Some of the cataclysms we have explored occurred on Earth. Some unfolded near us in space. But a faraway cataclysm that wreaks havoc in its own neighborhood and is too distant for us to even see wouldn’t affect us, right?
Not quite.
Sometimes the violence is so extreme, it sends dangerous debris flying at speeds that are off-the-chart high, shrapnel that can travel all the way to our planet and even penetrate our bodies to kill some of us. Impossible? Well, gather round the campfire and hear about a peril unmentioned in your insurance policy’s fine print.
The bullet fragments are cosmic rays, arguably the most intimidating label in all of science. They definitely live up to it. Even their history is intriguing.
Their presence was first suspected over a century ago, when physicists noticed that some electrified laboratory objects slowly lost their positive or negative charge for no apparent reason. Some electrical entity was apparently sneaking into the lab. That it came from the sky was not at all obvious until 1912, when Austrian physicist Victor Hess carried a charge-measuring instrument aloft in a balloon and found that charges increased as he ascended. The culprit was assumed to be some kind of invisible beam from outer space, and thus the term cosmic ray was born.
It took until 1950 for scientists to prove that these were not rays—meaning electromagnetic beams—but solid particles, although the original name remains. Further studies showed that 89 percent of them were ordinary protons while 10 percent were alpha particles—packages of two protons and two neutrons. The remaining 1 percent were electrons. All carried electric charges and were thus influenced by magnetic fields.
A proton is a hydrogen’s nucleus. An alpha particle (two protons and two neutrons) is a helium’s nucleus. Their cosmic-ray proportions match the relative ratio of hydrogen to helium in the cosmos. It’s as if pieces of the universe are leaking into our bedrooms. What doesn’t make sense is why electrons—more abundant than protons and alpha particles in the universe—are so underrepresented in cosmic rays. But that’s only the first of the mysteries.
A cosmic ray’s energy depends on its speed, since a faster-moving object does more damage than a slower one. Cosmic rays (CRs) arrive in an amazing range of velocities; their power is expressed in electron volts, or eVs. The commonest, slower ones were created in the sun and present little mystery. The higher-energy CRs come from deep space. Then there are ultrahigh-energy cosmic rays, or UHECRs, which are so incredibly powerful that they’re utterly baffling.
Humans, happily, are protected from most cosmic rays by our planet’s atmosphere and, to a lesser extent, by its magnetic field. Still, enough CRs reach your body to deliver about twenty-six millirem of radiation exposure annually. It’s probably not harmful and possibly even beneficial, even if it’s 10 percent of a dose that would definitely sicken you.
You get an extra five millirem for every one thousand feet above sea level that your home is located. CRs really crank up their intensity in the upper troposphere at 35,000 to 42,000 feet, which is why you receive an extra millirem for each thousand miles you travel by plane. It’s frequent-flier radiation. Thanks to their extended time in that high-CR environment, airline crews have a 1 percent higher lifetime cancer incidence—twenty-three cases in one hundred, instead of twenty-two.
Astronauts who leave our planet’s protective magnetic field to venture to the moon or, maybe someday, to Mars face a fearsome cosmic-ray environment. In space, five thousand cosmic rays tear through the body each second. For astronauts on a multiyear mission, this will probably create an enormously elevated risk of cancer and the wholesale destruction of brain neurons. Future Mars explorers had better start out with high IQs because they might have trouble with graduate-school entrance exams upon their return.
Shielding is problematic. To achieve the same cosmic-ray blockage that Earth’s atmosphere provides, you’d need to huddle beneath sixteen feet of water or something with its mass equivalent.
While most cosmic rays have energies between ten million and ten billion electron volts, the much more energetic ones intermingled with them are utterly inexplicable. Cosmic rays of over one hundred million trillion eVs have been detected periodically since 1991, and these are forty million times more powerful than anything we can create in a particle accelerator. A single such cosmic-ray particle can deliver a wallop equal to a tennis ball hitting you at a hundred miles an hour, even though it’s far smaller than an atom. They’re assumed to be protons traveling at just under the speed of light.
How does a proton with its substantial mass get accelerated that crazily? No known process can do it. For years, the leading candidates for such UHECRs have been supernova remnants, but that can’t explain truly ultrahigh-energy particles. Recently, colliding galaxies like the Antennae mentioned in chapter 8 have gained favor as the source, but there are problems with this theory too. Today, the leading candidates are active galactic nuclei (AGN). They’re inside the most massive galaxies, like the superheavy colossus named M87 that lies in the center of the Virgo cluster fifty million light-years from us. It’s assumed these galaxies’ supermassive black holes play a pivotal role in slingshotting these bullets to their fantastic speed and power. Recent measurements indicate a link between the direction of UHECRs and the location of AGN galaxies, although these are so nearly ubiquitous, it’s hard to say for sure.
Perhaps the most intriguing idea is that UHECRs materialize when theoretical dark-matter particles hypothetically decay into high-speed proton pairs, one of which falls into a black hole while the other is shot across the cosmos. But is this a case where desperate, baffled astronomers are using the speculative as evidence for the exotic?
