[riggsdk]

(not a physicist) One thing that I always wondered about that I never see "debunked" anywhere is any discussion about whether or not entanglement is actually just because the two entangled particles are put into a pretty predictable state (opposite of each other). If one is measured "up" the other will measure "down".

To me that just screams "particle physics are predictable(determinant) as long as the particles are shielded from outside noise, not because they are connected/bound together by some mysterious force or law of physics."

I suppose a thought experiment to prove/disprove that would be to send one of the entangled particles, particle A, around a black hole to slow it's time down and then afterwards measure if the entangled particles still give opposite results but consistent with the time delay.

[thrance]

You want to read up on Bell's Inequalities, which were experimentally confirmed[1] by Alain Aspect who won the physics nobel prize for it.

In short, you can never properly describe what happens by assuming the particles states are already determined after being entangled.

[1] https://en.wikipedia.org/wiki/Aspect%27s_experiment?wprov=sf...

[newpavlov]

There is a very important and often forgotten caveat to the Bell's inequalities: it covers local hidden states. If we are to assume that measurement and particle creation apparatuses are entangled with each other (after all, they had plenty of opportunities since the big bang), then we have a global hidden state and the quantum measurment randomness becomes a simple artifact of removing global state from the picture. This interpretation of QM is usually called "superdeterminism" and, personally, I like it much more than the black vodoo measurement magic with collapsing wave functions or creation of whole new worlds on each tiny measurement. This video can be a good introduction to this topic: https://youtube.com/watch?v=dEaecUuEqfc (don't mind the clickbaity title, the video itself is good)

[rcxdude]

superdeterminism is deeply weird, just in a different way that most of the other quantum mechanics interpretations are deeply weird. The main thing is that it implies that this global quantum state is just-so arranged that no matter how you make your decisions about what to measure, it's always correlated with the underlying quantum state.

[newpavlov]

I would say it differently. Suprdeterminism opposes to the deeply ingrained assumption that we can design experiments in a way which removes influence of a measurement apparatus (including experementators themselves) on the measured process.

[rcxdude]

It's more than that, though. Just 'having an influence' on the measured process doesn't explain the bell inequality. Super-determinism basically requires that there is some common state from the big-bang which means that if I were to decide to e.g. seed the random number generator I'm using in an experiment with a description of what I had for breakfast that morning, the particles in that experiment (which could in principle come from far enough away they had no way of causally interacting with me or said breakfast) somehow 'know' that I had made that decision, what I had for breakfast, and the details of the random number generator and act accordingly. Absent some mechanism by which this might occur, it requires an incredibly complex kind of setup to the universe to create that result, one that has so many free variables it could explain almost any universe with any physics.

[newpavlov]

It's not a some sort of particle conspiracy. The idea is not so different from the Laplace's demon. We have an initial state of the Universe at the moment of Big Bang (a PRNG seed, if you will) and a set of differential equations (QM is not different in this regard). Theoretically, it allows the demon to predict everything in the Universe. The wave nature of QM equations introduces a certain quirk to it, but, effectively, with your example the breakfast was already "preordained" at the moment of the Universe creation.

Surprisingly, this idea makes many physicists very umcomfortable and they start to object to SD using philosophical arguments about "free will".

[mrguyorama]

And since that precludes it from ever being testable, falsifiable, or making predictions different from a Bell's Inequality based theory, it just isn't physics

[newpavlov]

It's no different from any other interpretation.

[simonh]

Just to try and summarise the issue. It turns out measurements of one of the entangled particles (particle A) are correlated to the settings of the measurement apparatus. For example the axis on which the polarisation of a photon is measured affects the measurement you get.

That setting is not known when the particles become entangled, and so in principle cannot affect the state of particle B. However since the setting does in fact correlate with the measured state of particle A, it also correlates with the state of particle B.

[superb-owl]

You're looking for the Bell Experiment: https://en.wikipedia.org/wiki/Bell_test

It proves there's no "local hidden variable"--the state is indeterminate until they are measured. It's proven through some pretty simple probability theory, and is (relatively) easy to follow. There's a great video of Leonard Susskind explaining it somewhere

[stouset]

Imagine I send you a box. The box has three buttons: one on the top, one on the front, and one on the side. You can press only one of these buttons. When pressed it will either light up green or red. Pressing other buttons afterward causes no effect, the box is disabled.

I send your friend a copy of the same box. I tell you that the result of pressing each button is random but no matter what, if you both press the same button you will see the same result. If you press two different buttons, the results will be uncorrelated.

