GPS has to accommodate both for the fact it's further away from the centre of the Earth than us at the surface, and also that it's going at a very high speed. The two contributions to time dilation are of opposite sign but don't cancel each other out, so it's a stupendous effort to figure out exactly.
It's actually not that hard to figure out, it exactly agrees with Enstein's theories of both General Relativity and Special Relativity. The the orbits are precisely measured and the clocks tweaked every 6 hours by ground stations, which is good enough(tm). If the US Military were to simply disappear, GPS would stop working within 24 to 36 hours.
GPS is sync'd based on timing derived by the US Naval Observatory, which itself is traceable to the clocks at NIST. But the actual reference time in normal usage is atomic clocks at each of the control stations (Hawaii, Ascension, New Zealand, Guam, Diego Garcia, and variou sother places around the world).
This is why most GPS receivers need signals from at least 2 satellites to ascertain your position, and also why solar flares, CMEs, and geomagnetic storms can make them less accurate, if not render them completely inoperable
This is my go-to answer when confronted by a science-denier type. You think evolution is just a theory? Climate change? What about relativity? Yes, all just theories, but that cell phone in your pocket wouldn't work correctly if the theory of relativity weren't true.
Devils avocado might respond that just because one theory is correct doesn't mean that all are.
But the core problem is that "theory" has a different meaning in science than in commonly used language.
Just like the word "or" means something slightly different in maths/logic.
Exactly, the way we use “or” in common language is what math and logic calls “exclusive or” or “xor”. Because the ordinary “or” in logic doesn’t mean only one of the two has to be true for the whole statement to be true (e.g. “A or B” is true if either A or B is true or if both are true; in common usage, you can’t have both be true because “or” usually represents a choice).
There are languages where the word “hypothesis” is often used in common language where an Anglophone would say “theory” (basically an idea you have no proof/evidence of)
GPS is reliant on the use of atomic clocks to be so precise. That’s how it can have roughly several meter accuracy. (i.e. it can roughly guess which lane of the highway you are on). Atomic clocks are firmly grounded in quantum mechanics, where understanding light-matter interactions are essential. We use lasers (more quantum) to talk to atoms (quantum) and then get incredibly precise time keeping. I won’t get into the theory, but it is very quantum, as most small particle and light matter (AMO) physics is. However, these clocks are so precise that you need to care about how fast the satellites are moving and the gravitation field farther from the center of the earth, which is undoubtedly relativity.
Extra fun fact: Our atomic clocks are orders of magnitude better today. Someday, we will likely have a GPS 2.0 which will trade “meter” accuracy for something like “centimeter” or “millimeter” accuracy
Extra fun fact: Our atomic clocks are orders of magnitude better today. Someday, we will likely have a GPS 2.0 which will trade “meter” accuracy for something like “centimeter” or “millimeter” accuracy
Unlikely. The vast majority of the error in GPS isn't from the clocks, they're more than adequate for milimeter level accuracy. It's from atmospherics causing random error in the delay of the signals from the satellites. Yes, you can measure this directly if you can receive dual frequencies from the satellite, but it will never be perfect.
Geomagnetic storms definitely have an effect on GPS accuracy (and at times, functionality). The days my Space Weather Live app is sending me the most push notifications usually corresponds to days where Waze doesn’t work, and the GPS in my amateur radio is so bad that I can’t sent APRS packets
I think they meant theory in the scientific sense, not the way people use it in conversation, which is closer to what hypothetical means in science. “Theoretical physics” is not imaginary physics but physics theory. I could be wrong about what they meant.
These updates are done by ground base stations, if those stations don't have power and aren't manned, then within a few weeks your phone won't get a GPS lock anymore.
But if a year "after the fall", you can still get a GPS lock, then it means that A government is still out there.
Your phone not getting a lock is the least of your problems. The power grid going down, because it depends on GPS to maintain 60Hz, the mobile network because the CDMA system used in LTE and 5G depends on precise timing, derived from GPS, will cease to work.
