APOD: Andromeda over Patagonia (2020 Nov 25)

[heehaw]

I remember when I got my SkyScope, many decades ago, and I looked at Andromeda ... hardly impressive! ... and ... I discovered M32, by accident! Cool!

[MarkBour]

Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.

Chris? Any thoughts on how much the Milky Way would be inclined if viewed from the Andromeda Galaxy?

Ann

That was a very interesting point about the movement of light through media, Chris.

Ann --
In spite of my love for geometry and trigonometry, those relatively simple branches of mathematics that should be able to give a precise answer, I find my lack of astronomical knowledge to be my limitation. The question you're asking, of course, is the same question as drawing a line from the center of our galaxy to the center of the Andromeda galaxy, and determining how much such a line is inclined from the plane of our galaxy. If that line was perpendicular to our galactic plane, then the Milky Way would appear face on from Andromeda. If the line is right along our galactic plane, then the Milky Way is edge on, when viewed from Andromeda.

I can give an imprecise answer, from looking at the Milky Way and Andromeda in the sky. The Andromeda galaxy is pretty near the band of the Milky Way. So, it is 2.5 million light years away, in a direction that is not very different than the direction we look to see our galactic plane. If Andromeda had been over by Arcturus, or Merak, or Formalhaut, or some such place, it would be different, but our view of it is much closer to a line we would draw from Earth across a part of our home galaxy. My rough conclusion is that from Andromeda, the Milky Way galaxy is seen mostly edge on.

I have read that Earth is some 50-100 light years off of the center of the galactic plane of the Milky Way at the present time. That would skew our view a bit, but compared to the radius of our galaxy, it is tiny, so I don't think it has a large effect.

At the right is a diagram I found at: https://www.handprint.com/ASTRO/IMG/satellites.gif
In that diagram, they indicate Andromeda (M31) at the far left. (Of course it would actually be a great distance away, but in that direction.)

Also, here's a glorious simulation of the eventual collision of Andromeda and the Milky Way that has been predicted. I note that in the simulation, they initially draw the Milky Way as edge on.
https://upload.wikimedia.org/wikipedia/ ... lision.ogv

[neufer]

So if Andromeda Galaxy is the oldest light my unaided eye can see, my inquisitive mind naturally wonders, what is the youngest light that my unaided eye can see? It is not daylight since that is some 8 and a half minutes old. Could be even 100,000 years old since they say a photon can take that long to reach the surface of the Sun. Can it be Moonlight at 2 seconds old? No since the Moon only reflects light so Moonlight is still 8 and a half minutes old. So we learn that the greater the distance light travels, the older it is. So the youngest light must form the closest to our eyeballs. My guess is the light from the flashlight that the optometrist shines in our eyes from a distance of about an inch away during an eye exam would be the youngest. .

Photons travel through empty space at the speed of light.

From the viewpoint (inertial frame) of photons, it is the length contraction effect of
special relativity which reduces empty space distances to essentially zero distance

Hence the photons do not age at all during their space travels.

Click to play embedded YouTube video.

<<The muon is an unstable subatomic particle with a mean lifetime of 2.2 μs, much longer than many other subatomic particles. Muons arriving on the Earth's surface are created indirectly as decay products of collisions of cosmic rays with particles of the Earth's atmosphere.

• About 10,000 muons reach every square meter of the earth's surface a minute; these charged particles form as by-products of cosmic rays colliding with molecules in the upper atmosphere. Traveling at relativistic speeds, muons can penetrate tens of meters into rocks and other matter before attenuating as a result of absorption or deflection by other atoms.

When a cosmic ray proton impacts atomic nuclei in the upper atmosphere, pions are created. These decay within a relatively short distance (meters) into muons (their preferred decay product), and muon neutrinos. The muons from these high-energy cosmic rays generally continue in about the same direction as the original proton, at a velocity near the speed of light. Although their lifetime without relativistic effects would allow a half-survival distance of only about 456 meters ( 2.197 µs × ln(2) × 0.9997 × c ) at most (as seen from Earth) the time dilation effect of special relativity (from the viewpoint of the Earth) allows cosmic ray secondary muons to survive the flight to the Earth's surface, since in the Earth frame the muons have a longer half-life due to their velocity. From the viewpoint (inertial frame) of the muon, on the other hand, it is the length contraction effect of special relativity which allows this penetration, since in the muon frame its lifetime is unaffected, but the length contraction causes distances through the atmosphere and Earth to be far shorter than these distances in the Earth rest-frame. Both effects are equally valid ways of explaining the fast muon's unusual survival over distances.

Since muons are unusually penetrative of ordinary matter, like neutrinos, they are also detectable deep underground (700 meters at the Soudan 2 detector) and underwater, where they form a major part of the natural background ionizing radiation. Like cosmic rays, as noted, this secondary muon radiation is also directional.>>

[Ann]

Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.

