How is it possible to know a proposed perpetual motion machine won't work without even looking at it?

Question

It is often said that perpetual motion is impossible. Many people try to invent perpetual motion machines. Apparently every such attempt is doomed to failure. People don't even need to look at one to know this. What is it about perpetual motion that makes it impossible?

Answer 1

There are two main types of perpetual motion machines:

Perpetual motion machines of the first kind: These machines would create energy out of nothing. This is impossible because it violates the law of conservation of energy, which says that energy can't be created or destroyed, only changed from one form to another.

Perpetual motion machines of the second kind: These machines would convert all heat into work, without any waste. This is also impossible because it violates the second law of thermodynamics. This law says that heat always flows from a hotter object to a colder object, and you can't convert all that heat into useful work.

Another answer explains why perpetual motion machines of the first kind are not possible. Regarding the impossibility of perpetual motion machines of the second kind, it has not been definitively proven that the second law of thermodynamics cannot be violated. However, it is one of the most well-tested and fundamental laws of physics. The second law is deeply rooted in statistical mechanics, a branch of physics that deals with the behavior of systems with many particles. It is a consequence of the statistical nature of these systems.

But there is no definite widely accepted proof of their impossibility.

Answer 2

Practically, it's not a good use of time for a physicist to examine every purported perpetual motion machine.

Most perpetual motion machines are developed by people with basic misconceptions about physics or con artists. The machines are either frauds or are obtaining their energy from some source.

Conservation of Energy and the Second Law of Thermodynamics have been explicitly and implicitly tested in thousands of experiments. A large part of our current understanding of physics is built on top of these laws, and most of our predictions would not work if these laws were not nearly always followed. They appear to be obeyed for a wide range of systems, from human-sized machines, to galaxies, to atomic nuclei. If there are some cases where they violated, it would have to be somehow exceptional or somehow hidden so that it wasn't noticed before, despite centuries of experimentation.

It is very unlikely that some new, unexpected violation will appear for some magnets taped to a wheel. However, violations of these basic laws are sometimes considered to explain new phenomena.

Scientists are naturally going to be skeptical of claims of violations of these laws, and it is just not worth it to investigate every one. If you think you have a system that violates these laws, it would be very extraordinary and you'd need to have a mountain of evidence showing that the energy wasn't coming from somewhere to get the attention of a working physicist. Extraordinary claims require extraordinary evidence.

Answer 3

A perpetual motion machine at worst maintains the exact amount of energy that it begins with, otherwise it would lose energy and stop moving, hence not perpetual motion.

A device which maintains its exact original amount of energy is not, in theory, forbidden by the laws of classical Hamiltonian mechanics. Indeed, an ideal pendulum will swing forever. However, in practice, any system is always coupled to the "environment" somehow. This coupling often comes in the following forms:

1. surface contact friction (this limits, for example, mechanical watches)
2. Air resistance
3. Ohmic resistance
4. Radiative losses
5. Thermal losses (conductive, convective, radiative)

Any proposed perpetual motion machine will have some sort of environmental coupling like the above examples which will cause energy loss over time, thus spoiling the perpetual motion. A device which "looks like" a perpetual motion machine likely has some hidden energy source that is pumping energy into the system at a similar rate that energy is being lost due to the environmental couplings.

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There are some proposals for perpetual motions machines which actually generate more energy than the energy that is put in. These machines ARE forbidden by the laws of physics which state that energy must be conserved.

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So for the former case I can tell you that there can be no such perpetual motion machine because there is ALWAYS some external environmental coupling that damps energy from the system.

For the latter case I can tell you there can be no such perpetuatl motion machine because it would break conservation of energy.

In neither case do I need to see the details of the proposal to tell you that perpetual motion is impossible. In some cases it may be tricky to identify the energy loss mechanism, or a secret energy pumping mechanism, and this can make studying perpetual motion machine proposals interesting despite the fact that we know before starting that the result is impossible.

Answer 4

A perpetual motion machine is something that does some motion over and over, and never stops. They are divided into two kinds, both impossible.

With each repetition, a perpetual motion machines of the first kind has more energy that before. Usually the idea is to remove that energy and do something useful with it.

The immediate response from most physicists is that this won't work because it violates the law of conservation of energy. This is the idea that no matter what you do, you have the same amount of energy before and after.

A generator might work by burning fuel. This turns chemical energy in the fuel into electrical energy. The generator makes the same rotation over and over, producing energy with each turn. But this doesn't count as perpetual motion because the generator doesn't return to the same state after each turn. The fuel level is a little lower. Eventually the machine will stop because it runs out of fuel. A generator transforms energy, but does not produce it from nothing.

