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Only partly kidding...

Let's suppose that a thousand, or a million, years ago there was an explosion of some gigantic star (or other celestial object) that was strong enough to destroy Earth and Life As We Know It in this sector of the Universe. But it happened a thousand, or a million, light years away, so the event has not yet come to our attention.

What would be the implications of this (beyond Stack Exchange being wiped out)? Would humanity likely get a year's warning? A hundred years? (I won't ask what Humanity would do with the knowledge, because we all know it would make the stupidest decisions possible.)

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    $\begingroup$ Comment to the question (v1): It is preferred if the title question (about the end of the Universe) and the question in main text (about the end of the Earth) ask the same question. $\endgroup$
    – Qmechanic
    Mar 4, 2015 at 0:43

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Fortunately, explosions tend to decay in intensity like $1/r^2$, so no known celestial events (e.g. supernovae) would really be able to do that. In any case, with these sorts of things, we could presumably see that such an event was "about to happen" and had simply not happened yet.

There are actually some awesome doomsday cosmologies which are related to this idea, though! One of the best is the false vacuum decay event. This is the possibility that the zero-point energy of our vacuum is not the true minimum possible energy, but is instead separated by some barrier from a lower minimum. If the barrier is finite then eventually, quantum mechanics dictates that some part of space will tunnel through the barrier, relaxing to the lower energy state. As it decays, however, it makes it possible (definite, even) that nearby points of space will also decay. It turns out that this will lead to a bubble of death expanding at the speed of light. The laws of physics within this bubble are different from those of our laws and probably do not support the existence of atoms, so any particles that are hit by the wall of death just instantaneously dissolve. Since it expands at the speed of light, there is no actual way to see it coming with any possible experimental apparatus -- and of course you couldn't stop it even if you could detect it. Just one day, the wall of death passes through the solar system and everything goes bye-bye without any fanfare or even the slightest inkling that something is wrong. The best part about this: by looking at the mass of the recently-discovered Higgs boson you can find out whether this is possible, but the discovered mass is right on the threshold: we can't totally rule it out.

Your only real hope in such a scenario is that the many-worlds interpretation of quantum mechanics is correct. In that case, the only universes where you will continue to exist will be the ones where the wall of death has not yet hit Earth. These universes are vastly outnumbered by the ones where we are being destroyed by the wall of death, but fortunately we are not gods and therefore we don't have the global multiverse perspective needed to see this perpetually-ongoing destruction of the Earth.

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    $\begingroup$ since the wall's radius only expands with c, but the universe itself expands superluminal, such an event will never kill the whole universe, only the ingredients of one event horizon (roughly one hubble sphere). $\endgroup$
    – Yukterez
    Mar 4, 2015 at 6:48
  • $\begingroup$ But if this is possible, then eventually some part of space within our Hubble sphere will tunnel through the barrier. $\endgroup$
    – mmesser314
    Mar 8, 2015 at 15:44
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A gamma-ray burst is rather more likely than false vacuum - we observe them on a regular (daily) basis. If one happens in our galaxy, and it's pointed at us, we had a good run.

The mechanics are fairly simple - massive star collapses, huge amount of energy squirts out in 2 directions, not affected by $1/r^2$ spherical expansion. Anything in the way gets microwaved. Things on the far side of the planet might survive the initial event, but the destruction of over half the ecosystem means they just might have enough time to document it for future settlers.

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  • $\begingroup$ So, is there any sign that a gamma-ray burst is about to happen? And how long would the burst last, and would it build slowly over (at least) seconds or be done in a (literal) flash? $\endgroup$
    – Hot Licks
    Mar 4, 2015 at 0:31
  • $\begingroup$ en.wikipedia.org/wiki/Gamma-ray_burst - it depends on type of the burst. $\endgroup$
    – Mithoron
    Mar 4, 2015 at 0:43
  • $\begingroup$ For supernovas' we'd perhaps have a few hours warning due to neutrino radiation reaching us first--but this technique wouldn't work for GRBs at least with today's detectors. IceCube put some severe limits on high energy neutrino production from GRBs: arxiv.org/abs/1204.4219 $\endgroup$
    – Andrew S.
    Mar 4, 2015 at 1:06
  • $\begingroup$ So (answering my original question), a GRB in the Milky Way might be 50,000 light years away, meaning that one with Earth's name on it could have occurred 50,000 years ago, but we haven't heard of it yet. But reading the Wikipedia article it doesn't appear that a GRB would necessarily destroy all life on Earth -- just make things "interesting" for a few millennia. $\endgroup$
    – Hot Licks
    Mar 4, 2015 at 1:10
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Why do you think there would be any warning at all? There's not enough matter out there to propagate a blast wave, thus it pretty much has to be radiation. Radiation that's traveling at virtually lightspeed (even intergalactic space isn't a perfect vacuum) and that we would sense by means of radiation traveling at the same speed.

Only something that propagates below lightspeed could possibly be sensed. What would that be?

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  • $\begingroup$ Well, it's possible that the blast could build slowly, or that there would be some sort of event that "telegraphed" an impending blast. $\endgroup$
    – Hot Licks
    Mar 4, 2015 at 13:12

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