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Thread: Question on Gravity

  1. #1 Question on Gravity 
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    Here is the scenario.

    You place a room 12X12X12 an body of mass that exerts a gravitational pull of exactly 1 m/s2. This room is completely enclosed containing gravity experts and access to all current technologies.

    You place another room with the exact same conditions on a spaceship accelerating at exactly 1 m/s2.

    Assuming ideal conditions, would the scientist be able to determine if they are on a body of mass or if they are in a spaceship that is accelerating?


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  3. #2 Re: Question on Gravity 
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    Quote Originally Posted by mjr150
    Here is the scenario.

    You place a room 12X12X12 an body of mass that exerts a gravitational pull of exactly 1 m/s2. This room is completely enclosed containing gravity experts and access to all current technologies.

    You place another room with the exact same conditions on a spaceship accelerating at exactly 1 m/s2.

    Assuming ideal conditions, would the scientist be able to determine if they are on a body of mass or if they are in a spaceship that is accelerating?
    Given sensitive enough equipment, yes. Put an atomic clock at the floor and ceiling of each room. Wait and see how much time difference accumulates between floor and ceiling of each room. It will be larger in the room on the spaceship.


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  4. #3 Re: Question on Gravity 
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    Quote Originally Posted by Janus
    Quote Originally Posted by mjr150
    Here is the scenario.

    You place a room 12X12X12 an body of mass that exerts a gravitational pull of exactly 1 m/s2. This room is completely enclosed containing gravity experts and access to all current technologies.

    You place another room with the exact same conditions on a spaceship accelerating at exactly 1 m/s2.

    Assuming ideal conditions, would the scientist be able to determine if they are on a body of mass or if they are in a spaceship that is accelerating?
    Given sensitive enough equipment, yes. Put an atomic clock at the floor and ceiling of each room. Wait and see how much time difference accumulates between floor and ceiling of each room. It will be larger in the room on the spaceship.
    That is basically the Pound-Rebka experiment.

    http://en.wikipedia.org/wiki/Pound%E...bka_experiment
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  5. #4  
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    To determine which room is on the body of mass and which one is on the spaceship, is communication between the rooms required?
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  6. #5  
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    Quote Originally Posted by mjr150
    To determine which room is on the body of mass and which one is on the spaceship, is communication between the rooms required?
    A window should suffice.
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  7. #6  
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    How odd. To the observer, the gravitational pull in either rooms are almost exactly the same, but they are two completely different phenomena.
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  8. #7  
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    Wouldn't the temporal variance be between the clocks in the stationary room on the mass? In the room on the accelerating spaceship the floor and ceiling are both uniformly translational.
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  9. #8  
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    Quote Originally Posted by mjr150
    To determine which room is on the body of mass and which one is on the spaceship, is communication between the rooms required?
    No. If you had a third clock placed halfway between the ceiling and the floor, its accumulated time compared to the other two would suffice to tell the difference.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  10. #9  
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    Quote Originally Posted by GiantEvil
    Wouldn't the temporal variance be between the clocks in the stationary room on the mass? In the room on the accelerating spaceship the floor and ceiling are both uniformly translational.
    The thing to keep in mind is that gravitational time dilation is related to difference in gravitational potential, not a difference in gravitational acceleration. It can be thought of of being related to the amount of work it would take to move a clock from one position in a gravity field to another.
    For example, if you had a uniform gravity field that did not change strength with altitude, it would still take work to lift something from one height in the field to a higher one. Thus, clocks sitting at those two height will run at different rates even though they are feeling the same force of gravity.

    Due to the equivalence principle, from the frame of the ship, the acceleration of the ship behaves just like such a uniform field, and a clock at the ceiling will run faster than one on the floor, even though both are accelerating at the same rate.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  11. #10  
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    Janus said;
    The thing to keep in mind is that gravitational time dilation is related to difference in gravitational potential,
    What a devilish subtlety. Thanks for the answer.

    Is the following statement from the Wiki correct?
    In general relativity, the gravitational potential is replaced by the metric tensor.
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  12. #11  
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    For example, if you had a uniform gravity field that did not change strength with altitude, it would still take work to lift something from one height in the field to a higher one. Thus, clocks sitting at those two height will run at different rates even though they are feeling the same force of gravity.
    What would that mean ? A uniform gravityfield ? It wouldn,t be a gravityfield then would it ? How would you know upside from downside ? What direction would it have if it doesn,t change in a particular direction ?

    So this scenario can be ruled out allready with the anthropic - principle. That,s what this principle is usefull for, in the negative sense, to rule out non realistic propositions such as this one.
    It,s not even imaginary or fantasy ..... just untoughtfull.
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  13. #12  
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    Suppose this scenario, instead of the room being on a spaceship, what if it's inside the edge of a large container (say the size of a football field) rotating on it's axis at a speed that creates pull of exactly 1 m/s2.

    When I think about it, there are 3 things right away that tells the observer that they are inside a container.

