How did they know splitting the atom, fission, would release so much energy? And why would the opposite be also true, fusion?
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The short answer is both fusion and fission reactions can result in atoms that are more stable, and therefore have lower energy levels. This energy was released during the reaction.
The release of energy from fission is a prediction that comes from Einstein’s relativity, and his famous E=mc^2 equation. It says that the energy and mass are equivalent, and that the energy of an object is equal to the object’s mass times the speed of light squared. This equation gives massive amount of energy for very small amounts of mass. When you break apart an atom like Uranium, the resulting atoms and particles have less mass than the uranium atom had in the first place. This lost mass has been converted to pure energy, which is released by the fission reaction.
Fusion is the same idea, except you’re putting two atoms together into one, larger atom. How can both combining and splitting atoms release energy? Because in both cases the resulting atoms are more stable than the one we started with ( with uranium it’s more accurate to say that the products exist on the path towards the more stable atom, but that’s getting into a more complicated explanation).
Small atoms release energy when they combine, big atoms release energy when they break, and both gravitate towards ‘medium’ size. That ‘medium’ size is the iron atom.
Fusing large atoms or splitting small ones does not release energy, but actually absorbs energy.
Scientists studied radioactive decay a while before the manhattan project. They noticed things like induced radioactivity and nuclear fission, and found that these released energy as heat. From there, the idea of nuclear fission chain reactions emerged and countries then started research programs on the idea.
Einstein, his famous theory of E=MC^2 is a formula which states that mass is highly condensed energy, and so “splitting” or forcefully decaying mass will release a shitton of energy, equal to that mass multiplied by the speed of light in a vacuum squared.
Hence, Nuclear energy and Nuclear bombs. The harnessing and/or weaponization of highly unstable forms of mass and the energy they released when their mass decays into more stable forms.
It is down to the size of the atom, splitting a large atom releases energy fusing two small atoms releases energy.
I recommend the excellent PBS show about Einstein called “E=mc²” including the scientists who inspired his theory. One segment portayed Lise Meitner, after escaping Nazi Germany, realized Otto Haun’s lab was unknowingly splitting the atom.
So to start off in a nucleus there are protons and neutrons, which get stuck together into a single lump of stuff. It turns out the “glue” that holds those protons and neutrons together is made partially from the mass of the nucleus. Einstein noticed that mass could be converted into energy at a rate of E=mc^2 where C is the speed of light(3×10^8 m/s). Because the speed of light squared is such a big number even small amounts of mass become a large amount of energy.
So with fission the bigger and heavier the initial atom the more “glue” it needs and when you smash it into smaller pieces each of those smaller pieces needs less “glue” and as a result some of the energy used to hold the parent atom together gets released as heat. Even if the total amount of matter converted to energy is quite small because E=mc^2 it is still dramatically more energy then is released with conventional fuels.
With fusion it’s the same idea, you want to take a few light elements and fuse them into a slightly heavier element. With there being a difference in the amount of “glue” required. The only fusion reaction we can currently preform bis done using a very dense form of hydrogen. After the fusion is complete you have a helium atom and some discarded neutrons the the total reduction in the energy required to hold this helium vs its parent atoms is released as heat.
To add on, you’re not splitting one atom, you’re causing a chain reaction of atoms getting split.
To answer your question, they KNEW because Einstein gave them the equivalency of mass to energy with his famous equation E= MC^2. C being the speed of light at 3,000,000 meters per second. Square 3,000,000, You get 9 000,000,000,000. So if you have one pound of anything, even feathers, and you convert all of that one pound (454 grams, give or take) You multiply 454 times 9 trillion and you get 4,086 trillion. That unit is in joules, the basic unit of energy. A joule is one watt for one second. So you’ve got 4,086 trillion watts of power released in a millisecond. Makes a big spark.
Putting it simply, it is because they made a lot of calculations to arrive at that conclusion, specially Albert Einstein’s E= MC² (Energy = Mass times the square of the Speed of Light).
E = MC^2
Energy = Mass times the speed of light squared
If you know the mass of an object before it is split (fission) and the mass of the atoms after they are split then the rest is released as energy.
If you know the mass of two objects before you combine them, and the mass of the atom that is created when combined, the rest has to be energy.
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