Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Gravity dilemma (explained)

  1. Jan 4, 2010 #1
    A guy told me this theory which will counter act gravity and here is an explanation to destroy his theory:

    You are on a satellite orbiting the earth at high speeds using centripetal force (acceleration) to overcome gravity (same characteristics as acceleration). Now, relative to the satellite (you can't have an accelerated frame of reference, like a satellite, for relativity but I'll get to that later) you are looking down and it just looks like the earth is rotating at a certain speed, it could even look still if the satellite is moving in the same direction as the earths rotation.

    That is the scenario and from that base, there are a few things to understand. First off, the entire scenario is theoretically possible though you can't take a frame of reference because everything is spinning, thus constantly changing direction, hence accelerating, therefore It would be an accelerated frame of reference, in which case using relativity would defy the laws of physics. So, this guy said something like: now imagine you have the earth and the satellite, and you are looking at both of them from thousands of miles away. You are RELATIVE TO THE SATELLITE (we discussed that earlier) so the satellite "isn't moving". Now, you reach out your arm (something like a 2,000 mile long arm) and you spin the earth faster. The faster the earth spins, the lesser the gravity, because you are making more centripetal force and overcoming gravity (acting as the "centriFUGAL" force).

    At the time, I didn't know what to say to him so I thought about it for a few days and the more i thought about it, the more it made sense. After I thought for a good long time I remembered YOU CAN'T HAVE AN ACCELERATED FRAME OF REFERENCE. And now I feel stupid for not realizing that sooner.
     
  2. jcsd
  3. Jan 6, 2010 #2
    Sure you can use an accelerated reference frame. But, discounting small tidal effects, the satellite isn't accelerating in GR, it's in inertial motion. It's only accelerating relative to earth's rest frame, not its own. Neither centripetal force or gravity are forces in GR.

    But the only way for an observer far away to stay at rest relative to the satellite would be to accelerate himself using powerful rockets, ie a real force of some kind.
     
  4. Jan 6, 2010 #3
    The spectator could be on another "earth" and in another satellite with the same trajectory as the satellite orbiting earth.
     
  5. Jan 6, 2010 #4
    Spinning the Earth faster will probably have much less effect on the satellite than you imagine. For example doubling the spin of the Earth will have very little effect on the satellite except for a minor frame dragging effect, while doubling the orbital speed of the satellite will have a very significant effect on the satellite.

    If the two satellites are exactly synchronised, the Earth satellite will appear to be stationary from the point of view of the observer on the distant satellite. To him it will look like the Earth is orbiting around the satellite and whether or not the Earth is spinning or not will have little effect on its orbital speed around the satellite or on any perceived centripetal or centrifugal force.
     
  6. Jan 6, 2010 #5
    Spinning the Earth faster will have more effect that I imagined, because I imagined that it would have pretty much no effect. And the bottom of the satellite is always facing the earth so it would actually look like the earth is spinning. But this thread is kind of stupid if viewed by anybody but the "guy" (my friend). I should have just emailed him the explanation. lol
     
  7. Jan 6, 2010 #6
    Well, that's not the way I understood your initial post. In this case, the observer would be at rest with and local to the satellite, not "far away". It is impossible to stay at rest with the satellite, but not local to it, without using some means of acceleration.
     
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook