How Fast Was I Going, Officer?

Your morning isn’t off to a great start.

You’ve been pulled over by the most pedantic traffic cop ever. This cop is a physics nerd, so when you ask her how fast you were going, she is careful with her answer, but she tries to be as honest as possible.

Her radar gun says you were going 75 MPH (miles per hour) in a 55 MPH zone. KM-wise, that’s 120 in a zone where you should be going 88 km/h or slower.

That’s pretty fast! Of course, the planet you were driving on isn’t just sitting there itself: it’s spinning at roughly 1000 MPH (1600 km/h). After doing a little math and determining that the car was moving perpendicular to the planet, both you and your traffic cop agree to ignore the initial 75 MPH speed, especially given that the car’s motion was off at a right angle, so the speed of the planet is what really matters.

So, you were going about 1000 miles per hour. Case closed!

Well… maybe not quite closed. Sure, Earth is spinning at 1000 MPH, but it’s also hurtling through space, making a track around our Sun. It’s doing so at a speed that dwarfs the rotation of the Earth, just as the spinning of the planet dwarfs the speed of your terrestrial vehicle. This speed is roughly 67,000 MPH (107,000 km/h).

So, there you go: you were going 67,000 in a 55 MPH zone. It’s starting to seem less likely that you’re going to be let off with a warning, but at least the physics lecture is done.

Or is it? Astronomy nerds will already know the answer: that the Sun is also ripping through space on its own track, ultimately orbiting our galaxy’s center around 26,000 light years away.

You might already know that this implies a stupefying speed, since our Sun is only 8 light minutes away from Earth, and when you scale up distances, you often scale up speeds, too. However, even at the face-ripping 490,000 MPH, it takes our Sun (and the Earth, dragged right along with it) a whopping 230 million years to complete one “galactic year.”

You can see right away that the bigger number is the one that really matters, and by this point in your traffic cop’s impromptu popular physics lecture, you’re ready for the other shoe to drop.

You’re right to wait, because our lecturer is now describing the expansion of the universe itself: that cosmic flow that acts like a river’s current, but constantly forcing everything wider and wider apart. Thanks to the way light reaches us from very far away, we can determine how stretched out the light is, and work out how fast everything is moving away from everything else.

One way to think about this is as though you’re standing on a moving walkway, but then you start walking, too. Your speed in that direction and the speed of the moving walkway are going together, adding up into a turbo-charged speed from the perspective of a person on the ground.

Now imagine this entire walking track is being towed by a giant truck, and it’s traveling down a highway going south. You and the walkway are facing north, but from the perspective of a person on the ground, it looks like you’re still moving in the direction of the vehicle: south.

This “turtles all the way down” concept may not seem very satisfying, but it’s certainly closer to the way nature actually is than imagining that things have some absolute speed.

The universe’s expansion is 163,000 miles per hour, per megaparsec. That’s a mouthful, but you might notice right away that this is a rate of expansion, not a speed. That’s because the speed at which things are flying apart is actually increasing over time. We call this unexplained phenomenon dark energy.

This means that if two galaxies are pretty close to one another, they’re not moving all that far apart. However, the further they are apart, the faster they are moving away from one another, or relative to one another.

Ever hear the phrase everything is relative? Well, when you’re talking about speed, this is undeniably true. And, because space and time are inextricably woven together into one substance, the same thing is true about time. If you want to know which thing happened first, it depends on who you ask:

After a few minutes of exposition and an excellent lecture on cosmological principles, your traffic cop returns to answer your original question:

How fast was I going, officer?

She tears her ticket up in a moment of realization, and responds: “You know what? It depends on who you ask.”

*this is not legal advice