If I have two legos, always put them in opposite orientations, then walk away with a random one, being careful to not change its orientation, are the two legos "linked"? When I look down at the one in my hand, I will know the other ones orientation. How is the different with entangled particles? Is "entangled" just a way to say "still related because it's undisturbed"?
If you are referring to violations of Bell's theorem, then what they show is that hidden variables are incompatible with locality. In other words, non-local hidden variable theories are compatible with these violations and have not been shown to be false.
I'm being a little loose about casualty but that's because I don't want to write extra paragraphs about what "simultaneous" means.
In your latest comment, you say that "The whole point is to show that something non-local is happening". I'm not sure what thing's "whole point" you're referring to, but I suspect in the context of our conversation you're talking about the "whole point" of the implications of violations of Bell's inequality. But in that case, it's also not true that the point is that "something non-local is happening". There are QM interpretations that are entirely local, and compatible with the observed violations of Bell's inequality.
I found this paper to be a helpful breakdown of how the matter might be resolved, with some views embracing locality, and some rejecting it (some parts of the paper went over my head): https://arxiv.org/abs/1503.06413
People usually assume locality. I was talking about local hidden variable theory.
I was using unclear language to say something true, which is different from saying something false.
> There are QM interpretations that are entirely local, and compatible with the observed violations of Bell's inequality.
They are either breaking locality or doing something equivalent. This is getting into metaphysics, not physics, isn't it? Unless a conversation is deep into the weeds of quantum physics, I feel comfortable making answers that assume a particular commonplace interpretation.
We probably are into metaphysics. We're talking here about different theories that are all compatible with observations to date. Theory selection is very much a philosophical pursuit, though it's something physicists will do to.
Anyway, my main claim is that, as I understand it, the most popular or common QM interpretation (Copenhagen) embraces locality, so it's incorrect to say that "They are either breaking locality or doing something equivalent".
Here's an example of that from an article by Reinhard Werner ('What Maudlin replied to'), arguing (as I understand him) for locality. Some particles are entangled, separated, and then one particle is changed so as to impact (via entanglement) the other. However, the change on one to the other isn't real until later. "The state change only becomes effective when the results from the two labs are brought together and are jointly analyzed, which can happen centuries later."
In the other paper I linked you to, it has the realists (such as hidden variable theories) rejecting locality, with the operationalists (which, as I understand it, is the most common view) embracing it.
Note that if you have two observers a light-second apart, and they only choose what measurement to do 0.01 seconds before performing it, at least one of those choices affects both results.
Whenever the two measurements are made, they will be correlated in a certain way.
You cannot communicate information from one to the other via how you choose to measure them, but you can in some cases coordinate on how your actions are correlated by choosing how to measure based on some info learned on one side.
So, if the direction along which A chooses to measure the spin depends on something A learned after A and B went far away from each-other, this can’t change the probabilities of B getting a result of 0 or 1, but it can change the probabilities of things like “A and B both get 1” or “A gets 1 and B gets 0”, etc. , even though not in a way that changes how likely it is for A to get 1, nor how likely for B to get 1.
This could conceivably be useful if uh, how useful it is for both parties to do something vs only one party doing it, changes based on the info, I guess?
This behaviour makes zero sense to me otherwise.
That makes me think that all particles are actually the same particle, and we're only observing them at different time references. The twinning only objectifies that, so as observers, we are viewing them at time references which allow for known state.
Yes, this has a gazillion holes in it.
edit: to add to this, there is no reason to infer that because things are separated by distance, they are not the same thing. The universe does not have to play nice, to ensure that humanly derived, evolved logic thinks that things make sense from a classical perspective.
If all electrons, for example, are the same electron? Just observed at different time references and locations? Then things such as entanglement make more sense.
edit2: What I mean by 'the same particle', is that theoretically all matter in the universe was one ball of whatever prior to the big bang. Everything already is one particle, prior to diffusion and differentiation due to cooling.
Thus, every electron in the universe was at one point in time the same, single thing. Therefore, they are the same thing. If time does not exist, it means that this electron here, and over there, can be the same electron, for it can spend <unlimited time> here, and then <unlimited time> there, while at the same moment co-spanning the same time reference as we view it.
OK, I'll shut up now, because this is gibberish.
This sounds similar to Quantum Field Theory[1], which might interest you. I won't try to summarise it here because I don't have a good grasp on it myself, as a layperson (with a below-average understanding of physics, realistically!)
PBS Space Time - Quantum Entanglement and the Great Bohr-Einstein Debate https://youtu.be/tafGL02EUOA (and then watch the referenced Veritasium video)
I assume it's been proved that it's in superposition, rather than in a static state? If so, how? Wouldn't knowing require measuring?
Or, maybe to make myself more productive, what's a good search term for this? :)
Search for "bell inequality".
You can tell the particles are in a superposition because they behave differently than those in a static state:
- Double-split experiment: https://en.wikipedia.org/wiki/Double-slit_experiment - only particles in a superposition create interference patterns
- Bell's paradox: https://www.youtube.com/watch?v=v7jctqKsUMA, https://www.youtube.com/watch?v=hiyKxhETXd8 - entangled particles in a superposition can be measured to make predictions with more confidence than if they were simply static particles with different states
I guess a good search term would be "Polarized light and quantum mechanics" because IMO polarized light is a good example which you can even simulate yourself if you have a polarizing filter (https://www.youtube.com/watch?v=zcqZHYo7ONs&t=736s, https://www.youtube.com/watch?v=ZudziPffS9E, https://chem.libretexts.org/Bookshelves/Physical_and_Theoret...)
It appears that their math of correlation is correct for the math of correlation, all independent of any connections with physics.
But from all I can tell, what physics they are really talking about is lack of independence. Two random independent random variables have correlation zero. But correlation zero does not mean independent.
So, if physicists keep using correlation and find correlation zero, sorry, but there can still be dependence.
So, net, in short, for what physics is considering in the real world, they want to know if two measurements, each a random variable, are independent or not, not have zero correlation or not.
2 entangled particles are 2 particles whose states are unknown, except that they are linked: e.g. 2 electrons which have opposite spins, or 2 photons which have opposite polarization. So when you measure one particle, you instantly know the state of the other even if it's light-years away (this is not FTL communication as you can't communicate this information FTL).
It's sort of like putting an "up-spin" and "down-spin" electron in a box, having someone shift around the boxes behind your back, putting the boxes far away from each other, and then opening one: before you open the box you have no idea what either box contains, after you open one you know both including the far away one.
Except it's not the same as putting an "up-spin" and "down-spin" electron in a box, because of Bell's Theorem (https://en.wikipedia.org/wiki/Bell%27s_theorem#Theorem). Electrons' spin is actually determined when you measure them: you can measure up/down, but also left/right or any other 3D direction. And if you measure both electrons at a slightly different angle, their measurements probabilistically correlate in a way which means the electrons never really had any true internal "spin" (hidden variable) until you measured one of them. The fact that measuring one electron changes the others' "spin" faster than light seemingly violates the principle of locality.
The part which confuses me in the article is that, once you measure an entangled particle, they are no longer entangled. So I figure in order for this clock to be feasible you would either need an unbelievable amount of entangled atoms, you'd need to instantaneously re-entangle, or you'd need to use a "standard" atomic clock and only break entanglement after a certain number of ticks, to re-synchronize.