The Delayed Choice Quantum Eraser, Debunked

The delayed choice quantum eraser is one of the weirdest, if not THE weirdest, experiments in quantum mechanics. It supposedly rewrites the past because the choice of a measurement changes what happened in another measurement earlier. In this video I explain why this is not what's happening. The quantum eraser isn't remotely as weird as you may have heard.

Technical remark: You can find a lot of webpages saying that the envelope of the double-slit interference pattern is that of the single-slit diffraction pattern. Note that this is is only approximately correct.

0:00 Intro
1:45 The Double Slit
4:17 Entanglement
4:53 The Quantum Eraser
9:03 What they didn't tell you
11:45 Sponsor Message

#physics #quantum

[pbsspacetime]

Sabine, this is amazing. You are, as usual, 100% right. The delayed choice quantum eraser is a prime example of over-mystification of quantum mechanics, even WITHIN the field of quantum mechanics! I (Matt) was guilty of embracing the quantum woo in that episode 5 years ago. Since then I've obsessed over this family of experiments and my thinking shifted quite a bit. And then I procrastinated on filming the retraction! Thanks for laying it out more clearly than I could have. I have some thoughts to share that might add one more nail to the coffin ... coming to a video real soon!

[liesdamnlies]

I’m glad you recognized the most important thing here: You can never have enough videos of kittens in boxes.

[johneagle]

"I think they are all wrong.." You've got to love Sabine. I believe the number of friends she has decays as fast as Fr-87.

[sgalas]

So the combined photons detected by D3 + D4 produce a non-interference pattern. But why, when viewed separately, do they each produce an interference pattern at all? Why not just a random subset of photons from the combined pattern (since the detections at D3 and D4 are randomly determined by a beam-splitter)?

[mureebe]

"Ackchyually", upon returning to point A, the captain can be any age 46 or above, as we do not know the length of the voyage.

[samsam]

Sabine calling out the YT physics boys. You gotta love it.

[atlanticx]

You are the first one on youtube that I have seen to take the magic out with a fuller explanation. Thank you.

[paulyberk]

"You can find any pattern you want if you're willing to ignore enough data." -Matt Parker

Wonderful explanation of this experiment and eye opening to me.

[feynstein]

Everyone: The delayed choice quantum eraser is impossible to understand.
Sabine: Hold my Bier

[DeSinc]

I've looked on several occasions for a video like this to describe what is actually going on but all I could find was videos of people saying the particles act differently based on if you observed them or not which just always frustrated me to hear. finally now I have a video that I don't completely understand but at least can point to when people bring this topic up.

[blawrence]

Sabine, love your channel, but I think some clarification is needed here. Yes, I think I understand the explanation being offered here that retro-causality need not be invoked to explain the outcome of the delayed choice quantum eraser experiment. And yes, I read Sean Carroll's article that offers essentially the same explanation that you do here. After all, "we can trust Sean with the quantum stuff" as you so aptly put it. But, Sean Carroll's explanation (and by extension yours) invokes the Everettian "Many Worlds" interpretation to explain away the need for retro-causality. The wave function doesn't "collapse" as it would in the Copenhagen interpretation, but rather there was a decoherence that branched the multiverse along all possible paths.

But, the Many Worlds interpretation of quantum mechanics has been something that you've been very highly and very publicly critical of here in the past. If we can really trust Sean Carroll with the quantum stuff, and his explanation invokes Many Worlds to explain the delayed choice quantum eraser, then does it not follow that we should trust Sean Carroll's opinion of the Everettian "Many Worlds" multiverse upon which it depends?

"There’s no need to invoke retro-causality to explain the delayed-choice experiment. To an Everettian, the result makes perfect sense without anything traveling backwards in time." - Sean Caroll, "The Notorious Delayed-Choice Quantum Eraser", Sep 21, 2019

[maxmax]

10:15 is where it falls apart. You are treating the act of selecting certain data groups as if it's some sort of arbitrary cherry picking maneuver. But I fail yo see how this is at all arbitrary or some sort of obfuscatory move. You are merely constraining your observations to photon certain instances. Why can't you do that, especially if its what reveals the strangeness so many people find perplexing? Your coin analogy on the other hand is an example of arbitrary data cherry picking to get certain patterns at your whim. The consoquence is that you sidestep the obvious philosophical implications (which, by the way, don't have to reduce to retrocausality).

[Paxsali]

Joe Scott shitting his pants about being called "always amazing" by the always amazing Sabine Hossenfelder... well done!

[kingo_friver]

I've already given up to understand this topic after watching all those videos several times. Finally, today I got satisfaction, even though I still need to rewatch this several more times. Thank you so much!

[patrickegan]

Would love to see someone film the actual experiment rather than just seeing it animated. Thank you for the clear explanation

[dAvrilthebear]

Please, please don't debunk this Sabine, I need a hope that one day I can erase my life an rewrite the past!

[Disappearation]

Finally someone actually explains it instead of pretending to explain it while mystifying it further.

[PSG_Mobile]

I still dont understand why D3 and D4 have complementary patterns that look like interference instead of complementary patterns that look like normal distribbution.

[UnderTheGoldenGate]

The problem with many of these 2-slit explanations is that nobody I have ever encountered when talking about this has described what they mean by "measured which slit the particle went through". How do you do that? In order to "see" or "measure" a particle you need to have it interact with another particle. This in turn will change the particle, e.g. the wave function collapses. (e.g. when you see something with your eye, light is bouncing off the object). The mystery here is why nobody (nobody I've ever met) has yet answered the question how you measure the particle, not so much the mystery of quantum mechanics.

We need to start there before proceeding to the quantum eraser issue. And then we can ask after the particle comes out of the slit and hits that material, it's similarly interacting with the material?

[mrIceblink]

I like quite a few of Sabine's talks but this one is just a naive (local) deterministic explanation of quantum mechanics, which we're pretty sure HAS been debunked - of course if you believe in such an "interpretation" then there is no mystery for you, but you also do not understand QM. I suggest Sabine reads some of the original papers properly and/or gets an expert to help. The experiments use extremely precise coincidence counters for a reason, which I don't think Sabine understands.