Notion of Absolute Clock Synchronization in SR
in [Relativity]
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From: Inertial on 15 Mar 2010 06:33 "harald" <hvan(a)swissonline.ch> wrote in message news:b4db03e0-4bbb-4ab5-8aca-497de387f1fe(a)g19g2000yqe.googlegroups.com... > On Mar 12, 5:13 pm, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: >> harald wrote: >> > [clocks] can also be >> > synchronized at a certain point in time (only at that time) without >> > having the same frequency. >> >> We do not call this "synchronized". The whole point of synchronizing >> clocks is >> so they can be used together to make related measurements of something. >> Two >> clocks that indicate the same time only once cannot be used together. >> >> After all, a stopped "clock" is correct twice a day! >> (In physics, we don't call such a device a "clock"; it is >> merely a clockface with unmoving hands.) >> >> Tom Roberts > > That misses the point of my correction of the OP (both of which you > snipped) and my last remark was in general*. The OP wrongly thought > that two clocks that have the same frequency are necessarily > synchronized. The OP thus missed an essential point about > synchronizing clocks. Yeup > * It's unclear who "we" are. For physicists it's rather standard to > synchronize running clocks at the start of an experiment without > assuming that they run perfectly in sync during the experiment Then there is not much point in doing so :) > , and > this is also common language of textbooks (see for an online example > http://mamacass.ucsd.edu/people/pblanco/physics2d/handout1/index.html: > "we can arrange for the clocks to synchronize when O' passes O, i.e. > t'=t=0"). Yes .. you can synchronize them momentarily. But they are not e-synced then. There problem here is two different uses of the term. Synchronizing clocks in the first sense means making their readings show the same time AT the same time. Such clocks may only be synchrnoized for a moment. E-sync means that the ELAPSED times of both clocks correspond, as well as the readings at some time. E-sync'd clocks remain in synch.
From: harald on 15 Mar 2010 06:53 On Mar 15, 11:33 am, "Inertial" <relativ...(a)rest.com> wrote: > "harald" <h...(a)swissonline.ch> wrote in message > > news:b4db03e0-4bbb-4ab5-8aca-497de387f1fe(a)g19g2000yqe.googlegroups.com... > > > > > On Mar 12, 5:13 pm, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > >> harald wrote: > >> > [clocks] can also be > >> > synchronized at a certain point in time (only at that time) without > >> > having the same frequency. > > >> We do not call this "synchronized". The whole point of synchronizing > >> clocks is > >> so they can be used together to make related measurements of something.. > >> Two > >> clocks that indicate the same time only once cannot be used together. > > >> After all, a stopped "clock" is correct twice a day! > >> (In physics, we don't call such a device a "clock"; it is > >> merely a clockface with unmoving hands.) > > >> Tom Roberts > > > That misses the point of my correction of the OP (both of which you > > snipped) and my last remark was in general*. The OP wrongly thought > > that two clocks that have the same frequency are necessarily > > synchronized. The OP thus missed an essential point about > > synchronizing clocks. > > Yeup > > > * It's unclear who "we" are. For physicists it's rather standard to > > synchronize running clocks at the start of an experiment without > > assuming that they run perfectly in sync during the experiment > > Then there is not much point in doing so :) Perfect clocks for experiments don't exist - we just have to account for the errors in practice. In theoretical descriptions we can work with "perfect" clocks, but even those can deviate as illustrated below. > > , and > > this is also common