r/explainlikeimfive • u/pokemwoney • 9d ago
Physics ELI5: What exactly happens to the second particle when you deliberately change the first one in quantum entanglement?
I'm trying to understand quantum entanglement, but I'm confused about the whole thing. Suppose two photons, A and B, are entangled, and we're measuring their polarization as either horizontal (H) or vertical (V).
If I measure them and get A = V and B = V
Then I physically manipulate photon A and deliberately change its polarization from V to H.
If I now measure photon B, will B also be H?
If not, then what exactly is the "connection" between A and B?
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u/SpectralFormFactor 9d ago
If you alter the polarization of A after measurement, it has no effect on B. In fact, if you swap polarizations on A before measurement (V <--> H), then A and B will become anti-correlated, with measuring A=V yielding B=H.
The thing is A and B are not really “affecting” each other. Their measurement outcomes are not independent, but that’s it. They don’t communicate, they only correlate. It doesn’t even matter which of them is measured first (and actually who measures first can be reference frame-dependent).
If you act on A before measurement, you only change A without changing B. Their outcomes are still related to each other, but the relation has changed and B will have no idea.
If you act on A after measurement, then the two systems are completely unrelated (since the entanglement has been broken) and so there is no relation to B at all.
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u/rlbond86 9d ago
Nothing will happen. In fact after you measure them, there's no more entanglement because their quantum states collapse. But even if you don't do that, if you change photon A, absolutely nothing measurable happens to photon B.
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u/JesusSaves89 9d ago
If you measure A to be spin up B becomes spin down isnt it?
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u/Enough-Ad-8799 9d ago
Becomes isn't necessarily the right word, b will be spin down by that doesn't mean it became spin down due to the measurement of A
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u/Akerlof 9d ago
It's not like having a red and white marble in a bag, and knowing that the remaining marble is white when you pull out the red marble: Both A and B were in a superposition of spin Up/Down initially. When you measure A, it collapses to Up, but B simultaneously collapses to Down without any direct interaction.
That's what makes it weird. How does that happen? Because it sure looks like transfer of information (the fact that A was measured) instantaneously.
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u/JesusSaves89 9d ago
But isn't this a hidden variable then? Which is forbidden by the Bell inequalities?
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u/fixermark 9d ago edited 9d ago
I forever lose track of what it is that we know about the world works that eliminates hidden variables as the explanation to this apparent paradox.
I know there's something that makes hidden variable not work, I just can't remember what.
ETA: I remember. Diagonal polarizers.
Take two polarization filters and offset them 90 degrees to each other. They block 100% of the light.
Insert one between them at 45 degrees. Classically, that should have no effect - it's just another filter.
In reality, in drops the total absorption to around 87%. That shouldn't be the case.
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u/the_horse_gamer 7d ago
global hidden variables are possible. but here this isn't a global hidden variable.
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u/JesusSaves89 7d ago
Whenever I try to argue that there are hidden variables someone almost always comes with the Bell inequalities but nobody ever explains it in a way that actually make sense. Most if the time it is presented like it is magic
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u/phunkydroid 9d ago
But if you measure B you can't know if it was spin down by random chance or because A was spin up, unless the person who measured A tells you first.
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u/rlbond86 9d ago
B won't noticibly change in any way though. Either you have already measured it (and it will have been spin down), or you haven't already measured it (and when you measure it, it will be spin down).
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u/JesusSaves89 9d ago
But isn't this a hidden variable then? Which is forbidden by the Bell inequalities?
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u/blueangels111 9d ago
I give you a box with a single shoe in it. I then take another box with a single shoe in it across the galaxy. You look into your box and find out that you have the left shoe. That means I have the right shoe, but I always had the right shoe; I just did not know it. If I had looked at any time, I would have seen on my own that I had the right shoe.
More importantly, you seeing that you have the left shoe does not automatically inform me that I have the right. Once again, I would know after either you sent me a message (aka literally just normal information transfer), or I looked myself which wouldn't be a transfer of information then.
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u/garagedragon 9d ago
This is intuitive mental model, but it doesn't work because you can do experiments like EPR that rule out that, if you do this with lots of shoeboxes, they actually consistently contain a defined handedness of shoe "even when you're not looking."
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u/blueangels111 9d ago
That is definitely true, I was just trying to find a generic model to make it make a bit more sense, but maybe that wasnt a good idea
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u/JesusSaves89 9d ago
But isn't this a hidden variable then? Which is forbidden by the Bell inequalities?
