In my earlier question, regarding airships, I mentioned fictional gas, as light as hydrogen but non-flammable. Is it possible to make chemical element, with atoms made not from nucleons, but with some exotic particles, thus creating elements with densities and atomic masses same as "normal" elements, but with diferrent properties? Are there any particles other than nucleons in existence, that are stable enough to do that?
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Not that we know of. Basically, the Big Bang made every possible particle and the resulting stew then reacted with itself and broke down and ultimately cooled enough that only particles that are stable over billions of years survive from that period. We have not, so far, found anything other than the classic proton, neutrino, electron, and photon that are stable. (The neutron is not stable by itself, but is so weakly unstable that it can be stabilized in a nucleus. We understand how that happens, and no other known particle can be stabilized that way.)
Theory -- which works very well -- makes detailed predictions about the stability of particles and explains why only the particles we observe to be stable are stable. So the combination of theory and observation reinforce our belief that what we see is what there is.
Note that the theory (The Standard Model (=SM)) also hints that it is not complete and that a more complete theory (which would not change what the current theory says in the areas of physics that we have studied so far) might predict new, stable particles. Most physicists believe that dark matter (if it exists) is made up of some of these new particles. Many extensions to the SM have been proposed and the predicted new particles searched for. So far with a 100% failure rate.
Nearly all the simple extensions to the SM have been proven experimentally to be false, so increasingly baroque new extensions are being developed which predict new particles just out of reach of current experiment. (To date, what's happened is that each time this is done the experimentalists do a more sensitive experiment and the new theory is ruled out, too. So the theoreticians add one more bit to their model and try again. Rinse, lather, and repeat.)
In any event, dark matter -- if it exists at all (there are other possibilities) -- is characterized by the property that it doesn't interact meaningfully with ordinary matter, so it wouldn't replace nucleons, anyway.
Bottom line: Current theory and current experiment all say (resoundingly) "No. Not here."
(It's worth noting that under some very extreme conditions -- inside a neutron star, for example -- some particles probably become stable which are not stable in mild conditions like my desk or the center of the Sun.)
You might be looking in the wrong place. Instead of trying to find a gas lighter than $H_2$ or $He$, you might gain more value from finding lighter structural materials for the rest of the airship.
The Hindenburg’s structural mass was much greater than the mass of its lifting gas. A few-percent change there would pay off much more than an exotic gas. Finding a material with the strength of aircraft aluminum but half the weight would be a game-changer.
For fictional purposes, how about a world with aluminum isotopes that don’t have any neutrons? They’d be really light, but chemically almost identical.
You already got a well-reasoned negative answer, so other than agreeing with it I'll skip that.
On the positive possibilities, I think the best option is Helium, which is lightweight, stable and does exist. You could make He more abundant in your world in a number of ways, e.g.:
- An accident in your planet's remote past with a Helium planet. Of course, with some care: if your atmosphere is too rich in He, it will have even less lift.
- An overabundance in your planet (or in your planet's recent past) of alpha-emitting isotopes: given that alpha particles are Helium nuclei that will translate into cheap(er) He.
Additionally, one can play with (almost) realistic chemistry, to make sure He has less tendency to escape through walls than with our current materials.
On lighter-than-$^4$He options, there's
Expanding upon Mark Olsen's answer:
While there are plenty of other particles out there that behave similarly to what we consider normal matter there are two big problems:
1) The main one: The matter of our world is in the base energy state. All the other particles we know of are basically higher energy versions of the basic ones. Since said particles have a lower-energy analogue they have a considerable tendency to become that analogue and release considerable energy in the process. None of them are remotely stable.
2) You wanted something light: E=mc^2. In chemical and nuclear terms the high energy versions act pretty much like (except for the decay issue) low energy version--but since they have a lot more energy they have a lot more mass. Lets take "hydrogen" gas but made out of the next higher energy state:
The electron (500 keV) is replaced with the muon (105MeV, half life around a microsecond) and I'm not sure what the proton (938MeV) is replaced with but the lowest energy candidate is 1115MeV and has a half life below a nanosecond. Our particle went from 939MeV to 1220MeV--it's 30% heavier.
It is in fact possible to create such atoms. One example is muonium: an analogue of hydrogen with an anti-muon replacing the proton: https://en.wikipedia.org/wiki/Muonium Other particles such as pions can also form "atoms", either with electrons or with other particles: https://en.wikipedia.org/wiki/Pionium Perhaps the ultimate lifting gas would be positronium, the bound state of an electron and its antiparticle.
The problem with actually using any of them to provide lift is that they're highly unstable. Muon decays within a couple of microseconds, positronium undergoes self-aniihilation within nanoseconds, releasing gamma ray photons. But if you can hand-wave some way to make them stable...
However, you're up against diminishing returns. Air has an average molecular weight of ~29 g/mol. Hydrogen is 2 g/mol, helium 4 g/mol. So the best possible case, replacing air with a perfect vacuum (and ignoring structural issues) only gets you about a 7% increase in lift over hydrogen.