Talk:Perpetual motion machine

Perpetual motion machine

I'm sorry, but I think you're just wrong about the meaning of the phrase "perpetual motion" as used in discussions of thermodynamics, statements about the impossibility of perpetual motion, etc.

    The perpetual motion, then, which has been the subject of such anxious and laborious search, is not a mere motion which is continued indefinitely. If it were, the diurnal and annual motion of the earth, and the corresponding motions of the other planets and satellites in the solar system, as well as the rotation of the sun upon its axis, would be all perpetual motions....

    In short, a perpetual motion would be a watch or clock which would go for so long as its mechanism would endure without being wound up: it would be a mill which could grind corn or work machinery without the action upon it of water, wind, steam, animal power, or any other moving force external to it.

—Dionysis Lardner, 1857, Natural Philosophy for Schools, p. 70

    Perpetual motion is of course possible, as is stated in the first law of motion, "Motion continues in a straight line undiminished in velocity unless acted on by some external force." A top set spinning under conditions where there is no friction will never cease to spin. These conditions are very nearly realized in the motions of the planets and stars. The statement that energy cannot be destroyed asserts this possibility.

    But by "perpetual motion" is generally meant a machine which will do work and keep going though energy equivalent to the work done is not supplied to it. Many attempts have been made to construct such a machine. They have all failed. The statement that energy cannot be created denies the possibility of such a machine.

—S. Lawrence Bigelow (1912), "Theoretical and Physical Chemistry," Theoretical and Physical Chemistry By S Lawrence Bigelow p. 26

Dpbsmith 18:20, 1 January 2007 (EST)

Retrieved from "http://www.conservapedia.com/User_talk:Aschlafly"

Dpsmith, I moved this discussion to the talk page for perpetual motion, so others can benefit and contribute as desired to this.

Your quotes are helpful, and I agree that there are different definitions for a perpetual motion machine. But everyone agrees that a machine that does work without energy is impossible. So the above definitions do not lead to productive discussion. Besides, the earth is not really a perpetual motion machine. Our orbit would eventually become unstable, and presumably we would slow down eventually.

What is a fascinating question is whether motion can continue indefinitely in a closed system. One quote above implies it can, but I doubt most would agree. I wouldn't. --Aschlafly 18:59, 1 January 2007 (EST)

He's right Mr. Schlafly; the impossibility of perpetual motion is proven by the 1st law of thermodynamics, not the second.

Ben

OK, Ben, but define perpetual motion machine in a trivial away and it only results in a trivial answer.

How about this, let's define "perpetual motion machine II" that simply means a machine capable of perpetual motion (without a perpetual energy supply).

Now that's an interesting question that is difficult to answer fully! --Aschlafly 19:05, 1 January 2007 (EST)

Theoretically it is possible (the first law of motion), but practically it is impossible just because you wouldn't be able to eliminate all outside forces.

I also have a comment on your proof using the 2nd law of thermodynamics:

"The only argument that a perpetual motion machine is impossible is based on an interpretation of the Second Law of Thermodynamics. If entropy is always increasing, even in closed (and isolated) systems, then indefinite motion is impossible because an increase in the disorder of the system will inevitably disrupt the motion"

Firstly, as Dpbsmith has demonstrated, this is not the only argument against perpetual motion.

Secondly, though it is almost inevitable that increaced disorder will disrupt motion, it still isn't logically proven; it is always possible that it might go on just a little bit longer.

Ben

So, Ben, you seem to be saying that a "perpetual motion machine II" (as defined above) may be possible to build.

Really think so? If you doubt it, then maybe you can discovery a way to prove it is impossible. I don't think anyone has proven that yet, and no one else seems to be trying at this time either. But it would be worth proving. --Aschlafly 23:53, 1 January 2007 (EST)

Aschlafly: you said "But everyone agrees that a machine that does work without energy is impossible." Well, no. Historically the whole debate arose because for centuries people have believed that they have found a way to build a machine that does work without energy, something for nothing. There are still people who believe it. They tend to use the terms "free energy" and "overunity" to avoid the onus of the term "perpetual motion." An example of a modern machine claimed by its inventor to generate more power than it consumes is the Adams Motor[1], [2]. A classic fraud was the Keely motor.