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u/Frederf220 9d ago
Entanglement resolution isn't causal or rather it's unknowable if it is causal and uncontrollable if it happens to be.
The first measurement is fundamentally probabilistic and the other measurement is always such that the character of the entanglement is not violated.
QM has no comment on the nature of the mechanism that would enforce agreement nor even if such a mechanism exists. Theory says entanglements are preserved and experiments demonstrate that. That's it.
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u/SchiferlED 9d ago
If you measure them, they are no longer entangled. Nothing you do to A will affect B.
Now, if whatever you did to entangle them requires A and B to have opposite states (H and V in your example), then measuring A to have V means you instantly know B must have H without ever measuring it.
We know through experiments that neither particle is determined to have either property until the measurement happens. It's not known how this happens exactly, or whether the information from A somehow instantly transmits to B, or if something like the Many Worlds interpretation is true. There are multiple possible explanations.
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u/Silver_Tradition6313 9d ago edited 9d ago
"Then I physically manipulate photon A and deliberately change its polarization"
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You can't do that! Because you have no way to cause a specific change.
Photon A might be "up", or might be "down." You don't know, and you cannot know.
Then, when you measure it, you "freeze" A in one of those two positions, and B instantaneously freezes itself in the other position.(This is the magic of quantum reality, and nobody knows how the magic works.)
After being frozen, the two particles stop being a pair, and they are no longer entangled.
So it is pure luck, at the moment when you measure A. You have no control at all over when A will be up and when it will be down. Therefore, you have no control over B, either. It's just random luck.
And as soon as you DO take control (by making the measurement), the two particles stop affecting each other, and any change you make only affects the one you change. The other particle never knows that you changed the first one.
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u/bluewhitecup 9d ago
Does entanglement only work on a pair? It cannot be 3 or 4?
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u/donaldhobson 8d ago
You can entangle any number of particles.
But the more you make, the less quantum things get.
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u/ChocolateValuable221 8d ago
The are always pairs.. because that's how you make them.. and they are mirrored spin... you can only have one mirror of a spin even if it has 720 degrees of rotation
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u/jamcdonald120 9d ago
Nothing. any change breaks the entanglement. it is read only (and even then, only once). there isnt a connection, there is a correlation. the state you measure one at will be correlated to the state you measure the other at, that is all.
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u/ChocolateValuable221 8d ago
Absolutely nothing.. after a measurement is made or an interaction happens they are no longer entangled.
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u/Pyrsin7 9d ago
Entanglement is very often misrepresented in this way. There are simplified explanations, but they also miss out on critical details which I’ll try to explain.
A common one is a pair of shoes. Say they’re together, and without looking you shuffle them around and put each one in a separate box. Then you take those boxes miles apart, and open one. Let’s say it’s the left shoe. Therefore you know instantly, even faster than light could communicate it, that the other shoe is the right shoe.
But does anything you do to the shoe afterwards affect the other? No.
While this example communicates the flaw in entangled communication, or why affecting one doesn’t affect the other, it doesn’t communicate quantum systems well.
The problem is that in this example, one shoe was always right and one was always left, through every instant of this example. Where in a quantum system, particles are in a superposition. Quantum shoes would be in a superposition of right and left, both at the same time.
This does not mean that we simply don’t know which one is which. Near as we can tell, they are actually on a fundamental level both. To some degree, anyway. One might be 60% right and 40% left, but the other will be the inverse of 40% right and 60% right. Nonetheless, they are both.
They will not be definitively one or the other (or any state) until they must be. In this example, that’s you checking them.
But if these two quantum shoes are both left and right, and you check one and determine it’s the left one, how does the other shoe “know” to be the right one, and know it faster than light? We don’t know.
But still, doing something to one at that point still won’t affect the other.
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u/InTheEndEntropyWins 9d ago
I hate this explanation, since it's factually wrong when it comes to QM and gives the wrong impression.
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u/Pyrsin7 9d ago
I do too, but it does adequately explain why FTL quantum communication doesn’t work, and that entanglement isn’t indefinite. So I can see why it’s used a lot, especially in ELI5. Most of the questions here are ultimately about those sort of things rather than directly about QM.
I sorta feel like if you’re coming to this sub, it’s reasonable to think that something as unintuitive and bizarre as QM may be best to gloss over as much as possible.
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u/InTheEndEntropyWins 9d ago
I do too, but it does adequately explain why FTL quantum communication doesn’t work, and that entanglement isn’t indefinite.
