Thread: Galaxies and the speed of light ...

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 Serkat

(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...

I guess ...

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

... Words cannot express my embarrassment.

Originally Posted by Serkat

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.

This is the correct answer. Even if the path of the light is curved due to objects moving at immense speeds, it won't matter because what we're seeing is a snapshot, so nothing will be visually distorted.

Objects can not move faster than the speed of light, as far as we know. But if you see an object that is moving at say 10% the speed of light, the location it will appear to be will be the location it was at when the photon you see was reflected off of it's surface (or emitted). You will still be able to trace it's path across the sky, only with a delay of however many light years.

Originally Posted by Umbrasquall

This is the correct answer. Even if the path of the light is curved due to objects moving at immense speeds, it won't matter because what we're seeing is a snapshot, so nothing will be visually distorted.

You're not getting it. Even if it's a snapshot, different ends of the galaxy will not be showing the same time. But since no visible galaxy can travel faster than light, the largest possible skew is 2x.

OK,

the way I look at it is that:

the only galaxies we have seen are ones that are very very far away, we have never seen our own *because we are in it*. So when we look at a galaxie far away, the distance between the back end and the front of the galaxie is so insignificant compared to the distance the light has to travel to get here that we can't differenciate between the light from the front and the oh so slightly old light from the back. PLUS, when you see the spiral of a galaxie, its like watching a flat object so there is no back and front end, do you get it. Lokking at it from the side looks like a line. and even then you still can't tell front from back. So this intire question really is pointless.

NOW, if we were to take pictures of our own galaxie... maybe we could encounter such an issue since the ratio of the size of the galaxie vs how far ithe light is going to travel is way smaller.

My grain of salt.

So many people should never have entered this thread.

The question's a good one, the answer's

a) the effect simply isn't that big due to the distances and speeds involved
b) we see pretty galaxies 'face on' so all visible points are pretty much the same distance away; for the effect to occur the plane of the galaxy would have to be facing us and then you wouldn't be able to notice the effect very well anyway.

Originally Posted by drewmandan

You're not getting it. Even if it's a snapshot, different ends of the galaxy will not be showing the same time. But since no visible galaxy can travel faster than light, the largest possible skew is 2x.

You will see the light from both 'ends' of the galaxy at the same time. Light travels at a constant speed, so the light from either end will be emitted at the same time, and arrive at the same time.

There are special cases, but the above is the norm.

Also there is the possibility that something is traveling at a significant portion of the speed of light. In which case you may get some superluminal measurements, but that is just an illusion.

Light travels at a constant speed, so the light from either end will be emitted at the same time, and arrive at the same time.

But the other end is further away, so the light won't arrive at the same time at all. The light from the back end of the galaxy is older, and if the galaxy's moving, it will come from a different position in the sky.

Originally Posted by Umbrasquall

You will see the light from both 'ends' of the galaxy at the same time. Light travels at a constant speed, so the light from either end will be emitted at the same time, and arrive at the same time.

Fail

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.)

Originally Posted by Neruo

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.

I read an interesting scientific american article recently talking about this. It turns out that since the space metric itself is expanding faster than light, as time goes on, we will see less and less of the surrounding universe. According to the article, in 100 billion years, there will be only one visible galaxy (the Milky Way-Andromeda super galaxy) in the whole observable universe.