Perpetual motion and other impossible dreams. By Tom Napier Condensed from a talk given to the Regional Gathering of the Central New Jersey Mensa Group on March 7, 1998. Copyright © 1998, all rights reserved. Everything in the universe, ourselves included, is entirely composed of matter or energy. Thus an understanding of physics, which studies the interactions of matter and energy, lies at the root of all knowledge. Everything which has happened, or will happen, is a consequence of the basic reactions between matter and energy. This gives physics great predictive power. One of the most useful things you can learn from physics is what cannot happen. If you shuffle a deck of cards a great many times they may eventually end up in numerical order. However, you can predict with total certainty that, whatever the result, it will contain 52 different cards. No method of shuffling a deck will ever produce, for example, 52 Aces of Hearts. This result is not within the capabilities of the system. Those laws which say what is impossible are the best tested, the most general and the most absolute of the laws of physics. If someone comes to you with a theory which depends on an "unknown force" you can be sure they are talking nonsense. Anyone introducing an unknown force must explain why it has never been detected and why it has never before been found necessary. So here is a firm statement. If something is forbidden by a well-established physical law, then it ain't going to happen, no matter how much you might want it. All gravitational forces are attractive, there is no repulsive gravitational force. You can't screen against gravity, H. G. Wells' Cavorite is impossible. Since nothing can shield or cancel gravity no amount of wishing, or meditating, is going to make you lift off the ground without mechanical support. You can't accelerate an object without applying a force to it. Flying saucers can't hover motionless in thin air and if they made instantaneous stops and starts they would squash their crews to a pulp. Another absolute law is that no information or material object can travel faster than light. Sorry, Star Trek fans, there are no warp drives and it's never going to be possible to travel from one star system to another in the course of a one hour TV show. By impossible I mean impossible, not just that we don't yet know how to do it. Travel between stars is always going to take many years. We may some day receive radio transmissions from alien civilizations but we can be sure that if any aliens visit us they have spent hundreds or thousands of years getting here. We know that the laws of physics are the same on distant planets as they are here. Whatever technology aliens may have, they are limited by the same laws of physics as we are. For example, even if the aliens could get here, they couldn't abduct people by passing them through bedroom walls. No amount of hand- waving about alien technology can overshadow the fact that people and walls just don't behave that way. As a physicist, I am skeptical of claims that telepathy has been demonstrated. This is partly because historically every positive claim has turned out to be an error or downright fraud. However, if direct mind-to-mind communication can take place, it can only be via electromagnetic radiation. The human mind is an electrochemical machine and it takes chemical or electromagnetic energy to influence it. One can calculate the minimum amount of energy which has to be received to see a picture. From that you can calculate how much energy has to be transmitted. If the human brain could transmit or receive this energy it would have been detected by this time. Making claims which defy physical laws makes as much sense as claiming that 2+2 = 5 or that a triangle has four sides. Another absolute is that no machine can provide an energy output without a corresponding energy input. The most fundamental of the physical laws is the conservation of mass/energy. Energy can be changed from one type to another, and under some circumstances mass can be changed into energy and energy can be changed into mass, but the sum total always remains the same. The reason for this is that there are no reactions at the atomic level in which energy is gained or lost. This has been confirmed to very high degree of accuracy. At the most basic level, inputs and outputs always balance. There is no way of adding equalities and ending up with a surplus. Energy is always conserved. Anyone who thinks otherwise is like the gambler who is looking for some way of combining bets at roulette so as to come out ahead. Every bet pays a percentage to the casino. You can't combine losses and get a gain, no matter how clever you are. Perpetual motion exists in one sense. If something has no way of losing energy then it can continue in motion indefinitely. The planets are going to continue to circle the Sun for a long time and atoms and molecules are continuously vibrating. Unfortunately, you can't extract useful work from something which is moving without using up its energy. However, ignorant people are still trying to build machines which generate energy. An early favorite was the overbalanced wheel. Rather than say "it's impossible" it is interesting to work out why no machine like this will generate energy. The secret is to ignore the sideways motion of the parts. Apart from friction, moving horizontally neither generates or uses energy. You have to look at the vertical motions. Take any part of the machine. After one revolution it will be back where it started, let's say at the bottom of the wheel. In the course of one revolution the weight is raised to a certain height, work has to be done on it to raise it against the Earth's gravity. On the way down again gravity does work on the weight. The work which went into raising the weight is exactly equal to the work gained when lowering it, less any frictional losses. At the end of one revolution work has gone into the weight and the same work has come back out. No energy has been gained. The same argument applies to every part of the wheel. In the course of a revolution energy has gone in and energy has come out but the net gain has been zero. The sum of many zeros is still zero and there's no useful output, indeed there is a loss since some energy has gone into heating up the bearings of the wheel. This argument is sometimes put in the form that the gravitational field is conservative, that is, any work which is done against gravity can be completely recovered. The same rules apply to magnetic fields. Any push or pull a machine gets from a magnetic field delivers energy but it takes just as much energy to put things back where they started unless you use electrical power, an external energy input, to turn the magnets on and off. Another popular idea is the so-called Zero Point Energy. This term actually refers to two different things. The conventional