A machine, device or system which continues in motion for ever. Also known as a perpetuum mobile, the latin translation of perpetual motion machine.
Scientists and engineers are taught from an early age that it is impossible to make a practical perpetual motion machine because of the three key laws of thermodynamics.
We’ll start with the second law of thermodynamics. (non-techies should skip this boring, pedantic paragraph and go straight to the next) This says that for any given process in a closed system, the change in entropy of the complete system will always be equal to, or greater than zero. Thornton gives a number of ways to express it mathematically in his write-up.
Basically that means anything and everything which moves or otherwise changes, will somehow convert useful energy (motion, electricity, etc) into less useful forms of energy (heat, for example), except in one special case, which I’ll talk about later.
Now we have to look at the first law of thermodynamics. This says that in any closed system there is no way to create energy out of nothingness. To put that another way, the total energy within the system will remain the same, no matter what you do to it.
Combine these two, and it’s easy to see that in any closed system, the total energy within that system will remain the same, but if anything changes, then some of the energy will be converted into useless forms of energy, such as heat.
The rather depressing conclusion is that all things eventually run down, converting more and more of their useful energy into less useful forms. Thus , no machine can go on for ever, unless it has some kind of external energy source, which constantly tops up the energy 'lost' in various energy conversion processes.
But what about that special case where energy is converted with zero loss? That is where the third law comes into play. The third law basically says that friction is a fact of life, and that whenever you get into the practical world of making and building stuff, then it is physically impossible to build something which can convert energy without some small fraction of that energy being changed into heat.
OK, so the engineers dismiss perpetual motion by invoking the god of thermodynamics and declaring that perpetual motion is impossible at all times, in all places and in all environments. End of story. No arguments allowed.
Hey! You’re such a smartass! Tell me why the laws of thermodynamics are so inviolable.
Here’s the easy answer: If someone made a perpetual motion machine, it would be so useful and so valuable that it could not be hidden or forgotten. Throughout history no records indicate that such machines have ever been built, despite great intellectual efforts by many millions of people.
Nope, sorry, that’s not good enough
Hmmm, thought you might say that. OK, let me put it another way. The only way we humans can analyse how the universe works is by looking at our experience, thinking about those experiences and drawing conclusions. From this we can formulate some rules about how the world works and use those rules to make predictions about what should happen in the future.
The second law—along with its two relations—is one of those rules which is based on the sum total of human experience. People have pummelled it with all the other theories of how stuff works, tested it with physical experiments and thought experiments designed to find the tiniest crack in the wall of certainty surrounding it. Yet. despite all this, they have never found a way to break it: never even found the tiniest aberration which might make it give some weird and unexplained result.
That is as much as anyone can say about any of our so-called fundamental laws of physics. We have never seen or thought of anything which might imply that they are in any sense fallible. The reason that the second law is so revered by engineers and scientists is that it is absolutely fundamental to all branches of engineering. You might imagine that if there were anything wrong with it, then all the different types of test, experiment and theory would have revealed some kind of crack, but no. Nothing has ever dented our certainty in the second law of thermodynamics.
Hmm, doesn’t sound like much to base a whole way of life on?
Well, you might be right, but that’s the way the scientific business works, most of the time. If you don’t like it, start reading more on the philosophy of science.
So, err, does that mean I might be able to actually build a perpetual motion machine?
No, not here on earth. However, there might be places in the universe where things are so different that the experience we have accumulated here on earth in the last couple of millennia simply do not apply. And in those places, I suppose, it might well be possible.
Example of a machine (and why it won’t work)
Designs for perpetual motion machines fall into two broad categories: the ones invented by people thinking they have stumbled on the secret of eternal wealth, and those thought up by scientists and engineers to help explain the issues involved.The latter group are usually the more interesting. Almost always, the scientists produce ideas which are much harder to de-bunk. Here is a nice one from one of my heroes.
Feynman’s gas unit
| _______________________ |
--- | \ / / |
/ \ | \ / / |
| |__________\_/ / |
| | /_______________/ |
| |————------——————/\ |
\ / | / \ \ |
-| | /____\_______________\ |
| | |
_____|____ | |
| | | Low pressure gas |
| Ratchet | |______________________________|
The above diagram shows a machine proposed by Prof. Richard Feynman in his book, The character of Physical Law. It is a shaft with a series of vanes in a low-pressure atmosphere. The shaft is attached to a ratchet mechanism which allows rotation in one direction, but not in the other. Although pressure is normally evenly spread, the statistical nature of molecular motion means that every now and again, the pressure on one side of the vane will be higher than on the other, causing a slight positive torque on the shaft, leading to a small positive rotation. Because of statistics, there will be just as many occasions when the pressure is reversed, generating a negative torque on the shaft. But there will be no motion on those occasions, because the ratchet prevents it.
Although the motion will be very slow, and with a low torque, this appears to be a perpetual motion machine, driven by the kinetic energy of the molecules in the gas.
Can you spot the fallacy?
I first thought that the energy comes from the temperature of the gas, which will fall very slightly every time the vanes move. If a gas atom hits the vane, it will rebound with some energy. When the vane remains stationary, then the atom rebounds elastically. However, when the vane recoils as the gas atom hits, then the atom's rebound speed will be slightly slower, leading to a cooling of the gas. Eventually there will be no energy left in the gas, or not enough to turn the vanes, in any case.
Read Professor Pi's explanation (below) of why Feynman is right and I am wrong. This is incredibly subtle and shows how many traps there can be for the unwary when de-bunking such machines.