We measure space objects speed in relation to earth. But since earth is also moving through space, how do we know the true speed of objects without a universal “non moving” point?

Question

The speed of an object is measure relative to the observer. But since all objects are moving in relation to all objects, how can we know the true speeds of things? Wouldnt two observers on different planets measuring an object in space come up with different answers?

In: Physics

Answer 1

Everyone here has told you that all speed is relative, and there’s no “true speed”. Which is true!

*But* it is possible to measure an object’s speed relative to the cosmic background radiation. The cosmic background radiation is the afterglow from the big bang, it is pretty much the same (with small variations) in all directions. If there is a universal rest frame in the universe, that’s it.

Our velocity relative to the cosmic background radiation is about 370 km/s, in the direction of the constellation Leo. That’s about 0.1% of the speed of light.