Why is quantum mechanics weird? The bomb experiment

I have done quite a few videos to demystify quantum mechanics. In this video I want to explain just why quantum mechanics is weird. Is it entanglement or superpositions? Schrödinger's cats? Not quite, really. But the famous Elitzur-Vaidman Bomb Experiment demonstrates well just why quantum mechanics is weird indeed.

0:00 Intro
0:55 Psi
2:09 Dead-and-Alive cats
3:43 Entanglement
5:20 The Bomb Experiment
9:36 Sponsor Message

#physics #education #science

[SabineHossenfelder]

Hi everyone. As several people have pointed out, the audio doesn't quite sound right. I can't fix this -- I can only take the video down and replace it in the next couple of days. As it seems to at least be clearly understandable I'll let it up. Will make sure that next week we're back to the normal quality. Sorry about that.

[kpaasial]

Quantum mechanics: how I learned to stop worrying and love the bomb.

[OniSMBZ]

I think its pretty amazing that quantum mechanics is both weird and not weird at the same time.

[anatomicallymodernhuman]

I had a quantum mechanic once. He was definitely weird. And he was never sure whether he’d actually fixed my car or not.

[poposterous]

"Quantum mechanics isn't weird."
"Oh, that's nice."
"However, quantum mechanics is terribly weird."
"OH

[Korat]

I'm happy that my teacher of quantum physics was Paweł Horodecki, he showed us in 2008 at one of the first lessons this insane topic of Elitzur and Vaidman bomb experiment. I still treat my notes from it like some sacral artifact. I talked to all my friends about it even when they had nothing to do with physics .Great memories

[Cashman]

Sabine: QM is not weird, just unintuitive.
Me: Ok...
Sabine: But it is weird!
Me: Oh, the plot twist!

[astrobiojoe]

"Two paths diverged by a beam splitter, and I took the one without the bomb.... and that has made all the difference." - Robert "Photon" Frost

[EricBLivingston]

I thought the main weirdness of entanglement was the idea of no hidden variables. That is, it’s not that the two particles have correlated values the whole time that are simply measured at some point for particle A, thus implying what B’s value was all along, but rather that A and B do not in any way have those values, or have them in a non-privileged way along with every other option in superposition, and only when you measure A does B in fact take on the correlated value at that instant, where prior to the measurement it could not have been said to have that value at least not exclusively.

[tawe]

Things like delayed-choice quantum eraser always bugged me the most in QM. Maybe you could make a video about it?

[fafnirbane]

7:32 - is this correct, though? There's a 50% chance it gets blocked by the dud, and in the rest of the cases it either goes into detector A or detector B, with 25% chance each. Exactly the same as if the bomb were live.

Unless the wave function somehow magically passes through the dud and recombines with the upper ray to cancel out...?

[BladeRunner-tdbe]

My brain can tell me something that didn't happen, which is, understanding this lecture. Whenever I think I'm overly intelligent, I simply watch a video on this channel, and I'm immediately brought back to reality.

[drmocm]

It would sound less weird if you had mentioned what being a dud means, i.e. there is no functioning detector in the "bomb" path. So the weird thing is that the presence of a detector, or anything that can interact with the photon, leads to the change .

[simin]

The reasoning is a quite off and unnecessarily confusing : Sabine repeatedly said that there is nothing weird about entanglement or superpositions. However this bomb tester is simply an application of these tools to achieve the bomb test task. If these two tools aren't weird, then there is certainly nothing weird at all in exploiting the tools' unique properties to reveal information about a path even though the particle didn't take that path. Because that's basically what the tools enable: revealing the lower path has a detector while the particle travels via the upper path. Do you find that weird? well that's what's exactly weird about the tool itself, not the logical application of it!

[BigZebraCom]

Sabine Hossenfelder: Her science is up-to-the-minute up-to-date---and her clothes are from the future.

[NoirpoolSea]

Your channel is wonderful, a much needed counter to the Deepak Chopra type folks who keep misusing Feymans's "nobody understands quantum mechanics mechanics" statement. Also- I never had heard of the bomb experiment till this time. You are a true educator in the finest way.

[FalkFlak]

Concluding that an event didn't happen doesn't sound that weird to me.

[kingbeauregard]

About the "weirdness" of wave functions: I think it's worthwhile to remember that wave functions are NOT the particles themselves, rather they are mathematical models for some aspects of the particles' behavior. It's easy to lose track of the distinction, but it is important. Among other things, it means that there may be aspects of a particle's behavior that the wave function doesn't model very well, and that's fine. It just means we need to understand the limits of our model.

We've been down this road before incidentally. Remember the Bohr model of the atom, and how it was derived with the concept of an electron traveling in a circular path around the nucleus? The model was initially held to be correct, since it proved useful for many purposes. In fact it was based on some faulty assumptions about how electrons function, but it so happens that the math worked out pretty well anyway, and the Bohr model was accepted as the correct model. Until we understood that it wasn't as correct as we thought and discovered its shortcomings.

Wave functions seem to be holding up much much better than that, and they remain a useful model. But it's only a model.

[MisterWillow]

I tried to understand that experiment earlier, but couldn't wrap my head around it.
After your explanation, it is much clearer (but still totally weird). Thank you (again) for being such a great teacher!

[antirussia_org]

QM doesn't tell you anything about path not taken. All information about it came from the probabilities you gave earlier.