This answer has been selected and edited by New Scientist staffA back-of-the-envelope calculation suggests the whole Earth might freeze solid within 45 days, radiating away its thermal energy according to the Stefan-Boltzmann law, which relates energy loss of a body to its temperature. My calculations assume the vast bulk of the Earth's captured solar energy is stored in the oceans, which have an average temperature of 15 °C down to a depth of 35 metres. Energy carried by water at greater depths doesn't count because it would rapidly become isolated from the surface by ice floes.With its smaller heat capacity, the land would freeze much more quickly than the oceans. Air over relatively warm oceans would rise, pulling in cold air from the continents. This would chill the surface waters and might increase the circulation of water, exposing it to the chilling, perpetual night.Interestingly, the volcanic dust thrown up by the eruption of the Tambora volcano in 1815 acted both for and against cooling. The dust blocked out the sun, but it also reduced the escape of thermal radiation from the Earth by dint of a greenhouse effect. Sunlight dimmed by 25 per cent for a while, leading to a dip in global temperatures of 0.7 °C in 1816. But the fall in temperature was small despite a big reduction in sunlight, suggesting that the Earth might take longer to freeze than 45 days. Indeed, freezing may well be delayed further by the natural greenhouse effect that comes with our atmosphere and the thermal inertia of our oceans.Nevertheless, Earth would still be able to support a colony of humans. There would still be plenty of energy in the form of fossil and nuclear fuels, and geothermal heat mines. But without plants to replenish our oxygen supply, it would quickly run out, so we would need to build biospheres with artificial light for plant photosynthesis.Thankfully, switching off the sun is an experiment the Earth will not undergo for another 5 billion years.Mike Follows, Willenhall, West Midlands, UK

This answer has been selected and edited by New Scientist staffThe sun can't be turned off like a light bulb. It glows because its surface is about 5500 °C and is heated by the nuclear fusion inside its core which is even hotter - about 15,000,000 °C. Even if fusion in the core could be switched off suddenly, the sun would continue to radiate light just as the heating element on an electric stove gives off heat for a time after you switch it off.Obviously, the sun is bigger and hotter than a stove, so would continue to radiate heat and light for a long time. In addition, the energy produced in the core of the sun takes time to work its way out - millions of years if you track the energy by following the paths of individual photons. The sun would cool a tiny bit each year, but as the sun cools it would contract, releasing gravitational energy that would heat it and offset some of the cooling. That's how white dwarf stars continue shining. Suffice it to say that it would take many millions of years before our descendants even noticed.However, if the sun suddenly vanished, the Earth would cool quickly. Unprotected people would start freezing in days, but they could survive much longer if they went down into deep mines, warmed by the Earth's heat.Jeff Hecht, New Scientist contributing editor, Auburndale, Massachusetts, US

This answer has been selected and edited by New Scientist staffYou can estimate how long it would take us to freeze by extrapolating from the rate of cooling that happens overnight. In areas with clear skies, the temperature can drop to freezing in less than 12 hours. In places with heavy cloud cover, water vapour traps infrared radiation before slowing down the rate at which it radiates away into space, so cooling takes much longer. Here, the temperature drops by perhaps 5 °C in 12 hours. However, without the thermal energy of the sun constantly evaporating water, this insulating cloud cover would quickly disappear. It is likely that most parts of the Earth's surface would be frozen within a few days. The only exception would be near the coastline, where it might take a few weeks because of the amount of heat stored in the oceans.Could we stop this? Perhaps we could quickly burn the world's forests to release large amounts of carbon dioxide to help trap infrared thermal radiation. However, with only fossil fuels, nuclear and geothermal energy left to rely on, we'd still freeze quickly. And if we didn't, we'd soon run out of food and oxygen.Simon Iveson, UPN Veteran University, Yogyakarta, Indonesia

This answer has been selected and edited by New Scientist staffEven if the sun went out, ecosystems around hydrothermal vents along the Earth's mid-ocean ridges would continue to chemosynthesise using geothermal energy for a few thousand million years. So, business as usual for tube worms.Allan Mann, Alnwick, Northumberland, UK

As mentioned by previous answers, the land would freeze relatively quickly, but the oceans, with their huge heat capactity, would take a long time to freeze solid. Initially, the ice caps from the poles would spread south and north, eventually meeting at the equator. This I predict would take a few months to years. Hovever, ice is a good insulatior so the planetwide glaciers would seal off the oceans from the cold. As the oceans freeze, the volume of water drops, the salt of the remaining water would get more concentrated, thus lowering the freezing point of water. However, there will come point when even the supersaturated seas will freeze. I reckon a good 20 years will need to pass to reach this point. Add to this geothermal energy, plus weak tidal heating from the moon, will mean the water never totally freezes. One only has to look at Europa, covered in ice, with a surface temperature at -150C, but display evidence of a global ocean under the ice.After a few decades, the atmosphere would start to condense and the Earth would have seas of liquid air. Further on, even these would freeze, as the surface approaches the tempearture of space, around 3 Kelvin. The only hotspots now would be around volcanic vents which would spew gasses and water into what would be a near vacuum. Further from hotspts, cryo-volcanism would occur - eruptions of liquid air and water-ice slush.Brrrr!!!