4 ms·
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 spee
by _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 agoA 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 agoUnfortunately, 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 agoI 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 agoThat 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 agoThe 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 agoThis is an optical lattice clock which is a newer and more accurate technology than the older cesium beam type of that "wristwatch".
- ChuckMcM 9y agoThey 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 agoYes, 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 agoThat's not really a big deal. Most GPS satellites have two in them and they're not all that huge.
- ceejayoz 9y agoGPS 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 agoI'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 agoThat’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 agoIt'd require careful calculation between the base of the mountain and the top, plus some other location for calibration purposes.
- madengr 9y agoI thought they used rubidium, with cesium standards on the ground to broadcast corrections.