The inertia of inertia: bowling in space

My friend Shorty, from down South, bought me this marvelous textbook on physics for my birthday. It’s titled Conceptual Physics, by Paul Hewitt. It’s a marvelous big ol’ textbook that takes a learner from the very basics all the way into relativity, does it with minimal equations, a nice human touch, some cartoon work by the author, and lots of questions and exercises.

Which brings me to inertia, which I started reading about today, or more specifically, Newton’s First Law of Motion. (Newton’s stuff gets capitalized a lot.) Anyway, NFLOM says, "Every object continues in a state of rest or of uniform speed in a straight line unless acted on by a nonzero net force."

Hewitt notes, in a sidebar, "Inertia isn’t a kind of force; it’s a property of all matter to resist changes in motion."

He also notes, "We don’t know the reason for objects persisting in their motion when no forces act upon them."

Now here’s the thing. If you roll a bowling ball across a rug it will soon stop due to friction with the rug. If you roll it down a shiny bowling alley it will go a lot farther and if you’re lucky do some damage to a structure of tenpins before it stops. But if you were to take that ball out into space, beyond the gravitational influences of, say, a solar system, two things can happen.

If you just let go, applying no force, the ball, perhaps a bright red one, will sit there forever until something, some force or other, does something to it. Or, if you give it a push it will move at the same speed in a straight line, barring outside force, until the end of time.

As a practical matter, things will happen to disturb the ball’s inertia. It might run into a star, or pass by a planet, or get hit by a comet or asteroid. It might travel undisturbed for a trillion years except for hitting an atom head on once a year and thus be brought to a stop eventually.

But here’s the thing. Think about that ball’s motion. Out there in deep space all you did was give it a little push, using your arm muscles. That’s the only force applied to it. If nothing gets in its way, if no other force ever acts on it, the ball will just keep moving at the same speed in the same direction forever. If you gave it a five mile per hour push, it will cross the universe at five miles an hour.

How can that be?

The physicists say, "We don’t know."

How can that be? How can they not know? But I’ll take their word for it.

But I still feel mindboggled. My brain keeps asking the question, "What keeps it going?"

And comes up with strange answers, or questions. For example, did the initial push impart sub-atomic particles that carry force and that then react with the quantum fabric of space-time as constantly recycling drivers? Particles carry other kinds of forces, why not inertial motion?

See what I mean?

Part of the problem is that our frame of reference is so earthbound. Here, friction rules. Bowling balls slow down and stop because they constantly fight opposing forces like friction (and tenpins). Friction is in the blood of our brain’s synapses. It is a Law of Nature: bowling balls slow down and stop.

We don’t live in deep space. Stuff acts differently out there. Inertial motion is just too alien. Too out there. Too cosmic, if you will. The human brain will accept the theory, but the fact of an infinitely traveling bowling ball, no, neurons will not wrap themselves around that in a cocooning embrace. They will fight it, synapse by synapse, until the last synaptic soldier lies dead on the beaches of time.

Newton could really tick off an average brain. I can hardly wait to see what else this book is going to do to get my synapses in a twist.