3 ms·
Yes. In the experiments in our lab, we are sensitive, through gravity-gradients, to the annual variation of the water table in the hill adjoining the instrumen
by ISL 7y ago
Yes. In the experiments in our lab, we are sensitive, through gravity-gradients, to the annual variation of the water table in the hill adjoining the instrumentation.
Viewed another way, the hill beside our lab deflects the direction of "down" by about 50 nanoradians for every meter you move vertically in the lab. (If you kept the hill, and got rid of the earth, a satellite would orbit the hill.)
The really crazy thing about the experiments described in the article is that they depend on the difference in potential, which is much harder to estimate. As these clock-based experiments reach still-higher sensitivities, we're going to see improved and interesting tests of gravity emerge alongside the increasingly-difficult problems of time-transfer at these levels of precision.
- jpmattia 7y ago> by about 50 nanoradians for every meter you move vertically in the lab. Holy moly. Just what kind of measurements are you doing in your lab?
- ISL 7y agoPrecision tests of gravity: https://www.npl.washington.edu/eotwash/node/1 https://www.npl.washington.edu/eotwash/node/1 Our most-recent measurement: https://news.ycombinator.com/item?id=22797729 https://news.ycombinator.com/item?id=22797729 Once you know how, it's not as hard as you'd expect to measure a nanoradian ;).
- jpmattia 7y agoThat looks like all kinds of fun! Thanks for the links.
- ISL 7y agoIt is fun -- it requires an interesting blend of patience and impatience. Each individual step in the measurement process is slow, but those who are most successful at it are very hungry to get at reliable answers quickly. Gravity is, by most measures, our oldest force, and while GR is a very successful theory, we know essentially nothing about the underlying mechanism. This is very much unlike the rest of the Standard Model, where we have very-successful quantum theories.
- RHSeeger 7y agoThis was really interesting to read, thank you. The "a satellite would orbit the hill" and " direction of "down" parts of the comment, in particular, really helped understand/visualize what you were describing. Also, I immediately went into the next room and told my wife about it, because I thought it was that interesting. Thanks for chiming in.
- ISL 7y agoThanks -- it's so cool! (We care about this difference-in-the-direction-of-down because we need to lock one of our turntables to "local vertical" with nanoradian stability. It was initially a surprise, before my time, when it was realized that it was impossible to do so simultaneously for two sensors on the same turntable but at different locations on the turntable axis.) Also, if you think that's cool, depending on your latitude, the "down" you experience doesn't point in the direction of the center of the earth. The earth is rotating, so there is a substantial centrifugal contribution to the acceleration you experience. Off the top of my head, I believe the effect here in Seattle is about 14 milliradians. It is zero at the pole and equator (the apparent centrifugal acceleration makes you effectively lighter!), though :).
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