It may seem strange to begin a site about the mysteries of physics with a chapter on motion. After all, moving is something we do every day, all of the time. It doesn’t usually seem very mysterious. You may think that you are sitting still right now, staring at your computer, possibly with your legs propped up on the corner of your desk or a small, obedient child.
But that is not the case. In fact, just beneath you there is a fantastically large piece of rock which is spinning around in space. Being on the surface of this hunk of rock, you are moving with it — at speeds of up to 1,674 kilometers per hour (or 1,040 miles per hour). That is a few hundred kilometers per hour faster than the speed of sound.
And the planet does more than just spin — it’s also orbiting the sun. It may take a while to complete one full orbit (one year), but in that time it has gone a long way — almost a billion kilometers. We’re moving around the sun at a speed of over one hundred thousand kilometers per hour.
As if that weren’t enough, the Earth is located about halfway out from the center of our galaxy, which happens to be spinning too. The Earth, along with the rest of our solar system, is thus moving very quickly relative to the center of the galaxy. This galaxy of ours (the Milky Way) is also moving through space in a number of interesting and terrifying ways relative to some other local galaxies, such as towards Andromeda.
The point I’m trying to make here is that while you sit comfortably somewhere feeling snug and secure and completely motionless, you are actually travelling through the frozen, dead blackness of space at speeds of up to six hundred kilometers per second. That is not a typo. To really get a feel for what this means, take a deep breath and hold it for about nine seconds. Now exhale. You have just travelled the distance from New York to London, and you didn’t even have to take off your shoes at security. And you were holding your breath the entire time. And here’s the thing that gets me feeling really dizzy: the plane didn’t stop at London. Our planet just keeps on hurtling through space, and it’ll continue to hurtle long after humans have disappeared from this planet and the Milky Way smashes into the Andromeda galaxy in the biggest and slowest explosion in local history.
Now, my mention of airplanes might have made you curious about something. “Why can’t I feel the Earth’s motion?” you might be asking yourself. After all, we certainly feel it when an airplane takes off, and that’s when it accelerates to a puny 170 kilometers per hour, which is a mere 0.008% of our speed through space.
This question has a somewhat simple answer: it’s because the Earth is actually moving at a mostly constant speed in a mostly constant direction, while a plane speeds up as it takes off. But to the great minds in the world of physics, that is exactly the kind of answer that only raises more questions. When Galileo thought about motion, he laid the groundwork for Isaac Newton, who basically invented what we know of today as physics. Albert Einstein was thinking about motion when he developed his Theory of Relativity, which changed the way we think about reality.
For the purposes of this site, though, I have much more modest reasons for starting off with a chapter on motion. The first is that it is a surprisingly difficult thing to fully understand. The second is that without an understanding of the ways things move, it would be impossible to discuss why things move the way they do. And that, in the end, is what physics is all about.