The Problem with Quantum Measurement

Today I want to explain why making a measurement in quantum theory is such a headache. I don't mean that it is experimentally difficult, I mean that it is theoretically not understood just what is going on. And that is even though the problem is more than 100 years old.

In this video I briefly go through the most important postulates of quantum mechanics, which is the Schrödinger equation, the Born rule, and the measurement postulate. I then explain why the measurement postulate is problematic and why re-interpreting it does not remove the problem.

[sjzara]

I really like that you get right down into the technicalities. You don't hand-wave away issues like so many physics popularisers.

[timjackson]

When I was a young physicist trying to get my head around these concepts, asking questions like "what actually happens to the wave function when it collapses", appeared to be taboo. In undergraduate class the response varied from "come and see me after the class" to "I don't know". And don't get me started on Copenhagen! It's lovely to see Sabine breaking the taboos and stating clearly what we know and what we don't, drawing a solid line around the limits of the various theories.

[joshuacoppersmith]

The clarification of superposition and mixed state is pure gold. Thanks!

[justchecking]

I have just binge-watched several of your programs and find them extremely well done - your clear explanations, the grphics used, and your view of the unreliability of scientific objectivity are fantastic. With my background in physics and science I was able to immediately understand everything you presented. I look forward to watching more of them tomorrow.
John D. German, retired scientist, engineer, and professor of physics.

[mikecrafton]

Your videos are wonderful!! I love how you make your explanations make sense and are so clear and understandable even for those of us who are not professional scientists!!
Thank you!! Always looking forward to your next video!!

[peterh.]

Good explanation of the problem with quantum measurement by you Sabine. You make the difficult illustrative and easy to understand.

[quantumcat]

Bonjour Sabine, I appreciate more and more your (mini)lectures. I love your approach to physics. The only major drawback is that 6:56 min. of you is just a teaser for me. I would love something like one hour of your interesting lecture on a subject like string theory or any subject that you personally like. Sabine, I believe that I'm getting addicted to you and quite frankly I love it.

[baterickpatman]

Sabine, you never cease to amaze me.. after trying to pour through scientific articles that were over my head, doing useless Google searches to read uninformed incorrect answers, and everything in between, I found the best answer here. THANK YOU :)

This is the best, most accessible to the "casual physicist" explanation of The Measurement Problem I've ever seen/heard!

[Rospajother]

Great video, that has always perplexed me concerning the measurement of the interference when measuring it collapses it :)

[hckytwn]

Would love for you to continue this Sabine... you left us hanging at the end!

[jonathandamonte]

I bought your book about a week ago & just finished reading this chapter tonight when I saw this video just posted today. Great timing. Great book. Great video. Thanks!

[anubhavdec]

This issue has been bothering me since I joined college. It's agonizing. Thanks for seriously addressing this issue and not discarding it like some of the poppy physicists of the west.

[robertoiannucci]

Come for the music, stay for the physics.

[madderhat]

I understood everything except for the bit after " In this video..."😶

[foolo]

Hi. Great video! There are a few popular science channels out there that have made videos recently, that seem to suggest that many-worlds solve the measurement problem. It seems to be a trending topic. You mentioned in the end of the video "What does it take to solve the measurement problem? We will talk about this some other time." I would love to see this update video.

[Jehannum]

I look forward to the future video where you explain your solution to these problems. I wonder what it will be. For me, the Transactional Interpretation answers these questions and ties it all together nicely: The Born Rule and the measurement problem are no longer such a mystery.

[CaptainGuntu]

I love your channel so much. It's hard to comprehend everything you say, but I love trying, and thinking about these topics.

[nossocc]

Awesome, this is great. Its never been clear to me what constitutes a measurement and how it can be "outside" of quantum mechanics. Your topic seems to be addressing this issue. Looking forward to the next video :)

[reframer]

Frau Hossenfelder, vielen Dank für dieses Video!! Seite Jahren versuche ich das mit Leuten zu besprechen. Ich weiß nicht, ob ich immer an die falschen Leute geraten bin, aber Sie sind tatsächlich die erste, auf die ich treffe, die dieses einfache Problem verstanden zu haben scheint. Sie haben es perfekt formuliert, ich könnte es nicht besser. Die Inkompatibilität der beiden Prinzipien (bei Ihnen 1. und 3.) und die einfach Feststellung, dass ein Observer ebenfalls aus mikroskopischen Bestandteilen aufgebaut sein muss, die der Dynamik der Schrödinger Gleichung folgen (und damit 3. eben NICHT erfüllen können) liegen eigentlich auf der Hand. Dennoch habe ich in der Vergangenheit festgestellt, dass sich die Physikerwelt zum einen nicht dafür interessiert oder zum anderen teilweise das Problem einfach nicht versteht, was wirklich sehr frustrierend ist. Danke also nochmal für dieses Video! Jetzt fühle ich mich nicht mehr ganz so alleine^^
Und - halleluja - die eindeutige Aussage, dass Decoherence daran nichts ändert. Darauf werde auch ich immer verwiesen - und natürlich, es ist leicht, ein Problem einer Theorie mit einem Berg kompliziertem statistischen thermodynamischen Ballast zu überhäufen und dann zu sagen, das löst das Problem, es ist nur nicht direkt offensichtlich verständlich - natürlich. Das ist wie wenn ich ein Fake-Unternehmen mit irgendwelchen Zahlenrechnereien versuche schön zu rechnen. Das ändert nichts an dem Grundproblem, das das Unternehmen fake ist. Genauso ist es mit der Inkonsistenz der Quantenmechanik und der Dekohärenz.

[lezhilo]

A student asked about the measurement problem and tried to interpret the wavefunction with what sounded like the neo-Copenhagen interpretation you mentioned. My argument against it went like this: if the wavefunction really only corresponds to information that observers know, then consider electrons going through a double slit towards a screen and two observers(A&B). Observer A does not measure the electrons' position, so to A, the electrons' wavefunctions do not collapse, and a double-slit interference pattern forms at the screen. Observer B measures the electrons' position, so he has gained information about the electrons' positions. The electrons' wavefunctions collapse and the double-slit interference pattern is destroyed. Thus we arrive at a contradiction: if two observers possess different information, then quantum mechanics predict contradictory screen patterns according to the neo-Copenhagen interpretation in the presence of observers having different information.


Is my understanding of neo-Copenhagen interpretation correct?