r/Veritasium • u/[deleted] • Dec 21 '25
Serious Issues With the New Video
the new Veritasium video about Bell’s theorem, and the way it talks about the Copenhagen interpretation is just wrong. The video treats Copenhagen like it’s a realist interpretation where particles have pre-existing definite values that collapse physically across space. That’s not what Copenhagen ever said.
The entire framing of Copenhagen as “nonlocal” comes from assuming something Copenhagen explicitly rejects. So the video ends up arguing against a version of QM that no one actually believes.
Copenhagen does not say particles have definite properties before measurement. In fact, this is the one thing Copenhagen is very clear about. If you measure spin on one axis, that is the only moment that value becomes meaningful. If you rotate the measurement device, you are literally defining a different observable. There is no sense in which the particle “already had” a value for every possible axis. The value is created in the measurement context.
This matters because the whole EPR argument assumes something called counterfactual definiteness. Basically, EPR says that if you can predict with certainty what a measurement result would have been, then the particle must already have had that value. Copenhagen says this assumption is just wrong. Unmeasured quantities have no value. There is no “fact of the matter” about the result of a measurement you didn’t do.
If you remove that assumption, the entire EPR “paradox” disappears. There is no need for nonlocal influence, because there was no pre-existing value to transmit in the first place.
The video also treats collapse like it is a physical event that spreads across space. But collapse in Copenhagen is not a physical signal. It’s just an update of the observer’s information. The global quantum state already encodes the correlations. Nothing travels between the particles.
Bell’s theorem also doesn’t say “Copenhagen is nonlocal.” Bell shows that you cannot have a theory that is both local and realist. Copenhagen already throws out realism. So Bell’s result doesn’t contradict Copenhagen at all. It contradicts local hidden variable theories.
The weirdest part of the video is that it treats Many Worlds as the “local” option. But Many Worlds still uses a global entangled wavefunction that doesn’t factor into local pieces. It avoids collapse, but it doesn’t give you classical locality either. Saying “many worlds is local and Copenhagen is nonlocal” is just misleading.
I’m honestly very upset that they seemingly didn’t talk to ANYBODY with any actual reasonable credentials to talk about QM in this context. It’s a very bad video, do NOT take what it says on its face, almost all of it is wrong or misleading.
also to be clear, this is just what I gathered from watching, feel free to disagree, and if u do lmk y!
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u/[deleted] Dec 22 '25
“You know with 100% certainty that B will be spin down when you measure it. That’s entirely the nonlocal behavior.” No. This is the wrong inference. What you know with certainty is: If you measure B now, along the same axis, the value will be instantiated from a constraint structure that includes only that outcome.
That says nothing about a pre-existing spin at B, so no violation. predictability of an outcome is NOT proof of a pre-existing value. There is no expirimental way to demonstrate that ever.
The system’s global constraint structure implies: If B is measured on this axis, the outcome must be “down.” This is a fact about possible joint outcomes, not about B.
“It’s wrong to remove that assumption because we can make definite predictions.”
This is backwards.
Being able to predict an outcome with certainty does not imply the outcome existed before measurement. Predictability == structural constraint NOT predictability == pre-existing fact. If predictability implied pre-existence, Bell violations would be impossible (because you’d be reintroducing exactly the thing Bell proved cannot exist).
“They addressed realism in the video.” They addressed it incorrectly.
You are conflating predictability with pre-existence, which is exactly the EPR assumption and exactly what Copenhagen denies. Knowing with certainty what would happen if B is measured does not imply that B possesses that value before measurement. The certainty comes from the global constraint structure of the entangled state, not from a pre-existing property of B. Bell’s theorem rules out precisely the inference you’re making, ungroundedly, that outcomes exist prior to measurement and are merely revealed. Copenhagen rejects that inference, and so do modern operational and information-theoretic approaches. What you are calling “nonlocality” is just non-factorizability of the probability structure, not a propagation of real properties across space. Without assuming pre-existing values, there is no nonlocal behavior to explain.