# Fermilab

## Inquiring Minds

Motion in the Universe

Question:
I am interested in the concept of absolute 'still' vs. all the speed references we have. I have been attempting to calculate the speed at which an individual is traveling through the universe when standing 'still'. i.e., the rotation speed of the earth, the speed of the orbit of the earth around the sun, the solar system withing our galaxy, the galaxy...etc. The odd question that struck me was that given all the relativity applications and explainations, is there such a measurement or 'thing' as absolute STILL?

In our universe, there is really one reference frame that I would call an "absolute rest frame" - it is the frame that cosmologists refer to as the "co-moving frame".

In this reference frame, an observer is a rest, apart from the expansion of the universe, that is, someone in this reference frame is not moving with respect to distant galaxies.

Think of the universe like the surface of a balloon, with pennies stuck onto the surface. These pennies are the clusters of galaxies. As you blow the balloon up, the pennies get further and further apart - this is the expansion of the universe. (The pennies/galaxies don't get bigger, because they are held together by gravity, or gravitationally bound.) The co-moving frame is the reference frame in which the coordinates expand along with the balloon. This is the closest you can get to having an absolute rest frame in an expanding universe.

So, after you account for the motion of the earth around the sun, the solar system within the galaxy, the galaxy within a cluster of galaxies, etc, eventually all you are left with is the expansion of the universe...and that's about as close as you could get to being absolutely still!

Regards,
Nicole Bell
Astrophysicist

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