If two pieces of paper, one crumpled up and one flat, were to be dropped, which one would reach the ground first? Without much deliberation, most people would likely say the crumpled piece of paper, and they would be correct. Air resistance slows the descent of the flat piece of paper given its larger surface area. Another favourite physics question is, in the absence of air resistance, whether a bowling ball or basketball would reach the ground first when dropped. In this case, while it might seem intuitive to assume that the bowling ball lands first, many graduates of high school physics would explain that the balls actually reach the ground at the same time. 

What might be more surprising to students, however, is that Aristotle, one of the greatest thinkers of his age, was similarly stumped by falling objects. Of course, there were no bowling balls when Aristotle was alive, but he did believe that the heavier an object was, the faster it fell.  

In a new study in the journal Perception, a team of researchers studied human perceptual bias when watching an accelerating object. Perhaps, as the researchers suspected, humans have difficulty perceiving such an acceleration, leading them to erroneously conclude that all objects fall at a constant velocity; this would help to explain Aristotle’s error. 

Fred Kingdom, professor in the Department of Ophthalmology at McGill, came up with the idea for the experiment after noticing his own inability to see the effects of gravity on falling objects, despite knowing that the object must be accelerating according to the laws of physics.

“I’ve always felt, when I look at an object that’s in free fall, I don’t actually get a very strong impression that its accelerating, even though the physics tells us that it is accelerating, and I thought that maybe there’s a bias in our perception of a free falling object,” Kingdom shared in an interview with The Tribune. 

The team set up an experiment, asking more than a hundred students at York University to watch videos of a basketball on their computer screens. 

“We showed a range of different accelerations, and a range of different decelerations, and all the subject had to do on each trial was to say, ‘do you perceive it to accelerate?’ or ‘do you perceive it to decelerate?’” Kingdom said. 

Analyzing the data, it appeared that people do have a bias that makes it harder to determine whether an object is accelerating downward when compared to an object moving up. 

“What we found was that, for the object that was going downwards, you needed to have it accelerating more than if it was going upwards in order to perceive it,” Kingdom said. 

Kingdom’s hypothesis for what causes this bias is just as fascinating as the experiment. Apparently, when we perceive acceleration often enough, our brains can get “tired” of it and, in turn, inaccurately calculate the acceleration of moving objects. A moving train, for example, will seem to be moving in the opposite direction when, in fact, it is merely slowing down.  

“If you are perceiving downwards movement all the time, it might fatigue the neurons which perceive downwards movement, but not the ones that perceive upwards movement,” Kingdom explained. 

Our brains are not seeing objects thrown up in the air as much as objects falling through the air. In turn, the part of our brain that can perceive downward acceleration is “tired” more often the part that can take in upward movement. 

Our brains do their best to analyze the world according to the laws of physics, having evolved over millions of years to do so, but just as computers have glitches, so do the world-generating capabilities of our brains. Of course, this is only one such bias in perception among many, further adding to our understanding of the complexity of our mind’s eye.