Tuesday, December 3, 2013

Newton's Weird World

If you go to a bookstore and look through the popular science section, the books on physics will mostly be about the most mind-bending modern theories: quantum mechanics, relativity, big bang cosmology, black holes, and so on. You won't find many books about the basic classical physics you learn in school. Newton's laws of motion, prisms and rainbows, magnets and electric motors--these things don't seem to strike the popular imagination. I think that's because modern physics seems so exotic (and because reading about it makes you look smart). Space and time bend and meld, single particles go through two holes at the same time (unless you try to catch them at it), black holes slow time, capture light, and may even lead to other universes...that is freaky stuff.

Lately, though, I've been realizing how freaky and counter-intuitive the old-school physics of Newton and Galileo can be. People have an intuitive understanding of physics, and it's good enough to let us navigate the surface of this particular planet most of the time, but in the grand scheme of things, it's wrong. Sometimes dramatically wrong. Most of the people who ever lived went to their graves thinking the earth is flat. They were wrong about the shape of the surface they lived on every day of their life. I would too, if I hadn't been taught differently. It's a humbling thought. 

People also assumed for thousands of years that heavy rocks fall faster than light rocks. After all, doesn't a feather fall more slowly than a boulder? It's just common sense. But Galileo thought he would test the idea anyway, and it turned out common sense was wrong. Light things may fall more slowly on Earth because of air resistance, but the deeper law of nature is that in a vacuum, feathers fall just as fast as boulders. That's well known these days, of course, so we don't really feel how surprising it is. But for people living at the time, it was earth-shattering.

It's shocking to discover that natural law runs counter to our intuition, even for things we see every day. When you start thinking about all the ways that's true, dusty old textbook physics starts to gleam a little more. Here are some examples I like:*

When I go for one of my brief, agonizing runs, I always think about how strongly the earth is pulling down on me. But I never consider how I'm pulling up on it with the same amount of force. Every object with mass creates a gravitational field, and I assuredly have mass. And every force comes in pairs--nature is symmetrical that way. I tug on the Earth just as hard as it tugs on me. It's just that the earth is so much more massive that it accelerates me a lot more than I accelerate it.

I just said the earth is pulling "down" on me. That's because I go around thinking there's a real up and a real down. But of course there isn't. And north certainly isn't up, no matter how hard it is to think otherwise. "North is up" is just a convention, and early cartographers often drew maps "upside down" before that convention was established, as in this map of Europe and North Africa from 1459. I can look at that map and tell myself it's just as valid as "right side up" ones, but it's still looks wrong. It's not, though. I am.

The most basic rules of motion can be totally surprising. For example, if you hold a rifle five feet above the ground, and I hold a bullet in my hand at the same height, and I drop it at the same moment you shoot, the two bullets will hit the ground at the same time (disregarding air resistance, etc). The high-velocity bullet falls just as fast as the low-velocity bullet. You would think the lateral motion of the bullet from the gun would somehow interfere with its downward motion ("downward" I should say) but it doesn't. The two motions do combine to create a curved path, but their magnitudes are independent.

Speaking of falling objects, the moon is falling. It's dropping like the giant rock it is. It's just that its lateral motion is balanced with its "downward" motion in such a way that it falls around the earth instead of into it. It's been plummeting for billions of years, but it's never managed to land. We've been plummeting into the sun all that time, too. It makes me a little queasy thinking about it.

Another illusion I have is that when I throw a rock, I always feel like I'm giving it a certain amount of energy. I imagine this energy fades as the rock progresses, so it finally slows down and lands. But that's not what's happening at all. When the rock leaves my hand, it's going at a particular velocity, and it would keep going at the same velocity indefinitely in the absence of other forces. As Newton taught us, objects in motion tend to stay in motion. The rock slows and falls because air resistance exerts a force that decelerates it, while gravity works to return it toward the earth. Energy is always conserved, so the energy I give to the rock doesn't fade away. Some of it is transformed into heating the air that slows down the rock, but none of it disappears.

As for heat, it's funny, counterintuitive stuff, too. In fact, it's not even stuff. It's molecular motion, and it behaves in unexpected ways. When I step out of the shower and put one foot on the tile floor and the other on a bath mat, I could swear the bath mat is warmer than the floor. But it can't be--they're both at the same temperature as the rest of the room. It's just that the tile conducts heat better than the fibers in the mat, so it sucks heat away from that foot more efficiently. That's why it feels colder, even though it isn't.

Of course, I'm speaking metaphorically when I say the tile "sucks heat", even though I may not realize it. It's not really what happens. In fact, the idea of suction is an illusion. If I take a drink through a straw, I'm not exerting a "force of suction". I'm lowering the air pressure in the straw, and that allows the pressure of the atmosphere (a surprisingly high 14.7 pounds per square inch) to push the drink into the straw. It doesn't sound right, does it? It goes against common sense.

And that's the problem with common sense.


* You probably know these things as well as I do. My point isn't to say "Did you know that....", but to say, "We both know this; let's stop and think about how amazing it really is." That's how most of my posts are intended, actually.