definition is that it is the energy which would remain if you could cool something to zero degrees absolute. For example, it is because of zero point energy that liquid helium remains a liquid. The term is also used to mean the postulated energy density of a vacuum. Some people think that there is a tremendous amount of energy which exists throughout space and that somehow one can tap off a little bit of it and use it. This is used to "explain" how some devices with no obvious input derive their output power. There are two problems with this idea, one is that if space really contained all that energy then its gravitational effect would make the universe about ten inches in diameter. The other is that a uniform energy density cannot supply energy any more than a completely flat lake can drive a waterwheel. This brings me to the laws of thermodynamics. Some inventors would take that flat lake and say that it contains a huge amount of heat energy and therefore it must be possible to tap some of it. Unfortunately, a large body of water tends to be in thermal equilibrium, that is, it is all at the same temperature. You can only get useful work from heat when a temperature difference exists. Just for the record, the First Law of Thermodynamics says that when work is done on a system the temperature change is independent of how the work is done. That is, the change in thermal energy equals the input of mechanical energy, no matter how the mechanical energy is applied. Heat out equals energy in. There's no trick method to be discovered. Unfortunately, the reverse does not apply. Yes, if you take work from something it gets colder, but this does not imply that things will spontaneously become colder while generating energy. You must have a place to which to transfer heat energy, and it must be colder than the starting point. This is a crude statement of the Second Law of Thermo- dynamics which was recently described in the Mensa Bulletin as the most depressing discovery in science. It can be phrased in several ways. The official way is that no engine can be built which has no effect other than to generate energy and make something cooler. I like to restate it as, you can't extract energy from a temperature difference if there is none. To generate useful power you need to have both a source of heat energy and a lower temperature sink to transfer the energy to. The energy entering your engine is proportional to the absolute temperature of the source. The wasted energy leaving it is proportional to the absolute temperature of the sink. The difference between the two is the energy which can be converted into useful work. That means that the hotter your source and the colder the sink, the more of the energy flowing through your system can be tapped off as useful output. It's the same as money, the higher your income and the lower your fixed expenses, the more money you have available to do what you like with. If your expenses equal your income, you have no money left over to spend. An expression you may come across is thermodynamic efficiency. This is just the ratio between the output mechanical energy and the input thermal energy. Since you are always losing heat energy to the sink, the thermodynamic efficiency of a machine is always less than 100%. The efficiency of a generating station, for example, is in the 35 to 40% region since it is working between the temperature of burning oil and the air temperature in its cooling towers. A very bright Frenchman called Sadi Carnot published his only scientific paper in 1824. In it he explained how this effect limited the efficiency of steam engines. He introduced the concept of the ideal heat engine, one which converted every bit of the available heat energy into mechanical energy. He even described how, in principle, one could build such an engine by heating and cooling gas in a cylinder. No other engine, even in theory can be more efficient than this so-called Carnot Cycle engine. It converts all the available heat and you can't do better than "all." No practical engine runs on the Carnot Cycle so no real engine is quite as efficient as is theoretically possible. Many free energy proponents will say that the equations for thermodynamic efficiency don't apply to their engine since it doesn't run on a Carnot Cycle. That's just another way of saying that it is not as efficient as it could be, not that it generates more output than is theoretically possible. One free energy machines promotion is based on the idea that a heat pump puts out six times as much energy as it takes to run it. This is true in a sense but only because five-sixths of the heat comes from the outside air. As the spiel goes, if you could be only 33% efficient in converting that heat back to energy, you would get out two units of electricity for every one you put in. One unit would run the system, the other is yours to run your house or to sell back to the electric company. This is a classical case of comparing apples and oranges. Under some circumstances a heat engine can be 33% efficient. Under some circumstances a heat pump can have a coefficient of performance of six. Unfortunately, these are not the same circumstances. The heat pump's performance is best when the difference between its output temperature and the temperature of the outside air is small. A heat engine's efficiency is best when the temperature difference between its input and output is high. If the temperature difference is the same for both, the gain and loss balance exactly. You never come out ahead. You can do worse than break even but you can never do better, no matter how ingenious you are. It can be difficult to explain to people why these schemes won't work, particularly if they have invested their life's savings in them. You could just say, "Trust me, physics says it can't happen," but that's a little unconvincing. It is usually possible to work out where the flaw lies, provided the inventor is not so secretive that you can't get the necessary information. Unfortunately, even if you can come up with an explanation, it may be over the heads of the very people who are most susceptible to such a scam. They fell for it in the first place because they didn't know enough science to see that it wouldn't work. A little education in the sciences pays off in the long run. Another problem is that, even if you debunk one claim, others pop up. People can't accept the general principle that none of these machines will work. They are so sure that there is a free lunch out there somewhere if they just look hard enough. Much of what I have said here about the laws of physics and what is impossible can be found in greater detail in two books by PhACT member and one-time physics professor, Milton Rothman. These are "A Physicist's Guide to Skepticism" and "The Science Gap." Both are available from Prometheus Books, 700 East Amherst St., Buffalo, NY 14215.