Spontaneous Emission: The Radiation Episode

Hello and welcome to Physical Attraction, the show that explains physics… one chat-up line at a time.

This episode is about radiation.

[Television, the drug of a nation, breeding ignorance and feeding radiation]

So, just to clarify, exposure to radiation does not give you superpowers, the only superpower it gives you is being dead; Bruce Banner, Daredevil, Captain Atom, dead, dead, dead, and rather horribly. Okay; now we’ve got that out of the way…

What is radiation? It really, really depends on how you want to look at it. Sometimes it’s helpful to think about electromagnetic radiation as particles — photons — that swoop around and bash into atoms and walls and so on. Sometimes it’s helpful to think of it as ripples in the electrical and magnetic fields that surround us and permeate us at all times; disturbances in the Force, if you like. Sometimes it’s helpful to think about it as another form of energy, just like matter — atoms, protons, neutrons, electrons, all particles — are a form of storing energy. Sometimes it’s helpful to think of it as one of the components that makes up the Universe as a whole. In reality, it’s all of these things. Physics will never exactly correspond to reality; so if I say something like “electromagnetic radiation is made up of photons, and these are like little billiard balls that fly around and hit stuff” — it’s a useful description sometimes, and can help us understand sometimes, but it’s incomplete.

One thing we can be pretty clear about is that radiation is a spectrum, and that spectrum depends on the wavelength of the radiation. The longest-wavelength radiation is radio waves; then, anything from about 100cm to 0.1cm we categorize as microwaves; anything smaller than 1mm but bigger than 700nm is infrared where a nanometer is 1 over a billion metres, and this is the bit of the spectrum that we feel as “heat”. Then there’s the magical tiny bit between 700nm and 390nm that our eyes are actually sensitive to and we call visible light; then the UV below this; X-rays, and, anything with a really tiny wavelength are gamma rays. And, as wavelength goes down, energy goes up, so gamma rays are the most energetic while radio is the least energetic.

It’s important to remember that these subdivisions are totally down to humans. As in, they’re completely arbitrary and convenient to us. There are all kinds of ways you could subdivide the scale, if you wanted to — but in reality, it’s just one continuous scale, it’s all electromagnetic radiation, and we’ve chosen to kind of “split it down the middle” with the middle range being the tiny range our eyes are sensitive to, that we can actually detect. The different wavelengths of radiation will interact differently with matter — gamma rays, for example, are usually absorbed by matter while radio waves can pass straight through most of the time — but a radio wave isn’t fundamentally a different thing than a gamma ray, it’s just a question of energy. And there are processes where radio-photons can gain energy in collisions, and actually get scattered-up into being gamma rays; it happens a lot in pretty extreme outer space conditions.

So you can talk about “discovering radiation”, and the Curies are rightly praised for doing that, but really what they discovered was the bits of radiation that we couldn’t already see.

But what does it mean to say that radiation is emitted, or radiation is absorbed? How does radiation get made, and how does it get destroyed? There are two main ways that energy gets converted into radiation. One is when matter collides with antimatter — that’ll have to have its own episode, I think, but when that happens, they annihilate each other and the energy in that matter is turned into radiation.

One of the things we know about electrical charges — like the charge on an electron, or a proton — is that when you accelerate them, radiation is produced. One way to think about this that I find helpful is — imagine an electron. We know that it has an electric field around it, and this field will push away negative charges and pull in positive charges. We can imagine, to help us, field lines that spread out from the electron like the spokes of a wheel. Now imagine wiggling the electron back and forth. All of those field lines have to point at the electron, even as you change its position.

But special relativity tells us that no information can travel faster than the speed of light — than the speed of electromagnetic radiation. (There’s going to be a relativity episode soon, so hopefully we can get onto this.)

A long long way from the wiggling electron, the electric field can’t “know” that you’ve moved the electron. Otherwise, information has travelled faster than the speed of light. The information about the wiggling electron has to travel outwards — like a ripple — at the speed of light. That is your electromagnetic radiation. So you can have this amazing picture: when you’re looking at a sunset, or someone cute, or a garbage can full of trash, you’re actually receiving — at the speed of light — live, up-to-date information about wiggling electrons. That’s what’s bombarding us all the time. Even when we close our eyes.

The more energetic the process, the faster you wiggle the electron, the greater the frequency of the electron’s vibrations. Therefore the electromagnetic radiation will have a greater frequency, and so it will have a smaller wavelength — because the wavelength times the frequency is always the speed of light — and so it has more energy. So this simple picture, of wiggling electrons, can actually explain quite a lot about how radiation works.

