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I'm aware of hard radiations from space can wipe out life but I believe that would also causes the planet to be inhabitable for a period of time. Is there any other kind of energy that could destroy life without any "cool down" needed for new life to develop?

This energy could be known or theoretical or just plausible that spans entire galaxy if possible.

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  • $\begingroup$ The "kill everything" part seem possible. But not spanning a whole galaxy. That is just too big. $\endgroup$ – James K Feb 10 '17 at 22:53
  • $\begingroup$ Not necessarily whole galaxy at once but eventually? $\endgroup$ – Curious Smith Feb 10 '17 at 22:55
  • $\begingroup$ I'm on my phone right now so I can't do much research but maybe a neutron bomb might do the trick. So the trick would be to have something release enough neutrons to kill what you need to kill. $\endgroup$ – Erik Feb 11 '17 at 1:12
  • $\begingroup$ ISTR a gamma-ray burst with its beam aligned on our galaxy taking place a few galaxies out would be catastrophic for life throughout the galaxy. Extinction for Earthlike multicellular terrestrial life. $\endgroup$ – nigel222 Feb 15 '17 at 20:44
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Funnily enough, the energy burst with the shortest cooldown that I can think of would be heat.

Most other radiation sources tend to kill living things by ionising their atoms. However life will suffer continued radiation for extended periods as ionised elements will often become radioactive to some extent.

Heat of course kills by providing activation energy for chemical reactions (such as burning), and once a heat burst has ceased, life will recover reasonably rapidly.

There is however a hard limit to this: If you need to kill all water dwelling life, you will need to boil oceans, at which point you are likely to see water acting as a greenhouse gas, creating a runaway effect, ultimately causing permanent uninhabitability. It could be fine on a desert planet perhaps, especially if you melt enough sand to a mirrored finish, thus reflecting heat back out.

Edit: I also realised that microwaves are a great delivery method for heat, generally not heating up most inorganic matter. This will lead to the sand not melting in the desert world scenario though.

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  • $\begingroup$ That could work very well. I was just reading up on evolution of Earth, it seems like water didn't show up until later when it was freed from underground. There could have been some kind of galactic heat wave that swept through the galaxy before Earth had water. $\endgroup$ – Curious Smith Feb 10 '17 at 22:49
  • $\begingroup$ I'm aware I'm not really covering the kill the galaxy aspect, but frankly that requires a MK - 4 plot device. Perhaps a kardashev type III + civilization deciding that they want a galaxy free of life. Whatever fits your story and a lot of arm waving. More background and I'll try and cover that too. $\endgroup$ – Dubber Rucky Feb 11 '17 at 12:07
  • $\begingroup$ There is however a hard limit to this: If you need to kill all water dwelling life, you will need to boil oceans -- yes and no. Heating above 45 degrees Celsius, well short of boiling point, will kill most lifeforms through protein denaturation. 55 °C will kill also most "thermo-resistant" life forms. On the other hand, if all life includes also the Archaea termophiles and hyperthermophiles, then boiling the oceans might actually not be enough. $\endgroup$ – LSerni Feb 11 '17 at 12:45
  • $\begingroup$ @LSerni I was thinking of the need to boil in reference to bacteria that dwell around thermal vents. I'm not sure of exact required temps, but once the sea is boiled, the greenhouse effect should leave the planet something like venus, which I believe will sort out those pesky thermophiles eventually. $\endgroup$ – Dubber Rucky Feb 11 '17 at 12:50
  • $\begingroup$ New thinking on the asteroid that killed the dinosaurs: after the impact kicked a bunch of stuff into space, re-entering pieces heated the atmosphere up to the point that it killed any air breather that did not have a deep tunnel to hide in. So all the big water creatures but not crocodilians because some of them estivate in mud tunnels. Ditto turtles. $\endgroup$ – Willk Feb 11 '17 at 21:31
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Its possible for radiation to damage life, but it's harder to wipe it out, and it's just not possible for it to happen on a galactic scale.

A gamma ray burst is a massive explosion that releases huge amounts of Gamma rays in a beam. We have observed very distant ones, from half-way across the observable universe. Fortunately, we have not had one close to us, since it could kill a large fraction of land life. However, gamma rays are not very penetrating of water. They can travel a metre or so before being blocked. So animals under the sea would probably survive. Those animals that weren't killed outright would be hurt by the collapse of food chains and the damage to the ozone layer. But I don't think you can kill all life this way.

Heating the planet to boil the oceans would probably kill nearly everything, but this would require a massive amount of energy (10^25J). If you took all the energy of the sun over two years, it would be enough to boil the oceans, and probably kill all life. A nearby supernova could do this, but there wouldn't be a habitable planet left afterwards, it would probably kick start a runaway greenhouse effect. It certainly would take some time for the world to cool, even if this is avoided.

