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killshot
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Assuming a universe with only space and two motionless masses, would the two masses start to move toward each other?
I don't think that the OP meant "space" to mean only the spatial dimensions. I rather think that they meant "nothing but empty space and two masses".chrisphd said:No, because without time, no motion can occur.
Nonsense. There are several measurements that observers on either mass could make that would show that the masses were not only accelerating, but accelerating at an increasing rate.chrisphd said:I see. In that case we have a more tricky question on our hands.
So firstly i'll assume the particles are "point" particles. Then there exists only 1 "ruler", if you like, in space in which distance can be measured with. That is, the distance between these two particles is the only possible measuring device.
This to me implies that describing the particle's as moving towards one another is meaningless, because there is no concievable way to infact identify that the distance between the particles is decreasing.
What analogy can I use to make my picture clear. Immagine that tommorrow, everything in the universe is exactly half the size it was today. That is, I am now half my height, and the ruler I use to measure my height is also half the height of what it was today, and the expectation value of the radius of an electron around the universe is halved. Clearly in this situation, because everything in the universe has halved, there is absolutely no way to tell at all that anything has changed.
In fact, because in the previous paragraph, the consequences of everything in the universe halving in size, is identical to the theory that nothing in the universe changed at all, we would use occam's razor to say that in fact nothing in the universe changed at all.
Similarly in your example of a two particle universe. Whether the distance between the particles is decreasing or staying the same, because the only distance unit in your scenario is the distance between the two particles, these scenarios would be equivalent.
Therefore, it is reasonable to suggest that no motion occurs.
chrisphd said:A body in free fall cannot detect that it is in free fall.
An analogy is that when you stand in an elevator that is free falling, the laws of physics are such that the elevator is approximately an inertial reference frame.
Free fall (or more precisely the equivalence principle) only applies when the acceleration is constant, which is not the case here.chrisphd said:A body in free fall cannot detect that it is in free fall.
An analogy is that when you stand in an elevator that is free falling, the laws of physics are such that the elevator is approximately an inertial reference frame.
So technically, the equivalence principle doesn't apply here. However, the gravitational field can be approximated as uniform near the Earth's surface. The correction is so small that the elevator can be approximated as free-falling. Once once again, the equivalence principle only applies in a uniform field.chrisphd said:In fact, the force acting on the free falling elevator is also not constant because as the elevator gets closer to the Earth, the acceleration will increase.
chrisphd said:Please read my previous post. It explains why your intuitive view is flawed. The "jerk" you feel in an elevator is due to the additional force between the elevator and the human. The "jerk" is not experienced when it is an external force that is causing the changing acceleration. Do read my previous post.
Any object of non-zero dimensions in a non-uniform gravitational field would experience a tidal force, which is most certainly detectable.My previous thought experiment in the spaceship however was showing that there is no interaction between the human and the spaceship even if their accelerations were changing, to show why born2bwire's intuitive argument was wrong. I did neglect the subtleties of tidal effects.
None the less, if this is the measurement you are going to use, it will not apply to the "point" particles i was referring to in my original post.
end result is that there will be change in weight which is detectableRead the following carefully born2bwire, because i'll not exlpain this part again.
Actually in my spaceship thought experiment, the human in the spaceship at all times will appear to be weightless. This is because his velocity will always be equal to the velocity of the spaceship at any time, since they are both being accelerated at the same rate at any time. Therefore there is no interaction between the spaceship and the human. We can therefore neglect the spaceship from the picture.
We now have a picture of just a human moving towards this planet. Now the human is comprised of atoms. Each atom in the human is also accelerating at the same rate as any other atom at any time, and thus will all have the same velocities as each other at any time. This means there will be no "intra-body" forces, if you like, to detect the changing acceleration. The picture can then be further simplified, because there is no interaction between the atoms, we can remove all atoms from the picture except for one atom.
We can continue simplifying the picture indefnitely, and there will be no way to determine the changing acceleration.
Of course this neglects tidal effects, as ultimately the discussion should converge to the point particles mentioned in my earliest post.
chrisphd said:I see. In that case we have a more tricky question on our hands.
So firstly i'll assume the particles are "point" particles. Then there exists only 1 "ruler", if you like, in space in which distance can be measured with. That is, the distance between these two particles is the only possible measuring device.
