APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Comments and questions about the APOD on the main view screen.
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johnnydeep
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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by johnnydeep » Mon Jul 11, 2022 7:29 pm

Ann wrote: Mon Jul 11, 2022 7:15 pm
DL MARTIN wrote: Mon Jul 11, 2022 3:35 pm Thanks to all for clarifying my "hanging out" remark on dark matter and energy. I'm not an astronomy whiz so I appreciate the feedback. What, I guess I find questionable is we don't seem to acknowledge the total lack of scrutiny between the time frame that the observed entity represents and the present time that we are observing it. For example and to reiterate, how does astronomy account for the intervening 2.5 million years of existence of Andromeda? Or, for that matter, the 8 minutes applied to the Sun.
It seems that there is a fundamental gap in knowledge between the 'ago' and the 'now' that is not explained.
What you are really complaining about is the fact that the speed of light is so slow, only 299792458 metres per second (approximately 300000 km/s or 186000 mi/s).

That's why it takes 2.5 million years for light emitted by the Andromeda Galaxy to travel all the way to us. (And that's why we don't know the present state of Betelgeuse, of Sgr A*, the black hole at the center of our galaxy, or of Voyager I and II, or even of the Sun! And that's why there will be a fourteenth of a second of delay when you speak on the phone with someone on the other side of the Earth!)

Isn't that a bother? Wouldn't it be better if the speed of light was infinite? Then we would really know of any major developments going on in Andromeda right now. We would know right away if Sgr A* was up to anything unusual. We could listen to the news on the radio about the current developments in the Virgo Cluster regarding supernovas and outbursts and such things, and the news would really be current! Wouldn't that be a good thing?

Maybe. Or not. I found this question and an answer at Quora:
What would the universe be like if the speed of light was infinite?

Adam Wu, Amateur scientist answered:

The speed of light is actually the speed of causality.

A universe where the speed of causality is infinite is a universe with no time. Every effect happens instantaneous with its cause. Both the beginning and the end of the universe happen simultaneously, so the universe would never exist, and nor could anything else.
Oh wow. Well, that would not be so good, would it? Not all things considered.

I guess we will just have to take our Universe as it is. We may mutter and complain that we don't know what Andromeda is like now, but maybe we can find some consolation in the realization that our ignorance regarding Andromeda (and everything else that is some distance away from us - what did your auntie say on the phone during that one fourteenth of a second of delay during your conversation with her?) is the price for our existence.

Ann
I don't understand the "infinite c <=> everything happens all at once" argument, but that's probably only because I don't really understand relativity. But, let's assume it's true.

So, instead of infinite c, how about c = 10100 m/s. Does that avoid the predicament, or would it just mean that we'd already be experiencing the heat death of the universe? I guess so. [ Hmm, doesn't the multiverse theorize that there could be many universes where c is any arbitrary speed at all? ]
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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by Chris Peterson » Tue Jul 12, 2022 2:02 pm

johnnydeep wrote: Mon Jul 11, 2022 12:49 pm
Chris Peterson wrote: Mon Jul 11, 2022 12:06 pm
heehaw wrote: Mon Jul 11, 2022 9:31 am

Picky! But I can be picky too: " its photons are 2.5 million years old when they reach you" No, in their own frame, it takes the photons no time at all to reach us.
Even more picky. We don't observe those old photons. They undergo scattering in the atmosphere and especially in our eye. Each scattering event creates a new photon. The light that hits our retinas is brand new.
Are the photons hitting the detectors in telescopes in space even the same ones that left their source in a far away star or galaxy?
No. Many scatter off of dust or atoms in their path while in transit (and are therefore replaced). But I doubt any make it through filters without being scattered. Or even structures like transparent oxide layers or passivating layers on the detectors themselves. (It is because of scatter in transparent materials that they have a lower speed of light than c.)
Chris

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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by Chris Peterson » Tue Jul 12, 2022 2:05 pm

DL MARTIN wrote: Mon Jul 11, 2022 3:35 pm Thanks to all for clarifying my "hanging out" remark on dark matter and energy. I'm not an astronomy whiz so I appreciate the feedback. What, I guess I find questionable is we don't seem to acknowledge the total lack of scrutiny between the time frame that the observed entity represents and the present time that we are observing it. For example and to reiterate, how does astronomy account for the intervening 2.5 million years of existence of Andromeda? Or, for that matter, the 8 minutes applied to the Sun.
It seems that there is a fundamental gap in knowledge between the 'ago' and the 'now' that is not explained.
It is fully explained by special relativity.

