7 ms·

Scientists use an atomic clock to measure the height of a mountain

45 points by montrose 9y ago

_rpd 9y ago
The innovation here is that they made the atomic clock portable enough to be taken to the top of a mountain, so that they could use the difference in clock speed to calculate the height by general relativity.
- programmarchy 9y ago
  A portable vacuum chamber. Pretty cool. So unlike a pressure altimeter, this could also work in space. And unlike a radar altimeter, you don't need a direct line of sight to the surface. Although seems like the measurement could also be distorted by large masses like the moon...
  - deleted 9y ago
    [deleted]
  - mikeash 9y ago
    Unfortunately, you still need some communication with the outside world. To the local observer, the clock will run at the same rate no matter where you are. Only distant clocks will change.
    - 7952 9y ago
      I wonder if you could measure the difference between two clocks on the same craft at a distance from one another. The strength of gravity follows the inverse square law so presumably the difference in time between the two points will change also.
      - mikeash 9y ago
        That should work, and give you the local gravitational gradient, which would correspond to the altitude. You might want four arranged in a tetrahedron so you don’t need to orient them.
- PoachedSausage 9y ago
  The atomic wrist watch has been around for a while though. http://www.leapsecond.com/pages/atomic-bill/ http://www.leapsecond.com/pages/atomic-bill/
  - lopmotr 9y ago
    This is an optical lattice clock which is a newer and more accurate technology than the older cesium beam type of that "wristwatch".
- ChuckMcM 9y ago
  They made a strontium clock portable enough to take to the top of a mountain. There are already "chip scale" cesium clocks (see https://www.microsemi.com/product-directory/clocks-frequency-references/3824-chip-scale-atomic-clock-csac https://www.microsemi.com/product-directory/clocks-frequency...) 1x10^-11 (cesium) vs 1x10^-18 for strontium clocks [1]. The additional 7 orders of magnitude get you to measuring time dilation at very very small scales (which it what they tried to do in the article). [1] https://www.nist.gov/news-events/news/2015/04/getting-better-all-time-jila-strontium-atomic-clock-sets-new-records https://www.nist.gov/news-events/news/2015/04/getting-better...
  - starpilot 9y ago
    Yes, I think GE's "time machine" employs four cesium clocks in a compact configuration. With six, you can reduce the worldline divergence considerably.
- astrodust 9y ago
  That's not really a big deal. Most GPS satellites have two in them and they're not all that huge.
  - ceejayoz 9y ago
    GPS uses cesium (deviation, per Wikipedia: 10^−13), rather than strontium (deviation: 10^-17). Cesium clocks were already pretty small back in the 1970s (https://en.wikipedia.org/wiki/File:President_Pi%C3%B1era_receives_ESO%27s_first_atomic_clock.jpg https://en.wikipedia.org/wiki/File:President_Pi%C3%B1era_rec...), but if you're going to measure relativity-caused differences between the top and bottom of a mountain, I suspect the extra accuracy is important.
    - astrodust 9y ago
      I'm not sure how this experiment works as the strength of gravity is variable across the planet anyway. Accounting for this could be tricky. https://news.nationalgeographic.com/news/2011/04/110406-new-map-earth-gravity-geoid-goce-esa-nasa-science/ https://news.nationalgeographic.com/news/2011/04/110406-new-...
      - lttlrck 9y ago
        That’s across the entire surface of the plant. It isn’t going to vary much at all across the distance relative to the base and peak where they made their measurements would it?
        astrodust 9y ago
        It'd require careful calculation between the base of the mountain and the top, plus some other location for calibration purposes.
    - madengr 9y ago
      I thought they used rubidium, with cesium standards on the ground to broadcast corrections.
tahw 9y ago
How the hell did they measure clock speed if the passage of time itself is different?
- _rpd 9y ago
  Relative to a twin clock at sea level.
- phyzome 9y ago
  "Dear tahw, what time and date is it over there? Please write back. I shall write again next month. Sincerely, phyzome" ... "Dear tahw, over the past year, I have noticed something interesting in our correspondences -- more hours have passed on your end than mine, on average. I believe our clocks are ticking at different rate. Yours, phyzome" (except measuring number of ticks, as communicated over fiber optics, and possibly with other tricks to cancel out communication lag)
nyc111 9y ago
Is it possible that they measured the distance not height to the reference clock? It takes time for the signal to travel to the reference clock. But I’m not sure about the exact geometry.
- ChuckMcM 9y ago
  I would guess that one would send a clock signal (think of it as a square wave at a very precise frequency) and measure the phase shift between the two clocks. (also very precisely)
- empath75 9y ago
  Latency would be constant, time dilation would continually drift.
timb07 9y ago
A similar experiment (but to only detect the effect of altitude, not to measure it) was done about 12 years ago, described here: http://www.leapsecond.com/great2005/ http://www.leapsecond.com/great2005/
sebtoast 9y ago
