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Union University Department of Physics

The Science Guys



Science Guys > October 2003

October 2003

I am on a rotating planet, orbiting a star, which is revolving around in a galaxy in an expanding universe; how fast am I actually moving as I sit here?

This question vexed physics for years. Galileo, Newton, Maxwell, Michelson, and Einstein all made essential contributions toward answering this fundamental question.

Quantifying motion of any kind requires a reference frame. Suppose you are flying in a plane at 300 mph. If the air was smooth, the engine was very quiet, and the windows were covered, how could you tell you were moving? You would be in what physicists call an inertial frame- the plane would be your frame of reference and as far as you are concerned you are at rest. However, someone on the ground is at rest relative to the ground, but to the earth-bound observer the plane and you are moving rapidly.

Galileo and Newton would have insisted (and correctly so) that all inertial frames are physically equivalent- no experiment can be done that will show one frame to be preferred over another. But during the late 1800's, physicists asked the question, "Just what is the absolute reference frame for the universe?" This question was prompted by a famous set of electromagnetism equations (Maxwell Equations).

Maxwell’s Equations predicted a constant value for the speed of light, so the question became, "What reference frame does this speed refer to? After all, all speeds are with respect to something!" The answer that came back from the physics community was, "The speed is with respect to the light-carrying ether! There is an invisible, luminiferous substance (ether) that fills the universe and the speed of light predicted by Maxwell is with respect to the ether."

Albert Michelson and Edward Morley in 1887 conducted an experiment to find the ether. They found no ether! The experimental consensus is that the speed of light is the same for all observers, no matter what their motion relative to the light source. There is no ether and thus no preferred or absolute reference frame in the universe.

The Earth is rotating and we move in a circle around the axis of the Earth at about 840 mph, which is with respect to the Earth’s axis. But, the Earth orbits the Sun with a speed of approximately 67,000 mph with respect to the Sun. But, our solar system is embedded in a galaxy that is rotating with respect to the galactic center at 425 000 mph. In addition, our galaxy moves with respect to other galaxies! So, how fast do we move, really? To answer this question, the individual must specify a reference frame. You could correctly say you were going 425 000 mph if the galactic center is your reference point.

This very question led to a revolution in man’s view of space and time leading to Einstein’s relativity, but that will have to wait for another column!