Re: Physics: reactionless drive

Dan Fabulich (
Tue, 2 Nov 1999 16:16:53 -0500 (EST)

'What is your name?' '' 'IT DOESN'T MATTER WHAT YOUR NAME IS!!!':

> To cut to the chase....somehow by applying heat to gas... velocity is
> imparted to individual molecules.

Well, the fact of the matter is that when we're talking about gasses, the temperature of the gas just IS the mean kinetic energy of the molecules involved, in a very palpable sense. So it's not that heat SOMEHOW imparts velocity, it's that heat IS the act of imparting velocity to the molecules.

Similarly, it's not like shaking hands somehow *causes* hands to be shook; it's that shaking hands just IS the shaking of hands.

> I may be missing something here....but where is the "for every action
> there is an equal and opposite reaction".

I'm not sure I'm understanding your question... Maybe you're asking how the mean kinetic energy is increasing without obvious force being applied?

Suppose that the gas is in a canister. Note that even though the kinetic energy of the gas is increasing, the momentum of the *container* of gas is still zero. Those little particles are all zipping around, on average, in every direction at exactly the same magnitude, so the center of gravity of the whole can (that is, the average location of all of the particles in the can) doesn't move, even if the individual particles are bouncing all over everywhere.

On account of this, no force need be applied to the canister to get this change, though heat must be applied (and, as you're surely aware, heat doesn't come from nowhere).

> Even if i'm not missing something......<great leap>....suppose that all of
> the gas molecules could be coaxed to go in the same direction? (Campbell's
> "Mightiest machine(?)" <early sixties?>)

This reminds me of Maxwell's demon. He was noticing that you can get work by allowing heat to flow from a hot place to a cold place. (You can think of it as something like a water wheel or turbine, which extracts energy from the flow of water). When the two places are at the same temperature, no more energy can be extracted in this way. This is the fundamental theory behind an engine.

Well, being "hot" just means that you're a fast particle; being "cold" just means that you're slower that the fast hot particles. So, Maxwell imagined, what if we had a box with two insulated compartments, and a tiny little door between them, which was opened and closed by a tiny little Demon, or nanobot if you want a more modern version. Then suppose that you put the gas, of one constant temperature, on the right. Now "constant temperature" is something of a misnomer: what this really means is that exactly half the particles are faster than the average, and half the particles are slower than the average, and the average determines the temperature.

The Demon/nanobot, imagined Maxwell, could simply watch the particles heading towards the door. If the particles were sufficiently fast, he'd open the door and let them through. Otherwise, he'd keep the door closed and they'd remain in the original canister. After a while, one compartment would be filled with fast, hot particles and the compartment on the right would be filled with slow, cold particles. At that point, you turn that system into an engine, allowing heat to flow from one side to the other. Free energy results from nothing, since opening/closing the door takes very little energy at all in comparison to the amount which the engine would generate. Entropy, in this case, would decrease in the closed system, for free.

Maxwell's Demon doesn't work, and the law of conservation of energy still holds, however. Why? Since it was revealed that the act of OBSERVING the particles, of figuring out how fast the individual particles are, this observation act ITSELF required energy to run. In fact, it requires much, much, more energy than the engine would generate after the Demon/nanobot had done its work. (I think that entropy, on account of this, is better thought of as "uncertainty" than as "chaos," though some disagree with me.)

I tell this little anecdote since what you're describing seems like a similar project to Maxwell's Demon: it seems like it would work, but the devil is in the details [pun intended]. Coaxing all of the molecules to travel in one direction would GREATLY reduce the entropy of the system of gasses; the attempt to reduce the entropy in this way, even by a nanobot/Demon, requires much more energy than you get out of it.

If it's any consolation, you should feel proud for inventing yourself one of the most interesting physical puzzles of this century, IMO.


-unless you love someone-
-nothing else makes any sense-

e.e. cummings