My gravity theory
- Thread starter Motor Daddy
- Start date
Oh, come on. What is it - .08 od one second ? They make all the stupendous time dilation calculations. Surely they could make this one simple, once only one ! Satelite clock recieves 12.00.00, and is set to read it as 12.00.08 - how easy is that ?
However, if it the signal from earth was sent every second, the difference would NOT be cumulative, and would be so miniscule as to be negligble. And even if it wasn't negligible, seeing as it was a new signal every second, it too could be compensated for by one simple adjustment.
Actually I made a mistake about what Tom thought. It wasn't the speed of light it was the speed of gravity he felt wasn't restricted in GR. He felt it wasn't important that it was c like the theory predicts. It is important but it's really something that's difficult to measure. I must have the speed of light on the brain. The following was a very interesting attempt to measure the speed of gravity
Aberration and the Fundamental Speed of Gravity in the Jovian Deflection Experiment
http://arxiv.org/abs/astro-ph/0311063
Over the years following there was controversy on what was actually measured. The speed of gravity as Kopeikin and Fomalont claimed and denied by Clifford Will, Steve Carlip, and others who felt it was a 2nd order measurement of the speed of light. The disagreement exists to this day. If you look in the cite base #13 is a paper by Steve Carlip. It was all interesting to me since I started studying GR a couple of years before the experiment and I'm always interested in tests of the theory. Cool experiment. Most argued the speed of gravity was not what was measured.
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The difference is ~38,400 nanoseconds a day. Figure out how many days it would take for that to equal 1 second. The reason we had to account for the gravitational and relative motion time dilation for such a small number is light travels .333564095 meter/nanosecond.
.333564095 m/ns * 38,407 ns/day [my calculation from the project A, Exploring Black Holes] is 12,811.1962 meters/day signal error.
So it's not negligible and it's definitely accumulative. That's why the scientist make these decisions rather than citizens who don't know anything about it.
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As you would know, there is Coordinated Universal Time (UTC)
http://en.wikipedia.org/wiki/Coordinated_Universal_Time
The current version of UTC is defined by International Telecommunications Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions.[1] and is based on International Atomic Time
Note, based on International Atomic Time.
So, my question continues to be unanswered, and it is - why couldn't that be sent to the GPS satelites, not once a day, but every second (say), resulting in the satelites having the exact time every second, and doing away with all the atomic clock stuff.
I would suppose there is some good explanation, but I haven't heard it here yet.
http://en.wikipedia.org/wiki/Coordinated_Universal_Time
The current version of UTC is defined by International Telecommunications Union Recommendation (ITU-R TF.460-6), Standard-frequency and time-signal emissions.[1] and is based on International Atomic Time
Note, based on International Atomic Time.
So, my question continues to be unanswered, and it is - why couldn't that be sent to the GPS satelites, not once a day, but every second (say), resulting in the satelites having the exact time every second, and doing away with all the atomic clock stuff.
I would suppose there is some good explanation, but I haven't heard it here yet.
@ lakon,
6 reasons off the top of my head,
a) no means of communication is instant. This is critical because all 24 of the satellites would require to be synchronous or calculations will be off. How do you ensure all of the various satellites receive the new updated time at precisely the same nanosecond. It would not need to be simultaneous updates, but you can see the problem.
b) line of sight is required unless bouncing signals around again at less than instant speed.
c) Atomic Clocks are more accurate than radio clocks as a rule (by about a second), and triangulating/calculating positions on earth timing is a critical element.
d) Using atomic clocks allows scientists to view/confirm relativity in action.
e) Another problem is any device requiring a signal could be "hacked" with a false signal or jammed. Someone might not bother doing this to your little GPS device, but they may fool around with a satellite. Perhaps some terrorist that want you to be late for your cousins bar mitzvah.
f) The sixth reason is the most important and likely the main reason GPS satellites use atomic clocks. To calculate your position with GPS you need INCREDIBLY accurate timing NOT ONLY WITH SATELLITE but also with your receiver GPS device. So how does your device get the accuracy of an atomic clock without charging you $50000 for your dashboard device? Your device looks at the time sent from all the signals it receives and uses the atomic clocks comparisons to update your simple quart clock.
Some "Atomic Clocks" you see in stores are really "radio clocks", they are likely mislabeled unless you pay $50k+. I am sure they could create an accuracy much higher than 1 second off transmitting from earth, but if you were designing a satellite would you rather put in a really accurate clock or worry about sending a hackable signal from earth every second to 24 or more satellites. It seems like a good time for the KISS rule. Broadcasting could work, but it would involve headaches and concerns. A high end atomic clock is about $100k, and very cheap in comparison to the cost of a satellite and launch.
I read here that someone said time updates were sent from Earth on occasion. I did not know this was done (or necessary), but like anything signals can be corrected if they are broadcasting incorrectly. There would be a team of people who correctly tell the satellites to adjust the signals they send (This is all I thought was done).
