Re: Animation illustrating time dilation in satellites

In sci.physics.relativity, Androcles
<Engineer@xxxxxxxxxxxxxxxxxx>
wrote
on Mon, 10 Dec 2007 01:12:38 GMT
<ao07j.22084$kt3.8608@xxxxxxxxxxxxxxxxxxxxxxxxx>:

"The Ghost In The Machine" <ewill@xxxxxxxxxxxxxxxxxxxxxxx> wrote in message
news:8aeu25-86g.ln1@xxxxxxxxxxxxxxxxxxxxxxxxxx
: In sci.physics.relativity, Androcles
: <Engineer@xxxxxxxxxxxxxxxxxx>
: wrote
: on Sun, 09 Dec 2007 20:00:42 GMT
: <KPX6j.20677$kt3.16253@xxxxxxxxxxxxxxxxxxxxxxxxx>:
: >
: > "The Ghost In The Machine" <ewill@xxxxxxxxxxxxxxxxxxxxxxx> wrote in
message
: > news:5tmt25-dcj.ln1@xxxxxxxxxxxxxxxxxxxxxxxxxx
: >: In sci.physics.relativity, Paul B. Andersen
: >: <paul.b.andersen@xxxxxxxxxxxxxxxx>
: >: wrote
: >: on Fri, 07 Dec 2007 14:55:01 +0100
: >: <475950B5.5000005@xxxxxxxxxxxxxxxx>:
: >: > This is an applet illustrating time dilation in bodies moving
: >: > in the vicinity of the Earth.
: >: >
: >: > http://home.c2i.net/pb_andersen/Satellites.html
: >: >
: >: > Good ol' Newton is used to determine the motion of the bodies,
: >: > while the Schwarzschild metric is used to determine their
: >: > proper times.
: >:
: >: The GUI needs a bit of work but the idea is an
: >: interesting one. I'm also curious as to whether
: >: there's an explanation of the formulas used;
: >: http://relativity.livingreviews.org/open?pubNo=lrr-2003-1
: >: though might be a starting point were I to pursue this as
: >: an applet. Unfortunately page 5 warns that Keplerian orbits
: >: are a poor approximation at low altitude.
: >
: > If Andersen drew a picture of a cat and wrote "woof-woof"
: > beside it that would be as convincing a demo that a cat was a dog
: > as a satellite with the wrong time beside it convinces everyone
: > of time dilation.
: >
: >: It's also not clear what the rotating thing is, unless
: >: that represents a spherical planet of about 6378 km in radius,
: >: superimposed on a stationary polar coordinate system.
: >
: > I just can't see what is supposed to be happening.
: >
: > Hey look, this is length contraction:
: >
: > 0|_______________________|11
:
: There *is* no length contraction in SR. One cannot
: *measure* length in SR (or any other theory, really).
: The best one can do is measure *time* contraction, with
: an accurate velocity determination (which *is* possible
: if one assumes constant lightspeed), if the moving item
: is going too fast.
:
: >
: > See, my 12" ruler has contracted to 11 inches because I wrote 11
: > at the end instead of 12. Very convincing, I'm sure.
:
: Affix the ruler to the side of a car moving at 67 mph (30 m/s)
: and get back to me on how you'd measure it. ;-)
:

A simple engineering technique that has you non-thinking
physics types baffled or you wouldn't ask. I'd get in the car.

That changes the problem. The requirement is to measure
the ruler from a stationary point as the car is moving
past that point.

Getting into the car allows one to measure the ruler from
a moving point as the car is moving past the stationary
point.

It is naive to think the two measurements will be identical.


: >
: >
: >:
: >: >
: >: > A short comment to the scenarios in the applet:
: >: >
: >: > Two ballistic rockets:
: >: > ----------------------
: >: > Illustrates that two inertial bodies may experience
: >: > different proper times between the same two events.
: >: > (And the non inertial ground clock yet another.)
: >: >
: >: > Geostationary + GPS:
: >: > --------------------
: >: > Illustrates the time dilation in said satellites
: >: > (Both are 'running faster' than the ground clock)
: >: >
: >: > ISS and zero time diff. orbit:
: >: > ------------------------------
: >: > Clocks in the International Space Station are
: >: > running slow, while clocks in a satellite in
: >: > circular orbit at just the right altitude are
: >: > running at the same rate as the ground clock.
: >: >
: >: > Rockets in opposite directions
: >: > ------------------------------
: >: > Illustrates that the orbits of two rockets
: >: > launched at exactly the same way, but in
: >: > opposite directions, will be different.
: >: >
: >: > Two zero time diff. orbits:
: >: > -----------------------------
: >: > Even clocks in elliptical orbit may
: >: > - on average - run at the same rate as
: >: > a ground clock.
: >: >
: >: > Twin scenario - kind of:
: >: > ------------------------
: >: > Two twins meeting regularly comparing clocks
: >: > are ageing differently.
: >: >
: >: > Missing a scenario?
: >: > Give me a hint, and I may add it.
: >: >
: >:
: >: A scenario you might want, if you can get the precise
: >: data for it, would be a reprisal of Hafele & Keating.
: >: You might also consider a moon shot, though that might
: >: complicate things a bit since the moon warps and twists
: >: space as well. I have no idea if anyone's pursued time
: >: deltas in a modern moon probe. For that matter you
: >: could try the moon itself as an orbiting item.
: >:
: >: You might also want to embed the applet in a Web Page
: >: along with these scenarios and the formulas and/or a link,
: >: plus the usual disclaimers regarding computer modeling.
: >:
: >: There's also a typo in "two oppsosite orbits".
: >:
: >: And of course the results computed here might be compared
: >: (via a link) to real-world results.
: >
: > If he's going to do a demo then he needs to slow down
: > light to a manageable speed as every other demo does.
: > You know full well that all the paradoxes begin with letting
: > v = 0.866c or 0.9c or 0.6c or some such value.
: > I want to see the twins demo kind of where satellite A meets
: > satellite B before satellite A meets satellite B as measured
: > by their own clocks. Y'know, one hour passes for A and it
: > meets B, but two hours pass for B before it meets A.
: > That would be really convincing, especially if they used
: > sundials synchronized to sundials on the ground.
: >
:
: I'll refer you to Hafele-Keating for an experimental verification
: of a variant of the twin paradox.

More noise than signal.


Ah, yes, of course. Noted.

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