Thanks to my friend Darlene for pointing me to this article!

I’ve talked a lot about invisibility on this blog and have written papers on invisibility physics and even written a book on the history of invisibility physics, but still there are fun new discoveries that surprise me in a fun way!

Take, for example, the recent news from Northwestern University that engineers there have made a small drone that is, if not invisible, pretty dang hard to see while in flight! Their video of the “Phantom Twist” drone is quite spectacular.

The drone relies on motion blur, in which a rapidly vibrating of spinning object can appear transparent simply because enough light passes through every location where the object would otherwise block it, and the object is spinning too quickly for the human eye to follow its motion. The drone itself is mostly thin sticks held together, which in motion become very hard to see. The engineering team used machine learning (an actual useful form of AI) to optimize the drone design for both stable flight and minimum visibility.

There isn’t much insight I can provide about the drone that isn’t covered well in Northwestern’s press release linked above, but I wanted to share one reason this story delighted me: science fiction got there first! In 1930, the story “Invisible Death” by Anthony Pelcher appeared in Astounding Stories of Super-Science, a story of murder aided by invisibility technology.

In the end, it is revealed that a murdered scientist named Darius Darrow had invented invisibility based on, you guessed it, motion blur:

Darrow got his idea from watching a rapidly revolving wheel. He noticed that the spokes and rim blended into a blurred disc when a certain speed was reached. The entire wheel was practically invisible, under certain lighting conditions, when a higher speed was attained.

Darrow went further and reached the conclusion that there was a rate of vibration that would produce invisibility. This was accepted in practically all engineering research plants, long before it was perfected by Darrow.

“The facts are that any rapidly vibrating object becomes more and more difficult to outline as its rate of vibration increases. All that was left for Darrow was to arrive at the exact mathematical time, tone, or rate of vibration producing invisibility and to construct a vibrator tuned to produce this condition.

In the story, the motion blur is imagined as a form of vibration, but the analogy is made to a rapidly revolving wheel, which is basically the strategy used by the drone! As has happened a number of times, science has caught up with science fiction and at least partly realized its vision.

Do we need to worry about invisible drones spying on us all the time? Probably not; it would be hard to upscale this design to something large enough to travel long distances and remain invisible — among other things, the bigger the spinning object is, the stronger the forces become trying to pull it apart. Also, it makes a heck of a lot of noise! I expect that they will find ways to reduce the noise, but rest assured that you don’t have to worry that invisible drones are watching you as you read this!