Re: Clock synch


jem skrev:

Nicolaas Vroom wrote:
"Tom Roberts" <tjroberts137@xxxxxxxxxxxxx> schreef in bericht
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Nicolaas Vroom wrote:

"Tom Roberts" <tjroberts137@xxxxxxxxxxxxx> schreef in bericht
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Nicolaas Vroom wrote:

"Nicolaas Vroom" <nicolaas.vroom@xxxxxxxxxx> schreef in bericht
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"Tom Roberts" <tjroberts137@xxxxxxxxxxxxx> schreef in bericht
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There is no "time dilation", in the sense of a clock physically
ticking slower, or in the sense of "time moving slower". There is
no "shortening/elongating", in the sense of a given object
physically getting shorter or longer.

The effects called "time dilation" and "length contraction" in SR
are not physical effects, and are due to different measurement
procedures used by differently moving observers.

Suppose there are two Observers A and B at the same position P1
which both have identical clocks and which initially show the same
value.
How would you call the effect, that, if those two Observers move
from P1 to P2 along a different path, that when they meet,
they realize that the two clocks show a different value ?
What is wrong by calling that a physical effect ?

There is nothing wrong with that, indeed that is what we mean by
"physical effect". But that is NOT "time dilation".

Accordingly to the book "Introducing Einstein's relativity" by
Ray d'Inverno at page 33 "Thus moving clocks go slow by a factor
SQR(1- v*v/c*c). This phenomenon is called time dilation."

Yes. That is NOT what you described. <shrug>


And accordingly to you what Rat described is not a physical effect ?


IMO this is the situation I have described above.

Then you need to learn how to distinguish different situations better.

Inverno's definition, and mine, involves the comparison of a single moving
clock's elapsed time to the coordinate clocks of a specified inertial
frame -- this NECESSARILY requires two different clocks in that frame,
because the moving clock is MOVING (relative to that frame).

The use of two coordinate clocks of that frame is essential in performing
the geometric PROJECTION that is known as time dilation.

Your scenario is completely different from this. <shrug>


Let me rephrase my previous example slightly:

Suppose there are two Observers A and B at the same position P1
which both have identical clocks and which initially show the same value.
How would you call the effect, that, if Observer A stays at home
and Observer B moves from P1 to P2 and back to P1,
that when they meet,
they realize that the two clocks show a different value ?
What is wrong by calling that a physical effect ?

And is this time dilation ?

>>> As I have said so often, time dilation is the difference in projections

onto different coordinates.

How would you describe an experiment that demonstrates above

In inertial frame A an observer arranges for two assistants to be
pre-positioned along the path that moving clock B will take relative to
frame A. These assistants are given standard clocks, and they are
synchronized using Einstein synchronization in frame A. When the clock B
passes each assistant, they record the values on their own clock and on
clock B. Afterwards, both assistants bring their records to the observer,
who then computes the time different on clock B and in frame A.


IMO this is now identical with Observer B moving from P1 to P2
which uses the "moving clock B"
Except in the rephrased example moving clock B also moves back
and there is no asistant at P2.

IMO my example is a clear case were time dilation is involved
and because the moving clocks ticks slower IMO it is also
correct to call it a physical effect

Time dilation refers to the effect where two observers each measure* the
other's clock to be ticking at a slower rate than their own clock. As
to whether or not this should be called a "physical effect", bear in
mind that term has no exact definition, so whether time dilation is said
to be a physical effect because it refers to a measureable phenomenon,
or whether it's said to be a non-physical effect because no changes
occur in the clocks' tick mechanisms, doesn't much matter, just as long
as what's going on is understood.

How could you claim it to be understood if you really can not tell if
it is real world phenomena or an artefact of math that really adds up
to nothing?


* using the specific measuring procedure that Roberts described above,
or its equivalent.

BTW, "time dilation" is not the effect that results in different clock
readings in the Twins Paradox (since in the usual formulation of the TP,
the time dilation effects are symmetric between the two twins). The
asymmetry in the TP appears when the two twins actually watch each
other's clocks (through telescopes say). When measured in this way, the
clock rate comparisons of each twin are slower for a while and faster
for a while, except that the Earth-bound twin sees the slower rate for a
longer while than the traveling twin does.



It is the use of multiple clocks SYNCHRONIZED IN FRAME A that makes this a
projection onto the time coordinate of frame A.


Nicolaas Vroom



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