- #36
Nick O
- 158
- 8
Ah, I guess I had no idea what kind of output an accelerometer produces. I thought that 0 would also be the "rest" value, resulting in no net change when falling.
I never saw Einstein make such a claim about gravitational fields. To the contrary, according to Einstein he was "struck by the fact that the force of gravitation possesses a fundamental property, which distinguishes it from electro-magnetic forces. [..] All bodies fall in a gravitational field with the same acceleration"inertiaforce said:According to this video, a bowling ball and a feather fall at the same rate because according to Einstein, they aren't falling:
[..]
The video actually said that Einstein's view was that no force was acting on the feather or on the ball.
Nugatory said:Let's assume for simplicity that the two balls have the same mass.
Using Newton's model, the forces between each ball and the Earth would be of equal magnitude and opposite direction, so the Earth would experience no net force and the balls would be drawn towards the Earth according to Newton's ##F=ma## by the forces acting on the balls.
Using Einstein's model, the geodesic paths of the two balls and the Earth would intersect. The curvature of spacetime bends the paths of the two balls towards the path of the earth. Eventually they collide.
rajeshmarndi said:I didn't get the above answer, when we say it is the Earth that moves towards the ball, the answer given above, is that the balls followed their curved path towards the earth.
rajeshmarndi said:Suppose two balls were "dropped" at the same time on opposite sides of the Earth. Which way would the Earth fall?
LitleBang said:Interesting how people can be experts about gravity without being able to tell us the mechanism of gravity.
LitleBang said:In my opinion when he said they were not falling he meant that in their frame of reference the Earth was moving toward them.
So if a quantum theory of gravity doesn't happen should we look for some other method or just throw up our hands?stevendaryl said:General Relativity views curved spacetime as the most
LitleBang said:So if a quantum theory of gravity doesn't happen should we look for some other method or just throw up our hands?
1) Movement is relative. The Earth's surface moves in the frame of the ball.rajeshmarndi said:I didn't get the above answer, when we say it is the Earth that moves towards the ball, the answer given above, is that the balls followed their curved path towards the earth.
stevendaryl said:How is it possible for opposite sides of the Earth to be accelerating in different directions, without the Earth expanding? I think it's helpful to think of a lower-dimensional analog. Think of spacetime as the surface of a globe, and think of the time axis as being measured North-South, while spatial distances are measured East-West. Suppose you have two objects on the equator, a distance of 10 meters apart. As time moves on, those objects move north along lines of longitude. If there were no forces acting on those objects, they would be getting closer and closer together, until they collide at the North Pole. It requires a force to keep the two objects 10 meters apart as they move North.
This video shows the effect of acceleration in flat space time, far from gravity sources. What they call "gravity" is a inertial force in an accelerated frame.inertiaforce said:Guys, watch this video from 12:00 to 12:46.
No. That is an inertial frame, so there is no inertial force there. The Earth surface accelerates in their direction, because of the EM-repulsion from below, which is a real force, that causes proper acceleration.inertiaforce said:Now imagine this explanation from the frame of reference of the ball and feather. From the ball and feather's point of view, could it be said that there is a gravitational field causing the Earth to be accelerated in their direction?
inertiaforce said:Guys, watch this video from 12:00 to 12:46. Now imagine this explanation from the frame of reference of the ball and feather. From the ball and feather's point of view, could it be said that there is a gravitational field causing the Earth to be accelerated in their direction?
stevendaryl said:How is it possible for opposite sides of the Earth to be accelerating in different directions, without the Earth expanding?
stevendaryl said:Suppose you have two objects on the equator, a distance of 10 meters apart. As time moves on, those objects move north along lines of longitude. If there were no forces acting on those objects, they would be getting closer and closer together, until they collide at the North Pole. It requires a force to keep the two objects 10 meters apart as they move North.
rajeshmarndi said:I didn't understand "It requires a force to keep the two objects 10 meters apart as they move North." How does this correspondence to acceleration of Earth surface in different direction.
What is the source of Earth acceleration in all direction.
LitleBang said:Interesting how people can be experts about gravity without being able to tell us the mechanism of gravity.
What is the term referred to the above phenomenon.stevendaryl said:Pressure. The material that the Earth is made out of is under pressure, and that pressure exerts an outward force on all points on the surface of the Earth.
stevendaryl said: ↑georgir said:As to "What is the term referred to the above phenomenon.", can you say which phenomenon that is?
rajeshmarndi said:What is the term referred to the above phenomenon.
So, the pressure of the material, which actually causes experiences of the weight?
But we studied, we feel weight because of Earth gravity. It seems gravity and weight are two different thing.
I understand due to gravity or due to curvature of space, as each points on Earth follow this curved path, they build internal pressure, which exert on our body and we feel it as weight.
