Driving around with my little sister in the back seat recently, I noticed something odd. She had, well, "appropriated" a helium balloon from a display at the supermarket (stealing is wrong, Hanna) and I watched it float back and forth as we stopped and started on the road. The weird thing was that the balloon was always moving in the exact opposite way as us.

So what was going on here? Turns out the odd behavior of balloons in cars has to do with Newtonian physics, Einstein's theory of General Relativity, and, ultimately, the behavior of gases. Sounds complicated, but it's actually pretty simple. And cool.

When you accelerate and decelerate your car, you feel yourself getting pulled back and forth. As Isaac Newton explained 350 years ago, a body at rest or in motion tends to stay that way until acted upon by an outside force. So let's say you're driving forward at 40 miles per hour and you accelerate to 55 mph, feeling yourself pressed back into the seat. What's actually going on is that your body wants to stay at the constant speed of 40 mph but the car starts going faster than that, catching up to you and running into your back.

Consider the opposite situation. You're driving along at a constant speed but something (rabbit, deer, or child) runs in front of your path. You slam on the brakes, quickly decelerating and getting thrown forward. Your body was totally happy traveling at a constant speed and wanted to keep doing so. But the car around it suddenly slowed down, falling behind you and leaving you dangling in the air.

Now, if you play out any of these same situations with a helium balloon in the car, you'll notice something peculiar. Whenever you get pushed backwards, the balloon goes forward. If you get thrown forward, the balloon goes back. And, if you make a sharp turn, you lean outward while the balloon leans inward. It's like the balloon always wants to do the opposite of what you're doing.

As Einstein showed with his theory of General Relativity, the force you feel when accelerating is indistinguishable from the force of gravity. Known as the Equivalence Principle, this finding helps explain what's up with the balloon's behavior. Think about what a helium balloon does in a gravitational well, like the one we find ourselves here on Earth. We humans get pulled downward by our planet but the balloon floats upward. If you set up an acceleration (which remember has the same effect as gravity), the balloon is going to move opposite to this pseudo-gravitational force.

Now wait a minute, you're probably thinking right now, the balloon is just a dumb piece of rubber filled with gas. How could it possibly "know" which way gravity is? Of course it doesn't. It's responding to the air around it. Helium floats because it is buoyant; its molecules are lighter than the nitrogen and oxygen molecules of our atmosphere and so they rise above it. In the car, it's the air molecules that are actually getting pulled and pushed around by gravity as the result of the accelerating frame.

So you accelerate in the car, pushing a bunch of air molecules backwards. This sets up a tiny pressure gradient, with slightly denser air at the back of the car and slightly thinner air at the fore. And now the balloon rises "upward" toward the thinner air like a diver coming up from deep water.