# The far side of the galaxy?

This question has been nagging at me: How are we able to get a clear picture of a galaxy if the light from the far side of a galaxy is arriving 100,000 years or so after the light coming from the near side. With a 100,000 year difference the stars that we are seeing in the far side wouldn't even be the ones that are actually there. It would seem to me that it would just be a blur or we wouldn't see a beautiful spiral.

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mathman
There wouldn't be a blur. What we are seeing is a clear picture that is 100,000 years old.

Janus
Staff Emeritus
Gold Member
100,000 years is a very short time when compared to galaxy movement. It can take over 200 million years for a galaxy to complete a revolution. So in 100,000 yr it will have turned ~0.15°, this is about 1/38 of the angle between the minute marks on a clock.
This is so small, that for a typical galaxy, it is beyond the resolution that we can achieve with our telescopes.

There wouldn't be a blur. What we are seeing is a clear picture that is 100,000 years old.
I think FelixBiel is correct in his thinking about what we see. As I read it, he is not talking about a galaxy 100,000 ly away, but one whose near edge is 100,000 ly closer than the far edge.

FelixBiel: Consider that the phenomenon you describe occurs with everything we see. When I look up from the keyboard, I see my mug, my cat looking out the window and trees on the far side of my back yard. I am seeing in a single moment the mug as it looked about 3 nanoseconds ago, the cat as she was 7 nanoseconds ago, the window at 8 nanoseconds ago and the trees at 50 nanoseconds ago. The light all arrives at the same time, but there is no need for that to make a blurred image.

Or are you thinking about the fact that the trees are blurred for me when the cat is in focus?

EDIT: Janus posted while I was typing. His point is a good one.

bcrowell
Staff Emeritus
Gold Member
I don't think you'd see a blur, you'd see a very tiny distortion. Each star would still appear pointlike (assuming it's a galaxy with some stars in it that you can resolve), but they would be systematically displaced relative to one another in some way.

So in 100,000 yr it will have turned ~0.15°, this is about 1/38 of the angle between the minute marks on a clock.
This is so small, that for a typical galaxy, it is beyond the resolution that we can achieve with our telescopes.
It might also cause some theoretically observable skewing of surface brightness relative to the true surface brightness. I guess you could correct for these effects, but the correction would be so tiny that you probably wouldn't care.

You can also have general-relativistic effects such as magnification or reduction due to the universe's slightly nonzero mean spatial curvature. Also maybe gravitational lensing by intervening objects, and Sachs-Wolfe effects...?

When I typed blur I was immediately visualizing this to be wrong. Maybe more of an elongation? But I guess that it so slight to not make much difference. I really feel like we actually don't understand the true nature of light, but I guess applies to a whole slue of things in the very tiny world of the particle.

DaveC426913
Gold Member
When I typed blur I was immediately visualizing this to be wrong. Maybe more of an elongation? But I guess that it so slight to not make much difference. I really feel like we actually don't understand the true nature of light, but I guess applies to a whole slue of things in the very tiny world of the particle.
This does not involve "the true nature of light"; it is simply optics.

I don't know why you think there would be some elongation. The fact that the light travelled a greater distance over a longer time does not mean it would elongate the image.

This does not involve "the true nature of light"; it is simply optics.

I don't know why you think there would be some elongation. The fact that the light travelled a greater distance over a longer time does not mean it would elongate the image.
no but it does mean that you are seeing a different age of the galaxy from one side to the other. and galaxies are moving as a whole, so you would think that the far side would almost look bent back towards where the galaxy is coming from and the near side would look bent to where it is going. There for slightly elongated.

DaveC426913
Gold Member
no but it does mean that you are seeing a different age of the galaxy from one side to the other. and galaxies are moving as a whole, so you would think that the far side would almost look bent back towards where the galaxy is coming from and the near side would look bent to where it is going. There for slightly elongated.
Where is it coming from? Where is it going to? From our reference frame within it, the galaxy is stationary, going nowhere.

Yes, due to the galaxy's rotation, stars 100,000ly away would not quite be where they would seem but, as Janis pointed out, it is negligibly small.

Where is it coming from? Where is it going to? From our reference frame within it, the galaxy is stationary, going nowhere.

Yes, stars 100,000ly away would not quite be where they would seem but, as Janis pointed out, it is negligibly small.
What kind of question is that? All galaxies are moving (in fact everything is moving relative to us), it doesn't matter where to or where from just that it is. Maybe it is moving so slow that the effect is negligible at best?

DaveC426913
Gold Member
What kind of question is that? All galaxies are moving (in fact everything is moving relative to us), it doesn't matter where to or where from just that it is. Maybe it is moving so slow that the effect is negligible at best?
I state again: from our reference frame within the galaxy, it is not moving with respect to us.

As far as any optics are concerned, the Milky Way (and us within it) are "at rest" in the universe, and everything else is moving around us. This is a perfectly valid viewpoint, and any optical phenom you wish to examine had better be consistent with it.

Therefore I ask again: what do you mean when you say "where it is coming from" and "where it is going to"?

Images from our galaxy do not get slowed down or sped up because our galaxy is moving wrt to other galaxies. There's some flaw in your conceptions about light and optics that needs to be rooted out.

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I state again: from our reference frame within the galaxy, it is not moving with respect to us.

Therefore I ask again: what do you mean when you say "where it is coming from" and "where it is going to"?

Images from our galaxy do not get slowed down or sped up because our galaxy is moving wrt to other galaxies. There's some flaw in your conceptions about light and optics that needs to be rooted out.
I thought I was clear but I guess not. When we are looking at another galaxy it is moving. The far side of that galaxy is 100, 000 years or so older there for where it was 100, 000 years ago. The question makes perfect sense to me and I am sure there is a simple answer.

DaveC426913
Gold Member
I thought I was clear but I guess not. When we are looking at another galaxy it is moving. The far side of that galaxy is 100, 000 years or so older there for where it was 100, 000 years ago. The question makes perfect sense to me and I am sure there is a simple answer.
The nearest galaxy is 2 million light years away; 20 times further than its diameter. Virtually all other galaxies are significantly farther than that.

True, there may be some room for a little distortion but really, even the nearest one it can't be more than 1 part in 20.