Chris? Any thoughts on how much the Milky Way would be inclined if viewed from the Andromeda Galaxy?

Ann

Well, Andromeda has a galactic latitude of about -10°. This is the angle below the galactic plane referenced to the Sun, not the galactic center, but they're not going to be very different. So that means that from Andromeda, the Milky Way looks like an edge-on galaxy tipped about 10°. So even more flattened than Andromeda appears to us.

Thanks!

Ann

[Ann]

Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.

Chris? Any thoughts on how much the Milky Way would be inclined if viewed from the Andromeda Galaxy?

Ann

That was a very interesting point about the movement of light through media, Chris.

Ann --
In spite of my love for geometry and trigonometry, those relatively simple branches of mathematics that should be able to give a precise answer, I find my lack of astronomical knowledge to be my limitation. The question you're asking, of course, is the same question as drawing a line from the center of our galaxy to the center of the Andromeda galaxy, and determining how much such a line is inclined from the plane of our galaxy. If that line was perpendicular to our galactic plane, then the Milky Way would appear face on from Andromeda. If the line is right along our galactic plane, then the Milky Way is edge on, when viewed from Andromeda.

I can give an imprecise answer, from looking at the Milky Way and Andromeda in the sky. The Andromeda galaxy is pretty near the band of the Milky Way. So, it is 2.5 million light years away, in a direction that is not very different than the direction we look to see our galactic plane. If Andromeda had been over by Arcturus, or Merak, or Formalhaut, or some such place, it would be different, but our view of it is much closer to a line we would draw from Earth across a part of our home galaxy. My rough conclusion is that from Andromeda, the Milky Way galaxy is seen mostly edge on.

satellites.gif

I have read that Earth is some 50-100 light years off of the center of the galactic plane of the Milky Way at the present time. That would skew our view a bit, but compared to the radius of our galaxy, it is tiny, so I don't think it has a large effect.

At the right is a diagram I found at: https://www.handprint.com/ASTRO/IMG/satellites.gif
In that diagram, they indicate Andromeda (M31) at the far left. (Of course it would actually be a great distance away, but in that direction.)

Also, here's a glorious simulation of the eventual collision of Andromeda and the Milky Way that has been predicted. I note that in the simulation, they initially draw the Milky Way as edge on.
https://upload.wikimedia.org/wikipedia/ ... lision.ogv

Thank you so much, Mark, even I can sort of visualize it now!

Ann

[Chris Peterson]

So if Andromeda Galaxy is the oldest light my unaided eye can see, my inquisitive mind naturally wonders, what is the youngest light that my unaided eye can see? It is not daylight since that is some 8 and a half minutes old. Could be even 100,000 years old since they say a photon can take that long to reach the surface of the Sun. Can it be Moonlight at 2 seconds old? No since the Moon only reflects light so Moonlight is still 8 and a half minutes old. So we learn that the greater the distance light travels, the older it is. So the youngest light must form the closest to our eyeballs. My guess is the light from the flashlight that the optometrist shines in our eyes from a distance of about an inch away during an eye exam would be the youngest. .

Photons travel through empty space at the speed of light.

From the viewpoint (inertial frame) of photons, it is the length contraction effect of
special relativity which reduces empty space distances to essentially zero distance :!:

Hence the photons do not age at all during their space travels.

Photons can only travel through empty space. They always travel at c. So from their frame, they don't travel at all and do not age at all. They are created, travel some distance (in our frame) and are destroyed... and from their perspective it's all over the instant it begins. Weird, no?

[johnnydeep]

Well, the light from Andromeda isn't really old light. In fact, no light that you see is old. The photons that strike your retina are never more than a few picoseconds old.

Photons always travel at c. But light travels slower than c in a medium. That's because a photon in a medium will get scattered, meaning it will be absorbed and re-emitted, a process that takes time. So the net speed of light is reduced, even though every photon still travels at c. A photon coming from Andromeda may or may not make it to Earth without scattering. But once it gets to our atmosphere, it will scatter many times before it reaches your eye, and then scatter many more times as it moves through your eye. Each scattering event consumes the original photon and produces a new one. So everything you see is the product of a photon produced by a scattering event in your vitreous a few micrometers from your retina. You detect a brand new photon.

A-ha! So how is it that glass and other transparent materials seem not to change the direction of the light passing though it? Yes, I know about refraction, but even so, I would have thought that any photon re-emitted by an atom after being absorbed would be in some random direction? or is it a conservation of momentum thing?

Also, I never understood how neutrinos can apparently pass unimpeded through ludicrous thicknesses of material. I'm guessing they don't get absorbed and re-emitted, which means they somehow are able to wind their way through an obstacle course of electrons and atomic nuclei!

There are different scattering mechanisms, but yes, there is very little change of direction for interactions of optical wavelength photons and the electrons in transparent media.

