Debunked: delayed choice quantum eraser + What to know about quantum mechanics

C C Offline
The delayed choice quantum eraser, debunked

INTRO: A lot of you have asked me to do a video about the delayed choice quantum eraser, an experiment that supposedly rewrites the past. I haven’t done that simply because there are already lots of videos about it, for example Matt from PBS Space-time, the always amazing Joe Scott, and recently also Don Lincoln from Fermilab. And how many videos do you really need about the same thing if that thing isn’t a kitten in a box. However, having watched all those gentlemen’s videos about quantum erasing, I think they’re all wrong. The quantum eraser isn’t remotely as weird as you think, doesn’t actually erase anything, and certainly doesn’t rewrite the past. And that’s what we’ll talk about today... (MORE)

What should everyone know about quantum mechanics?

EXCERPTS (Ethan Siegel): Before there was quantum mechanics, we had a series of assumptions about the way the universe worked. We assumed that everything that exists was made out of matter, and that at some point, you’d reach a fundamental building block of matter that could be divided no further. In fact, the very word “atom” comes from the Greek ἄτομος, which literally means “uncuttable,” or as we commonly think about it, indivisible. These uncuttable, fundamental constituents of matter all exerted forces on one another, like the gravitational or electromagnetic force, and the confluence of these indivisible particles pushing and pulling on one another is what was at the core of our physical reality.

The laws of gravitation and electromagnetism, however, are completely deterministic. If you describe a system of masses and/or electric charges, and specify their positions and motions at any moment in time, those laws will allow you to calculate — to arbitrary precision — what the positions, motions, and distributions of each and every particle was and will be at any other moment in time. From planetary motion to bouncing balls to the settling of dust grains, the same rules, laws, and fundamental constituents of the universe accurately described it all.

Until, that is, we discovered that there was more to the universe than these classical laws.

1.) You can’t know everything, exactly, all at once. If there’s one defining characteristic that separates the rules of quantum physics from their classical counterparts, it’s this: you cannot measure certain quantities to arbitrary precisions, and the better you measure them, the more inherently uncertain other, corresponding properties become...

[...] 2.) Only a probability distribution of outcomes can be calculated: not an explicit, unambiguous, single prediction.

[...] 3.) Many things, in quantum mechanics, will be discrete, rather than continuous...

[...] 4.) Quantum systems exhibit both wave-like and particle-like behaviors...

[...] 5.) The act of measuring a quantum system fundamentally changes the outcome of that system.

[...] 6.) Entanglement can be measured, but superpositions cannot...

[...] 7.) There are many ways to “interpret” quantum physics, but our interpretations are not reality. This is, at least in my opinion, the trickiest part of the whole endeavor. It’s one thing to be able to write down equations that describe the universe and agree with experiments. It’s quite another thing to accurately describe just exactly what’s happening in a measurement-independent way.

Can you? I would argue that this is a fool’s errand. Physics is, at its core, about what you can predict, observe, and measure in this universe. Yet when you make a measurement, what is it that’s occurring? And what does that means about reality? Is reality:
  • a series of quantum wavefunctions that instantaneously “collapse” upon making a measurement?
  • an infinite ensemble of quantum waves, were measurement “selects” one of those ensemble members?
  • a superposition of forwards-moving and backwards-moving potentials that meet up, now, in some sort of “quantum handshake?”
  • an infinite number of possible worlds, where each world corresponds to one outcome, and yet our universe will only ever walk down one of those paths?
If you believe this line of thought is useful, you’ll answer, “who knows; let’s try to find out.” But if you’re like me, you’ll think this line of thought offers no knowledge and is a dead end. Unless you can find an experimental benefit of one interpretation over another — unless you can test them against each other in some sort of laboratory setting — all you’re doing in choosing an interpretation is presenting your own human biases. If it isn’t the evidence doing the deciding, it’s very hard to argue that there’s any scientific merit to your endeavor t all... (MORE - missing details)
confused2 Offline
Re quantum eraser..
She starts off with the screen (whatever) always shows 'no interference' - the interference patterns emerge from choice of which photons we choose to look at using coincidence detector (and a delay). So far so good but how do the slits know we have the technology to determine which way the photon went - the technology is all (in time and distance) after the photon has hit the screen?
C C Offline
Belated adjustment. I should have first posted the most recent Don Lincoln video (about it) that she referenced at the start. He in turn is going to advise first looking at the video below it.
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(2) Fermilab: The super bizarre quantum eraser experiment

(1) The video he said to go back and look at before proceeding
confused2 Offline
Many thanks CC - the DSE and derivatives are one of my favourite things. I'll 'whatever' when I've gone through them - usually ten times or so. I do like Sabine though - especially after ''The end of the world as we know it".
confused2 Offline
OK. What we would like to see is a screen at the location of D0 and an interference pattern that comes and goes as you fiddle about with which way path information later on in time. Nobody does that because (I guess) it doesn't happen. Sabine points out the trick with coincidence detection with D0 and other detectors which can construct (or not) an interference pattern  from (her claim) no interference pattern. The photons that go through one slit and the photons that go through the other wouldn't normally be closely related - they don't include (for example) the distance between the slits EXCEPT they are from a laser where one photon is very much like another .. so could this be a two photon effect - a misrepresentation of a single photon effect? I don't really think so (Sabine!) but that is where I am right now.


[Image: 1280px-Kim_EtAl_Quantum_Eraser.svg.png]

[Image: 1280px-Kim_EtAl_Quantum_Eraser.svg.png]


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