To get the flavor of the kind of mistake that is made in proposed perpetual motion machines and why they are dismissed out of hand, consider this example.

Suppose If you drive from a high altitude point, A, to a low altitude point, B. Altitude comes with potential energy. You can coast from A to B without using fuel. You will speed up on downhill stretches and slow down on uphill stretches. But if you lost altitude overall, you have gained more speed than you lost. You have converted potential energy into kinetic energy.

To turn this into a perpetual motion machine, all you have to do is find a route that goes from A back to A while losing altitude. Somebody spends time with a map, and comes up with a proposed route.

A physicist says it doesn't matter how clever or intricate the route was. If you go from A to A, the altitude is the same when you left and when you arrived. The potential energy is the same. You cannot have gained kinetic energy. He does not need to look at the route to see this. He only needs to look at the start and end altitude. All loop trips come back to the same altitude. So no loop trip is a perpetual motion machine.

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A perpetual motion machine of the second kind doesn't gain energy with each repetition. But it never loses energy either.

This is impossible because you can't make anything with moving parts perfect. This isn't at all obvious and isn't a very satisfying reason. It isn't as obviously true as the impossibility of getting something from nothing. It is tantalizing because you can almost make one by making something that is almost perfect. It leaves the feeling that if it falls short, maybe you can just improve the shortcomings.

The route from A back to A is an example of this kind of perpetual motion machine. You come back to the same height as you left. This means you have the same potential energy as when you left. At first glance, it seems like this is a perpetual motion machine. But how would you make it?

A car has moving parts, and moving parts have friction. A more careful look at the energy shows that you convert potential energy to kinetic energy as you coast downhill faster and faster. And friction converts kinetic energy to heat. This slows you down. You don't have enough kinetic energy left to coast all the way back up to the original altitude.

So you set about eliminating all sources of friction. Instead of wheels, you magnetically levitate the car. You run it in a vacuum to eliminate wind resistance. But there is always something you cannot eliminate perfectly.

To be a perpetual motion machine, there must be parts that move with respect to each other. The parts must exert forces on each other. Parts are made of atoms. This means atoms in one parts exerts forces on atoms in the other parts. Parts have thermal energy, which means the atoms are always jiggling randomly.

If one part rubs on another, friction converts kinetic energy into heat. Atoms in one part bump into atoms in the other and make them jiggle harder.

A principal of thermodynamics says that once energy is converted to heat, you would have to use energy to convert it back. The total jiggling always goes up in this process. This is why you have to be perfect. You cannot allow any energy to be converted to heat because a perpetual motion machine cannot spend kinetic or potential energy to convert it back.

Answer 5

There are already a lot of good answers, but I'll add one slightly different perspective. Since you can't actually build a perpetual motion machine, when people claim that one such machine works, they are always making theoretical arguments. Those arguments usually invoke basic results in physics, such as the forces between magnets, the buoyant force, torque balance, and so on.

However, those results in turn are derived from the basic laws of physics, and from those laws, you can also derive that energy is conserved! So the argument saws off the branch it's standing on.

If you think about it, there are only four logical possibilities:

1. The known laws of physics contain an internal logical contradiction. It would seem very unlikely for nobody to have noticed this, especially because these machines are often made of very common parts, like magnets and ropes.
2. The machine exploits a violation of the known laws of physics. We can usually ignore this possibility because perpetual motion machine designers assume the laws of physics work in order to argue their machine works in the first place. Sometimes machines do violate the known laws of physics, but in that case they're not machines, they're physics experiments discovering new effects. Generally, such experiments are rare and difficult, and unlikely to be built in some retired engineer's garage.
3. The machine works, but only by using some other source of energy which its designer didn't account for. A lot of "working" perpetual motion machines actually just inefficiently harvest solar energy, or the Earth's rotational or magnetic field energy, or something else like that. These aren't interesting because they're usually extremely uneconomical compared to other energy generation mechanisms, like solar power or nuclear power.
4. The machine doesn't work.

Physicists dismiss perpetual motion machines because the first two options are exceedingly unlikely, and the last two are boring.

