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Is it possible for humans to survive on the surface of the Earth, assuming the Sun was, for some reason, not emitting any light/heat/other radiation? Would whether the Sun was actually present affect this survival?
EDIT: By survival I mean live long enough to give birth to a further generation.
Short answer is NO.
A slightly longer one is that the Earth's entire ecosystem (with a couple of very specific exceptions which I'll get to) is solar powered.
Plants use photosynthesis to produce O2 and carbohydrates. Photosynthesis is an endothermic (energy storing) reaction that takes water and CO2 and produces Oxygen and carbohydrates. The important point here is that the reason that plants use sunlight to do this is that they need to harvest energy to power the reaction. That energy is effectively 'stored' in chemical form, which they then reverse to extract their own energy and produce enough by-products (O2 and carbohydrates in their own biomass) that we in turn can also use those chemical forms to power us as we convert them back to CO2 and water.
So without the sun as a source of energy, no plants and no us. We'd suffocate first, but plants wouldn't last that long either as eventually all the atmospheric O2 would be converted to CO2 (via carbohydrate -> water conversion also) and even the plants would eventually suffocate or starve.
That said, there are some specific ecosystems on Earth, very deep in the ocean, where there is a large amount of life surrounding Black Smokers, a form of hydrothermal vent. These ecosystems use the thermal energy from these vents to support their life cycle, proving that the sun is not essential to ALL life on Earth, but these vents are unlikely to be able to support even a small human population even if we were down there.
Of course, we haven't even got onto the topic of planetary surface temperatures without the sun; the surface of the Earth would effectively go into a deep freeze, meaning that humans would not be able to survive even if O2 and food was available.
What all this means is that the Earth's life requires energy to survive. All animal life is parasitic to some degree, insofar as it consumes vegetation to survive. Even vegetation is parasitic as far as the Earth is concerned, harnessing the heat of the sun and the mineral content of the Earth to survive. Without that sun, the entire system breaks down as there is no energy entering the ecosystem to harvest or exploit.
We live on a planet that orbits its star at just the right distance to make life possible, introducing enough energy to keep water in a liquid form and to support the processes that are essential to life. This will not always be the case as the sun slowly expands to be a red giant. That will be the exact opposite of the scenario described in this question, but it will be no less lethal to life on this planet. When you get right down to it, our existence is a lot more fragile (and more improbable) than one might think.
The sun is an essential part of that existence, which would be impossible without it.
A small population might survive by going underground and using geothermal and nuclear power to survive and grow food via artificial sunlight.
That being said long term survival (beyond a few decades) would be unlikely due to exhaustion of supplies, lack of raw resources for repairs and reactor fuel.
