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What Einstein missed: The EPR paradox

By John Fish
This paradox calls into question two of the most fundamental pieces of quantum mechanics: superposition and entanglement.

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Video transcript

imagine being able to talk to someone 'we instantly there's been a lot of discussion about how quantum entanglement could result in faster-than-light communication sounds great right this guy didn't think so Albert Einstein discovered the speed limit of the universe the speed of a light he showed that nothing even information could go faster than light if that's true why do people say that quantum entanglement goes faster than the speed of light we're getting a bit ahead of ourselves let's introduce a few quantum concepts that are going to make this easier to understand if I flip the croydon in a classical world we get either heads or tails in the quantum world we get heads and tails it's as if the coin never stops spinning we call this a superposition when an object is in all possible states at once it's in a superposition now when we measure a superposition click the coin stop spinning we measure one of the possible outcomes we call that collapsing the superposition observing collapses superposition an example of this on the quantum scale is the spin of a particle you can think of the spin of a particle has just been another property that we can measure up down left right as tails in the quantum world our particle is in a superposition of all possible spins before we measuring superposition is weird but it's not the weirdest thing that quantum world has to offer let's talk about quantum entanglement quantum entanglement is a prediction made by the math of quantum mechanics that says that you can entangle two particles so that their properties are always correlated to give an example of classical entanglement let's take a look at these two golf balls one's orange and one's white I'm going to put them in this bag and I'm going to shake it up now without looking I'm going to take one out we're going to put the bag behind my back and as soon as I look at my golf ball it's white I know that the one behind my back is orange but I didn't have to measure it and that makes sense because the colors of the balls are predetermined this is how Einstein tried to justify quantum entanglement Einstein was thinking about the spin of two entangled particles when you measure the spin of a particle it can either agree or disagree now our two particles are in superposition meaning that when we measure them they should have equal probability to either agree or disagree with our measurement so if we have two particles it makes sense that half the time they both should be the same so they would both agree or both disagree and half the time one no degree in the other would disagree or one would disagree and the other would agree so 50% of the time our results will be correlated but that's not quite what the experiments show the experiments show 100% correlation that is when one of the particles agrees the other one disagrees on one of the particle disagree the other agrees and this is what the mathematics shows so let's think about this two measurements that are supposed to be completely random are somehow entangled so that they're always correlated that's a little spooky the way that Einstein explained it is he said there was some variable for the particles so that no matter what direction we measure it in there is some variable that predetermined the spin just like the colors of our golf balls were predetermined the measurement is predetermined as well seems fine right problem solved well John Bell came and he devised a test that would show definitively whether there was a hidden variable or not and experimentalist conducted that test and it turns out there is no hidden variable so now we're back to where we were the only way that we can explain quantum entanglement is with the math of quantum mechanics which Einstein and many others said that if there was no hidden variable information must travel faster than the speed of light so quantum mechanics is incomplete this is what we call the EPR paradox and many science fiction writers have misinterpreted it as saying that we can have faster than light communication through quantum entanglement let's talk about why this isn't the case what Einstein didn't think of it here's a device that I built to explain this what you're looking at is two particles one on the left and one on the right the two LEDs inside the particles are just representing the possible outcomes of measuring the particle dead or alive 1 or 0 red or blue it doesn't matter the point is that these two particles are entangled and we can see this by measuring so if we measure just 1 we see that the other one is the immigrants and if we had measured it in the opposite so if the one on the left was blue then we see the one on the right turns red so they're in tanked here's the fun part in between them I've put galaxies so they're light-years apart and yet they're still correlated they're still entangled we can see that so if this is true then how do we explain that information is traveling faster than light because obviously if they're light years apart and we measure them at exactly the same time and they're opposite some information must have traveled faster than light to get here or has it let's say that we gave one of our entangled particles to Einstein and the other to our fish friend and we sent them galaxies apart in this example the LEDs represent the spin of the particle whether it agrees or disagrees with the measurement if Einstein makes a measurement he knows instantly what the measurement of our fish friends particle will be the superposition is collapsed but here's the key to everything our fish friend doesn't know that the spin of his particle is no longer in superposition because you can't measure whether a particle spin is in superposition or not because measurement destroys a superposition so there's no way for our fish friend to know that Einstein measured his particle as far as our fish friend or Einstein can tell their measurements are just coin flips the only correlation shows up when they compare their measurements which they can only do classically but obviously this classical comparison isn't taking advantage of any spooky quantum effects and it follows the universes speed limit the speed of light even though Alice and Bob could be measuring at exactly the same time light-years apart and still have correlated results there's no information traveling between them and as a result we're not violating relativity it also means though that we can't use quantum entanglement communication so unfortunately looks like science fiction is going to remain fiction at least for now