I am confused, I always thought that moving faster makes time go slower but the article says this: "For example, a clock on top of a tall mountain — far from the center of the Earth — will move a tiny bit faster than a clock at the base of that mountain, where the gravity is stronger. It's not a mechanical error. Time itself actually passes faster at the top of the mountain." From what I can understand from the Wikipedia article is that time does go slower the faster you move: "Special relativity indicates that, for an observer in an inertial frame of reference, a clock that is moving relative to him will be measured to tick slower than a clock that is at rest in his frame of reference." I did a little search and half the sites I saw said one thing and the other half said the opposite. Would anyone mind explaining like I'm five? Or maybe share a trustworthy sources I could read on the subject? Thanks.
- nikanj 9y ago
  https://en.wikipedia.org/wiki/Gravitational_time_dilation https://en.wikipedia.org/wiki/Gravitational_time_dilation Spoilers follow: This is a theme in Pohl's Gateway, as the protagonist shoots his friends into a black hole to give himself enough velocity to escape it's pull. Later, he realizes they're still falling in, as the immense gravity of the black hole stretches time for them.
  - liberte82 9y ago
    Isn't it true that if you were falling into a black hole, you would see the end of the universe before you crossed the event horizon? Therefore from our frame of reference nothing has ever actually fallen into a black hole? Or maybe I'm thinking of reaching the singularity at the center of the black hole.
    - computerex 9y ago
      That's not true. If you are falling into a black hole, to you time will flow normally until your sphaghettified demise. To an observer at infinity, they'll never observe you going through the event horizon because no light can reach the observer from within the event horizon and because gravitational time dilation.
btilly 9y ago
I wouldn't think this would be exceptional as a way to measure altitude. However it is exceptional as a way to measure gravity differences arising from stuff inside of the Earth. See https://academo.org/demos/gravity-map/ https://academo.org/demos/gravity-map/ for a picture of how that varies over the Earth. (Though you wouldn't be measuring the strength of gravitational pull directly. You would be measuring the gravitational potential. The strength of gravity is the gradient of the potential field.)
- marknadal 9y ago
  Why is the Indonesia area so red on that map? I get that other red spots have tall mountains... but why Indonesia?
  - wahern 9y ago
    Continental crust is less dense than marine crust. So just because the Himalayas are tall, they also displace denser mantle below, so they don’t contribute much more to gravitational pull. The Andes and Indonesia are strong subduction zones. Besides the crust on the surface, there’s another layer of (dense oceanic) crust below adding to the pull. -- Darren #44 comment at http://blogs.discovermagazine.com/badastronomy/2011/03/31/the-earths-lumpy-gravity/ See, also, the following paper, which catalogs how the angle of subduction effects the gravimetric profile. Harabaglia, Paolo & Doglioni, Carlo. (1998). Topography and gravity across subduction zones. Geophysical Research Letters - GEOPHYS RES LETT. 25. 703-706. 10.1029/98GL00137.
_adamb 9y ago
"It didn't work as nicely as we hoped, but we learned a lot and it's a start," Lisdat said. "Sometimes you just have to begin, and then you can figure out how to improve." Good sentiment to live/work by.
- cr0sh 9y ago
  > Good sentiment to live/work by. Definitely. I've found in my personal life that just getting started on something can be the hardest part about any given task (whether something physical, or implementing an idea as code or something like that). But once you get rolling on it, things tend to come along easier. You might not reach the end, things might fail (sometimes spectacularly), or things will change midstream and you might not end up where you had planned. Regardless, though, the journey can end up being better than the original goal. ...you just have to begin.
westurner 9y ago
Quantum_clock#More_accurate_experimental_clocks: https://en.wikipedia.org/wiki/Quantum_clock#More_accurate_experimental_clocks https://en.wikipedia.org/wiki/Quantum_clock#More_accurate_ex... > In 2015 JILA evaluated the absolute frequency uncertainty of their latest strontium-87 optical lattice clock at 2.1 × 10−18, which corresponds to a measurable gravitational time dilation for an elevation change of 2 cm (0.79 in) on planet Earth that according to JILA/NIST Fellow Jun Ye is "getting really close to being useful for relativistic geodesy". AFAIU, this type of geodesy isn't possible with 'normal' time structs. Are nanoseconds enough? "[Python-Dev] PEP 564: Add new time functions with nanosecond resolution" https://mail.python.org/pipermail/python-dev/2017-October/149944.html https://mail.python.org/pipermail/python-dev/2017-October/14...
- inteleng 9y ago
  The thread you linked makes a good point: there isn't really any reason to care about the actual time, only the relative time. These sorts of clocks just use 256- or 512-bit counters; it's not like they're having overflow issues.
prewett 9y ago
The article said that this was the first time atomic clocks were taken out of the lab to do this experiment, but definitely not the first time: http://www.leapsecond.com/great2005/tour/ http://www.leapsecond.com/great2005/tour/ My favorite part is where he notes that with kids everything has to be exactly equal: "three kids, three clocks, three sodas." My somewhat fuzzy arguments with my brothers make me think he is a wise man.

