On Mon, Aug 12, 2019 at 3:48 AM Bruno Marchal <[email protected]> wrote:
> On 9 Aug 2019, at 13:15, Bruce Kellett <[email protected]> wrote: > > On Fri, Aug 9, 2019 at 7:49 PM Bruno Marchal <[email protected]> wrote: > >> On 9 Aug 2019, at 04:07, Bruce Kellett <[email protected]> wrote: >> >> From: Bruno Marchal <[email protected]> >> >> On 8 Aug 2019, at 13:59, Bruce Kellett <[email protected]> wrote: >> >> On Thu, Aug 8, 2019 at 8:51 PM Bruno Marchal <[email protected]> wrote: >> If the superposition are not relevant, then I don’t have any minimal >> physical realist account of the two slit experience, or even the stability >> of the atoms. >> >> Don't be obtuse, Bruno. Of course there is a superposition of the paths >> in the two slit experiment. But these are not orthogonal basis vectors. >> That is why there is interference. >> >> >> But each path are orthogonal. See the video of Susskind, where he use 1 >> and 0 to describe the boxes where we can find by which hole the particles >> has gone through. Then, without looking at which hole the particle has gone >> through, we can get the interference of the wave which is obliged to be >> taken as spread on both holes, and that represent the superposition of the >> two orthogonal state described here as 0 and 1. >> >> I seldom watch long videos of lectures. But if Susskind is saying that >> the paths taken by the particle through the two slits are orthogonal then >> he is flatly wrong. Writing the paths as 1 and 0 does not make them >> orthogonal. And if they were orthogonal they could not interact, and you >> would not get interference. Two states |0> and |1> are orthogonal if their >> overlap vanishes: <0|1> = 0. Interference comes from the overlap, so if >> this vanishes, there is no interference. >> >> Either Susskind is terminally confused, or you have misrepresented him. >> >> Or maybe you are wrong. Slit one is orthogonal to slit two, as much as >> spin in different direction. >> > > When you observe which slit the particle went through, then yes -- the > slits are then orthogonal eigenstates of the position operator. > > > OK. But without collapse, the observation of which slit the particle was > taking is only self-entanglement, and it makes the whole history “the > particle went through slit A + me seeing the particles going through slit > A” orthogonal to the whole history “the particle went through slit B + me > seeing the particles going through slit B”. > So, like I said, the slits are orthogonal if measured. This is irrelevant to the interference at the screen, because it is the photon waves at the screen that interfere, not the slits. Orthogonal states do not interfere. Decoherence is only self-entanglement. It spread at the speed of light, or > a bit below, in the environment, making hard to fuse the histories again, > through “amnesia”, but that explains why the superposition states are are > hard to maintain accessible. FAPP, we can forget the parallel histories, > but, only FAPP! > FAPP is the way we do physics. Metaphysics is for the birds that can't fly! > The interference comes from the fact that we get a superposition of going >> through slit one + going through slit two when we send a planar >> monochromatic wave on the wall with the two slits, and don’t measure which >> slit the particle go through. >> > > Yes, then the states that we are measuring are not orthogonal. You do not > get interference between orthogonal states. > > >> That is how Susskind explains the two slit experiment in term of >> entanglement. You don’t need to look at the whole video, I gave the >> position of this sub-talk in the video. >> >> Any crisp measurement, like “which slit” gives rise to orthogonal state, >> which can interfere when superposed. >> > > Which is essentially what I said -- orthogonal states do not interfere. > > > In the sense you mention I am OK, but we have a slight vocabulary problem. > Not important, if you agree that measurement are self-entanglement, so that > the superposition of the orthogonal state SlitA and SlitB, say some oblique > (with sqrt(2) = 1) SlitA + SlitB is inherited by the observer “looking” > which is which. > If you do not measure which slit the photon went through, then the superposition of slits is not broken by decoherence. But the interference at the screen depends only on things like the wavelength of the light, the separation of the slits, and the distance between the slits and the screen. If you refine this calculation by taking the finite width of the slits into account, you convolute the interference pattern with the diffraction pattern due to finite slit width. This is an elementary calculation in physical optics, not even requiring quantum mechanics. But the waves at the screen cannot be orthogonal, or else they would not interfere. Bruce -- You received this message because you are subscribed to the Google Groups "Everything List" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/everything-list/CAFxXSLRmoV5fKbbyLUfhbrZtzGUF23d%3Dq_RHyuyzDfq%3DPP2arA%40mail.gmail.com.
