Notion of Absolute Clock Synchronization in SR
in [Relativity]
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From: BURT on 20 Mar 2010 23:49 On Mar 20, 8:44 pm, GSS <gurcharn_san...(a)yahoo.com> wrote: > On Mar 20, 5:27 pm, harald <h...(a)swissonline.ch> wrote: > > > > > > > On Mar 19, 7:25 pm, GSS <gurcharn_san...(a)yahoo.com> wrote: > > > > On Mar 19, 6:12 pm, "Inertial" <relativ...(a)rest.com> wrote:> "GSS" <gurcharn_san...(a)yahoo.com> wrote in message > > > > ..... > > > > >> Let us consider two identical precision atomic clocks, positioned at > > > >> points A and B, separated by a distance of about 30 km along east- > > > >> west direction, on the surface of earth. > > > > > As you are talking SR, you must be assuming that the surface of the earth is > > > > an inertial frame (so we'll ignore it rotating, or orbitting, and ignore > > > > gravity). > > > > No, I meant to ignore gravity effects only. > > > > >> Assume the two clocks A and B > > > >> are mutually synchronized through Einstein convention such that the > > > >> time taken, T_ab, by a laser pulse to propagate from A to B (as > > > >> measured from the clock readings of B and A) is the same as the time > > > >> taken, T_ba, by a laser pulse to propagate from B to A. > > > > > Which is, of course, obviously true. > > > > >> That means, > > > >> T_ab - T_ba = 0 which indicates e-synchronization of the two clocks. > > > > > Only if that is what the clocks show. The fact light takes the same time to > > > > travel the same distance at the same speed doesn't make the clocks > > > > synchronized. > > > > >> In your opinion, will this synchronization remain valid at least for a > > > >> 24 hour period? That is, if we take to and fro signal propagation time > > > >> readings at hourly intervals, will all readings show, > > > >> T_ab - T_ba = 0 > > > > > It will be valid forever, if they remain at rest in the inertial frame. > > > > Will it still be valid for ever, if they remain at rest only in the > > > local or lab frame fixed on the surface of earth? What do you think is > > > the effect of earth rotation on the mutual e-synchronization of two > > > clocks in the lab frame? > > > > >> Perhaps you may like to call this mutual synchronization of clocks A > > > >> and B as the 'local clock synchronization' valid in the local or lab > > > >> frame of the two clocks. > > > > > If you want to. > > > > >> Kindly explain the procedure for e-synchronization of the same two > > > >> atomic clocks A and B in the ECI or the GCRF frame. How exactly will > > > >> it be different from the local clock synchronization in practical > > > >> terms? > > > > > The clocks would have to be at rest in those frames. > > > > > If not, then you can of course have a whole series of appropriate clocks > > > > that ARE at rest in the frame you want (so A and B will be comoving past > > > > those clocks), and chose a time on those clocks (say 12:00), and whichever > > > > rest clocks A and B are adjacent to when those rest get to 12:00, copy that > > > > time to A and B. Then A and B will be in sync in that frame, and show the > > > > same time in that frame. But they won't be measured as ticking at the > > > > correct rate, so they will get more and more out of synch with other rest > > > > clocks they pass. > > > > I don't think you really mean what you write. > > > Do you seriously think there are some atomic clocks that are actually > > > at rest in the ECI or BCRF frames? Or can you ever make any atomic > > > clock ever at rest in the ECI or BCRF frames? > > > > >> Since the two clocks under consideration are simultaneously known to > > > >> be co-moving in the solar system at about 30 km/s, you may kindly > > > >> explain the procedure for e-synchronization of the same two atomic > > > >> clocks A and B in the solar system BCRF frame. > > > > > The clocks would have to be at rest in that frame. (Or see above) > > > > But how? > > > > >> How exactly will it be > > > >> different from the local clock synchronization in practical terms? > > > > > No different at all. But the same clocks are not going to be both at rest > > > > in more than one (non-eqivalent) inertial frame. > > > > >> Going one step still further,the two clocks under consideration are > > > >> simultaneously known to be co-moving in the Galactic reference frame > > > >> at about 200 km/s. Kindly explain the procedure for e-synchronization > > > >> of the same two atomic clocks A and B in the Galactic reference frame. > > > > > The clocks would have to be at rest in that frame. (Or see above) > > > > What I make out from your response is that two clocks A and B > > > considered above, can be e-synchronized in ECI or BCRf or Galactic > > > reference frames only if these clocks can be brought to rest in these > > > frames. Since it is practically impossible to bring the two clocks (at > > > rest on the surface of earth) to rest in ECI or BCRF or Galactic > > > reference frames, it implies that it is practically impossible to > > > mutually e-synchronize the two clocks in any of these inertial > > > reference frames. That means two atomic clocks A and B can be mutually > > > e-synchronized only in their local or lab frame and none else. All > > > talk of e-synchronizing two or more atomic clocks in different > > > inertial reference frames in relative uniform motion, is just > > > hypothetical day dreaming or gedanken. > > > The synchronisation procedure in the ECI "frame" accounts for the > > speed of the clocks on the Earth's surface; that's also how GPS works > > (commonly called "Sagnac correction"). There is no reason to make any > > clocks "rest" in the ECI "frame". > > > Regards, > > Harald > > In essence, Sagnac correction accounts for the effect of motion of the > receiver during the transit time of the signal pulse from transmitter > to the receiver. Here the main issue is that if you mutually > synchronize two atomic clocks A and B (separated by distance D) on > earth surface, can the *same* two clocks be mutually synchronized in > the ECI and BCRF frames without first bringing them to *rest* in these > frames? Inertial says you cannot. > > If you think these two clocks can be mutually synchronized in the ECI > and/or BCRF frames without bringing them to *rest* in these frames, > kindly explain that synchronization procedure. If you do succeed in > synchronizing these two clocks in ECI and/or BCRF frames (while they > are still at rest in their local or lab frame on the surface of earth) > will their mutual synchronization in their lab frame be broken or > maintained? > > GSS- Hide quoted text - > > - Show quoted text - Waves fall. Mitch Raemsch
From: valls on 22 Mar 2010 07:19 On 20 mar, 07:15, harald <h...(a)swissonline.ch> wrote: > On Mar 19, 5:39 pm, va...(a)icmf.inf.cu wrote: > > > > > > > On 19 mar, 04:23, harald <h...(a)swissonline.ch> wrote: > > > > On Mar 18, 11:27 pm, va...(a)icmf.inf.cu wrote: > > > > > On 18 mar, 10:49, harald <h...(a)swissonline.ch> wrote: > > > > > > On Mar 18, 3:04 pm, va...(a)icmf.inf.cu wrote: > > > > > > > On 17 mar, 10:34, harald <h...(a)swissonline.ch> wrote: > > > > > > > > On Mar 17, 3:34 pm, va...(a)icmf.inf.cu wrote: > > > > > > > > > On 16 mar, 17:20, harald <h...(a)swissonline.ch> wrote: > > > > > > > > > > On Mar 16, 3:30 pm, va...(a)icmf.inf.cu wrote: > > > > > > > > > > > On 15 mar, 05:53, harald <h...(a)swissonline.ch> wrote: > > > > > > [..] > > > > > > > > > > > > > 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) > > > > > > [..] > > > > > > > > The topic of this thread is concerned with the fact that according to > > > > > > > all inertial reference systems in which the inertial reference system > > > > > > > with its synchronized clocks is moving, those clocks are out of sync > > > > > > > with each other (see also below). > > > > > > > > > I mentioned the ECI of GPS, taken for granted that the e- > > > > > > > > synchronization method of all its clocks is well-known. Let us > > > > > > > > remember that all the moving clocks show the unique ECI time, > > > > > > > > corresponding to the same time that a similar clock at rest in the > > > > > > > > relevant ECI point would show. If now we consider the ECI moving at a > > > > > > > > constant velocity with respect to an (imaginary) inertial frame B, > > > > > > > > Note: the ECI "frame" itself is already an imaginary frame... > > > > > > > I dont understand why you consider the ECI an imaginary frame. > > > > > > The whole Earth is rotating relative to it; there isn't any material > > > > > frame that is pretended to be "in rest". > > > > > I have a doubt here about what do you mean by material frame. > > > > A stiff thing made up of atoms (such as earth, wood, steel or > > > concrete). > > > > [..] > > > > > > > Yes, an inertial observer at rest in the moving system (moving GPS > > > > > > satellite) appreciates all ECI clocks out of synchronism, but who > > > > > > care that? > > > > > > OK, perhaps I misunderstood what you tried to communicate - in which > > > > > case I don't know what it was! > > > > > I prefer to put the emphasis in what we are in agreement now. Our own > > > > ideas can be evolving somewhat in the time. > > > > > > In fact, you here agree with the SRT claim that clock synchronisation > > > > > (along x) is "relative", in the sense that it is meant. > > > > > Yes, without any doubt synchronization is relative to the inertial > > > > frame you select to do it. But let us take some care here, I > > > > distinguish a real inertial system (the centre of mass one associated > > > > to some well-determined body set) from an imaginary one (as all of > > > > them in the 1907 Minkowski view). > > > > > > > I feel now very happy with your very valuable reference to 1905 > > > > > > Einstein first paper on Relativity. Now we can make real the imaginary > > > > > > inertial frame B identifying it with a moving GPS satellite (the real > > > > > > inertial frame B is the centre of mass one corresponding to the > > > > > > satellite and all bodies in its interior). > > > > > > You can choose it as you wish, according to SRT (as long as it isn't > > > > > rotating, which is incompatible with GPS satellites!). > > > > > An inertial frame can never be rotating. The space belonging to the > > > > ECI (or any other inertial frame) has always all its points at rest.. > > > > > > Consider now the inertial Solar System (the centre of mass one of all > > > > > > its bodies). In principle, we can synchronize clocks in all its > > > > > > planets, showing all of them the same unique time defined by 1905 > > > > > > Einstein. > > > > > > It is "unique" for the solar system, just as the pair of shoes that I > > > > > wear are "unique" for me... > > > > > I dont think so. You can change your shoes, but not the unique time > > > > corresponding to the Solar System as long as it is maintained as a > > > > closed one (I forgot to mention explicitly that basic condition when > > > > talking about real inertial frames). > > > > Sure we can - we can set t=0 whenever we want, and also choose our > > > time standard. > > > From your last answer I deduce that we dont share a common > > interpretation about what is the time defined by 1905 Einstein. Let > > us take two very well-known real inertial systems, the ECI and the > > Solar System (SS). Let me ask you a very crucial question. Can be two > > GPS clocks (for example one in a satellite and another in the Earths > > surface) e-synchronized with respect to the ECI and at the same time > > also e-synchronized with respect to the SS? > > The essential difference between the (1905 Einstein) times of two > > different inertial systems has no relation at all with the totally > > arbitrary selection of initial instants and time standard units in > > both systems. > > Perhaps you meant with "unique time", "unique synchronization"? Most > people would call that not "absolute" but "relative" synchronization. > Anyway, I'm not at all interested in debates over words. > I didnt use the word unique in the comment where I made you a clear question. I repeat it: Can be two GPS clocks (for example one in a satellite and another in the Earths surface) e-synchronized with respect to the ECI and at the same time also e-synchronized with respect to the SS? I am referring to the synchronization described by 1905 Einstein in his first relativity paper (30 June 1905). As you, I am also not interested at all in any debate about simple words. If it helps you to answer the question, my opinion is that it must be answered with a clear no. I want to know if we are in agreement about this point or not. RVHG (Rafael Valls Hidalgo-Gato) > [..] > > Regards, > Harald- Ocultar texto de la cita - > > - Mostrar texto de la cita -
From: Androcles on 22 Mar 2010 07:39 <valls(a)icmf.inf.cu> wrote in message news:c7f6d7b8-bbb7-4a86-8af8-c9b01bdf8e09(a)t20g2000yqe.googlegroups.com... On 20 mar, 07:15, harald <h...(a)swissonline.ch> wrote: > On Mar 19, 5:39 pm, va...(a)icmf.inf.cu wrote: > > > > > > > On 19 mar, 04:23, harald <h...(a)swissonline.ch> wrote: > > > > On Mar 18, 11:27 pm, va...(a)icmf.inf.cu wrote: > > > > > On 18 mar, 10:49, harald <h...(a)swissonline.ch> wrote: > > > > > > On Mar 18, 3:04 pm, va...(a)icmf.inf.cu wrote: > > > > > > > On 17 mar, 10:34, harald <h...(a)swissonline.ch> wrote: > > > > > > > > On Mar 17, 3:34 pm, va...(a)icmf.inf.cu wrote: > > > > > > > > > On 16 mar, 17:20, harald <h...(a)swissonline.ch> wrote: > > > > > > > > > > On Mar 16, 3:30 pm, va...(a)icmf.inf.cu wrote: > > > > > > > > > > > On 15 mar, 05:53, harald <h...(a)swissonline.ch> wrote: > > > > > > [..] > > > > > > > > > > > > > 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 > > > > > > > > > > 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. 