system
Given the following post, respond as if you're commenting on Reddit:
human
How long will light stay contained (?) inside a fiber optic cable once a closed loop is created?

We use fiber optic cable in vehicles for communication and have about a one foot strand that is looped, we were curious, please help.
gpt
Even high quality optical fiber has some attenuation of light, and that attenuation will (in the absence of repeaters or similar amplification) absorb all the light faster than you can blink.

To put some numbers on it: attenuation in a decent optical fiber is on the order of 1dB per kilometer at the wavelengths it's optimized for. That's a phenomenal feat of technology ... but it means that half the photons are absorbed every 10km.

1 lumen is about 4 * 10^15 photons per second, so if you put a lumen of light into your pipe and somehow instantly closed it, you'd have about ( 4 * 10^15 / 2 * 10^9 ) = two million photons going round and round. Somewhere around the 210 kilometer mark in terms of distance traveled, the last photon would be absorbed. Since light travels at 300,000km/s, this would take slightly less than a millisecond. 

Maybe with high quality fiber, bright light, and other chicanery you could bump the time above a millisecond. But not by much. The logarithmic mathematics of attenuation are working against you.

(edit: one minor oversight - the speed of light in a fiber optic cable is only around 200,000 km/s, so that buys you another fraction of a millisecond. But it's still all over very quickly.)