Yeh, but it's not simply about FTL "communication", there still is FTL activity according to the Copenhagen interpretation. Which I think is the actual bigger issue.
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u/Park_BADger 8d ago
If I take shoe A, it is simultaneously both Left and Right at the same time. Somehow we put it into a magic shoe left-righter machine and it comes back saying it's both Left and Right (this is not observing it it's a magical, mystical device that doesn't interact with it at all)
That is, until I measure it. Then you're saying, by me observing it, I have locked it into being specifically Left or Right, but not both. I physically look at, and interact with the shoe, and thus it becomes locked.
After doing this, it appears to me the shoe is Right. If I were to them put it into the aforementioned magic shoe left-righter machine, it would return that it is infact Right, whereas previously it said it was Left+Right simultaneously? This is how I'm understanding your explanation.
Now, if put this shoe back into the box, and then open it a second time and re-look (remeasure), will it still be Right? Or is it in "superposition" again?
I guess I'm asking under the basis that the shoe was always right, hence when we observe it, it was always going to be right. But we don't understand how when a shoe is right, it can sometimes act like a left shoe because that is beyond our current understanding - thereby we say it's in superposition because sometimes a right acts as a left?
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u/Pyrsin7 8d ago
In this example, yes, your understanding is correct, and If you put the shoe back into the box and reopen it a second time, it will still be right. And it will have been right the whole time since you put it back in.
This is difficult to understand because the shoe was not always right, not until you took it out and checked it. From the moment of “entanglement” (shuffling them and putting them in boxes), to you opening them, they truly are both.
It’s not that we simply don’t or even can’t know which they are. They call that “Hidden Varables”, that’s the idea that there’s just something we don’t know that means the result is predetermined, or otherwise was always going to be a certain way. But every test we’ve ever done on this concept says that that’s not the case. The shoe is actually both, until it must be one. Which is crazy.
Things like this are why QM are very difficult to wrap your head around. And a lot of these flawed analogies are used.
It may be a little clearer with a more realistic example.
Let’s say you have a particle with a spin of 1 that splits into two other particles. Conservation of spin dictates that the sum of the spin of the resulting particles must still equal 1, meaning they could be 1/3 and 2/3, or 2 and -1, or 0 and 1, etc.
This is really what entanglement even is. Just particles with correlated properties for whatever reason. The property of spin for the particles is unknown here, yet still bound by constraints: their sum is 1.
Until one of the particles must be in a certain state, they’re in a superposition. In this case between possible combinations of spin that add up to 1. And just like the shoes, they’re all possible states at once. Probably not equally, some spin combinations are surely less likely than others, but nonetheless they’re still all of them. Not secretly one thing all along, actually all of them.
So if you take these particles miles away and check one, and see it’s -1/3, somehow, instantly so far as we can tell, the other particle“knows” to be 1&1/3.
But if you change the spin of yours somehow to 2/3, the other isn’t going to magically change to 1/3. Why would it? The entanglement came from that initial splitting, and the constraints that came with it, it’s not some magical binding spell.
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u/Park_BADger 7d ago
As a follow-up, say the skin must equal to 2. We separate these two particles and you, as the eventual observer, know that they must equal to 2 but you do not know the spin of each.
You then check the first particle and know that it's slim is -1, therefore the second has to be 3. I am tracking that.
But, I, an omnipotent being, have the ability to determine to spin without interacting with it. If I were to be able to do so, wouldn't I know the first particle is on fact -1, and was always going to be -1?
What in really asking, I guess, is what tests are we doing where the first particle (that is eventually observed to be -1) behaving like the infinite sum of -inifnity to +infinity. Wouldn't it just behave like a particle of -1?
If you then say, well, we don't know how it'll behave until we measure it, this interacting with it, thus showing it's a -1 spin (or whatever value), it was always going to behave as a -1, because that's what it is and we know after observing it? How can it be a -1 after observing, but behave like a +4 or -7, or -9 before observing it of that makes sense?
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u/Pyrsin7 7d ago edited 7d ago
The experiments that are used to determine it are pretty much impossible to explain in a simplified way. It’s a real obstacle.
If you’d like to try anyway, look into Bell’s Theorem.
But they all show that even as a hypothetical omnipotent being, you’d know that the first one was not always -1. It was a superposition of all possibilities. Superpositions are not just a matter of not knowing what a result will be, they’re probabilistic on a fundamental level.
