You define it to be accurate, and then measure the precision.
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.
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.