language of textbooks (see for an online example > >http://mamacass.ucsd.edu/people/pblanco/physics2d/handout1/index.html: > > "we can arrange for the clocks to synchronize when O' passes O, i.e. > > t'=t=0"). > > Yes .. you can synchronize them momentarily. But they are not e-synced > then. > > There problem here is two different uses of the term. Indeed, the LT's t0 sync is a different synchronization. > Synchronizing clocks in the first sense means making their readings show the > same time AT the same time. Such clocks may only be synchrnoized for a > moment. > > E-sync means that the ELAPSED times of both clocks correspond, as well as > the readings at some time. E-sync'd clocks remain in synch. Thanks for the elaboration. But more precisely: e-synched "perfect", "stationary" clocks remain in sync (if at the same gravitational potential). Now, let's hope that the OP will understand this. :) Cheers, Harald
From: Inertial on 15 Mar 2010 06:54 "harald" <hvan(a)swissonline.ch> wrote in message news:447bc019-e8d6-40ab-a095-f6c7bed72c03(a)a18g2000yqc.googlegroups.com... > On Mar 15, 11:33 am, "Inertial" <relativ...(a)rest.com> wrote: >> "harald" <h...(a)swissonline.ch> wrote in message >> >> news:b4db03e0-4bbb-4ab5-8aca-497de387f1fe(a)g19g2000yqe.googlegroups.com... >> >> >> >> > On Mar 12, 5:13 pm, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: >> >> harald wrote: >> >> > [clocks] can also be >> >> > synchronized at a certain point in time (only at that time) without >> >> > having the same frequency. >> >> >> We do not call this "synchronized". The whole point of synchronizing >> >> clocks is >> >> so they can be used together to make related measurements of >> >> something. >> >> Two >> >> clocks that indicate the same time only once cannot be used together. >> >> >> After all, a stopped "clock" is correct twice a day! >> >> (In physics, we don't call such a device a "clock"; it is >> >> merely a clockface with unmoving hands.) >> >> >> Tom Roberts >> >> > That misses the point of my correction of the OP (both of which you >> > snipped) and my last remark was in general*. The OP wrongly thought >> > that two clocks that have the same frequency are necessarily >> > synchronized. The OP thus missed an essential point about >> > synchronizing clocks. >> >> Yeup >> >> > * It's unclear who "we" are. For physicists it's rather standard to >> > synchronize running clocks at the start of an experiment without >> > assuming that they run perfectly in sync during the experiment >> >> Then there is not much point in doing so :) > > Perfect clocks for experiments don't exist - we just have to account > for the errors in practice. In theoretical descriptions we can work > with "perfect" clocks, but even those can deviate as illustrated > below. But we 'assume' that they do. >> > , and >> > this is also common language of textbooks (see for an online example >> >http://mamacass.ucsd.edu/people/pblanco/physics2d/handout1/index.html: >> > "we can arrange for the clocks to synchronize when O' passes O, i.e. >> > t'=t=0"). >> >> Yes .. you can synchronize them momentarily. But they are not e-synced >> then. >> >> There problem here is two different uses of the term. > > Indeed, the LT's t0 sync is a different synchronization. > >> Synchronizing clocks in the first sense means making their readings show >> the >> same time AT the same time. Such clocks may only be synchrnoized for a >> moment. >> >> E-sync means that the ELAPSED times of both clocks correspond, as well as >> the readings at some time. E-sync'd clocks remain in synch. > > Thanks for the elaboration. But more precisely: e-synched "perfect", > "stationary" clocks remain in sync (if at the same gravitational > potential). Now, let's hope that the OP will understand this. :) Yeup .. I think we're all on the same track here.
