Eli5 what is bells inequality and EPR paradox
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Well, not sure I could explain this to a five year old, but anyways… Let’s look at the Heisenberg uncertainty principle. Back when people were trying to figure quantum mechanics, there was some uncertainty about what that meant. Did an uncertainty in position mean a particle was in many places, or it was in one place but we just had no way of knowing until we went out and measured it. The second way is more classical, and relies on so-called “hidden variables” – things that exist, but that we can’t measure. It turns out that the existence of such hidden variables does lead to measurable predictions. Basically, Bell’s inequality lets you test this – if you combine probabilities of carefully set up events in various ways, you can rule out *any* hidden variable theory.
The EPR paradox is often called “spooky action at a distance”. Let’s say I generate a pair of electrons that go shooting off in opposite directions. To conserve angular momentum, those electrons have to have opposite spins, but in general which one has what spin is totally random, they just have to be opposite. Now wait for a while. If someone measures the spin of the first electron, then someone measuring the spin of the electron at the same time *has* to get the opposite result. But this is strange! Measuring one electron all of a sudden set the spin of the other electron, but if you want until the electrons are far apart and measure at the same time, either measuring one electron sends information to the other electron *faster than the speed of light*, or the electrons somehow already knew what their spins are – a hidden variable. However, if we find a system that violates Bell’s inequality, we can rule out those hidden variables.
So what’s going on? Well, Einstein thought that quantum mechanics was incomplete. What’s the answer? I don’t know! I do know that it’s hard to explain this to graduate students, let alone 5 year olds…
Well, not sure I could explain this to a five year old, but anyways… Let’s look at the Heisenberg uncertainty principle. Back when people were trying to figure quantum mechanics, there was some uncertainty about what that meant. Did an uncertainty in position mean a particle was in many places, or it was in one place but we just had no way of knowing until we went out and measured it. The second way is more classical, and relies on so-called “hidden variables” – things that exist, but that we can’t measure. It turns out that the existence of such hidden variables does lead to measurable predictions. Basically, Bell’s inequality lets you test this – if you combine probabilities of carefully set up events in various ways, you can rule out *any* hidden variable theory.
The EPR paradox is often called “spooky action at a distance”. Let’s say I generate a pair of electrons that go shooting off in opposite directions. To conserve angular momentum, those electrons have to have opposite spins, but in general which one has what spin is totally random, they just have to be opposite. Now wait for a while. If someone measures the spin of the first electron, then someone measuring the spin of the electron at the same time *has* to get the opposite result. But this is strange! Measuring one electron all of a sudden set the spin of the other electron, but if you want until the electrons are far apart and measure at the same time, either measuring one electron sends information to the other electron *faster than the speed of light*, or the electrons somehow already knew what their spins are – a hidden variable. However, if we find a system that violates Bell’s inequality, we can rule out those hidden variables.
So what’s going on? Well, Einstein thought that quantum mechanics was incomplete. What’s the answer? I don’t know! I do know that it’s hard to explain this to graduate students, let alone 5 year olds…
Well, not sure I could explain this to a five year old, but anyways… Let’s look at the Heisenberg uncertainty principle. Back when people were trying to figure quantum mechanics, there was some uncertainty about what that meant. Did an uncertainty in position mean a particle was in many places, or it was in one place but we just had no way of knowing until we went out and measured it. The second way is more classical, and relies on so-called “hidden variables” – things that exist, but that we can’t measure. It turns out that the existence of such hidden variables does lead to measurable predictions. Basically, Bell’s inequality lets you test this – if you combine probabilities of carefully set up events in various ways, you can rule out *any* hidden variable theory.
The EPR paradox is often called “spooky action at a distance”. Let’s say I generate a pair of electrons that go shooting off in opposite directions. To conserve angular momentum, those electrons have to have opposite spins, but in general which one has what spin is totally random, they just have to be opposite. Now wait for a while. If someone measures the spin of the first electron, then someone measuring the spin of the electron at the same time *has* to get the opposite result. But this is strange! Measuring one electron all of a sudden set the spin of the other electron, but if you want until the electrons are far apart and measure at the same time, either measuring one electron sends information to the other electron *faster than the speed of light*, or the electrons somehow already knew what their spins are – a hidden variable. However, if we find a system that violates Bell’s inequality, we can rule out those hidden variables.
So what’s going on? Well, Einstein thought that quantum mechanics was incomplete. What’s the answer? I don’t know! I do know that it’s hard to explain this to graduate students, let alone 5 year olds…
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