    Number one, the center of the floor is slightly closer to the axis which would mean that at the center, the pull would be a little less.

    Second, in the middle of the floor, the angle of pull would be straight down but at the edges of the room (2 of the 4 edges that are parallel with the axis), the pull would have a slight angle.

    Third, atomic clocks on the floor and the ceiling would go out of sync because the bottom of the floor has to cover more distance to go around the axis than the top.

    Anything else?
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  14. #13  
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    Quote Originally Posted by Ghrasp
    For example, if you had a uniform gravity field that did not change strength with altitude, it would still take work to lift something from one height in the field to a higher one. Thus, clocks sitting at those two height will run at different rates even though they are feeling the same force of gravity.
    What would that mean ? A uniform gravityfield ? It wouldn,t be a gravityfield then would it ? How would you know upside from downside ? What direction would it have if it doesn,t change in a particular direction ?

    So this scenario can be ruled out allready with the anthropic - principle. That,s what this principle is usefull for, in the negative sense, to rule out non realistic propositions such as this one.
    It,s not even imaginary or fantasy ..... just untoughtfull.
    A uniform gravity field would not change in strength from place to place, but this would not prevent it from having a direction.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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  15. #14  
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    Quote Originally Posted by mjr150
    Suppose this scenario, instead of the room being on a spaceship, what if it's inside the edge of a large container (say the size of a football field) rotating on it's axis at a speed that creates pull of exactly 1 m/s2.

    When I think about it, there are 3 things right away that tells the observer that they are inside a container.

    Number one, the center of the floor is slightly closer to the axis which would mean that at the center, the pull would be a little less.

    Second, in the middle of the floor, the angle of pull would be straight down but at the edges of the room (2 of the 4 edges that are parallel with the axis), the pull would have a slight angle.

    Third, atomic clocks on the floor and the ceiling would go out of sync because the bottom of the floor has to cover more distance to go around the axis than the top.

    Anything else?
    Rotations are significantly different than linear accelerations. Check the Coriolis effect. You wouldn't need clocks to tell it apart from the other scenarios.
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  16. #15  
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    Surely all this means is that the force exerted by gravity is indistinguishable from the force exerted by inertia, not that gravity and inertia are themselves identical.?

    You could charge your observer and the ceiling and floor appropriately (like the charged oil drop experiment where the drop is held in balance electrostatically against gravity) and perfectly offset the forces of gravity or inertia so the observers float in the room. It does not mean that charge and gravity and inertia are related, just that force is generically anonymous unless you can observe the source.
    You learn something new each day if you're not careful!
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  17. #16  
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    Quote Originally Posted by muppet
    Surely all this means is that the force exerted by gravity is indistinguishable from the force exerted by inertia, not that gravity and inertia are themselves identical.?

    You could charge your observer and the ceiling and floor appropriately (like the charged oil drop experiment where the drop is held in balance electrostatically against gravity) and perfectly offset the forces of gravity or inertia so the observers float in the room. It does not mean that charge and gravity and inertia are related, just that force is generically anonymous unless you can observe the source.
    Inertia is not a force.

    Gravity is indistinguishable from acceleration -- that is the "equivalence principle" of general relativity.
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  18. #17 gravity is created by? 
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    so gravity is created by the force of motion, and mass? What if kinetic energy/motion exerts the illusion of extra mass via gravity pulling it's mass/weight in a direction of time-space, where as velocity make as if it's heavier/dense/temporal increase of mass?

    kind of like a carnival ride, or roller coster... a increase of speed feels like a increase in gravity where as you can experience increased pressure/weight while being on the roller coaster.

    http://assets.nydailynews.com/img/20...nival-ride.jpg
    http://www.bubblingwithenergy.com.au...al_Roundup.jpg
    http://www.youtube.com/watch?v=iTpldgih4Vg
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  19. #18  
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by muppet
    Surely all this means is that the force exerted by gravity is indistinguishable from the force exerted by inertia, not that gravity and inertia are themselves identical.?

    You could charge your observer and the ceiling and floor appropriately (like the charged oil drop experiment where the drop is held in balance electrostatically against gravity) and perfectly offset the forces of gravity or inertia so the observers float in the room. It does not mean that charge and gravity and inertia are related, just that force is generically anonymous unless you can observe the source.
    Inertia is not a force.

    Gravity is indistinguishable from acceleration -- that is the "equivalence principle" of general relativity.
    but inertia could concentrate force, storing/building momentum? does inertia absorb force to like exert it as kinetic energy?
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  20. #19  
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    Quote Originally Posted by FuturePasTimeCE
    but inertia could concentrate force, storing/building momentum? does inertia absorb force to like exert it as kinetic energy?
    Inertia does not concentrate anything.

    You will need to study some mechanics. You will find that mechanics deals with mass, force, position, velocity, acceleration and quantities derived from them. Inertia is not on the list. Inertia is just the tendancy of a mass in motion to remain in that state unless a force changes it.
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