APOD: Total Solar Eclipse 2020 (2021 Jan 07)

Comments and questions about the APOD on the main view screen.
Javachip3

Re: APOD: Total Solar Eclipse 2020 (2021 Jan 07)

Post by Javachip3 » Sat Jan 09, 2021 2:25 pm

johnnydeep wrote:
Thu Jan 07, 2021 6:22 pm
... probably 99.999% of all light emitted by stars is not absorbed by anything, near or far...
If the Universe is sufficiently or infinitely large, with a sufficient or infinite number of massive objects, then it follows that every single photon emitted by our star or any star is absorbed or annihilated by something, somewhere, eventually.

One night 2.5 million years ago, octillions (poetic license) of photons escaped from the Andromeda Galaxy. Last night, hundreds of them were absorbed by my eyeball.

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Chris Peterson
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Re: APOD: Total Solar Eclipse 2020 (2021 Jan 07)

Post by Chris Peterson » Sat Jan 09, 2021 2:28 pm

Javachip3 wrote:
Sat Jan 09, 2021 2:25 pm
johnnydeep wrote:
Thu Jan 07, 2021 6:22 pm
... probably 99.999% of all light emitted by stars is not absorbed by anything, near or far...
If the Universe is sufficiently or infinitely large, with a sufficient or infinite number of massive objects, then it follows that every single photon emitted by our star or any star is absorbed or annihilated by something, somewhere, eventually.

One night 2.5 million years ago, octillions (poetic license) of photons escaped from the Andromeda Galaxy. Last night, hundreds of them were absorbed by my eyeball.
Actually, not a single photon that your eye recorded from Andromeda was likely emitted by Andromeda.

In an expanding Universe, with much of it expanding away from photon sources at greater than the speed of light, there is no reason to think that most photons will ever interact with matter.
Chris

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Javachip3

Re: APOD: Total Solar Eclipse 2020 (2021 Jan 07)

Post by Javachip3 » Sat Jan 09, 2021 6:25 pm

Why are photons reaching us from the Andromeda Galaxy not the same photons that left the Andromeda Galaxy? Is that because photons from the Andromeda Galaxy interact with intervening dust clouds or other matter on their way to Earth? Or do you refer to particle physics (light on light scattering, photon-photon collisions to produce electron-positron pairs, or the reverse reaction) or quantum effects (virtual photons)? In any of the latter examples, "new" photons from M31 arriving at my eyeball would not contribute to the sharp optical image of M31 that my brain or my camera sensor creates.

M31 is very nearby on the cosmological scale and is approaching us, not receding. All photons escaping M31 in our direction (allowing for 2.5 million years of galactic motion) should reach us unless something happens to them en route.

If Earth is at the center of a sphere, at what radius do objects recede from us due to cosmological inflation at faster than c, so that their photons can no longer reach us, nor ours reach them? Would that be 13.8 billion light years? This idea is difficult for me to conceptualize because the hypothetical sphere in the question is defined within space-time that is not fixed, but expanding. There aren't many opportunities to get intelligent answers to these questions, so thank you. :ssmile:

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Chris Peterson
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Re: APOD: Total Solar Eclipse 2020 (2021 Jan 07)

Post by Chris Peterson » Sat Jan 09, 2021 6:37 pm

Javachip3 wrote:
Sat Jan 09, 2021 6:25 pm
Why are photons reaching us from the Andromeda Galaxy not the same photons that left the Andromeda Galaxy? Is that because photons from the Andromeda Galaxy interact with intervening dust clouds or other matter on their way to Earth?
Very few photons interact with matter between Andromeda and Earth. But most interact with Earth's atmosphere before they reach your eye, and probably all the photons that enter your eye are absorbed and re-emitted many times before they reach your retina.
M31 is very nearby on the cosmological scale and is approaching us, not receding. All photons escaping M31 in our direction (allowing for 2.5 million years of galactic motion) should reach us unless something happens to them en route.
Of course, that represents a vanishingly small percentage of all the photons leaving Andromeda.
If Earth is at the center of a sphere, at what radius do objects recede from us due to cosmological inflation at faster than c, so that their photons can no longer reach us, nor ours reach them? Would that be 13.8 billion light years? This idea is difficult for me to conceptualize because the hypothetical sphere in the question is defined within space-time that is not fixed, but expanding.
The edge of the observable universe (that is, the horizon beyond which the expansion rate is greater than c) is about 46 billion light years away, which accounts for the expansion that has occurred over 13.8 billion years.
Chris

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Cloudbait Observatory
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