8 ms·
Scientists use the Tokyo Skytree to test general relativity
- grenoire 7y agoI have a question for all the physics nerds here: The surface of the Earth is not quite flat. When you consider the sheer radius of it, it can be approximated as such, but it isn't. Do geographical features (massive mountains and deep oceans) impact the gravity of Earth, or are they truly irrelevant?
- yrro 7y agoWhich Way Is Down? https://www.youtube.com/watch?v=Xc4xYacTu-E https://www.youtube.com/watch?v=Xc4xYacTu-E
- dvh 7y agoDown is towards enemy.
- ISL 7y agoYes. 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 :).
- deleted 7y ago[deleted]
- terramex 7y agoTheir effect is minuscule, but measurable: https://svs.gsfc.nasa.gov/11234 https://svs.gsfc.nasa.gov/11234
- hinkley 7y agoIf memory serves, there’s a spot over the Marianas Trench where gravity is about 1% lower. Said that way, it doesn’t sound like much, but 0.1 m/s² sounds pretty substantial to me.
- deusexepsilon 7y agoI really feel sorry for a bunch of people who have said that, but it was a long one, and I think that's mostly true. Well, a lot of people do, but I don't have their money — my brain. I know why... just to say that, my brains are really dumb. What if this fact has changed? Well... let's just get our dirty little world back.
- scatters 7y agoThe geoid deviates from the ellipsoid by an amount on the order of tens of metres - from -106m in southern India, to +85m in Iceland. This has effects on satellite orbits as well as on ocean surface, although that is affected further by ocean circulation (meaning it is not locally flat) and subsequently by tides.
- codethief 7y agoRelevant link: https://en.wikipedia.org/wiki/Geoid https://en.wikipedia.org/wiki/Geoid
- dharmab 7y agoGravity is used by the oil and mining industry to find deposits. https://www.hartenergy.com/exclusives/gravity-and-magnetic-geophysical-methods-oil-exploration-176138 https://www.hartenergy.com/exclusives/gravity-and-magnetic-g...
- daxfohl 7y agoYes, the mass of the earth was estimated in 1774 by measuring how much a nearby mountain deviated measurements. https://en.wikipedia.org/wiki/Schiehallion_experiment https://en.wikipedia.org/wiki/Schiehallion_experiment
- basementcat 7y agoMason and Dixon reported a systematic error in their measurements of the border of Maryland and Pennsylvania to the Royal Society. This was suspected to be the mass of the Allegheny Mountains and eventually led to subsequent experiments. https://en.m.wikipedia.org/wiki/Mason%E2%80%93Dixon_line https://en.m.wikipedia.org/wiki/Mason%E2%80%93Dixon_line
- daxfohl 7y agoI learned about that when I was reading the novel by Pynchon. I can't remember whether it was actually in the novel or if I stumbled across it while cross referencing.
- perl4ever 7y agoThere was an experiment done to measure the gravity of a mountain in Scotland before the United States was even a thing: https://en.wikipedia.org/wiki/Schiehallion_experiment https://en.wikipedia.org/wiki/Schiehallion_experiment "The Royal Society formed the Committee of Attraction to consider the matter, appointing Maskelyne, Joseph Banks and Benjamin Franklin amongst its members"
- swixmix 7y agoNavy used gravity measurements to help inertial navigators. https://www.navy.mil/submit/display.asp?story_id=31940 https://www.navy.mil/submit/display.asp?story_id=31940
- hinkley 7y agoThere is something that has always bugged me about the space curving analogy and how it relates to relativity. Gravity in a multi body system or in special cases of a two body system, has counteracting forces. If you found the Lagrange point between a binary black hole system, you could stay there. Or if you hollowed out the core of the Earth, you would float because you are being pulled in all directions at once (essentially the 2 body problem doesn’t “behave” for concave objects or distance less than diameter). Is time slower or faster at the Earth’s core? Is it slower or faster at the Lagrange point of a three body system?
- ISL 7y agoThe essential observation/consequence of General Relativity is that is that everyone (in the absence of non-gravitational interactions) is just floating, or travelling in a straight line as-measured-locally. We travel in orbits because space is curved. (“Space-time tells matter how to move; matter tells space-time how to curve.” -- Wheeler) A clock at the center of the Earth runs more slowly than a clock on Earth's surface. The answer to your Lagrange point question is probably indeterminate. The differences in clock rates depend on the relative depths in potential not the magnitude of gravitational force (as measured by anyone).
- knzhou 7y agoIn the limit of weak gravity, gravitational time dilation depends only on the gravitational potential, and not at all on the local gravitational field. So going down to the center of the Earth just has the opposite effect of going up to space, i.e. the clock will run slower. Nothing special happens just because the field happens to cancel out. The equivalence principle essentially tells us that local gravitational field values don't have meaning, anyway.
- ars 7y agoYou are claiming you can distinguish between freefall inside the center of the earth, from freefall traveling between the stars, solely by the effect on time dilation?
- DeusExMachina 7y ago> The scientists from RIKEN and their collaborators took up the task of developing transportable optical lattice clocks that could make comparably precise tests of relativity, but on the ground. The ultimate purpose, however, is not to prove or disprove Einstein. What they are testing is their ultraprecise clocks, not the theory of general relativity like the headline says.
- knzhou 7y agoSure, but all of these things are always interlinked. It's entirely possible that the first deviation we ever find from general relativity will come in the form of just trying to make better and better clocks, but then finding some funny systematic effect that prevents different atomic clocks from properly syncing up, and then ruling out all other explanations. This kind of discovery is not without precedent in physics.
- fsh 7y agoThe clocks were of course compared in previous experiments with no height difference between them. The Skytree experiment is directly measuring gravitational time dilation which is a central prediction of General Relativity. Combined with an accurate measurement of the height difference and local gravitational acceleration, this makes it a very accurate test of GR.
- TomGullen 7y agoDoes time run faster at night time when you are further away from the sun?
- mcnamaratw 7y agoGeneral info is available here: https://en.wikipedia.org/wiki/Gravitational_time_dilation https://en.wikipedia.org/wiki/Gravitational_time_dilation