system
Given the following post, respond as if you're commenting on Reddit:
human
If fossil fuels ever becomes too expensive to be a viable source for air travel, what are some alternatives we can use to power planes in the future?

I'm not aware of any energy source with the same potential energy to weight ratio that even comes close to fossil fuels. Even if battery technology came close, wouldn't we have to take a step back to using propellers? I have never been able to get a good answer to this.
gpt
Synthetic kerosene made from biomass at $9/gal is not a heck of a lot more expensive than Jet A (approaching $7/gal)

But in answer to your question, compressed hydrogen at 70MPa ( at 123MJ/kg) has nearly 3 times the energy density of jet a (at 42MJ/kg) by mass.

As far as alternative fuel vehicles go, aircraft are well suited to conversion to run on compressed hydrogen since its biggest problem is confinement and aircraft have high rates of consumption and low storage life demands, so leakage isn't as much of a problem.