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  • If you take a laser and shine it at a wall with two holes in it, you have the famous

  • double slit experiment - where waves coming through two slits interfere with each other

  • to be bright in some places on the wall, and dark in others.

  • And this works with quantum particles, too, since they behave in wave-like ways: send

  • a cat towards the slits, and it'll show up at a point on the wall.

  • Send a bunch of cats, and their accumulation reveals the same interference pattern as a

  • light wave .

  • Now imagine you add another, competing double slit experiment, with another cat, that shares

  • one of the slits with the first setup.

  • Of course, if you send the second cat over and over towards these two slits , the points

  • where it hits the wall will give a similar interference pattern.

  • And at this point here for one cat, and this point here for the other, those cats never

  • show up, no matter how many times you send them through the slits.

  • Their wavelike behavior causes what's calleddestructive interference”; if cats were

  • light, these points would be in cat-darkness.

  • But weird things happen if you send both cats at their double slits at the same time.

  • The top cat goes through the top and middle slits and then towards the wall, and the bottom

  • cat goes through the bottom and middle slits and then towards the wall; and things would

  • again go as expected, of course, except that the cats get in each others' way going through

  • the middle slit.

  • Maybe the slit's too small for two cats to fit through simultaneously, or maybe, yeah!

  • maybe one cat is actually made of antimatter so if both go through the middle slit, they

  • annihilate each other and never make it to the wall.

  • Either way, the situation is now this: the cats traverse the slits in a quantum superposition

  • of top cat top/bottom cat middle, top cat top/bottom cat bottom, and top cat middle/bottom

  • cat bottom.

  • There's noboth cats in the middlein the superposition, since the cats can't

  • traverse the middle slit together.

  • And since the superposition is missing theboth cats in the middleoption, the

  • interference patterns change and it's possible for the cats to end up in the places on the

  • wall where before there was cat darkness.

  • This isn't at all surprising for waves - I mean, different amounts of wave coming through

  • the slits means a different interference pattern.

  • But there's something weird about this when particles are involved.

  • To see why, remember that individually, the cat darkness arose because the cat's wave-particle

  • superposition went through both slits and interfered with itself to result in zero probability

  • of the cat ending up there.

  • So if the bottom cat DOES end up there, it must not have been able to interfere with

  • itself, so it must not have gone through both slits, so there must have been something blocking

  • one - the other cat, in the middle.

  • And if the top cat ends up in its previously cat-free spot, then it must not have gone

  • through both slits either - the bottom cat must have been blocking the middle slit.

  • This wouldn't be a problem, except that when you actually do this experiment , some

  • of the time BOTH cats end up in the previously cat-free spots.

  • And we know they can't both have gone through the middle slit, because they would have annihilated

  • each other - so each cat must have been blocked from going through the middle slit by the

  • other cat having gone through the middle slit, simultaneously.

  • Which of course seems impossible, and is why this situation has been called a paradox . And

  • it's certainly thought-provoking if you like to think about local realism or contextuality

  • or weak measurement values or the interaction between classical logic and quantum mechanics.

  • But it's not really that surprising, as long as you believe that quantum particles

  • can be in superpositions (which happens all the time and has been incredibly well experimentally

  • confirmed).

  • As we said earlier, the two cats pass through the slits in the superpositiontop middle

  • plusmiddle bottomplustop bottom”, which include both apparently necessaryblockages

  • of the middle slit by one cat or the other, and it's this superposition that results

  • in the changed interference pattern that allows for the possibility that both cats simultaneously

  • end up in the previously cat-dark locations.

  • If all of this seems a bit weird - yeah, it is!

  • But it's worth remembering that weirdness and paradox are not one and the same . And

  • the quantum cat/antimatter-cat double double-slit experiment is fully consistent with the predictions

  • and experimental results of quantum mechanics.

  • Sometimes the universe is just weird!

  • If you (or somebody you know, who maybe you're looking for a present for?) are the kind of

  • person who likes to dive deep into quantum paradoxes, or the cool things waves do when

  • they interfere, then you should definitely check out Brilliant.org, this video's sponsor,

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If you take a laser and shine it at a wall with two holes in it, you have the famous

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B2 slit quantum middle superposition bottom interference

Hardy's Paradox | Quantum Double Double Slit Experiment

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    Summer posted on 2021/02/18
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