3 ms·

I still don't get it. Why do the two waves add up? Why can't they just coexist? And if they do add up, doesn't that mean that the atoms are piggyback riding on

by air7 6y ago

I still don't get it. Why do the two waves add up? Why can't they just coexist? And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible?

cygx 6y ago
Why do the two waves add up? Because the electric field only has a single value at any point in space. The equations that describe this are linear (ignoring quantum stuff irerelevant at these scales), so the waves combine by simple addition. Why can't they just coexist? In a way, they do continue to coexist (again due to the linearity): For example, if two waves pass each other by, you get back the original waves once you leave the region of interference. And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible? Rather, the electrons around the atoms start wiggling around, making new waves. I wonder if one could set up a mechanical analogue by putting some floating balls in a pool and attaching them to the bottom so that they breach the surface if a wave passes through, creating new waves...
l33tman 6y ago
I'm not sure that video helps anything. At the very least it confuses the QFT level description of light and electrons with the EM classical description. The "waves" you mention add up because each photon sent in through the glass takes all the possible paths, and you need to add up the contributions from all these paths to see where this single photon wants to go. This looks like wave propagation mathematically. The reason the waves add up is because they are all the same photon. If you send in two unrelated photons, they get their own waves and in that case they do coexist without interference, for all practical field strengths (there are light-on-light scattering effects that come into play at immense field strengths, just now starting to get probed).
- rstuart4133 6y ago
  Yeah, it didn't help anything. But I don't have to invoke QFT and EM to explain why: 1) He's saying the light moves the electrons and the electrons set up a 2nd wave that interferes with the original to produce a lower frequency. That's simple enough. But then he says the lower frequency wave travels slower. Eh, what? We are try to explain is why light travels slower? Ergo, you can use "because it travels slower" in your explanation. 2) That said, the lower frequency does travel slower because light gets split into a spectrum by the glass prism. To me it seems a satisfying answer would not only explain why it got slower, but why slowdown depends on frequency.

by air7 6y ago

cygx 6y ago

Why do the two waves add up? Because the electric field only has a single value at any point in space. The equations that describe this are linear (ignoring quantum stuff irerelevant at these scales), so the waves combine by simple addition. Why can't they just coexist? In a way, they do continue to coexist (again due to the linearity): For example, if two waves pass each other by, you get back the original waves once you leave the region of interference. And if they do add up, doesn't that mean that the atoms are piggyback riding on the light wave and essentially slowing it down which shouldn't be possible? Rather, the electrons around the atoms start wiggling around, making new waves. I wonder if one could set up a mechanical analogue by putting some floating balls in a pool and attaching them to the bottom so that they breach the surface if a wave passes through, creating new waves...

l33tman 6y ago

I'm not sure that video helps anything. At the very least it confuses the QFT level description of light and electrons with the EM classical description. The "waves" you mention add up because each photon sent in through the glass takes all the possible paths, and you need to add up the contributions from all these paths to see where this single photon wants to go. This looks like wave propagation mathematically. The reason the waves add up is because they are all the same photon. If you send in two unrelated photons, they get their own waves and in that case they do coexist without interference, for all practical field strengths (there are light-on-light scattering effects that come into play at immense field strengths, just now starting to get probed).

rstuart4133 6y ago
Yeah, it didn't help anything. But I don't have to invoke QFT and EM to explain why: 1) He's saying the light moves the electrons and the electrons set up a 2nd wave that interferes with the original to produce a lower frequency. That's simple enough. But then he says the lower frequency wave travels slower. Eh, what? We are try to explain is why light travels slower? Ergo, you can use "because it travels slower" in your explanation. 2) That said, the lower frequency does travel slower because light gets split into a spectrum by the glass prism. To me it seems a satisfying answer would not only explain why it got slower, but why slowdown depends on frequency.