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From: jbriggs444 on 5 Mar 2010 14:17
On Mar 5, 1:54 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
[discussing the scenario in which two clocks symmetrically accelerate
toward each other and this is observed from the point of view of one
of the clocks]

> Your initial assertions are correct.  Both will report a slow down....
> During part of the trip.  And a different slowdown  during a different
> part of the trip.

I should correct myself here.

The part I wrote about "a different slowdown during a different part
of the trip" was an erroneous reference to a scenario in which a clock
is reporting the Doppler shift that is _seen_ (i.e. without accounting
for transit delays).

In such a scenario, the clock sees an interval after it has
accelerated and before its peer has accelerated and then a second
interval after the peer has accelerated. The Doppler shifts for the
intervals are, of course, different.

In the scenario at hand we are trying to discuss what is _observed_
(i.e. accounting for transit delays and adopting particular standards
of simultaneity)

What is _observed_ is only a single interval during which the two
clocks are in constant relative motion, not two such intervals. [From
an "observed" point of view, the period when the peer clock remains
motionless does not fall within the interval of the journey. Instead,
it is in the relative past]

From: mpalenik on 5 Mar 2010 15:07
On Mar 5, 2:17 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
> On Mar 5, 1:54 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
> [discussing the scenario in which two clocks symmetrically accelerate
> toward each other and this is observed from the point of view of one
> of the clocks]
>
> > Your initial assertions are correct.  Both will report a slow down.....
> > During part of the trip.  And a different slowdown  during a different
> > part of the trip.
>
> I should correct myself here.
>
> The part I wrote about "a different slowdown during a different part
> of the trip" was an erroneous reference to a scenario in which a clock
> is reporting the Doppler shift that is _seen_ (i.e. without accounting
> for transit delays).

In fact, what I was trying to describe to Ste was the effects that are
specifically not due to Doppler shifting, as I was specifically making
the point that the predictions of SR for time dilation are
mathematically different than those which are due to the observed rate
of change on a ticking clock to transit delays.


>
> In such a scenario, the clock sees an interval after it has
> accelerated and before its peer has accelerated and then a second
> interval after the peer has accelerated.  The Doppler shifts for the
> intervals are, of course, different.
>
> In the scenario at hand we are trying to discuss what is _observed_
> (i.e. accounting for transit delays and adopting particular standards
> of simultaneity)

No, that's not actually what we were talking about at all.

>
> What is _observed_ is only a single interval during which the two
> clocks are in constant relative motion, not two such intervals.  [From
> an "observed" point of view, the period when the peer clock remains
> motionless does not fall within the interval of the journey.  Instead,
> it is in the relative past]

The relevant quantity was the time that each clock displays after the
two are brought into comoving frames, which once again, depends on the
frame that they are brought into.

And yes, as the clocks accelerate, due to Doppler effects, they will
each see an apparent change in the other clock's rate that brings it's
reading into what it is supposed to be for whichever frame they are
accelerating into.

But I fear if Ste reads any of this, he's going to slip back into his
whole "relativity is due to propagation delays" thing again. Let's
tackle one problem at a time.
From: jbriggs444 on 5 Mar 2010 15:47
On Mar 5, 3:07 pm, mpalenik <markpale...(a)gmail.com> wrote:
> On Mar 5, 2:17 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
>
> > On Mar 5, 1:54 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
> > [discussing the scenario in which two clocks symmetrically accelerate
> > toward each other and this is observed from the point of view of one
> > of the clocks]
>
> > > Your initial assertions are correct.  Both will report a slow down.....
> > > During part of the trip.  And a different slowdown  during a different
> > > part of the trip.
>
> > I should correct myself here.
>
> > The part I wrote about "a different slowdown during a different part
> > of the trip" was an erroneous reference to a scenario in which a clock
> > is reporting the Doppler shift that is _seen_ (i.e. without accounting
> > for transit delays).
>
> In fact, what I was trying to describe to Ste was the effects that are
> specifically not due to Doppler shifting, as I was specifically making
> the point that the predictions of SR for time dilation are
> mathematically different than those which are due to the observed rate
> of change on a ticking clock to transit delays.

Ok good. So we're both not talking about that.

> > In such a scenario, the clock sees an interval after it has
> > accelerated and before its peer has accelerated and then a second
> > interval after the peer has accelerated.  The Doppler shifts for the
> > intervals are, of course, different.
>
> > In the scenario at hand we are trying to discuss what is _observed_
> > (i.e. accounting for transit delays and adopting particular standards
> > of simultaneity)
>
> No, that's not actually what we were talking about at all.

> > What is _observed_ is only a single interval during which the two
> > clocks are in constant relative motion, not two such intervals.  [From
> > an "observed" point of view, the period when the peer clock remains
> > motionless does not fall within the interval of the journey.  Instead,
> > it is in the relative past]
>
> The relevant quantity was the time that each clock displays after the
> two are brought into comoving frames, which once again, depends on the
> frame that they are brought into.

In the scenario in question they are not brought into co-moving frames
(whatever that means -- the notion of things being "brought into"
frames is very questionable) They are brought _TOGETHER_.

The time displayed on each clock when they become adjacent is an
_observable_. It's not frame dependent. The numbers don't change if
you decide to adopt a different frame of reference. You take a
snapshot of the clocks side by side and you look at the numbers.
There is no ambiguity. All frames get the same answer.

