6 ms·

Let's assume they manage to make a nuclear clock out of this, with an Allan drift that's low enough to be useful. Once that's done, it'll take years of observat

Let's assume they manage to make a nuclear clock out of this, with an Allan drift that's low enough to be useful. Once that's done, it'll take years of observation to measure any meaningful differences and gather enough data to notice something. Meanwhile, moving the height of anything a centimeter, the position of the moon, and a whole other host of noise sources have to be canceled out. I have no doubt this will be done... and it will be awe inspiring to hear it all told after the fact. While you're waiting... I found this really cool meeting documented on YouTube[1] that has the clearest explanation of how Chip Scale Atomic clocks work I've ever seen. I look forward to Chip Scale Optical Lattice clocks [1] https://www.youtube.com/watch?v=wHYvS7MtBok https://www.youtube.com/watch?v=wHYvS7MtBok

incompatible 2y ago
> Meanwhile, moving the height of anything a centimeter, the position of the moon, and a whole other host of noise sources have to be canceled out. Because time runs slower the stronger gravity becomes? I don't think it would be a problem, as long as the entire experimental apparatus is within the same gravity field for the duration of a particular measurement.
- wongarsu 2y ago
  The gravity field we are in isn't that constant. The gravitational influence of the moon is strong enough to move a lot of water here on Earth. The other planets are a lot further away, but not completely without gravitational influence. Earth's orbit around the sun isn't a perfect circle and has ~3% difference between lowest and highest point. The seasonal shift in mass distribution on Earth is big enough that we used to correct for it in astronomical time observations (the up to 30ms or so between UT1 and UT2). On the other hand, I don't think this experiment is really all that sensitive to gravity since we aren't really measuring time.
  - incompatible 2y ago
    But it would depend on how long it takes to make a single measurement. Perhaps the moon wouldn't move far.
- mikewarot 2y ago
  Optical lattice clocks are so precise these days, you can detect the difference in clock rates caused by a 2 centimeter difference in height. The higher clock will run faster. In the famous thought experiment you can't tell the difference in an elevator in either a gravitational well, or accelerating frame. It turns out that is only true if the elevator is sufficiently small. Sufficiently small is getting smaller every year.
  - incompatible 2y ago
    The lower gravity clock will run faster, but the experiment should give the same result, regardless of which frame it's running in. The same way that the caesium-133 atom transition frequency is 9192631770 Hz, regardless of the gravitational field.
    - deleted 2y ago
      [deleted]
    - mikewarot 2y ago
      The time shift of a Cesium beam atomic clock at different altitudes is a well established experimental result. With Optical Lattice clocks, they are much more stable, and the output is light, so a phase difference can be detected much, much quicker. According to [1], a difference of 1 centimeter can be measured. The quote of interest: An optical lattice clock with a frequency accuracy of 1 × 10^−18, which is currently the most accurate in the world, has a detectable gravitational potential equivalent to an elevation difference of approximately 1 cm. If you can access it, here's the related letter in Nature[2] from 2018. [1] https://www.rd.ntt/e/research/JN202304_21619.html https://www.rd.ntt/e/research/JN202304_21619.html [2] https://www.nature.com/articles/s41586-018-0738-2 https://www.nature.com/articles/s41586-018-0738-2
      - incompatible 2y ago
        Yes, nevertheless each clock in its own frame of reference is measuring 9192631770 Hz, or whatever. It's only with multiple communicating clocks that you'd observe the difference: that 9192631770 Hz in one place is different to 9192631770 Hz somewhere else.
marcyb5st 2y ago
Can't they do something similar to Ligo/Virgo setups? I.e. Multiple experiments running the same or similar hardware so that you can remove the type of noise you mentioned easily enough. Additionally, this feels like it is much cheaper to deploy compared to the interferometer hardware used by those experiments, so you can put enough replicas around the world to cancel out any local source of noise.

incompatible 2y ago

> Meanwhile, moving the height of anything a centimeter, the position of the moon, and a whole other host of noise sources have to be canceled out. Because time runs slower the stronger gravity becomes? I don't think it would be a problem, as long as the entire experimental apparatus is within the same gravity field for the duration of a particular measurement.

wongarsu 2y ago
The gravity field we are in isn't that constant. The gravitational influence of the moon is strong enough to move a lot of water here on Earth. The other planets are a lot further away, but not completely without gravitational influence. Earth's orbit around the sun isn't a perfect circle and has ~3% difference between lowest and highest point. The seasonal shift in mass distribution on Earth is big enough that we used to correct for it in astronomical time observations (the up to 30ms or so between UT1 and UT2). On the other hand, I don't think this experiment is really all that sensitive to gravity since we aren't really measuring time.
- incompatible 2y ago
  But it would depend on how long it takes to make a single measurement. Perhaps the moon wouldn't move far.
mikewarot 2y ago
Optical lattice clocks are so precise these days, you can detect the difference in clock rates caused by a 2 centimeter difference in height. The higher clock will run faster. In the famous thought experiment you can't tell the difference in an elevator in either a gravitational well, or accelerating frame. It turns out that is only true if the elevator is sufficiently small. Sufficiently small is getting smaller every year.
- incompatible 2y ago
  The lower gravity clock will run faster, but the experiment should give the same result, regardless of which frame it's running in. The same way that the caesium-133 atom transition frequency is 9192631770 Hz, regardless of the gravitational field.
  - deleted 2y ago
    [deleted]
  - mikewarot 2y ago
    The time shift of a Cesium beam atomic clock at different altitudes is a well established experimental result. With Optical Lattice clocks, they are much more stable, and the output is light, so a phase difference can be detected much, much quicker. According to [1], a difference of 1 centimeter can be measured. The quote of interest: An optical lattice clock with a frequency accuracy of 1 × 10^−18, which is currently the most accurate in the world, has a detectable gravitational potential equivalent to an elevation difference of approximately 1 cm. If you can access it, here's the related letter in Nature[2] from 2018. [1] https://www.rd.ntt/e/research/JN202304_21619.html https://www.rd.ntt/e/research/JN202304_21619.html [2] https://www.nature.com/articles/s41586-018-0738-2 https://www.nature.com/articles/s41586-018-0738-2
    - incompatible 2y ago
      Yes, nevertheless each clock in its own frame of reference is measuring 9192631770 Hz, or whatever. It's only with multiple communicating clocks that you'd observe the difference: that 9192631770 Hz in one place is different to 9192631770 Hz somewhere else.

marcyb5st 2y ago

Can't they do something similar to Ligo/Virgo setups? I.e. Multiple experiments running the same or similar hardware so that you can remove the type of noise you mentioned easily enough. Additionally, this feels like it is much cheaper to deploy compared to the interferometer hardware used by those experiments, so you can put enough replicas around the world to cancel out any local source of noise.