6 reasons off the top of my head,
a) no means of communication is instant. This is critical because all 24 of the satellites would require to be synchronous or calculations will be off. How do you ensure all of the various satellites receive the new updated time at precisely the same nanosecond. It would not need to be simultaneous updates, but you can see the problem.
b) line of sight is required unless bouncing signals around again at less than instant speed.
c) Atomic Clocks are more accurate than radio clocks as a rule (by about a second), and triangulating/calculating positions on earth timing is a critical element.
d) Using atomic clocks allows scientists to view/confirm relativity in action.
e) Another problem is any device requiring a signal could be "hacked" with a false signal or jammed. Someone might not bother doing this to your little GPS device, but they may fool around with a satellite. Perhaps some terrorist that want you to be late for your cousins bar mitzvah.
f) The sixth reason is the most important and likely the main reason GPS satellites use atomic clocks. To calculate your position with GPS you need INCREDIBLY accurate timing NOT ONLY WITH SATELLITE but also with your receiver GPS device. So how does your device get the accuracy of an atomic clock without charging you $50000 for your dashboard device? Your device looks at the time sent from all the signals it receives and uses the atomic clocks comparisons to update your simple quart clock.
Some "Atomic Clocks" you see in stores are really "radio clocks", they are likely mislabeled unless you pay $50k+. I am sure they could create an accuracy much higher than 1 second off transmitting from earth, but if you were designing a satellite would you rather put in a really accurate clock or worry about sending a hackable signal from earth every second to 24 or more satellites. It seems like a good time for the KISS rule. Broadcasting could work, but it would involve headaches and concerns. A high end atomic clock is about $100k, and very cheap in comparison to the cost of a satellite and launch.
I read here that someone said time updates were sent from Earth on occasion. I did not know this was done (or necessary), but like anything signals can be corrected if they are broadcasting incorrectly. There would be a team of people who correctly tell the satellites to adjust the signals they send (This is all I thought was done).
Let's turn this around slightly. Are you asking for an answer to that question because you genuinely think all of the complicated relativistic atomic clock modelling is pointless and could be done away with in the trivial manner your describe or are you asking because you realise that given the trivial alternative you're proposing your realise that your grasp of the details is perhaps insufficient and thus you'd like someone to relieve you of a small piece of ignorance? If the latter then fine, we can go into details. If the former than not fine. So which is it?
There are 5 ground stations and 24-32 GPS satellites (depending on which spares are in use, what the latest launches were etc)
Ground stations are not "simple transmitters." And there are some obvious logistics issues with putting critical US military assets in (say) China.
Because it is cheaper, more reliable and more accurate to have the accurate clocks in the satellites.
If we could really put cheap reliable accurate etc transmitters all over the Earth then we wouldn't need GPS at all. Loran would work just fine.
I would imagine that an array of Coordinated Universal Time transmitters (which probably already exists) would ensure this.
No bouncing - line of site would be quite possible
Read the wiki article I linked earlier. The earth based UTC's are indeed atomic clocks.
Yes ..
Ah so ! it would be far easier to mess with a bunch of fragile satelites than with a ground based, vast array of UTC transmitters sending the satelites the exact time every second.
As said above, so do the UTC clocks. Read the wiki article.
This lends neither here nor there to the arguement.
Neither does this. You are waxing when you should be waning.
.. and thus back to my original point. If they can update them every day and do it right, why not every minute or second, thus obviating the need for atomic clocks in satelites and all the time dilation hoo haa ?
PS - We all learn new things in our dialouges here, and that's a good thing (except of course, for the few here that are thoroughly educated and know everything already). I just learnt that there is not one, but SEVEN global positioning systems, including the American one we know as GPS. Check out the 'Other Systems' section on this wiki ..
http://en.wikipedia.org/wiki/Gps
That's starting to make a bit more sense now. Thanks.
This is the correction per second +4.4453E-10 s. +.00000000044453s. Which is safer and more practical? Telling the sat computer to make this correction every second or tell the onboard computer to make a correction of +38,407ns summed over every second of an earth day. IE the computer tells the clock to run .00000000044453s faster every second. The sat has the exact time. They don't tell it to make one 'Big' adjustment of 38,407ns. I'm not sure they even have to adjust the clocks in flight. I think they set the sat clocks to run fast before launch and have the capability to make adjustments in flight. Actually, I think, they've built a control model based on operational data where clocks are no longer needed. If you really want to learn about the GPS read the Living Reviews on Relativity article by Neil Ashby. I think I linked that for Scott. Or get all the details at the GPS site.
Yes, thanks. I have been reading up on it.
@ Lakon,
If you think about what you are saying you would have realized that once you broadcast the time to another clock it is no longer atomic time.
It does not matter even if the GPS satellites have the correct time, they will work just fine as long as they are all in sync with one another.