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) > > > > > > [..] > > > > > > > > The topic of this thread is concerned with the fact that > > > > > > > according to > > > > > > > all inertial reference systems in which the inertial reference > > > > > > > system > > > > > > > with its synchronized clocks is moving, those clocks are out > > > > > > > of sync > > > > > > > with each other (see also below). > > > > > > > > > I mentioned the ECI of GPS, taken for granted that the e- > > > > > > > > synchronization method of all its clocks is well-known. Let > > > > > > > > us > > > > > > > > remember that all the moving clocks show the unique ECI > > > > > > > > time, > > > > > > > > corresponding to the same time that a similar clock at rest > > > > > > > > in the > > > > > > > > relevant ECI point would show. If now we consider the ECI > > > > > > > > moving at a > > > > > > > > constant velocity with respect to an (imaginary) inertial > > > > > > > > frame B, > > > > > > > > Note: the ECI "frame" itself is already an imaginary frame... > > > > > > > I don�t understand why you consider the ECI an imaginary frame. > > > > > > The whole Earth is rotating relative to it; there isn't any > > > > > material > > > > > frame that is pretended to be "in rest". > > > > > I have a doubt here about what do you mean by �material frame�. > > > > A stiff thing made up of atoms (such as earth, wood, steel or > > > concrete). > > > > [..] > > > > > > > Yes, an inertial observer at rest in the �moving system� (moving > > > > > > GPS > > > > > > satellite) appreciates all ECI clocks �out of synchronism�, but > > > > > > who > > > > > > care that? > > > > > > OK, perhaps I misunderstood what you tried to communicate - in > > > > > which > > > > > case I don't know what it was! > > > > > I prefer to put the emphasis in what we are in agreement now. Our > > > > own > > > > ideas can be evolving somewhat in the time. > > > > > > In fact, you here agree with the SRT claim that clock > > > > > synchronisation > > > > > (along x) is "relative", in the sense that it is meant. > > > > > Yes, without any doubt synchronization is relative to the inertial > > > > frame you select to do it. But let us take some care here, I > > > > distinguish a real inertial system (the centre of mass one > > > > associated > > > > to some well-determined body set) from an imaginary one (as all of > > > > them in the 1907 Minkowski view). > > > > > > > I feel now very happy with your very valuable reference to 1905 > > > > > > Einstein first paper on Relativity. Now we can make real the > > > > > > imaginary > > > > > > inertial frame B identifying it with a moving GPS satellite (the > > > > > > real > > > > > > inertial frame B is the centre of mass one corresponding to the > > > > > > satellite and all bodies in its interior). > > > > > > You can choose it as you wish, according to SRT (as long as it > > > > > isn't > > > > > rotating, which is incompatible with GPS satellites!). > > > > > An inertial frame can never be rotating. The space belonging to the > > > > ECI (or any other inertial frame) has always all its points at rest. > > > > > > Consider now the inertial Solar System (the centre of mass one of > > > > > all > > > > > > its bodies). In principle, we can synchronize clocks in all its > > > > > > planets, showing all of them the same unique time defined by > > > > > > 1905 > > > > > > Einstein. > > > > > > It is "unique" for the solar system, just as the pair of shoes > > > > > that I > > > > > wear are "unique" for me... > > > > > I don�t think so. You can change your shoes, but not the unique time > > > > corresponding to the Solar System as long as it is maintained as a > > > > closed one (I forgot to mention explicitly that basic condition when > > > > talking about real inertial frames). > > > > Sure we can - we can set t=0 whenever we want, and also choose our > > > time standard. > > > From your last answer I deduce that we don�t share a common > > interpretation about what is the �time� defined by 1905 Einstein. Let > > us take two very well-known real inertial systems, the ECI and the > > Solar System (SS). Let me ask you a very crucial question. Can be two > > GPS clocks (for example one in a satellite and another in the Earth�s > > surface) e-synchronized with respect to the ECI and at the same time > > also e-synchronized with respect to the SS? > > The essential difference between the (1905 Einstein) times of two > > different inertial systems has no relation at all with the totally > > arbitrary selection of initial instants and time standard units in > > both systems. > > Perhaps you meant with "unique time", "unique synchronization"? Most > people would call that not "absolute" but "relative" synchronization. > Anyway, I'm