Starship Asterisk*

alter-ego wrote: ↑Fri Sep 08, 2023 1:53 am

johnnydeep wrote: ↑Thu Sep 07, 2023 1:02 pm

Chris Peterson wrote: ↑Wed Sep 06, 2023 8:55 pm

By quantum mechanisms, where a photon ceases to exist and another one comes into existence. It's QM... if you're looking for something intuitive, you're looking in the wrong place!

Alright. Dang - overmatched again: QM n+1, johnnydeep 0

I'm not sure how comfortable you are with classical wave mechanics math, but for me, Feynman's lecture on the development of the index of refraction and what it means is as intuitive as it gets (The Origin of the Refractive Index). In wave mechanics, electrons (even orbital electrons) are modeled as linear oscillators connected by springs. The result of all e-field interactions resulting from the incident light propagating through a dielectric is a cumulative phase delay accrued over the length of the medium. This derivation is a continuous wave (classical) solution, and cumulative phase retardation from the light generated from oscillating electrons is the origin of the apparent slowing of light in a dielectric.

johnnydeep wrote: ↑Thu Sep 07, 2023 1:02 pm

Chris Peterson wrote: ↑Wed Sep 06, 2023 8:55 pm

johnnydeep wrote: ↑Wed Sep 06, 2023 8:53 pm

Then I still don't get what you meant by "in a medium they [photons] undergo scattering (a photon is absorbed and a new one emitted) which takes time... hence, the light propagates at less than c". How are photons being absorbed/emitted if not via electrons?

By quantum mechanisms, where a photon ceases to exist and another one comes into existence. It's QM... if you're looking for something intuitive, you're looking in the wrong place!

Alright. Dang - overmatched again: QM n+1, johnnydeep 0

Chris Peterson wrote: ↑Wed Sep 06, 2023 8:55 pm

johnnydeep wrote: ↑Wed Sep 06, 2023 8:53 pm

Chris Peterson wrote: ↑Wed Sep 06, 2023 7:38 pm
To be clear, the photon's speed isn't reduced. Massless particles can only go one speed, c. And although the interaction of the photon with matter is called "scattering", it would be a mistake to picture this as balls on a billiard table. The "scattering" here is a mathematical construct derived from quantum mechanics, and isn't the same as more intuitive forms of scattering where the direction of the new photon is different. I'm offering a physical description that is helpful when thinking of light as a particle. Phase velocity is a wave analysis. Both work.

In any case, the scattering process here is unrelated to an atom absorbing a photon, transferring that energy to an electron in a higher state, and then re-emitting another photon when the electron drops again. This scattering is more akin to the creation and annihilation of virtual particles.

Then I still don't get what you meant by "in a medium they [photons] undergo scattering (a photon is absorbed and a new one emitted) which takes time... hence, the light propagates at less than c". How are photons being absorbed/emitted if not via electrons?

By quantum mechanisms, where a photon ceases to exist and another one comes into existence. It's QM... if you're looking for something intuitive, you're looking in the wrong place!

orin stepanek wrote: ↑Wed Sep 06, 2023 8:54 pm DSCF7208_HESS900c.jpg
It is amazing what Scientists learn about the universe! I don't
qualify; but I like to read about it! If we don't kill ourselves
first; man could have a forever home in the universe with all the
wonderful goodies that are there! What a dreamer I am!

johnnydeep wrote: ↑Wed Sep 06, 2023 8:53 pm

Chris Peterson wrote: ↑Wed Sep 06, 2023 7:38 pm

johnnydeep wrote: ↑Wed Sep 06, 2023 7:21 pm

Hmm. While I can understand the idea of a photon's speed being reduced in a medium due to having to be repeatedly absorbed and reemitted, why is direction or reemission predominantly along the same path as the original photon (at least in air, and lenses, etc)? And how is this process related to the phase velocity (whatever that is)? Also, I would think the absorption/reemission process would result in a MUCH slower speed than an unimpeded photon!

