[aceriscoolon]

i've had this question nagging me for a while, but i really have no one to ask it, but then i saw this thread, and i decided that i could ask the guys at fm!

so light travels pretty damn, fast, but it still cant traverse the mind boggling distances of space that fast (i hope that statement was right) the light from distant galaxies that is being emitted at this second will take millions of years (or more) to reach us, right? so what if (here we go) the light from other galaxies has yet to reach us for the first time, and when this light did reach us, would fill up the darks spots in the sky?

im not sure if i phrased that right, or if that at all follows any of the laws of the universe correctly, but it was bugging me, so i tried...

thanks in advance for any help there...

[saopinax]

i've had this question nagging me for a while, but i really have no one to ask it, but then i saw this thread, and i decided that i could ask the guys at fm!

so light travels pretty damn, fast, but it still cant traverse the mind boggling distances of space that fast (i hope that statement was right) the light from distant galaxies that is being emitted at this second will take millions of years (or more) to reach us, right? so what if (here we go) the light from other galaxies has yet to reach us for the first time, and when this light did reach us, would fill up the darks spots in the sky?

im not sure if i phrased that right, or if that at all follows any of the laws of the universe correctly, but it was bugging me, so i tried...

thanks in advance for any help there...

i know what you are saying, i think though, that with the immensity of space such as it is, and the vastness between the galaxies, that such distant light becomes very minute. Think about the milky way, when you look up and see it in the night sky, we are IN this galaxy, and it does not light up the night sky. I think for the effect you are wanting, pretty much the light from every start that has ever been would need to reach us all at the same time. Seeing that we are unable to see other galaxies (at least i think) without the aid of powerful telescopes, you have little fear of a white night sky.

[9rCR9hWL]

I would say the biggest factors would be the interstellar dust blocking the light, that there will be areas where there would be no light sources and then there is the inverse square law which would mean the light would be greatly reduced in intensity over distance.

[aceriscoolon]

so is it possible that the light from those galaxies has already reached us, but we do not have telescopes powerful enough (or exposures long enough) to see them?

[9rCR9hWL]

so is it possible that the light from those galaxies has already reached us, but we do not have telescopes powerful enough (or exposures long enough) to see them?

The light is indeed already reaching us, just too faint to see without very specialized observation equipment, like large array telescopes and the like.

You've probably heard about the astronomers looking at far distant galaxies, etc, this is because the light may be taking up to 10 billion years, or more, to reach us and they can see what the universe was like at the times the light left those galaxies - a sort of time machine to see the past.


This is really something for SladeX and Neeyik (and a couple of others) to comment on - a lot of nice stuff on the 'net, as well.

[Kk21pwa9]

so what if (here we go) the light from other galaxies has yet to reach us for the first time, and when this light did reach us, would fill up the darks spots in the sky?

There won't be many objects whose emitted light has not reached us yet - as pointed out by others, that light will be incredibly weak though, as well as heavily redshifted to infrared, The likes of the Keck telescope has picked out early galaxies at distances of around 13 billion light years, and that's about as old as you're going to get, in terms of galaxies. However, due to the expansion of spacetime, there is definite limit to how far one can see an object, and beyond this, one will never see anything (for as long as the universe continues to expand), so there will always be dark spots.

[avaiguite]

not to mention the light is taking different amounts of time to reach us, so we are never gonig to have a full picture of the universe

[Dayreive]

13 billion years of light traveling at 300.000.000m/s.


man[help]

[Kk21pwa9]

That's certainly how long the light has been travelling for (assuming the speed of light has been constant throughout that time, for which there is no concrete evidence or sound theory to suggest otherwise) but the actual distance is further than your figures suggest: during those 13 billion years of travelling, the universe has been expanding, increasing the "gap" between the galaxy and our eyes. The comoving distance for such a galaxy would be around 28 billion light years.

[fabrizioitwloch]

I read something quite interesting the other day, It was something in one of the papers, it basically said that the nearest Galaxy to us is Andromeda and to reach that Galaxy we would have to travel for 2 million years at the speed of light which is 186,282.397 miles per second.

