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In School (15 years ago) quantum physics really fascinated me. The implications on reality, what we know, and how the world behaves is amazing. In school it was mainly about how we change stuff just by observering (double slit, waves vs particles and even changing the past).

But as i rediscovered and read about some of these topics in the last couple of days i've also read about quantum entaglement, i don't quite get it, and don't see how it's any different from things we observe in everyday life. As I understood, the main experiment for quantum entaglement is somehow splitting a particle and we end up with two particles that complement each other. Particle A has either property X or Y. Particle B has either property X or Y. But both properties must be present, so if A = X then B = Y. Now if we measure A, and we can see it has X, if we measure B before light could travel from A-> B we see it has Y.

Why is this spooky, or sometimes even intreprted that A has influence over B state? Isn't it just applied logic?

See if I have a red lambo and a blue lambo in a warehouse. I instruct two drivers which are gps tracked, to pick a car at random and drive in opposite directions. If I now look at the gps tracking software, i see two lambos riding in opposite directions, if i follow car A, i don't know if its blue or red, but if i ask the driver to send me a picture of the car, and i see its the red lambo, i KNOW that the other car is blue, and if ask the other driver for a picture, i would see that the car is in fact blue.

In my mind, this is just the same as the quantum experiment, but i am sure im missing something, so I'm looking for an explanation what makes it special :)

Yalla T.
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    See also: Quantum Entanglement - What's the big deal? and many many more related posts here. Please use the search function. – Tobias Fünke Nov 15 '23 at 10:27
  • The answers to the other questions didn't help me to fully grasp why this is different from standard logic / correlation. But given the amount and quality of answers concerning this topic on this site, it may have been futile to think that asking a similar question would produce answers that would really help me grasp it. Sorry. – Yalla T. Nov 15 '23 at 11:02
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    I don't understand (especially the last part of your comment). Anyway, you can edit your question, link to an answer and explain what exactly is unclear, to start with. Other than that, you can also ask in a comment under a respective answer for clarification. Your question has been asked several times on this site already. – Tobias Fünke Nov 15 '23 at 11:07
  • https://physics.stackexchange.com/questions/330554/understanding-quantum-entanglement-help-me-validate-this-analogy/330841#330841 – alanf Nov 15 '23 at 11:13
  • If you have a pair of gloves, one must be left and one must be right, so If you split a pair at random and post a left handed glove to me, I know the glove you've kept is right-handed. That's not spooky. However, suppose you have two coins and post one to me. If I toss the coin and it comes up heads, and then I knew that if you tossed your coin it would come up tails, that would be spooky, and that's what entanglement is like. – Marco Ocram Nov 15 '23 at 12:34
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    Be careful what you read ..... even magazines and scientists love to make headlines! If we take a bogus theory and apply it some real experimental results you tend to get false conclusions. There is no spooky action. – PhysicsDave Nov 15 '23 at 13:36
  • @MarcoOcram : I don't think your example is well-chosen. It can be explained with hidden variables. I toss my coin; it comes up heads; this tells me that it's weighted to always come up heads; this tells me that its partner was the one that's weighted to always come up tails. In particular, there need be no Bell violation here. – WillO Nov 15 '23 at 15:35
  • @WillO sure, but I had assumed the coins were not weighted, etc. I see your point that they could have been weighted and that would account for the outcome. – Marco Ocram Nov 15 '23 at 15:46
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    Just to add to the correct answers by @ValterMorettl and others: You won't get a good answer unless and until you understand the Bell argument. Your example is what is called "perfect correlations", and those can be modeled with hidden variables and without "spooky" anything. But at many other relative angle settings, the same cannot be said. Understanding Bell requires a bit of math. I maintain a page that simplifies this if you are interested: https://drchinese.com/David/Bell_Theorem_Easy_Math.htm – DrChinese Nov 15 '23 at 15:47
  • @MarcoOcram : But that doesn't help at all. You've proved there are no hidden variables by assuming there are no hidden variables (i.e. the coins are not weighted). The whole point with entanglement is that we don't need to assume away the hidden variables; instead we can infer their absence from the experimental outcomes. – WillO Nov 15 '23 at 16:38
  • Here's how to fix your example: There are two lambos in a warehouse. One is driven to my house and one is driven to your house. We repeat this experiment many times. Whenever we both examine our lambos in our driveways, they are opposite colors. Whenever one of us examines his lambo in his driveway and the other examines it in his garage, they are opposite colors. Whenever we both examine our lambos in our garages, they are the same color. – WillO Nov 15 '23 at 16:40
  • (CONTINUED).. That is the essence of what happens with entanglement. The lambo example is oversimplfied because in quantum mechanics, the correlations are not perfect --- but they are still "spooky" in exactly the same sense that my preceding comment is spooky. The point is that the correlations change as a function of last-minute choices made by the experimenters. This is what's completely missing from your original example. – WillO Nov 15 '23 at 16:41

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As far as I understand, you are assuming that the values $A=X$ and $B=Y$ are already fixed before you check them. This idea is called "realism": the measurements simply record outcomes already present. Here nothing weird takes place as you point out.

