# Re: expansion of the universe (was Re: JOIN [sl4]: Hello, I'm Mercy.)

From: Perry E. Metzger (perry@piermont.com)
Date: Sat Apr 19 2003 - 00:13:08 MDT

"Jonathan Standley" <standley@rcn.com> writes:
> "No, I know that part already. What I don't understand is why it isn't
> a violation of special or general relativity for that to happen -- so
> far as I knew your relative speed to another object in space could
> never exceed c. (Or perhaps it is okay because of physics I don't
> know. I was sort of reaching for more detail on this.)"
> I'm not 100% sure on this myself, but I think it has do to with the fact
> that the universe was expanding, which is different from objects moving. If
> you were "moving" away from another object because of universal expansion,
> you would not have any gain in kinetic energy as a result of the expansion.
> You wouldn't be in motion at all.

In special relativity, I view myself as still always. I see the other
guy as moving. Since I see him as moving, I view him as having
kinetic energy. (I must admit I'm not entirely clear on the notion of
measuring space without reference to objects within the space...)

BTW, this is why in special relativity, if I have two objects that
have time bombs on them that will blow them up after ten seconds
(relative to the local frame of reference), and we send one of them at
high speed past the other, the observer on each of the objects
perceives himself as still, and therefore sees time as dilated for the
other guy, so both think the other blows up first.

> Again, I could be wrong, but I think this is a good analogy: imagine you are
> on a flat sheet of infinitely flexible material. Well, not on the sheet,
> but embedded within the sheet, with no way of escaping. You are at one end
> of the sheet, and want to travel to the other end. If the sheet is deformed
> through the 3rd dimension, stretching it out, the distance between them
> increases with no velocity change in the objects relative or absolute
> velocity.

Ah, but I was under the impression that the average geometry of the
expanding universe we're in is flat...

```--
Perry E. Metzger		perry@piermont.com
```

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