There’s a diagram of an elevator in my professor’s office and the clock is ticking faintly in the background.

“Show me how general relativity works,” he says.

After my initial fright subsides, I stare at the chalkboard for a couple more seconds, let the elevator start accelerating in my mind and finally begin to scribble down equations. Relativity is interesting stuff. You see, I’m a nerd.

If you’re driving at 60 miles per hour and spit out the window of your truck – not something I’d recommend – you might see your spit fly forward at 10 miles per hour (in the split second before the wind catches it and throws it back in your face). To the hitchhiker on the side of the road, your spittle appears to be traveling at 70 miles per hour.

Things get interesting when you consider that light won’t play by these rules. If you’re the person in the truck, light appears to be travelling away from you at 186,000 miles per second. If you’re the person on the roadside, light from the truck is still traveling away at 186,000 miles per second. To resolve the paradox, scientists say that time and space mean something different to the hitchhiker and the person in the truck. Their perceptions of time and space are “relative.” We end up having to say that time runs slower for the person in the truck, from the person on the roadside’s point of view.

Of course, if you’re the person in the truck, it looks as though the hitchhiker is moving by at 70 miles per hour. If you were the truck driver, you’d be forced to say the other person is moving and her clock is running slow. At first it may seem silly to say the person is moving. It’s the truck that’s on the freeway. But zoom out for a second and you’ll see the truck and the hitchhiker are sitting on the surface of a spinning planet whirling around a sun that’s pacing around the center of a galaxy drifting through space. Is there really a preferred frame of reference? A physicist will tell you no.

All this describes something called “special relativity,” the idea that moving clocks run slow. Think of it as the law of physics that says the bus can’t be on time. There is another kind of relativity that physicists talk about, called general relativity. It’s the one that deals with the elevator on the chalkboard.

Imagine for a second that the elevator is floating out in space and that it’s accelerating in a direction that we’ll call “up” for the hell of it. The fluorescent light bulb at the top of the elevator is shining light on the floor. Say the bulb is flickering and lets out a pulse of light every half second or so. If the elevator is accelerating upward, you’re running into these pulses before they’ve traveled the full length of the elevator and consequently you see more of them in a shorter amount of time than if the elevator were standing still. It would be like parking on the side of a freeway and counting the southbound cars that passed you. If you’re driving northbound, you’ll pass more cars in the same amount of time.

To an observer at the bottom of the elevator, the light bulb is emitting more pulses – actually flashing faster. From the light bulb’s point of view, this makes no sense. Again, light travels at the same speed from the point of view of both observers, but the timing of the pulses is suddenly different. Again, we must say that time and space are relative. This time, the light bulb’s clock appears to be running faster from the floor clock’s point of view.

Remember there is no preferred point of view. This means any frame of reference in which things are accelerating will result in the same set of perspectives. In the case of the Earth, gravity is constantly accelerating things downward. This means we are constantly on the floor of the elevator – and that the clock sitting high on the wall, ticking faintly in the background, is actually ticking faster than the one on my wrist.

The coolest example of this is the Global Positioning System, or GPS. GPS satellites are flying by overhead at thousands of miles per hour, which slows their clocks down by our standards. But gravity is acting on them too, speeding up their clocks even more than their speed slows them down. They’re ticking away up there at the very top of the elevator and a GPS receiver down here on the ground measures distance and position by catching regular pulses of light from those satellites – just like the broken light bulb in the elevator. Engineers actually set the satellite clocks to run slow and make up the difference. There are actually quite a few relativistic effects acting on the satellites and all of them give engineers headaches. But my editor and I have different conceptions of space. So we’ll leave that alone for today.

Josh Braun is the Daily Nexus science editor. He’s actually not sure what time it is.