May 8, 2021

I, Science

The science magazine of Imperial College

Earth-engulfing black holes, continent-crushing asteroids, crop-cremating supernovae. Sounds like science-fiction, or at worst the rant of a lunatic – and largely you’d be right. Not that these things don’t happen, it’s just that they’re extraordinarily rare and you’re probably better off minding that car screeching past you than worrying about our collective planetary plight; after all, these kind of events are out of our hands. Not content with fate, however, these mischievous hands of ours may inadvertently be bringing equivalent dangers into our backyard.

Black Holes

In Fritz Leiber’s short story A Pail of Air, Earth has been torn away from the Sun by a passing “dark star”, causing the Earth’s atmosphere to freeze. The only way to breathe is to thaw the atmospheric “snow” over a fire. Such a disaster scenario could actually occur if a rogue black hole passed close to the solar system. Similarly, it’s suggested that passing dark matter (the invisible stuff holding galaxies together and making up 80% of the mass of the Universe) could shake loose some comets in the outer solar system, firing them Earthwards.

The difficulty with understanding the risk from black holes and dark matter is in their name: we can’t see them directly and only infer their existence through their effect on surrounding luminous matter. It’s thought that there are hundreds of intermediate black holes (and by intermediate mass it is meant thousands of solar masses) roving around our galaxy. However, the chances of one passing close enough to throw the Earth out into deepest space are pretty slim.

As for being sucked into a black hole, the biggest contender is the supermassive black hole at the centre of our galaxy— a whopping four million solar masses. But it’s a misconception that black holes somehow act as cosmic vacuum cleaners; until surrounding stars fall over the event-horizon, the black hole appears to them like any other big ball of matter. If a single solar mass black hole were to suddenly replace the Sun, the orbit of the Earth would remain exactly the same.

So far, so safe. But that could all change, thanks to the work of meddling physicists working deep underground on the world’s most energetic particle accelerator: the Large Hadron Collider (LHC). Some speculative theories predict that the high energy proton collisions at the LHC could produce microscopic black holes, sucking the Earth into oblivion. Physicists stress, however, that any black holes produced would evaporate instantly before any mass accretion could occur. What’s more, cosmic rays of much greater energies than the LHC bombard our upper atmosphere every day, so even if these strange objects are produced, the consequences won’t be all that dramatic.

However, physicists are never content with their latest toy and we can be sure they’ll keep building bigger, more energetic particle smashers as long as humanity lasts. But as we move further into the realm of the unknown it would be wise to keep our eyes peeled— as President of the Royal Society Sir Martin Rees notes: “It isn’t good enough to make a slapdash estimate of even the tiniest risk of destroying the world”. Indeed, some physicists joke that high energy particle colliders may be the answer to the Fermi Paradox; we can’t find any evidence of extraterrestrial civilizations because they all wind up building particle smashers that suck their planets down a black hole.

Near-Earth objects and gamma ray bursts

On June 30 1908 a factory-sized meteoroid exploded in the air above Tunguska, Siberia releasing the same energy as a thousand nuclear bombs and leveling thousands of square kilometers of forest.

The rate of such events was estimated by Eugene Shoemaker to be one every 300 years, while he thought the rate of meteor explosions equivalent in energy to the nuclear bomb that destroyed Hiroshima to be one every year. On realising the source of the devastation, scientists were understandably concerned and initiated a host of asteroid tracking programs. NASA has since identified more than 800 large near-Earth objects with potentially devastating consequences for the human race.

It would take an object as big as five miles across to a cause a mass extinction on Earth like the one which is thought to have killed off the dinosaurs. However, it doesn’t take that much to send the world into turmoul. Asteroids only half a mile across would devastate the ozone layer for years after impact, producing a hole as big as that observed above the Antarctic in the 1990s and seriously harming global food production.

A gamma ray burst (produced when very high-mass stars collapse to form a black hole) occurring in the Milky Way could wreak similar havoc for our atmosphere. The vast flood of radiation would destroy up to half the Earth’s ozone layer and may stop photosynthesis occurring in the world’s oceans to a depth of 80 meters, starving the Earth of oxygen.

“Asteroids only half a mile across would devastate the ozone layer for years after impact”

But the chances of a humanity-annihilating meteor or blinding gamma ray burst heading our way anytime soon are pretty slim. Ozone-destroying asteroids only happen once every 200,000 years and the only reported fatality from a meteor impact is an Egyptian dog killed in 1911.  Likewise, the relatively small number of gamma ray bursts detected suggests they are extremely rare events, with only one burst per galaxy roughly every million years or so. Moreover, the intense radiation from a burst is the result of vast amounts of energy being squeezed into a thin beam. This means that, unless it’s pointing directly at us, we’re safe. Had it been left unchecked, the destruction of the ozone layer by CFC pollution, would have had the potential to cause as much damage as any meteorite or gamma ray burst. The risks of introducing a new technology without proper testing are also apparent in the first nuclear bomb tests; physicist Edward Teller speculated that the explosion could be energetic enough to ignite nitrogen in our atmosphere.

As the pace of technological innovation accelerates, rather than panicking about unknown cosmic catastrophes, perhaps we should be keeping check on our own potential for destruction.