I have wondered this for a long time now. A discussion with my
physics professor proved unsatisfying. Well, my curiosity goes likes
such. We imagine two objects that are apart but close to each other
in a remote part of the universe. If they are not revolving, then the
objects will come together by gravity. However, if they are revolving
(at the right speed), then the objects will stay apart, like there was
a force between them. Now I ponder, what do you measure the rotation
relative to? The background stars and the rest of the universe? What
if the two objects was the only thing in the universe. How can you
measure the rotation relative to nothing?

On 8 Jul 2003 21:08:08 -0700, (Hanroanu) wrote:

I have wondered this for a long time now. A discussion with my
physics professor proved unsatisfying. Well, my curiosity goes likes
such. We imagine two objects that are apart but close to each other
in a remote part of the universe. If they are not revolving, then the
objects will come together by gravity. However, if they are revolving
(at the right speed), then the objects will stay apart, like there was
a force between them.

There's a force between them in both cases, the same force. The
difference is the initial velocity vectors, which changes the path.

Now I ponder, what do you measure the rotation
relative to? The background stars and the rest of the universe? What
if the two objects was the only thing in the universe. How can you
measure the rotation relative to nothing?

You can still determine an angular momentum. Rotation is absolute,
meaning there are experiments you can do that determine whether you
are rotating or not. One of them would be in fact to observe the
orbital dynamics. Others might involve sending test spacecraft out
away from the local gravitational field. High precision laser
experiments would also indicate a motion.

Coriolis "force" on earth tells us we are rotating, even without
reference to anything else but the earth's surface. It causes things
launched straight to appear to curve. Foucault pendulums go through a
circle, hurricanes spin in characteristic directions, and artillery
shells go off to one side. No background of stars, moon, or sun needed
to observe these effects.

- Randy

Hanroanu wrote:

I have wondered this for a long time now. A discussion with my
physics professor proved unsatisfying. Well, my curiosity goes likes
such. We imagine two objects that are apart but close to each other
in a remote part of the universe. If they are not revolving, then the
objects will come together by gravity. However, if they are revolving
(at the right speed), then the objects will stay apart, like there was
a force between them. Now I ponder, what do you measure the rotation
relative to? The background stars and the rest of the universe? What
if the two objects was the only thing in the universe. How can you
measure the rotation relative to nothing?

Acceleration is an absolute quantity measurable in a hermtically
sealed and isolated/shielded environment.

Start with an empty universe. Add a neutron star with modest
rotational angular velocity omega. (Neutron stars are stiff - no
differential rotation vs. latitude, no equatorial bulge). It is
orbited by a planet with circular equilibrium astrosynchronous orbital
angular velocity omega, orbiting in the plane of the neutron star's
equator. The planet spins with in-plane angular velocity omega.

A naive observer enters the universe. He sees two bodies stationary
in space despite gravitation. How can this be?

OTOH, you can erect a few Foucault pendula (you obviously need at
least three. Why?) or three-axis ring laser gyros on various parts of
the planet and look for the obvious in both cases. OTOH, you can
embrace Mach's Principle and wonder how two bodies can be stationary
in space despite gravitation.

--
Uncle Al
http://www.mazepath.com/uncleal/
(Toxic URL! Unsafe for children and most mammals)
"Quis custodiet ipsos custodes?" The Net!

In article ,
Hanroanu wrote:
I have wondered this for a long time now. A discussion with my
physics professor proved unsatisfying. Well, my curiosity goes likes
such. We imagine two objects that are apart but close to each other
in a remote part of the universe. If they are not revolving, then the
objects will come together by gravity. However, if they are revolving
(at the right speed), then the objects will stay apart, like there was
a force between them. Now I ponder, what do you measure the rotation
relative to? The background stars and the rest of the universe? What
if the two objects was the only thing in the universe. How can you
measure the rotation relative to nothing?


Foucalt built a large pendulum that could swing for days, and the
direction that the bob moved turned a circle once per day, proof of the
rotation of the Earth. Today, Sagnac interferometers, made by interfering
laser beams through spools of fiber optics, can be made sensitive enough
to measure the Earth's rotation. Coriolis forces on large pools of water
allowed to still before draining can also show direction of rotation
(although the swirl of toilets has nothing to do with that one).

A more interesting question, I think, is how would the inhabitants
develop a theory of rotation given that what you've described is all they
know. Transform to an accelerated reference frame and you pick up
inertial forces--centrifugal force, Coriolis forces, etc. Science would
have to advance beyond our Galileo and Copernicus before the debates
started flaring about the nature of the force and movement of the planet
and sun.

All the while assuming a Newtonian analysis is sufficient. I don't know
what a cosmos consisting of two objects would be like in general
relativity.