Your problem is that you cannot separate what matters: seeing an object at a certain age, and what does not matter: the object as it is "now".
Chris

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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by johnnydeep » Tue Jul 12, 2022 2:14 pm

Chris Peterson wrote: Tue Jul 12, 2022 2:02 pm
johnnydeep wrote: Mon Jul 11, 2022 12:49 pm
Chris Peterson wrote: Mon Jul 11, 2022 12:06 pm

Even more picky. We don't observe those old photons. They undergo scattering in the atmosphere and especially in our eye. Each scattering event creates a new photon. The light that hits our retinas is brand new.
Are the photons hitting the detectors in telescopes in space even the same ones that left their source in a far away star or galaxy?
No. Many scatter off of dust or atoms in their path while in transit (and are therefore replaced). But I doubt any make it through filters without being scattered. Or even structures like transparent oxide layers or passivating layers on the detectors themselves. (It is because of scatter in transparent materials that they have a lower speed of light than c.)
I think I've had this confusion before, but how does a photon that's scattered make it to us at all? "Scattering" to me means that it is deflected out of our line of sight and so doesn't get to us. And for that matter, how does a photon that's absorbed and reemitted make to us either? Wouldn't it have to be reemitted along the same path as before, and wouldn't that be rare?
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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by Chris Peterson » Tue Jul 12, 2022 2:23 pm

johnnydeep wrote: Tue Jul 12, 2022 2:14 pm
Chris Peterson wrote: Tue Jul 12, 2022 2:02 pm
johnnydeep wrote: Mon Jul 11, 2022 12:49 pm

Are the photons hitting the detectors in telescopes in space even the same ones that left their source in a far away star or galaxy?
No. Many scatter off of dust or atoms in their path while in transit (and are therefore replaced). But I doubt any make it through filters without being scattered. Or even structures like transparent oxide layers or passivating layers on the detectors themselves. (It is because of scatter in transparent materials that they have a lower speed of light than c.)
I think I've had this confusion before, but how does a photon that's scattered make it to us at all? "Scattering" to me means that it is deflected out of our line of sight and so doesn't get to us. And for that matter, how does a photon that's absorbed and reemitted make to us either? Wouldn't it have to be reemitted along the same path as before, and wouldn't that be rare?
"Scattering" is a process whereby a photon interacts with an atom and another photon is emitted. The direction of the photon need not change. That's how much scattering works, in fact. What makes a transparent material transparent is that the scattered photons continue in the same direction.
Chris

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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by johnnydeep » Tue Jul 12, 2022 7:06 pm

Chris Peterson wrote: Tue Jul 12, 2022 2:23 pm
johnnydeep wrote: Tue Jul 12, 2022 2:14 pm
Chris Peterson wrote: Tue Jul 12, 2022 2:02 pm
No. Many scatter off of dust or atoms in their path while in transit (and are therefore replaced). But I doubt any make it through filters without being scattered. Or even structures like transparent oxide layers or passivating layers on the detectors themselves. (It is because of scatter in transparent materials that they have a lower speed of light than c.)
I think I've had this confusion before, but how does a photon that's scattered make it to us at all? "Scattering" to me means that it is deflected out of our line of sight and so doesn't get to us. And for that matter, how does a photon that's absorbed and reemitted make to us either? Wouldn't it have to be reemitted along the same path as before, and wouldn't that be rare?
"Scattering" is a process whereby a photon interacts with an atom and another photon is emitted. The direction of the photon need not change. That's how much scattering works, in fact. What makes a transparent material transparent is that the scattered photons continue in the same direction.
So, in the case of scattering off of dust, I would think it extremely rare for a photon to be scattered in the same direction it was going. Guess it depends on just what molecules and atoms the dust is made of? Is "scattering in the same direction" more common (or not) due to some underlying physical making it more likely?
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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by Chris Peterson » Tue Jul 12, 2022 7:12 pm

johnnydeep wrote: Tue Jul 12, 2022 7:06 pm
Chris Peterson wrote: Tue Jul 12, 2022 2:23 pm
johnnydeep wrote: Tue Jul 12, 2022 2:14 pm

I think I've had this confusion before, but how does a photon that's scattered make it to us at all? "Scattering" to me means that it is deflected out of our line of sight and so doesn't get to us. And for that matter, how does a photon that's absorbed and reemitted make to us either? Wouldn't it have to be reemitted along the same path as before, and wouldn't that be rare?
"Scattering" is a process whereby a photon interacts with an atom and another photon is emitted. The direction of the photon need not change. That's how much scattering works, in fact. What makes a transparent material transparent is that the scattered photons continue in the same direction.
So, in the case of scattering off of dust, I would think it extremely rare for a photon to be scattered in the same direction it was going. Guess it depends on just what molecules and atoms the dust is made of? Is "scattering in the same direction" more common (or not) due to some underlying physical making it more likely?
Basically, photons don't stop and change direction. If they scatter, they are absorbed and a new one emitted, with it's direction determined by material properties. If they don't scatter, they are simply absorbed and add to the heat energy of whatever absorbs them. Reflection is a form of scattering. And the nature of scattering isn't determined just by the material, but by the size of the particle. Most dust is large compared with the wavelength of light, while gas atoms or molecules are smaller. So those things tend to interact with light very differently.
Chris

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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by johnnydeep » Tue Jul 12, 2022 8:15 pm

Chris Peterson wrote: Tue Jul 12, 2022 7:12 pm
johnnydeep wrote: Tue Jul 12, 2022 7:06 pm
Chris Peterson wrote: Tue Jul 12, 2022 2:23 pm

