Chronology Current Month Current Thread Current Date
[Year List] [Month List (current year)] [Date Index] [Thread Index] [Thread Prev] [Thread Next] [Date Prev] [Date Next]

Re: Cosmology



At 09:13 AM 2/12/01 -0600, Philip Zell wrote:
> is not the expansion of space
time a non-Newtonian concept, and isn't gravity a Newtonian concept,
so is it proper to be invoking Newton to explain the non Newtonian?

Wow, these questions are making me think.

First of all, our best known description of gravity is non-Newtonian,
namely general relativity. So we could give a non-Newtonian description if
necessary. OTOH there is a "correspondence principle" that says we should
check to see that GR reproduces the well-known Newtonian results in
every-day cases... which it does. So simple calculations of gravity within
a galaxy in an expanding universe are 100% legitimate.

Secondly, there is nothing about the (present-day) expansion of the
universe that violates Newton's laws! We have velocities proportional to
position. What's the problem? (If we had velocities inexplicably
depending on _time_, that would require a revision of Newton's laws, but we
don't.)

If you chalk up the presently-observed expansion pattern to a special
initial condition, there's nothing non-Newtonian about it.

In GR, do you not explain the orbits of planets in terms of geodesics?

We do.

Are these geodesics not curved as a result of the mass distribution in
our solar system?

They are.

Yet, is not the space time in our solar system
flat, because there isn't that much mass contained within it?

It is not flat. It is non-flat just enough to produce the curved free-fall
that we observe.

Is flatness defined strictly in terms of the geodesic followed by light?

Not strictly. A geodesic is the shortest path from A to B. Light plays by
the same rules as everything else.