From: David Deutsch <david.deutsch@qubit.org>
Date: Fri, 08 Mar 2002 10:55:23 +0000
Subject: Re: Cancellation of Worlds

Universes don't cancel each other out. Universes don't disappear (except in the sense of 'change with time'). The same is true of particles.

'Universe' and 'particle' are emergent concepts. These terms are accurate only insofar as the universe or particle in question is *not* affected by its counterparts. This is a very good approximation in many situations, but never a perfect one.

When we speak of interference between universes, or interference between a particle and its counterparts, we are adopting a more approximate way of speaking. We are pretending that the multiversal object is composed of *no more than* a number of single-universe objects that affect each other. For instance, when we explain why partlcles in an interference experiment arrive at some places but not others, we say that they have been deflected totowards some directions and away from others by their other-universe counterparts. But this is only ever a coarse-grained description of what happens.

When we look even closer at the mechanism of what happens, the language of universes and particles-in-universes becomes even more approximate and begins to be misleading, because the detail of what happens is inherently multiversal. And when we speak of constructive or destructive interference between universes, or between a particle and its counterparts, we have reached the extreme of misleadingness, for we are talking about something that has no counterpart in reality at all. We are now in the territory of saying that the versions of the particle that were heading in certain directions 'cancel each other out' or 'cease to exist'. This is not just false but incoherent.

The 'cancellation' in destructive interference is not a physical process in which real entities cease to exist. It arises not when a complex phenomenon is *physically composed* of simpler ones, but when when we *explain* a more complex phenomenon by using simpler ones that we already understand, as stepping stones or intermediate, imperfect ways of understanding the complex phenomenon. For instance: we may understand that a particle starting (in all universes) at point A, ends up at X in some of those universes and Y in others. Likewise we understand that a particle starting at B ends up at X in some universes and Y in others. Our task is to explain why a particle starting at A in some universes and B in others, ends up at X in all of them. We may say: it is because the path from A to Y which would have occurred if it had started at A, and the path from B to Y which would have occurred if it had started at B, have "cancelled each other out". But those two entities that have "cancelled each other out" are not features of reality. They are hypothetical entities, which we invoke for convenience of exposition. They help us to understand the experiment we are doing in terms of two other *experiments which we are not doing*, but which are simpler.

If we chose to, we could calculate what happens at X without ever mentioning Y or paths that end up at Y.

-- David Deutsch
http://www.qubit.org/people/david/David.html

From: Henry Sturman <henry@sturman.net>
Date: Fri, 08 Mar 2002 15:09:05 +0100
Subject: Re: Cancellation of Worlds

>'Universe' and 'particle' are emergent concepts. These terms are accurate
>only insofar as the universe or particle in question is *not* affected by
>its counterparts. This is a very good approximation in many situations, but
>never a perfect one.
[...]

Would you agree, then, that your position that in a 4-slit photon interference experiment in each universe the photon goes through (at most) one of the slits (The Fabric of Reality, page 43), is incorrect? Does it not follow from your post that there is no meaningful way in which the multiverse can be split into 4 groups according to which of the 4 slits the photon goes through? That a universe cannot be considered separately until one either measures the slit the photon goes through or observes the photon on screen? So that when one observes the screen location one cannot meaningfully speak of a particle path through a particular split? So that the correct answer to the question "What slit did the photon go through?" is: "Through all 4 slits at the same time (in the multiverse)"?

From: David Deutsch <david.deutsch@qubit.org>
Date: Fri, 08 Mar 2002 20:04:52 +0000
Subject: Re: Cancellation of Worlds

Henry Sturman wrote on 8/3/02 2:09 pm:

> [David Deutsch wrote:]
>
>> 'Universe' and 'particle' are emergent concepts. These terms are accurate
>> only insofar as the universe or particle in question is *not* affected by its
>> counterparts. This is a very good approximation in many situations, but never
>> a perfect one. [...]
>>
> Would you agree, then, that your position that in a 4-slit photon interference
> experiment in each universe the photon goes through (at most) one of the slits
> (The Fabric of Reality, page 43), is incorrect?
>
No. At the time when the four instances of the photon are passing through the slits, they are unaffected by each other to enormous accuracy.

> Does it not follow from your post that there is no meaningful way in which the
> multiverse can be split into 4 groups according to which of the 4 slits the
> photon goes through?
>
No, it follows that there is (at the time when they are passing through).

> That a universe cannot be considered separately until one either measures the
> slit the photon goes through or observes the photon on screen?
>
Measuring the photon is, in practice, one way of ensuring that the various instances of it do not affect each other. Another way is to separate the various instances spatially, as in the slits experiment or in an interferometer.

> So that when one observes the screen location one cannot meaningfully speak of
> a particle path through a particular split?

One cannot truthfully speak of its having gone through a particular slit, because while what happens to the multiversal entity during the time of passing through the slits is extremely accurately described as the fourfold history of four causally autonomous photons, and what happens to it when it is observed on the screen is fairly accurately described as an n-fold history (one for each spot where it could appear -- or in an interferometer we could arrange for n to be 1), there is no such description that holds through the intermediate stage, because the multiversal entity just does not consist of n causally autonomous entities each resembling a classical particle (by having a trajectory) at that stage.

> So that the correct answer to the question "What slit did the photon go
> through?" is: "Through all 4 slits at the same time (in the multiverse)"?

Yes, exactly.

-- David Deutsch
http://www.qubit.org/people/david/David.html