5 ms·

I 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 a

I 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 ago
Which Way Is Down? https://www.youtube.com/watch?v=Xc4xYacTu-E https://www.youtube.com/watch?v=Xc4xYacTu-E
- dvh 7y ago
  Down is towards enemy.
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 ago
    Precision 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 ago
      That looks like all kinds of fun! Thanks for the links.
      - ISL 7y ago
        It 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 ago
  This 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 ago
    Thanks -- 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 ago
Their effect is minuscule, but measurable: https://svs.gsfc.nasa.gov/11234 https://svs.gsfc.nasa.gov/11234
- hinkley 7y ago
  If 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 ago
    I 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 ago
The 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 ago
  Relevant link: https://en.wikipedia.org/wiki/Geoid https://en.wikipedia.org/wiki/Geoid
dharmab 7y ago
Gravity 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 ago
Yes, 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 ago
  Mason 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 ago
    I 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 ago
There 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 ago
Navy 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

yrro 7y ago

Which Way Is Down? https://www.youtube.com/watch?v=Xc4xYacTu-E https://www.youtube.com/watch?v=Xc4xYacTu-E

dvh 7y ago
Down is towards enemy.

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 ago
  Precision 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 ago
    That looks like all kinds of fun! Thanks for the links.
    - ISL 7y ago
      It 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 ago
This 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 ago
  Thanks -- 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 ago

Their effect is minuscule, but measurable: https://svs.gsfc.nasa.gov/11234 https://svs.gsfc.nasa.gov/11234

hinkley 7y ago
If 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 ago
  I 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 ago

The 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 ago
Relevant link: https://en.wikipedia.org/wiki/Geoid https://en.wikipedia.org/wiki/Geoid

dharmab 7y ago

Gravity 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 ago

Yes, 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 ago
Mason 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 ago
  I 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 ago

There 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 ago

Navy 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