In atoms, the electrons are held in energy levels; because of quantum mechanics (yes, there’s an episode in that, as well), these are discrete. That means that the energy can only have certain values. The gaps between energy levels have certain, fixed values, too. If a photon hits the electron, it can tickle it juuuuust right to give it the energy to go to a higher level. And then, after a while, the electron will wiggle its way back down into the ground state, and release a photon again. So this is why objects have different colours, depending on what they’re made of; it all depends on the energy levels of the atoms, which tells us how the electrons are going to wiggle, which tells us what colour light they can absorb, and which ones get reflected.

One exciting thing to point out about this picture of radiation is that there are other ways atoms can get into excited states. One of these is by collisions with other atoms. And, since everything that has a temperature has molecules that are moving around very quickly and colliding with each other, this means that everything with a temperature is constantly exciting some of its atoms into excited states. And, depending on that temperature, and how hard the collisions are, those atoms will emit different frequencies of radiation. So you’re emitting radiation, right now, sitting in your chair; it’s just that it’s all in the infrared, because that’s what room temperature and body temperature correspond to. Unless you’re on fire, in which case, you might be emitting in the visible. Which gives rise to this episode’s chat-up line:

“Are you thermally emitting electromagnetic radiation in the visible spectrum? Because you look… unusually hot!”

Yeah, yeah, I know, I know. Restraining orders at the ready.

So that’s electromagnetic radiation. But things get a little bit more complicated. Basically in the grand definitions of things that were termed “radiation”, and things that are called “radioactive”, not everything that’s emitted is electromagnetic radiation; sometimes it’s particles. If a nucleus (the centre of an atom made up of protons and neutrons) is unstable, there are various different ways it can change to become more stable. Sometimes the neutrons and electrons rearrange themselves, and reduce in energy that way, and the extra energy is emitted as a gamma ray. This is what’s called gamma radiation. Sometimes a neutron changes into a proton, and emits an electron, which is what’s called beta radiation. And sometimes the whole nucleus can just say “oh for crying out loud” and kick out two neutrons and two protons, all stuck together: that’s alpha radiation. Only one of those is a photon, but they all get lumped together as “radiation.” This is basically for historical reasons; when all of these things were first discovered, we knew that there were certain substances that must be emitting some different kinds of particles, because we could see their effects. For example, X-rays lit up the coating on some glass, and Marie Curie discovered types of radiation from some of the elements because of how it affected photographic plates. Later on, Rutherford realized that the beta radiation could make it through longer distances of material than the alpha radiation. This is because alpha has twice the charge of beta, and so it gets stopped dead by electromagnetic interactions more quickly. So we knew that they were different before we knew what they were, and that’s why we’re stuck calling everything “radiation” even though some of it is particle radiation and some of it is electromagnetic.

So a lot of people are slightly concerned when you tell them that massive amounts of radiation are passing through their bodies all the time, 24/7, and that they’re in a sense “radioactive” and emitting infrared radiation. Of course, only certain kinds are actually harmful to humans. The difference is one of energy. Some kinds of radiation are powerful enough to knock electrons away from atoms entirely — ionizing radiation. So alpha particles, beta particles, gamma rays, X-rays and some UV can do this. An atom is electrically neutral; its protons, which have positive charge, are cancelled out by its electrons, which have negative charge. But when an atom is ionized, one electron is removed, and so it does have an overall charge. This is what makes it harmful to humans, because once it has charge, it can destroy chemical bonds, pull apart cells, and generally wreak havoc. So you could stand in front of a massive radio-wave generator and it wouldn’t make a difference — even if there’s loads of energy in those radio-waves. They simply don’t have the energy to knock electrons away from atoms and cause this kind of damage. An alpha-particle source is dangerous — but alpha-particles can be stopped stone dead by a piece of paper, so they’re easy to handle. (Although if you eat a source of alpha particles it will kill you. This is exactly what happened to Alexander Litvinenko, the Russian dissident — he was poisoned by an alpha-emitter, Polonium-210, and there was nothing the doctors could do. Getting into his life story is a whole other rabbit hole, but it is really interesting if you want some Wikipedia to trawl.) Beta radiation is ionizing and can penetrate quite far, but a thin sheet of metal should stop most of it. We can actually use this to our advantage, and people do; if you want to make sure sheet metal is of a uniform thickness, stick a beta source on top of it, and measure how much passes through; that’ll tell you how thick the sheet of metal is. Similar techniques are used to track leaks in oil pipes and so on underground.

But a gamma ray source is ionizing, and penetrative. Even if it has far less energy than the radio waves, it’s much much more harmful for you, and because it’s not charged, it takes far more material to stop it. Thick layers of lead. Disposing of the gamma-ray material is by far the most difficult part of the job for people who are dealing with nuclear waste, from nuclear power plants.

Here’s something that has always interested me. Nuclear waste is going to remain dangerous for thousands of years. We have no idea what human civilization is going to look like in thousands of years — we don’t even know if they’ll speak the same language as we do now, if they’ll use or understand the same symbols. So as well as physicists, people who dispose nuclear waste are employing the services of psychologists, and linguists. How do you communicate a very simple message: “PLEASE DON’T DIG HERE AND FOR THE LOVE OF GOD DON’T EAT ANYTHING” in a way that will remain culturally relevant to humans thousands of years from now?

