5 ms·

> It begins with a single polarized photon that, when measured, can have either a horizontal polarization or a vertical polarization. But before the measurement

by keyle 8y ago

> It begins with a single polarized photon that, when measured, can have either a horizontal polarization or a vertical polarization. But before the measurement, according to the laws of quantum mechanics, the photon exists in both polarization states at the same time—a so-called superposition. Pardon my basic understanding, but doesn't it meant that it's both simply because it hasn't been measured - and therefor we don't know? Meaning it's either one or the other; at a very fast rate, it's both.

shredprez 8y ago
Piggybacking on my similarly basic understanding of wave-particle duality: it is literally both, and the mode of measurement is what causes the superposition to collapse into one state or another. In other words, you can “force” a superpositioned particle into the desired state simply by measuring it appropriately. We infer from this odd arrangement that the particle contains both states at once. Pardon any incorrectly used terminology above, but I believe my explanation is conceptually sound.
colanderman 8y ago
"Exists in both states" is a very misleading description of how quantum states work. I wish introductory texts didn't use that phrasing (or "superposition", for that matter). A photon can have one of an infinity of polarizations. These states are describable as a mixture of any two orthogonal polarizations (a "basis"), such as "horizontal" and "vertical". "Left circular" and "right circular" are another choice. The funny thing happens when you measure "how much" a photon is polarized into any given orientation. It appears as being polarized either in that orientation, or in the polarization complementary to what you measured. E.g., horizontal or vertical. The outcome is determined with a probability based on the actual polarization of the photon prior to your measurement. Then – the funny thing – the photon takes on that polarization. Repeated measurement using the same basis gives the same results. Measure again with a different basis, and the game starts over again. You can see this in the macro world with a laptop screen and two pairs of polarizing sunglasses. Using one pair of sunglasses, you should be able to block the light from the screen entirely – you are holding them perpendicular to the polarization of the screen. Place the second pair in between the first and the screen, and rotate it. At certain orientations, you will be able to see the screen! The second pair (nearest the screen) is diagonally polarized, and will "convert" the light from the screen into that polarization, before it reaches the first pair (furthest from the screen), which is orthogonal to the screen only. (Note: Things change completely once a second photon enters the picture. It can be entangled with the first, and their states are no longer independently describable. Measurement of one affects the pair.)
folie 8y ago
What you're referring to is known as a "hidden variable theory," (the photon does have some fixed polarization, but the value is hidden from us). This was argued about for decades, until tests of Bell's inequality (https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem) essentially disproved hidden variables. Particles really can exist in a superposition of multiple states simultaneously, and it's not because of our lack of knowledge or our frame of reference. It's a delightfully strange result.
- improbable22 8y ago
  Yes. Although maybe it's worth adding that what was disproved is local hidden variables. The not-disproved hidden variable ideas must have explicit instantaneous action at large distances, i.e. faster-than-light communication among the hidden variables. And that is a pretty ugly thing to have; before Bell (and Aspect, maybe) one could hope that this would not be necessary.
chii 8y ago
> it's both simply because it hasn't been measured ... Meaning it's either one or the other A common misconception about quantum superposition is that the particle "has" some state, and we only use superposition to model that we "dont know". This is called the hidden-variable theory (https://en.wikipedia.org/wiki/Hidden-variable_theory https://en.wikipedia.org/wiki/Hidden-variable_theory). It's been proven that this hidden variable theory can't be true because of https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem. This is completely non-intuitive. See this video for a laymen's explanation: https://www.youtube.com/watch?v=z-s3q9wlLag https://www.youtube.com/watch?v=z-s3q9wlLag (you may also need to watch the prev. video about the EPR paradox https://www.youtube.com/watch?v=5HJK5tQIT4A https://www.youtube.com/watch?v=5HJK5tQIT4A). Edit: actually, this is a much better video https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c - less technical and more visual.
- keyle 8y ago
  Brilliant, thank you!
marcosdumay 8y ago
That is the sane reaction after learning some quantum physics, but the answer is that no, it doesn't exist on both states. It is easier to understand if you look at photons position. Try to calculate the interference between any two waves, and compare to the odds of finding a photon somewhere it they were dispersed proportional to the waves amplitudes at the start.

