> but how can we know for sure that it's intrinsic randomness?
We can't be sure (even though Luboš Motl disagrees). The universe may not be intrinsically random if superdeterminism is true. However, this isn't a popular idea in physics; in fact Gerard t'Hooft is the only Nobel prize winner I can think of that supports this idea.
Local hidden variable theories have almost all been ruled out by experiment in the last few decades (the same kind of theory Einstein was trying to find). There's still some loopholes, but, eh...
Colbeck and Renner have written a pretty powerful paper that given the assumption that measurements can be chosen freely, no extension of QM can have improved predictive power than the current theory of QM, regardless of whether this randomness "comes" from somewhere. Here's the paper: http://arxiv.org/abs/1005.5173
It's not a dumb question, that's what we should start by assuming because it makes a lot more sense. This is referred to as "local hidden variables".
Bell's theorem (http://en.wikipedia.org/wiki/Bell's_theorem) and the tests around it show this to be false, things just don't act like there's a variable we simply don't know.
That is exactly the experiment I couldn't remember the name of and tried to ramble my way around it via another route.
This is the clearest way to understand that it must be truly random and not just hidden ignorance of state, and has been tested to a remarkable degree of accuracy.
Not knowing the specific state - and being incapable of knowing - is the critical bit.
TL;DR: (edit) ignore the rest of this and just go read a lot on Bell's Theorem and the Double Slit Experiment. Both demonstrate concretely the essence of the weirdness.
(I'll leave the rest below just to keep me honest about my ignorance.)
it's a provably special state where randomness allows it to have the needed characteristics for the math to work at all. [1]
Also disclaimer: I'm not a PhD physicist, just a BS in engineering physics. This is merely how I cope with the idea.
Think of it like this: given time, chaotic systems become unpredictable and noisy. An electron whizzing around an atom doesn't need much time before it's state is utterly unpredictable at any measurable level (Heisenberg Uncertainty assures this must happen eventually, and pretty fast it turns out).
So what if we isolate that little electron from the outside world? Then even the universe becomes uncertain, and must assume ANY possible state is valid and happening at once, blurring them together based on how likely each is given the last known observation! Otherwise the next observation could be contradictory to expectations, and seg fault the universe (perhaps just locally). This way though, the universe can always say "well I thought it could have been that" and be correct, even if it only thought it really unlikely.
But until the universe interacts with the state then, once coherence [2] sets in only probability can hint at what state it is in. Thus it is random in the before-the-fact sense like the probability before rolling a die, as opposed to an after-the-fact sense like a coin that is already flipped yet hidden.
And - here's the catch - the math needs to treat it as unknown. To label the parts of an unlabeled system will break the algebra! And if a distribution of states is unknown in a mathematically precise ignorance, this is indistinguishable from true randomness. Such pure conceptual states are rare in our messy universe (at least observationally), but they do exist and can be proven to exist in the same way a one time pad can be proven to be cryptographically secure.
Thanks for being my duck, I needed to think about something interesting today!
[1] Perhaps the most amazing bit is that if it weren't random, then it would act differently, since the distribution of observations would vary from the expected random field after enough observations. And this has been done! Study the double slit experiment deeply, as it really is a fantastic empirical example of why interfering via observation adjusts our interpretation of the results.
[2]This term is jargon here, like quark colors. It's not coherence like a laser's.
Not dumb at all! Is a discussion that can be a little misleading, so be wary and don't blindly trust some random guy on a forum.
In the video they relate this randomness to the fact that qubits live in a superposition of states, in contrast to bits, which can be either 1 or 0, but not both. This means that when you measure a qubit, you would still obtain 1 OR 0, but you will obtain each with certain probability. So, how is this different to just picking a random bit and then measuring it? You could fabricate a way of picking those classical bit that yield the same distribution than the qubit.
We find a difference in one of the very first experiments that highlighted the quantum nature of the microscopic world; the double slit experiment: you have two screens, one with 2 small holes and the other is placed behind that. Then you send a bunch of classical particles through the punctured screen and you receive the counts on the screen behind it. Then you have a distribution of places where those particles hit. Then you do the same with quantum particles this time. You obtain a different pattern (this is important, is different to the classical pattern). This pattern is called "diffraction" pattern and means there was a wave like interaction between the particles. So now you try to do the same experiment with quantum particles, but sending them one by one, so no classical-like interaction could exist. Amazingly, you find the same diffraction pattern. This means one particle can "interact" with itself. The way physicists explain this is by allowing quantum states to exists on this superposition states which induces intrinsic randomness due to the fact that when measured, each particle could only have travelled through one of the slits, or each qubit (after measurement) can only be 1 OR 0, but not both.
This difference between classical and quantum description of nature goes deeper, and makes mechanisms as entanglement and teleportation possible. "Bell inequalities" are a great example of of some other, related ways in which both differ.
Pragmatically, we're kind of forced to assume simplicity until we can show otherwise. "Maybe we just haven't figured it out" applies to everything. Maybe we just haven't figured out how to violate energy conservation, or time travel, or escape the matrix.
