diagram one

[D H]

No. michel. You are misunderstanding the diagram. Yet another reason the diagram is a POS.

 

Time to take another look at a single frame of that movie. Imagine two stars, call them stars A and B. Star A is ten light years distant from us in the +x direction and has zero proper motion and zero radial velocity with respect to us. Star B is observed to be ten light years distant in the -x direction and is receding from us at 0.001 light speed.

 

Due to the null relative velocity between star A and us, star A is always ten light years away from us. If we plot star A's past, present, and future on our spacetime diagram the result will be a vertical line. The distance between this vertical line and the vertical time axis is 10 ly (or 10 ly scaled so that we can see it on our diagram).

 

Star B is moving away from us. The light we are seeing now was emitted by the star 10 years ago when it was 10 ly away from us. Right "now" it is 10.01 ly away. The light we see from the star now and where the star is "now" are two different spacetime events, and hence are two different points on the spacetime diagram. We can plot all of star B's past, present, and future on the spacetime diagram. In this case we get a line that is very slightly tilted from the vertical due to star B's 0.001 radial velocity with respect to us.

 

Suppose that unbeknownst to us, star A just "now" consumes the very last bit of its available hydrogen. The helium flash, and the star's subsequent expansion to being a red giant will be visible to us in ten years. Assume our spacetime diagram has godly powers, so the helium flash can be marked with an X and the expansion to red giant can be marked by coloring the future line of the star red. Now imagine a movie made out of a time-lapsed sequence of spacetime diagrams. Note that there is no motion here in the sense that we think of as motion. The star is perpetually 10 light years away from us. The movie would show motion. That X that marks the helium flash, along with the red line behind it, moves down the diagram as time marches on. Ten years from now that X will cross the diagonal line that represents our past line cone. At that point in time we will finally see the helium flash.

Edited February 11, 2011 by D H

[michel123456]

No. michel. You are misunderstanding the diagram. Yet another reason the diagram is a POS.

(...)

Thank you DH, it was a very clean explanation.

I see no divergence between your text and my understanding.

Your explanation is valid for a still frame of the diagram. When the diagram gets dynamic, it becomes weird, since "dynamic" means "time" and I suppose it is not so correct to have 2 times, one under "dynamic representation" and one under T axis.

 

 

 

(...)Ten years from now that X will cross the diagonal line that represents our past line cone. At that point in time we will finally see the helium flash.

 

So you agree that the right part of the diagram is not visible. (sorry but I would like a clear answer about the black zones).

 

Besides (a little bit out of subject) I don't understand how the Great Army progressed about 200 km in a single day between 6th & 7th of December, and gained 2000 soldiers in the meanwhile. (related to Minard's diagram).

Edited February 11, 2011 by michel123456