Is There A Zero-Point For Measuring Astronomical Speeds?

Since you know how fast stuff in the universe is moving, where is the point of reference that is not moving at all, to base the speeds at which stuff travels?
A solar storm approaches Mars (artist's concept). The Red Planet is thought to have lost much of its atmosphere to such extreme space weather. Credit: NASA/GSFC

A solar storm approaches Mars (artist's concept). The Red Planet is thought to have lost much of its atmosphere to such extreme space weather.

Originally posted at Forbes!

This one has a quick answer and a longer explanation. The quick answer is that there is no absolute point of reference that defines motion for all other objects in the Universe. All of our estimates of how fast objects in space are moving are relative to something specific, not absolute.

This lack of an absolutely unmoving object, or point, is one of the principles from the theory of relativity, and simply means that you can’t talk about any motion – at any scale – without also being clear about what that motion is in comparison to. Sometimes these comparisons are implied. When we talk about how fast your car is moving, we assume that we mean relative to the ground, because that’s clearly the relevant metric. But there’s nothing stopping you from measuring the speed of your car relative to the car in the next lane over.

If I sit inside a train car, or a bus, I’m not moving. At least, I’m not moving relative to the bus or train. But the train could be moving relative to the ground. Which speed are you interested in, for describing how I am moving?

You run into the same problem at every stage when you’re describing the Universe. As a point on the surface of the Earth, you are rotating around the Earth’s internal axis, which is revolving around the sun, which is in turn orbiting the center of our galaxy, which is rotating around the center of mass between Andromeda and the Milky Way. The speed of any given piece of this motion is entirely defined relative to some other, arbitrarily defined reference frame, so the speed we’re reporting depends on what piece of this motion we’re interested in describing.

Two galaxies can be crashing into each other with some speed – what speed? Probably the speed of one, relative to the motion of the other. Scientists won’t always spell out the reference frame, because there’s often a standard “relative to…” implied, but it always has to be there.

You can keep playing this game forever – you will always be able to define a speed relative to some defined point – but there’s no special point that will allow you to absolutely calibrate the motions of all things in the universe. There is no universal rest-stop. Any other observer of our universe should find the same thing. They will always be able to measure a velocity, measure a speed, but all of these measurements have to be relative measurements.

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