You ask me to send you and your friend a bunch of these paired boxes and start testing. You then both press random buttons on each box and record your results.

Comparing notes afterward you can see that every time you happened to press the same button you received the same result. You confirm that if you pressed two different buttons, you get uncorrelated results. No problem, you think. I have obviously preprogrammed each box to be one of GGG, GGR, GRG, GRR, RGG, RGR, RRG, or RRR.

But then you notice something strange. If this theory was true, for 2/8 boxes you would have an RRR/GGG box and would see the same answer no matter what. The remaining 6/8 boxes you should get the same answer 2/3 of the time. This means that your answers should agree 3/4 of the time. Even if you surmise that I never send you a box set to the same three values, your results should agree 2/3 of the time.

You crunch the numbers and find that your results agree precisely 50% of the time.

[danbruc]

The interesting thing about entanglement is not the correlation per se. You can take a pair of hand gloves, put each one into a box, and send them to opposite ends of the universe. When you open one box at one end of the universe and see the left glove, you immediately know that someone at the other end of the universe will find the right one. The interesting thing about entanglement is that decision which glove goes into which box is not made when you prepare the boxes before sending them to opposite ends of the universe but only at the moment you look into the first box.

[datavirtue]

And if it doesn't fit...you must acquit.

[tzs]

If all you could measure was "up" and "down" then I think entangled particles would be indistinguishable from unentangled particles that were created as up/down pairs. But particles can be measured in other directions, and that's where the determinism goes away.

A nice thought experiment is the CHSH game. It's a two player game where the players (player A and player B) cooperate to beat the house. It is played as follows:

1. Each player is assigned a referee.

2. The players, accompanied by their referee, go to separate rooms. Before going to the separate rooms the players can confer. They may also bring any equipment with them that they want. The rooms are shielded to block any communication between the players during their time in the rooms. You may assume that the communication blocking is 100% effective.

3. Each referee uses a true random number generator to generate a bit, and tells the player the value of that bit.

4. The player then generates a bit, by any means, and tells it to the referee.

5. The referee records the bit they generated and the bit provided by the player.

6. Steps #3-5 are repeated 999 more times.

7. After both players have gone through #3-5 1000 times, the referees confer and check their records. For each round the players win $100 in these two cases:

  The players generated different bits and the referees both generated 1
  The players generated the same bit and at least one referee generated 0

In a classical universe the best strategy for the players is simply to agree on an algorithm that will result in them picking matching bits every round, such as "always pick 0". 75% of the time the referees will generate 00, 01, or 10 and the players will win $100.

In a quantum universe the players can do better. They can generate 1000 pairs of entangled particles and each take one particle from each pair. Let's assume that the particles are linearly polarized photons polarized in the up/down direction.

When player A is given the referee's bit, A sends their particle from the first pair through a polarizing filter and reports a 1 if the particle makes it through the filter, and a 0 if it is blocked.

If the referee's bit was a 0 the player orients their polarizing filter along the up/down axis. If the referee's bit was 1 they orient their filter rotated 45° to the right.

Player B does a similar thing, except their filter is rotated 22.5° to the right if they got a 0 from the referee and 22.5° to the left if they got a 1.

Here's a diagram of their measurement angles, where X0 means player X got a 0 from the referee and X1 means they got a 1:

  B1                B0
  |        |        |
  |        |        |
  +--------+--------+--------+
  |-22.5   |0       |22.5    |45
  |        |        |        |
           A0                A1

They do this for each round, using the photons from the n'th entangled pair for round n.

Note that if either player receives a 0 from the referee the angle they use will be 22.5° apart from the angle the other player uses no matter what bit the other player got from the referee.

When the measurements on a pair of entangled particles are taken at an angle θ the results match the result you'd get at a 0° difference cos^2(θ) of the time.

For 22.5° that's 85.4% of the time, so when either referee generates a 0 they players will win 85.4% of the time.

If both referees generate 1, B measures at -22.5° and A measures at 45°. That's 67.5° apart and the player bits only match 14.6% of the time, but when both referees generate 1 the players want to generate different bits so that's good. They players win 85.4% of the time in this case.

That's an 85.4% win rate in all cases, which beats the 75% that they can get in a classical universe.

If you try to make some sort of classical-only thing that can take the place of entangled particle pairs you'll find that you can't make it work. You won't get past 75%.

Another thought experiment that might be clearer (or might just muddle things) involving two mysterious devices that you are trying to reverse engineer is here [1]. That puts it more mechanical/computational terms which may be easier to play around with.

[1] https://news.ycombinator.com/item?id=35905284