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

Note, that there is also “proper speed”. When you visit a star light years away (like alpha centuri), you yourself can be there in 3 days (considered you have enough energy). So talking very simply, from your point of view, you travel FTL in proper distances.

Observers on Earth, will see that you will take 3 years. And this is where time dilation and length contraction come into play. As you are near FTL, your way towards the star contracts, as such you only need some days.
However, for the observers on the earth, your way does not conract, instead they see in difference in time perceived.