[rymohr]

You seem to know a lot about this stuff. I have a question for you.

If fusion creates the potential for fission (radioactive waste) and radioactive waste can be used to build atomic bombs, how have we not figured out how to make mini perpetual-energy reactors?

[Robotbeat]

Both fission and fusion release net energy by having products with greater binding energy. The greatest binding energy is iron (and some surrounding elements). Once you get there, no more energy can be released by fusion (or fission).

See this graph: https://opentextbc.ca/universityphysicsv3openstax/wp-content...

So it’s not a perpetual motion machine. Iron is the bottom.

(Heavier stuff than iron can be created by fusion, but that absorbs energy instead of releasing it. Supernova create these heavier-than-iron elements like Uranium and gold endothermically... they’re also created by the decaying guts of neutron stars—which are essentially ginormous atomic nuclei held together by gravity instead of nuclear forces—when they collide and some of their guts are released into space.)

[chasil]

The S-Process creates elements with atomic numbers higher than Iron, and it does not rely on supernovas or neutron star dissolution.

https://en.wikipedia.org/wiki/S-process

[wahern]

S-process is still endothermic, though, right?

I'm not sure if endothermic is the best word. IANAP. It seems to usually be used when discussing fusion-based neutron generation. But AFAICT neutron generation, especially as it relates to the s-process, is still largely a thermal process--the greater the temperature, the more neutrons are generated, the faster the s-process evolves. (If you go back to the beginning of the universe all nuclear synthesis represents an endothermic process, right? Though, maybe such semantic games aren't particularly helpful when distinguishing nuclear synthesis processes.)

[pletnes]

Nickel-62 is the most stable isotope, if memory serves. It’s not efficiently generated in star fusion, however, so iron-56 is believed by many to be the most stable.

[FiatLuxDave]

The term you are looking for is "nuclear binding energy curve". Basically lighter isotopes can gain energy by fusing, and heavier isotopes can gain energy by splitting, but somewhere in the middle (around iron and nickle) the isotopes are the most stable. So you gain energy by moving towards iron, whether from the light end or the heavy end of the periodic table.

[7thaccount]

Thermodynamics has some laws (First or Second, I can't remember) that point out perpetual energy or motion machines are impossible.

[jahabrewer]

I really don't know, but aren't there some caveats about those assuming that space is flat or something about the rate of expansion?

[jschwartzi]

Nope. The three laws are unequivocal. The universe can only increase or maintain entropy through physical processes. It can never return to a lower-entropy state. The laws say nothing about the topography of the universe and it wouldn’t matter anyway.

[LeegleechN]

When you dig into it more you realize the second law of thermodynamics is more of a statistical statement and doesn't have the same status as say the laws of quantum mechanics or relativity.

It's possible to create hypothetical situations where all of the must fundamental laws are being followed but the second law of thermodynamics is violated (for example if there are many more 'ordered' states than 'disordered' ones). And there is some vanishingly small chance that it will be violated in our universe for a macroscopically observable length of time.

In practice you won't go wrong by treating it as absolute.

[mensetmanusman]

Actually, there is debate about the conservation of energy over cosmological length scales.

e.g. if new voxels of spacetime are created during, and they contain zero-point energy... may account for photons losing energy as they red shift over large distances.

[jerf]

Yes, at large scales space expansion can sort of make it so energy is not conserved: http://www.preposterousuniverse.com/blog/2010/02/22/energy-i... Depending on how you look at it, anyhow. One way or another you end up with some sort of unintuitive concept being introduced.

[bananabreakfast]

Fusion cannot create the potential for fission and radioactive waste cannot be used to build atomic bombs.

I'm afraid you're quite off base here.