not at all interested in debates over words. > I didn�t use the word �unique� in the comment where I made you a clear question. I repeat it: Can be two GPS clocks (for example one in a satellite and another in the Earth�s surface) e-synchronized with respect to the ECI and at the same time also e-synchronized with respect to the SS? I am referring to the synchronization described by 1905 Einstein in his first relativity paper (30 June 1905). As you, I am also not interested at all in any debate about simple words. If it helps you to answer the question, my opinion is that it must be answered with a clear �no�. I want to know if we are in agreement about this point or not. RVHG (Rafael Valls Hidalgo-Gato) ============================================ If you are not interest in words, why add confusion with "e-synchronized" ? That's not a word. Einstein said: 1.. If the clock at B synchronizes with the clock at A, the clock at A synchronizes with the clock at B. 2.. If the clock at A synchronizes with the clock at B and also with the clock at C, the clocks at B and C also synchronize with each other. He didn't say "e-synchronized". Can be two Global Positioning System (GPS) clocks (for example one in a satellite and another in the Earth�s surface)... The GPS doesn't ever use a clock on the Earth's surface, GPS receivers don't have atomic clocks or need clocks at all. However, atomic clocks upload position and time to the satellites and they are ALL synchronized.
From: Inertial on 22 Mar 2010 07:43 "Androcles" <Headmaster(a)Hogwarts.physics_w> wrote in message news:xGIpn.19115$DF1.2048(a)newsfe20.ams2... > > <valls(a)icmf.inf.cu> wrote in message > news:c7f6d7b8-bbb7-4a86-8af8-c9b01bdf8e09(a)t20g2000yqe.googlegroups.com... > On 20 mar, 07:15, harald <h...(a)swissonline.ch> wrote: >> On Mar 19, 5:39 pm, va...(a)icmf.inf.cu wrote: >> >> >> >> >> >> > On 19 mar, 04:23, harald <h...(a)swissonline.ch> wrote: >> >> > > On Mar 18, 11:27 pm, va...(a)icmf.inf.cu wrote: >> >> > > > On 18 mar, 10:49, harald <h...(a)swissonline.ch> wrote: >> >> > > > > On Mar 18, 3:04 pm, va...(a)icmf.inf.cu wrote: >> >> > > > > > On 17 mar, 10:34, harald <h...(a)swissonline.ch> wrote: >> >> > > > > > > On Mar 17, 3:34 pm, va...(a)icmf.inf.cu wrote: >> >> > > > > > > > On 16 mar, 17:20, harald <h...(a)swissonline.ch> wrote: >> >> > > > > > > > > On Mar 16, 3:30 pm, va...(a)icmf.inf.cu wrote: >> >> > > > > > > > > > On 15 mar, 05:53, harald <h...(a)swissonline.ch> wrote: >> >> > > > > [..] >> >> > > > > > > > > > > > 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 >> > > > > > > > > > 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. 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) >> >> > > > > [..] >> >> > > > > > > The topic of this thread is concerned with the fact that >> > > > > > > according to >> > > > > > > all inertial reference systems in which the inertial >> > > > > > > reference system >> > > > > > > with its synchronized clocks is moving, those clocks are out >> > > > > > > of sync >> > > > > > > with each other (see also below). >> >> > > > > > > > I mentioned the ECI of GPS, taken for granted that the e- >> > > > > > > > synchronization method of all its clocks is well-known. Let >> > > > > > > > us >> > > > > > > > remember that all the moving clocks show the unique ECI >> > > > > > > > time, >> > > > > > > > corresponding to the same time that a similar clock at rest >> > > > > > > > in the >> > > > > > > > relevant ECI point would show. If now we consider the ECI >> > > > > > > > moving at a >> > > > > > > > constant velocity with respect to an (imaginary) inertial >> > > > > > > > frame B, >> >> > > > > > > Note: the ECI "frame" itself is already an imaginary frame... >> >> > > > > > I don�t understand why you consider the ECI an imaginary frame. >> >> > > > > The whole Earth is rotating relative to it; there isn't any >> > > > > material >> > > > > frame that is pretended to be "in rest". >> >> > > > I have a doubt here about what do you mean by �material frame�. >> >> > > A stiff thing made up of atoms (such as earth, wood, steel or >> > > concrete). >> >> > > [..] >> >> > > > > > Yes, an inertial observer at rest in the �moving system� >> > > > > > (moving GPS >> > > > > > satellite) appreciates all ECI clocks �out of synchronism�, but >> > > > > > who >> > > > > > care that? >> >> > > > > OK, perhaps I misunderstood what you tried to communicate - in >> > > > > which >> > > > > case I don't know what it was! >> >> > > > I prefer to put the emphasis in what we are in agreement now. Our >> > > > own >> > > > ideas can be evolving somewhat in the