Why is quantum mechanics weird? The bomb experiment

i have done quite a few videos in which i've tried to demystify quantum mechanics because many things people say are weird about quantum mechanics aren't weird like superpositions or entanglement not with no really they're not weird just a little unintuitive but now i feel like i may accidentally have left you with the impression that quantum mechanics is not weird at all but of course it is and that's what we'll talk about today before we talk about quantum mechanics big thanks to our tier 4 supporters on patreon we greatly appreciate your help and you too can help us keep this channel going go check out our patreon page or support us right here on youtube by clicking on the join button just below this video now let's talk about quantum weirdness first i remind you what's not weird about quantum mechanics though you may have been told it is in quantum mechanics we describe everything by a wave function usually denoted with the greek letter psi the wave function itself cannot be observed we just use it to calculate the probabilities of measurement outcomes for example the probability that the particle hits the screen at a particular place some people say it's weird that you can't observe the wave function but i don't see anything weird with that you see the wave function describes probabilities it's like the average person you never see the average person it's a math thing that we use to describe probabilities the wave function is like that another thing that people seem to think is weird is that in quantum mechanics the outcome of a measurement is not determined calculating the probability for the outcome is the best you can do that is maybe somewhat disappointing but there's nothing intrinsically weird about it people just think it's weird because they have beliefs about how nature should be then there are the dead on the life cats a lot of people seem to think those are weird i would agree but of course we don't see that on the live cuts but then what's with particles that are in two places at the same time or have two different spins at the same time we do see those right well no we actually don't see them when we see a particle when we measure it it does have definite properties and not two at the same time so what do physicists mean when they say that particles can be at two places at the same time it means they have a certain mathematical expression called a superposition from which they calculate the probability of what they observe a superposition is just a sum of wave functions for particles that are in two definite states yes it's just a sum the math is easy it's just hard to interpret what does it mean that you have a sum of a particle that's here and a particle that's there well i don't know i don't even know what could possibly answer this question but i don't need to know what it means to do a calculation with it and i don't think there's anything weird with superpositions that just sums you add things like you know two plus two okay so superpositions of particles which are in two places are just a flowery way to talk about sums but what's with entanglement that's non-local right and isn't that weird well no entanglement is a type of correlation non-local correlations are all over the place and everywhere they're not specific to quantum mechanics and there's nothing weird about non-local correlations because they are locally created see if i rip a photo into two and ship one half to new york then the two parts of the photo are now non-locally correlated they share information but that correlation was created locally so nothing weird about that entanglement is also locally created suppose i have a particle with a conserved quantity that has value zero it decays into two particles now all i know is that the shares of the conserved quantity for both particles have to add to zero so if i call one of the shares x then the other one has to be minus x but i don't know what x is this means these particles are now entangled they are non-locally correlated but the correlation was locally created now entanglement is in a quantifiable sense a stronger correlation than what you can do with non-quantum particles and that's cool and it's what makes quantum computers run but it's just a property of how quantum states combine entanglement is useful but not weird and it's also not what einstein meant by spooky action at a distance check out my earlier video for more about that so then what is word about quantum mechanics what's weird about quantum mechanics is best illustrated by the bomb experiment the bomb experiment was first proposed by eliza and weidman in 1993 and it goes as follows suppose you have a bomb that can be triggered by a single quantum of light a single photon the bomb could either be life or a dut you don't know if it's a dirt then the photon doesn't do anything to it if it's life boom can you find out whether the bomb is live without blowing it up seems impossible but quantum mechanics makes it possible that's where things get really weird here's what you do you take a source that can produce those single photons then you send those photons through a beam splitter the beam splitter creates a superposition so a sum of the two possible paths that the photon could go to make things simpler i'll assume that the weights of the two paths are the same so it's 50 50. along each possible path there's a mirror so that the path meet again and where they meet there's another beam splitter if nothing else happens that second beam splitter will just reverse the effect of the first so the photon continues in the same direction as before the reason is that the two paths of the photon interfere like sound waves interfere in the one direction they interfere destructively so they cancel each other out in the other direction they add together to 100 percent we place a detector where we expect the photon to go and call that detector a and because we'll need it later we put another detector up here where the destructive interference is and called that detector b in this setup no photon ever goes to detector b but now now we place the bomb into one of those paths what happens if the bombs are dot that's easy nothing happens the photon splits takes both paths recombines and goes into detector a as previously what happens if the bombs live if the bombs live it acts like a detector so there's a 50 chance that it goes boom because you detected the photon in the lower path so far so clear but here's the interesting thing if the bomb is live but doesn't go boom you know the photons in the upper path and now there's nothing coming from the lower path to interfere with so then the second beam splitter has nothing to recombine and the same thing happens there as of the first beam splitter the photon goes both paths with equal probability it is then either detected at a or at b the probability for this is 25 each because it's half of the half of cases when the photon took the upper path in summary if the bombs live it blows up 50 percent of the time 25 percent of the time the photon goes into detector a 25 percent of the time it goes into detector b if the photon is detected at a you don't know if the bomb's life or a dead because that's the same result but here's the thing if the photon goes to detector b that can only happen if the bomb is life and didn't explode and that means quantum mechanics tells you something about the path that the photon didn't take that's the sense in which quantum mechanics is truly non-local and weird not because you can't observe the wave function and not because of entanglement but because it can tell you something about events that didn't happen you may think that this can't possibly be right but it is this experiment has actually been done not with bombs but with detectors and the result is exactly as quantum mechanics predicts this video was sponsored by brilliant which is a website and app that offers interactive courses on a large variety of topics in science and mathematics watching videos is fun and gives you an idea what quantum mechanics is all about but if you really want to understand what's weird about quantum mechanics you have to actively engage with the subject brilliant is a great place to do that all their courses are interactive so you are challenged with questions and can check your understanding along the way for this video for example i recommend their courses on linear algebra and quantum objects to support this channel and learn more about brilliant go to brilliant.org sabine and sign up for free the first 200 subscribers using this link will get 20 off the annual premium subscription thanks for watching see you next week

Channel: Sabine Hossenfelder

Views: 448,289

Keywords: quantum mechanics, quantum weirdness, quantum theory, why is quantum mechanics weird, quantum, bomb experiment, quantum mechanics bomb experiment, physics, science, superposition, entanglement, hossenfelder, science without the gobbledygook

Id: RhIf3Q_m0FQ

Length: 10min 40sec (640 seconds)

Published: Sat Aug 28 2021