My point is that the phrase "perpetuum mobile" goes back to the days before thermodynamics and refers to innumerable attempts to produce simple mechanical arrangements, typically involving shifting or pivoting weights, that on paper look as if they might really do work without an external input.

You seem to be interested in a different philosophical question, one which I don't know much about or whether there is any established name for it. I'm arguing strongly that you should either find out what that name might be, or invent something that doesn't use the phrase "perpetual motion" at all.

Here is my guess at what an answer to your question might be. If you use the best modern techniques, e.g. suspending a spinning object in vacuum via superconductive magnetism, you can get something that will move without additional energy input for a really long time. You can measure the rate at which it slows down very carefully or predict it theoretically.

My guess is that it may well be possible today to build a system in which the rate of energy loss is so slow that it can be predicted continue to run for hundreds of thousands of years if the apparatus remains intact; thus the limiting factor in how long it runs is not the apparatus itself, but unrelated external catastrophes (an asteroid hits it, the building it is in collapses in an earthquake, funding runs out and someone pitches the apparatus in the trash, breaking it, etc. etc.)

Probably the place to look for one of these things in real life would be the gyroscopes used in inertial guidance systems.

If the word "forever" is taken literally, then I don't know how you answer the question, because beliefs about the future lifetime of the universe change every generation or so. If the word "forever" means something like a mathematical limit—then I think the answer is: according to current understanding, it is possible to build a machine that will run indefinitely long, or as long as you like, receiving no external energy and performing no external work, where the actual limit on the running time is set not by the construction of the machine itself, but by the probability of external catastrophe.

I think. Dpbsmith 08:44, 2 January 2007 (EST)

OK, Dpsmith, you win on your point about what a perpetual motion machine really means. But I'm still interested in why perpetual motion (without producing extra energy) is impossible. The increase in entropy must prevent it. Underlying that may be the uncertainty principle in quantum mechanics.

More thought and research would be worthwhile here. I think we're all convinced that the motion would eventually stop. But why? What force stops it? --Aschlafly 22:36, 2 January 2007 (EST)

It's beyond my own knowledge. I suspect such questions are like the irresistable force and the immovable object, though. Provisionally, let's call your gadget an "endless coaster."

Point #1: At least according to Newtonian physics as I understand it, if you truly had a closed system, it would not stop. But if you truly had a closed system, there would be no way to see that it was still moving.

In order to observe it, there would have to be some energy exchange between the "perpetual motion" and the observer. I have an idea that since the observer is gaining information, the observed must be gaining entropy, but that's just handwaving and I don't know how to prove it.

What I don't know whether there are any theoretical reasons that would make it impossible to have a truly closed system.

Here's another angle. If we're considering, say, a ball bearing magnetically suspended in a vacuum by a superconducing magnet or something like that, if if there is a small amount of friction the result, according to classical physics, would be to make the ball bearing spin slower on an exponential decay curve. It would have a half-life, like radioactive decay. Perhaps it loses half of its spin every day. Well, according to classical physics, it would spin slower and slower but would never actually stop. Asymptotically approaches zero, never eaches it. Most likely (out of my depth again) quantum physics would say that at some point the spin becomes quantized, meaning that after some period of time it can't slow down any more. The spin must be either one quantum or none... and then you get all that crazy wave-function collapse stuff. You have a superimposed state in which the spin is one with some probability and zero with some probability, and the probability decreases over time.

But really, once you start talking about whether something can literally go on forever, you're outside the bounds of science. I can't keep track of the number of times the "scientific" narrative of cosmology has changed during my own lifetime, and it shows no signs of settling down. Is the universe closed? Open? Continuously expanding? Oscillating? Obviously, if physics predicts a finite lifetime for the universe, then a "perpetual" motion, meaning one that would last longer than the universe, is impossible. Dpbsmith 10:03, 3 January 2007 (EST)