by keyle 8y ago

shredprez 8y ago

Piggybacking on my similarly basic understanding of wave-particle duality: it is literally both, and the mode of measurement is what causes the superposition to collapse into one state or another. In other words, you can “force” a superpositioned particle into the desired state simply by measuring it appropriately. We infer from this odd arrangement that the particle contains both states at once. Pardon any incorrectly used terminology above, but I believe my explanation is conceptually sound.

colanderman 8y ago

"Exists in both states" is a very misleading description of how quantum states work. I wish introductory texts didn't use that phrasing (or "superposition", for that matter). A photon can have one of an infinity of polarizations. These states are describable as a mixture of any two orthogonal polarizations (a "basis"), such as "horizontal" and "vertical". "Left circular" and "right circular" are another choice. The funny thing happens when you measure "how much" a photon is polarized into any given orientation. It appears as being polarized either in that orientation, or in the polarization complementary to what you measured. E.g., horizontal or vertical. The outcome is determined with a probability based on the actual polarization of the photon prior to your measurement. Then – the funny thing – the photon takes on that polarization. Repeated measurement using the same basis gives the same results. Measure again with a different basis, and the game starts over again. You can see this in the macro world with a laptop screen and two pairs of polarizing sunglasses. Using one pair of sunglasses, you should be able to block the light from the screen entirely – you are holding them perpendicular to the polarization of the screen. Place the second pair in between the first and the screen, and rotate it. At certain orientations, you will be able to see the screen! The second pair (nearest the screen) is diagonally polarized, and will "convert" the light from the screen into that polarization, before it reaches the first pair (furthest from the screen), which is orthogonal to the screen only. (Note: Things change completely once a second photon enters the picture. It can be entangled with the first, and their states are no longer independently describable. Measurement of one affects the pair.)

folie 8y ago

What you're referring to is known as a "hidden variable theory," (the photon does have some fixed polarization, but the value is hidden from us). This was argued about for decades, until tests of Bell's inequality (https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem) essentially disproved hidden variables. Particles really can exist in a superposition of multiple states simultaneously, and it's not because of our lack of knowledge or our frame of reference. It's a delightfully strange result.

improbable22 8y ago
Yes. Although maybe it's worth adding that what was disproved is local hidden variables. The not-disproved hidden variable ideas must have explicit instantaneous action at large distances, i.e. faster-than-light communication among the hidden variables. And that is a pretty ugly thing to have; before Bell (and Aspect, maybe) one could hope that this would not be necessary.

chii 8y ago

> it's both simply because it hasn't been measured ... Meaning it's either one or the other A common misconception about quantum superposition is that the particle "has" some state, and we only use superposition to model that we "dont know". This is called the hidden-variable theory (https://en.wikipedia.org/wiki/Hidden-variable_theory https://en.wikipedia.org/wiki/Hidden-variable_theory). It's been proven that this hidden variable theory can't be true because of https://en.wikipedia.org/wiki/Bell%27s_theorem https://en.wikipedia.org/wiki/Bell%27s_theorem. This is completely non-intuitive. See this video for a laymen's explanation: https://www.youtube.com/watch?v=z-s3q9wlLag https://www.youtube.com/watch?v=z-s3q9wlLag (you may also need to watch the prev. video about the EPR paradox https://www.youtube.com/watch?v=5HJK5tQIT4A https://www.youtube.com/watch?v=5HJK5tQIT4A). Edit: actually, this is a much better video https://www.youtube.com/watch?v=ZuvK-od647c https://www.youtube.com/watch?v=ZuvK-od647c - less technical and more visual.

keyle 8y ago
Brilliant, thank you!

marcosdumay 8y ago

That is the sane reaction after learning some quantum physics, but the answer is that no, it doesn't exist on both states. It is easier to understand if you look at photons position. Try to calculate the interference between any two waves, and compare to the odds of finding a photon somewhere it they were dispersed proportional to the waves amplitudes at the start.