It sure would be a contradiction if we were talking about ordinary things, but for the vast reaches of the universe, it appears that new situations have to be taken into account. I tried to trash out some of these issues in an article I wrote for Sky and Telescope. There is nothing very intuitive about any of it, and the bottom line is that we cannot determine what motion is on the cosmological scale by any means we intuitively know how to apply. We do not detect shifts in the sky locations of any galaxy over a period of time so that we can form a quotient representing velocity ( distance divided by time). Furthermore, we have Einstein's theory of general relativity whose essential predictions seem to be confirmed, which predicts the outcomes of measurements made on the cosmological scale.
In Newtonian physics, the kind we all have lots of intuition about, if an object changes its coordinates in a set amount of time, we say that it has moved with a certain speed. What we are actually doing is measuring its distance at two different times and identifying the change in distance divided by the time as a speed. We can be fancier and talk about the changes in the three components to its distance so that we can compute a Velocity, but the essential operation is the same. This doesn't work for cosmology. What we have to work with according to general relativity is that every galaxy has an abstract coordinate address ( its so-called Gaussian coordinate) in 3-dimensional space, and that gravitational forces cause the distances between these coordinate addresses to change without altering the coordinates themselves. This is where we get the rough idea that 'space expands but the galaxies remain fixed'. The analogy often used is to paint a coordinate grid on the surface of a balloon, and glue pennies to a couple of the intersections in the grid. Now by blowing up the balloon, the pennies remain fixed to the intersection point where you glued them, but the distances between the pennies nevertheless increase. This is not the Newtonian idea of motion, but it is the kind of 'motion' you have to propose for galaxies in deep space if the world is built according to the basic principles of general relativity.
This is not the first time our intuitions about nature have been so roughly jerked around. Special relativity and its 'Twin Paradox' and quantum mechanics with all of its bizarre rules, are at least as alien to us as the idea of expanding empty space!!! Where all of this 'reeducation' of human intuition will lead us, no one knows, but we have certainly come a long way since the 19th century by just going with the flow, and leaving the loftier questions for brave philosophers of the 21st century to sort out, if in fact that will ever be possible. Evolution has not prepared us to intuitively understand many aspects of our physical world. All we can do is protest the new knowledge, and then get on with our lives.
Copyright 1997 Dr. Sten Odenwald
Return to Ask the Astronomer.