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

Another Artificial Gravity Question

  1. Jun 28, 2005 #1
    Lets say you're on a spaceship, and the floor of the ship is pulling you and the other objects in your ship toward it. (it might be easier to think of this in terms of a computer simulation)

    If you go beneath the floor of your ship, you are not affected by it's artificial gravity.

    Now think about this: If you put all of the objects in your ship towards the front end, would it start to dip down due to the 'weight'? Think about how a ship dips down in the ocean when more weight is added to the front or back.

    If it did start to dip down, would your ship start to spin- like a seesaw only without the ground underneath it?

    Maybe this makes no sense at all.
  2. jcsd
  3. Jun 29, 2005 #2


    User Avatar
    Science Advisor

    What is the source of your artificial gravity? In most scenerios, it is created by having the spaceship spin. In that case things are pulled to the outer wall.

    If there is no outside force of gravity involved, the oscillation you are talking about doesn't make sense.
  4. Jun 29, 2005 #3


    User Avatar
    Science Advisor

    So you have something (your floor) that attracts objects on one side only? This is impossible.
  5. Jun 30, 2005 #4
    Are you saying you don't believe in accelerating objects?
  6. Jun 30, 2005 #5


    User Avatar
    Science Advisor

    What do you mean by "down"? :biggrin:

    Normally "down" means "in the direction of gravitational force". Since the only gravity you mention is a floor in the ship, there would be know pull on the ship itself.
  7. Jun 30, 2005 #6


    User Avatar
    Science Advisor

    Say things fall to your floor from one side only. Set this device in space. Everything "above" this floor will feel an attraction to it, while nothing "below" will feel it. So this space ship accelerates "upwards" (i.e. in the direction normal to the plane of the floor) on its own just using gravity and no power source. Conservation of energy makes this is impossible.
  8. Jun 30, 2005 #7


    User Avatar
    Staff Emeritus
    Science Advisor

    A quick note: the electromagnetic way of generatign a field in a localized area involves a parallel-plate capacitor. You charge one plate up with a positive charge, and another plate up with a negative charge. The result is an electric field which exists only between the plates.

    If there were such a thing as negative mass, we could do the same with gravity. There probably isn't such a thing as negative mass, but there is some question as to how to formulate this principle without being over-restrictive - i.e. how to formulate the principle so that we don't have bulk negative mass, but still have the Casimir effect and black hole evaporation. Various proposals about the appropriate energy condition have been made, but all of them appear to be too restrictive.

    On the other hand, if we assume for the sake of argument that there is such a thing as negative mass, the gravitational equivalent of the "parallel plate" capacitor accelerates just as krab describes, without using any energy,. This happens because the positive energy and momentum of the positive mass is exactly balanced by the negative energy and momentum of the negative mass.

    Unfortunately we get a wide variety of bizarre unstable behaviors when we postulate the unrestricted existence of negative mass, the negative mass naturally tends to sink to it's lowest energy state, which is essentially negative infinity :-(.
    Last edited: Jun 30, 2005
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook