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

I am a physicist and there is a lot of bad information in this thread.

~~There absolutely is~~ We can easily define a universal reference frame. Yes relativity is a thing, but it’s pretty easy to define a fixed point in the universe. You can find this in several ways:

1. You can look at the average velocity of every object in the universe and define a universal reference frame based on the reference frame where that average velocity is zero.
2. You can also look at the Doppler shift in the cosmic microwave background (CMB) which is the remaining heat glow from the big bang. Doppler shift for the CMB is analogous to when you hear the pitch of a train horn change from higher to lower as it passes you. We have good reason to believe the universe is isotropic, meaning the same in every direction. So we should expect that the CMB is the same in every direction, too. However, in one particular direction the frequency of the light is slightly higher, and the opposite direction is slightly lower due to Doppler shifting.

These above methods can give us a good absolute reference frame to measure any velocity relative to. For our galaxy, we are moving though space at about 600 km per second.

To anyone doubting the validity of defining a fixed reference frame, I also wanted to mention something known to physicists as Mach’s principle. This principle tells us how it’s possible for a rotating object to “know” when it’s rotating. If you spin around, you can feel your arms being “pulled” outwards, but how could this ever happen without there being some kind of universal reference frame? Mach explained this effect as the distant stars acting as a fixed reference point for the rotation and centrifugal effects. Einstein further explained this effect in general relativity via “frame-dragging”. But honestly this effect is still stumping physicists to this day, and we don’t fully understand how something could ever “know” it’s rotating. Our best explanation is that all the matter in the universe somehow provide the necessary reference frame for the rotation.

Edit: Changed wording to clarify my point. Thanks for the silver.

Edit2: Spelling