Question about how we look at space

  • Thread starter dcbobo
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In summary: Can you please elaborate on what you don't understand?I don't understand how we can say what something looks like 10 billion years ago if we have not seen it that long ago and it is 10 billion light years away.In summary, if the space between Earth and the sun started expanding and we moved away, we would only be able to see what the sun looked like a few minutes ago if we were to move to 10 billion light years away.
  • #1
dcbobo
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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.
 
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  • #2
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
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
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 traveled for 10 billion years as measured by clocks on Earth, his own clock would elapse only 447 million years due to time dilation:

http://www.google.com/search?num=30...rs+/+(1/sqrt(1-(0.999*c)^2/c^2)))&btnG=Search

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
if it to move 10 billion light years away, then it takes time to get there. Here Ill show you the full calculation. Let's 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 can't travel at that speed, but I am just making a theoratical example that would have exact dilation)
 
  • #6
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, let's 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
dcbobo said:
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, let's 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?
If it takes you 1000 billion years to travel 10 billion light-years, then you are only traveling 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
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
dcbobo said:
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?
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
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
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
dcbobo said:
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...
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
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... :frown:
 
  • #14
dcbobo said:
OK, the sun and Earth move apart from each other at twice the speed of light.
This is not physically possible.

- Warren
 
  • #15
Aren't we being separated from some quasars by twice the speed of light?
 
  • #16
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
dcbobo said:
Aren't we being separated from some quasars by twice the speed of light?
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
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
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
dcbobo said:
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.
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
 
  • #21
dcbobo said:
If we viewed the sun now and it moved away to a point 10 billion light years away, let's 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?

This is essentially correct. However there are two complications:

1) General relativity. The universe is expanding. Cosmologists use a coordinate system which gives each location its proper time and describes distances between location in terms of the expansion factor of the universe. Thus when an object moves between locations it is hard to keep track of what is going on.
This isn't really a problem for nearby Galaxies such as Andromeda, but is when you are talking about times comparable to the age of the universe.

2) Special relativity. This describes how objects which are moving with respect to each other will see time differently. It does not agree with the coordinate system used for describing the expanding universe mentioned above. If you are thinking in terms of cosmological timescales then you should use GR and ignore SR.
 
  • #22
chronon said:
This is essentially correct.
No.. not it's not.

- Warren
 
  • #23
Why can't they communicate with light? Light would eventually reach the other as long neither object moved faster than the speed of light and as long as the expansion of space didn't expand faster than the speed of light, correct?
 
  • #24
If the expansion of space causes two distant objects to move apart faster than light, light from one object cannot ever catch up with the other.

- Warren
 
  • #25
If the expansion of space is less than the speed of light light can traverse it correct? And if two objects move through space fast enough to combine their speed with the expansion of space to create separation greater than the speed of light, light will eventually make it across.
Are you saying two objects cannot move through space without the expansion of space?
 
  • #26
dcbobo said:
If the expansion of space is less than the speed of light light can traverse it correct? And if two objects move through space fast enough to combine their speed with the expansion of space to create separation greater than the speed of light, light will eventually make it across.
No, it won't. How could it?
Are you saying two objects cannot move through space without the expansion of space?
I don't even know what this means, so I doubt I said it.

- Warren
 
  • #27
Ok, two rockets leave Earth each going 99% the speed of light, they are being being separated at a rate greater than the speed of light if you measure the rate from one ship to the other. Let's say light leaves one speaceship (this ship is going 99% the speed of light) going back towards Earth, at the speed of light. Once it passes Earth it goes towards the other rocket (which is traveling 99% the speed of light also). Eventually the light will catch it, even though the rocket that the light came from and the rocket it's going to are seperating faster than the speed of light, correct?
 
  • #28
  • #29
chroot said:
No.. not it's not.

- Warren

Sorry, yes you're right, I didn't read what dcbobo said properly. It won't be 10 billion years ago, it will be 10 billion years before the time of observation. So the observation takes place in 1000 billion years time, seeing the sun as it is in 990 billion years time. (Of course the sun will have gone out by then as you say)

In this example relativity (special and general) doesn't really come into the picture. The speed is much less than c and the 10 billion years will is much less than the age of the universe (1000 billion years and a bit).
 
