Einsteins time dialation

[Luke]

hey. so i heard someone say that einstein considered gravity not to be a force but actually time dialation. ive been thinking over it for a while and i think im on the right track but there still seems to be a lot of holes in my logic so can someone please clear this up for me.

 

if you let go of a ball on earth it will fall to the ground.

if you let go of a ball in space, inside a rocket that is travelling at a constant velocity, the ball will not move.

if you let go of a ball in space, inside a rocket that is accelerating it will appear to "fall" or move in the opposite direction of the acceleration of the rocket.

 

so from this einstein "linked"(linked is the incorrect word here, but bear with me) acceleration to the concept of gravity.

 

acceleration is equal to change in velocity over change in time. A= deltaV/deltaT

 

and with acceleration we see an increase in velocity, but one of the fundamental priniciles of physics implies that without a force (einstein was considering what the consequences would be if gravity was not a force) present we cannot have an increase in velocity. hence the only thing left in the equation to allow an increase in velocity without a force being present is time. therefore in order to have an acceleration time had to dialate.

and what caused this time dialation was the presence of a mass.

 

ok so theres my thinking on the concept, and it may be a bunch of poppycock, but please clear this up for me.

[Jay-qu]

Well for one thing Einstein did away with gravity as a 'force', gravitational effects arise in GR by objects travelling along 'straight' lines (geodesics). So the concept of a force does not have to be invoked. You are correct that time dilates, but time dilation is not enough to make an object move wrt to another.

[Little Bang]

You are correct that time dilates, but time dilation is not enough to make an object move wrt to another.

 

And how do you know this to be a truth?

[Jay-qu]

Just try to formulate a legitimate mathematical expression that can do otherwise. Dilations are like a change of scale, they do not boost a reference frame.

[Qfwfq]

The trick is that the scale changes locally i. e. depends on radius in the Schwartzschild solution. The missing bit is that the length scale in the radial direction also changes, as the reciprocal.

 

It does seem odd, very odd, that this can cause an initially still particle to commence motion. It's a bit less odd if you remember that, in spacetime, "still" really means that its 4-velocity has exactly the direction of the time axis so what's really happening is that this direction changes along the worldline, the geodesic is curved. IOW a line for which all points have the same coordinates except [imath]t[/imath] is not a geodesic so it can't describe the trajectory of a particle.