Quantum randomness has a quality that makes it hard to explain in other ways. The results of measurements can't be made up on the spot, because they need to correlate with remote measurements. They also can't be stored ahead of time, because of how the correlation depends on which measurement is performed.
Xcelerate|12 years ago
We can't be sure (even though Luboš Motl disagrees). The universe may not be intrinsically random if superdeterminism is true. However, this isn't a popular idea in physics; in fact Gerard t'Hooft is the only Nobel prize winner I can think of that supports this idea.
Local hidden variable theories have almost all been ruled out by experiment in the last few decades (the same kind of theory Einstein was trying to find). There's still some loopholes, but, eh...
Colbeck and Renner have written a pretty powerful paper that given the assumption that measurements can be chosen freely, no extension of QM can have improved predictive power than the current theory of QM, regardless of whether this randomness "comes" from somewhere. Here's the paper: http://arxiv.org/abs/1005.5173
IanCal|12 years ago
Bell's theorem (http://en.wikipedia.org/wiki/Bell's_theorem) and the tests around it show this to be false, things just don't act like there's a variable we simply don't know.
HCIdivision17|12 years ago
This is the clearest way to understand that it must be truly random and not just hidden ignorance of state, and has been tested to a remarkable degree of accuracy.
HCIdivision17|12 years ago
TL;DR: (edit) ignore the rest of this and just go read a lot on Bell's Theorem and the Double Slit Experiment. Both demonstrate concretely the essence of the weirdness.
(I'll leave the rest below just to keep me honest about my ignorance.) it's a provably special state where randomness allows it to have the needed characteristics for the math to work at all. [1]
Also disclaimer: I'm not a PhD physicist, just a BS in engineering physics. This is merely how I cope with the idea.
Think of it like this: given time, chaotic systems become unpredictable and noisy. An electron whizzing around an atom doesn't need much time before it's state is utterly unpredictable at any measurable level (Heisenberg Uncertainty assures this must happen eventually, and pretty fast it turns out).
So what if we isolate that little electron from the outside world? Then even the universe becomes uncertain, and must assume ANY possible state is valid and happening at once, blurring them together based on how likely each is given the last known observation! Otherwise the next observation could be contradictory to expectations, and seg fault the universe (perhaps just locally). This way though, the universe can always say "well I thought it could have been that" and be correct, even if it only thought it really unlikely.
But until the universe interacts with the state then, once coherence [2] sets in only probability can hint at what state it is in. Thus it is random in the before-the-fact sense like the probability before rolling a die, as opposed to an after-the-fact sense like a coin that is already flipped yet hidden.
And - here's the catch - the math needs to treat it as unknown. To label the parts of an unlabeled system will break the algebra! And if a distribution of states is unknown in a mathematically precise ignorance, this is indistinguishable from true randomness. Such pure conceptual states are rare in our messy universe (at least observationally), but they do exist and can be proven to exist in the same way a one time pad can be proven to be cryptographically secure.
Thanks for being my duck, I needed to think about something interesting today!
[1] Perhaps the most amazing bit is that if it weren't random, then it would act differently, since the distribution of observations would vary from the expected random field after enough observations. And this has been done! Study the double slit experiment deeply, as it really is a fantastic empirical example of why interfering via observation adjusts our interpretation of the results. [2]This term is jargon here, like quark colors. It's not coherence like a laser's.
olvar|12 years ago
In the video they relate this randomness to the fact that qubits live in a superposition of states, in contrast to bits, which can be either 1 or 0, but not both. This means that when you measure a qubit, you would still obtain 1 OR 0, but you will obtain each with certain probability. So, how is this different to just picking a random bit and then measuring it? You could fabricate a way of picking those classical bit that yield the same distribution than the qubit.
We find a difference in one of the very first experiments that highlighted the quantum nature of the microscopic world; the double slit experiment: you have two screens, one with 2 small holes and the other is placed behind that. Then you send a bunch of classical particles through the punctured screen and you receive the counts on the screen behind it. Then you have a distribution of places where those particles hit. Then you do the same with quantum particles this time. You obtain a different pattern (this is important, is different to the classical pattern). This pattern is called "diffraction" pattern and means there was a wave like interaction between the particles. So now you try to do the same experiment with quantum particles, but sending them one by one, so no classical-like interaction could exist. Amazingly, you find the same diffraction pattern. This means one particle can "interact" with itself. The way physicists explain this is by allowing quantum states to exists on this superposition states which induces intrinsic randomness due to the fact that when measured, each particle could only have travelled through one of the slits, or each qubit (after measurement) can only be 1 OR 0, but not both.
This difference between classical and quantum description of nature goes deeper, and makes mechanisms as entanglement and teleportation possible. "Bell inequalities" are a great example of of some other, related ways in which both differ.
Strilanc|12 years ago
Quantum randomness has a quality that makes it hard to explain in other ways. The results of measurements can't be made up on the spot, because they need to correlate with remote measurements. They also can't be stored ahead of time, because of how the correlation depends on which measurement is performed.