  • #30
chroot said:
If the expansion of space causes two distant objects to move apart faster than light, light from one object cannot ever catch up with the other.

- Warren

That is not true, although you might not believe it.:rolleyes:

In the coordinate system used by cosmologists to describe the expansion of space it is possible to calculate the speed of separation of two objects and this can be greater than c. This has no significance whatsoever. It corresponds to a redshift of 1 (or thereabouts, I'm not sure how the curvature of space affects the calculation) and there are plenty of objects visible which are receding faster than that. This coordinate system is not compatible with special relativity. In a sense this isn't the best coordinate system for describing the velocity of distant objects. One day I'll work out a GR coordinate system which does agree with our SR frame of reference and put it on my website. The important thing to remember is that GR allows a lot of freedom in the choice of coordinate system and SR type statements imply a different one to cosmological statements.
 
  • #32
dcbobo said:
Could I get your thoughts about this page

http://www.astro.ucla.edu/~wright/cosmology_faq.html#FTL

Thanks again for the patience.

This guy knows his stuff. It's good to find someone who points out that what you say in cosmology depends on how you define distance. I hadn't seen this website before, thanks for finding it.
 
  • #33
This is Ned Wright's cosmology page ... it's one of the best on the web (thanks to marcus for bringing it PF members' attention).

If you have a question about cosmology (universal expansion, "Hubble" redshift, etc), this is a good place to start looking for an answer. If you've looked here are still don't understand, just come ask!

Please note that SR and GR are topics that can be discussed without reference to the universe as a whole, but if you're looking to understand things like dcbobo's question - in the context of the real universe - you may find better answers more quickly by posting to the General Astronomy & Cosmology section ... it's like the universe is 'more' than 'just GR' :bugeye:
 
  • #34
When look at the cosmos we don't see how things are, but only how things were in the past, in fact, a collage of how things were at many different times, like a superposition of many concentrical spherical photos from different times each showing only some objects (each photo being older the further away we see).
To deal with this (with the issue of the finite speed of light in general), sometimes I find useful the following analogy:

Remember the middle ages, when phones and tv's did not exist and the faster men could travel was by horse. The king is in his castle, an attack to the south of his country breaks-in, and a messenger leaves by horse to tell the terrible news to the king, but it's a 3 day journey. The king sits comfortably enjoying life in his castle, unaware that his kingdom has been attacked. For him, no attack has happened at all. The way he perceives the world is not what the world really is at that moment.

You can refine the analogy by realising that different types of signals could travel at different speeds. If attack was not too far, maybe they could immediately see a smoke column, but didn't know if it would be an attack or just an accidental fire. Other news might arrive by a messenger pigeon quite quickly, others at horse speed, or walking speed etc.
In any case there was a time lag between events "really happening" and "knowing about them", and the information or image available at any moment was a mix of data from different times.

So what happens with our perception of the universe should not be so puzzling at all for us humans, who have had to deal with limited information speeds for most of our history.
 

1. How do we study space?

Scientists study space through various methods such as telescopes, satellites, and probes. They also use computer simulations and mathematical models to understand the behavior of objects in space.

2. What is the purpose of studying space?

The purpose of studying space is to gain a better understanding of our universe and its origins. It also helps us to develop new technologies and innovations, and to potentially find habitable planets outside of our own.

3. How do we measure distances in space?

Scientists use a unit called a light-year to measure distances in space. One light-year is the distance that light travels in one year, which is approximately 9.5 trillion kilometers.

4. How do we know what objects in space look like?

Scientists use various methods to gather information about objects in space, such as taking images with telescopes or using spectroscopy to analyze the light emitted by objects. They also use computer models and simulations to create visual representations of objects in space.

5. How has our understanding of space changed over time?

Our understanding of space has greatly evolved over time, thanks to advancements in technology and scientific discoveries. For example, we now know that the universe is expanding and that there are other galaxies besides our own. We have also discovered new planets and moons, and have a better understanding of the origins of our solar system.

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