Quite some time ago I read a claim that any activity that released energy involved the conversion of mass to energy. This seems to me a pretty broad claim and makes me wonder
Mechanical Energy; I compress a spring, then release it; (Just considering the spring and not my arm) am I doing an energy-to-mass and then mass-to-energy conversion when storing/releasing energy in the spring? Does the spring gain mass when I compress it?
Chemical Reactions; I light a match; On some tiny level is mass getting converted to energy? I thought the atoms were just getting moved around into new combinations, releasing energy in bonds.
Yes in all cases. Both are just atoms moving around, but the mass of a collection of atoms changes when they’re moved around.
It might help to realize that mass isn’t additive: That is to say, if you have a system of many parts, and you find the mass of each part and add all of them up, you won’t get the mass of the whole system. Now, for most situations that humans commonly deal with, adding up the pieces will give you an excellent approximation, but it won’t be exact.
For the benefit of the more Physics Challenged among us, a bit more clarification could be helpful.
Molecules are held together by electrical bonds, having to do with the arrangement of their various electrons. Those bonds themselves are energy, but they are also mass. At this level, mass and energy aren’t very distinct from each other, and are kinda-sorta equivalent and interchangeable according to the well-known E=mc[sup]2[/sup] formula.
Re-arranging atoms and molecules entails creating or breaking those bonds, and perhaps re-arranging them in different ways. Combustion, for example, breaks down molecules and re-combines them with oxygen into other molecules, typically releasing heat energy. When those molecules were formed originally (e.g., through photosynthesis), energy was absorbed and used to create high-energy bonds (in carbohydrates, for example); those bonds now give those molecules greater mass than the sum of the original atoms. Burning them reverses this process.
It gets even more extreme when you look inside the individual atoms, specifically inside the nucleus. There are powerful forces (Chronos can help fill in the right terminology) inside the nucleus holding all those positively-charged protons together, and those forces themselves contribute significantly to the weight/mass of the nucleus. Bust up a nucleus (atomic fission) and those forces get released as energy, with Hiroshima-level results.
Looking even deeper, it’s now known that the individual protons are composed of quarks, vaguely particle-like thingies that are held together with bonding energies. Here again, it’s those bonding energies that contribute the bulk of the mass of an individual proton.
One could speculate, then, that if you bust up the individual protons, you could get energy releases that would make Hiroshima look like a flickering candle. I read somewhere, though, that the forces holding quarks together in protons are so strong, and protons are thus so stable, that protons have an estimated half-life exceeding the estimated life-time of the universe. It’s the massive (literally) strength of those bonds that give protons most of their mass. Protons, it seems, are going to be around for the long haul.
It’s hard to add anything to the replies above. But here’s another example to drive the point home, with some follow-on questions.
Even lowering a weight in a gravitational field (eg moving a bowling ball from the shelf over your sofa to a trophy stand on the buffet) should decrease the mass of a system (by a miniscule amount).
(I never considered before, but is the bowling ball lighter, the Earth lighter, or both?)
There is a figure tossed around in black hole work, that matter infalling into a black hole undergoes ~10% conversion to energy, with no discussion of the specific process by which this happen (beyond friction in the accretion disc, which seems to me hard to calculate any specific numbers for). Is this 10% figure simply the representation of how much energy should be released in a fall from infinity to a black hole’s surface.
Neither. The system consisting of the Earth and the bowling ball together, however, is lighter.
And up to 50% of the mass infalling into a black hole can be radiated away as energy; if the actual figure is less, it’s because frictional heating and the other mechanisms involved are messy and inefficient. And friction might be difficult to calculate, but it can be done, and anyway we have some real accreting black holes that we can observe, too, so we’re not entirely reliant on calculations.
If the energy released by lowering the bowling ball is kept on Earth (like by wrapping your living room in a Dewar flask or something), I’d guess the Earth-bowling ball system would “weigh” the same? If the heat were allowed to leak away and radiate to space, then less?
You are exactly right that most of their mass comes from binding energy, but that isn’t why they have a long half-life. They have a long half-life because there is nothing available for them to decay to, thanks to the decay processes available in nature taken together with energy conservation.
The neutron is almost identical to the proton in its internal structure – just replace one “up” quark in a proton with a “down” quark – and yet the neutron is unstable.
The neutron isn’t the most telling example, though. There are many massive quark-built particles whose masses are almost entirely due to binding energy – more so than for the proton – that decay very quickly. In fact, the shortest lifetimes across all particle species come from these sorts of quarks-bound-together particles.
It’s natural that one we are familiar with – the proton – is the atypical one. If it could decay quickly like (most of) the others, we wouldn’t be familiar with it.
I didn’t need the video to know that proper response to analytical algebraic topology of locally Euclidean metrization of infinitely differential Riemannian manifolds is “Bozhe moi!”
Busting things up doesn’t always release energy. Busting a helium into two hydrogens takes energy, and going the other way (two hydrogen into helium), gives a lot of energy, literally a Hydrogen-bomb (for a little bit of hydrogen) or Sun (for a lot) amount of energy.
