rqr said:
[JesseM wrote:]
"However, this does not mean it the two ends will reach
the clocks very close to simultaneously in all frames,
so it is not an absolute synchronization method!"
Thanks!
I fully agree. Can we consider this rod case closed?
By closed, I mean that we seem to agree that a rod's
intrinsic length is change by its absolute motion
through space, so its endpoints cannot be used to
absolutely synchronize two clocks it the rod is
moving at a speed that differs from that of the
clocks.
I don't personally agree there is such a thing as "absolute motion through space", I just described things in terms of Lorentzian relativity since this is how
you apparently prefer to think about things. Of course the difference between the usual interpretation of SR and Lorentzian relatiity is just a philosophical one, they predict the same thing about all experimental results.
rqr said:
But before I go on to the next case, I have a few
things to say about something else you wrote.
Something's fishy.
[JesseM wrote:]
"The answer would depend on what frame we are using."
How a clock-synchronization experiment's result can
possibly vary depending on the frame in use?
I am speaking of the *general* result, i.e., either we
have a resulting absolute synchronization or we have
a resulting absolute asynchronization.
This sentence doesn't really make clear what you mean by "general result", the two alternatives you offer are identical...maybe you mistyped? Anyway, all I was talking about was the question you asked about whether the right end of the rod would be to the left or the right of the right clock at the "same moment" that the left end of the rod reached the left clock. Of course different frames define simultaneity differently, so they give different answers. If you prefer to imagine an absolute frame with absolute simultaneity, then the answer depends on whether the clocks or the rod have the higher absolute velocity, which we mere mortals would have no way of knowing, since there's no experimental method that can determine the absolute rest frame (see below).
rqr said:
(Remember your own warning that this must be a
closed-lab experiment; since it is, we do not have to
worry about any outside-observer viewpoints; all that
matters is that which occurs *within* the closed lab.)
Not true, the whole point of a physical definition of "absolute simultaneity" (as opposed to a purely philosophical one) is that
different closed-lab experiments must give the same definition of simultaneity. Closed-lab experiment means that as the experiment is performed the experimenter must have no access to information about anything outside the lab, but once the experiment is done and the clocks have been synchronized, you must open the different labs up and compare the clocks synchronized in different labs to see if they have arrived at the same definition of simultaneity or different ones. As long as the
physical aspects of the theory of relativity are correct--and these are entirely compatible with the
interpretation known as Lorentzian relativity, in which there is such a thing as absolute simultaneity--then it will be impossible to come up with a closed-lab method that is guaranteed to give the same definition of simultaneity for different labs.
rqr said:
Here, now, is the next case:
It is basically just the clock transport case.
In a given frame A, clocks C1 and C2 are at rest
and separated. Let a 3rd clock C3 that is moving
inertially toward C1 be set to match C1 as they
meet. As this 3rd clock moves on, it eventually
reaches clock C2. As they meet, C2 is made to match
C3. (This rigmarole eliminates all accelerations.)
matches C1
C3--->
C1------------------------------------C2
...........C3--->
C1------------------------------------C2
.........matches C3
Clocks C1 and C2 will be absolutely synchronized by
this simple, closed-lab procedure (given that the
physical internal atomic rhythms of all three clocks
are identical during the experiment).
They won't be absolutely synchronized, no. Presumably you want C3 to move very slowly relative to C1/C2 so there is little difference in the time dilation factor between C3 and C1/C2, but the difference in time dilation factors cannot be eliminated entirely. Assume the distance between C1 and C2 in their rest frame is 10 light-second, and the velocity of C3 in their rest frame is v. In this case, in their frame it takes a time of 10/v seconds for C3 to move between the two clocks. But C3 is slowed down by a factor of \sqrt{1 - v^2/c^2}, so it advances forward by
\frac{10 \sqrt{1 - v^2/c^2}}{v}
in that time. So, what's the difference between this, the time on C3 as it reaches C2, and the time that C1 reads at the same moment in this frame, namely 10/v? Well, it'd just be:
\frac{10(1 - \sqrt{1 - v^2/c^2})}{v}
If we take the limit of this quantity as v approaches zero, it will work out to zero (use
L'Hospital's rule along with the
chain rule of calculus to prove this), which means C2 will get arbitrary close to being synchronized with C1 in this frame as the velocity of C3 becomes very slow compared to c.
