Thread: Galaxies and the speed of light ...

If a galaxy is so big that it takes millions of light years for a single photon to cross from one end to the other, how can we see photos of galaxies when the light from the back end will be out of place when the front one's light arrives?

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Okay, that might not have come out properly, but I hope you get the idea. How can we see Galaxy photos?

First, know that when we see galaxies, we are seeing the past. We see what they looked like millions of years ago, because they are an equal number of millions of lightyears away. The "front end" will look more recent than the "back end," because the front end is closer. It also depends on the viewing angle. If the galaxy is perpendicular to our viewing, then they the viewed galaxy should be from the same time period.

Of course, I'm no astronomer...

Originally Posted by Abra

First, know that when we see galaxies, we are seeing the past. We see what they looked like millions of years ago, because they are an equal number of millions of lightyears away. The "front end" will look more recent than the "back end," because the front end is closer. It also depends on the viewing angle. If the galaxy is perpendicular to our viewing, then they the viewed galaxy should be from the same time period.

Of course, I'm no astronomer...

The point I am trying to get across, however, is that when the light from the back eventually reaches us, it will not be lined up with the light from the front. The Galaxy will appear to be skewered.

Well, since that's not the case, you are obviously wrong.

Read your words again. When the light from the back reaches us, the light from the front is long gone. It won't wait for the light from the back to catch up.

The universe is pretty big.

Originally Posted by Keeper

The point I am trying to get across, however, is that when the light from the back eventually reaches us, it will not be lined up with the light from the front. The Galaxy will appear to be skewered.

That might be a problem if galaxies were millions of light years across, but they're not. They go up to around 100,000 light years, give or take. Since galaxies don't move very fast, 100,000 years won't produce any really noticeable skewing effect.

By the way, light year = distance
year = time

EDIT: Before some smart ass comes in and 'informs' me that distant galaxies travel close to the speed of light, I will say that the maximum skew is about 2x. Simple geometry. And that's for galaxies that would be so red-shifted that they wouldn't even be visible.

Originally Posted by Kushna Mufeed

Well, since that's not the case, you are obviously wrong.

Or light is a lot faster then we think.

Read your words again. When the light from the back reaches us, the light from the front is long gone. It won't wait for the light from the back to catch up.

Yes, and?

Originally Posted by drewmandan

That might be a problem if galaxies were millions of light years across, but they're not. They go up to around 100,000 light years, give or take. Since galaxies don't move very fast, 100,000 years won't produce any really noticeable skewing effect.

Are you sure? I've heard differently, and that is what got me thinking on that to begin with.

By the way, light year = distance
year = time

I know. Where did I use it incorrectly.

(I'm not an astronomer)

I think for a 2D photo you just use the light you're getting. So the back of the galaxy is older than the front when you look at it. It's just a snapshot. The effect of the delay isn't that important, given the immense size of those bastards.

You'd only need the light from the front to the end of the galaxy if you were to create a full 3D-model of the galaxy... which is impossible because it would take several hundreds of thousands of years to create and some crazy algorithms to align all the light data to positions and trace them back through time...

Also, you misused "light years" in the first sentence, but the point got across anyway.

Originally Posted by Keeper

Are you sure? I've heard differently, and that is what got me thinking on that to begin with.

I take astronomy in university. Don't even go there.

Originally Posted by Keeper

Or light is a lot faster then we think.

Big @ good ol' Keeper.

Luckily, quite some people know enough about astronomy to be able to clear it up. But really, this kind of stuff isn't as hard as it sounds, just browse around wikipedia for a few minutes, or read 'a brief history of time' by Stephen Hawking and you have pretty much all the knowledge you can have about astronomy before it gets hardcore-physics-wacky.

Anyhow, it is certain that we can in fact not see most of our universe. It is simply impossible because the universe has and is expanding so fast: when trying to look far across the universe after a while, it would take 13,7 billion years for the light from there to reach the earth (or the hubble). All you would see is... the big bang, that happened 13.7 billion years ago (we can actually see this big-bang residue). Galaxies that lie beyond that 13.7 billion year mark are simply imposslbe to see, just about by definition of 'sight'. In a billion years, we still probably couldn't see the universes that are today 14.7 billion lightyears away, because they are all still moving away from us.

Anyhow, what we really see when we look at something, for instance, 12 billion lightyears away, is matter (or some sort of traces, radio-wave-like-stuff) from 1.7 billion years after the big bang. Young galaxies, at most.

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Anyhow, it is all kind of hard to get your head around it, since when you are in a room, there is no huge delay between something happening and seeing, nor is the room expanding at speeds greater than light.

We know pretty much a giant cool lot about the history of our universe. Mixed in with some general relativity, this kind of subject is just awesome. Really, 'A brief history of time' is a pretty understandable, great book to understand the basics of astronomy / weird physics stuff that has to do with it.

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edit/addition: ShadowNightWing, maybe wendylove exaggerated a bit, but really, you can't beat creationists -especially the young earth kind- when it comes to weird astronomical assumptions. Such as that God created a whole bunch of photons in mid-flight 6000-years ago, that now do indeed seem to trace back to celestial bodies.

(also, I saw I wasn't the only one that thought that 'A Brief History of Time' is indeed a good way to learn about the basics of such stuff.)

Or light is a lot faster then we think.

Keeper, no. A big no.

Light can only move slower, it cannot move faster it is mathematically impossible and contradicts Einstein.

No light hasen't slowed down, only creationist believe this. Thats because they get basic scientific knowledge incorrect. Which, would explain this thread Keeper, I would read A brief history of time by Stephen Hawking.

I guess ...

Is that it.

Originally Posted by wendylove

Keeper, no. A big no.

Light can only move slower, it cannot move faster it is mathematically impossible and contradicts Einstein.

No light hasen't slowed down, only creationist believe this. Thats because they get basic scientific knowledge incorrect. Which, would explain this thread Keeper, I would read A brief history of time by Stephen Hawking.


Is that it.

That's the most one track minded, idiotic thing that I've ever heard. You're making the assumption that all Astrophysicist are atheist. Damn how dumb can you be?

Not being creationsist does not automatically make you atheist...

You're an idiot if you think all non-creationists (IDiots) are Atheist.

That said, care to provide a citation for an astrophysicist that is an ID-advocate?

Originally Posted by Seismosaur

Not being creationsist does not automatically make you atheist...

You're an idiot if you think all non-creationists (IDiots) are Atheist.

That said, care to provide a citation for an astrophysicist that is an ID-advocate?

Stop following me Troll. You got some kind of obsession with me or something? That post is directed to the wendylove not you LameO!

That's the most one track minded, idiotic thing that I've ever heard. You're making the assumption that all Astrophysicist are atheist.

Well, there is no evidence that light speeds up and I pretty sure if it did, they would have found it now. Only creationist believe it does so they can believe the universe is young.

Also, the person who actually came up with the big bang theory was a christian. So I don't believe all cosmologist are atheist.

I think the reason we can see entire galaxies even though the light from the back is out of place from the light at the front, is because the distortion is gradual. This gradual distortion, combined with the crazy shapes of the galaxies, makes it look continuous.