From: louisnews Newstrom (louisnews@comcast.net)
Date: Sun Aug 25 2002 - 13:07:26 MDT
From: scerir <scerir@libero.it>
> The problem here is the same. Imagine a symmetrical situation with
> two non-inertial frames. Two clocks go in opposite directions, they
> accelerate, decelerate, come back. Which one is slow? Why?
If they accellerate and decellerate similarly, then they will both show
the same time, as both have experienced the same relative effects.
> Einstein avoided this situation. He imagined (1905) just one clock
> moving, or (later) accelerating. What about the other?
This is because symetrical situations are boring. Both clocks would
keep the same time.
> Btw SR can manage non-inertial frames.
> Can Special Relativity handle accelerations?
It can "handle" all situations fine. It states that non-inertial frames
might not have the same laws of physics.
A simple example of a non-inertial frame is a space station rotating for
artificial gravety. Imagine a person on the space station let's go of a
hammer. He perceives that the motionless hammer is acted upon by some
force which pushes it to the floor.
An observer from outside of the space station would perceive that the
hammer was "flung" with a velocity equal to the floor of the space
station. The hammer then travelled in a straight line at that velocity
until the curving floor of the space station intersected its path.
In this case, one view perceives a force, the other view says there is
no force. Both "handle" the situation, just differently.
Relativity says that all inertial frames are the same. Non-inertial
frames may be different.
> My point was just to show that the situation is still messy,
> after all these years.
Only becuase people, like Dingle, quote the theory wrong, and then show
that (their version of) it is contradictory.
> In general you can not easily introduce a
> preferred frame, a preferred geodesic, trying to explain the
> asymmetrical (experimental) effect.
That is correct. The theory of relativity says that there is no
preferred frame, so any attempt to introduce one gets messy.
> From the experimental pov Hafele and Keating, who performed that
> famous experiment, say something interesting.
...
> Thus, unless additional information is supplied, the statement
> that 'moving clocks run slow' is an artifact completely without
> physical significance.
...
> It is purely a symmetric, definitional
> artifact of measurement created by the adoption of the Einstein
> simultaneity convention
What they say is true, but I wouldn't have said "no physical
significance". To illustrate, let's imagine a totally spatial
situation. (I find many points of relativity easier to visualize with
two spatial dimensions...)
Imagine two guys who just got GPS's. Being obsessed with their new toys
they focus totally on the coordinates (latitude and longitude). One
walks mostly northwards, drifting to the east. The other walks mostly
eastward, drifting to the north.
One guy will say that he is walking faster, because he is reaching
Northern latitudes at a faster rate than his friend.
The other guy will say that he is walking faster, because he is reaching
Eastern longitudes at a faster rate than his friend.
>From the quote above, Hafele and Keating would say that "neither one is
walking slower" and "there is no physical signifigance".
How they are brought together will determine who is "right". If the
Northward walking guy would walk straight south to meet with his friends
tracks, then yes he would be "way behind" his friend along the eastward
tracks.
On the other hand, if the eastward walking guy would walk straight west
to meet his friends northward tracks, he would be "way behind" along the
northward track.
This is why in our example, it matters which clock accellerates.
Because which track they converge on will determine whose coordinate
system is being used to measure who is "behind".
(I find that most of relativity can be more easily understood by using
two spatial dimensions instead of one spatial and one time dimension.
With two spatial dimensions you can visualize the results of going
"diagonal" as opposed to straight along one of the coordinate directions.)
--- Louis Newstrom louisnews@comcast.net
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