# Why do blinking lights (such as car signals) synchronize briefly over time?

Asked by Male (1353) January 28th, 2012

If you’ve ever been in a position where you can see a line of cars in front of you, all using the same left or right turn signal, you’ll notice that most of them will synchronize in blinking for a brief period.

Let’s say there are two cars in front of you (A and B), and both have their left signal blinking. At first, it may seem like A goes on when B goes off, and B goes on when A goes off. Over time, B will appear to “catch up” to A, which shortens the interval between A going off and B going on.

Eventually, A and B will both blink simultaneously in synchronization. Afterwards, B may appear to “slow down” from A, which lengthens the interval between A going off and B going on. Then the cycle repeats itself. You could also interpret this vice versa.

Car signals are just an example. I’ve seen this phenomenon appear in most things that have a fixed pattern.

Does anyone know why this happens? What’s this called?

Observing members: 0 Composing members: 0

The phenomenon you are describing is called aliasing . The slowly repeating frequency of when the signals line up is called the beat frequency . While you might not recognize it at first, you can see other examples of these two phenomena every day: the slowly varying volume of two closely tuned guitar strings, the wavy lines in digital images of just the right(wrong) resolution, the slow rumble of two airplane engines. The mathematics of the process are well understood and are used in many different applications like: communication devices, radar ranging and speed measurements, signal processing of acoustic elements.
Keep your eyes and ears open throughout the day and see how many other examples you can find.

LuckyGuy (40630)

Also our brains are tuned for pattern recognition. So we look for (unconsciously) and recognize patterns.

marinelife (62485)

@LuckyGuy nailed it. It’s not that the lights synchronize, which would mean that they adopt the same on-off frequency and maintain that frequency. What you’ve noticed is the point at which the frequencies temporarily align before diverging again.

For another very common example, and one that I notice all the time, notice in public bathrooms how the floor tiles and wall tiles (at least the molding tiles along the base of the wall) start at a common spot in a corner, but then the (usually longer) wall tiles don’t match any more until farther down the wall, when a wall and floor tile joint will once again align the same as they do in the corner. (You can sometimes do the same observation with ceiling and floor tiles.)

And thanks, @LuckyGuy, for the term. I didn’t know that what you described was “aliasing”.

CWOTUS (26092)

@LuckyGuy Got it right.

Here is an example so you can see how it works visually:
x.x.x.x.x.x.x.x.x.x.x.x.x.x
x_x_x_x_x_x_x_x_x_x_x
time—>

The time it takes for the lights to line up perfectly is given by AB / GCD(AB), where GCD is the greatest common denominator, and A B are the lengths of time between the flashes of each light. There will also be many near misses in between.

PhiNotPi (12677)

Ahh…thanks for all the replies. The visual example really helped.

I’m gonna do some more reading to get a better understanding, but it should much much easier now since I get the gist of it.

Male (1353)

I heard an example of it today. We are having a surprising warm spell right now so snow is melting from the roof and spilling over the frozen rain gutters. The drips in several spots are quite regular and the splash sounds they make go in and out of phase in a regular pattern.
Isn’t Physics fun?

LuckyGuy (40630)