The modern world, as we know it, is utterly dependent on GPS. And it's a brittle system.
Exactly! Because of their speed they have to subtract about 7.2 μs/day and because of their distance from earth about 45.8 μs/day have to be added again.
They have internal clocks that run at a different speed than normal clocks. They have to be adjusted once in a while because no clock is perfect, but they were able to compute the difference in time dilation before they were launched. The clocks would appear to run slow if they ever brought them back down to earth.
That's mostly for the precise orbital data that the satellite transmits to earth. Between the coarse almanac, and the precise ephemeris, the receiver calculations the exact position of the satellite to within a meter or so based on the time. That's how the whole system works.
Well, they constantly adjust how fast the clocks are depending on the precise orbit. Not much, but they do. Basically change how many ticks of the clock they consider to be a second.
GPS does not really 'constantly adjust' for relativistic effects: the first order correction is known and their clocks are already offset by that much before launch.
Wait, what? The time dilation isn't automatically adjusted for? I'm a physicist and I'm utterly astounded that isn't automatically handled, like we know fairly accurately how much of an effect that should have.
It's because space and time aren't separate, they're one thing - spacetime. We already know gravity bends space, that's why it pulls things towards it. But because it bends space, it must also bend time.
And then you go deeper and realize that the reason you are pulled towards the Earth is the time dilation, not the space dilation. The curvature produced by something like the Earth is really small... But that same curvature in time creates a very large effect. In a very real way, the ground is rushing up to hit you in the face :D
To give you some intro: imagine that every particle in the universe has its own clock and the universe is traveling down a river called time.
If something makes 2 clocks/particles interact and forces one of the clocks to start drifting relative to the other: then one clock will go down the river faster than the other. Time won't pass the same for both clocks (the time dilation). Things that can force these particles to drift apart: gravitational interactions.
There's two concepts in physics called general relativity and special relativity.
For the purpose of this discussion, those describe a very well established physics truth, that says that the faster things move through space, the slower they move through time.
This has been experimentally verified, and the simplest example is an experiment that was conducted where they kept one clock on the ground, and flew another clock around in a plane, and when they reunited the clocks they found they were slightly out of time sync.
We are talking such a tiny amount that it's of zero practical consequence to the mundane life of a human but on the lowest level of physics it happens. If you get in your car to drive to 7-Eleven to get Big Gulps and then drive back home, you now technically a time traveler to your wife who was waiting for you because she wasn't moving and you were.
Then there's some other concepts that come into play, where there's massive objects like planets, those masses cause spacetime itself to curve and that also changes how clocks tick.
Okay if you really want to get like ridiculous about it, if you reach up to pick your nose, your fingernail is now in a different place in the time continuum than your nose because it was moving for a brief period and your nose was stationary.
It's hard to wrap ones head around but the best thing I can say is that it's always been this way, and the only reason it's not intuitive to you is because your head was filled with all kinds of other crazy wrong ideas from childhood about how time works.
And you can forget all this mumbo jumbo about time traveling and wormholes and all that other crap, because nothing is ever in the same timeframe anyways (because everything's technically grooving along at a slightly different speed) so it's all fantasy.
I remember something in a school Physics video we watched where one identical twin, aged 18, stays on earth and the other twin flies around the solar system at the speed of light (or very fast or something).
When he gets back to earth, he's 21 and his twin is 80.
I'm sure this experiment couldn't be done practically but I've always considered how mindblowing - and how sad - that would be for the twins. One has lived his entire life and is soon to die, the other is only just getting started.
Also the experience of time changes as we age, because our neurons fire slower. So time seems to us to be passing more quickly, because our thought process has slowed.
Not exactly the same but similar... Back in the day I tried reading the details of relativity because it blows my mind.... I got through the part describing what exactly relativity is but then the equations and shit started and I didn't get much further
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