So that theory, pretty much means that if Aliens don't reside in this Galaxy then they are going to have to be travelling a hell of a lot faster than the speed of light to be able to pop on over and visit us.

http://en.wikipedia.org/wiki/Andromeda_Galaxy

[bestworkother]

I read something quite interesting the other day, It was something in one of the papers, it basically said that the nearest Galaxy to us is Andromeda and to reach that Galaxy we would have to travel for 2 million years at the speed of light which is 186,282.397 miles per second.

So that theory, pretty much means that if Aliens don't reside in this Galaxy then they are going to have to be travelling a hell of a lot faster than the speed of light to be able to pop on over and visit us.

http://en.wikipedia.org/wiki/Andromeda_Galaxy

Yes, it is very unlikely that aliens from another galaxy can reach us by travelling linearly through 3D space. Even nomadic species who "live" on a ship would have trouble crossing intergalactic space without some kind of hibernation system and a very long lasting energy source.

[nanyaHgoc]

I read something quite interesting the other day, It was something in one of the papers, it basically said that the nearest Galaxy to us is Andromeda and to reach that Galaxy we would have to travel for 2 million years at the speed of light which is 186,282.397 miles per second.

So that theory, pretty much means that if Aliens don't reside in this Galaxy then they are going to have to be travelling a hell of a lot faster than the speed of light to be able to pop on over and visit us.

http://en.wikipedia.org/wiki/Andromeda_Galaxy

In Bill Bryson's A Short History Of Nearly Everything, if I remember correctly, I read that even if you use the worst case scenarios when calculating the probability of intelligent life existing elsewhere in the Milky Way, then the number of planets containing intelligent life could well be in the millions. Considering there are 200 billion stars in our galaxy, I can believe this could well be true.

We may not have to worry about aliens in other galaxies, as we may have enough in our own galaxy.

[bestworkother]

In Bill Bryson's A Short History Of Nearly Everything, if I remember correctly, I read that even if you use the worst case scenarios when calculating the probability of intelligent life existing elsewhere in the Milky Way, then the number of planets containing intelligent life could well be in the millions. Considering there are 200 billion stars in our galaxy, I can believe this could well be true.

We may not have to worry about aliens in other galaxies, as we may have enough in our own galaxy.

Well, the "worst case scenario" still depends on one key factor, which is the probability that random molecules will combine together in such a way as to produce a "living" organism. This is still unknown, as we do not know of any other independently started life forms anywhere else. If this probability is very very small then the "worst case scenario" would be a lot worse...

[77Dinaartickire]

As far as space travel goes, isn't there a theory that says as you approach the speed of light time speeds up relative to you?

[bestworkother]

As far as space travel goes, isn't there a theory that says as you approach the speed of light time speeds up relative to you?

As you travel time will always remain constant for you, but your observation of the speed of time of an observer will change.

Therefore, it will still take you 2 million years to travel 1 million light years if you were travelling at 0.5C.

However, an observer on earth observing the space ship would see time passing a lot faster on the space ship than on earth, and vise versa. But anyway, I don't wanna go into another one of these discussions again

[Kk21pwa9]

As you travel time will always remain constant for you, but your observation of the speed of time of an observer will change.

That's only the case for constant motion; under acceleration, "your" time physically slows down.

Therefore, it will still take you 2 million years to travel 1 million light years if you were travelling at 0.5C.

However, an observer on earth observing the space ship would see time passing a lot faster on the space ship than on earth, and vise versa. But anyway, I don't wanna go into another one of these discussions again Each would observe the other's time slow down, not speed up - this seems to generate a paradox, but this is solved by the fact that the space ship would think that they've travelled a much shorter distance than they actually have.

[bestworkother]

That's only the case for constant motion; under acceleration, "your" time physically slows down.

I thought that under acceleration your inertial reference frame is constantly changing, i.e. the point of simultaneity between you and the observer is changing. So it's not a case of whose time is "physically" slower, but a case of the accelerating observer effectively "seeing" time moving faster for the constant observer, and the constant observer seeing time moving slower for the accelerating observer.

