> On 6 Jul 2021, at 09:21, smitra <smi...@zonnet.nl> wrote:
> 
> On 05-07-2021 12:18, Bruce Kellett wrote:
>> On Mon, Jul 5, 2021 at 7:39 PM smitra <smi...@zonnet.nl> wrote:
>>> On 05-07-2021 09:00, Bruce Kellett wrote:
>>>> On Mon, Jul 5, 2021 at 2:23 PM smitra <smi...@zonnet.nl> wrote:
>>>> I don't think this is actually done in the experiment. What is
>>>> observed is the presence or absence of the interference pattern on
>>> the
>>>> screen where the balls hit. The photons are not detected. But if,
>>> in
>>>> principle, they are of suitable wavelength to resolve the slit
>>>> difference, then the interference pattern vanishes. The experiment
>>> is
>>>> convincing in that they start wil cold buckyballs which show a
>>> clear
>>>> interference pattern. They then gradually heat the balls so that
>>> the
>>>> typical wavelength of the photons decreases. This gradually washes
>>> out
>>>> the interference pattern. (Because at lower temperatures, the
>>>> wavelength distribution of the IR photons is such that a few of
>>> them
>>>> have shorter wavelengths.) As the temperature is increased so that
>>>> most IR photons have short enough wavelengths, the interference
>>>> pattern disappears completely. The paper by Hornberger et al. is
>>> at
>>>> arXiv:quant-ph/0412003v2
>>> This is then what I said previously, what you denied, i.e. that you
>>> are
>>> only considering part of the system which is defined by the reduced
>>> density matrix. The complete system of buckyball plus photons will
>>> show
>>> interference, even if the wavelength is small enough to resolve the
>>> slits provided you perform the right sort of measurement on the
>>> balls
>>> and photons.
>> That is false.
> 
> This is easy to see. Denote the buckyball state of a buckball moving through 
> the left slit by |L> and moving through the right slit by |R>. Suppose that a 
> photon is emitted by the by the buckyballs such that the ball moving through 
> the left slit emits a photon in a state |PL> that will be orthogonal to the 
> state |PR> of the photon emitted by the ball moving through the right slit . 
> The state of the system after the ball passes the slits is then:
> 
> |psi> = 1/sqrt(2) [|L>|PL> + |R>|PR>]
> 
> This state then evolves under unitary time evolution, we can write the state 
> just before the ball hits the screen as:
> 
> |psi_s> = 1/sqrt(2) [|L_s>|PL_s> + |R_s>|PR_s>]
> 
> There is then no interference patter on the screen for the buckyballs because 
> |PL_s> and |PR_s> are orthogonal, the unitary time evolution preserves the 
> orthogonality of the initial states. The probability to observe a buckyball 
> on position x on the screen is:
> 
> P(x) = ||<x|psi_s>||^2 = 1/2 [|<x|L_s>|^2 + |<x|R_s>|^2] + Re[<x|L_s> 
> <x|R_s>* <PR_s|PL_s>]
> 
> And the last interference term is zero because <PR_s|PL_s> = 0
> 
> But if we also observe the photon on another screen and keep the joint count 
> for buckyballs landing on spot x on the buckyball screen and for photons 
> landing on spot y on the photon screen as a function of x and y, then we do 
> have an interference pattern as a function of x for fixed y. If we de note by 
> U the unitary time evolution for the photons until they hit their screen, and 
> put |PL_t> =U|PL_s> and |PR_t> = U|PR_s>, then the probability distribution 
> is:
> 
> P(x,y) = |<x,y|U|psi_s>|^2 = 1/2 [|<x|L_s>|^2|<y|PL_t>|^2 + 
> |<x|R_s>|^2|<y|PR_t>|^2] +Re[<x|L_s> <x|R_s>* <y|PL_t><y|PR_t>*]
> 
> The interference term Re[<x|L_s> <x|R_s>* <y|PL_t><y|PR_t>*] does not vanish 
> as it involves evaluating the components of  the buckyball and photon states 
> in the position basis and so there is no inner product involved anymore. For 
> fixed y the quantity <y|PL_t><y|PR_t>* will have some value that will be 
> nonzero in general, so if we keep y fixed then there will be an interference 
> term.
> 
> So, we can conclude that invoking escaping IR photons does not male any sense 
> in this discussion because all it does is it scrambles the interference 
> pattern to make it invisible in a way that allows it to be recovered in 
> principle using measurements on those IR photons. You can, of course, erase 
> the interference patter by measuring the observable for the photons that has 
> |PR> and |PL> as its eigenstates. But even in that case the information will 
> still be there in the state of all the atoms of the measurement apparatus for 
> the photons. But if you don't perform any measurement then the information 
> will simply continue to exists in the escaping photons.
> 
> So, in general we can conclude by generalizing this to any large number of 
> particles that even with what we consider to be permanent records, you don't 
> get rid of the theoretical possibility of interference between the sectors 
> where those records are different. So, the existence of parallel worlds 
> cannot be made fully 100% irrelevant if QM is rigorously correct, and we 
> cannot therefore argue that QM is exactly equivalent to an alternative theory 
> that leaves out parallel worlds. Even though the difference may be almost 
> 100% insignificant FAPP, it's not exactly 100% even in the macroscopic realm.
> 
> The argument against the existence of parallel worlds by invoking decoherence 
> that makes superposition hard to detect for complex systems is thus analogous 
> to the defense of creationists when they invoke a God of ever smaller gaps of 
> things that have not yet been fully explained.