From: valls on 16 Mar 2010 10:30 On 15 mar, 05:53, harald <h...(a)swissonline.ch> wrote: > On Mar 15, 11:33 am, "Inertial" <relativ...(a)rest.com> wrote: > > > > > > > "harald" <h...(a)swissonline.ch> wrote in message > > >news:b4db03e0-4bbb-4ab5-8aca-497de387f1fe(a)g19g2000yqe.googlegroups.com.... > > > > On Mar 12, 5:13 pm, Tom Roberts <tjrob...(a)sbcglobal.net> wrote: > > >> harald wrote: > > >> > [clocks] can also be > > >> > synchronized at a certain point in time (only at that time) without > > >> > having the same frequency. > > > >> We do not call this "synchronized". The whole point of synchronizing > > >> clocks is > > >> so they can be used together to make related measurements of something. > > >> Two > > >> clocks that indicate the same time only once cannot be used together.. > > > >> After all, a stopped "clock" is correct twice a day! > > >> (In physics, we don't call such a device a "clock"; it is > > >> merely a clockface with unmoving hands.) > > > >> Tom Roberts > > > > That misses the point of my correction of the OP (both of which you > > > snipped) and my last remark was in general*. The OP wrongly thought > > > that two clocks that have the same frequency are necessarily > > > synchronized. The OP thus missed an essential point about > > > synchronizing clocks. > > > Yeup > > > > * It's unclear who "we" are. For physicists it's rather standard to > > > synchronize running clocks at the start of an experiment without > > > assuming that they run perfectly in sync during the experiment > > > Then there is not much point in doing so :) > > Perfect clocks for experiments don't exist - we just have to account > for the errors in practice. In theoretical descriptions we can work > with "perfect" clocks, but even those can deviate as illustrated > below. > > > > , and > > > this is also common language of textbooks (see for an online example > > >http://mamacass.ucsd.edu/people/pblanco/physics2d/handout1/index.html: > > > "we can arrange for the clocks to synchronize when O' passes O, i.e. > > > t'=t=0"). > > > Yes .. you can synchronize them momentarily. But they are not e-synced > > then. > > > There problem here is two different uses of the term. > > Indeed, the LT's t0 sync is a different synchronization. > > > Synchronizing clocks in the first sense means making their readings show the > > same time AT the same time. Such clocks may only be synchrnoized for a > > moment. > > > E-sync means that the ELAPSED times of both clocks correspond, as well as > > the readings at some time. E-sync'd clocks remain in synch. > > Thanks for the elaboration. But more precisely: e-synched "perfect", > "stationary" clocks remain in sync (if at the same gravitational > potential). Now, let's hope that the OP will understand this. :) > (Hello Harald, nice to meet you again). In the ECI frame of GPS all the clocks remain synchronized, even if they have different velocities and gravitational potentials. Then, taking into account that huge experimental evidence, I dont see any other alternative that to accept that absolute clock synchronization exists in SR with the following meaning. Once perfect and stationary clocks are e-synchronized in some inertial frame, they remain showing the same time lecture at any local instant in all the others inertial frames. Of course, that equal time lecture does not correspond to the local time in each of the others inertial frames, where according to SR rules, the now moving clocks (all with the same velocity) are running slower than the local perfect and stationary e-synchronised ones. RVHG (Rafael Valls Hidalgo-Gato) > Cheers, > Harald- Ocultar texto de la cita - > > - Mostrar texto de la cita -
From: Tom Roberts on 16 Mar 2010 15:01
valls(a)icmf.inf.cu wrote: > In the ECI frame of GPS all the clocks remain synchronized, even if > they have different velocities and gravitational potentials. This is not true of STANDARD clocks. The clocks in the GPS satellites have been modified so this is approximately true. That can only be done for certain highly-symmetric situations, such as clocks in circular orbits around a planet in which other effects can be neglected. Indeed, in the GPS those other effects are not neglected, but are corrected for. These modified clocks are not what we normally mean when we say "clock". > Then, > taking into account that huge experimental evidence, I don�t see any > other alternative that to accept that �absolute clock synchronization� > exists in SR with the following meaning. Once �perfect and stationary� > clocks are e-synchronized in some inertial frame, they remain showing > the same time lecture at any local instant in all the others inertial > frames. This is simply not true. Synchronizing clocks in frame A, and looking at them from frame B, they are not in synch if B is moving wrt A. The modified clocks of the GPS are synchronized only in the ECI frame. That, of course, is sufficient for the GPS, but is completely and utterly inadequate to claim "absolute clock synchronization". Tom Roberts |