# Thread: I don't understand the second part of Hubble's Law

1. Member
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Jan 2007
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16

## I don't understand the second part of Hubble's Law

I understand the first part about all the galaxies around us receding from us and this means the universe is expanding.

I don't understand the second part, about how the farther a galaxy is from us, the faster it is receding. What does this mean?

I think I have found way to picture the situation that works for me. It is like this: if you were cut a rubber band into a strip, place a dot on each end and another one in the middle. If you start stretching it, no matter how fast the dots on ends are moving from each other, the dot in the middle is only going to move half as far and appear to be moving half as fast.

Thank you for all the help. Anyone who wants to take a stab at making it even clearer is welcome to write.
Last edited by pschroeter; 2012-Jun-20 at 06:52 PM.

2. Draw dots on a rubber band, and stretch it between your hands.

The overall rubber band expands in length at the same "rate", but you'll note that the distance between dots that are close to each other grows slower than the distance between dots that are far apart.

Code:
```1:       A B C D E F
2:     A  B  C  D  E  F
3:   A   B   C   D   E   F
4: A    B    C    D    E    F```
Each step:
A and B get one space further apart.
B and C get one space further apart.
C and D get one space further apart.
D and E get one space further apart.
E and F get one space further apart.
So: A and F get 5 spaces further apart.

3. Originally Posted by pschroeter
I don't understand the second part, about how the farther a galaxy is from us, the faster it is receding. What does this mean?
Or another analogy. Suppose that you are at a rocket base, and lots of rockets are being launched in various directions, all with unlimited fuel. You will find that the ones that are close by are moving rather slowly, whereas those far away are moving quickly. And you will find that the further away a rocket is, the faster away from you it is moving. The situation is kind of like that.

4. Originally Posted by pschroeter
I understand the first part about all the galaxies around us receding from us and this means the universe is expanding.
Also, just a slight modification, but it's not true that all the galaxies are receding from us. Nearby galaxies (like Andromeda, notably) can be moving toward us. It's true that beyond a certain distance, we never find galaxies moving toward us.

http://spiff.rit.edu/classes/phys301...nd/expand.html

I suggest you play with the "raisin bread" applet near the end of the
lecture.

6. Member
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Jan 2007
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Thank you, I think the raisin bread did the trick. I picked three raisins in a row and finally got an idea of what is going on.

7. Member
Join Date
Jan 2007
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Thank you for your help, it, well... helped. What I was really impressed by is you avatar. I have been going to see YES since '78 and they are my favorite band, Steve Howe my favorite guitarist. I'm still waiting for them to add a Ohio date to their summer tour.

8. Originally Posted by StupendousMan
I suggest you play with the "raisin bread" applet near the end of the
lecture.
I always understood the raisin bread analogy, but I've never actually seen a clear animation like that. Nice.

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