4 ms·

I can give it a shot... So Einstein, Podolsky, and Rosen were sitting around (presumably) looking over the mathematical foundations of Quantum Mechanics, when

I can give it a shot... So Einstein, Podolsky, and Rosen were sitting around (presumably) looking over the mathematical foundations of Quantum Mechanics, when they noticed something. In certain situations, you could end up with one wave function describing two physically separated particles in a mutual superposition of states. The consequence is that altering the state of one particle would instantaneously cause the state of the other to become resolved. Effectively they "discovered" quantum entanglement (which has since been verified as a real phenomenon, not just a mathematical curiosity). What really makes the EPR paradox important, though, is what it implies about reality. Einstein, et al. realized that there were only two ways to explain quantum entanglement. One possibility is that the entangled particles contain extra information, inaccessible to normal observation, about their respective states and which way the superposition will resolve. This is the so-called "hidden variables" solution. The other possibility is that an action on one particle is, in fact, instantaneously causing an effect on the other particle. While this is not, strictly speaking, a violation of General Relativity as no information is exchanged, Einstein found this possibility so unsettling that he famously coined it "spooky action at a distance". The conclusion from the EPR paper was that the "hidden variables" solution was more likely, implying that Quantum Mechanics was an as-yet incomplete theory. Fast-forward a couple of decades, and we're just beginning to appreciate that "spooky action at a distance" is actually more likely to be the correct explanation. As I mentioned before, what this implies about reality itself is pretty mind blowing. To go even further, we've since learned that entanglement can not only occur between particles separated in space, but also between particles separated in time. Quite literally, the future and the past may be linked by this "spooky action at a distance". We still don't fully grasp what, exactly, this means. One possible implication of this is that our existence as sentient beings may simultaneously be a consequence and the cause of a universe that can give rise to sentient beings. Needless to say, even though it's likely that Einstein was wrong about "hidden variables", the course of investigation that the EPR paper set the physics community down is at least as important as Relativity.

lindenr 13y ago
The EPR paradox was resolved a while ago, strongly in favour of Einstein's "spooky action at a distance". The first experiments were done in the 70's. See: https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem Also the last paragraph of the introduction at https://en.wikipedia.org/wiki/EPR_paradox https://en.wikipedia.org/wiki/EPR_paradox
- jballanc 13y ago
  Yes, but as with most things in physics, the first signs that an idea is dead usually come decades before the ghost is truly given up. Take, for example, this work from 2012 that was still dealing with a variation on the "hidden variables" formulation: http://arstechnica.com/science/2012/10/quantum-entanglement-shows-that-reality-cant-be-local/ http://arstechnica.com/science/2012/10/quantum-entanglement-...
  - einhverfr 13y ago
    I think you have a larger problem though. A theory is (as Heisenberg explained in his book "Physics and Philosophy") an interpretation of data based upon unscientific, non-falsifiable, a priori assumptions on the part of the theorist. You can't really disprove something as nebulous as "hidden variables." That's not a falsifiable statement. What you can do is disprove some theory based on hidden variables. Such evidence does not apply to the possibility of other theories of hidden variables that are yet unformulated.
    - dllthomas 13y ago
      My understanding is that any sort of local hidden variables have been ruled out (at least, if you want to preserve causality).
      - GregBuchholz 13y ago
        I'm not a physicist, but I'd be interested in people's opinions on: "Chaotic Ball" model,local realism and the Bell test loopholes. http://arxiv.org/abs/quant-ph/0210150 http://arxiv.org/abs/quant-ph/0210150
cynicalkane 13y ago
Not to be "that guy", but a lot of this is terribly incorrect: * General relativity did not come from "grasping the mathematics" of special relativity. SR can be completely understood by an undergraduate and is an internally consistent description of mechanics and electromagnetic phenomena. Like Newtonian mechanics before it, it doesn't need anything more to be consistent. * Local hidden variables are not "likely wrong", they are provably impossible. It is impossible to have hidden variables without breaking the principle of causality. * Entanglement does not break the principle of causality. "Spooky action at a distance" is not an "effect" in a well-defined physical sense. It cannot be used to send information or cause things to happen. * All of this could be--and was--understood without the EPR paper. Relativity was the most important thing since Newton. EPR was minor in comparison. * I have no idea what connection you're trying to draw between sentient life and QM.
- jostylr 13y ago
  Note local hidden variables is impossible. The pilot wave theory is the theory of "hidden variables" that led Bell to his theorem. Basically, EPR shows that either nature is nonlocal or there were hidden variables. Bell showed that hidden variables had to have something nonlocal about them. So Einstein created relativity and helped to highlight how nature has an aspect that seems incompatible with it (basically, there is a "now" which, however, may be undetectable and is not our "now"). For those curious, the nonlocal hidden variables are the positions of the particles. Very hidden. So hidden, they are the only thing we see in experiments! The particles are guided by the wave function. This resolves all the weird paradoxes such as Schrodinger's cat. It provides a great way to understand and investigate nonlocality, spin, identical particles, etc. in a very precise theory that even has broadly applying existence and uniqueness of solution theorems, unlike classical mechanics. http://plato.stanford.edu/entries/qm-bohm/ http://plato.stanford.edu/entries/qm-bohm/
  - tomp 13y ago
    Hi! I'm a mathematician, I've always been curious about physics, but I've never understood some of the concepts of QM, such as what is "observation" in the Schrodinger's cat paradox, why QM means the universe is not deterministic, why hidden variables cannot exist, ... mostly because every physicist that I was able to talk to has been unable to properly explain these concepts. Do you know any books/articles/sources about QM that could understand these concepts, without going in unnecessary mathematical and physical details (i.e. using the least physics and mathematics necessary to explain the paradoxes of QM)?