Neutrinos interact only very weakly with baryonic matter, which is another way of saying they don't often get absorbed. Most likely that's what makes dark matter dark, as well.

I guess I'm still hung up on the outdated "little billiard ball" model of particle physics. Do neutrinos travel in perfectly straight lines through, say, 10 feet of lead? If so, I would think any such line would intersect at least one proton or neutron in at least one atom (and probably millions of them). How is it then that such a neutrino can avoid colliding with such a proton or neutron?

[Chris Peterson]

A-ha! So how is it that glass and other transparent materials seem not to change the direction of the light passing though it? Yes, I know about refraction, but even so, I would have thought that any photon re-emitted by an atom after being absorbed would be in some random direction? or is it a conservation of momentum thing?

Also, I never understood how neutrinos can apparently pass unimpeded through ludicrous thicknesses of material. I'm guessing they don't get absorbed and re-emitted, which means they somehow are able to wind their way through an obstacle course of electrons and atomic nuclei!

There are different scattering mechanisms, but yes, there is very little change of direction for interactions of optical wavelength photons and the electrons in transparent media.

Neutrinos interact only very weakly with baryonic matter, which is another way of saying they don't often get absorbed. Most likely that's what makes dark matter dark, as well.

I guess I'm still hung up on the outdated "little billiard ball" model of particle physics. Do neutrinos travel in perfectly straight lines through, say, 10 feet of lead? If so, I would think any such line would intersect at least one proton or neutron in at least one atom (and probably millions of them). How is it then that such a neutrino can avoid colliding with such a proton or neutron?

Yes, they travel in straight lines. A straight line (that is, a true line with a cross-sectional area of zero) has virtually zero probability of intersecting a proton, neutron, or electron in 10 meters of lead. Keep in mind that a 10 meter block of lead is almost entirely empty space. And a neutrino is on the order of a million times smaller than an electron, so it nearly does trace a perfect line in this context.

Your billiard ball model is fine... as long as you recognize that it's being played on a table that is light years across.

[neufer]

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A straight line (that is, a true line with a cross-sectional area of zero) has virtually zero probability of intersecting a proton, neutron, or electron in 10 meters of lead. Keep in mind that a 10 meter block of lead is almost entirely empty space. And a neutrino is on the order of a million times smaller than an electron, so it nearly does trace a perfect line in this context.

A lead nucleus has a diameter of about 10.34 fm
[where a fermi (fm) = 10^-15 m].

A lead atom has a diameter of about 350 pm = 350,000 fm.

Hence, a neutrino will on average penetrate a lead nucleus after
going through only about (350,000)³/(10.34)² fm ~ 40 cm of lead.

However, technically,
only elementary point particles of zero dimension
actually do the interacting.

[Chris Peterson]

---

A straight line (that is, a true line with a cross-sectional area of zero) has virtually zero probability of intersecting a proton, neutron, or electron in 10 meters of lead. Keep in mind that a 10 meter block of lead is almost entirely empty space. And a neutrino is on the order of a million times smaller than an electron, so it nearly does trace a perfect line in this context.

A lead nucleus has a diameter of about 10.34 fm
[where a fermi (fm) = 10^-15 m].

A lead atom has a diameter of about 350 pm = 350,000 fm.

Hence, a neutrino will on average penetrate a lead nucleus after
going through only about (350,000)³/(10.34)² fm ~ 40 cm of lead.

However, technically,
only elementary point particles of zero dimension :arrow:
actually do the interacting.

But a neutrino will usually pass through an atomic nucleus without interaction, in part because the nucleus is largely empty, as well.

I believe that a neutrino has a 50% chance of being absorbed passing through a light year of lead.

[neufer]

But a neutrino will usually pass through an atomic nucleus without interaction, in part because the nucleus is largely empty, as well.

I believe that a neutrino has a 50% chance of being absorbed passing through a light year of lead.

A light year of lead for 10 MeV Supernova neutrinos

with effective interactive cross sections of ~4 x 10^-43 cm²

(vs. 840 x 10^-27 cm² for the physical cross section of a lead nuleus).

UC Berkeley P290e How to “see” a neutrino? Alan Poon 2017.09.27

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[neufer]

From the viewpoint (inertial frame) of photons, it is the length contraction effect of
special relativity which reduces empty space distances to essentially zero distance

Hence the photons do not age at all during their space travels.

Photons can only travel through empty space. They always travel at c. So from their frame, they don't travel at all and do not age at all. They are created, travel some distance (in our frame) and are destroyed... and from their perspective it's all over the instant it begins. Weird, no?

Quantum entangled photons are

• born simultaneously,
  live for zero seconds and
  then die/wave function collapse simultaneously
  (give or take a Planck time unit).

So where is "spooky action at a distance" taking place (for photons)