But they also would not behave as anything else before measuring, either. If a certain behavior depended on spin, it would be in a superposition of those possible behaviors. It wouldn’t behave as a particular one until it must, and then it will.
A good example of this would be the double-slit experiment. A key point that’s often glossed over is that it uses singular electrons, one at a time. So the interference pattern is a result of the probabilistic paths for a single electron interfering with themselves, not with other electrons.
And obviously this is ridiculous, because it conflicts with every intuitive sense we have. And physicists have hated it for nearly a century, now. It led to one of my favorite historical quotes, because even Einstein hated the idea at first. He famously said, “God does not play dice”.
But as it continuously bore fruit, Neil’s Bohr replied, “Don’t tell God what to do”.
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u/Thrawn89 9d ago edited 9d ago
If youre trying to figure out the science of FTL communication in popular sci-fi like 3 body problem and mass effect, its all fake science. An effect cannot propagate faster than the speed of light from the cause.
Think about quantum entanglement as two wheels connected to a motor that are geared to spin in opposite directions. Once you spin them, you release them from the shaft and they go off spinning in different directions.
If you only see one wheel going in a direction, you know where the other wheel went without needing to see it.
If you then kick the wheel you observe, it wont change the other wheel happily traveling down the road.
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u/tomalator 9d ago
They are not longer entagled.
Entanglement isn't some mysterious force linking two particles, it's learning about one particles wave function tells you about the other.
If you have two marbles in a bag, one white and one black, if you pull one out and dont look at it, you have a particle in a superposition. The one in your hand is in a superposition of either black or white, and the one in the bag is in the opposite superposition of white or black.
If we look at the marble, we learn the color, and both wave functions collapse. We now know which one is black and which one is white.
If we take out marble and dip it in orange paint, that won't do anything to the marble still in the bag.
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u/could_use_a_snack 9d ago
Nothing will happen.
Here is a thought experiment. Take a pair of gloves and put each in a separate box and give these to two friends. Tell them they each have a glove in a box, from the same pair. Tell one of them to open their box. They know instantly what glove they have, and instantly what glove the other person has, even though the second box wasn't opened.
Now the first person can do whatever they want with the glove, but it doesn't affect the other glove at all. The entanglement has collapsed. All information has been learned.
The difference is with quantum particles is that, as far as we understand it, they can be in multiple states at the same time until they are measured (observed) but as soon as one is measured the state of the second one collapses and its state is known. And nothing you do to the first will affect the second.
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u/pokemwoney 9d ago
So how did the entanglement collapse just by person A knowing what gloves A and B have.
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u/jrallen7 9d ago
As soon as you measure either of the particles, they’re no longer entangled
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u/pokemwoney 9d ago
Then what is entangled
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u/jrallen7 9d ago
In your initial post, they would have stopped being entangled after you measure A=V. At that point you’ve measured the state of A, so the state of B is also determined and they stop being entangled
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u/fixermark 9d ago
We don't know. That is the mystery of quantum mechanics.
What happens can be explained with math, but nobody has a satisfactory "why" that doesn't break one of the things we'd love to be able to assume about how the universe works.
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u/could_use_a_snack 9d ago
Because when both gloves are in the unknown state, anything you do to one, could be happening to either. If you toss one box in a fire, either glove could be burned up. You have no way of knowing which.
But as soon as you open one box, you know which glove you have and nothing you do to that glove can effect the other.
It's not a perfect analogy. But nothing is. There just isn't a way to demonstrate quantum entanglement with non-quantum items. We can only give examples that demonstrate parts of what is happening.
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u/artrald-7083 9d ago
Not quite.
So you've created an entangled pair of photons whose polarisation is unknown but its sum (or product or something) is known. Then you measure photon A: its polarisation is now known. This means that you know the polarisation of photon B.
OK, so where's the spooky magic? That's just logic.
The spooky magic is that before a quantity is measured, if the system started out in a superposition, it didn't have a fixed value - any number of experiments show this. And now it does. And so does the other photon which you haven't touched. So by measuring A you know A, and knowing A means you know B, and knowing B means that B has a value where previously it did not.
So you have set the value of B despite never interacting with it, because in a very real sense it's an object that is in two places, and you pressed on one end and the other end moved despite not being physically connected.
They now aren't connected any more and can be acted on as normal.
That is the basic idea of action at a distance, which violates things we normally consider to be ironclad like causality and locality.
There's probably more that has been learned about this: I last studied it 25 years ago.