"Scattering" is a process whereby a photon interacts with an atom and another photon is emitted. The direction of the photon need not change. That's how much scattering works, in fact. What makes a transparent material transparent is that the scattered photons continue in the same direction.
So, in the case of scattering off of dust, I would think it extremely rare for a photon to be scattered in the same direction it was going. Guess it depends on just what molecules and atoms the dust is made of? Is "scattering in the same direction" more common (or not) due to some underlying physical making it more likely?
Basically, photons don't stop and change direction. If they scatter, they are absorbed and a new one emitted, with it's direction determined by material properties. If they don't scatter, they are simply absorbed and add to the heat energy of whatever absorbs them. Reflection is a form of scattering. And the nature of scattering isn't determined just by the material, but by the size of the particle. Most dust is large compared with the wavelength of light, while gas atoms or molecules are smaller. So those things tend to interact with light very differently.
Ok, thanks. I keep reverting to thinking of photons as just little billiard balls, but clearly that's dead wrong.
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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by Chris Peterson » Tue Jul 12, 2022 8:18 pm

johnnydeep wrote: Tue Jul 12, 2022 8:15 pm
Chris Peterson wrote: Tue Jul 12, 2022 7:12 pm
johnnydeep wrote: Tue Jul 12, 2022 7:06 pm

So, in the case of scattering off of dust, I would think it extremely rare for a photon to be scattered in the same direction it was going. Guess it depends on just what molecules and atoms the dust is made of? Is "scattering in the same direction" more common (or not) due to some underlying physical making it more likely?
Basically, photons don't stop and change direction. If they scatter, they are absorbed and a new one emitted, with it's direction determined by material properties. If they don't scatter, they are simply absorbed and add to the heat energy of whatever absorbs them. Reflection is a form of scattering. And the nature of scattering isn't determined just by the material, but by the size of the particle. Most dust is large compared with the wavelength of light, while gas atoms or molecules are smaller. So those things tend to interact with light very differently.
Ok, thanks. I keep reverting to thinking of photons as just little billiard balls, but clearly that's dead wrong.
Always good to remember that at the scale these things operate, nothing works according to our intuition, which obviously evolved for a very different scale. And your comment is exactly why I refer to this way of looking at things as the Billiard Ball Fallacy. Physical intuition is a powerful bias that we have to consciously put aside when dealing with very small things (and sometimes, very large things).
Chris

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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by johnnydeep » Tue Jul 12, 2022 8:28 pm

Chris Peterson wrote: Tue Jul 12, 2022 8:18 pm
johnnydeep wrote: Tue Jul 12, 2022 8:15 pm
Chris Peterson wrote: Tue Jul 12, 2022 7:12 pm

Basically, photons don't stop and change direction. If they scatter, they are absorbed and a new one emitted, with it's direction determined by material properties. If they don't scatter, they are simply absorbed and add to the heat energy of whatever absorbs them. Reflection is a form of scattering. And the nature of scattering isn't determined just by the material, but by the size of the particle. Most dust is large compared with the wavelength of light, while gas atoms or molecules are smaller. So those things tend to interact with light very differently.
Ok, thanks. I keep reverting to thinking of photons as just little billiard balls, but clearly that's dead wrong.
Always good to remember that at the scale these things operate, nothing works according to our intuition, which obviously evolved for a very different scale. And your comment is exactly why I refer to this way of looking at things as the Billiard Ball Fallacy. Physical intuition is a powerful bias that we have to consciously put aside when dealing with very small things (and sometimes, very large things).
Like colliding galaxies perhaps?
--
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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by Chris Peterson » Tue Jul 12, 2022 8:31 pm

johnnydeep wrote: Tue Jul 12, 2022 8:28 pm
Chris Peterson wrote: Tue Jul 12, 2022 8:18 pm
johnnydeep wrote: Tue Jul 12, 2022 8:15 pm

Ok, thanks. I keep reverting to thinking of photons as just little billiard balls, but clearly that's dead wrong.
Always good to remember that at the scale these things operate, nothing works according to our intuition, which obviously evolved for a very different scale. And your comment is exactly why I refer to this way of looking at things as the Billiard Ball Fallacy. Physical intuition is a powerful bias that we have to consciously put aside when dealing with very small things (and sometimes, very large things).
Like colliding galaxies perhaps?
Probably the biggest problem there is that we see them as if they were a sort of liquid, when in reality they are almost entirely empty space.
Chris

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Re: APOD: Andromeda over the Sahara Desert (2022 Jul 11)

Post by johnnydeep » Tue Jul 12, 2022 9:04 pm

Chris Peterson wrote: Tue Jul 12, 2022 8:31 pm
johnnydeep wrote: Tue Jul 12, 2022 8:28 pm
Chris Peterson wrote: Tue Jul 12, 2022 8:18 pm

Always good to remember that at the scale these things operate, nothing works according to our intuition, which obviously evolved for a very different scale. And your comment is exactly why I refer to this way of looking at things as the Billiard Ball Fallacy. Physical intuition is a powerful bias that we have to consciously put aside when dealing with very small things (and sometimes, very large things).
Like colliding galaxies perhaps?
Probably the biggest problem there is that we see them as if they were a sort of liquid, when in reality they are almost entirely empty space.
Were you thinking of different types of very large things?
--
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