To be clear, the photon's speed isn't reduced. Massless particles can only go one speed, c. And although the interaction of the photon with matter is called "scattering", it would be a mistake to picture this as balls on a billiard table. The "scattering" here is a mathematical construct derived from quantum mechanics, and isn't the same as more intuitive forms of scattering where the direction of the new photon is different. I'm offering a physical description that is helpful when thinking of light as a particle. Phase velocity is a wave analysis. Both work.

In any case, the scattering process here is unrelated to an atom absorbing a photon, transferring that energy to an electron in a higher state, and then re-emitting another photon when the electron drops again. This scattering is more akin to the creation and annihilation of virtual particles.

Then I still don't get what you meant by "in a medium they [photons] undergo scattering (a photon is absorbed and a new one emitted) which takes time... hence, the light propagates at less than c". How are photons being absorbed/emitted if not via electrons?

Chris Peterson wrote: ↑Wed Sep 06, 2023 7:38 pm

johnnydeep wrote: ↑Wed Sep 06, 2023 7:21 pm

Chris Peterson wrote: ↑Wed Sep 06, 2023 7:07 pm
Other ways of looking at it. The "speed of light" is not the same thing as "c". Photons always travel at c, but in a medium they undergo scattering (a photon is absorbed and a new one emitted) which takes time... hence, the light propagates at less than c. If a charged particle (matter) travels faster than this reduced speed of light, you get Cherenkov radiation.

Hmm. While I can understand the idea of a photon's speed being reduced in a medium due to having to be repeatedly absorbed and reemitted, why is direction or reemission predominantly along the same path as the original photon (at least in air, and lenses, etc)? And how is this process related to the phase velocity (whatever that is)? Also, I would think the absorption/reemission process would result in a MUCH slower speed than an unimpeded photon!

To be clear, the photon's speed isn't reduced. Massless particles can only go one speed, c. And although the interaction of the photon with matter is called "scattering", it would be a mistake to picture this as balls on a billiard table. The "scattering" here is a mathematical construct derived from quantum mechanics, and isn't the same as more intuitive forms of scattering where the direction of the new photon is different. I'm offering a physical description that is helpful when thinking of light as a particle. Phase velocity is a wave analysis. Both work.

In any case, the scattering process here is unrelated to an atom absorbing a photon, transferring that energy to an electron in a higher state, and then re-emitting another photon when the electron drops again. This scattering is more akin to the creation and annihilation of virtual particles.

johnnydeep wrote: ↑Wed Sep 06, 2023 7:21 pm

Chris Peterson wrote: ↑Wed Sep 06, 2023 7:07 pm

AVAO wrote: ↑Wed Sep 06, 2023 6:49 pm

Well Ann

I think you are right. The somewhat cumbersome statement in the original link has been oversimplified.

"Cherenkov radiation is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wavefront in a medium) of light in that medium."

Small but important difference ;-)

Jac

Other ways of looking at it. The "speed of light" is not the same thing as "c". Photons always travel at c, but in a medium they undergo scattering (a photon is absorbed and a new one emitted) which takes time... hence, the light propagates at less than c. If a charged particle (matter) travels faster than this reduced speed of light, you get Cherenkov radiation.

Hmm. While I can understand the idea of a photon's speed being reduced in a medium due to having to be repeatedly absorbed and reemitted, why is direction or reemission predominantly along the same path as the original photon (at least in air, and lenses, etc)? And how is this process related to the phase velocity (whatever that is)? Also, I would think the absorption/reemission process would result in a MUCH slower speed than an unimpeded photon!

Chris Peterson wrote: ↑Wed Sep 06, 2023 7:07 pm

AVAO wrote: ↑Wed Sep 06, 2023 6:49 pm

Ann wrote: ↑Wed Sep 06, 2023 4:33 am I coughed when I read this: Blue light, how nice... but - charged particles moving slightly faster than the speed of light??? ... Ann
Edit: I just watched the video, but I didn't see any blue light in the sky. What a pity.

