Movement and spacetime

matthewmussen · Jun 11, 2008

what would be the difference between the idea of being at point A and moving toward B in space-time; and being at point A and not moving and B not moving but the space-time between them moving and warping out of the way?

Fredrik · Jun 11, 2008

Didn't you just answer it yourself while asking the question? In the second case, space-time is curved. I assume that's not what you wanted to hear, since you said so yourself. What sort of answer are you looking for?

DaveC426913 · Jun 11, 2008

matthewmussen said: ↑

what would be the difference between the idea of being at point A and moving toward B in space-time; and being at point A and not moving and B not moving but the space-time between them moving and warping out of the way?

The difference is that is scenario 1, the subject would have the epxreince of moving from A to B, whereas in scenario 2, the subject would ahve the experience of not moving while the two points would move to coincide.

It would be easy to tell the difference merely by observing external reference points, such as distant stars.

MeJennifer · Jun 12, 2008

matthewmussen said: ↑

what would be the difference between the idea of being at point A and moving toward B in space-time; and being at point A and not moving and B not moving but the space-time between them moving and warping out of the way?

Particles have wordlines in spacetime and cross events which are points. In a curved spacetime worldlines can cross even if at some point the lines were parallel to each other.

But spacetime itself is fixed, it does not change.

matthewmussen · Jun 12, 2008

Fredrik said: ↑

Didn't you just answer it yourself while asking the question? In the second case, space-time is curved. I assume that's not what you wanted to hear, since you said so yourself. What sort of answer are you looking for?

would the idea change the way the motion, or lack thereof, of particles, is conceived; where as all particles are motionless, only space-time warps, muli-dimentionally. (i.e. - "my arm is not moving my fingers to my nose, but the amount of space time is curving or moving so there is less between them.) we would perceive in 3+1 dimentional space, as my arm is moving my fingers to my nose, using Einstein's perspective.

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DaveC426913 said: ↑

The difference is that is scenario 1, the subject would have the epxreince of moving from A to B, whereas in scenario 2, the subject would ahve the experience of not moving while the two points would move to coincide.

It would be easy to tell the difference merely by observing external reference points, such as distant stars.

fredrik, thank you sir, but this idea is theoretical. visible external reference other than point A and point B (the sight of my fingers and the feel of my nose) are not applicable in this theoretical scenario.

MeJennifer said: ↑

Particles have wordlines in spacetime and cross events which are points. In a curved spacetime worldlines can cross even if at some point the lines were parallel to each other.

But spacetime itself is fixed, it does not change.

Thank you MeJennifer, but your answer is based on part on Einstein's "Absolute Spacetime theory"in which this thread is written to challenge.

thanks to you all for helping

Fredrik · Jun 12, 2008

matthewmussen said: ↑

would the idea change the way the motion, or lack thereof, of particles, is conceived;

I guess you can say that it changes the meaning of velocity. Consider e.g. the case of galaxies moving away from each other due to the expansion of the universe. I'm quoting myself from another thread:

Fredrik said: ↑

When we say that galaxy A and galaxy B are moving apart with speed v, it really means that "the proper distance between A and B along the shortest possible path in the hypersurface of constant time coordinate changes by v units of length for each unit of time that we change the time coordinate".

This speed is something very different from the speed an object has in a local inertial frame. That's why it can be >c.

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