes are separate and do not interact with each other. If these universes can’t interact with each other how can an interference pattern be created?

Is it the case that there is a single wave function that is a superposition of all possible outcomes (or measurements).  This wave function evolves as described by the Schrödinger equation.  The Copenhagen interpretation differs from MWI in that what happens when a measurement (or observation) causes the wave function to ‘collapse’ or ‘split into many worlds’?

[joigus]

23 minutes ago, scuddyx said:

Is it the case that there is a single wave function that is a superposition of all possible outcomes (or measurements).  This wave function evolves as described by the Schrödinger equation.  The Copenhagen interpretation differs from MWI in that what happens when a measurement (or observation) causes the wave function to ‘collapse’ or ‘split into many worlds’?

Exactly. In the Copenhagen interpretation, every partial wave, tagged by a potential outcome, keeps evolving until a measurement is made. When such measurement is made, a sudden change in the wave function is produced, called the wave packet reduction, wave function collapse, or normalized projection of the state. This mathematical operation is incompatible with the Schrödinger equation. The partial waves that carried with them other possible results simply stop evolving.

In the MWI, on the contrary, every time a measurement is made, a universe implementing that result is set in motion, so to speak.

[Halc]

2 hours ago, scuddyx said:

Is it the case that there is a single wave function that is a superposition of all possible outcomes (or measurements).  This wave function evolves as described by the Schrödinger equation. 

One should also mention Everett's RSF interpretation, which posits pretty much what you said there, and no more. More precisely: "All isolated systems evolve according to the Schrodinger equation". That's it.  No wave function collapse, and no metaphysical spawning of new worlds.

[Don410]

Hmm. I think the universes would have to split at measurement in order for Many Worlds to work. I guess somebody could argue that different universes can briefly interact, but that would kind of beg the question of exactly when universes split. (Are they really separate universes as long as they can interact?)

With that being said, the universes splitting at measurement seems to eliminate any advantage MWI has over Copenhagen. It seems to result in MWI having the same Copenhagen issue of a measurement basically creating reality. 

Edited July 20, 2020 by Don410

[Halc]

29 minutes ago, Don410 said:

With that being said, the universes splitting at measurement seems to eliminate any advantage MWI has over Copenhagen. It seems to result in MWI having the same Copenhagen issue of a human measurement basically creating reality. 

Neither view posits a special role for humans. You're thinking the Wigner interpretation which has a special role for humans, but even Wigner backed off support for it since it can be shown to lead to solipsism.

If humans (or biological consciousness) are necessary, then the universe never collapsed into say the existence of Earth until humans came along to do the first measurement. Kind of a chicken/egg problem there.

[Strange]

20 hours ago, Don410 said:

With that being said, the universes splitting at measurement seems to eliminate any advantage MWI has over Copenhagen. It seems to result in MWI having the same Copenhagen issue of a measurement basically creating reality. 

As they are both descriptions of the same theory, that probably isn't surprising. Anything that happens in MWI also happens in Copenhagen (or any other interpretation), it is just described differently.

These are just "human stories" (fairy tales?) to attempt to explain what the theory says in understandable terms. It is not clear that they have anything to do with what "really happens". (I don't even know what "really happens" means.)