You are right. Some people invoke their personal ontological commitment, which 
is invalid in this setting.

To make a parallel world disappearing is impossible as long as 2+2=4.

Bruno



> 
> Saibal
> 
> 
>>>> This is not what happens. Read the paper referenced above.
>>> It's not what happens in that experiment, but you can in principle
>>> demostrate an interference pattern also when photons are emitted by
>>> the
>>> balls.
>> Provided the  wavelength of the IR photons is too large to resolve the
>> inter-slit distance. When you heat the balls further, the interference
>> disappears.
>>>> This is all totally irrelevant to the actual experiment in
>>> question.
>>> And that experiment is in turn irrelevant to the question of whether
>>> or
>>> not a real superposition actually exist. You can always perform a
>>> measurement involving more particles where an interference has
>>> vanished,
>>> that only demonstrates that the reduced density matrix described a
>>> mixed
>>> state, the entire system is still in a pure state.
>> Of course real superpositions exist. The experiment shows that
>> decoherence need not involve large numbers of degrees of freedom.
>>>> These considerations do apply to each and every case. I mentioned
>>> the
>>>> buckyball experiment because it makes things obvious. But the
>>> general
>>>> principle is always true. Experiments that produced recorded
>>> results
>>>> are not reversible. Because, for example, they are not thermally
>>>> isolated, and IR photons can always escape to infinity and be
>>>> irretrievable.
>>> Even if IR photons always escape to infinity, the complete quantum
>>> state
>>> of the entire system is still a pure state. There is still a
>>> superposition between the balls going through one and the other
>>> slit.
>> Maybe that is not what is observed. That superpostion has decohered.
>>>>> For example, one may object by invoking that the universe is
>>>>> filled with a plasma and that the IR photons travel at a speed
>>>>> slightly below the true vacuum speed of light.
>>>> What difference would that make. The IR photons are still faster
>>> than
>>>> any material object sent after them to capture them. They will
>>> always
>>>> escape. And because the universe is expanding, they will
>>> eventually
>>>> pass over the Hubble horizon and be forever lost from sight!
>>> But the observations on the balls will be completed long before
>>> that, so
>>> how is this relevant for the existence of parallel worlds?
>> I think it is relevant to the question of reversibility.
>>>> Whether this means that the off-diagonal terms of the density
>>> matrix
>>>> (in the appropriate basis) do actually vanish, or if this is
>>> achieved
>>>> by some other means, your theory has to adapt to the reality of
>>>> irreversibility or your theory does not describe the real world.
>>> It is
>>>> clear that for many reasons, pure Everettian QM, based solely on
>>> the
>>>> Schrodinger equation, fails to explain many important features of
>>> the
>>>> world we observe.
>>> Which would mean that QM cannot be correct as a fundamental theory.
>> It is very probable the QM, in its current form, is not the correct
>> fundamental theory.  In the history of science it is never the case
>> that the dominant theory at one time survives unaltered into the
>> future. The negative induction against scientific realism is that all
>> scientific theories are ultimately shown to be false.
>> Bruce
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