h a l f b a k e r y
Incidentally, why isn't "spacecraft" another word for "interior design"?

meta:

account: browse anonymously, or get an account and write.

 user: pass:
register,

# Double-Double Slit Experiment

Will particles turn back into waves?
 (+1) [vote for, against]

In a double slit experiment a wave can become what seems to be a particle by being "observed" at the slit. What would happen if such a particles were NOT observed thorough the second series of slits? Would they still be particles or would they turn back into waves?

This experiment would determine whether the observation "took something away"

Drinking Tim Horton's double double during the experiment is optional

 — ixnaum, May 09 2016

Until the photon is collapsed it will be both. The incident of collapse (observation) is terminal and cannot be reversed.
 — WcW, May 10 2016

 //Until the photon is collapsed it will be both

 That's the plan ... first pair of slits collapses, and the second pair of slits will help us determine whether we have a wave or whether we have a particle.

What I'm trying to get at is this... right now we assume that photon is changed to a particle because the interference pattern disappears. That's a reasonable conclusion. But it doesn't answer this question: Does that mean it lost it's wave property forever? Or will the interference pattern come back once the photons continue through the second pair of slits? (no observation at the second pair)
 — ixnaum, May 10 2016

 Here is a fairly simple explanation for what goes on during a two-slit experiment.

 Start by realizing the space through with a photon moves is not empty; it is full of "virtual particles" popping into temporary existence, and vanishing again. And it is known that real particles like photons can interact with those virtual particles.

 A photon carries momentum. If it hits a virtual particle, it is possible for some of the photon's momentum to be temporarily transferred to the virtual particle. The one virtual particle might even transfer it to another. Remember, the virtual particles MUST disappear, and the Law of Conservation of Momentum requires that the photon get its momentum back, eventually.

 During a two-slit experiment, just imagine the photon as a particle passing through one slit. However, some of its momentum can get carried by virtual particles through the other slit. THAT suffices to lead to an interference pattern.

Do remember that the two-slit experiment has been performed with electrons and even whole atoms. That should make it easier to imagine some of their momentum getting carried by virtual particles through the slit that the real particles don't go through.
 — Vernon, May 10 2016

Vernon, it's not fair to just make this stuff up.
 — WcW, May 10 2016

Idea should be recast as a joke greeting card for International Physicists Day. ( With appropriate slits, emitters, detectors, and instructions. )
 — popbottle, May 10 2016

 // is it possible that light is a wave, or waves, of statistical likelihood// That's exactly what a photon is. In fact it's exactly what everything is.

// A wave of probability of particles materialising?// Not really. It's a wave of probability of particles being detected.
 — MaxwellBuchanan, May 10 2016

 [WcW], since this is the HalfBakery, I'm free to make up any stuff I want. In this particular case, though, the "stuff" FITS.

 It is most certainly a fact that real particles constantly interact with a surrounding sea of virtual particles. And logical consequences are logical consequences.

The way momentum can be distributed/passing through the two slits can perfectly explain the interference pattern, without needing to care which slit was used by the real/moving particle.
 — Vernon, May 10 2016

This thing about atoms behaving in quantum ways is very interesting. Is it correct to deduce that the result of this double double slit experiment would be that photons would behave as particles through the first slits and as waves through the second pair of slits?
 — ixnaum, May 10 2016

 //Is it correct to deduce that the result of this double double slit experiment would be that photons would behave as particles through the first slits and as waves through the second pair of slits?//

 Photons behave as photons. Which behave very much like all other things. If you ask where something is, you will be given an answer, plucked out of nowhere on the basis of a probability density function. As soon as you stop asking for a definite position, the thing will not be anywhere - it will just be a probability density function.

 Ultimately, it comes down to the computational capacity of spacetime. If you consider all the factors that could influence the position of, say, a photon, it's obvious that spacetime cannot possibly calculate exactly where that photon is in realtime. So, spacetime works like an overworked accountant - it can't keep track of every pound, but if you say "give me a figure for revenue", it will spit out a plausible and precise number.

 And when spacetime does pluck that precise number out of thin air, it only does it for the person who asked the question. If the person asks for the revenue, the accountant will say "£345.67"; but if another person asks the same question, and if they're out of earshot of the first person, the accountant will grab another number out of the air and will say "£389.23".