45 points by montrose 9y ago

_rpd 9y ago

The innovation here is that they made the atomic clock portable enough to be taken to the top of a mountain, so that they could use the difference in clock speed to calculate the height by general relativity.

programmarchy 9y ago
A portable vacuum chamber. Pretty cool. So unlike a pressure altimeter, this could also work in space. And unlike a radar altimeter, you don't need a direct line of sight to the surface. Although seems like the measurement could also be distorted by large masses like the moon...
- deleted 9y ago
  [deleted]
- mikeash 9y ago
  Unfortunately, you still need some communication with the outside world. To the local observer, the clock will run at the same rate no matter where you are. Only distant clocks will change.
  - 7952 9y ago
    I wonder if you could measure the difference between two clocks on the same craft at a distance from one another. The strength of gravity follows the inverse square law so presumably the difference in time between the two points will change also.
    - mikeash 9y ago
      That should work, and give you the local gravitational gradient, which would correspond to the altitude. You might want four arranged in a tetrahedron so you don’t need to orient them.
PoachedSausage 9y ago
The atomic wrist watch has been around for a while though. http://www.leapsecond.com/pages/atomic-bill/ http://www.leapsecond.com/pages/atomic-bill/
- lopmotr 9y ago
  This is an optical lattice clock which is a newer and more accurate technology than the older cesium beam type of that "wristwatch".
ChuckMcM 9y ago
They made a strontium clock portable enough to take to the top of a mountain. There are already "chip scale" cesium clocks (see https://www.microsemi.com/product-directory/clocks-frequency-references/3824-chip-scale-atomic-clock-csac https://www.microsemi.com/product-directory/clocks-frequency...) 1x10^-11 (cesium) vs 1x10^-18 for strontium clocks [1]. The additional 7 orders of magnitude get you to measuring time dilation at very very small scales (which it what they tried to do in the article). [1] https://www.nist.gov/news-events/news/2015/04/getting-better-all-time-jila-strontium-atomic-clock-sets-new-records https://www.nist.gov/news-events/news/2015/04/getting-better...
- starpilot 9y ago
  Yes, I think GE's "time machine" employs four cesium clocks in a compact configuration. With six, you can reduce the worldline divergence considerably.
astrodust 9y ago
That's not really a big deal. Most GPS satellites have two in them and they're not all that huge.
- ceejayoz 9y ago
  GPS uses cesium (deviation, per Wikipedia: 10^−13), rather than strontium (deviation: 10^-17). Cesium clocks were already pretty small back in the 1970s (https://en.wikipedia.org/wiki/File:President_Pi%C3%B1era_receives_ESO%27s_first_atomic_clock.jpg https://en.wikipedia.org/wiki/File:President_Pi%C3%B1era_rec...), but if you're going to measure relativity-caused differences between the top and bottom of a mountain, I suspect the extra accuracy is important.
  - astrodust 9y ago
    I'm not sure how this experiment works as the strength of gravity is variable across the planet anyway. Accounting for this could be tricky. https://news.nationalgeographic.com/news/2011/04/110406-new-map-earth-gravity-geoid-goce-esa-nasa-science/ https://news.nationalgeographic.com/news/2011/04/110406-new-...
    - lttlrck 9y ago
      That’s across the entire surface of the plant. It isn’t going to vary much at all across the distance relative to the base and peak where they made their measurements would it?
      - astrodust 9y ago
        It'd require careful calculation between the base of the mountain and the top, plus some other location for calibration purposes.
  - madengr 9y ago
    I thought they used rubidium, with cesium standards on the ground to broadcast corrections.

tahw 9y ago

How the hell did they measure clock speed if the passage of time itself is different?

_rpd 9y ago
Relative to a twin clock at sea level.
phyzome 9y ago
"Dear tahw, what time and date is it over there? Please write back. I shall write again next month. Sincerely, phyzome" ... "Dear tahw, over the past year, I have noticed something interesting in our correspondences -- more hours have passed on your end than mine, on average. I believe our clocks are ticking at different rate. Yours, phyzome" (except measuring number of ticks, as communicated over fiber optics, and possibly with other tricks to cancel out communication lag)

nyc111 9y ago

Is it possible that they measured the distance not height to the reference clock? It takes time for the signal to travel to the reference clock. But I’m not sure about the exact geometry.

ChuckMcM 9y ago
I would guess that one would send a clock signal (think of it as a square wave at a very precise frequency) and measure the phase shift between the two clocks. (also very precisely)
empath75 9y ago
Latency would be constant, time dilation would continually drift.

timb07 9y ago

A similar experiment (but to only detect the effect of altitude, not to measure it) was done about 12 years ago, described here: http://www.leapsecond.com/great2005/ http://www.leapsecond.com/great2005/

sebtoast 9y ago

I am confused, I always thought that moving faster makes time go slower but the article says this: "For example, a clock on top of a tall mountain — far from the center of the Earth — will move a tiny bit faster than a clock at the base of that mountain, where the gravity is stronger. It's not a mechanical error. Time itself actually passes faster at the top of the mountain." From what I can understand from the Wikipedia article is that time does go slower the faster you move: "Special relativity indicates that, for an observer in an inertial frame of reference, a clock that is moving relative to him will be measured to tick slower than a clock that is at rest in his frame of reference." I did a little search and half the sites I saw said one thing and the other half said the opposite. Would anyone mind explaining like I'm five? Or maybe share a trustworthy sources I could read on the subject? Thanks.