But as commonly understand, our body is pulled to the Earth surface and it exert pressure on the surface, in return the surface exert equal opposite force on us.
inertiaforce said:According to this video, a bowling ball and a feather fall at the same rate because according to Einstein, they aren't falling:
https://testtube.com/dnews/which-falls-faster-a-feather-or-a-bowling-ball/?utm_source=FB&utm_medium=DNews&utm_campaign=DNewsSocial
What does this mean exactly? The Earth comes up to the ball and the feather?
PeroK said:Suppose two balls were "dropped" at the same time on opposite sides of the Earth. Which way would the Earth fall?
Stevendaryl said nothing about the "Earth expanding", which is exactly the wrong way to explain this, and leads people to conclude that GR is a bunch of nonsense.Wes Tausend said:The trick is to temporarily imagine Earth expanding as stevendaryl briefly mentioned earlier.
stevendaryl said:Think of spacetime as the surface of a globe, and think of the time axis as being measured North-South, while spatial distances are measured East-West. Suppose you have two objects on the equator, a distance of 10 meters apart. As time moves on, those objects move north along lines of longitude. If there were no forces acting on those objects, they would be getting closer and closer together, until they collide at the North Pole. It requires a force to keep the two objects 10 meters apart as they move North.
I agree Stevendaryl does a good job explaining an aspect of gravity.A.T. said:Stevendaryl said nothing about the "Earth expanding", which is exactly the wrong way to explain this, and leads people to conclude that GR is a bunch of nonsense.
He says that the surface accelerates away from the center. But that doesn't imply movement away from the center (expansion). He also gives a much better analogy:
stevendaryl said:There is a sense in which it is correct to say that the surface of the Earth is accelerating upward: there is an upward force on the surface of the Earth, and this force causes the surface to accelerate upward relative to a freefall path (or geodesic). All parts of the Earth are accelerating upward, in this sense (I wouldn't call it "falling" upward, falling means traveling in the absence of any forces holding you up, and that is not the case with the surface of the Earth; the surface of the Earth is held up by contact forces from the rocks below). This notion of acceleration, relative to a geodesic, or freefall path, is local, so different spots on the Earth are accelerating in different directions.
How is it possible for opposite sides of the Earth to be accelerating in different directions, without the Earth expanding? I think it's helpful to think of a lower-dimensional analog. Think of spacetime as the surface of a globe, and think of the time axis as being measured North-South, while spatial distances are measured East-West. Suppose you have two objects on the equator, a distance of 10 meters apart. As time moves on, those objects move north along lines of longitude. If there were no forces acting on those objects, they would be getting closer and closer together, until they collide at the North Pole. It requires a force to keep the two objects 10 meters apart as they move North.
But the definition of inertial frame depends on which theory of gravitation you use. In GR "inertial frames" are just local approximations and there are no valid global inertial frames, when gravity sources are around.PeroK said:...inertial frame ... whatever your theory of gravitation...
In GR the free falling ball does remain at rest in an local inertial frame, and so does the center of the Earth.PeroK said:...then that ball would remain at rest in the inertial frame we have established...
Unfortunately I have seen many concluding that GR is nonsense after being exposed to such non-sequitur explanations, and I cannot even blame them. We know the Earth doesn't expand, so assuming that it does explains nothing.Wes Tausend said:I think most people know the difference beween reality and a thought experiment, so no one need be "mislead", or think GR non-sense.
PeroK said:If the Earth moved to the first ball, then that ball would remain at rest in the inertial frame we have established.
PeroK said:We could also establish an inertial frame in which the Earth is at rest
A.T. said:But the definition of inertial frame depends on which theory of gravitation you use. In GR "inertial frames" are just local approximations and there are no valid global inertial frames, when gravity sources are around.
In GR the free falling ball does remain at rest in an local inertial frame, and so does the center of the Earth.
PeroK said:from my naive perspective, one should be able experimentally to conclude that the Earth orbits the sun and not vice versa.
PeroK said:He concluded by observation alone that the planets orbited the sun.
Then you know more than Poincaré. He wasn't so absolutely sure we would be able to know. Poincaré eventually came within an inch of solving SR before Einstein and I admire him too.A.T. said:Unfortunately I have seen many concluding that GR is nonsense after being exposed to such non-sequitur explanations, and I cannot even blame them. We know the Earth doesn't expand, so assuming that it does explains nothing.
From bad analogy to the worst analogy.Wes Tausend said:Pretty much like the bowling ball stretching the blanket close to large mass.
PeterDonis said:More precisely, the solar system, to a very good approximation, can be described as an isolated system of matter surrounded by empty space, with a definite center of mass, and that the object whose trajectory is closest by far to the trajectory of that center of mass is the Sun. We describe this informally as the planets orbiting the Sun (though a more precise description would be that the planets and the Sun all orbit their common center of mass). ...