There’s nothing quite like looking up at the sky on a nice, sunny day, admiring the clouds and pondering the heavens, and realizing that our entire existence could be wiped out by some random, indifferent gamma rays from the skies. If their source was close enough to Earth and aimed directly at us, gamma rays could destroy our ozone layer and potentially pack the same punch as an atomic bomb deployed on the half of the planet facing them. We’d quite literally be toast. Though it’s an interesting (yet not so fun) scenario to think about, and can grant you some unique perspective, do we have reason to panic?
I’ll let you wallow in existential dread for a bit, but meanwhile, let’s get some vocabulary and principles out of the way:
Energy, in physics, is the ability to do work.
Work ‘is done whenever a force moves something’, like shoving a box across the floor (it can be quantified by multiplying force and distance, but that isn’t important for now).
So, energy is nothing but a measurement of this ability. It has many different forms, including kinetic (when an object is in motion) or potential (like water behind a dam). Energy cannot be created or destroyed; it can only change between its forms.
Radiation is ‘the emission or transmission of energy in the form of waves through space or through a material medium’.
Radiation has many forms too; for example, sound radiation travels in waves, but needs a medium, such as air or water, to do so, which is why there is no sound in outer space. Another form of radiation is extremely quick subatomic particles (particle radiation).
Electromagnetic radiation is energy that travels through space in waves; it is the visualization of an ‘electromagnetic disturbance’ in the so-called electromagnetic field, which can be described as ‘invisible areas of energy’. In short, it is simply light.
Electromagnetic radiation – light – and its different levels, or frequencies (how often the light wave completes a cycle), can be quantified by the electromagnetic spectrum which divides them up by wave length. The longest waves have the lowest frequencies and the least energy. These include radio waves, microwaves, and infrared light. Visible light falls somewhere in the middle. Then comes UV light, X-rays, and finally gamma rays with the shortest waves/highest frequencies.
Somewhat straightforward so far – but then quantum physics came along. I won’t delve into it too much here, but in quantum mechanics, radiation is thought to be emitted in set quantities – quanta – depending on its properties. Picture quanta as Legos: they come in predetermined sizes, and at a certain point, there is a limit after which you can’t break them down into smaller parts.
Gamma rays are very energetic, and are like very, very big Lego bricks. Bringing about something like a quantum, or package, of gamma rays requires passing a certain (very large) threshold of energy (like needing lots of little Legos to build something the size of a big one), which is why your cup of coffee doesn’t zap you to a crisp: it just cannot emit whole packages of energy large enough to constitute a package of gamma.
For light, the quantum is a photon, a massless particle of light: the smallest possible quantity. Its energy varies depending on the type of light. However, that must mean that light is made up of particles – photons – and not waves. But it’s kind of both. Physicists have accepted something known as the wave-particle duality to explain why light behaves as a particle at some times, and as a wave at others. Therefore, the two are comparable; in other words, a photon’s energy is proportional to the electromagnetic frequency, or inversely proportional to the length of the light wave. For example, a blue visible light photon’s energy lies at around 3.1 eV (electric volts, a measure of their energy), while a gamma ray's can be over 100,000 eV. Proportionally, the wavelength of visible light can be around one-thousandth of a human hair wide, while gamma wavelengths are smaller than the nucleus of an atom. So, when talking about light’s wavelength or energy, the terms are somewhat interchangeable.
So back to gamma rays; why are they generally to be avoided? Due to their short wavelength, they are simply too energetic to be stopped by things in its path. They can pass through your body no problem; they’ll just squeeze through the atoms. Don’t bother holding up a mirror to deflect them as they’ll pass right through it. X-rays, while a little bulkier, are similar. Both can be stopped by dense material such as lead, which is why you often wear that when getting X-rayed to avoid excessive radiation.
But what makes them dangerous to humans is that the high energy of gamma rays, X-rays, and even UV rays means they are so-called ionizing radiation; basically, they can break electrons off an atom. If you get hit with them, they can damage not only your organs and bones, but your very DNA. Ionizing radiation (which can take many forms, including particle radiation as well as light/electromagnetic) is the reason for the cancers and other extreme health consequences caused by the atomic bomb, which gives off a lot of it.
