Why does my ceiling glow in the dark?

fro2g said:

TL;DR Summary: My ceiling glowed in the dark. Can you explain this?

Can you explain this for my grandmother?

Four years ago I rented an apartment for her on the 7th floor in a new building. I then decided to take care of my grandmother, so I lived with her. Everything was fine, but a year later the light bulb in the guest bathroom burned out. Since I'm the only one using this room, we didn't change the lamp. I used a flashlight. One day I was taking a shower in complete darkness and saw a light on the ceiling around the sewer pipe. Not a flash, but a white soft light, which is noticeable only in complete darkness at any time of the day.

For reference: there are no windows or other light sources in the room. It's just a concrete ceiling + water-based paint (or lime, I'm not sure). Everything was fine in the other bathroom. Of course, I asked my grandmother and she also saw this light. I couldn't photograph it because when the light comes on the spot disappears. If the lamp in the bathroom hadn't burned out, I would never have noticed it.

However, the light didn't bother us in any way, so we ignored it.
This light was visible for about a year or two and then disappeared. One morning my grandmother called me and said that there was no more light. After that he never appeared.

We moved six months ago, but now my grandmother constantly asks me about it. I don't know what to answer her. I'm worried that she might develop mental problems. I don't believe in the supernatural, so I'm looking for a scientific explanation.

I hope for your help (sorry for mistakes, English is not my native language)

.Scott said:

Phosphorescent and Fluorescence are consistent with what you have described is common in many materials - including those used in house paints. But that would be my second guess.

My first guess is that the sewer pipe stretched from floor to ceiling (or wall to wall) and that the entry from the floor was not light tight. So what you were seeing was light from the bathroom in the unit below yours leaking through the crack between the concrete floor and the pipe - then projecting onto the ceiling above.

A few things are worth noting:
1) Once healthy human eyes have dark adapted, they are really good at detecting photons. Rod cells (vs. the color sensitive cone cells) are more numerous (by a factor of about 100 million) and more sensitive (by a factor of about 300). So even if the downstairs resident can just barely detect color with the available light, you will need less that a billionth of that to detect it.
2) Because the sewer pipe will expand and contract from temperature changes, it must be able to slide against the opening in the concrete floor. This makes it very difficult to precisely mate a metal sewer pipe and a cement floor without a soft flexible material - like calking or a rubber gasket. And those soft materials can work free.
3) Although some paints and other chemicals are phosphorescent, the effect isn't very long-lasting.

fro2g said:

I thought about it, but the light didn't go away for a year, 24/7. I bought a new lamp for the bathroom only 10 months later...

Averagesupernova said:

One other thing that comes to mind is if the ceiling is truly concrete, it seems unlikely it was poured with the pipe in place. Yes, pipes come out of concrete floors all the time. Usually a slab laid on the ground or a basement floor. What I assume would have been done is a sleeve was placed in the form prior to the pour. Then the pipe was slipped through and some caulking or firestop material (if required) was put around the pipe. That could leak light through from the other side.

fro2g said:

this light disappeared a year later.

fro2g said:

I don't know, but my grandma is trying to convince me that it was spirits, aliens, radiation or spies, haha

fro2g said:

worried that she might develop mental problems.

fro2g said:

sounds logical, but this light was visible 24/7. I know the neighbors don't leave it on.

DaveC426913 said:

Also, how do you know it was on 24/7? All you know is that it was on when you checked, no?