time. >> >> > > > > In fact, you here agree with the SRT claim that clock >> > > > > synchronisation >> > > > > (along x) is "relative", in the sense that it is meant. >> >> > > > Yes, without any doubt synchronization is relative to the inertial >> > > > frame you select to do it. But let us take some care here, I >> > > > distinguish a real inertial system (the centre of mass one >> > > > associated >> > > > to some well-determined body set) from an imaginary one (as all of >> > > > them in the 1907 Minkowski view). >> >> > > > > > I feel now very happy with your very valuable reference to 1905 >> > > > > > Einstein first paper on Relativity. Now we can make real the >> > > > > > imaginary >> > > > > > inertial frame B identifying it with a moving GPS satellite >> > > > > > (the real >> > > > > > inertial frame B is the centre of mass one corresponding to the >> > > > > > satellite and all bodies in its interior). >> >> > > > > You can choose it as you wish, according to SRT (as long as it >> > > > > isn't >> > > > > rotating, which is incompatible with GPS satellites!). >> >> > > > An inertial frame can never be rotating. The space belonging to the >> > > > ECI (or any other inertial frame) has always all its points at >> > > > rest. >> >> > > > > Consider now the inertial Solar System (the centre of mass one of >> > > > > all >> > > > > > its bodies). In principle, we can synchronize clocks in all its >> > > > > > planets, showing all of them the same unique time defined by >> > > > > > 1905 >> > > > > > Einstein. >> >> > > > > It is "unique" for the solar system, just as the pair of shoes >> > > > > that I >> > > > > wear are "unique" for me... >> >> > > > I don�t think so. You can change your shoes, but not the unique >> > > > time >> > > > corresponding to the Solar System as long as it is maintained as a >> > > > closed one (I forgot to mention explicitly that basic condition >> > > > when >> > > > talking about real inertial frames). >> >> > > Sure we can - we can set t=0 whenever we want, and also choose our >> > > time standard. >> >> > From your last answer I deduce that we don�t share a common >> > interpretation about what is the �time� defined by 1905 Einstein. Let >> > us take two very well-known real inertial systems, the ECI and the >> > Solar System (SS). Let me ask you a very crucial question. Can be two >> > GPS clocks (for example one in a satellite and another in the Earth�s >> > surface) e-synchronized with respect to the ECI and at the same time >> > also e-synchronized with respect to the SS? >> > The essential difference between the (1905 Einstein) times of two >> > different inertial systems has no relation at all with the totally >> > arbitrary selection of initial instants and time standard units in >> > both systems. >> >> Perhaps you meant with "unique time", "unique synchronization"? Most >> people would call that not "absolute" but "relative" synchronization. >> Anyway, I'm not at all interested in debates over words. >> > I didn�t use the word �unique� in the comment where I made you a clear > question. I repeat it: > Can be two GPS clocks (for example one in a satellite and another in > the Earth�s > surface) e-synchronized with respect to the ECI and at the same time > also e-synchronized with respect to the SS? > I am referring to the synchronization described by 1905 Einstein in > his first relativity paper (30 June 1905). > As you, I am also not interested at all in any debate about simple > words. > If it helps you to answer the question, my opinion is that it must be > answered with a clear �no�. I want to know if we are in agreement > about this point or not. > > RVHG (Rafael Valls Hidalgo-Gato) > ============================================ > If you are not interest in words, why add confusion with "e-synchronized" > ? > That's not a word. > Einstein said: > > 1.. If the clock at B synchronizes with the clock at A, the clock at A > synchronizes with the clock at B. > 2.. If the clock at A synchronizes with the clock at B and also with the > clock at C, the clocks at B and C also synchronize with each other. > He didn't say "e-synchronized". > > Can be two Global Positioning System (GPS) clocks (for example one in > a satellite and another in the Earth�s surface)... > The GPS doesn't ever use a clock on the Earth's surface, GPS receivers > don't have atomic clocks or need clocks at all. > However, atomic clocks upload position and time to the satellites and > they are ALL synchronized. So if "GPS doesn't ever use a clock on the Earth's surface" then where are these "atomic clocks" that "upload position and time to the satellites"?
