There are cosmic snipers firing at random into the universe. What are they and what happens if they hit us?
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00:00Imagine if you could gather the energy from every star within a hundred million light years from thousands of galaxies each with billions of stars.
00:10Imagine you could take this kind of power and use it to fire the biggest super weapon in the universe.
00:16Imagine the damage you could do.
00:19It turns out you don't need to imagine it.
00:22These exist and they're called gamma-ray bursts.
00:26But what are these cosmic snipers?
00:28And what happens if one takes a shot at Earth?
00:37To understand gamma-ray bursts, we first have to understand gamma rays.
00:42Gamma rays are electromagnetic radiation, waves which carry energy just like visible light.
00:48Visible light is a tiny part of the electromagnetic spectrum.
00:52It's the part your eyes can see.
00:54At lower energies, there are radio waves, microwaves and infrared.
00:59And at higher energies, ultraviolet, x-rays and gamma rays.
01:04Gamma rays are incredibly powerful.
01:07A single gamma ray photon is more energetic than a million visible photons combined.
01:13Their high energy makes gamma rays a form of ionizing radiation.
01:18Meaning they are energetic enough to break apart atomic bonds.
01:23This makes them dangerous to you and me.
01:26Ionizing radiation disrupts the delicate biochemical machinery that keeps us alive like a 9mm bullet through a clock.
01:33Fortunately, on Earth, the ozone layer blocks gamma rays, filtering them out before they can harm us.
01:39But if the atmosphere blocks gamma rays from space, how were gamma ray bursts or GRBs from space ever discovered?
01:46During the Cold War, the USA sent up spy satellites which could detect gamma rays from Soviet nuclear tests in space.
01:53They didn't see any bombs, but they did observe faint bursts coming from space, lasting only a few seconds.
02:01To date, this may be the only major scientific discovery made by spy satellites, that we know about anyway.
02:08Astronomers use telescopes that see different kinds of light to make their discoveries.
02:13And these spy satellites gave them a new pair of eyes.
02:16They were a mystery for 30 years, but eventually we discovered the source of a GRB, a galaxy 6 billion light years away.
02:24If a GRB can be seen from such a distance, then it must be incredibly energetic, releasing more energy in a second than the Sun will in its entire 10 billion year lifetime.
02:35Making GRBs the brightest events in the universe.
02:38So, where did they come from?
02:41GRBs accompany some of the most violent cataclysmic deaths in the universe, and the birth of black holes.
02:48There are two types of gamma ray bursts, short and long, and each has their own source.
02:54Long GRBs last about a minute, and scientists think they are produced by supernova, when the core of a massive star collapses to become a black hole.
03:03Short GRBs last a second, and are produced when two neutron stars in a binary merge.
03:09Over millions of years, their orbits decay by emitting gravitational waves.
03:13Once they are close enough to touch, they crash and splash into each other, forming a black hole.
03:19Both supernova and neutron star mergers make the same thing.
03:23Black holes surrounded by a magnetized disk of gas left over from their parent stars.
03:28In these environments, the rotation winds up the magnetic field, which funnels hot jets of particles traveling at nearly the speed of light.
03:37The gas in this funnel creates two tight jets of high-energy gamma rays, like a celestial laser gun.
03:43So, unlike other cosmic explosions, which spread out and fade, GRBs stay focused and can be seen from much further.
03:51Any more detail would require too much mathematics for a YouTube video.
03:56The universe is full of these cosmic snipers, firing blindly and randomly into the dark, and they're hitting us all the time.
04:04On average, we detect one per day. Fortunately, most are harmless.
04:09All the bursts we have detected so far originated outside the Milky Way, too far away to hurt us.
04:16But a nearby GRB could be disastrous.
04:19If one goes off within a few light-years of us, it would totally cook the surface of the Earth, or at least the half that's facing it.
04:26But even a more distant GRB could still end life on Earth.
04:30And it wouldn't need to score a headshot to kill us.
04:33If originating from a few thousand light-years away, it would be a hundred light-years wide by the time it reaches us, washing over the solar system like a tidal wave.
04:43Again, the ozone layer protects us, but it's better equipped to handle the trickle of ultraviolet from the Sun.
04:49A gamma-ray burst would overwhelm it, leaving us exposed to deadly solar radiation.
04:54Ozone takes years to replenish itself by natural processes, which is more than enough time for the Sun to burn the Earth sterile, or at least to kill most complex life.
05:04In fact, this may have already happened.
05:07A GRB has been suggested as one possible cause of the Ordovician extinction 450 million years ago that eradicated almost 85% of all marine species, although it's pretty much impossible to prove.
05:21Gamma-ray bursts could even be one reason we don't see life anywhere else in the universe.
05:26They might be wiping clean huge chunks of it on a regular basis.
05:31It's been suggested that because of GRBs, only 10% of all galaxies might be hospitable to life similar to us.
05:39So, are they going to kill us? Probably not.
05:43In a galaxy like ours, there may only be one GRB per millennia, and to harm us, they must be close and directed at us.
05:51But since gamma-rays travel at the speed of light, we won't know one is headed our way until it arrives.
05:56So, there could already be a GRB on its way to kill us all, and we won't know it until it hits us, and we're dead.