nikanj 9y ago
https://en.wikipedia.org/wiki/Gravitational_time_dilation https://en.wikipedia.org/wiki/Gravitational_time_dilation Spoilers follow: This is a theme in Pohl's Gateway, as the protagonist shoots his friends into a black hole to give himself enough velocity to escape it's pull. Later, he realizes they're still falling in, as the immense gravity of the black hole stretches time for them.
- liberte82 9y ago
  Isn't it true that if you were falling into a black hole, you would see the end of the universe before you crossed the event horizon? Therefore from our frame of reference nothing has ever actually fallen into a black hole? Or maybe I'm thinking of reaching the singularity at the center of the black hole.
  - computerex 9y ago
    That's not true. If you are falling into a black hole, to you time will flow normally until your sphaghettified demise. To an observer at infinity, they'll never observe you going through the event horizon because no light can reach the observer from within the event horizon and because gravitational time dilation.

btilly 9y ago

I wouldn't think this would be exceptional as a way to measure altitude. However it is exceptional as a way to measure gravity differences arising from stuff inside of the Earth. See https://academo.org/demos/gravity-map/ https://academo.org/demos/gravity-map/ for a picture of how that varies over the Earth. (Though you wouldn't be measuring the strength of gravitational pull directly. You would be measuring the gravitational potential. The strength of gravity is the gradient of the potential field.)

marknadal 9y ago
Why is the Indonesia area so red on that map? I get that other red spots have tall mountains... but why Indonesia?
- wahern 9y ago
  Continental crust is less dense than marine crust. So just because the Himalayas are tall, they also displace denser mantle below, so they don’t contribute much more to gravitational pull. The Andes and Indonesia are strong subduction zones. Besides the crust on the surface, there’s another layer of (dense oceanic) crust below adding to the pull. -- Darren #44 comment at http://blogs.discovermagazine.com/badastronomy/2011/03/31/the-earths-lumpy-gravity/ See, also, the following paper, which catalogs how the angle of subduction effects the gravimetric profile. Harabaglia, Paolo & Doglioni, Carlo. (1998). Topography and gravity across subduction zones. Geophysical Research Letters - GEOPHYS RES LETT. 25. 703-706. 10.1029/98GL00137.

_adamb 9y ago

"It didn't work as nicely as we hoped, but we learned a lot and it's a start," Lisdat said. "Sometimes you just have to begin, and then you can figure out how to improve." Good sentiment to live/work by.

cr0sh 9y ago
> Good sentiment to live/work by. Definitely. I've found in my personal life that just getting started on something can be the hardest part about any given task (whether something physical, or implementing an idea as code or something like that). But once you get rolling on it, things tend to come along easier. You might not reach the end, things might fail (sometimes spectacularly), or things will change midstream and you might not end up where you had planned. Regardless, though, the journey can end up being better than the original goal. ...you just have to begin.

westurner 9y ago

Quantum_clock#More_accurate_experimental_clocks: https://en.wikipedia.org/wiki/Quantum_clock#More_accurate_experimental_clocks https://en.wikipedia.org/wiki/Quantum_clock#More_accurate_ex... > In 2015 JILA evaluated the absolute frequency uncertainty of their latest strontium-87 optical lattice clock at 2.1 × 10−18, which corresponds to a measurable gravitational time dilation for an elevation change of 2 cm (0.79 in) on planet Earth that according to JILA/NIST Fellow Jun Ye is "getting really close to being useful for relativistic geodesy". AFAIU, this type of geodesy isn't possible with 'normal' time structs. Are nanoseconds enough? "[Python-Dev] PEP 564: Add new time functions with nanosecond resolution" https://mail.python.org/pipermail/python-dev/2017-October/149944.html https://mail.python.org/pipermail/python-dev/2017-October/14...

inteleng 9y ago
The thread you linked makes a good point: there isn't really any reason to care about the actual time, only the relative time. These sorts of clocks just use 256- or 512-bit counters; it's not like they're having overflow issues.

prewett 9y ago

The article said that this was the first time atomic clocks were taken out of the lab to do this experiment, but definitely not the first time: http://www.leapsecond.com/great2005/tour/ http://www.leapsecond.com/great2005/tour/ My favorite part is where he notes that with kids everything has to be exactly equal: "three kids, three clocks, three sodas." My somewhat fuzzy arguments with my brothers make me think he is a wise man.