If all humans died, when would the last light go out?

This question comes from Alan, who asks:

If every human somehow simply  disappeared from the face of the earth,  

how long would it be before the last  artificial light source would go out?

We'll start with the obvious:  most lights wouldn't last long,  

because the major power grids  would go down relatively quickly.

Without people, there would  be less demand for power,  

but our fridges and air conditioners  and lava lamps would still be running.

Fossil fuel plants, which supply the  vast majority of the world's electricity,  

require a steady supply of fuel, and their  supply chains do involve people doing things.

As coal and oil plants started shutting down in  the first few hours, other power sources would get  

hit with the extra load. This kind of situation  is difficult to handle even with human guidance.

And the result would be a rapid  series of cascading failures,  

leading to a blackout of  all the major power grids.

Nuclear reactors, of course, don’t  require a steady supply of fuel:  

one reactor operator I talked to said that  if their core settled into low-power mode,  

it could continue running almost indefinitely;  a cube of uranium contains about six million  

times as much stored energy as  a similar-sized cube of coal.

Unfortunately, although there's enough fuel,  most nuclear reactors wouldn't keep running  

for long. As soon as something went wrong,  the core would go into automatic shutdown.

Every part of a reactor control system is designed  so that a failure causes it to rapidly shut down.

This would happen quickly; many  things can trigger shutdown,  

but the most likely culprit would  be the failure of the power grid.

However, plenty of light comes from  sources not tied to the major power  

grids. Let's take a look at a few of  those, and when each one might turn off.

Many remote communities, like those on far-flung  islands, get their power from diesel generators.

These can run until their tanks run out of fuel,  

which in most cases could be  anywhere from days to months.

Off-grid generating stations that don't need a  

human-provided fuel supply  would be in better shape.

Geothermal plants can run for a fair  bit of time without human intervention.

According to the maintenance schedule for the  Svartsengi Island geothermal plant in Iceland,  

every six months the operators must change the  

gearbox oil and re-grease all  electric motors and couplings.

Without humans to perform these  sorts of maintenance procedures,  

some plants might run for a few years, but  they'd all succumb to corrosion eventually.

Lights relying on wind power  would last a bit longer. Wind  

turbines are designed so that they  don't need constant maintenance,  

for the simple reason that there are a  lot of them and they're a pain to climb.

The Gedser Wind Turbine in  Denmark was installed in the  

late 1950s and it generated power  for 11 years without maintenance.

Modern turbines are typically rated  to run for 3 years without servicing,  

and there are no doubt some  which would run for decades,  

and one of them would probably have  at least a status LED in it somewhere.

Eventually, most of the wind turbines  would be stopped by the same thing that  

would destroy the geothermal plants:  Their gearboxes would seize up.

Generators that convert falling water  into electricity will also keep working.

An operator at the Hoover Dam once  said that if everyone walked out,  

the facility would continue to run  on autopilot for several years.

Though if the power grid is down, all  that electricity would have nowhere to  

go. In the end the dam would probably  succumb to clogged intakes or the same  

kind of mechanical failure that hit the  wind turbines and geothermal plants.

Battery-powered lights wouldn’t fare much  better, and will all be off in a few dozen years.

Even without anything using their power,  batteries eventually self-discharge.

Some types last longer than others, but  even batteries advertised as having long  

shelf lives typically only hold  their charge for a decade or two.

Solar power is probably the most promising  candidate. There are many off-grid solar-powered  

buildings, weather stations, and other remote  infrastructure around the world. Emergency call  

boxes, often found along the side of the road in  remote locations, are frequently solar-powered.

They usually have lights on them, which provide  illumination every night. Like wind turbines,  

they're hard to service, and  they last for a long time.

Solar panels will generally last as long  as the electronics connected to them,  

and as long as the panels are  kept free of dust and debris.

The wires and circuits will eventually succumb  to corrosion, but solar panels in a dry place,  

with well-built electronics, could easily  continue providing power for a century if  

they're kept free of dust by occasional  breezes or rain on the exposed panels.

If we follow a strict definition of  lighting, solar-powered lights in  

remote locations could conceivably be  the last surviving human light source.

But there's another contender, and  it's a weird one: spent nuclear fuel.

Contrary to popular portrayals,  radioactivity isn't usually visible,  

which is part of why we need warning signs  around areas with radioactive materials or waste.

Sure, watch dials used to be coated in  small amounts of radium to help them  

glow in the dark, but the glow didn't  come from the radioactivity itself.

It came from the phosphorescent paint on top of  the radium, which glows when it’s irradiated.

Once the phosphorescent paint breaks down,  

the watch dials are still  radioactive, but no longer glow. 

Watch dials, however, are not our  only radioactive light source.

When radioactive particles travel  through materials like water or glass,  

they can emit light through  a sort of optical sonic boom.

This light is called Cherenkov radiation,  and it's seen in the distinctive blue  

glow of nuclear reactor cores. Some of our  radioactive waste products, such as cesium-137,  

are melted and mixed with glass, which cools  into a solid block before being wrapped in  

more shielding for transport and storage. And in the dark, these glass blocks glow blue.

Cesium-137 has a half-life of 30 years,  which means that two centuries later,  

the blocks will still be glowing with  1% of their original radioactivity.

Since the color of the light depends only on  the particle decay energy, and not the amount  

of radiation, it will fade in brightness  over time but keep that same blue color.

And thus, we arrive at our  answer: Centuries from now,  

deep in concrete vaults, the light from  our most toxic waste will still be shining.