This is why so many of us are fascinated by them. We want to learn about black holes—even see them—but we certainly don't wish to get close to one!
Except ... when the 'black hole' in question is just a super cool optical illusion. Like the one at the top of the page!
Does it look like it's expanding to you?
The illusion in this case is not an animated GIF, we promise. Instead, it involves many static, or motionless, objects.
In the background is a pattern of simple black ovals on a white background. But in the middle is a much larger black oval. The edge of the middle oval is not sharp or hard. Instead it is what is called a gradient—it gradually goes from full black to white.
For most people—86% of those in a recent study conducted by a trio of researchers from Norway and Japan—this image creates a feeling of expanding darkness. For a few seconds, it feels as though the black hole is growing.
Go back and take a look. How do your eyes react?
Getting ready for change
So why is this happening? And why is it only really happening with a black oval, and not one that is white or other colours (which were tested by the study). The researchers have a theory—our brains are getting prepared for a change in darkness!
In their research, the scientists were careful to observe the eyes of the people who saw the image. The people who said that they saw the hole expand also had their pupils dilate, or expand. And what's more, the longer a person said that the hole was expanding, the wider their pupils were dilating.
In other words, the illusion of growing darkness and the dilating pupils went hand in hand!
Our pupils dilate in lower light environments so that our eyes can get more light in. It is an involuntary reaction that helps us to see (better) in the dark. So in effect, the expanding hole in this illusion is likely just an example of our brain being briefly tricked into trying to let in more light because it thinks that a darkness is coming!
Which makes this optical illusion a real eye opener!