I see the statement “GPS won’t work without relativity” a lot and to me this implies that if Relativity was not a fact GPS wouldn’t work. Surely the statement should be more along the lines of “Without accounting for Relativity GPS wouldn’t work”? If the whole Universe worked according to Newtonian Mechanics wouldn’t GPS work just fine without taking Relativity into account?
enigmatic, I came across that link when I was searching for examples. It’s just that when people like Phil Plait (The Bad Astronomer) and Pamela Gay who are two relatively prominent astronomers on the Internet make this statement I wonder who is wrong here, me or them?
Anyhow off to sleep now, I’ll check again in the morning for any more feedback.
As I understand it, what with being a physicist and having studied Einstein’s special relativity and also being interested in GPS, without accounting for Relativity, GPS wouldn’t work right. There isn’t anything about GPS that specifically relies on any relativistic effects per se. You are correct, I believe.
I think you’re misreading their statements. They are effectively saying “GPS was built with relativity in mind; if it turned out that relativity didn’t describe the universe, GPS as designed never would have worked in the first place.”
It’s a bit like saying “airplanes require air at 15-100 kPa to fly properly”. The statement is true because we’ve designed airplanes to fly in this range. We could design planes to fly in a 1000 kPa atmosphere–in fact, it would be easier–but airplanes as they currently exist would not work. The fact that current airplanes do work as designed is evidence that we were right about our measurements.
Yes, obviously, if the laws of physics were different (Newtonian as you present as an example) then GPS would use those laws of physics. If the laws if physics said GPS had to be pink, then GPS satellites would be pink. But they don’t, and they’re not.
What the phrase you’re quoting actually means is: “We make GPS systems according to physical laws incorporating relativity because if we didn’t they wouldn’t work”.
You have cause and effect kinda backwards. They’re made that way because those are the laws of physics. If the laws if physics were different, they’d be made another way.
So the answer to your question is: yes. Absolutely. If the universe was Newtonian then they’d be built to follow only Newtonian laws. It’s not, though, and they’re not.
Maybe. I don’t think we know how to describe gravity outside the framework of general relativity. It might be that there’s no way to get gravity out of a universe where general relativity isn’t true.
If you look at the BA example in context, it is fairly obvious that he’s using “relativity” as shorthand for “Einstein’s theories of special and general relativity”. The very next sentence is:
“Without the theory of relativity GPS wouldn’t work” is pretty much equivalent to "Without accounting for Relativity GPS wouldn’t work.
But it still doesn’t answer the underlying philosophical question, becuase it’s like saying “parchutes wouldn’t work without gravity”; it is so true that it doesn’t tell us anything special about the parachute-gravity raletionship that also doens’t include every other thing in the universe.
If we “turned off” relativity, the whole universe would stop working, so the statement “GPS wouldn’t work without relativity” is uninformative, even if 100% true, without a specific things or how without relativity it wouldn’t.
GPS satellites contain atomic clocks which are astonishingly accurate. GPS works by having each satellite broadcast a signal indicating the time its clock has. The satellites travel on orbits which are known to a high degree of accuracy. GPS works by having the receiver read the signal from multiple satellites, calculate how far away each must be based on the difference in times received, and calculate the position of the receiver relative to the satellites.
If the universe worked according to strictly Newtonian rules, time would pass at the same rate on the satellites as it did on the ground. However, in a universe which works according to relativity, time passes at a different rate on the satellites, due to their movement and due to them being further out of the Earth’s gravity well. The calculations which are performed by the satellites and the receivers take this into account. If they didn’t, if the math was done assuming a strictly Newtonian universe, the math wouldn’t work out the same.
So it isn’t so much that they wouldn’t work without relativity, as that relativity had to be taken into account when the system was developed.
Well, yes. But our hypothetical questioner doesn’t believe this, so it’s not very helpful in an argument. The whole point to the “GPS wouldn’t work without relativity” is that it’s an answer to “prove to me that relativity describes the universe”. GPS was designed with relativity in mind because that’s how we think the universe works. If they were wrong and relativity didn’t describe the universe, GPS wouldn’t have ever worked.
I once heard that the original GPS systems – that is, when the whole system was being set up and tested and such – were NOT designed with relativity in mind. Their calculations were very straightforward, based on current time, speed of light, distance to each satellite, etc. But it didn’t work, and it took them a while to realize that they neglected to account for relativity causing the clocks on the satellites to go at a different speed than our clocks.
If that is true, then although today’s GPS systems are are designed with relativity in mind, the original ones were not.
I’ve heard this before; often (and insultingly) with a story like “the engineers didn’t believe the scientists when it came to relativity so the didn’t account for it initially, and later they had to fix their mistake”. I’ve never seen a citation, though, so I’m highly inclined to disbelieve it.
Total layperson here, pretending to tell you all How It Is.
Yes, I read that too somewhere.
Now here’s a question I have on the subject: Just how precisely accurate are those GPS atomic clocks (and our measurements of them), even with Relativistic accounting? My “intuitive” guess is that they must be much more accurate than without that accounting (obviously), but probably still have some residual inaccuracy. I should imagine that they still need to recalibrate regularly by empirically measuring drift from benchmark sites of known location. Is this so?
Consider: One of the cites mentions that the on-board atomic clocks must be accurate to within 20-30 nanoseconds. Twenty to thirty? There’s 50% slop in that specification right there! I’m thinking that the clock might be running at, e.g., 23.47563859376 nanosecond accuracy while the real required accuracy might be 27.853757563 nanoseconds. So there would still be drift in the observed earth measurements. Surely, there must be stationary earth sites of precisely known location, which are measured regularly to detect such residual drift, so that empirical corrections can be made?
Can somebody tell me how right or wrong I am in thinking this? Or, to ask it another way: How much does GPS accuracy rely on on-going empirical measurements, Relativity adjustments notwithstanding?
I am aware that GPS readings are also greatly affected by atmospheric conditions. To get from that 50-100 meters accuracy down to centimeter accuracy, they do it like this, basically: Take readings from a site of unknown (or mobile) location. Also take readings from a nearby stationary site. (Location not necessarily known precisely. The important thing being that it’s stationary.) The stationary site appears to be wandering around, to do shifting atmospheric conditions. The amount of movement can be applied to adjust the readings of the other (unknown or mobile location) to obtain its exact location precisely.
I have a friend who designed software to track runners in real-time orienteering activities, which entailed doing this. He demostrated and explained all that to me recently.
Then, there’s always things like this. Taking Relativity into account is, apparently, not the entire solution.
I think it’s just semantics. People don’t always say things in the best way.
It’s more accurate to say “GPS won’t work without accounting for relativity” and it’s more informative as well. “GPS won’t work without relativity” isn’t a very descriptive statement. The next logical question would be “Why?” and the answer would be “Because it accounts for it.”
The theory of a GPS system itself doesn’t need relativity, but if we didn’t know about relativity, we couldn’t build a GPS system that worked. If relativity wasn’t a fact, our current GPS system wouldn’t work, but we could build a new one or take out the relativity correcting stuff from the current one and it would work. GPS works either way, you just have to know which universe you’re living in, one with relativity or one without, and make your GPS to match or else it won’t work.
This is the way I see it. Our GPS requires relativity to work because it was designed to work in a universe where relativity was real. However, if the universe was based on Newtonian physics, it would have been possible to design a GPS that worked in that universe.
So a GPS in general doesn’t require relativity to work. But the specific GPS we use is designed to work only in a relativity-based universe, so the fact that it works proves that relativity is real.
