Critical Question:
- What are perpetual motion machines, and why are they impossible?
There was an episode of The Simpsons in which Lisa made a perpetual motion machine, which angered Homer because “it just keeps going faster and faster.” Later, he called her into the room and yelled, “In this house, we obey the laws of thermodynamics!”
He had every right to be angry, but Lisa is not alone in her fascination with the idea of a machine that never stops. As we’ve already seen, motion requires energy, and energy isn’t always easy to come by. But if we had a machine that could keep moving forever without assistance, the possibilities would be endless.
Alas, this is one of those cases that really is too good to be true.
A perpetual motion machine (it’s a long phrase; let’s go with PMM) is something that runs forever and can do useful work (i.e, provide energy) without needing any energy input.
Even if you knew nothing about the formal physics of energy and motion, such a machine would probably seem unlikely to you. But according to our current understanding of the laws of physics, not only is a PMM difficult to create in practice, it is actually theoretically impossible. This follows directly from the principle of the conservation of energy. Energy cannot be created or destroyed. No machine, no matter how cleverly it is designed, can provide the world with energy unless it steals some from somewhere else.
This may seem a bit abstract, or it might seem obvious. But the reason I bring it up is that our world’s relationship with energy has changed rapidly over the last few hundred years, and since so much of our future depends on how we power our technology, it’s important to understand exactly how that energy works.
This story begins, one might say, with the invention of the steam engine and the onset of the industrial revolution. One reason why the steam engine was so important was that it finally made use of a kind of energy which is everywhere but which is actually quite difficult to harness: heat. From that point on, humans kept discovering more and more ways to make things move, and each fuel source seemed more abundant than the last: first we had fire and water pushing locomotives, and then gasoline came to be used in the engines of cars – a substance that in some places bubbled out of the ground faster than we could burn it. As nineteenth-century physicists began to understand the mysteries of electricity and magnets, these two sometimes seemed almost mythically powerful, especially when Nikola Tesla rode into town and set up his enormous coils which sent artificial lightning bolts sparking across the room. In the twentieth century, Einstein discovered that an enormous amount of energy are stored within the nucleus of every atom, and once people figured out how to release that energy, they could power entire cities – or level them – with relatively small amounts of radioactive material.
Energy seemed to be everywhere, and scientists seemed to be able to get as much as they wanted from anywhere they pleased. The world became power-hungry. But soon, it became apparent that most of our energy sources tended to cause significant problems for the world around us – some of them were even beginning to run out, even though they’d once seemed infinite.
Now the search is on for energy sources that won’t disappear or damage the environment. But an explanation of the impossibility of PMMs and of entropy (which we’ll get to in the next section) gives an indication as to why it probably makes more sense to just use less energy.
Traditional PMMs make use of a kind of circular logic. Imagine, for example, a string of plastic balls filled with air. These are attached to an apparatus as in the diagram below. As the ball on the bottom enters the water, it floats to the top. This continuous action turns the wheels, which can then be used to turn something else forever.
In this case, the machine doesn’t work because the energy needed to pull the bottom ball into the water is actually more than the floating balls can provide. When hooked up, this machine would only turn until one ball got stuck at the bottom, and then the whole thing would stop.
As it turns out, every idea anyone has ever had for a PMM fails for similar reasons. We have never found any process anywhere that can power such a machine, because any transfer of energy is always zero-sum: you can only get out of it what you put in.[1. In fact, you can’t even get that much! Damned entropy.] In other words, there is no such thing as “free” energy.
What that means for us is that whenever we think we’ve found a new source of energy, one important thing to consider is where we’re taking energy from, and how long we have before that source runs out. Even the miraculous new energy sources discovered since the dawn of the industrial revolution obey these laws – I’ll discuss this a bit more two sections on.
In the next section, we’ll go one step further in discussing the problems involved in using energy to power machines to do things for us.
Big Ideas:
- Perpetual motion machines violate the law of conservation of energy: they would, in theory, provide “free” energy without taking it from somewhere else. This is impossible.
- Every source of energy must eventually run out.
Next: 5.5 – Heat and Temperature
Previous: 5.3 – Conservation of Energy