You say,
So now not only do we need to synchronize the time between the satellites, but now you propose we have broadcasting stations all around the globe that must also have synchronous time. So how do we broadcast the atomic time of an American station to a station in Australia without bouncing signals? You're suggesting we bounce the time all over the world and then send it to various satellites from various stations? Is this what you are suggesting? It doesn't make much sense.
I said,
This demonstrates the importance of the satellites being synchronous and extremely accurate, and demonstrates why your GPS device does not need its own atomic clock.
You can argue the decision to equip the satellites with atomic clocks all you like, but it has likely been discussed by the people spending buying a few million dollars worth of atomic clocks. It seems your decisions seem less practical. You are arguing for the losing side.
Synchronized atomic clocks will be a lot more accurate than clocks receiving time updates, and it is a lot more simple. Your idea I don't understand. Do all of your proposed worldwide stations have atomic clocks, or do they all receive updates from bounced signals? Then they broadcast this already inaccurate time to the satellites in line of sight? Maybe I cannot grasp your systems logistics, but this sounds like what you are proposing. Your idea would make more sense to me if you broadcast a time and then let the satellites send the signals to each other instead of bouncing to a dozen stations on earth first.
If they do change a time on a given satellite it would not be something they do to all at once. They may twig a satellite by a nanosecond or two once a year, but I doubt they would ever try to "update" all of their satellites at once. This is just a guess, but it makes sense.
You said,
If you think about what you are saying you would have realized that once you broadcast the time to another clock it is no longer atomic time.
It does not matter even if the GPS satellites have the correct time, they will work just fine as long as they are all in sync with one another.
You say,
So now not only do we need to synchronize the time between the satellites, but now you propose we have broadcasting stations all around the globe that must also have synchronous time. So how do we broadcast the atomic time of an American station to a station in Australia without bouncing signals? You're suggesting we bounce the time all over the world and then send it to various satellites from various stations? Is this what you are suggesting? It doesn't make much sense.
I said,
You reply,
(after I fixed the spelling)
This demonstrates the importance of the satellites being synchronous and extremely accurate, and demonstrates why your GPS device does not need its own atomic clock.
You can argue the decision to equip the satellites with atomic clocks all you like, but it has likely been discussed by the people spending buying a few million dollars worth of atomic clocks. It seems your decisions seem less practical. You are arguing for the losing side.
Synchronized atomic clocks will be a lot more accurate than clocks receiving time updates, and it is a lot more simple. Your idea I don't understand. Do all of your proposed worldwide stations have atomic clocks, or do they all receive updates from bounced signals? Then they broadcast this already inaccurate time to the satellites in line of sight? Maybe I cannot grasp your systems logistics, but this sounds like what you are proposing. Your idea would make more sense to me if you broadcast a time and then let the satellites send the signals to each other instead of bouncing to a dozen stations on earth first.
If they do change a time on a given satellite it would not be something they do to all at once. They may twig a satellite by a nanosecond or two once a year, but I doubt they would ever try to "update" all of their satellites at once. This is just a guess, but it makes sense.
Lol .. you're imagining a battle where there is none. I was asking questions - albeit possibly in a direct way. Posts earlier to yours seem to have now provided meaningful answers on the question of why GPS satelites need atomic clocks.
Thanks Bruce; Very good, but this is over my head a bit. No worries. I think the only way to discriminate between GR and SR, is the math, personally, unless this experiment claims to have found otherwise? I find it most important to use GR math, due to our difficulty with frame of reference in SR, FIRST. General relativity FIRST, then the remainder can be attributed to SR. I don't quite understand how the difference can be observed through any other means, than the math. What was observed exactly, physically? I’m not very concerned whether it was Gravity (time dilation) or the speed of light (time dilation) that was observed. I’m more interested in what would distinguish our observation of inertial effects, due to relative velocity, or strict inertial mass.
I love hijacking a thread, by the way. The GPS discussion is excellent!
What do you mean? We can test SR in particle accelerators and GR using stellar dynamics, there are phenomena which predominantly tests one and not the other.
What 'difficult with frame of reference' are you referring to? The choice of frames in SR is a matter of choosing coordinates. GR has the same thing but to a much more complicated degree.
GR contains SR as a local symmetry, ie every set of local coordinates has Lorentz invariance about the point so you cannot separate them like that really. For something like the GPS network you have effects due to both the gravitational field and the motion through the field and while you can say "The former is GR and the latter is SR" there is no clear line in the sand separating them. Which brings me onto your next comment....
We can, however, consider phenomena where the gravitational field is important and cannot be ignored, ie planetary dynamics, which SR wouldn't be able to handle, and then consider phenomena where the gravitational field is unimportant and can be ignored, ie particle accelerators. We can use GR to model particle accelerator dynamics in a weak field approximation but it would be so close to SR to be indistinguishable.
More productive than listening to MD's nonsense.
Motor Daddy
Valued Senior Member
I'll bite.
Listening to me for the last two years has been the best thing that's ever happened to you as far as being at the right place at the right time. Luck is when preparation and opportunity meet.