# Question about how we look at space

1. Aug 23, 2004

### dcbobo

If the space between earth and the sun started expanding and we moved away. Once it got 10 billion light years away, we would look at it and say that is what the sun looked like 10 billion years ago. How could we say that if we saw it 10 billion years ago?

Sorry to use such an extreme example. I just don't understand, if other galaxies are moving away from us that are so-and-so miles away from us, how do we say what we are looking at happened so-and-so many years ago unless nothing is moving?

If anyone could explain to me or point me to a site that explains why we view things in space the way we do, i'd be thankful.

2. Aug 23, 2004

as we lok at the sun right now, we are seeing what happened on it about 8 minutes ago. If we were to move away from the sun, our distance from it would increase, causing light to take more time to get to earth. Your little trick to remember is that if we were to move to 10 billion light years away from the sun at light speed. Time would dilate and the time passed for the sun would be 10 billoin years, while the time that passed for us would be like 1 second. Do you understand.

3. Aug 23, 2004

### dcbobo

No, sorry I don't understand at all. What if it doesn't move away at the speed of light?

I'm asking if it moved away from us right now, and then 10 billion light years from now we looked at it, we'd say that is what it looked like 10 billion light years ago, I don't understand how we get this....if we already saw it.

4. Aug 23, 2004

### chroot

Staff Emeritus
Actually Nenad, it would not be possible for us to move away from the Sun at the speed of light, nor is it possible for any massive body to move at the speed of light.

dcbobo,

You do seem to have a slight misconception.

To take a realistic example, imagine if a man took off in a starship capable of going 99.9% of the speed of light, as seen by Earthlings. If he travelled for 10 billion years as measured by clocks on Earth, his own clock would elapse only 447 million years due to time dilation:

When he finally came to a stop, he would see the Sun as it were only 447 million years after he departed.

If this sounds funny, think of it this way: the image of Sun is carried by photons, which travel at the speed of light. If our starship captain travels at very close to the speed of light, the photons have only a slight speed advantage over him. He's effectively getting very close to outrunning the light. If he could actually go the speed of light (which isn't possible), he'd be flying right alongside the photons from the Sun. When he came to a stop, it'd look to him as if the Sun hadn't aged a day!

- Warren

5. Aug 23, 2004

if it to move 10 billion light years away, then it takes time to get there. Here Ill show you the full calculation. Lets say that the earth were to move at a considerable speed of 0.01c. Then the time it would take us to get to 10 billion light years away at that speed would be about exackly 10 billion years in front of the time that the sun has been radiation, since out time dilates, and the suns time is longer. And by the time we stop moving at that distance, the difference in time between up and the sun would be 10 billion years, (the sun willwave been burning for 10 billion years longer than it would have of we were stationary). Do you get it now?
BTW (I know that we cant travel at that speed, but Im just making a theoratical example that would have exact dilation)

6. Aug 23, 2004

### dcbobo

heh chroot, I still don't undertand. I do understand what you are trying to say that traveling at the speed of light that the we are traveling as fast as the particles that allow us to see the sun. So all we would be doing is actually looking at the same particles that we saw when we started....

But i'm not talking speed. I'm talking time, I think (heh, really not sure). I see now my original post may have be to crude of an example with the sun. If we viewed the sun now and it moved away to a point 10 billion light years away, lets say it took 1000 billion years to get to that point. When we viewed the sun 1000 billion years from now at this point, how are we saying that is what the sun looked like 10 billion years ago?

I agree with you on my misconception, I am completely confused.

7. Aug 23, 2004

### chroot

Staff Emeritus
If it takes you 1000 billion years to travel 10 billion light-years, then you are only travelling 0.01 c, and relativistic effects are negligible. The speed of light is much, much greater than the speed of your spacecraft.

When you stopped at the end of your trip, it'd look like the Sun was 1000 billion years old -- i.e., dead and gone.

- Warren

8. Aug 23, 2004

### dcbobo

No Nenad, I am even more confused :(

The particles from the sun would still be coming towards us. As it moved away, would the speed of the light particles change?

It seems to me that the only way for us to view something 10 billion light years away from us, and claim that is how it appeared 10 billion years ago, is if the object releasing the light moved away from us more than the speed of light.

Chroot, thanks but evidently I'm missing something. I'm not making making my question clear enough for some reason. I guess I should have used realistic numbers that would apply to a star.

