My husband is watching YouTube and there’s a man discussing quantum entanglement.
His description: There are two particles. They can be either green or red, but they are both colors until they’re measured. Once you measure one, though, it automatically determines that the other is the same. No matter how many times you measured, or how far you separated the particles, the two would always be the same color.
Why does one being one color guarantee that the other one would be? How do they “know” to always be that color? And what sort of implication does that have for science/real world, other than being really cool?
In: Physics
A high level description could be “particles who’s state cannot be described independently of the state of each other.” In fact, that’s the tagline on Wikipedia. The fact is, in the quantum scale, things can behave very strangely. So much so that under certain circumstances, we can’t guarantee an outcome no matter how much we know about the initial state. Unlike in classical mechanics, if we know the speed, direction, and height of a thrown ball, we can guarantee where it will land. Sometimes, the best we can do in the quantum realm is to provide odds of outcome A and outcome B.
Particles may become entangled under a variety of methods which can lead to all sorts of behaviors. The most common method actually yields two particles that will always have opposite spin, not the same. However, you could make a pair that were entangled together in the same state.
I’m going to give an analogy first. Like all analogies, it is imperfect, but I think it will go a long way to answering your question. Let’s say, I hold out in front of me a red ball and a green ball. Then, I turn my back and put the balls into different boxes and close the lids. I shuffle them around a bit, then hand one each to you and your friend. You are instructed to travel as far apart as possible, to opposite sides of the galaxy and only then, open the boxes. You do so, look inside, and discover you have the red ball. You know INSTANTLY that your friend has the green ball without ever actually sending or receiving any information from them (assuming I play fair)
In this way, it is not entanglement which causes the particles to measure alike or differently. They are particles that are made to be either mutually exclusive or mutually inclusive (probably not the best choice of words, but close enough). Nor does it matter that you know they have the green ball because you can do nothing about it, nor can you do anything with that information. You could meet up later and compare notes and discover that your knowledge was correct, they did have the green ball, but it’s little more than a parler trick. Maybe you could do a magic show where you open your box, then predict what’s in theirs if you hid your tomfoolery well enough.
Let’s talk about an actual example now. Say enough energy concentrates in one location, that the energy is spontaneously converted into matter (via E=mc², we know that matter and energy are more closely related than at first glance it seems). They might have some spin, but if their spin was in the same direction, then the total, universal angular momentum would be changed a little in that direction. It’d be like a free energy machine that just generated momentum from nothing. So of course the spins must be in opposite direction. If one of them is spin up, you know instantly the other must be spin down, otherwise, the conservation of angular momentum would have been broken.
It gets weirder than that, though, in the quantum realm, because the spin is not in a pre-decided direction. It is not until the particle is measured that it’s spin is true in that direction. Before measuring, it’s spin is equally likely to be measured in any direction and so must be in all directions at the same time. It’s so bonkers it’s hard to wrap your mind around even for a seasoned physicist, let alone the eli5 level. However, if you can trust that we have been able to do experiments that prove there’s no predetermined information. There’s no map of which direction the particle’s spin will measure depending on the direction of the measurement device or anything like that. Hopefully you can just accept that the spin of the particle is not in a single, defined direction until it is measured.
But that means its partner was not in a single, defined direction either. If you measure particle A, you have actually collapsed the superposition of both A and B at the same time. It seems like you have reached a hand out across space and affected the other instantaneously. Faster than the speed of light. However, just like before, that’s all you can do. You cannot use this to communicate. Nor can you use it to affect the other particle.
Moreover, there’s something in that description that isn’t quite right. Or could be potentially misleading. If you continue measuring the particle in the same axis. (You don’t move your measurement apparatus) Then you will get the same measurement over and over and over again. Always spin up or spin down. (I’m sorry, I seemed to have skipped over that part. This isn’t really an opportune place to put it, but nowhere else is better: the particles can’t measure anything BUT up or down. No left, right, forward, or backwards. It will either align with the detector (Up) or disalign (Down). This is important for how we proved that there is no hidden information for the particles to know how to be measured. Also, the entanglement can only be observed if they are measured on the same axis. If you place both particles in detectors aligned with each other, they will always measure opposite. If you rotate one detector sideways, then all bets are off. It’s random. If they are perfectly misaligned (180⁰ apart) then they will actually be guaranteed to measure both particles the same (both up or both down)).
Anyway, sorry for that. So I was saying, if you measure the same particle repeatedly in differing directions, you no longer can guarantee they will both be opposite in those directions, even if you always keep the two detectors perfectly aligned to each other. Only the first measurement is guaranteed to be opposite (or the same if you entangled the particles to be that way). Remember, you can’t actually affect the other particle other than collapsing the superposition, which simultaneously breaks entanglement, so any further measurements have no correlation other than the correlation added by the way you measure. (Like measuring in the same direction as particles don’t typically spontaneously switch spin).
And even collapsing the superposition can’t be used for anything because how would they know? You can’t probe a superposition without measuring it, which collapses the superposition. So how would you know if you just collapsed it, or it was already collapsed by your partner in the other lab measuring this particle’s twin? You can’t. You can’t do anything or know anything other than predict the other particle’s spin and only if measured in the same axis.
At the moment, no, it’s not useful for anything but a parler trick. We’ve been able to design experiments which confirm some of our theories about quantum mechanics, and it has opened the road for new theories, furthering our understanding, and it has also just brought more questions that we don’t know how to answer. But for the moment, and almost certainly forever, it’s just a cool trick and nothing more.
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