By Vladimir Brezina
I love paddling at night. Not only for eminently practical reasons—to avoid the heat and humidity of New York’s summer days, for instance—but because of a remarkable visual illusion. As dusk falls, I feel myself paddling faster and faster, until I am simply flying over the water through the darkness. It’s an exhilarating feeling. If you paddle at night, no doubt you know exactly what I am talking about.
But when I look at my GPS, or for a moment emerge into the glare of shore lights, the illusion is shattered. I find that I am paddling at my usual daytime speed, if not slower.
I’ve often idly wondered what the basis of this illusion was. It seems that it’s by no means limited to paddling. According to this article, runners run faster at night, and cyclists ride their bikes faster at night. Even car drivers drive faster at night—although that might not be just an illusion :-).
The explanation given in the article is a relatively plausible one based on well-established neurological mechanisms. When we move through the world, we judge our speed by the speed of the optic flow, the coherent apparent motion of the objects in our visual field past us. But not all objects appear to move at the same speed. Nearby objects appear to move past faster than distant objects. (Indeed, this motion parallax helps us decide which objects are nearby and which are distant, a calculation that itself can generate some potent visual illusions.) Our brain balances the apparent fast movement of nearby objects and the slow movement of distant objects to determine our most likely true speed.
But at night this balance is disturbed. We see, dimly, only the fast-moving nearby objects—the waves around the kayak—and not the distant objects—the distant shoreline whose slow movement would in daytime provide a corrective balance. At night, all the objects that we see are moving fast. Consequently, we conclude that we are moving fast through the world.
So that mystery is solved. Now I just have to worry about why time goes faster as you get older…