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The principle of superposition is that any addition of quantum states is also a valid quantum state. For example, you can have a system that has a state which i
by bowsamic 4y ago
The principle of superposition is that any addition of quantum states is also a valid quantum state. For example, you can have a system that has a state which is both "the particle is at x = 1m" and "the particle is at x = 2m". What really happens physically when you measure the position of the particle is not specified by the theory, but the Copenhagen interpretation says that one of those positions will be chosen depending on the coefficient in front of each state, e.g. if we have a state |psi> = 1/sqrt(2) |x = 1m> + 1/sqrt(2) |x = 2m> then we get either 1m or 2m with 50/50 probability. Then if we measure 1m we will find that afterwards the state will be |psi> = |x = 1m>. Again, what actually happens during this process is not well understood by physics yet, and also is one of the things that differentiates different interpretations of quantum mechanics, so yes that is "philosophy".
> The cat is dead and alive because you just don't know it. But it surely is one or the other, isn't it?
The Copenhagen interpretation says it is both dead or alive, or that the above particle really was in two places at once. Other interpretations, such as the Many World Interpretation (MWI), say different things. For example, MWI says that upon measurement the universe actually branches into many universes, one for each possible outcome, and that we live in one such universe (the universe with a living cat, for example), but that the state itself is not ontologically a mixture of those two orthogonal possibilities.
Entanglement is a very different beast. Basically it means that two systems interact such that measurements on them are highly correlated, i.e. by measuring something on system A you immediately know something about system B, even if they are very far apart. Again, how you interpret entanglement (i.e. whether or not "spooky action at a distance" actually happens) depends on your interpretation of quantum mechanics.
- friendzis 4y agoMy go-to ELI5 explanation for entanglement: suppose you take an object, cut it along non-symmetry axis (e.g. cut a bill), place both halves in different envelopes. Upon opening one envelope (measuring) you immediately know what is in the other envelope. Not because of "spooky action at a distance", but rather due to prior knowledge of the system[s].
- bowsamic 4y agoThat is not correct though, at least not in the Copenhagen interpretation. The reason is that the quantum state is supposed to be a complete representation of the system right now. It doesn't quantify our ignorance, it should actually correspond completely to the physical state of the system (check the PBR theorem for a strong justification of this) Therefore, according to Copenhagen, the state really is collapsed for the other system upon measurement of the first. There is spooky action at a distance, because the entangled state does not encode our lack of information or ignorance, it encodes an actual physical thing. The most common example is two entangled spin systems A and B in an initial state: |psi> = 1/sqrt(2) (|upA upB> + |downA downB>) if you then measure the spin of A and find it to be |upA>, you then know system B is in |upB> BUT it wasn't |upB> all along, because |psi> before measurement is the complete physical state of the quantum system. |psi> is not an epistemological construct, it is an ontological one. It does not correspond to our lack of knowledge of the system, but the actual physical system.
- gryn 4y agoso god doesn't following best practices and uses global interdependent variables. maybe our universe was a PoC and the Universe 2.0 is going to be well refactored version.
- maskedinvader 4y agoNot quite, entanglement is indeed spooky action at a distance given bell’s inequality theorem showed that it can’t be local hidden variables (prior knowledge)
- bowsamic 4y agoAlso not quite, since that depends on your interpretation of quantum mechanics. For example, in the Many Worlds Interpretation there is not spooky action at a distance, since there is no wavefunction collapse in the first place, i.e. measurement does not cause a projection operator to occur, but instead occurs through unitary decoherence. Therefore, no spooky action at a distance (although it is still non local)
- hoseja 4y agoIt's not really about "measurement" but "interaction with the broader reality".
- bowsamic 4y agoNo, certain interpretations (such as Many Worlds Interpretation (MWI)) say that "measurement" is just "interactions with the broader reality". Copenhagen says that "measurement" is a primitive feature of the theory that cannot be explained via such interactions. I.e. Copenhagen interpretation says that Quantum Mechanics cannot describe the physics of the photodiode entirely, since it cannot describe the measurement process. MWI says that measurement occurs due to decohering interactions with the environment