#### Rog

• 69
##### Re: Reasoning behind the Universal Accelerator
« Reply #80 on: January 06, 2022, 07:05:36 AM »
You are conflating proper acceleration and coordinate acceleration (unless you are suggesting that the jumper is subject to some outside force). An inertial observer (like our jumper) will have coordinate acceleration, but zero proper acceleration unless he is accelerated by a force. Proper acceleration is absolute, objective, physical and is the result of a force.  Coordinate acceleration is frame dependent, subjective and has no physical basis. Its simply an artifact of differing reference frames.  You can make it appear or disappear simply by changing your frame of reference. Proper acceleration is absolute and invariant.

From the perspective of the jumper he is inert and the ground could be moving up to meet him and from the perspective of an external observer, the jumper could be falling to the ground and the ground is stationary. Or maybe they are both moving towards one another. All of those perspectives are equally valid, subjectively, but only one of them is objectively correct.

Motion is relative, but it is not subjective.  If an asteroid appears in the sky and then meets the ground and no one observes it, the earth and the asteroid have still moved relative to one another.  That is an objective fact.  But no observer is required to establish that fact or how the earth and the asteroid moved relative to one another or to establish the cause . Those are also objective facts established solely by the forces acting on the asteroid and the earth. Those forces don’t change according to subjective perception.  And coordinate acceleration is a subjective perception.

At what velocity would an outside observer see the jumper moving? Keeping in mind you have acknowledged that initially the jumper would be inert, or have no velocity, and  without conflating proper acceleration and coordinate acceleration.  If no force has acted on the jumper, he has no proper  acceleration. If he has an accelerometer, it would read zero. No acceleration, no velocity. There is either a physical, material force acting on the jumper or there isn’t. If there isn’t he has no proper, or objective, physical acceleration.  Perception doesn’t give rise to forces that don’t exist.

#### Rog

• 69
##### Re: Reasoning behind the Universal Accelerator
« Reply #81 on: January 06, 2022, 07:52:12 AM »
Quote
Velocity quite literally only has a definition as one body WITH RESPECT TO another body.
Thanks for the insight. Can you please just answer a simple question?

If B is our jumper, then
At what velocity would an outside observer see the jumper moving? Keeping in mind Pete  acknowledged that initially the jumper wouldn’t have any velocity and  without conflating proper acceleration and coordinate acceleration, like he has done. Taking that into consideration, enter whatever you think the value Vb should be.

Then can you tell me what you think the value of Va should be.? That would be the speed of the earth..

Notice that it doesn’t say “speed of the secondary reference frame relative to B .  Just the speed of A. Think of it as if the earth had a speedometer on it.  A car’s speedometer reads its velocity WRT to the road. And it reads it within the car’s reference frame. If the earth had a speedometer and it was measuring its velocity relative to something else (pick whatever “else” you want), what would it read? Enter whatever value you think Va should be. (doesn't have to be exact, just an approximation)

That will give us what you believe is the relative velocity between A and B, or our jumper and the earth.