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girhen t1_j0f8rc8 wrote

Reply to comment by nsjr in Does rotation break relativity? by starfyredragon

No, you're onto the idea.

Reference frames are in relation to what makes sense. It's easy to choose either an inertial frame with reference to your car or the ground when you want to talk about movement in relation to the two. It's hard to make a fixed point for something as big as galaxies.

The thing is, when it comes to calculating position of objects that are as far away as galaxies, you can basically consider their position fixed. If I put you on a merry go round and said there was a car moving at 5mph 1000 miles away, its movement wouldn't be enough to matter to you much over the course of a couple hours.

The solar system moves at 140 miles per second. That's fast. But the closest star to us is Proxima Centauri - 4.25 light years away. 140 miles per second is .000751547c (c is speed of light). It would take 5,655 years for us to reach Proxima Centauri if it were stationary and we moved directly at it.

So yes, there are many differentials to consider. One of the ways we consider speed is by using the doppler effect of light - red shift and blue shift - to determine speed we're gaining/closing on it based on known colors we expect from stars.

So yeah... it's complicated. Astrophysics is not known to be an easy field to comprehend, much less do.

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Game_Minds t1_j0gt1ju wrote

We can actually use the doppler effect on nearby stars to determine how fast they are rotating! The light from one half of the star is bluer than the other because part of the surface of the star is rotating towards us, and part away! Add up the difference in frequency (and do math), that tells you the difference in their speeds, half of that is how fast it's rotating! This is also handy when determining things like a star's absolute color and luminosity, as its spectrum can appear shifted or blurry if the star is rotating very fast or at a funny angle

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NeverPlayF6 t1_j0gizvi wrote

> If I put you on a merry go round and said there was a car moving at 5mph 1000 miles away, its movement wouldn't be enough to matter to you much over the course of a couple hours.

This is a small angle approximation, right?

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girhen t1_j0gxhv7 wrote

Absolutely. There are two orders of power between the observer and the two objects. Typically, 15 degrees is acceptable to use it, or 10+ times the distance of the observer from the two objects. 100+ times the distance is preferable, which is still the case of the car after moving for 2 hours.

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