Even the frame in which the two clocks are mutually at rest.

From: mpalenik on 5 Mar 2010 15:55
On Mar 5, 3:47 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
> On Mar 5, 3:07 pm, mpalenik <markpale...(a)gmail.com> wrote:
>
>
>
>
>
> > On Mar 5, 2:17 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
>
> > > On Mar 5, 1:54 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
> > > [discussing the scenario in which two clocks symmetrically accelerate
> > > toward each other and this is observed from the point of view of one
> > > of the clocks]
>
> > > > Your initial assertions are correct.  Both will report a slow down....
> > > > During part of the trip.  And a different slowdown  during a different
> > > > part of the trip.
>
> > > I should correct myself here.
>
> > > The part I wrote about "a different slowdown during a different part
> > > of the trip" was an erroneous reference to a scenario in which a clock
> > > is reporting the Doppler shift that is _seen_ (i.e. without accounting
> > > for transit delays).
>
> > In fact, what I was trying to describe to Ste was the effects that are
> > specifically not due to Doppler shifting, as I was specifically making
> > the point that the predictions of SR for time dilation are
> > mathematically different than those which are due to the observed rate
> > of change on a ticking clock to transit delays.
>
> Ok good.  So we're both not talking about that.
>
>
>
>
>
> > > In such a scenario, the clock sees an interval after it has
> > > accelerated and before its peer has accelerated and then a second
> > > interval after the peer has accelerated.  The Doppler shifts for the
> > > intervals are, of course, different.
>
> > > In the scenario at hand we are trying to discuss what is _observed_
> > > (i.e. accounting for transit delays and adopting particular standards
> > > of simultaneity)
>
> > No, that's not actually what we were talking about at all.
> > > What is _observed_ is only a single interval during which the two
> > > clocks are in constant relative motion, not two such intervals.  [From
> > > an "observed" point of view, the period when the peer clock remains
> > > motionless does not fall within the interval of the journey.  Instead,
> > > it is in the relative past]
>
> > The relevant quantity was the time that each clock displays after the
> > two are brought into comoving frames, which once again, depends on the
> > frame that they are brought into.
>
> In the scenario in question they are not brought into co-moving frames
> (whatever that means -- the notion of things being "brought into"
> frames is very questionable)  They are brought _TOGETHER_.

All I can say is read what I originally wrote again.

BTW, "together" is ambiguous. Together can mean comoving or it can
mean "they pass each other." Each of those warrants a different
answer.
From: mpalenik on 5 Mar 2010 16:15
On Mar 5, 3:55 pm, mpalenik <markpale...(a)gmail.com> wrote:
> On Mar 5, 3:47 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
>
>
>
>
>
> > On Mar 5, 3:07 pm, mpalenik <markpale...(a)gmail.com> wrote:
>
> > > On Mar 5, 2:17 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
>
> > > > On Mar 5, 1:54 pm, jbriggs444 <jbriggs...(a)gmail.com> wrote:
> > > > [discussing the scenario in which two clocks symmetrically accelerate
> > > > toward each other and this is observed from the point of view of one
> > > > of the clocks]
>
> > > > > Your initial assertions are correct.  Both will report a slow down....
> > > > > During part of the trip.  And a different slowdown  during a different
> > > > > part of the trip.
>
> > > > I should correct myself here.
>
> > > > The part I wrote about "a different slowdown during a different part
> > > > of the trip" was an erroneous reference to a scenario in which a clock
> > > > is reporting the Doppler shift that is _seen_ (i.e. without accounting
> > > > for transit delays).
>
> > > In fact, what I was trying to describe to Ste was the effects that are
> > > specifically not due to Doppler shifting, as I was specifically making
> > > the point that the predictions of SR for time dilation are
> > > mathematically different than those which are due to the observed rate
> > > of change on a ticking clock to transit delays.
>
> > Ok good.  So we're both not talking about that.
>
> > > > In such a scenario, the clock sees an interval after it has
> > > > accelerated and before its peer has accelerated and then a second
> > > > interval after the peer has accelerated.  The Doppler shifts for the
> > > > intervals are, of course, different.
>
> > > > In the scenario at hand we are trying to discuss what is _observed_
> > > > (i.e. accounting for transit delays and adopting particular standards
> > > > of simultaneity)
>
> > > No, that's not actually what we were talking about at all.
> > > > What is _observed_ is only a single interval during which the two
> > > > clocks are in constant relative motion, not two such intervals.  [From
> > > > an "observed" point of view, the period when the peer clock remains
> > > > motionless does not fall within the interval of the journey.  Instead,
> > > > it is in the relative past]
>
> > > The relevant quantity was the time that each clock displays after the
> > > two are brought into comoving frames, which once again, depends on the
> > > frame that they are brought into.
>
> > In the scenario in question they are not brought into co-moving frames
> > (whatever that means -- the notion of things being "brought into"
> > frames is very questionable)  They are brought _TOGETHER_.
>
> All I can say is read what I originally wrote again.
>
> BTW, "together" is ambiguous.  Together can mean comoving or it can
> mean "they pass each other."  Each of those warrants a different
> answer.- Hide quoted text -
>
> - Show quoted text -

Just to elaborate, what I said was that if one clock or the other
accellerates, the situation is no longer symmetric and will affect the
time each clock reads respectively.

And the notion of comoving frames is not questionable at all. It's
standard terminology in the literature.
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