One more try, If we look the sun 200 million years from now and it is 10 million light years away. That means, we are looking at this star as it appeared 10 million years ago, correct? It just seems to me that if we can see how it appears 8 minutes ago, how do I come to the conclusion that i'm looking at the sun as it was 10 million years ago if we saw it 200 million years ago and could observe it as it moved away....

9. Aug 23, 2004

### chroot

Staff Emeritus
You really need to define this problem better if I'm to make any sense out of it. You're leaving out the mechanism by which the observer gets from here to there, and it matters a great deal.

Why don't you make up a complete scenario, including the travel times and speeds, so that we can help you understand it fully.

- Warren

10. Aug 23, 2004

### dcbobo

chroot,

That my problem, I don't understand the mechanism of how we view space. We look through a telescope and say a given star or system is so many million light years away. I don't understand how we got that. How are we measuring that?

I don't mean how things travel through space... do we account for the travel when view a certain star or system?

I'm confused when I try to understand how it works by using the sun(something close) as an object that if moved a distance from us how do we say it appears x amount of light years away if at that many years ago it was right next to us to view....

11. Aug 23, 2004

### chroot

Staff Emeritus
What travel are you talking about then?

We can use a simple technique called parallax to measure the distance to nearby stars. We look at a star in January, for example, and measure its angular position relative to much more distant stars. Then we repeat the process in June, when the Earth is on the opposite side of its orbit, and measure the star's angular position relative to the same distant stars. The difference in the position allows us to determine the distance to the star with only basic middle-school trigonometry.

If a star is 100 light-years away, its light takes 100 years to reach us from the time it is emitted. I'm frankly still not sure what is confusing you.

- Warren

12. Aug 23, 2004

### chroot

Staff Emeritus
Once again, this problem is not well-defined enough to be answered. You must define exactly how the object is to be moved.

- Warren

13. Aug 24, 2004

### dcbobo

Well thanks guys, sorry I couldn't be better with description. But you guys gave me enough information to learn what I wanted after some looking around. You got me in the right direction. I'm just having a hard time accepting or making sense of what I've read.

So, i'll keep trying until I get it...

OK, the sun and earth move apart from each other at twice the speed of light. If in 1 billion years that means they are 2 billion light years away from each other, then in 1 billion years, what are looking at?

Btw chroot, I never even made it to pre-algebra, sooo, i'm trying to keep this simple as my mind can handle...

14. Aug 24, 2004

### chroot

Staff Emeritus
This is not physically possible.

- Warren

15. Aug 24, 2004

### dcbobo

Aren't we being seperated from some quasars by twice the speed of light?

16. Aug 24, 2004

### pervect

Staff Emeritus
Here's a complete scenario that might be of some interest.

A spaceship starts out at earth, accelerating at a constant proper acceleration of 1g = 1.03 light years/year^2. (Or use 1 light year/year^2 if you like, it's simpler).

I think that the ship should enter the "rindler horizon" at t=.97 years, so that photons emitted after this time will not reach the ship until the ship enters the deacceleration phase.

The ship contiunes to accelerate for 21 years, ship time. At this point, we turn the ship around.

In the earth/sun frame, the rocket is 1.2 billion light years away at turn-around. In the ship frame, the earth/sun is roughly 20 light years away.

The ship now deaccelerates. After another 21 years of ship time, the ship stops accelerating, and comes to rest. At this point, the ship is 2.4 billion light years away from the earth.

17. Aug 24, 2004

### chroot

Staff Emeritus
No. There is a simple geometrical illusion that occurs when a high-velocity jet of material is aimed nearly directly at us; it results in the jets appearing to move superluminally. Of course, no actual matter is moving superluminally.

- Warren

18. Aug 24, 2004

### dcbobo

Ah well, i'm lost again. Maybe it's time for bed.

It thought if two objects are moving away from each other, each with a speed close to the speed of light and the space between them is expanding at a rate close to the speed of light, they would easily be seperating twice as fast as the speed for light.

19. Aug 24, 2004

### pervect

Staff Emeritus
I'm not super confident in my answer yet, but I seem to be getting that after this massive amount of travel, the spaceship at rest will be receiveing light rays that were emitted approx 2 years after the spaceship left earth.

20. Aug 24, 2004

### chroot

Staff Emeritus
It is possible for the expansion of space to make two distant galaxies recede from each other at a relative speed > c. The two objects cannot communicate with each other.

- Warren