- #1
Stephanus
- 1,316
- 104
Dear PF Forum,
I've been wondering about how on Earth (and I do mean it, from earth
) that we know there's a galaxy 20 billions light away. Considering that https://en.wikipedia.org/wiki/Age_of_the_universe is 13.8 billions years old. But before I'm asking about Supernova Ia and Hubble Flow, I'd like to ask the following question.
There are two comoving observers W at west and E at east, separated 100 light years away. A third observer, R, is running away from east to west at 0.6c.
The moment R reaches W, R sends a signal to E. This signal will reach E in 100 years wrt E and W clock. Not in 100/(1-0.6) = 250 years wrt EW!
This I already learned from PF Forum![Smile :smile: :smile:](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
And supposed E and W are separated 100 millions light years away.
And again supposed, just supposed that the sepace between them is not expanding the moment W sends a signal to E.
Then after that space is expanding between them at, say 0.6c. That means that now W travels to the west at 0.6c wrt E.
Perhaps I can draw a simple picture. s is the signal sent by W
W....s..........E
then the space between them is expanding at x
W....s.....xx......E
W....s.....x..x......E
W....s.....x...x......E
W....s.....x...x......E
Now I'm lost here. Not in space but in thought.
How is s speed?
s relative to W is 300,000 km per second? I think so.
But what is s speed relative to E?
300,000 km/s or 120,000 km/s.
But the motion of everything to light is always 300,000 km/s, right?
For example.
Alice move to the west wrt Bob 100,000km/s. Bob moves to the west wrt Charlie 50,000 km/s, Charlie moves 200,000 km/s wrt Delta to the east.
And then Alice, Bob and Charlie sends a signal to Delta. All of their signals travel at 300,000 km/s wrt Delta.
Now, my question is this
=====================================================
What is s speed to E?
=====================================================
Thank you very much.
I've been wondering about how on Earth (and I do mean it, from earth
There are two comoving observers W at west and E at east, separated 100 light years away. A third observer, R, is running away from east to west at 0.6c.
The moment R reaches W, R sends a signal to E. This signal will reach E in 100 years wrt E and W clock. Not in 100/(1-0.6) = 250 years wrt EW!
This I already learned from PF Forum
And supposed E and W are separated 100 millions light years away.
And again supposed, just supposed that the sepace between them is not expanding the moment W sends a signal to E.
Then after that space is expanding between them at, say 0.6c. That means that now W travels to the west at 0.6c wrt E.
Perhaps I can draw a simple picture. s is the signal sent by W
W....s..........E
then the space between them is expanding at x
W....s.....xx......E
W....s.....x..x......E
W....s.....x...x......E
W....s.....x...x......E
Now I'm lost here. Not in space but in thought.
How is s speed?
s relative to W is 300,000 km per second? I think so.
But what is s speed relative to E?
300,000 km/s or 120,000 km/s.
But the motion of everything to light is always 300,000 km/s, right?
For example.
Alice move to the west wrt Bob 100,000km/s. Bob moves to the west wrt Charlie 50,000 km/s, Charlie moves 200,000 km/s wrt Delta to the east.
And then Alice, Bob and Charlie sends a signal to Delta. All of their signals travel at 300,000 km/s wrt Delta.
Now, my question is this
=====================================================
What is s speed to E?
=====================================================
Thank you very much.