Placeholder Image

Subtitles section Play video

  • 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, this video's sponsor,

  • which has courses and quizzes about all sorts of fascinating wave mechanics, and quantum

  • mechanics, and math puzzles, and more.

  • Unlike a youtube video, Brilliant gives you the opportunity to actually practice and hone

  • your problem solving and scientific thinking skills, for school, work, or because understanding

  • the universe is awesome!

  • And if you go to you can get 20% off a premium subscription

  • to brilliant for yourself or for somebody else as a gift - it's non-materialistic

  • AND arrives instantly!

  • Again, that's for a deeper understanding of the weird but

  • true physics that underlies our universe, for you, or your friends and loved ones.

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

Subtitles and vocabulary

Operation of videos Adjust the video here to display the subtitles

B2 slit quantum middle superposition bottom interference

Hardy's Paradox | Quantum Double Double Slit Experiment

  • 5 0
    Summer posted on 2021/02/18
Video vocabulary