In Fahrenheit 451, what does Beatty explain that Man wants to invent, but never could?

Speaking to Montag, Beatty talks about the properties of fire, and why it is so important to mankind:

"What is there about fire that's so lovely? No matter what age we are, what draws us to it?" Beatty blew out the flame and lit it again. "It's perpetual motion; the thing man wanted to invent but never did. Or almost perpetual motion. If you let it go on, it'd burn our lifetimes out."
(Bradbury, Fahrenheit 451, Google Books)

Fire consumes whatever it can, burning without care as long as fuel exists. In stars, there is so much fuel that the star can burn for billions of years; in human terms, this is as close to perpetual motion as makes no difference. Man has always wanted to create perpetual motion, but cannot because of the laws of physics; fire allows the pretence of perpetual motion, the feeling that something has been created that cannot be controlled or stopped, and so it fascinates man beyond normal reason.