After all, when we crack open the pyramids, we don’t understand or just ignore the many curses that say that anyone who does so is going to be doomed. Indeed, if you go on too much about how dreadful it is, people will probably think you’re hiding something special — that’s one aspect of human nature that probably won’t change, we really hate doing what we’re told. What if some apocalyptic event means that future humans forget everything they knew about radiation, and so we can’t just say “there’s nuclear waste down here and digging is probably a bad idea?” The oldest known languages have barely been in use for a few thousand years. This needs to be understandable 100,000 years from now. So they put notices down in every conceivable language, in case civilization manages to remember Esperanto while forgetting everything else. You might think — just put a skull and crossbones, that’s pretty threatening — but it was pointed out that for some people, this is associated with the Latin American Day of the Dead festivities. So they might think this is the way to the underground party. What if human civilization in 100,000 years is fanatically obsessed with pirates, and they see the Jolly Roger sign and think it’s buried treasure? The best solution we have at the moment is an information centre with a series of graphic videos that shows the effects of radiation on the human body — that way, humans 100,000 years from now will be as clued-up to the general message of “BAD!” if they’re super-advanced or if they’ve regressed into cavemen. It’s a pretty haunting idea: visceral warnings buried underground to save us from our former selves, to tell them that this is a bad and shameful place. And we have no idea if it would work.

Doses of radiation in the human body are measured in Sieverts. They can also be measured in bananas. Bananas contain potassium, and some potassium is naturally radioactive, so every time you eat a banana, you’re slightly irradiated. It’s a really tiny effect — hardly likely to make a difference. In fact, you’d need to eat 35 million bananas to die of radiation poisoning from their potassium. Obviously, eating 35 million bananas has its own problems. In fact, 35 million bananas would weigh around 3.5 million kilos which is easily enough to crush you to death. In case you have enemies, a quick google of the historical technique of crushing people — which has done nothing to help my search history or amazon recommendations — suggests that a mere 3,200 bananas might be enough weight to crush someone to death. The banana equivalent dose, though, has actually helped allay a lot of public fears about radiation. So for example, the Three Mile Island meltdown exposed local residents to 800 Banana Equivalents. Which doesn’t sound nearly as bad.

I hope I’ve cleared up the distinction between electromagnetic radiation and other types of radiation. We’ve mainly been talking about the electromagnetic spectrum this episode, but I couldn’t title it “radiation” without explaining that.

The last thing I want to talk about in terms of radiation is cosmic rays. Because Earth is being bombarded, all the time, by cosmic rays from unknown sources in our space. And some of them have ridiculous amounts of energy. These are both electromagnetic and particle kinds of radiation. One particle that hit the upper atmosphere was so energetic that it had around 48J of energy: so physicists dubbed it the “Oh-my-God” particle, for obvious reasons. They got very excited.

What does it mean for a nucleus to have 48J of energy? Well, first off, the thing is moving at the speed of light, basically. Nothing with mass can ever move at the speed of light, because of special relativity, but this is as close as anything with mass is likely to get. If it raced a light beam, it would take 21,500 years for the light beam to pull ahead by a millimetre. 48J is about as much energy as there is in a baseball pitched at 60mph. All packed into an atomic nucleus that is a million million million million million million times smaller than a baseball. The thing has so much energy that the second it hits the atmosphere, it disintegrates into a massive shower of particles; only by detecting this shower can we figure out what energy the original particle had.

So if you’re like me, a couple of questions come to mind. The first one is — if these energies are so huge, why can’t we use them instead of the large hadron collider? And, indeed, the energies are much much larger than the LHC. The problem is that we can’t predict when and where these events will occur, to stick our sensors there — they’re really incredibly rare. An event comparable to the OMG particle might only occur once every few years. And the other problem is that not as much energy can go into making new particles. Collisions have to conserve energy and momentum. So when you have a superhigh energy cosmic ray smashing into the atmosphere, which is basically sitting still, whatever it hits has to carry off a lot of the energy in recoil. Not so in the LHC, where the two beams of protons whizzing around smash into each other with the same energy — no recoil required.

The other question is — well — where on earth did the particle get this kind of energy? And the answer to that is we really have no idea. There are some candidates — like the magnetars with really strong magnetic fields like we found in the episode on stellar formation. Or maybe at the super-energetic, supermassive black holes at the heart of galaxies, you might be able to accelerate particles to this kind of energy. Supernovae might also be a decent candidate. But the reality is that it’s a total mystery. Nobody really knows how these particles come to have these insane amounts of energy. All we do know is that, somewhere out there, something WILD is going on.