Such an event can't happen galaxy wide. For a start, the galaxy is so big that it would take 100000 years even for light to cross it. And the scale is much bigger. It has a cross section that is 10^28 times bigger, so you would need 10^28 times more energy (10^53 J). This puts it outside the realm of reality. It is the sort of energy you get if you make every star in the galaxy into a massive giant star, and then make them all go supernova at once.

Becoming more speculative. It is sometimes hypothesised that the physics of the universe are not truly stable, and could change False vacuum. A bubble of new physics could spread out from a point at the speed of light. Now usually we suppose that this bubble of new physics would be fundamentally different from our universe, but suppose that the new universe was identical, except that gold was highly radioactive. The tiny amount of gold that we have in our body (0.2mg) would breakdown in radioactive decay in a few seconds, subjecting us to a massive radiation dose (about 1000Sv) The same thing would happen to the rest of life. The planet would be sterilised, and the bubble would spread, killing of any life in the universe (given enough time) New life could evolve inside the bubble. However, nothing could travel into the bubble if it had even a trace amount of gold. And there is no way for anyone to know of the existence of the bubble in order to prepare for it.

This scenario is not scientifically plausible, but if you wave your hands fast enough, people might not notice.

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  • $\begingroup$ Well, gold is a suitable candidate - fourmilab.ch/documents/golden_glow $\endgroup$ – LSerni Feb 11 '17 at 0:50
  • $\begingroup$ I was skeptical that it would take all of the sun's energy for 2 years to boil the oceans when the sun is about a million times larger than the earth. I think your figures are inaccurate. This article says that it would take 4.7x10^26 J of energy to boil the oceans, while the sun outputs 3.8×1026 J each second, so it would only take about 1.2 seconds of the Sun's output to boil the oceans. $\endgroup$ – Johnny Feb 11 '17 at 4:28
  • $\begingroup$ Only the sun's energy that hits the earth: solar constant*earth cross section. The 4.7*10^26 number is heat of vapourisation, but the water first needs to be heated, and water has a huge heat capacity. $\endgroup$ – James K Feb 11 '17 at 9:23
  • $\begingroup$ The 4.7*10^26 number is 10X higher than your unsourced number, it takes around 350 J to heat a gram water from 17C to 100C, then 2260J to vaporize it, so it doesn't make a huge difference in the numbers. The earth receives around 173,000 TW of continuous solar radiation, or 1.73e+17 J/sec. If it all went to heating the oceans it would take 86 years to boil the oceans. $\endgroup$ – Johnny Feb 11 '17 at 19:12
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No cool down period? Not even one nanosecond? Sounds like there's a lack of understanding of reality here. Life exists on Earth from high in the atmosphere to deep underground. What you want to do apparently (but you're not clear) is irradiate the entire planet so that there's a zero survival rate even for organisms heavily shielded and isolated. And do it with such intensity that the dose is supplied virtually instantly. We don't know how deep underground microbes exist, we've found evidence for them as deep as 12 miles. The most obvious sterilization methods I can think of which could penetrate 12-15 miles of rock would be X-rays but it would leave radioactives behind. I'm not capable of calculating the extent such energy would transmute atoms, but it's fairly trivial to calculate the intensity of a beam of x-rays (of a given frequency (distribution)) which would effectively sterilize through 15 miles of rock. The issues with waste heat might be addressed by cooling the Earth by passing an opaque (or semi-opaque) cloud between the Earth and the Sun. So, I'd say runaway greenhouse effect is a red-herring. What the surface would be like after such an attack, I've no idea. Perhaps someone else can calculate the effects of an extreme x-ray burst, if you ask that specific question. (heat and radioactivity are the most important effects, there may be secondary effects (such as loss of atmosphere))

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  • $\begingroup$ No, X-rays do not leave radioactive substances behind. Not even most Gamma or Beta radiation can do this. You need a particle energy of above 10 Mega-electronVolt (MeV) for that and most gamma and beta cannot reach that high. Some Alpha radiation can but Alpha has really poor penetrating power so it cannot reach more than 3-5 cm in air. The only radiation that practically can induce activity is neutron radiation. And again: the atmosphere is an issue, not to mention that free neutrons have a half-life of 14 minutes, making it near impossible to get a significant amount to Earth. $\endgroup$ – MichaelK Feb 11 '17 at 12:17
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Thanks everyone for your suggestions. I see now that there are many limitations no matter what form of energy I use. Since my ideas are set in far future, I guess I will work up an alternative previously undiscovered energy which results in superluminal velocity that effectively sterilize everything it encounters.

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