No matter which theory is correct, the origin must be some sort of cataclysmic event. That’s because ultra-extreme conditions are needed to produce the required ultra-velocity. In the case of a black hole, you’d start with a star much more massive than the sun—probably weighing between ten and twenty suns—that runs out of its nuclear fuel in its old age. We saw this same scenario in chapter 5 when we explored type 2 supernovas, since this is exactly the initial start-up conditions that cause them. More precisely, we begin with a star that weighs more than eight suns but less than forty or fifty. Such superheavy stars make up less than 0.1 percent of the Milky Way’s stellar inventory. The aging, heavy star, its wild youthful years behind it, is running out of fuel in its core, and loses its fierce outward-pushing energy. Now each day is worrisomely spent with all its massive outer layers hovering like a sword of Damocles, barely in equilibrium with the energy emitted by its core. Then one day, the core falters ever so slightly, and that’s the final straw. The star collapses.
Collapse creates heat, so the fresh sizzle can ignite its remaining core material to perform new kinds of nuclear reactions, and the star gets a life extension. But eventually these longevity boosts run out. If the star weighs between half a sun and about 1.44 suns, it will collapse to become a white dwarf. These are common stars, which is why we observe little Earth-size white dwarfs all around us, even through backyard telescopes.
If the star is heavier than 1.44 solar masses, the stronger gravity forces a greater collapse; that can result in a type 2 supernova, which leaves behind its tiny packed core as a bizarre neutron star. We explored these in chapter 6 and came away convinced that they’re the strangest observable things in the whole universe. The star is now just twelve miles wide, and its material is so packed together that a speck the size of a poppy seed would outweigh an aircraft carrier.
Yet things can get even weirder. If a star has even a greater mass, it won’t stop imploding when it’s twelve miles across. Instead, it will keep getting smaller with its surface gravity growing ever stronger, the implosion truly getting out of hand and showing no sign of stopping until the speed needed to escape its gassy surface reaches 186,282 miles per second, the speed of light. At that point, not even light can escape and the star instantly grows dark. It is now a black hole, although it doesn’t care that it reached that stage and still keeps collapsing smaller and smaller.
Our science fails when we ask the obvious question: When does it stop? Einstein’s theory of relativity says that it continues until its density is infinite and its size is smaller than a speck. Meaning, until it occupies zero space.
But since neither infinitely high density nor existence in a zero-volume space has any physical meaning, it’s reasonable to ask what really happens. No one knows. Some think that an unknown process must step in and halt the collapse. Maybe the “strong force” that operates at atomic-nucleus scales comes to the rescue to stop the shrinkage. It’s an unanswerable mystery.
Meanwhile, it has certainly been a cataclysm for the poor star and for any life on any planets orbiting it. Not because they’d get sucked into the black hole, because actually they wouldn’t. No, it’s a disaster because all the light has gone out, and as far as we know, there cannot be life without an energy source.
But violence begets violence, and this black hole can cause trouble in an entirely different way. If the black hole is a member of a double-star system—and about half the suns in the galaxy are binary stars—then ordinary emissions from the normal star, which include a stellar wind analogous to the solar wind blowing from our sun (made up of a steady stream of subatomic particles like electrons and protons), can be captured by the black hole’s fierce gravity.
Some of this atom-stuff is pulled into the ultradense collapsed star, the singularity at the exact center, never to escape. But if its angle of approach and speed are just right, it can instead be flung away as if by a slingshot and hurled off at superhigh speeds that can approach that of light. You guessed it—it’s now a cosmic ray.
There are theoretically enough of these cataclysmic black holes in binary systems to provide a steady supply of cosmic rays. Nonetheless, astrophysicists think the most energetic cosmic rays come largely or entirely from supernovas.
We explored both types of exploding stars, so we know that the mega-violence unleashed when the star explodes is almost unimaginable. It equals the brightness of five billion suns, and with type 1, there’s actually very little variation in this brilliance from supernova to supernova. But it isn’t just light that’s being emitted. The star has been torn to shreds, and the atoms of its body are hurled outward. In some cases, the superstrong magnetic field around the new supernova accelerates charged particles for years, decades, and centuries after the initial brilliance has vanished. It’s this detritus, too, that provides the cosmic-ray flux currently zooming through the entire universe and, unfortunately, your body.
Some of the particles collide with atoms in Earth’s atmosphere, typically about thirty-five miles above us, which is far higher than the flights of commercial jets. When they do so, they break apart air atoms like a rack of billiard balls to create a cascade of subatomic items like muons, which some physicists regard as cosmic rays in their own right.
On average, 240 muons penetrate your body per second. This nonstop 24-7 violation could be prevented only if you put massive shielding between yourself and the sky, say by living in an underground parking garage. If you’re not inclined to do that, you’ll keep being the dartboard.
It hardly seems fair. A cataclysm many light-years away in the form of a supernova that has destroyed its own solar system now brings the destruction into your own beloved body. And you have a right to protest; a muon isn’t always harmless. Occasionally, one will strike the wrong bit of genetic material in a DNA strand. This is one reason humans, even those who buy everything from health-food stores, can be plagued by spontaneous tumors.
We may enjoy the titillation of hearing about cataclysms that are distant, or that struck our world in the remote past (like those asteroid-induced mass extinctions), or that pose a threat only in the far future (like the sun swelling up so that it engulfs our planet). But the muon/UHECR business is an ongoing reality. Caused by a distant cataclysm, it continuously presents the unlikely but nonzero risk of delivering disaster into our everyday lives.