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  4. Is spacetime always understood in relation to the speed of light (em radiation)?
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Is spacetime always understood in relation to the speed of light (em radiation)?

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Offline jeffreyH

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Re: Is spacetime always understood in relation to the speed of light (em radiation)?
« Reply #20 on: 22/05/2019 19:30:02 »
Objects free fall at the same rate in a gravitational field if starting from a stationary position at the same altitude at the same time. That is, objects uniformly accelerating through space. I didn't think I would have to be that pedantic.

Ok I'll give it to you on the inertia versus inertial motion. Did you miss the point on purpose? They both exhibit types of uniform motion. Therefore no force is felt to be present in both situations. They both act like inertial frames but in radically different ways.

Your accelerometer won't show a force because all parts of it move uniformly through space under the influence of the force of gravity.
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Offline jeffreyH

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Re: Is spacetime always understood in relation to the speed of light (em radiation)?
« Reply #21 on: 22/05/2019 19:35:01 »
BTW The above implies that gravity can only be fully understood in terms of quantum mechanics. Or maybe quantum field theory. That we think of gravity in only macroscopic terms is our oversight.
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Re: Is spacetime always understood in relation to the speed of light (em radiation)?
« Reply #22 on: 22/05/2019 19:52:42 »
Quote from: jeffreyH on 22/05/2019 19:30:02
Objects free fall at the same rate in a gravitational field if starting from a stationary position at the same altitude at the same time. That is, objects uniformly accelerating through space. I didn't think I would have to be that pedantic.
That would be identical acceleration, not uniform acceleration.  Uniform means a constant acceleration rate in the same direction, such as 1G forward for a period of time.  A thing in freefall due to gravity is rarely uniform acceleration since the acceleration changes all the time.  It can be done, but you don't see it much or at all in nature.

That said, objects with different mass will take a different trajectory, all else being equal.  It makes little difference until the mass of the 'falling' thing begins to have mass on the same order as the thing it orbits.  So the moon takes a different path than would a pebble with the same altitude and velocity, mostly because the moon drags Earth around significantly and the pebble doesn't.

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Ok I'll give it to you on the inertia versus inertial motion. Did you miss the point on purpose? They both exhibit types of uniform motion. Therefore no force is felt to be present in both situations. They both act like inertial frames but in radically different ways.
But the point being discussed concerned gravity being a force or geometery.  Inertial motion behaves like no force acting upon it, thus gravity (acting on a point mass) should also behave like it isn't a force.  That's why Einstein pushed the bent-spacetime interpretation of gravity, not the 4th force in need of unification with the other 3.
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Re: Is spacetime always understood in relation to the speed of light (em radiation)?
« Reply #23 on: 22/05/2019 20:05:50 »
Did pedantry die and you felt obliged to give it CPR? Ok not uniform acceleration but falling in lock step all the way to the ground.

No propagation of perceived force since all atoms are affected identically in the absence of extreme tidal forces. Also, mimicking an inertial frame, although in reality not an inertial frame since the value of potential of the field is not constant along the objects worldline.
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Re: Is spacetime always understood in relation to the speed of light (em radiation)?
« Reply #24 on: 22/05/2019 20:25:33 »
Quote from: jeffreyH on 22/05/2019 20:05:50
mimicking an inertial frame, although in reality not an inertial frame since the value of potential of the field is not constant along the objects worldline.
The potential of a gravitational field is not locally detectable, so that part is quite identical to the inertial object.

That makes for an interesting question to illustrate it.  What is the gravitational potential of a KG rock at sea level?  It's negative, but how much?  Feel free to use non-local methods to measure it, but it still isn't intuitive.  Sure, it has a computable energy due to Earth gravity (I get -62 megajoules), but Earth is not all there is.
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