How do you measure the accuracy of a clock? What if every clock is wrong, just a little bit?
You can build two and see how much they shift relative to each other. That gives you precision.
So what's the point of a clock if you just define it to be correct? Again, having two clocks is what makes it interesting. Some people have commented that according to general relatively there will be measuralbe time dilation, but there are other fun experiments, e.g.
- Measure shift of fundamental "constants": If you have two clocks that use different elements, the frequency ratio can be related to some things we thought were constants in the universe. If they shift, they aren't constant.
- Look for preferred directions in space: does one clock give a different reading if you turn it on its side?
- Some theories predict that dark matter might induce a frequency shift in these clocks. Put the clocks far apart and look for spacial modulations in the dark matter density.
- Measure anything else that had to be tweaked to make the clock stable. This includes the magnetic field, for example, so the clock is also a really sensitive magnetometer.
Consider the construction of a precision clock, then build 2 (or more) of them. Take a pair set them to the same initial time, and then let them count time (same height, etc).
Ideally, a pair of perfect clocks would display the same time over time. In practice you see the clocks slowly walk off with respect to each other (a systematic component and a random component), a reasonable first order approximation is to pretend the difference in displayed time shows a random walk behavior. A collection of a large number of clocks would behave like a collection of random walk instances, diffusing in delta-time space.
A poor clock construction would diffuse more quickly, and a better clock construction would diffuse more slowly.
One doesn't need a perfect clock reference to measure the random walk of a clock type / construction. Just compare 2 identically constructed and used ones.
For example even very small magnetic fields will change the clock speed, thermal changes will as well (so will lots of other things). So you try to shield from that, and keep the temperature stable (and of course you need to figure out every other things that could add noise).
Then you measure all those influences that you just are unable to control, and calculate what affect they have on the clock, and that's your accuracy number.
One way to directly measure that, instead of calculate it, is to have two identical clocks, synchronize them, and let them run. Then compare them, and see if they differ. (Watch out for relativity messing with time.)
For example, every electron is exactly the same as every other electron, they do not vary in the slightest. You utilize properties like that to make exact references to time.
it's a human construct so whatever is agreed upon is correct.
If "all the clocks are wrong" it doesn't matter as long as they are consistent. (in the case of atomic clocks, frequency of energy transitions within atoms)
All ntp servers get the average of atomic clocks, which is then distributed to all phones and computers.
If the constants from these atomic clocks "are a little bit wrong" it does not matter (for most human activities)
That's why we average them and distribute the average.
For physics related research, this new clock being more precise does have use, but for pretty much everything else, whatever constant we have is good enough as long as it's consistently used.
Back in the day it was someone just running around with a pocket watch giving everyone the time from the clock tower which was calibrated from a sundial and that was good enough.
Replace the sun's shadow with electron transitions and the timekeepers with ntp servers and that's what you have today.