4 ms·
susskind described entanglement in a bit different way than I'd heard - "that entanglement allows one to know everything there is to know about a system of part
by platz 10y ago
susskind described entanglement in a bit different way than I'd heard - "that entanglement allows one to know everything there is to know about a system of particles (the whole), while knowing nothing about it's parts."
In the classical version of an information experiment, if i randomize placement of objects A and B into 2 boxes, and send one of those boxes to someone else - upon opening my box I instantly know whether the other box contains object A or B. The Quantum Mechanical version of the above experiment is very similar, except there could be several different degreees of freedom to measure on upon opening my box (what angle of spin measure on, etc..)
So to me, that doesn't suggest that something "traveled" to the other box, just like in the Classical version.
Rather, somehow I only "knew" the system at the beginning without knowing "any of it's parts" (due to entanglement & superposition).
Then, just observing a degree of freedom in one the parts finally reveals what the corresponding degree of freedom in the other part was.
This (intuitively) makes more sense to me than saying "information traveled" and "action at a distance"
- equivrel 10y agoThe objects in boxes scenario you have described is useful to think about when coming to grips with QM and entanglement. However, one needs to be careful, because there is an additional subtlety: QM can have measurement scenarios that are not only entangled, but also non-local [1]. This means that it is provably impossible to endow the variables that you are measuring with prior, local assignments, e.g. like in the boxes scenario. So there is actually another phenomenon present in QM that makes it weird, and is strictly speaking distinct from entanglement. [1] - https://en.wikipedia.org/wiki/Quantum_nonlocality https://en.wikipedia.org/wiki/Quantum_nonlocality
- platz 10y agolocality is looking suspect for other reasons too, as it's one of the only ways to avoid 'black hole firewalls' [1] (the other way to avoid firewalls involves giving up conservation of information which would be bad for QM theory). [1] Black hole Firewalls with Sean Carroll and Jennifer Ouellette https://www.youtube.com/watch?v=_8bhtEgB8Mo&t=3634s https://www.youtube.com/watch?v=_8bhtEgB8Mo&t=3634s
- danbruc 10y agoSo to me, that doesn't suggest that something "traveled" to the other box, just like in the Classical version. Rather, somehow I only "knew" the system at the beginning without knowing "any of it's parts" (due to entanglement & superposition). Then, just observing a degree of freedom in one the parts finally reveals what the corresponding degree of freedom in the other part was. This (intuitively) makes more sense to me than saying "information traveled" and "action at a distance" But exactly this is not the case, exactly this is the difference between classical physics and quantum physics. Classically you place one coin in each of the two boxes, heads in one, tails in the other. Nothing strange happens here, the outcome is determined when setting up the boxes. When you look in one of the box, you will see either heads or tails as determined by the setup. The other box will have the coin showing the opposing side, also determined by the setup. The quantum case is very different. You again place one coin in each of the two boxes, again one heads and one tails, but this time the coins are constantly flipping. It is no longer predetermined whether you will get heads or tails and only when you look in one of the boxes does the coin in there stops flipping and you see either heads or tails. But more importantly the other coin stops flipping at the very same time so that you will see the opposing side when you look into the second box. The correlation is in both cases established when the boxes are setup, the coins will always show opposing sides. The quantum cases of course also requires both coins flipping synchronously. But in the classical case the exact outcome is also established during the setup while in the quantum case the exact outcome is only established when one looks in the first box.
- platz 10y ago> the coins are constantly flipping ... the other coin stops flipping at the very same time I'm not sure this corresponds to my understanding of the wave function before observation. Just because the probabilities for each state is less than 1, to me doesn't mean that it's "constantly flipping". Only that upon measurement there's a probability that one of those states gets defined/constrained. This is what i think susskind meant by understanding the whole (probabilities of the wave function of the whole system), without understanding the parts "the particles" i.e. it's kind of not right to think of just the parts here. It's only the system+parts that must be thought of together (which of course is why one of the reasons no-one truly understands it).
- M_Grey 10y agoVery little about QM is intuitive at the ontological level; what matters is that it's logical at the mathematical and practical level. Lasers work, the math works, the ontology doesn't.
- platz 10y agosure, but I'm not sure we should be saying that it doesn't have an ontology (even if it is a very weird one), just because the only model that we've been able to come up with thus far is mostly a calculational one (as QM "calculates" probabilities).
- M_Grey 10y agoIt doesn't have an ontology with a current prayer of being falsifies, which is the same thing if you're interested in the science and not the popsci. Exceptions exist, hence AdS/CFT correspondence and M-Theory, but they are exceptions. Even then, all that can be said is various tools have been created with value beyond their fields, not that they're in any way falsifiable yet. At any rate, "intuitive" strikes me as a bad test for QM.