From: Tom Roberts on 22 Mar 2010 11:45
GSS wrote: > Let us consider two identical precision atomic clocks, positioned at > points A and B, separated by a distance of about 30 km along east- > west direction, on the surface of earth. Assume the two clocks A and B > are mutually synchronized through Einstein convention such that the > time taken, T_ab, by a laser pulse to propagate from A to B (as > measured from the clock readings of B and A) is the same as the time > taken, T_ba, by a laser pulse to propagate from B to A. That means, > T_ab - T_ba = 0 which indicates e-synchronization of the two clocks. OK, that's clear enough. But note there is no inertial frame in which these clocks are synchronized, and that is usually a prerequisite for calling them "synchronized". Your measurements and operations are clear, but applying the word "synchronized" to this is not so clear. In particular, you could repeat this multiple times, performing this same procedure on pairs of clocks until you come back to the starting place -- the first and last clocks will NOT be "synchronized" in this sense. > In your opinion, will this synchronization remain valid at least for a > 24 hour period? That is, if we take to and fro signal propagation time > readings at hourly intervals, will all readings show, > T_ab - T_ba = 0 Modulo drifts in the individual clocks, yes. In gedankens we invariably use ideal clocks, so they don't drift, and the answer is an unqualified yes. That's because in such gedankens we also ignore such tiny things as the variation in earth's rotational rate (which would turn that into a "no", but the variations would be far below present technology's resolution). > Kindly explain the procedure for e-synchronization of the same two > atomic clocks A and B in the ECI or the GCRF frame. How exactly will > it be different from the local clock synchronization in practical > terms? To do this, one must set the clocks so that they display the time of a coordinate clock of the ECI that is located where the moving clock is located at the instant in question. ECI coordinate clocks, of course, are at rest in the ECI frame, and because the ECI is supposed to be an inertial frame in the sense of SR (not GR), gravitation does not affect ECI coordinate clocks. This means that the moving clocks must all be modified so that effects of their motion and gravitational potential are compensated. The result is that all such moving clocks are modified and initialized so they always indicate the time that an ECI coordinate clock would indicate at the moving clock's current location. This is how the GPS was set up. I am ignoring tiny corrections, such as due to the gravitation of sun and moon, variations in earth's density, mountains, etc.; these are all treated as corrections in the GPS. > Since the two clocks under consideration are simultaneously known to > be co-moving in the solar system at about 30 km/s, you may kindly > explain the procedure for e-synchronization of the same two atomic > clocks A and B in the solar system BCRF frame. How exactly will it be > different from the local clock synchronization in practical terms? You would have to do the same thing described above, but in the BCRF frame. But corrections for gravitation would be more serious, as the region covered is larger and the sun is much more massive than the earth. Astronomers essentially do this when generating a planetary ephemeris, but they don't achieve nearly the accuracy of the GPS. > Going one step still further,the two clocks under consideration are > simultaneously known to be co-moving in the Galactic reference frame > at about 200 km/s. Kindly explain the procedure for e-synchronization > of the same two atomic clocks A and B in the Galactic reference frame. > How exactly will it be different from the local clock synchronization > in practical terms? Apply the same technique, but in the galactic frame. The difficulties will be larger still.... > For the sake of simplicity of discussions, you may completely neglect > the gravitational effects on clock synchronization and only account > for the SR effects. That's insufficient. Gravitational effects are much larger than velocity effects for the GPS satellite clocks. Tom Roberts |