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Is the world going to end on December 21, i.e. today? I’m glad I asked, and not just because having a question like that in your lede is total SEO-bait. No, the world is not going to end, at least not for the reasons you’d expect. One segment of the Mayan Long Count calendar ends on the 21st, but another one is all set to begin — heck, there are actual Mayans still out there who are planning on having a big party to celebrate the beginning of a new era. There’s no Planet X or rogue star coming to ruin our day — if there were, we’d have totally seen them by now. They wouldn’t even be a secret — you can’t exactly block the sky, and amateur astronomers would have been all over it.

Relax.

Now, here’s two ways we could all die any minute now that have nothing to do with December 21.

Relativistic kill vehicles
Science fiction lies to you all the time about alien invasions. Let’s say there’s an alien species out there who wishes us ill. Maybe they don’t care for our television shows. Maybe they’re space racists. Maybe their particular religion encourages the wholesale slaughter of all nonbelievers.

More likely, what would happen is this: If they were thinking of invading, they’d likely have some kind of lightspeed or faster-than-light travel, so they’d assume that, given enough time, we’d be able to develop similar technology. They’d further assume that we didn’t get to the top of our food chain without being somewhat ruthless (they would have a lot of evidence if they cared to investigate). They’d be forced to conclude that, given enough time, we’d become a threat to them.

So, they’d strike first.

What they wouldn’t do, of course, is do something as silly as invade. Invasion is messy and risky and really not terribly necessary. Instead, they’d shoot us.

Once you can make something move at the speed of light (or faster, if indeed that’s even possible), you can turn pretty much anything into a really scary weapon. The proper term for this is a relativistic kill vehicle, or RKV. Strap a lightspeed engine to an asteroid, say — maybe one as big as a small moon, point it at what you want to be dead, and turn it on. Now you have a bullet that can’t be tracked (since it’s moving at lightspeed, you literally won’t see it coming until it’s right on you) and therefore can’t be stopped. You’d be able to launch it from anywhere at anytime, with no warning, and there would literally be nothing anyone could do about it. Even if you somehow managed to stop the first one, what’s to prevent your enemy from launching another one? Or another thousand?

Considering all the horrible stuff we’ve come up with in the past, the aliens would figure that we’d try to use RKVs on them if we could. They’d probably be right.

Anyways, if they did happen to know where we were, they could have already sent a bunch of RKVs hurtling at us. Better to take care of the problem now, before we start getting any ideas. Any time now, right?

Gamma-ray bursts
Imagine a massive star, much bigger than the Sun. Actually, give this video a quick watch to picture the scale I’m talking about:

When these stars go supernova at the end of their lifetimes, they’re going to very temporarily give off an awful lot of energy — say, 10 billion times as much as our sun will emit in its entire lifetime. It emits these bursts of energy from its poles — so, the top and bottom of the star. They come out in a focused beam that moves at somewhere awful close to the speed of light.

We’ve seen these happen before, all outside our own galaxy. That’s pretty good, because they’re way too far away to do any damage when they hit. Let’s say one did go off in our galaxy, close by, and we were unlucky enough to be in the path of the beam. It’d blow off much of the atmosphere like the seeds of a dandelion, which would be somewhat disruptive to things like crops and global temperatures and pretty much everything else. Everyone on the side of the Earth facing the blast would be so heavily irradiated that we’d probably all end up looking like this guy from the first RoboCop movie:

Maybe it won’t be quite that dramatic, but we’d all be in for a very bad time. There’s some thought that a gamma-ray burst might have been the cause of an earlier extinction, some 450 million years ago. We’d honestly have about as much warning as the trilobites did.

So, enjoy the rest of today. The doomsday prophets, as usual, were wrong. The apocalypse might come someday, but take heart: it’ll be random, probably very quick, and you won’t have to deal with irritating posts on Twitter and Facebook in the weeks leading up to it.

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