PDA

View Full Version : Relative Time

TBaker
2006-Jun-13, 08:25 PM
Assume we observe a distant galaxy, and determine from its distance and recessional velocity that it is of a certain minimum age, say 10bn years. What is to prevent an observer from that object spotting "us," plotting our distance and recessional velocity, and assuming we are also 10bn years old, even though obviously we are not? I assume this scenario is not possible, because the light from "us," since we are "younger," hasn't reached the other observer as of yet. Can someone confirm or provide a better explanation of this question?
Thanks.

HenrikOlsen
2006-Jun-13, 11:05 PM
us him many years ago
\ /
\ /
X
/ \
/ \
/ \
/ \
us him
now now

He sees our old light "at the same time" we see his.

Murff
2006-Jun-14, 10:38 PM
what about: ( -----> = light)

<------------------------------------------------------- his 10bn years
.....................Our 4bn years--------------->

When we were born, his light was already here, but since he is 10bn light years away, our light has not reached him yet.

Does that work?

(any reference to specific numbers, the words "us", "our", "we", "his", are strictly to make the sentence easy to read)

TBaker
2006-Jun-14, 10:49 PM
Yes, that's how I was looking at it. Most helpful.
Thanks.

Murff
2006-Jun-14, 11:00 PM
I am not in any way, shape, or form saying my view is right. It is just how I see one way in which it might occur.

The other post would work as well.

The mathematics that go into it (speed of light, rate of expansion, speed at which we are moving away from one another) are astounding :think:

Ken G
2006-Jun-15, 05:26 AM
You have it right on Murff (welcome to the forum). Also, keep in mind that there is no guarantee that "he" is even still here "now"-- that galaxy might not have survived the 10 billion years since it emitted the light we are seeing, it might have been "eaten" by a neighboring galaxy, say, 5 billion years ago, yet still we'd see it as it was 10 billion years ago (so we don't see it as being 10 billion years "old", we see it at whatever age it had 10 billion years ago).