On 1/3/2014 8:10 AM, Edgar L. Owen wrote:

Jason,## Advertising

Thanks for your several posts and charts. You really made me think and I like that! I'm combining my responses to your multiple recent posts here.First though there are two ways to analyze it, GR acceleration, as opposed to SR worldlines, is the most useful because it makes the following argument re present time easierto understand.Imagine a new experiment in which Pam is completely still relative to Sam but somewhereway off in the universe and in a gravitational field of exactly the same strength. Inthis case both Pam's and Sam's clock times run at exactly the same rates and both agreeto this. Therefore it is clear they inhabit the exact same present moment even by yourarguments, and their identical clock times correlate to this.

`No, that doesn't follow at all. Running at the same rate doesn't mean at the same time.`

`My watch runs at the same rate as my grandfathers - but not at the same time. All you can`

`conclude is that, by exchanging signals Pam and Sam can set their clocks to *the same time`

`in their frame* and by symmetry they will run at the same rate.`

Now assume Pam's gravitational field increases to the point where her clock time runshalf as fast as Sam's. Again there is no relative motion so again both agree that Pam'sclock time is running half as fast as Sam's. And again both exist in the exact samepresent moment, it's just that Sam's clock time is running twice as fast through thatcommon present moment. Again clock time correlates with present moment time...

`First, they are in relative motion in spacetime. Second, there is no "present moment".`

`Pam and Sam are at different locations, so even aside from gravitational effects, their`

`agreement on how to set their clocks is arbitrary, it holds only in their frame, and`

`another observer moving relative to them will see their clocks as NOT reading the same`

`time even when their gravity fields were the same.`

This gravitational time slowing is a GR, not SR effect,

`They are actually the same effect, except in GR the path lengths are measured over a`

`non-flat geometry. See Epstein's book "Relativity Visualized".`

and GR effects are absolute in the sense that they are permanent real effects that allobservers agree upon. They must be distinguished from SR effects which make thesituation more difficult to understand in terms of a present moment.An acceleration equivalent to the gravitational field would produce the exact same GReffect, but also introduces an SR relative velocity effect.Now consider an pure SR effect in which Pam and Sam are traveling past each other atrelativistic speeds but there is no acceleration. Velocity is relative, as opposed toacceleration which is absolute, therefore both observers think the other is movingrelative to them, and both views are equally true. Now because of this relativity ofvelocity both observers see the clock of the other observer slow and by equal amounts.But the absolutely crucial thing to understand here is that this SR form of timedilation is not permanent and absolute like GR time dilation is. It vanishes as soon asthe relative motion stops, whereas GR time differences are absolute and persist evenafter the acceleration stops.

`The effect on *rate* stops, but the integrated effect of the rate having been different`

`over some duration is real. That's why the twins are different ages when they re-unite.`

This is why the SR versus GR model is more useful in understanding what is going onparticularly with respect to the common present moment.

`You "common present moment" is just an arbitrary inertial frame choice which you use to`

`label events with a t-value. It's just coordinate time.`

So during relative motion between Pam and Sam there most certainly is a common presentmoment,

`There is a whole range of moments which will be at the same coordinate time depending on`

`what inertial frame is chosen to define coordinates.`

but trying to figure out what clock times of Pam and Sam correspond to that presentmoment leads to a contradiction (as you quite rightly pointed out with your diagrams)because Pam and Sam see clock time differently and do not agree on it. They did agree ontheir GR relativistic time differences

There was no gravity in my diagrams.

and thus knowing which of their clock times corresponded to the same present moment waseasy.

`No, there is the same arbitrariness of "now" in your GR example. You just chose to`

`privilege the frame in which both are at rest (in space). In any other inertial frame`

`their clocks will still be seen to run at the same rate, but they will no longer be set to`

`the same time.`

With SR, equal and opposite, time dilation it is impossible to correlate both observers'clock times to the same present moment.

Sure it is, when they are at the same event.

Nevertheless that's just an artifact of SR clock time which doesn't falsify a commonpresent moment. A common present moment exists, it just isn't correlated with clocktimes the same way by both observers.

`It isn't correlated because clock's measure real physical intervals while present moments`

`are artifacts of coordinate choices and are arbitrary.`

All the nice chart examples you took the time to produce demonstrate this. They aretrying to assign an agreed upon clock time to the common present moment time during SRrelative velocities and thus they correctly lead to the contradiction you pointed out.

They are showing that different coordinate choices change what you count as "present moments".

However once you understand how this works you understand that fact doesn't falsify acommon present moment as you implied.

You can't falsify an arbitrary choice.

Now consider the twins from the original example. In this case there is both lots ofrelative velocity SR effects between both twins, and there is the absolute GRacceleration effect on Pam only.Now the SR effects persist only during relative motion and when the twins meet up againthat leaves ONLY the GR acceleration effect which is the only cause of the twins' clocktime difference.

`Nonsense. There is no gravity in by diagram, but a twin who traveled the white/blue world`

`line comesback 2.67 ticks younger than the stay-at-home twin.`

All SR relative velocity effects must vanish when the relative velocities cease.

`Only effects on rate vanish. The effect of different path length are measured by clocks`

`and biological processes and they are not arbitrary.`

Otherwise we would have Pam and Sam meeting up again with each claiming the other'sclock time was going slower than theirs. That is impossible. At rest in the same presentmoment all observers must be able to agree on their clock time differences. Both agreePam's clock time passed more slowly than Sam's and both agree as to the amount, basedONLY on GR (acceleration) effects.

`They will also agree that there is a difference if one simply traveled a shorter path`

`through spacetime to the comparison event - whether it was shorter because the space`

`wasn't flat or because they just took a non-straight path.`

Assume again the twins passing each other at a constant (no acceleration) velocity. Bothsee the other's time passing slower than theirs and thus both see each other at anearlier clock time date than themselves. This is contradictory and cannot last when theymeet. It is the acceleration that brings the relative velocities to zero that producesthe only absolute persistent time effect

`No, it is the difference in path length. That's the point of my diagram. The clocks of`

`White and Blue never experience any accelerations, but they measure a shorter world line`

`between two events than does Black's clock. Acceleration has nothing to do with it except`

`that it can be integrated to get distances - given a couple of constants of integration.`

and when, and only when, that happens will the twins agree as to their time differences,as always in a shared universal present moment.This is why is is possible to correlated clock times to present moment time for GRacceleration time dilation, but NOT for SR relative velocity time dilation.Hope this is clear. It may be a little difficult...

`It's clear to me. What's difficult is getting you to see it. Introducing GR is just`

`obfuscation.`

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