Open thread, Apr. 10 - Apr. 16, 2017

MrMind

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If it's worth saying, but not worth its own post, then it goes here.

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1. Please add the 'open_thread' tag.

2. Check if there is an active Open Thread before posting a new one. (Immediately before; refresh the list-of-threads page before posting.)

3. Open Threads should start on Monday, and end on Sunday.

4. Unflag the two options "Notify me of new top level comments on this article" and "Make this post available under..." before submitting.

Do you think, the Lorentz factor id somehow present in z, or not?

z represents frequency differences which is the same as apparent slowdown (or speedup for blueshift). Note, this is apparent slowdown in the sense of what we see through a telescope, not how much it is "really" slowed down in "our" reference frame.

Now, when we imagine an extending our reference frame to that other galaxy in a particular way such that in that particular extension of our reference frame the slowdown is caused by motion rather than by universe expansion, then we use the relativistic doppler shift formula to get a speed. Th... (read more)