Talk:Einstein–Podolsky–Rosen paradox

Section under "the crux of the matter" is wrong

The section under "the crux of the matter" is wrong. By only measuring the x or z axis you cannot distinguish between a classical system with hidden variables and a quantum system. One has to measure at 45 degrees also. The description is wrong.

Sorry, I meant to put this somewhere else, but I do not know how to delete this.

Observer as Variable

The impact of consciousness is unquestioned, how much more abstract is consciousness greater will its power both in size and complexity?

particles are not states...

In the first line of description of the EPR experiment it is currently written:

The thought experiment involves a pair of particles prepared in what would later become known as an entangled state.

I think it is misleading to link "particles" to "states". In quantum mechanics (not quantum field theory), a particle is in a state or it has a state, but it is not a state. In addition, there is a single quantum state not a pair of them involved in the EPR thought experiment. Since @Johnjbarton: undid my change in the article, I'm starting the discussion here to come to a better formulation. I think it's uncontroversial that

The thought experiment involves a pair of particles prepared in what would later become known as an entangled state.

The only question is where to put the wikilinks. I object to linking "particles" to "quantum state" since the two are very different objects; if we don't and link "entangled state" to "quantum entanglement" then the concept of quantum state doesn't appear in the sentence (which is why I linked "entangled" to entanglement and "state" to "quantum state"). An alternative could be:

The thought experiment involves a pair of particles prepared in a state of the kind that would later become known as an entangled state.

Would that be ok? --Qcomp (talk) 09:26, 2 September 2023 (UTC)

Please check the history, your version is the current one. Jähmefyysikko (talk) 09:36, 2 September 2023 (UTC)

@Qcomp thank you for this great explanation. my change was an error which I undid immediately afterwards. I'm sorry for the mistake and confusion. Johnjbarton (talk) 15:05, 2 September 2023 (UTC)

ok, sorry for the fuss - I got the notice about the revert and didn't notice that it had already been undone again. Thanks to all for caring about the article. --Qcomp (talk) 15:13, 2 September 2023 (UTC)

Uncertainty principle

The article states, "... it is impossible to measure both the momentum and the position of particle B exactly; however, it is possible to measure the exact position of particle A." This is an error. The uncertainty principle also says it is impossible to measure the exact position of particle A! The idea that knowing the position of particle A allows calculation of the exact position of particle B is also highly suspect. Please, would some Quantum Mechanics guru (which I am not) check this article carefully. Meanwhile, I will take the whole article with a grain of salt. Wcmead3 (talk) Wcmead3 (talk) 03:13, 23 February 2024 (UTC)

You claim:

• "The uncertainty principle also says it is impossible to measure the exact position of particle A!"

Do you have a source for this assertion? If not then I think the article is not in error.

If one places a small metal cup in the area where one expects particle A, then the detection of a particle in the cup fixes its position. Such a cup cannot be made arbitrarily small, but the QM theory discussed in the EPR article is not about such practicality. Rather it is about the position operator, which can be arbitrarily exact. Johnjbarton (talk) 03:58, 23 February 2024 (UTC)

For myself, I'd regard having to divide Planck's constant by zero to get the uncertainty in momentum after making my perfectly precise position measurement as pretty decisive. Surely it it means at the very least that if a particle's position is precisely known, the uncertainty in its momentum is undefined. So we can approach perfect measurement arbitrarily closely, watching the uncertainty in the momentum tend to infinity as we do so, but things break down once we get to zero uncertainty. Musiconeologist (talk) 23:21, 22 March 2024 (UTC)

The uncertainty principle is about measurements as it says in this article as well. Johnjbarton (talk) 23:43, 22 March 2024 (UTC)

@Johnjbarton Well yes, but it states a mathematical relationship between their uncertainties, which becomes a contradictory one if either uncertainty is zero. Musiconeologist (talk) 23:54, 22 March 2024 (UTC)

Putting it more worryingly, if ΔpΔx=h/2π (or is it 4π? It's too many years since I learnt this stuff) and Δx=0, then h=0. But it's not. Musiconeologist (talk) 23:48, 22 March 2024 (UTC)

Well is physics not math and its greater than rather that equal to, so we can rest easy. Johnjbarton (talk) 00:22, 23 March 2024 (UTC)

@Johnjbarton Ah of course. Though multiplying two things one of which is zero to get an answer greater than h/2π still bothers me. It still seems to me to imply straight away that Δx ≠ 0, i.e. x can't be precisely measured. Anyway my guess is that this is where the statement about the impossibility of a perfect measurement comes from. But I suppose it might be reasonable to say that since the relation breaks down for a perfect measurement, it can't be applied to one and therefore doesn't rule it out. But I only learnt the electronic engineering version of this stuff, and it was a few decades ago . . . Musiconeologist (talk) 01:21, 23 March 2024 (UTC)

The "Paradox" paper

" According to Heisenberg's uncertainty principle, it is impossible to measure both the momentum and the position of particle B exactly; however, it is possible to measure the exact position of particle A. By calculation, therefore, with the exact position of particle A known, the exact position of particle B can be known. Alternatively, the exact momentum of particle A can be measured, so the exact momentum of particle B can be worked out. "

Well, no. As per Heisenberg's uncertainty, no measurement is absolute, neither combined nor separate. There are always digits missing, and thus exactness is never reached. However, both can be approximated at nearly the same approximated time, which all measurements are. Hence, no true predictions are possible, and yet approximations are, which will invariably vary over the entire extrapolation, increasing in variablility over the main components, time and distance, if and when these apply.