# Do contracting objects show red shift?

by Yashbhatt
Tags: contracting, objects, shift
Thanks
P: 3,674
 Quote by Drakkith True, but I'm not talking about the intersection of the circle and the sphere. Just the way the edges of the sphere (aka the "horizon") get closer to the center.
No, I have to take what I said back. Now that I see what you're saying, you're right.

If the distance between me and the central point of the contracting object is ##R## and at some moment the radius of the contracting object is ##r##, then the points on the edge of the disk of the object will be at a distance ##\sqrt{R^2+r^2}## from me, and this will become smaller (the point is moving towards me) as ##r## diminishes with the contraction.

Of course this effect is well and thoroughly negligible if ##R\gt\gt{r}##, whereas the effect on the points that are directly on the line between the two centers is independent of ##R##.
P: 169
 Quote by Drakkith Consider a circle. Every point on the circle is equidistant from the center. Now imagine you are at the center of a circle which has a radius starting at you and ending at the center point of the sphere. Since the circle curves towards you, it passes through the edges of the sphere closer to you than a straight line drawn from the edges of the sphere through the center. So when the edges of the sphere contract, they get closer to the center point and thereby closer to you.
Sorry, but I still din't get you. If any point on the sphere is moving towards us we won't be able to observe it since it will be on the other side of the sphere.
Mentor
P: 11,839
 Quote by Yashbhatt Sorry, but I still din't get you. If any point on the sphere is moving towards us we won't be able to observe it since it will be on the other side of the sphere.
I'm sorry, I'm not sure how to explain it any better than I already have. Maybe someone else can?
 P: 1,857 lets try this your observer C on earth, take the image you posted on post number 9. with a and b. Now that circle you posted is 3D, a planet. So as B moves towards point A on that 3D image. the distance between B and C decreases, hence its moving towards you=blueshift.
 P: 1,857 In Drakkiths example observer A is the center of a circle, B is a point on the surface. As the surface contracts B moves towards A =blueshift
 Sci Advisor PF Gold P: 9,435 It depends on the observer. I remain convinced there is no privileged observer in the universe.
P: 1,857
 Quote by Chronos It depends on the observer. I remain convinced there is no privileged observer in the universe.
were setting the observer points in our examples, so privileged observer isn't even an issue, not sure where your coming from in this thread?

"It depends on the observer" 100% correct

"I remain convinced there is no privileged observer in the universe" 100% unrelated to the context and examples in this thread
 P: 169 @Mordred That's exactly what I was thinking. I din't know you were talking about rotating objects. Got it now.

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