Each would observe the other's time slow down, not speed up - this seems to generate a paradox, but this is solved by the fact that the space ship would think that they've travelled a much shorter distance than they actually have.

I was taking into account relativistic Doppler shift, and assuming the aliens are travelling directly towards us, we would observe their time to be must faster relative to ours.

[Kk21pwa9]

I thought that under acceleration your inertial reference frame is constantly changing, i.e. the point of simultaneity between you and the observer is changing. So it's not a case of whose time is "physically" slower, but a case of the accelerating observer effectively "seeing" time moving faster for the constant observer, and the constant observer seeing time moving slower for the accelerating observer.

The accelerating observer would see the rest of the universe "slow down" as his rate of time does slow down compared to the rest of the universe (it's part of the equivalence principle, between acceleration and gravity, as described in general relativity). But even in constant motion, nobody ever sees anyone else's time run faster - time dilation, relative or absolute, always means the one observes another's time running slower. The "fix" for such an apparent paradox in constant motion (there is none in accelerative motion) is length contraction - for example, we can observe muons at the surface of the Earth, created by high energy electrons colliding with particles at the upper edges of the universe. Their "life span" is in the order of microseconds, so even moving close to the speed of light, they shouldn't be able to reach the surface - however, due to that high speed, we observe the muon's time running slower (hence it lives long enough). Now if we could travel as the muon, what would we see? Time dilation says the muon should see the Earth's clock running slow, hence why it's clock can tick away like mad, until it decays, while the Earth's grinds away. This would seem like a paradox: how can both be right? The answer is solved by the fact that special relativity offers more than just time dilation, there's length contraction too; so from our perspective of the muon's perspective, it doesn't see our clock run slow, instead it sees the distance it has to travel as being much shorter than that we see.

[bestworkother]

The accelerating observer would see the rest of the universe "slow down" as his rate of time does slow down compared to the rest of the universe (it's part of the equivalence principle, between acceleration and gravity, as described in general relativity). But even in constant motion, nobody ever sees anyone else's time run faster - time dilation, relative or absolute, always means the one observes another's time running slower. The "fix" for such an apparent paradox in constant motion (there is none in accelerative motion) is length contraction - for example, we can observe muons at the surface of the Earth, created by high energy electrons colliding with particles at the upper edges of the universe. Their "life span" is in the order of microseconds, so even moving close to the speed of light, they shouldn't be able to reach the surface - however, due to that high speed, we observe the muon's time running slower (hence it lives long enough). Now if we could travel as the muon, what would we see? Time dilation says the muon should see the Earth's clock running slow, hence why it's clock can tick away like mad, until it decays, while the Earth's grinds away. This would seem like a paradox: how can both be right? The answer is solved by the fact that special relativity offers more than just time dilation, there's length contraction too; so from our perspective of the muon's perspective, it doesn't see our clock run slow, instead it sees the distance it has to travel as being much shorter than that we see.

That's true. Each would observe the other's time slow down during constant relative motion, but in a case where an alien ship is travelling towards earth, the aliens would have aged less than the ppl on earth, due to their change in inertia from the accelerating and decelerating of their space ship. Therefore, the overall effect should be that the aliens would see our time running faster. All the speeding up happens during the inertia changes, so the majority of the time the aliens would still see Earth time running slow.

[Kk21pwa9]

That's true. Each would observe the other's time slow down during constant relative motion, but in a case where an alien ship is travelling towards earth, the aliens would have aged less than the ppl on earth, due to their change in inertia from the accelerating and decelerating of their space ship. Therefore, the overall effect should be that the aliens would see our time running faster. All the speeding up happens during the inertia changes, so the majority of the time the aliens would still see Earth time running slow.

Yes, of course, you're quite right - my brain's fuzzy with a cold at the mo'! Mind you, although this is technically what happens (and has been proven experimentally, on the atomic scale), I think it's not so simple to assume that one's faculties remain as "normal" during such acceleration - I'm not talking about the accelerative "forces" on the body, but how the mind would actually operate in a slower rate of time frame. How your brain would interpret what's going on during the "time speed up" phases is something I've never seen tackled in depth.