But that's just for the C1/C2 rest frame; now consider what happens in another frame where C1 and C2 move at some nonzero velocity, say 0.6c, and C3 moves with some slightly different velocity (0.6c + v)--again, we're going to be taking the limit as v approaches zero. In this frame the distance between C1 and C2 is shrunk to 8 light-seconds.
So if C3 and C1 start at x=0 light-seconds at time t=0 seconds, C3's position as a function of time x(t) is:
x(t) = (0.6c + v)*t
And if C2 starts at x=8 l.s. at time t=0 s, C2's position as a function of time is:
x(t) = 0.6c*t + 8 l.s.
So to figure out when C3 catches up with C2, set them equal:
0.6c*t + vt = 0.6c*t + 8 l.s.
...and solving this for t gives t=(8 l.s.)/v. So, this is the time it takes for C3 to go from C1 to C2 in this frame. Now, since C1 is slowed down by a factor of 0.8 in this frame, it will have elapsed a time of (0.8)*(8 l.s.)/v in this time. And C3 is slowed down by a factor of \sqrt{1 - (0.6c + v)^2 /c^2} in this frame, so it will have elapsed a time of:
\frac{\sqrt{1 - (0.6c + v)^2 /c^2} * 8 \, l.s.}{v}
So, the difference between the reading of C1 and the reading of C3 at the moment C3 reaches C2 will be:
\frac{(0.8 - \sqrt{1 - (0.6c + v)^2 /c^2}) * 8 \, l.s.}{v}
Now we want to know what this difference will approach in the limit as v approaches zero. Since both the numerator and the denominator of this fraction individually approach zero in the limit as v approaches zero, we can again use
L'Hospital's rule, taking the derivative of both the top and bottom and seeing what the new fraction approaches in the limit as v approaches zero...to take the derivative of the numerator we must also use the
chain rule again. This gives us the following complicated-looking fraction:
\frac{8 \, l.s. * [-(1/2) * (1 - (0.6c + v)^2 / c^2)^{-1/2} * (\frac{-1.2c - 2v}{c^2})]}{1}
Which simplifies to:
\frac{8 \, l.s. * (\frac{0.6c + v}{c^2})}{\sqrt{1 - (0.6c + v)^2 / c^2}}
And if we take the limit of
this as v approaches zero, it just turns out to be:
\frac{8 \, l.s. * (\frac{0.6c}{c^2})}{0.8}
Which is 6 seconds. So, even in the limit as the velocity of C3 relative to C1 and C2 gets arbitrarily small, this method will still leave C1 and C2 6 seconds out-of-sync in this frame (and you're free to imagine that this frame in which C1 and C2 move at 0.6c also happens to be the absolute rest frame of Lorentzian relativity). And not-so-coincidentally, it turns out that if you had synchronized C1 and C2 using the Einstein synchronization convention, they would also be 6 seconds out-of-sync in this frame. So again, this method is useless for
absolute synchronization, all it does is to replicate the same type of synchronization as the Einstein convention, which will cause clocks that are in-sync in one frame to be out-of-sync in another (or, if you prefer, will cause clocks that are moving relative to the absolute rest frame to be absolutely out-of-sync).
Incidentally, if you don't trust my calculus, feel free to take the equation I gave earlier for the difference between the reading of C1 and C3 at the moment C3 reaches C2:
\frac{(0.8 - \sqrt{1 - (0.6c + v)^2 /c^2}) * 8 l.s.}{v}
...and instead of taking limits, just plug in some very small v like v=0.000001c, you should end up with an answer very close to 6 seconds.