This to me implies that describing the particle's as moving towards one another is meaningless, because there is no concievable way to infact identify that the distance between the particles is decreasing.
What analogy can I use to make my picture clear. Immagine that tommorrow, everything in the universe is exactly half the size it was today. That is, I am now half my height, and the ruler I use to measure my height is also half the height of what it was today, and the expectation value of the radius of an electron around the universe is halved. Clearly in this situation, because everything in the universe has halved, there is absolutely no way to tell at all that anything has changed.
In fact, because in the previous paragraph, the consequences of everything in the universe halving in size, is identical to the theory that nothing in the universe changed at all, we would use occam's razor to say that in fact nothing in the universe changed at all.
Similarly in your example of a two particle universe. Whether the distance between the particles is decreasing or staying the same, because the only distance unit in your scenario is the distance between the two particles, these scenarios would be equivalent.
Therefore, it is reasonable to suggest that no motion occurs.
Actually in my spaceship thought experiment, the human in the spaceship at all times will appear to be weightless. This is because his velocity will always be equal to the velocity of the spaceship at any time, since they are both being accelerated at the same rate at any time. Therefore there is no interaction between the spaceship and the human. We can therefore neglect the spaceship from the picture.
killshot said:Assuming a universe with only space and two motionless masses, would the two masses start to move toward each other?
Evolver said:Matter attracts matter with a gravitational fall off rate of the inverse of r2 or 1/r2.
"The gravitation attraction force between two point masses is directly proportional to the product of their masses and inversely proportional to the square of their separation distance. The force is always attractive and acts along the line joining them."
The only things that affect gravitation are mass and distance. Since two objects in space would fit those prerequisites... they would experience gravitation and would attract towards each other based on their distance to one another. This is also assuming that this hypothetical "space" we are talking about would not some how interfere with the masses via quantum fluctuations or virtual particles.
Since our understanding of what causes gravity is limited, it very much may have to do with quantum interactions of this sort, and this "space" you have deemed the medium for these interactions of mass may very well be a moot hypothetical argument.
The problem with this question is that is attempts to harness our limited understanding of gravity (they have never even seen proof of the theorized graviton force carrying particle). So to say what causes gravity itself is unknown at this point. Your question of putting two objects of mass in space with nothing else may very well defy some law of gravitation that we are currently unaware of.
But to the best of my knowledge, with our modern view of gravity... yes, as long as they have mass they will affect one another proportianally to the distance between them.
killshot said:Concurring that this all hypothetical;
Assuming the masses can feel(?) the curvature of the space.
Could G be a potential energy when they are motionless and therefore the masses remain motionless? Then if one mass is set in motion in any direction would the two masses eventually come together?
Could motion some how be part of the gravitation mystery?
I know this begs the question about G as a constant (but that maybe a matter of scale).
He did not mention any commentators either.Dadface said:The OP described a universe with only space and two masses but it seems that people have been adding to this universe.Where, for example, did the observers come from?
Dadface said:The OP described a universe with only space and two masses but it seems that people have been adding to this universe.Where, for example, did the observers come from?
Buckleymanor said:He did not mention any commentators either.
Maybe I should shut up.![]()
Good question mankillshot said:Assuming a universe with only space and two motionless masses, would the two masses start to move toward each other?
The gravitational attraction of motionless masses is the force of attraction between two objects with mass, even when they are not in motion. This force is described by Newton's law of gravitation, which states that the force of gravity between two objects is directly proportional to the product of their masses and inversely proportional to the square of the distance between them.
The gravitational attraction of motionless masses is responsible for keeping objects in orbit around larger objects, such as planets around the sun. It also affects the trajectory of objects in free fall, causing them to accelerate towards the center of the larger object.
No, the gravitational attraction of motionless masses depends on the mass of the objects and the distance between them. Objects with larger masses will have a stronger gravitational pull, and objects that are closer together will have a stronger gravitational attraction.
No, the gravitational attraction between two objects cannot be shielded or canceled out. However, the strength of the gravitational force can be reduced by increasing the distance between the objects or by decreasing the mass of one or both objects.
Einstein's theory of relativity explains that gravity is not a force between masses, but rather a curvature of spacetime caused by the presence of mass. This means that the gravitational attraction of motionless masses is a result of the curvature of spacetime and not a direct force between the objects.