APOD: HESS Telescopes Explore the Sky... (2023 Sep 06)

[APOD Robot]

HESS Telescopes Explore the High-Energy Sky

Explanation: They may look like modern mechanical dinosaurs, but they are enormous swiveling eyes that watch the sky. The High Energy Stereoscopic System (H.E.S.S.) Observatory is composed of four 12-meter reflecting-mirror telescopes surrounding a larger telescope housing a 28-meter mirror. 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. This light is emitted when a gamma ray from a distant source strikes a molecule in Earth's atmosphere and starts a charged-particle shower. H.E.S.S. is sensitive to some of the highest energy photons (TeV) crossing the universe. Operating since 2003 in Namibia, H.E.S.S. has searched for dark matter and has discovered over 50 sources emitting high energy radiation including supernova remnants and the centers of galaxies that contain supermassive black holes. Pictured in June, H.E.S.S. telescopes swivel and stare in time-lapse sequences shot in front of our Milky Way Galaxy and the Magellanic Clouds -- as the occasional Earth-orbiting satellite zips by.

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[Ann]

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.

[cesarclark]

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.

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

[Ringo Supernova]

What is this song? It's as amazing as the video!

[Alietr]

What is this song? It's as amazing as the video!

Shazam says it is "Jiangnan Girl" by Haixiao.

And those are more than occasional satellites. They are a blight.

[Roy]

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”.

I think there is a stunning amount of admirable technology on the bottom end of this sensorium/system, and a stunning amount of something else on the top end. YMMV.

[Chris Peterson]

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”.

Nothing hand-waving about it at all. These are all solid theoretical ideas, scientific in the sense of being explanatory and falsifiable. And it is instruments like the ones under discussion that will serve to either disprove or support those theories and get us closer to some real truth.

[AVAO]

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

[johnnydeep]

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.

I believe that's a satellite. Any Cherenkov radiation would only be detected by the 5 telescopes, not the more mundane camera taking this video. As for those telescopes, it's cool to me that their large mirrors are compose, not of closely fitted hexagonal or square segments but by small circular mirrors. How common is it to do that? I presume it's at least much less expensive whatever may be lost in sensitivity. PS - and those small mirrors apparently even overlap!:

Click to view full size image

[Chris Peterson]

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

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.

[johnnydeep]

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.

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]

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!

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]

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.

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?

[orin stepanek]

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!

[Chris Peterson]

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?

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!

[AVAO]

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!

Well said Orin. I couldn't do better. As mankind we are the only organism on the planet that has eyes of entirely different wavelengths to marvel at the beauty of the universe. I've only been really interested in astronomy for 7 years. And if I've learned anything in that time, it's humility. We are so tiny, unimportant and don't even manage to deal properly with our own mother planet. Jac

[johnnydeep]

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!

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

[alter-ego]

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

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]

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.

Thanks, I'll have to check it out, though my comfort with math has waned considerably since my graduate days. I actually have the paperback Feynman Lectures on Physics in a box somewhere that I bought 35 years ago with the best of intentions, but sadly, I never managed to wade through it.

[HamayounAstro]

It would be great if someone could give a description of what we are seeing throuhout out the video, especially the brighter stars and constellations