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Technology, Science & Alt Science / Re: Clarification on Gravity
« on: September 24, 2008, 12:44:07 AM »Orbit is not local.
What would be your definition of 'local'? I was merely referring to making measurements at a spacetime point to avoid any arguments over tidal forces and so on.
No, I am still saying it's wrong to say someone on the earths surface has a net acceleration of anything other than zero. I am also saying it's wrong to say the earth accelerates up to meet a skydiver.
Sadly both of those positions are incorrect, for reasons we have already agreed upon. It may not make sense to someone standing on the ground to tell them that they are accelerating upwards, but that is exactly what is happening. They are constantly deviating from a geodesic in space-time because (let's forget the Earth's own motion for a moment) that path goes down through the ground beneath them.
It is not that the geodesic is a bungee cord pulling them down, but more that it is the path they would follow if the Earth wasn't in the way. There is no difference between that case and them hovering above the ground in a rocket, or flying in a rocket in deep space. All 3 observers measure the same force and all of them agree they are accelerating upwards.
In that sense, the skydiver and the person on the ground could agree that the Earth accelerated up to meet them, because that is what the only sensible coordinate system says happened. If you choose to constantly reset the ground to be 'zero' in your co-ordinate system then of course you will get a different result, but that co-ordinate system is a non-inertial one.
You put to objects with gravitational fields next to each other they will accelerate to each other. Gravitation is word that people use to define the reason why objects attract each other. This attraction causes acceleration.
To you, the external observer, yes - the objects appear to accelerate towards each other because you have defined a co-ordinate system according to your own frame (which may or may not be inertial). If you formulate your observations in an inertial frame you will see that neither object accelerated, but that the distance between them decreased because they were both at rest on geodesics. This is not the same as acceleration, which would have caused them to deviate from those geodesics.
He did not state the obvious.
I would agree - he stated something profound and made it obvious, which is why he was a genius.
Einstein did not want to say 5=5, he wanted to say 1+4 =2+3. He took different ideas and related them, he did not take the something and relate it to itself.
Now once again, I know what the equivalence principle is and I know it doesn't take something and relate it to itself. You have to get over the fact that two things can be equal and not be the same thing.
The EP tells us how to treat co-ordinates and forces in a sensible way. The implication is that the only sensible co-ordinate system is an inertial one, in which case the observer on the surface of the Earth is accelerating upwards always. Of course the Earth isn't exploding outwards, and that is because the co-ordinate system we choose says that as it accelerates outwards, we move our co-ordinates inwards along geodesics.
