You are standing on the sidewalk, up to no good, when a police car comes rushing toward you. You probably have warrants out, but that is not what I’m here to discuss. I’m hear to talk about the Doppler effect, and I expect you to take notes.

The Doppler effect, named after Christian Andreas Doppler, is the change in frequency of a wave relative to the source of the waves. More precisely, it’s the reason that a passing cop car sounds precisely like this: “weeoo, weeoo, weeeooo, weeeeeeeeee oooooooo, weeeeeeeeeeeeeeeeeeeeee oooooooooooooo …”

You people love it when I talk science. Nothing excites you more than long-winded diatribes on quantum theory or the elegant beauty of relativity.

The Doppler effect is encountered in common experience by anyone who roams his surroundings. In a city like Lewiston, the effect is evident in the form of conversations as you walk into and out of another person’s life in a span of seconds. The volume of that ephemeral dialogue rises and falls before disappearing altogether. The sound waves are gone and so is your fleeting involvement in the conversation.

If you were a downtown reporter, passing a pair of men having a conversation on Pine Street, it might sound like this.

“Is it one-way glass?”

“Naw.”

“Oh. So they can see you, then?”

“Yeah, but if you have a situation where you’re …”

Distance pulls the sound waves apart, stealing the context at the same time. And you will either dismiss this conversation entirely or spend a few moments trying to deduce why these men so desired tinted windows in that van.

On Ash Street, a girl with a stamp-sized cell phone pressed against her ear, moving with great velocity: “She’s 16 now. You’d think she’d know better than to …”

The voice faded into obscurity and the meaning was lost. The girl shot me a hostile look as though I was eavesdropping. Which I was.

A block away, on Bates Street, a woman was yelling across the street to a man on the other side. I passed through those sound waves until they were impossibly stretched and could no longer be deciphered.

“He can’t come out right now. He’s on the computer looking at a …”

Best that I don’t know the rest of that one.

In Kennedy Park: “They don’t listen. Every time I go down there, nobody will listen.”

Not to worry, ma’am. I’m listening.

The science of sound is not as interesting as the words that are produced by it. Downtown, it is like walking through a snowstorm of thought.

“… not my damn fault you can’t hold down a job. If you’d stop …”

“If I ever see that bastard again, I’m going to …”

“… doesn’t call the cops, I will. I mean, how long are we going to put up with this …”

Doppler gives you a few seconds of microdrama and then pulls those sound waves away, like a magician yanking away a tablecloth. The effect leads to misunderstandings and confusion, such as this one from a young man on Bartlett Street who said either: “I want my toilet back and I plan to get …” Or, “I’m really sad because I don’t have anything to pet …”

How your brain processes incoming sound waves depends on your velocity, weather conditions and earwax, among other contributing factors. It also depends on the person who delivers the sound. If you could view the sound waves, they would sometimes appear oddly jagged and wobbling. These are the sounds of the drunk. They make up for the slurring vibrations with volume.

On Walnut Street, just after sundown. High-energy waves, robust with volume and emotion, as the inebriated speaker tries to send her words the length of a city block.

“Yeah, you keep walking. You keep walking, you bastard. And don’t bother coming home tonight because I …”

Or the playful, drunken cat-call of a man trying to entice a drinking buddy by sending sound waves toward the top of a four-story building.

“Manny! Maaaaaaaaany! Come on outside, you baby. We’re all heading down to the …”

If you make yourself aware of Doppler, the effect is dramatic. Cosmologists use the science to gauge the expansion of the universe. I use it to gauge the temperament and nuance of the city. If this kind of science does not appeal to you, tune in next week: how obscene hand gestures are transmitted with photons.

Mark LaFlamme is the Sun Journal crime reporter and author of the novel, “The Pink Room,” which explores String Theory.