Re: The beginning of space-based light pollution?

This post isn't to Paul in particular, but just anyone who's still
listening...

Paul Schlyter wrote:
> From a General Relativity point of view though, photons always
> travel in "straight lines", as does any other object in free fall
> (i.e. any other object upon which no forces except gravity are
> acting -- e.g. our own Earth, or the Moon, or the planets....).
> But since spacetime itself is curved, these "straight lines" (or
> geodesics, which is the proper way to label them) appear curved
> to us.

This has always seemed so odd to me that I decided to work out what it
means in broad terms, and the result is here:

http://astro.isi.edu/notes/gr.pdf

I can't call it a primer or an introduction to general relativity, since
it talks about it in a rather old-fashioned way. But as long as most of
us aren't going to be studying general relativity in any depth anyway, I
figure it's worthwhile bringing the topic out with analogies that, even
if they are outmoded, are at least technically correct, unlike most I've
seen in lay explanations.

If, by the way, you're interested in thinking abouut space and geometry,
I also have a small document on Newtonian gravity--in particular, a
geometric demonstration of the extraction of Newton's inverse square law
from Kepler's laws of planetary motion. Despite the subject matter,
this document is actually a bit more involved than the other.

http://astro.isi.edu/notes/newton.pdf

> Rest mass greater than zero ==> always travel slower than light
>
> Rest mass zero ==> always travel at precisely the speed of light
>
> Rest mass imaginary ==> always travel faster than light
> (these are the so-called "tachyons" - hypothetical particles which
> never have been observed. Trying to slow down a tachyon to light
> speed would require an infinite amount of energy - if tachyons exist)

Incidentally, one of the original formulators of tachyonic theory
pointed out that if there existed tachyonic observers, they would always
seem themselves as being sub-light-speed, and *us* as tachyonic. You
can never measure yourself as exceeding the speed of light (leaving
photons behind, something like that).

Brian Tung <brian@xxxxxxx>
The Astronomy Corner at http://astro.isi.edu/
Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt
.

Relevant Pages