Well Ann

I think you are right. The somewhat cumbersome statement in the original link has been oversimplified.

"Cherenkov radiation is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wavefront in a medium) of light in that medium."

Small but important difference

Jac

Other ways of looking at it. The "speed of light" is not the same thing as "c". Photons always travel at c, but in a medium they undergo scattering (a photon is absorbed and a new one emitted) which takes time... hence, the light propagates at less than c. If a charged particle (matter) travels faster than this reduced speed of light, you get Cherenkov radiation.

AVAO wrote: ↑Wed Sep 06, 2023 6:49 pm

Ann wrote: ↑Wed Sep 06, 2023 4:33 am I coughed when I read this:

They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air.

Blue light, how nice... but - charged particles moving slightly faster than the speed of light??? ... Ann
Edit: I just watched the video, but I didn't see any blue light in the sky. What a pity.

Well Ann

I think you are right. The somewhat cumbersome statement in the original link has been oversimplified.

"Cherenkov radiation is electromagnetic radiation emitted when a charged particle (such as an electron) passes through a dielectric medium at a speed greater than the phase velocity (speed of propagation of a wavefront in a medium) of light in that medium."

Small but important difference ;-)

Jac

cesarclark wrote: ↑Wed Sep 06, 2023 9:02 am

Ann wrote: ↑Wed Sep 06, 2023 4:33 am I coughed when I read this:

They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air.

Blue light, how nice... but - charged particles moving slightly faster than the speed of light??? What's your opinion on that, Einstein?
...
Oh! Charged particles moving faster than light in air. Yes, light moves at almost 300,000 kilometer per second in a vacuum. Not when it moves through a medium like air.

I guess you can go back to sleep now, Albert. I don't need you here any more.

Ann

Edit: I just watched the video, but I didn't see any blue light in the sky. What a pity.

I don't know if it's really blue light. It's at 0:34 in video.

Ann wrote: ↑Wed Sep 06, 2023 4:33 am I coughed when I read this:

They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air.

Blue light, how nice... but - charged particles moving slightly faster than the speed of light??? ... Ann
Edit: I just watched the video, but I didn't see any blue light in the sky. What a pity.

Roy wrote: ↑Wed Sep 06, 2023 5:37 pm OK, I went to the HESS website to get the process straight in my mind, then to look at some of the articles they produced. The five observation machines are in the Namibian desert (once known as German South-West Africa), very dry.
1. Detect the flash-track when a high energy gamma ray hits the atmosphere.
2. Triangulate the direction it may have come from when the gamma ray hit atmosphere 10 kilometers up.
3. Infer that it came from something interesting in that direction, which is easy to do, because there is something interesting in every small part of the sky.

Now come the articles, which seem to me to be “we don’t know” hand-wavium. Here are some sample phrases:
“Dark matter annihilation”. “Evaporation of primordial black holes”. “Binary black hole coalescence events”
And one article, which had 240 authors, referred to a “hadronic scenario”, a “leptonic scenario”, and a “binary scenario”.

Ringo Supernova wrote: ↑Wed Sep 06, 2023 1:04 pm What is this song? It's as amazing as the video!

Ann wrote: ↑Wed Sep 06, 2023 4:33 am I coughed when I read this:

They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air.

Blue light, how nice... but - charged particles moving slightly faster than the speed of light??? What's your opinion on that, Einstein?

---

bloxd io


Oh! Charged particles moving faster than light in air. Yes, light moves at almost 300,000 kilometer per second in a vacuum. Not when it moves through a medium like air.

I guess you can go back to sleep now, Albert. I don't need you here any more.

Ann

Edit: I just watched the video, but I didn't see any blue light in the sky. What a pity.

They are designed to detect strange flickers of blue light -- Cherenkov radiation --emitted when charged particles move slightly faster than the speed of light in air.