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
Because “they are two particles” is an oversimplification. They are one quantum system, until they are disturbed, for example by measuring them. It’s a very complex concept, so simplification seems like a good idea, but too much simplification eliminated essential facts.
At some point in time it is determined that two particles have the same “vibe” and it stays that way forever no matter how far apart they go. Kind of like soulmates 🫶🏼
Quantum entanglement is pretty wild. Basically, you’ve got two particles that can be either green or red. But until you measure them, they’re both colors at the same time. Once you measure one and see it’s green, the other one will be green too, no matter how far apart they are. They’re linked in a way that if you know the state of one, you instantly know the state of the other.
Why this happens is still a bit mysterious, but it’s because they share a connected state. This has big implications like super secure communication (quantum cryptography) and super-fast computers (quantum computing). It also challenges our basic ideas about how the world works, showing that things can be connected in ways we can’t easily see or explain.
basically, you start with 1 particle that you put into a superposition between red and green, you dont know if its red or green, and the math behind it says the particle isnt secretly definitively one or the other
Now you make another particle interact with that one so the new particle will be red/green depending on the other one (which even it doesnt know)
These particles are entangled now. unless they are disturbed, when 1 collapses, the other will for sure collapse to the corresponding state.
We have no idea how the particles communicate. it could be that they can “talk” to each other, but if they can, it has to be faster than light. it could be that there is some hidden variable that both now share, but if it is, we have no idea what this hidden variable is. This has huge scientific implications because no matter what the answer is, we are wrong about something that we currently think is true.
This has major implications for the real world, the biggest practical one being cryptography. Right now, if you want to connect to amazon, you have to use an asymmetric encryption protocol to exchange keys with amazon, but in a quantum network you could exchange entangled qbits instead. when these collapse both you and amazon will have the same random sequence of bits, which you can use to encrypt messages. no one else can intercept these bits since the collapse is random.
One pop-sci implication that is actually impossible is FTL communication. its tempting to say “Oh, just entangle 2 qbits, and then I set mine to red, so yours automatically flips, and vice versa, now I can send messages!” But this is 100% impossible. the act of setting a qbit to a value breaks existing entanglements.
Because all physical interactions follow conservation laws. If you have a good enough understanding of how two particles interact, you can predict the outcomes following an actual experimental interaction. For example, if you shoot a photon into a fancy crystal (a bean splitter), and it is split into two lower energy photons, you know that the total spin of the system must be conserved. For such a pair of photons in this example, the total spin must be zero. Therefore, if you measure the spin of one of the photons, and get a spin of 1/2, you then “instantaneously” know the spin of the other particle must be -1/2.
Nobody has talked about how particles become entangled, which is important for understanding why they behave as they do. The key is conservation.
Particles have certain potential properties. The properties of a particle can affect those same qualities of any other particle it may interact with. These properties have relationships with one another that must be conserved throughout the interaction.
So, because the two particles interacted in the past, their future property states must always be equivalent for any observer in order for the laws of conservation to be maintained.
It is a counter-intuitive idea because, with the inclusion of quantum uncertainty, we appear to be able to *change* the action of one particle by changing how we observe its
entangled partner. In reality these states are set by that initial interaction, not our observation.
Conceptually quantum entanglement is fairly simple and a lot of the “weirdness” comes from a lack of understanding of what is going on.
Imagine you have 2 boxes that each weigh one pound. You know that one of the boxes contains 1/2 a pound of feathers and 1/2 a pound of corn. The other box contains 1/2 of a pound of lead and 1/2 of a pound of stone. You are handed those boxes but are not told what is in either box.
From your perspective, both the boxes and everything in them are now entangled. Before you open a box, both boxes could contain either feathers and corn or lead and stone. Because you have no way of determining which box is which without opening them, both boxes are “mystery boxes” to you before you open them.
You now open one of the boxes and see a layer of feathers in it. Both boxes instantly stop being mystery boxes. The box you opened is the feather/corn box and the box you didn’t open is the lead/stone box – and this is true regardless of where the boxes are in the universe.
This isn’t a particularly meaningful concept in your everyday life. Its meaning comes from the fact that there are certain ways that you can interact with those boxes that will reveal what IS NOT inside without revealing what is actually inside of the box. For example, lets say you had 100 boxes and 1 of them contained drugs. You could walk a drug dog around some of the boxes and, if the drug dog didn’t signal, then you would know that drugs were not inside of those boxes. Despite the drug dog sniffing the boxes, all 100 of those boxes remain “mystery boxes” to you, but you have gained a lot of information about the boxes that the dog sniffed. IE, the boxes remain entangled despite you having extracted information about their contents.
Conceptually, being able to tell what IS NOT inside the box is useful in certain mathematical functions. For example, computers operate by telling you what something IS. This is good for most general computing tasks, but is terrible for trying to break cryptography. Quantum computers can use entanglement’s ability to tell you what something IS NOT to easily break cryptographic functions by asking simple questions like “is the decoded number bigger than 100?”
If the answer that comes back is no, then you’ve narrowed your range of possible answers as to what the thing is from infinity (which will take your normal computer trillions of years to solve) to 100 (which can be solved in a fraction of a second).
Outside of that, there are some philosophical implications of entanglement if you believe that the entangled particles truly don’t take on a definite form until they unentangle. That being said, that philosophical interpretation of both quantum theory and quantum entanglement isn’t necessary for either to work.
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.
I’m not sure I’m asking the correct question but what benefit is this to the nature of the universe? Is it necessary for the universe to function?
Quantum Entanglement is really fucking weird and not fully understood yet. The idea is that if you do some special trickery to two or more atoms, they become sort of “linked” together in such a way that all their properties are now related to their partner’s properties. The why is even more complicated so we’ll leave that there.
Now that you have 2 or more atoms linked together, you can move them all around in space and different speeds and any other way you can think of. No matter how far apart these partners are, you will ALWAYS be able to determine the state of the other simply based on the one in front of you.
To be clear, this is not a transference of information. It’s more like we’re deducing from really far away.
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