5 ms·

Here is one of my favorites. Take a light source and shine it at a filter that is polarized up/down that is in front of a filter that is polarized left/right. N

Here is one of my favorites. Take a light source and shine it at a filter that is polarized up/down that is in front of a filter that is polarized left/right. None of the light will get through: the first filter removes all of the L/R components of the light, and the second filter removes all the remaining light. Now add a third filter, between the two, which is polarized at 45 degrees. Now some of the light goes through! If this doesn't surprise you, imagine there was a man firing a machine gun at a pair of walls. The two walls are thick enough that they absorb all the bullets. But when you add a third wall in between them, some of the bullets go through.

marcosdumay 5y ago
Not surprisingly, this is related to the observer effect of Quantum Mechanics.
ffhhj 5y ago
For me this is as surprising as the double slit experiment, but much easier to reproduce.
jiggawatts 5y ago
It makes more sense if you think of the polariser sheets as fences with vertical bars, and light like a string going through between two bars and being wiggled. If the direction of the waves in the string are at right-angles to the slit, the wave can't propagate through the bars. If the wave is at an angle, then some of the wave gets through, with a maximum of 100% when the wave wiggles up and down in the same direction as the slit. In this model you can visualise how three 45-degree slits will allow some light to go through even though two 90 degree slits don't -- the light is transformed by its passage through the polarisers. Where QM people get themselves confused is that they assume that the light is not transformed, even though it clearly is... I.e.: If the polarisers didn't rotate the angle of the light waves, then successive aligned polarisers would result in very close to 0 light making it through, but this is not what happens. (Excluding losses due to non-polariser-related effects) All of this goes back to a fundamental confusion between two possibilities: 1) Is it the EM field in free space that is quantised? OR 2) Is it just the interactions of the EM field with matter that are quantised? If you actually run the experiments to check which one, it turns out that (2) is true, but most QM people think (1) is true because most of the time they're indistinguishable. See this video for such an experiment: https://www.youtube.com/watch?v=SDtAh9IwG-I https://www.youtube.com/watch?v=SDtAh9IwG-I
- joshuamorton 5y ago
  IIRC, you can grok this with linear transforms: given X = [[1,0],[0,0]] and Y = [[0,0],[0,1]], aXY = 0 for all a (because XY == [[0,0],[0,0]]). But, given V = [[1,1],[1,1]] (or, in fact, a lot of other matrices/linear transforms), XVT = [[0,1],[0,0]], which blocks all vertical components and rotates the horizontal component 90 degrees. So if you imagine light as a wave with a horizontal and vertical component, and the polarizing filter applies a linear transform to the light.
- amelius 5y ago
  Can you explain the double slit experiment with this approach?
  - jiggawatts 5y ago
    Not really, the waves in the double slit experiment travel through vacuum (or air), not through a solid like with polariser sheets.
    - amelius 5y ago
      But they interact with the matter at the boundary of the slit.
_Microft 5y ago
You can do that 'continuously' by inserting many layers of slightly rotated polarizers to rotate the polarization plane by an arbitrary angle. An applied example of this are liquid crystal screens. Molecules take the role of the polarizers there. The rotation angle depends on an applied electric field and when you sandwich a layer of crystals between polarizers, you almost have a screen: https://www.britannica.com/technology/twisted-nematic-cell https://www.britannica.com/technology/twisted-nematic-cell
akomtu 5y ago
My hunch is that light does get thru the first two filters, but it gets "squeezed" so it's hard to see it. The third filter "unpacks" the light.

marcosdumay 5y ago

Not surprisingly, this is related to the observer effect of Quantum Mechanics.

ffhhj 5y ago

For me this is as surprising as the double slit experiment, but much easier to reproduce.

jiggawatts 5y ago

It makes more sense if you think of the polariser sheets as fences with vertical bars, and light like a string going through between two bars and being wiggled. If the direction of the waves in the string are at right-angles to the slit, the wave can't propagate through the bars. If the wave is at an angle, then some of the wave gets through, with a maximum of 100% when the wave wiggles up and down in the same direction as the slit. In this model you can visualise how three 45-degree slits will allow some light to go through even though two 90 degree slits don't -- the light is transformed by its passage through the polarisers. Where QM people get themselves confused is that they assume that the light is not transformed, even though it clearly is... I.e.: If the polarisers didn't rotate the angle of the light waves, then successive aligned polarisers would result in very close to 0 light making it through, but this is not what happens. (Excluding losses due to non-polariser-related effects) All of this goes back to a fundamental confusion between two possibilities: 1) Is it the EM field in free space that is quantised? OR 2) Is it just the interactions of the EM field with matter that are quantised? If you actually run the experiments to check which one, it turns out that (2) is true, but most QM people think (1) is true because most of the time they're indistinguishable. See this video for such an experiment: https://www.youtube.com/watch?v=SDtAh9IwG-I https://www.youtube.com/watch?v=SDtAh9IwG-I

joshuamorton 5y ago
IIRC, you can grok this with linear transforms: given X = [[1,0],[0,0]] and Y = [[0,0],[0,1]], aXY = 0 for all a (because XY == [[0,0],[0,0]]). But, given V = [[1,1],[1,1]] (or, in fact, a lot of other matrices/linear transforms), XVT = [[0,1],[0,0]], which blocks all vertical components and rotates the horizontal component 90 degrees. So if you imagine light as a wave with a horizontal and vertical component, and the polarizing filter applies a linear transform to the light.
amelius 5y ago
Can you explain the double slit experiment with this approach?
- jiggawatts 5y ago
  Not really, the waves in the double slit experiment travel through vacuum (or air), not through a solid like with polariser sheets.
  - amelius 5y ago
    But they interact with the matter at the boundary of the slit.

_Microft 5y ago

You can do that 'continuously' by inserting many layers of slightly rotated polarizers to rotate the polarization plane by an arbitrary angle. An applied example of this are liquid crystal screens. Molecules take the role of the polarizers there. The rotation angle depends on an applied electric field and when you sandwich a layer of crystals between polarizers, you almost have a screen: https://www.britannica.com/technology/twisted-nematic-cell https://www.britannica.com/technology/twisted-nematic-cell

akomtu 5y ago

My hunch is that light does get thru the first two filters, but it gets "squeezed" so it's hard to see it. The third filter "unpacks" the light.