 To summarize: (a) quantum fuzziness is spacetime's way of keeping up with all the maths.

 (b) asking for a number will produce a number, arbitrarily chosen from the fuzzy range of available answers. This is "collapsing".

(c) Quantum collapse is relative, like everything else. Just because spacetime has given you a definite answer only means that there's a definite answer for you; I may not be given a definite answer, or if I ask for one it might be a different answer.
 — MaxwellBuchanan, May 10 2016

 So can you DoS spacetime by collapsing spacetime way too frequently?

What if we built a machine that collapses spacetime trillions times a second in a certain area? Would it slow down time in that region relative to others?
 — ixnaum, May 11 2016

Maxwell, that way of looking at uncertainty is hubristic. Our primary problem is the presumption that we already understand all of the discrete components. When you pound a nail into dry wall you don't get to claim "computational conservation" when the nail goes through more easily in one place than the other, you basically have to concede that there are other things under the drywall we don't have a model for as yet. Tagging out with a complicated dodge like "it's all a big computer simulation" is simply a way to say "we think math can help us understand this".
 — WcW, May 11 2016

 //the presumption that we already understand all of the discrete components.// But it has been proven pretty conclusively that no "hidden variables" model is valid.

 //a complicated dodge like "it's all a big computer simulation"// No no no. I'm not a Matrix freak, and physics is not a computer simulation. Nevertheless, the actual physical behaviour of things is complex, and spacetime itself has to compute that behaviour. Think of spacetime as an analogue computer, where the computation *is* the phenomenon. It is pretty clear that if it takes five pages of maths to describe the behaviour of a proton in a magnetic field, spacetime has to work equally hard to make it behave like that. _That_ is the point I was making.

There is a limit to the ability of spacetime to figure out what everything is meant to be doing all the time, so it gets by with averages until you press it for a specific answer.
 — MaxwellBuchanan, May 11 2016

From my perspective that's a paradoxical view because the lively nature of the thing is very much the hidden variable that I am talking about, less a computer and more a writhing explosive underpinning to the makeup of the universe that seems uncertain and chaotic from the front side and looks orderly and predictable from the back. Spooky action at a distance is only confusing from the side that experiences distance, particle uncertainty only seem uncertain when you cannot see the fabric that twists together to make the particles and the places where they are not. I suspect that we will eventually be able to make models that allow for meaningful predictions that go much further than "measure closely and the quantum computer goes gonfable and spits out random data". It seems that true random behavior is abhorred which fits very well with concrete, "computational", models.
 — WcW, May 12 2016

Well, it would be lovely if you were right about that.
 — MaxwellBuchanan, May 12 2016

Wouldn't it? Seriously damnit; truth and beauty.
 — WcW, May 12 2016

Apparently, yes. Look up "quantum eraser".
 — MechE, May 12 2016

One thing that I still don't understand. Is it true that with a single photon you can't determine if it's behaving as particle or wave? .. because to have confirmation of a wave you need interference pattern which means a large amount of photons?
 — ixnaum, May 12 2016

 Quantum entanglement is a lot like marriage. The state of one spouse's correctness on any subject can be immediately be determined by the state of their other spouse's, even though no communication has taken place between them. If, for example, a man observes a tree falling in the forest and tells his wife, then it is immediately obvious that the tree did not fall, because his wife's opinion must collapse to the negative upon the revelation of her husband's informing her that it did.

Incidentally, this is why lesbian marriages should be impossible; they violate the laws of physics in that one partner must be wrong.
 — RayfordSteele, May 13 2016

 As we know, a "double-double" is an order at Tim's in Canada; your (usually) large coffee comes with 2 cream, 2 sugar already added.

 Combined with a simple dyslexic misreading of slit as 'silt', you have the image of a mad physicist peering into the cup, watching the Brownian motion, refusing to observe the wavicles in the second set of slits, contemplating what might be found at the bottom of the cup when the experiment is over.

 Tasty method of refusing to observe wavicles...unless the undulating coffee freezes in place as you take the Chi-Cheemaun to Manitoulin.