lindenr 13y ago

The EPR paradox was resolved a while ago, strongly in favour of Einstein's "spooky action at a distance". The first experiments were done in the 70's. See: https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem Also the last paragraph of the introduction at https://en.wikipedia.org/wiki/EPR_paradox https://en.wikipedia.org/wiki/EPR_paradox

jballanc 13y ago
Yes, but as with most things in physics, the first signs that an idea is dead usually come decades before the ghost is truly given up. Take, for example, this work from 2012 that was still dealing with a variation on the "hidden variables" formulation: http://arstechnica.com/science/2012/10/quantum-entanglement-shows-that-reality-cant-be-local/ http://arstechnica.com/science/2012/10/quantum-entanglement-...
- einhverfr 13y ago
  I think you have a larger problem though. A theory is (as Heisenberg explained in his book "Physics and Philosophy") an interpretation of data based upon unscientific, non-falsifiable, a priori assumptions on the part of the theorist. You can't really disprove something as nebulous as "hidden variables." That's not a falsifiable statement. What you can do is disprove some theory based on hidden variables. Such evidence does not apply to the possibility of other theories of hidden variables that are yet unformulated.
  - dllthomas 13y ago
    My understanding is that any sort of local hidden variables have been ruled out (at least, if you want to preserve causality).
    - GregBuchholz 13y ago
      I'm not a physicist, but I'd be interested in people's opinions on: "Chaotic Ball" model,local realism and the Bell test loopholes. http://arxiv.org/abs/quant-ph/0210150 http://arxiv.org/abs/quant-ph/0210150

cynicalkane 13y ago

Not to be "that guy", but a lot of this is terribly incorrect: * General relativity did not come from "grasping the mathematics" of special relativity. SR can be completely understood by an undergraduate and is an internally consistent description of mechanics and electromagnetic phenomena. Like Newtonian mechanics before it, it doesn't need anything more to be consistent. * Local hidden variables are not "likely wrong", they are provably impossible. It is impossible to have hidden variables without breaking the principle of causality. * Entanglement does not break the principle of causality. "Spooky action at a distance" is not an "effect" in a well-defined physical sense. It cannot be used to send information or cause things to happen. * All of this could be--and was--understood without the EPR paper. Relativity was the most important thing since Newton. EPR was minor in comparison. * I have no idea what connection you're trying to draw between sentient life and QM.

jostylr 13y ago
Note local hidden variables is impossible. The pilot wave theory is the theory of "hidden variables" that led Bell to his theorem. Basically, EPR shows that either nature is nonlocal or there were hidden variables. Bell showed that hidden variables had to have something nonlocal about them. So Einstein created relativity and helped to highlight how nature has an aspect that seems incompatible with it (basically, there is a "now" which, however, may be undetectable and is not our "now"). For those curious, the nonlocal hidden variables are the positions of the particles. Very hidden. So hidden, they are the only thing we see in experiments! The particles are guided by the wave function. This resolves all the weird paradoxes such as Schrodinger's cat. It provides a great way to understand and investigate nonlocality, spin, identical particles, etc. in a very precise theory that even has broadly applying existence and uniqueness of solution theorems, unlike classical mechanics. http://plato.stanford.edu/entries/qm-bohm/ http://plato.stanford.edu/entries/qm-bohm/
- tomp 13y ago
  Hi! I'm a mathematician, I've always been curious about physics, but I've never understood some of the concepts of QM, such as what is "observation" in the Schrodinger's cat paradox, why QM means the universe is not deterministic, why hidden variables cannot exist, ... mostly because every physicist that I was able to talk to has been unable to properly explain these concepts. Do you know any books/articles/sources about QM that could understand these concepts, without going in unnecessary mathematical and physical details (i.e. using the least physics and mathematics necessary to explain the paradoxes of QM)?