6 ms·

Atomic clocks compared with astounding accuracy

70 points by zeristor 6y ago

a11r 6y ago
Link to paper: https://arxiv.org/pdf/2005.14694.pdf https://arxiv.org/pdf/2005.14694.pdf
amenghra 6y ago
If you want to read more about atomic clocks, this is a great post from 2005: http://leapsecond.com/great2005/tour/ http://leapsecond.com/great2005/tour/
rkagerer 6y ago
According to Einstein’s theory of relativity, Earth’s gravity causes the frequency of an atomic clock to depend on its altitude. Consequently, the height difference between two remote clocks can be determined by measuring their difference in frequency. At the level of measurement uncertainty achieved in the latest work, clock comparisons could resolve centimetre-sized height differences. Therefore, clocks could provide new tools for long-term environmental monitoring of, for example, ice sheets or ocean levels.
thatcherc 6y ago
I got to visit JILA one time and talk to researchers working on a blue or ultraviolet atomic clock (possibly the strontium mentioned in the article - can't quite remember) and they had a great story about how sensitive their clocks were: In a part of the testing and commissioning phase of this new clock, they were comparing its frequency with the nearby NIST clock and noticed that some days it ran slower than other days. Even after accounting for the usual thing you might expect (position of the Moon, the tides [which have an effect on these clocks even all the way up in Colorado], and even variations in atmospheric density) they still saw this unexplained variation now and then. Eventually they realized it was correlated with home games at the football stadium: the mass of the fans in the bleachers was gravitionally redshifting their clock! I'm not 100% sure if that's a fully true story or whether it was embellished to impress prospective grad students, but I thought it was amazing. They also said the clocks went crazy any time there was a big earthquake in North America. The sensitivity of these devices is just incredible
- Chris2048 6y ago
  If that was true, shouldn't they also see significant differences during night/day, and holidays?
  - marcosdumay 6y ago
    Those cause dispersed changes on people's distribution on all directions around the clock, so no, they are probably not detectable. Also the sibling comparing with the Earth's mass isn't in a good direction, because the Earth's mass is mostly stable, and the clock only suffers disturbances from changes... But if the story is true, it should have an easy time detecting vulcanism.
    - mikewarot 6y ago
      Tides cause clocks to change time... even the tail end of electromechanical clocks were precise enough to detect it. It was detected in 1929 with the Shortt–Synchronome clock See section 3 of this: https://hgss.copernicus.org/articles/11/215/2020/ https://hgss.copernicus.org/articles/11/215/2020/
      - fsh 6y ago
        This is because Shortt clocks are pendulum clocks whose oscillation period directly depends on the gravitational acceleration. Quarz oscillators and atomic clocks are not affected by this.
        mikewarot 6y ago
        It took a while to sink in that gravity isn't anywhere near as simple as I usually think it is. I understand Gravitational Acceleration at the surface of the earth is on the order of 9.81m/sec^2 I also understand that if you take clocks to the mountains, they run slightly faster. What would happen if you put a clock at the bottom of a mine shaft a few miles down? (In an air-conditioned room, of course) How does the math work?
geenew 6y ago
The potential land surveying application of high-accuracy clocks is quite interesting. It would provide a measure of elevation that is (in the vertical scale, at least) completely independent of GNSS, at an accuracy comparable to what can be achieved by current field-deployed GNSS receivers, at ~0.02m I wonder how far that could be stretched. Current permanent ground-station GNSS receivers have vertical accuracies in the mm scale. They achieve that by solving for measurements taken over very long periods (~6 months). Would atomic clock height measurement tools be able to achieve similar accuracies, if the datasets for a single station were made over a similar length of time?
- tgb 6y ago
  It sounds like it would more be used for relative elevation differences. Is GNSS the gold-standard for that? I'd have thought you could do better with a laser and a level.
  - AnimalMuppet 6y ago
    I'm not sure that we have levels that are accurate enough for that.
    - tgb 6y ago
      Really? I can't tell what accuracy a good theodolite gets, but this consumer device [1] claims 3/16ths of an inch at 30ft. Obviously that won't scale to 1km distance like they're doing in this, a professional one seems competitive with the quoted ~2cm accuracy. [1] https://www.amazon.com/SKIL-Self-Leveling-Cross-Line-Laser/dp/B07DMK1NB8 https://www.amazon.com/SKIL-Self-Leveling-Cross-Line-Laser/d...
  - WJW 6y ago
    Absolute gold standard for use in a laboratory, probably not. But GNSS is definitely the instrument of choice for 99% of surveying and navigational purposes.
- fsh 6y ago
  GNSS and clock comparisons measure slightly different things and are complementary. GNSS gives the position of the receiver in space relative to some defined coordinate system. This allows accurate mapping of the earth's surface and measurements of continental drift. Clock comparisons measure (via the gravitational red shift) the difference in gravitational potential between the locations of the clocks. This is affected not only by the height difference, but also by changes in the mass distribution within the earth. Transportable optical clocks would allow real-time mapping of the earth's gravitational equipotential surface (geoid). Currently, the geoid has to be determined by painstakingly levelling accross the continents point by point. This can be surprisingly accurate (cm-level) but is obviously much too slow to observe fast variations, for example due to earthquakes.
mikewarot 6y ago
It sounds like they need to put an array of full tensor 3d gravity gradiometers in Boulder to monitor the real time movement of mass above, below, and on the ground. It wouldn't be that expensive, relatively speaking. ;-)
dkdk8283 6y ago
I ran several stratum 1 systems and was a member of the public NTP pool for quite awhile. I have since moved on but maintain my own stratum 1 clock in my rack and a high resolution clock in my office.