However, the other hypothesis in this issue is that a pair of events cannot be causally related (none can be cause of the other) if it is not possible to send information between them with velocity $\leq c$. This hypotesis is known as locality.

One considers an experimental setup where experiments are performed in two regions of the spacetime where two parts of a quantum system are respectively located.

It is possible to chose these two regions in order that no superluminal information can travel from the one to the other during the experiments.

The so called Bell theorem (in the modern interpretation due to Leggett) proves that if both the realism hypothesis and the locality hypothesis hold then, performing a certain type of measurments, a certain inequality on the possible outcomes cannot be violated.

Real experiments (with photons) prove that the inequality is actually violated.

And this violation is actually predicted by Quantum Mechanics.

In practice, dealing with entanglement of couples of particles, one should renounce to at least one between realism and locality.

Valter Moretti
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  • I think i don't quite understand realism. Does realism mean that nature and natures law are real, and not pre determined by the universe, some god, or whatever - that things in quantum mechanics are probabilties and are not set in stone or created in the mind of the observer? – Yalla T. Nov 15 '23 at 12:19
  • Going back to my car example, is this mor analog: Paint the car in a random color but it must be the opposite color of the other car. You must choose random at the point of painting, and i give them no way to communicate.. And now, if I see what color car A is, car B is the opposite This can only be true (after random chance is eliminated trough repetition) if: A) The rule of choosing a random color is not real: It was either pre determined beforehand by the drivers, or the random algorithm is influenced somehow. B)The rule of no communication was broken, they found a way to communicate – Yalla T. Nov 15 '23 at 12:21
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    But I get the feeling that I need far more understanding in quantum physics to fully grasp this, and it's not as simple as trying to find real world analogies, since thats the point, its different hten what we learn and experience and expect from everyday life. – Yalla T. Nov 15 '23 at 12:29
  • @Yalla T. I think you can grasp the essence of the so-called "spookiness" with simple examples that require essentially no understanding of quantum physics --- though of course that's not the same as grasping why that spookiness must arise from more fundamental assumptions. I hope the two comments I just left on you original post will give you what you need. – WillO Nov 15 '23 at 16:45
  • @YallaT. Another way to see things: the quantum prediction for correlation depends solely on a future choice of measurement settings by our usual example experimenters Alice & Bob. Regardless of distance between them, it is the relative difference between their settings that dictates the results. These settings are called a "context" . It can be nonlocal, which gives rise to the "spooky" term because it's a parameter that depends on distant settings. Their choices can even be selected in midflight. See for example: https://arxiv.org/abs/quant-ph/9810080 or https://arxiv.org/abs/1508.05949 – DrChinese Nov 15 '23 at 20:56
  • @Valter Nice readable, thanks. „a certain inequality on the possible outcomes cannot be violated“ means what? – HolgerFiedler Nov 16 '23 at 04:35
  • @HolgerFiedler For instance the CHSH inequality https://en.wikipedia.org/wiki/CHSH_inequality – Valter Moretti Nov 16 '23 at 04:55
  • Let me explain, what I not understood: If the realism takes place (the states are „up“ and „down“ from the beginning), the inequality is what? Zero? If we are not able to measure up and down without destroying the process and not able to justify our measurement instrument to up and down, the result will be a statistical only over time and only give the relation, that there is up and down. And where was no manipulation on the states if the outcome is between 25 and 50%? Which point of thi is the inequality? – HolgerFiedler Nov 16 '23 at 05:05
  • @HolgerFiedler An inequality cannot be "zero" or anything else. The CHSH inequality is derived under the assumptions of locality and realism (you actually have to be precise in a mathematical way here). Any theory with these constraints must obey this inequality. QM violates it. – Tobias Fünke Nov 16 '23 at 09:07
  • Sorry I am very busy with my administrative duties and I cannot participate in the discussion. – Valter Moretti Nov 16 '23 at 10:57
  • @HolgerFiedler In a Bell test of photons with correlated linear polarization, you see HH or VV outcomes at any and all identical angles. If there is realism, then those outcomes must (according to EPR 1935) be predetermined. It was not obvious, but Bell deduced that there is no set of possible outcomes that match the quantum predictions for many angle combinations. For example: for 0, 120 and 240 degrees, those outcomes cannot match less than 1/3 of the time (HHV, HVH, HVV, HHH, VHV, VVH, VHH, VVV, any 2 of the 3). The quantum prediction is 1/4. That is one of many possible inequalities. – DrChinese Nov 16 '23 at 16:14