My knowledge about school level physics is admittedly not that fleshed out, but we were told that it is possible to convert mass to energy. My google-fu has sadly left me for my question here 🙁
So why can’t we just take e.g. nuclear waste and convert it to energy? After that so is my understanding it wouldn’t simply exist as matter anymore and wouldn’t require to store dangerous trash if you can convert it all to energy.
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It is possible to convert mass to energy, but it isn’t possible to just convert *any* mass to energy. The main methods we have of doing this are nuclear fission and nuclear fusion. Nuclear fission is the thing that causes nuclear waste in the first place. It involves bombarding uranium atoms with little particles called *neutrons*. This causes the uranium atoms to split apart into a couple different pieces, and it turns out the pieces together have less mass than the original uranium atoms did. The rest of the mass has turned into energy.
The thing is, though, that this mass was actually energy all along. It was the energy that held the uranium atoms together (the so-called *binding energy*), and it got released when they broke into pieces. And that’s kind of the point. Objects always gain mass if they have more energy. A spring has more mass when it is under compression or tension, because it has stored up potential energy. When you release the spring, that energy is released and the spring’s mass decreases again. Warmer objects also have more mass than colder ones, and spinning balls have more mass than stationary ones. So in a way, there’s not anything special about nuclear fission. It’s just a matter of how much energy you can harvest. And with nuclear fission, that happens to be a *lot* of energy from every single gram of nuclear fuel.
Anyway, the point is that we don’t have some kind of machine that you can put mass into and get energy out of, with all the mass turned into energy. You have to be able to release the energy somehow, and that’s not always possible.
A useful comparison is burning fuel. Let’s say you burn methane (natural gas). If you burn it hot enough all you’re left with is CO2 gas and water vapor. These products will have (very very slightly) less mass than the original methane did. The (tiny) difference in mass is the chemical bond energy (now released as heat) that held some of the atoms together inside the methane molecules (really quite similar to nuclear fusion, only there is a lot less harvestable chemical energy in a single gram of methane than there is usable binding energy in a gram of uranium). Okay, so now we have CO2 and water. They still have mass – can’t we convert more of that mass to energy? Well, how would you do that? You can’t keep burning them – they’re not combustible (water is rather famously used to put out fires, not fuel them). And that’s the problem: you have to have a way to harvest more energy from them – there is no magical mass-to-energy machine.
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