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grav
2009-Aug-07, 01:28 AM
Let's say we have a cube in space with eight light sources, one on each corner, each with the same luminosity. If we place a stationary observer in the center of the cube, he will feel no force from the light pressure that acts upon him. But now let's say he is travelling from the center of one side to the center of the other one on the opposite side. When he reaches the center of the cube, aberration from each of the light sources will push their apparent positions around toward the front of the observer from his perspective and he will feel a force of drag from the light pressure slowing him down from the front, is that right?

Now let's say, instead of light sources, we place eight equal masses on each of the corners. When stationary in the center, he feels no force of drag. When moving from the center of one side to the other, GR says that the gravity field of each of the masses will be centered upon the current position of each of the masses, so no aberration will take place. From the perspective of the observer, the distances will be length contracted by the same amount forward and back, so the actual positions of the masses in each direction is the same, and the gravity should be the same from either direction if there is no aberration. So does that mean that the observer will feel no force of drag, or no gravity slowing him down, even while travelling at a relative speed to the cube at its center?

grav
2009-Aug-07, 01:36 AM
Since "when the observer reaches the center" would mean a different thing for the observer than another observer stationary to the cube, let's just simplify things and only observe from the perspective of an observer that is stationary with the cube. When the moving observer reaches the center of the cube according to the cube observer, will he be seen to be slowing down when at the center of the cube with the lights? Will he be seen by the cube observer to be slowing down at all with the masses placed at the corners when the moving observer is at the center?

67champ
2009-Aug-07, 01:48 AM
Wow, that is an interesting question! I have no idea, but thanks for making my brain try to work for a minute or two... :-)

dt

grav
2009-Aug-07, 02:34 AM
Wow, that is an interesting question! I have no idea, but thanks for making my brain try to work for a minute or two... :-)

dt:)

korjik
2009-Aug-07, 08:32 PM
What you describe is called dynamic friction.

Basically when a large mass object is moving through a sea of evenly distributed low mass objects, the large mass will lose kinetic energy to the low mass objects due only to how the two interact with one another.

grav
2009-Aug-07, 09:58 PM
What you describe is called dynamic friction.

Basically when a large mass object is moving through a sea of evenly distributed low mass objects, the large mass will lose kinetic energy to the low mass objects due only to how the two interact with one another.Thanks, korjik. I believe that would be friction that occurs with collision of the bodies, though, like moving through a fluid. I want to know what would be the case for gravity without any physical interaction otherwise.

grav
2009-Aug-07, 10:04 PM
Let me try this real quick. If an observer were travelling from one mass to another, then according to an observer that is stationary to the masses, will the moving observer be slowing down at all when he reaches the center of the two masses, or will there be no force of drag at that point? In other words, does the relative speed of the observer to a mass produce a difference in the acceleration of gravity as it is received from the same distance when travelling directly toward or away from the mass, or is it the same in either direction?

StupendousMan
2009-Aug-08, 01:07 AM
Thanks, korjik. I believe that would be friction that occurs with collision of the bodies, though, like moving through a fluid. I want to know what would be the case for gravity without any physical interaction otherwise.

Dynamical friction occurs due to gravity alone, with no physical contact between the bodies. It causes massive stars to sink to the centers of globular clusters, for example, and small galaxies to sink to the centers of large ones during mergers.

grav
2009-Aug-08, 02:02 AM
Dynamical friction occurs due to gravity alone, with no physical contact between the bodies. It causes massive stars to sink to the centers of globular clusters, for example, and small galaxies to sink to the centers of large ones during mergers.Oh, okay, thanks. I looked it up on Wiki after korjik posted and it said it was the same as kinetic friction on a surface or through a fluid. In any case, I can see such a friction acting in an unbalanced system, sure, but what about at the moment of equilibrium when centered between masses? The aberration of light pressure due to the relative motion would slow down the moving observer at that point, I believe, but what about with gravity that has no such aberration? In order for orbits to remain stable, the way the planets receive that gravity from other planets and the sun must come from at least very close to the actual positions of the other masses, which would be exactly the same position if the masses remained inertial, I think, and with very nearly or exactly the same strength as well, without any regard to the relative motions otherwise, is that right?

korjik
2009-Aug-08, 07:04 AM
Oh, okay, thanks. I looked it up on Wiki after korjik posted and it said it was the same as kinetic friction on a surface or through a fluid. In any case, I can see such a friction acting in an unbalanced system, sure, but what about at the moment of equilibrium when centered between masses? The aberration of light pressure due to the relative motion would slow down the moving observer at that point, I believe, but what about with gravity that has no such aberration? In order for orbits to remain stable, the way the planets receive that gravity from other planets and the sun must come from at least very close to the actual positions of the other masses, which would be exactly the same position if the masses remained inertial, I think, and with very nearly or exactly the same strength as well, without any regard to the relative motions otherwise, is that right?

I poked around wiki for a while but couldnt find a good description. They may not call it dynamic friction, but it isnt kinetic friction.

I am not sure what you are going for here. The dynamic friction due to photons would be an extremely tiny force. I doubt it would be measureable over the lifetime of the planet. The bulk of the force would come from the sun, and it would be repulsive, since you would have to transform into the photon frame to see the effect. It wouldnt suprise me if the effects on the Earth due to each day's solar wind was a much larger total force than the total photon dynamic friction over the entire lifetime of the Sun.

astromark
2009-Aug-08, 09:11 AM
What are you talking about please... I see the phrase light pressure,and the glare of confusion overwhelms me... when and where did light become a force ?
I do understand kinetic friction. I do understand gravity force. I do not understand light having any force... what have I missed ?

Amber Robot
2009-Aug-08, 11:19 AM
Light has momentum. So, when a photon hits you and is either absorbed or reflected, there's a transfer of momentum. This is equivalent to a force. F=dp/dt.

trinitree88
2009-Aug-08, 03:21 PM
Light has momentum. So, when a photon hits you and is either absorbed or reflected, there's a transfer of momentum. This is equivalent to a force. F=dp/dt.

Amber Robot: Identically the same for neutrinos, except that they are absorbed or forward scattered. No neutrino is back-scattered, or reflected. Still..F=dp/dt pete