Eli5: Why do wheels look like they’re spinning the wrong way when going fast?
In: Physics
There are a lot of incorrect answers here.
It’s true that this is observed through cameras but it’s also something that we commonly perceive in real-time too, like when we’re looking at the blades of a helicopter in flight or the wheels of cars out the window on a highway. The formal name for this phenomenon is the [wagon-wheel effect](https://en.m.wikipedia.org/wiki/Wagon-wheel_effect).
The reason why this happens is twofold: It is based both on how the wheels spin and how our brain interprets images. Picture this: A wheel makes one full rotation in 50 milliseconds. But your brain “updates” and interprets your visual cues every 40 milliseconds. So when 50 milliseconds pass, the wheel will spin once fully, but because of the desync in timing you’ll see the wheel *almost* spin once fully. You’ll have to wait another 40 milliseconds because of the timing delay by your brain to see the wheel in its starting position.
Now imagine the same wheel spins twice, which would take 100 ms. At the end of the first wheel spin (as discussed above) you’ll see the wheel slightly behind, and 40ms later because the wheel is still spinning (because 100ms hasn’t elapsed yet) your brain will update itself again, and you’ll see the wheel even further back than you saw the first time.
At this point your brain is going to draw a conclusion. It’s going to interpret all those delays as “Hm, I only see the wheel slightly behind where it originally spins each time. Logically this must mean that the wheel is spinning backwards (opposite), not forward” and it relays this information to you accordingly, causing you to think what you think. If the wheel spins many times quickly (as wheels usually do) you’ll see this effect much faster, creating the phenomenon that you see.
Now it should be noted that in real life, our brain doesn’t have a “fixed” refresh rate. We aren’t computers after all, we can’t easily put a label on our brains. The 40ms refresh rate used above was only for simplicity’s sake, and in reality it constantly fluctuates based on many biochemical processes.
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