There was once upon a time that photography shutter times were so slow, they had never captured portraits or humans in pictures. The first photograph, 1826, was of a stationary scene. It was not until 1838, when humans were accidentally captured and observed on photograph:
This is a busy street with lots of carriages and pedestrians!
But where are they? They are invisible because of motion blur!
See the person at the shoeshine, standing still:
The rest of the photograph doesn't show anything because all the pedestrian traffic & horse carts are so motionblurred because camera exposure (persistence) was a full 7 minutes! At this level, blurring becomes so faint and prolonged that moving objects are rendered imperceptible. The photo film is so insensitive to momentary changes.
It apparently took many months (after this photograph) before people figured it out, that people had to stay still in order to be photographed! In this early photography era, people such as Sameul Morse (inventor of morse code) apparently thought it was very strange/amazing that moving objects were rendered invisible in photography.
As exposures shortened to less than a minute, you finally saw faint motion blur in moving objects. And exposures became shorter, there was less motion blur, and you finally could eliminate motion blur with ultrafast shutters or ultrafast flash photography. But when image persistence is several minutes, most moving objects becomes invisible to the human eye!
As a thought experiment, to understand this better, you take a camera photograph of a human walking at average speed:
Motion blur for 0.001s shutter -- clear photograph
Motion blur for 0.1s shutter -- slightly blurry
Motion blur for 1s shutter -- super blurry
Motion blur for 5s shutter -- very blurry and ghostly motion. Clear background shows through.
Motion blur for 15s shutter -- faint ghostly smear that's mostly see-through to background.
Motion blur for 1min shutter -- super faint smudge smear, almost no evidence of motion. Background perfect looking.
Motion blur for 5min+ shutter -- smudge is so imperceptible it completely disappears from human eye.
Motion rendered invisible by motion blur!