[HN Gopher] Atomic clocks compared with astounding accuracy
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Atomic clocks compared with astounding accuracy
Author : zeristor
Score : 57 points
Date : 2021-03-25 16:21 UTC (6 hours ago)
(HTM) web link (www.nature.com)
(TXT) w3m dump (www.nature.com)
| mikewarot wrote:
| 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. ;-)
| thatcherc wrote:
| 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 wrote:
| If that was true, shouldn't they also see significant
| differences during night/day, and holidays?
| macintux wrote:
| It's possible they had already measured and accounted for
| those variations.
| marcosdumay wrote:
| 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.
| SCHiM wrote:
| If we manage to increase the sensitivity a few more orders of
| magnitude, and put an array of these devices in a sphere, we
| can make gravitic sensors!
| mikewarot wrote:
| There are better sensors that are already deployed, they were
| developed in the 1970s so that submarines could navigate
| undersea with zero emissions by using a gravity gradient map
| of the seafloor, part of the Trident II system.
|
| https://en.wikipedia.org/wiki/Gravity_gradiometry#Lockheed_M.
| ..
| fsh wrote:
| Gravitational force can be measured very accurately by
| dropping a retroreflector in a vacuum chamber and measuring
| the acceleration with a laser interferometer. The clock
| comparisons explained in the article measure the difference
| in gravitational potential between two locations. This would
| normally require measuring and integrating the force all the
| way between the two locations.
| User23 wrote:
| Gravimeters are already a thing and they're sensitive enough
| to detect accumulating snowfall on the roof of the building
| they're in.
| fsh wrote:
| The stadium seems to be about 160 m from the closest part of
| JILA. Assuming that 100000 people arrive in a 2000 kg car each,
| the gravitational redshift would be around 1E-21. This is about
| 1000 smaller than the stability of the best JILA optical
| clocks.
| dekhn wrote:
| I built a microscope and most of the time I have it looked at a
| test object. It's in the garage; when my kids in the house walk
| around about 20 feet away, on a loosely coupled floor, the
| image jiggles noticeably.
|
| Michelson experiment was done deep in a building at night, with
| instrumentation floated on a pool of mercury to remove
| vibrations, but they still had problems due to horse-driven
| cargo a quarter mile away. Today, undergrad physics students do
| the experiment in a day on a tabletop (tools got much better).
| delecti wrote:
| I was curious and did some rough back of the envelope math.
|
| The stadium seats about 50,000. Lets assume it's half full of
| 100 kg people (or entirely full of 50kg people), or a total of
| about 2,500,000 kg. Additionally the stadium is about 300
| meters from JILA. Meanwhile Earth is 6x10^24 kg at 6000 km.
| That's about 2x10^18 times as massive at only 20,000 times as
| far. Accounting for squaring the distance, that's about 5x10^9
| times as much effect from Earth than from the crowd. It's
| plausible their instruments could pick up the effect of a
| difference of that much mass, though I suspect they embellished
| a bit the degree to which the clock went slower on game days.
| wrs wrote:
| According to the article these clocks are accurate to 2 parts
| in 10^18, so...
| ericbarrett wrote:
| Most people will drive to the stadium, so the local mass
| increase is a factor of ~10 higher. Assume 2 people per car
| and 1500kg/car (this is pretty conservative for Colorado,
| where SUVs and pickups are common), so each athlete,
| employee, and attendee would bring in ~800kg mass plus their
| own weight.
|
| EDIT: I was curious about the parking situation; seems to be
| a bunch of lots scattered around with JILA effectively in the
| center: https://en.parkopedia.com/parking/stadium/folsom-
| field-co/?a...
| [deleted]
| pdonis wrote:
| _> Accounting for squaring the distance_
|
| You actually don't _want_ to square the distance, since you
| are not trying to compute the acceleration due to gravity,
| you are trying to compute the gravitational potential (since
| that 's what affects clock rates), which goes like 1/r, not
| 1/r^2. So the Earth's effect should be about 10^14 larger
| than the effect of the people in the stadium. (Which is still
| several orders of magnitude larger than the sensitivity of
| the clocks, so it's entirely plausible that the clocks were
| detectably affected.)
| D-Coder wrote:
| I once worked with a guy who had worked on gravitational
| measurements in (I presume) grad school. They measured the
| attraction between spheres of lead. One of the adjustments they
| had to make was considering the change in the masses as _lead
| atoms evaporated_ from the surface of the spheres.
| adonovan wrote:
| The gravitational attraction between spheres of lead is
| strong enough to be measured in a high-school science lab (if
| you can remove the students and air currents); this is the
| famous Cavendish experiment of 1798.
|
| I remember it blowing my mind to realize that although a lead
| sphere is tiny compared to the Earth, it's also right there:
| whereas the Earth acts like a point mass four thousand miles
| beneath your feet.
| geenew wrote:
| 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?
| fsh wrote:
| 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.
| geenew wrote:
| GNSS and clock comparisons measure different things, one is a
| based on radio wave travel time between a receiver and the
| satellites in the constellation, the other is a physical
| measurement. That's what I was referring to wrt independence:
| a gravity measurement would avoid any potential error source
| in the whole chain of the GNSS system (which is quite
| lengthy).
|
| The geoid has been partially satellite-derived for some time,
| see the GRAIL, GRACE, and GRACE-FO missions. The US is also
| running a nationwide airborne gravity program at the moment,
| too, which an impressive undertaking and should be done soon-
| ish.
|
| Comparing a measurement of actual local gravity against the
| geoid-stated gravity should be enough to give the elevation
| relative to the geoid, which is the same output that you get
| from GNSS. That's my intuition, anyway.
| tgb wrote:
| 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 wrote:
| I'm not sure that we have levels that are accurate enough for
| that.
| tgb wrote:
| 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/d...
| WJW wrote:
| 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.
| amenghra wrote:
| If you want to read more about atomic clocks, this is a great
| post from 2005: http://leapsecond.com/great2005/tour/
| a11r wrote:
| Link to paper: https://arxiv.org/pdf/2005.14694.pdf
| rkagerer wrote:
| _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._
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