Does Someone Fast See Someone Slow as Wider or Thinner? The Physics of Apparent Size and Relative Motion
The perception of whether a moving person appears wider or thinner to an observer depends entirely on the direction of their movement relative to the observer's line of sight. This isn't a matter of simple "fast" versus "slow," but rather a fascinating interplay of how our brains process visual information and the principles of relative motion.
Let's break down the physics and the psychology behind this phenomenon:
The Role of Relative Motion
Imagine you're standing still, observing someone moving directly towards you. As they approach, their image on your retina expands. This expansion is interpreted by your brain as the person getting larger – appearing wider. Conversely, as the person moves away, their image contracts, creating the impression of them becoming smaller – appearing thinner.
This effect is independent of speed, to a degree. Whether they move slowly or quickly toward you, the relative change in size on your retina is what determines the perceived change in width. A fast-approaching person will experience this effect more dramatically, the apparent change in width occurring much more rapidly than a slow-approaching person.
Lateral Movement and the Illusion of Width
Now, consider the person moving perpendicular to your line of sight—sideways. Here, the perceived change in width is less straightforward. Their movement might trigger an optical illusion known as the phi phenomenon or apparent motion. Depending on the speed, the image might appear to elongate slightly in the direction of movement, potentially giving a subtly different perception of width. However, this effect is generally much less pronounced than the effect of approaching or receding motion.
The Brain's Interpretation
Our perception isn't simply a passive recording of retinal images. Our brains actively process and interpret visual information based on prior experience and context. We subconsciously account for factors like distance, perspective, and the known size of objects to create a coherent understanding of the world around us. This is why even small changes in retinal size can lead to significant perceived changes in the size or width of a moving object.
The Importance of Context
The perceived difference, if any, in width will also depend heavily on the context. A person moving at a high speed would trigger a more significant, and faster, change in the retinal image, potentially leading to a more pronounced perception of changing width. But this perceptual change wouldn’t be consistent across individuals; prior experiences and individual differences in visual processing could also play a role.
Conclusion: No Simple Answer
There's no single definitive answer to whether a fast-moving person appears wider or thinner to a stationary observer. The perceived change in width depends primarily on the direction of movement relative to the observer and the resulting change in the object's retinal image size. While a person moving directly toward you will appear wider, and a person moving away will appear thinner, lateral movement will produce far subtler and less consistent effects. The speed only affects the rate at which this apparent change occurs.