70 points by zeristor 6y ago

a11r 6y ago

Link to paper: https://arxiv.org/pdf/2005.14694.pdf https://arxiv.org/pdf/2005.14694.pdf

If you want to read more about atomic clocks, this is a great post from 2005: http://leapsecond.com/great2005/tour/ http://leapsecond.com/great2005/tour/

rkagerer 6y ago

According to Einstein’s theory of relativity, Earth’s gravity causes the frequency of an atomic clock to depend on its altitude. Consequently, the height difference between two remote clocks can be determined by measuring their difference in frequency. At the level of measurement uncertainty achieved in the latest work, clock comparisons could resolve centimetre-sized height differences. Therefore, clocks could provide new tools for long-term environmental monitoring of, for example, ice sheets or ocean levels.

thatcherc 6y ago

I got to visit JILA one time and talk to researchers working on a blue or ultraviolet atomic clock (possibly the strontium mentioned in the article - can't quite remember) and they had a great story about how sensitive their clocks were: In a part of the testing and commissioning phase of this new clock, they were comparing its frequency with the nearby NIST clock and noticed that some days it ran slower than other days. Even after accounting for the usual thing you might expect (position of the Moon, the tides [which have an effect on these clocks even all the way up in Colorado], and even variations in atmospheric density) they still saw this unexplained variation now and then. Eventually they realized it was correlated with home games at the football stadium: the mass of the fans in the bleachers was gravitionally redshifting their clock! I'm not 100% sure if that's a fully true story or whether it was embellished to impress prospective grad students, but I thought it was amazing. They also said the clocks went crazy any time there was a big earthquake in North America. The sensitivity of these devices is just incredible

Chris2048 6y ago
If that was true, shouldn't they also see significant differences during night/day, and holidays?
- marcosdumay 6y ago
  Those cause dispersed changes on people's distribution on all directions around the clock, so no, they are probably not detectable. Also the sibling comparing with the Earth's mass isn't in a good direction, because the Earth's mass is mostly stable, and the clock only suffers disturbances from changes... But if the story is true, it should have an easy time detecting vulcanism.
  - mikewarot 6y ago
    Tides cause clocks to change time... even the tail end of electromechanical clocks were precise enough to detect it. It was detected in 1929 with the Shortt–Synchronome clock See section 3 of this: https://hgss.copernicus.org/articles/11/215/2020/ https://hgss.copernicus.org/articles/11/215/2020/
    - fsh 6y ago
      This is because Shortt clocks are pendulum clocks whose oscillation period directly depends on the gravitational acceleration. Quarz oscillators and atomic clocks are not affected by this.
      - mikewarot 6y ago
        It took a while to sink in that gravity isn't anywhere near as simple as I usually think it is. I understand Gravitational Acceleration at the surface of the earth is on the order of 9.81m/sec^2 I also understand that if you take clocks to the mountains, they run slightly faster. What would happen if you put a clock at the bottom of a mine shaft a few miles down? (In an air-conditioned room, of course) How does the math work?

geenew 6y ago

The potential land surveying application of high-accuracy clocks is quite interesting. It would provide a measure of elevation that is (in the vertical scale, at least) completely independent of GNSS, at an accuracy comparable to what can be achieved by current field-deployed GNSS receivers, at ~0.02m I wonder how far that could be stretched. Current permanent ground-station GNSS receivers have vertical accuracies in the mm scale. They achieve that by solving for measurements taken over very long periods (~6 months). Would atomic clock height measurement tools be able to achieve similar accuracies, if the datasets for a single station were made over a similar length of time?

tgb 6y ago
It sounds like it would more be used for relative elevation differences. Is GNSS the gold-standard for that? I'd have thought you could do better with a laser and a level.
- AnimalMuppet 6y ago
  I'm not sure that we have levels that are accurate enough for that.
  - tgb 6y ago
    Really? I can't tell what accuracy a good theodolite gets, but this consumer device [1] claims 3/16ths of an inch at 30ft. Obviously that won't scale to 1km distance like they're doing in this, a professional one seems competitive with the quoted ~2cm accuracy. [1] https://www.amazon.com/SKIL-Self-Leveling-Cross-Line-Laser/dp/B07DMK1NB8 https://www.amazon.com/SKIL-Self-Leveling-Cross-Line-Laser/d...
- WJW 6y ago
  Absolute gold standard for use in a laboratory, probably not. But GNSS is definitely the instrument of choice for 99% of surveying and navigational purposes.
fsh 6y ago
GNSS and clock comparisons measure slightly different things and are complementary. GNSS gives the position of the receiver in space relative to some defined coordinate system. This allows accurate mapping of the earth's surface and measurements of continental drift. Clock comparisons measure (via the gravitational red shift) the difference in gravitational potential between the locations of the clocks. This is affected not only by the height difference, but also by changes in the mass distribution within the earth. Transportable optical clocks would allow real-time mapping of the earth's gravitational equipotential surface (geoid). Currently, the geoid has to be determined by painstakingly levelling accross the continents point by point. This can be surprisingly accurate (cm-level) but is obviously much too slow to observe fast variations, for example due to earthquakes.

mikewarot 6y ago

It sounds like they need to put an array of full tensor 3d gravity gradiometers in Boulder to monitor the real time movement of mass above, below, and on the ground. It wouldn't be that expensive, relatively speaking. ;-)

dkdk8283 6y ago

I ran several stratum 1 systems and was a member of the public NTP pool for quite awhile. I have since moved on but maintain my own stratum 1 clock in my rack and a high resolution clock in my office.