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: [Phys-l] Cosmic background radiation



On 11/06/2006 10:00 PM, Stefan Jeglinski wrote:
The distinction I'm making is important, because if our rulers
etc. expanded along with the universe, the expansion would be
unmeasurable and indeed meaningless.


Are you saying that if we only had the technology to measure an
expansion small enough (I suppose over a time scale necessary to
"see" it, namely one's lifetime), that we could measure the expansion
of the universe with a ruler?

In practice? No.

In principle? Yes.

I thought the issue was more tricky than this.

Overall, truly understanding the expansion of the universe requires
a tremendous amount of physics. It's a tour de force. But not
/everything/ about it is tricky.

When the universe expands:
Either the length between "here" and "there" changes, or it doesn't.
Either we can measure length with ruler etc., or we can't.

Is is not space that
is expanding, hence leading to a problem measuring the expansion with
a ruler, even in principle?

It's just like two bugs crawling south from the north pole,
following lines of latitude, i.e. great-circle routes, i.e.
geodesics. These are the straightest routes available to
them. (Assume for simplicity a perfectly spherical planet.)
The bugs will move apart from each other. They will not feel
any sideways force.

This does *not* mean that the bugs get bigger; they just move
apart. The bugs can carry rulers; they find that the number
of inches between one great circle and the next is non-constant.

There is a big difference between a collection of dust-particles
initially one inch a part, and a rigid ruler with markings one
inch apart. Each dust particle is an independent free particle.
The various parts of the ruler are not independent of each other!
The ruler is built out of not-very-stretchy material.

The inch-dust will move away from the inch-marks on the ruler.