May 18, 2026
If you've ever seen, heard or watched any public science explanation of how nuclear fission or fusion works, you've probably witnessed something like the following explanation:
Einstein discovered that E = mc², which means matter can be converted to energy and vice versa. In nuclear fission and fusion - for the appropriate starting materials - the combined mass of the resulting elements is less than the mass of the initial matter. Plug that difference into the equation above and you see there must be energy released. The speed of light is a big number so that means a lot of energy. Hence nuclear power generation and devastating weapons.
So none of that is wrong exactly. But it doesn't explain anything. If you read the above and thought you understood it, good for you, but that doesn't mean you understand nuclear physics, even at a superficial level.
Einstein's famous equation is saying, in essence, that mass and energy are more or less the same thing, and that depending on exactly how one looks at any particular physical system it might be useful to talk about one or the other. For example, if you excite some energy state of some particle by, say, making it vibrate a particular way, you are making that particle more massive in all the ways you can measure mass. So when you observe that the masses of some particles are lower than the masses of what you started with, that doesn't explain why you suddenly have a lot of energy - it's a tautology. We still have the same amount of mass-energy, because that is a conserved quantity. We have less of it in the form of actual massive particles, so we must have more of it in some other form - eg heat and light. This doesn't explain anything, it just states the obvious, it's saying the same thing in two different ways.
To get to something more like a real explanation, we have to explain where that energy comes from and therefore why the particles you get when you split a uranium atom, or fuse some hydrogen atoms, are less massive. This is harder to explain, so I have some sympathy for public explainers of nuclear physics, but I still think they need to do better. I'm not going to get into all the details - don't want to completely lose whatever tiny readership I might have - but I'll try to at least advance us in the direction of a better understanding of how it works.
Assuming you've had any sort of scientific education you must have encountered the concept of "potential energy", and even if you haven't it's not hard to follow. Hold a ball at some height, then let it go, it falls, gaining kinetic energy (ie motion). The Earth's gravitational field exerts that influence on it. So when it is held at a particular height and not released, we say it has potential energy, because it would gain energy if we released it. Put another way, the amount of energy is conserved, and when we release the ball the potential energy is converted into kinetic energy. A ball held at a lower height has less potential energy. Conversely, if you have an object at a certain height and you want it to be at a higher point you have to invest energy to make it so. Walk up a hill if you have any doubts on this point. This isn't the only way of looking at a ball in a gravitational field - you could also calculate the gravitational force, and then the acceleration of the ball, and how much force you have to exert with your hand to stop it falling, or make it rise. That would be saying the same thing. It's very useful in physics to know several different ways to say the same thing, because for any given problem one way is often much easier to think about than the other.
Gravity is just one of the forces described by physics. The others are the electromagnetic force, the strong nuclear force and the weak nuclear force. The electromagnetic force is familiar if you have played with statically charged objects and seen them attract or repel each other - or with magnets, though that is a more complicated story. The strong nuclear force is the main thing we are concerned with for now, however. This is the force that binds together the parts of an atomic nucleus. It is a very short range force - the range is of a similar order of magnitude to the dimensions of a hydrogen nucleus, and much shorter than a uranium nucleus. Just as for gravity, you can talk about potential energy when two objects that would be attracted by a force are further apart. Just like the ball and the Earth in the earlier example, we can talk about potential energy of particles attracted by the strong nuclear force. We usually call it binding energy, as it is the energy that binds them together. The quarks that make up the protons and neutrons of an atomic nucleus are bound by the strong force - another way of saying this is that if you want to separate them, you must apply energy. And if they had been separated, and then come together, they lose potential energy and thus that energy is available in some other form.
Now we can start to see how this might work for fusion of two hydrogen atoms. A hydrogen nucleus is typically just a single proton. Isotopes of hydrogen, deuterium and tritium, also have one and two neutrons respectively. If we could fuse together two hydrogen nuclei, we would have two separated protons coming together, thus losing the potential energy of the strong nuclear force, so you would expect energy to be released. The strong force is indeed strong, as its name suggests, so the amount of energy involved is large.
Keen readers will have already spotted at least one of the complicating factors - protons are positively charged, the electromagnetic force will cause them to repel each other, so in this case the electromagnetic potential works the other way and we would require energy to bring them together. The strong force is stronger than the electromagnetic forces, but shorter range - if you can get the particles close to each other then the strong force would overcome the electromagnetic, but at greater distances the electromagnetic force is stronger. This is a good thing for all of us - it's why keen hobbyists aren't building hydrogen bombs in their back yards - it's actually quite difficult to get fusion to happen. It's also why fusion power generation has been such a long and difficult enterprise - it's been "twenty years away" for as long as I can remember.
Adding some neutrons into the mix, eg using isotopes of hydrogen, makes things a little easier. The electromagnetic force has no interest in neutrons, but the strong force definitely does, so the balance between the two forces moves more in the strong force direction.
So we have a decent outline of how fusion can work and why it produces energy - or to put it another way, why the products of fusion have lower mass. Now to fission. Large atoms have nuclei bigger than the range of the strong nuclear force. The 92 protons and 143 or so neutrons in a uranium nucleus are only holding on to their neighbouring particles via the strong force, not the ones on the other side of the nucleus. The more protons you have in one place, the stronger the electromagnetic force, so the balance between the forces can tip towards the electromagnetic once a nucleus becomes big enough and you can gain energy from breaking the nucleus up, not from fusing more to it.
A nucleus that can produce energy by fission is inherently unstable. More neutrons increases stability because there is more strong force binding going on and the average distance between positively-charged protons increases - so heavier isotopes tend to be more stable than lighter ones. When naturally occurring elements are radioactive, it means their nuclei are falling apart of their own accord, usually at a slow rate, producing energy in the form of emitted particles. Bombarding radioactive nuclei with other particles can speed the process up, so if you have enough fission going on the process is self-sustaining and you get a large amount of energy released quickly - that's what people mean when they talk about "critical mass".
We've elided a lot of details, but that is in essence what I think a decent popular explanation of nuclear physics needs to cover. Just pointing out the mass difference doesn't explain anything. Given that the above wasn't especially short I can see why people might rather divert into a discussion of how long you could power a city by converting some arbitrarily small chunk of mass into energy, but I claim it is better to try to help people understand, not just try to impress them with random physics facts.