That’s just the direct damage. If an on-target gamma ray from 6,000 light years away hit the Earth for 10 seconds, it would severely damage our ozone layer on the exposed side of the planet, resulting in much more long-term UV light penetration from space – which, as we now know, is ionizing and DNA-damaging (hence the risk of skin cancer after too much sun exposure). The gamma rays would also interact with nitrogen in our atmosphere and turn it into the rusty colored nitrogen dioxide, effectively blocking out visible sunlight (though not the smaller, penetrating UV rays). To complete the charming scene, the nitrogen dioxide would also shower us with acid rain.
While we humans might be able to take measures dealing with this (depending on the severity), like perhaps living underground, plants and animals would have a much harder time adapting and might at least partially die out, causing an extinction event. In fact, there is a theory that a burst of gamma rays might have caused the demise of over two-thirds of species on Earth in the Ordovician extinction event 440 million years ago – the second largest ever. Considering that at the time, these were mostly marine species that had the extra protection of water against the UV, the prospect is not a great one.
Am I scaring you yet? Perhaps it helps to understand the origins of gamma rays. Apart from terrestrial sources such as radioactive decay, gamma rays can originate from a host of different sources in space. For one, the whole sky, in every direction, is full of gamma radiation, though it is not high energy enough to pose a significant threat; our dense ozone layer stops all but the most energetic.
This background of photons with gamma-level energy has long been a mystery, but now, it has been theorized that it is a result of cosmic rays colliding with other matter. Cosmic rays are protons or other ions (atoms with charge) with lots of energy, originating from high-energy sources such as the sun, supernovae, or galactic interiors. A recent study has shown that starburst galaxies (galaxies with high star formation, leading to high supernova rates) make great gamma factories: the supernovae produce lots of cosmic rays, and the surrounding dust and gas provide many points for collisions, creating gamma rays.
The above is only the common background gamma radiation, easily deflected by our atmosphere. A real danger to Earth would be a direct jet of the stuff from a single source. Though the sun sometimes emits them in solar flares, the most powerful origin of gamma rays would undoubtedly be a gamma ray burst: these are the brightest, most energetic events in the Universe (releasing more energy in a few seconds than the Sun will in its lifetime of ten billion years), save for the Big Bang. They can be long or short; the long ones range from about two seconds to a minute. These are thought to originate from extra powerful supernovae, sometimes called hypernovae. Short bursts of under two seconds come from the collisions of two neutron stars (ultra-dense remnants of a star; imagine the mass of the Sun squashed down to a size of around Manhattan), or a neutron star and a black hole.
As visualized in this video, both scenarios result in a black hole. The fusing of the former stars’ spins and magnetic shields translate onto the black hole, sending jets of gamma rays that came from the implosion into space in the form of two concentrated jets – concentrated enough to immediately fry the Earth from a few light years away. From slightly further distances, say a few thousand light years, we’ll still be fried, but it would take a little longer. This is because as the universe expands, it stretches light waves over long distances, expanding their wavelength and therefore reducing their energy (which is also responsible for the redshift of distant galaxies).
Astronomers detect around one to two of such gamma bursts a day. So why aren’t we dead yet? Basically, they are incredibly rare; even two bursts a day are not a lot in the vast expanse of space. In fact, we’ve never detected one from inside the Milky Way; the ones we know of all took place in galaxies millions if not billions of light years away, rendering them harmless to us. Based on those observations, we can calculate that these bursts are quite rare; in a million years, one galaxy would only produce a handful. And luckily for us, we reside in the uneventful suburbs of our galaxy, far away from the riffraff of supermassive stars and binary systems of neutron stars/black holes.
Scarily enough, the star Apep, a mere 8,000 light years away, is predicted to produce some gamma rays upon going supernova, but that is one star among millions in the galaxy. A study showed that the chances of a gamma ray burst occurring in the Milky Way are at 0.15%. Combine that with the beam having to strike us dead on to inflict any damage, and the odds of one hitting our tiny planet are pretty much zero.
So, don’t go selling your house and buying that superyacht just yet; the chances of us getting zapped by a gamma ray and cooking to death are literally astronomically low. But other threats to life on Earth, such as global warming and nuclear warfare (among others) are not only more realistic, but much more urgent; plus, those we can actually do something about. So, rest assured that the fate of humanity’s existence is (probably) in our hands. Actually, that might be much scarier than a death ray from the sky.
Good read!