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Fusion of hydrogen or helium normally requires at least the conditions found in the the cores
of stars . High temperatures and densities allow hydrogen and helium nuclei to get close
enough to fuse together into bigger nuclei and release a TON of energy, powering even
more fusion while releasing enough extra to power the star, or if you set this situation
up on earth, you might have a hydrogen bomb.
But it’s actually possible for fusion to occur at temperatures much, much lower than
the core of the sun - like, room temperature, for example.
Now, I’m not talking about the infamous “cold fusion” of the 1980s that hasn’t
been shown to work or involve any, well, fusion - no, I’m talking about the room-temperature
fusion of the 1950s that actually does work: fusion with the help of muons!
Nuclear fusion, of course, happens when atomic nuclei, like hydrogen nuclei , come close
enough together that their strong nuclear attraction can overcome their electric
repulsion, and they fuse together into a single, bigger nucleus - like helium .
This typically happens in a plasma, that is, a super hot soup of electrons and atomic nuclei,
where if it’s hot enough every once in a while two nuclei bump hard enough into each
other to fuse . But fusion can in principle happen in regular, non-plasma molecules, too
- like the hydrogen molecule, in which two hydrogen nuclei are kept relatively near to
each other by sharing electrons . The nuclei don’t stay separated a rigid distance apart,
though - they vibrate and wiggle and every so often, they can, in principle, get close
enough to fuse together.
But with hydrogen - or nitrogen, or oxygen, or pretty much all other molecules - this
happens exceedingly rarely (which is why our atmosphere, which has a fair amount of molecules,
isn’t a giant fusion bomb).
However, things are different if you replace the electrons with particles called muons,
which are basically exactly the same as electrons except 200 times heavier . Muons, being
essentially heavy electrons, form atoms and molecules in almost the exact same way as
electrons, but since they’re heavier, their orbits are much closer to the nucleus than
an electron with the same energy and angular momentum would be . And this means that atoms
and molecules held together with muons instead of electrons are about 200 times smaller,
and their nuclei are correspondingly about 200 times closer together.
And being closer together makes nuclei many many many times more likely to fuse together,
so much so that hydrogen molecules made with muons can fuse together at temperatures much
lower than the core of the sun - even room temperature!!
Which was predicted in 1947 and experimentally achieved in 1956 . Physicists have even managed
to achieve muon-aided nuclear fusion at temperatures close to absolute zero.
So at this point, you’re probably asking yourself: if room-temperature nuclear fusion
exists, why aren’t we using it to power modern civilization?
Well, while muon-facilitated fusion is indeed fully legit nuclear fusion at non-crazy temperatures,
there are some major problems which prevent it from being used as a power source.
First, muons don’t live very long . Unlike electrons which have an in principle infinite lifespan,
after about 2 microseconds muons spontaneously decay
into an electron and some neutrinos, so if you’re going to do anything with muons,
you have to do it real quick!
This turns out not to matter much for the purposes of facilitating fusion, but because
of their short lifespan, there aren’t a ton of muons around - so if you want a reliable
supply of muons, you pretty much have to make them with a high energy particle accelerator
, which takes a lot of energy per muon - at best about 5 giga electron volts , or about 50
times the E=mc^2 mass-energy of a muon itself.
Now, luckily you don’t need a muon for every single pair of hydrogen nuclei you want to
fuse, because after a pair of nuclei fuses into helium the muon can go off and help more
nuclei fuse …and then help more… and more… and more….
EXCEPT, every so often , the muon doesn’t - it’ll get stuck as part of the newly fused
helium atom , and can’t facilitate any additional fusing.
This means that each muon only helps an average of 150- fusions of nuclei before it gets stuck
. And since each fusion of nuclei releases about 18 mega electron volts of energy , this
means that, after 150 fusions, each muon facilitates an average of 2700 mega electron volts, or
2.7 giga electron volts, of energy generation.
Which means that, unfortunately, the numbers don’t add up - Remember it currently takes around
5 GeV of energy to produce a muon, but each muon only generates about two and a half GeV
of energy before getting stuck to a nucleus.
That is, muon-facilitated fusion is a net consumer of energy (rather than being a source
of energy).
This is the best case possible with current technology, and the numbers are still off
by a factor of 2 before even reaching any sort of break-even where muon-facilitated
fusion could generate as much energy as it consumes.
And we’d need to be much better than just breaking even, energy-wise, to make a viable
commercial power plant.
Pretty much the only hope for muon-facilitated-fusion is to figure out how to make muons for less
energy, or figure out how to have less of them stick to the helium nuclei, or how to
unstick them once they’re stuck - which are all hard problems limited by the unchangeable
physical properties of muons and nuclei, and so we’ve made quite slow progress in over
70 years of research.
The summary is that muon-induced fusion exists, it’s fascinating science, but it’s not
going to be powering the world any time soon.
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