Inertia

20th Century Fox

In Marvel’s X-Men series, there is a character known as Juggernaut. His power is that once he is in motion, nothing can stop him. In physics terms, this means his inertia can’t be changed by some external force. That’s not how physics works, though that’s a minor detail in a universe that has a talking raccoon. But Juggernaut’s use of his skill is just one demonstration of a common misconception about inertia.

The key aspect of inertia is that it doesn’t change unless some force acts on it. If an object is in motion, it will keep that motion indefinitely. Barring friction or other forces, an object won’t spontaneously slow down, nor does it need a force to keep moving. The common misconception is that the natural state of an object is to be “at rest” and a force is needed to keep it going. On Earth that seems reasonable because we experience air resistance and friction. To keep an object moving on Earth we do need to keep pushing it.

But that isn’t true in space. Often in science fiction movies you will see spaceships moving at cruising speed with their engines on, as if they need to burn fuel to keep their forward motion. Another common trope is that when a spaceship breaks or runs out of fuel, it begins to fall toward the Sun. Apparently without rockets to keep it moving, a spacecraft will slow down and “fall out of the sky”. Of course, planets don’t have rockets, and they orbit the Sun just fine due to their inherent inertia.

In astronomy, we often present the fact that galaxies are moving away from us as evidence that the universe is expanding. But this by itself is not evidence of an expanding universe. Galaxies have inertia just like anything else, and it would be possible for them to be speeding away from us in a non-expanding universe. The evidence for an expanding universe lies in how the galaxies move away from us. Specifically that more distant galaxies move away from us faster than closer ones.

Intuitively, it feels as if there must be some “at rest” frame, and that objects inherently move towards that state. It is the concept that Aristotle used to argue that the Earth doesn’t move, which was an idea that predominated for two millennia. But we now know there is no such rest frame. Instead everything in the universe moves relative to everything else, each with its own inertia.