--
"Is that plutonium on your gums?"
"Shut up and kiss me!"
-- Marge and Homer Simpson

(Hanroanu) wrote in message . com...
[snips]
How can you
measure the rotation relative to nothing?

The simple answer is, fictitious forces. Consider something
like launching a projectile from one mass to the other. Or
launching a projectile directly out from one mass. If there
is revolution and you don't know about it, you are going to
think projectile motion involves this extra motion to the
side. Motion from one mass to the other will be
this odd not-quite-a-spiral motion, for example. If you
had not realized the revolution motion, you would think
there had to be some very odd forces involved.
Socks

Hanroanu wrote in message
om...
I have wondered this for a long time now. A discussion with my
physics professor proved unsatisfying.

Learn to listen. Don't let preconcieved notions get in the way.

Well, my curiosity goes likes
such. We imagine two objects that are apart but close to each other
in a remote part of the universe. If they are not revolving, then the
objects will come together by gravity. However, if they are revolving
(at the right speed), then the objects will stay apart, like there was
a force between them.

This is correct only if one takes "revolve" to mean that the two
massive bodies are in free-fall orbit about their common center of
mass. If "revolve" means that the two bodies are at relative rest
while each body rotates or spins, then the statement is false.

Now I ponder, what do you measure the rotation
relative to? The background stars and the rest of the universe? What
if the two objects was the only thing in the universe. How can you
measure the rotation relative to nothing?

In Newtonian mechanics, rotation is absolute and self-referential.
In a rotating frame of reference, it is always possible to locally
measure that rotation (as with a Foucalt pendulum or laser ring
gyroscope) without reference to the "fixed stars". GTR predicts
that rotating mass/energy induces second order space-time
rotation, the effect being an order of magnitude less than that
predicted for precession of perihelion for elliptical orbits.
[Old Man]

"MarkK" wrote in message
m...
(Hanroanu) wrote in message
. com...
I have wondered this for a long time now. A discussion with my
physics professor proved unsatisfying. Well, my curiosity goes likes
such. We imagine two objects that are apart but close to each other
in a remote part of the universe. If they are not revolving, then the
objects will come together by gravity. However, if they are revolving
(at the right speed), then the objects will stay apart, like there was
a force between them. Now I ponder, what do you measure the rotation
relative to? The background stars and the rest of the universe? What
if the two objects was the only thing in the universe. How can you
measure the rotation relative to nothing?

I thought up that problem years ago:
Two disks spin in relation to each other. Which one (or two) exhibits
centrifugal force and why?
There has to be a some reference frame (an aether?).
I also wonder if other parts of the universe (galaxies, or even our
galaxy) may have slightly different reference frames (and therefore
different centrifugal forces) in relation to our frame.
Mark K.

Two bodies interact about a common point (The center of action),
in a common time (The system period).

This is explain in detail in the physics tutorial
which can be downloaded from my web site.

The tutorial starts with events as the basic quanta of reality,
and in six, simple, graphic steps,
develops a unique Physical Properties Chart
that shows the relationships between the physical properties
much as the Periodic Chart shows the relationships between the elements.

In fact, Lesson #1 of the tutorial,
graphically shows and explains
the relationships between "two objects revolving".

--
Tom Potter http://tompotter.us

(Gregory L. Hansen) wrote:

In article ,
Hanroanu wrote:
I have wondered this for a long time now. A discussion with my
physics professor proved unsatisfying. Well, my curiosity goes likes
such. We imagine two objects that are apart but close to each other
in a remote part of the universe. If they are not revolving, then the
objects will come together by gravity. However, if they are revolving
(at the right speed), then the objects will stay apart, like there was
a force between them. Now I ponder, what do you measure the rotation
relative to? The background stars and the rest of the universe? What
if the two objects was the only thing in the universe. How can you
measure the rotation relative to nothing?


Foucalt built a large pendulum that could swing for days, and the
direction that the bob moved turned a circle once per day, proof of the
rotation of the Earth. Today, Sagnac interferometers, made by interfering
laser beams through spools of fiber optics, can be made sensitive enough
to measure the Earth's rotation. Coriolis forces on large pools of water
allowed to still before draining can also show direction of rotation
(although the swirl of toilets has nothing to do with that one).

A more interesting question, I think, is how would the inhabitants
develop a theory of rotation given that what you've described is all they
know. Transform to an accelerated reference frame and you pick up
inertial forces--centrifugal force, Coriolis forces, etc. Science would
have to advance beyond our Galileo and Copernicus before the debates
started flaring about the nature of the force and movement of the planet
and sun.

All the while assuming a Newtonian analysis is sufficient. I don't know
what a cosmos consisting of two objects would be like in general
relativity.


Would it be more, or less complicated, for those inhabitants if the
two bodies rotated in addition to revolving, so they always presented
the same face to each other? Like our moon.

Jim