Please note that the social atmosphere aboard any public conveyance can be considered to be a vacuum for the purposes of this experiment.
 — Sgt Teacup, May 13 2016

 //One thing that I still don't understand. Is it true that with a single photon you can't determine if it's behaving as particle or wave? .. because to have confirmation of a wave you need interference pattern which means a large amount of photons?//

 No.

 First, you _can_ generate interference patterns with single photons. If you set up the double-slit experiment and fire one single photon through it, that photon will be detected hitting the screen at some point. You come back next week and fire another photon through it, and again it hits the screen at some point. If you do this over many many weeks, you will notice that the points where the photons hit make an interference pattern - bands of very few photon hits, and bands of many photon hits.

 The photon behaves like a wave when you look for wavelike properties (like, through two slits). It behaves like a particle when you look for particle- like properties (like, when it hits the screen).

 If you ask it to behave like a wave, but then set out to detect a particle, you will therefore detect a particle that could only have got there by acting as a wave.

 If you put your "passing photon monitors" on the slits, then you are looking for a particle at the slits, so you will see a particle. That will abolish the interference effect, because particles can't interfere.

 To come back to the original question: after the photon has gone through one of the slits (and has been detected as it goes by, so it's a particle), it will _thereafter_ behave just like any other photon. So, if you had a second double-slit (this time with no passing-photon- montitors) set up after the first double-slit, you would see interference in the second (un-monitored) double-slit but not the first (monitored) double slit.

Feynman was absolutely right when he said that the double-slit experiment is really the embodiment of quantum mechanics. Everything else is detail. Truly "understanding" what's happening in the double-slit experiment is like truly "understanding" a hypercube: however much you know about it, it's still not remotely graspable.
 — MaxwellBuchanan, May 13 2016

I belive the fundamental problem is that we still imagine that the photon takes a discrete course rather than that the photon exits in a field state not unlike the electron does, only this field state radiates away from the source until the photon field collapses. Instead of discreet points or pinpricks it is better to imagine that every photon emission is a spherical emission from the source that expands until it is collapsed. In one lightyear the photon field for an individual photon random emission is a sphere two light years across and could, thus be interacted with anywhere in a surface that is more than four square lightyears in size.
 — WcW, May 13 2016

 //the photon exi[s]ts in a field state not unlike the electron does//

 There's no real difference between a photon and an electron, but that's not the problem.

 The real problem is that the "field" (probability density function) can be made to collapse into a particle at one, and only one, point. Suppose I send a photon out from Earth. Its probability density function spreads out (not necessarily evenly, but it spreads) until it's, say, a light year across.

 Now I set up photon detectors in a big sphere around Earth, with a radius of one light year. One of those detectors will detect a photon - it will go click. But only _one_.

 Compare this with what we normally think of as radio waves. A radio station broadcasts a very weak "beep", and detectors in a 1 light-year sphere listen for the beep. Either all of the detectors will hear the "beep", or maybe only a few (if it's very weak), or maybe none - but it's not as if ONLY one detector can hear the "beep".

So, photons and electrons do _not_ behave like a classical field.
 — MaxwellBuchanan, May 13 2016

No, you see I meant exits, the vocabulary for emission is confusing. I wasn't trying to suggest that fields and photons shared other similarities than the peculiar way in which they propagate across what we measure as distance and time. The photon field is also prone to peculiar distortions in time which other fields are seemingly immune to. We need to stop teaching a vocabulary of the photon fired as a tiny cannonball though because that is clearly slowing how quickly people grasp photonics.
 — WcW, May 14 2016

 //stop teaching a vocabulary of the photon fired as a tiny cannonball//

We don't teach that, do we? Even in my day I don't think it was taught beyond primary school, if then. We're generally taught that photons have "wave/particle duality", which is still not perfect but is a better approximation. I presume nowadays that children are gently introduced to quantum mechanics in their early teens.
 — MaxwellBuchanan, May 14 2016

I think that still fails. The photon has features of all three; particle (mass), wave (frequency, polarity) and field (non discrete properties). This means that wave and particle still causes the student to imagine that each photon wiggles away in a specific vector.
 — WcW, May 14 2016

 [annotate]

back: main index