Another important factor is that it tends to be exhausting to debate the designers of such machines. It's usually pretty obvious, even to people without physics training, that a simple machine won't work. Most people just give up there, and the ones that don't are those who psychologically need their idea to work. They simply tack on more and more complications, until they can no longer tell at a glance that it doesn't work, and then triumphantly claim that it therefore does work. Not only does this complication make it hard to analyze their machine, but once you disagree, they simply add another complication and declare that the new machine must work. The process repeats forever, because these people are only interested in ways they can be right, rather than considering the possibility that they're wrong.

Answer 6

This is a form of inductive reasoning, which is generally how (experimental) science operates. Perpetual motion machines - which should better be called "perpetual work machines", IMO - are often said to be "forbidden" by the first and/or second laws of thermodynamics, viz. conservation of energy and the non-decrease of entropy in a closed system. However, we don't actually know if some cosmic agent like a deity is actually responsible or not for "forbidding" or enforcing these "laws" at least scientifically; instead what science does is it shows us that, again and again, we find, i.e. observe, that any attempt to go against the laws constantly fails to do so. The huge list of failed perpetual motion machines is, itself, a substantial body of evidence in favor of those laws' validity. That, in turn, is joined by the much larger body of observations of physical phenomena not directly or intentionally aimed at creating a PMM but nonetheless seen to conform to theory that can be mathematically shown to obey those two laws. And - and this may be last but is far from least - each time we discover something new, we find that it still obeys those laws!

What this means is that, given we have seen over and over that the first and second laws of thermodynamics are upheld each and every time, we can expect that it is likely they will continue to hold up forever. This is not a proof, though - it is experience. And that is the fundamental basis of science: experience, handled in a very careful and controlled fashion to ensure we faithfully record what actually happens and do not allow our recordings to become biased by our various psychological habits (e.g. dropping and/or adding spurious, data points).

So then from that huge body of aggregate experience, we likewise presume the next PMM thrown our way is just not gonna work, as that's what it means for those two laws to keep holding in that case. It's the same as when you know that the Sun rises every day, you expect it will keep rising; and that when you know that you go out each day you won't suddenly run into an invisible wall in front of your door. It's just something that continued experience keeps training us not to expect deviation from.

Answer 7

All perpetual machine projects that was checked back then and now - violate fundamental laws of thermodynamics, which has been established in Physics more than $150$ years ago and confirmed by countless number of experiments. Hence it's $99.99\%$ likely that next "big thing" in perpetual machines invention will have same thermodynamic cracks as their predecessors. And to be honest,- Physicists are tired to do a Sisyphean work of finding loopholes over and over again.

Answer 8

Many factors make these types of machines impossible because these machines break some of the laws of physics, especially thermodynamics. Let me show you:-

1. First Law of Thermodynamics:- this law states that energy can never be created nor be destroyed since Perpetual machines have to generate power on their own from nothing to run for unlimited time. This means the machine should not lose any energy in terms of friction, heat, etc.

2. Second Law of Thermodynamics:- The second law states that in a closed system, entropy tends to increase over time. Essentially, energy naturally spreads out and becomes less useful for doing work. In a perpetual motion machine, maintaining continuous motion would require a constant decrease in entropy, which contradicts this law.

Perpetual motion machines are impossible because they would require breaking the fundamental rules that govern energy conservation and the natural tendency of energy to disperse.

Thank You, Have a nice day

Answer 9

I vehemently disagree with mmesser314's portrayal that there is only the CoE violation. A perpetual motion machine's design can easily conform to CoE but violate 2LoT, and so I agree with Pato Galmarini's answer. However, the core of my answer below is not that we prove this or that. It is instead about understanding and organising principles.

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I think it is about the amount of effort needed to write a rebuttal, and that it is somewhat necessary.

By that, I mean that

1. Conservation of Energy (CoE), and
2. 2nd Law of Thermodynamics (2LoT)

are the fundamental building blocks for a modern physics understanding of our universe. Whereas we can easily point out when a certain scheme violates the CoE, it is a totally different situation when something obeys CoE but violates 2LoT.

After all, we do not teach entropy at the pre-university level, and even if we do upgrade to a level of civilisation whereby we teach entropy at the pre-university level, it takes 3 whole months of full-time education in statistical thermodynamics to finally begin to understand why it is that 2LoT is a fundamental organising principle for understanding how our universe is the way it is.

We cannot be expected to provide a long-winded university education for free, as much as we actually do wish or want to. It takes too long to write an answer that would satisfy the questioners in a way that they would not get to disagree with.

So, we have no choice but to close questions that violate 2LoT, as perpetual motion machine questions are.

Answer 10

The principle of perpetual motion is impossible under the laws of thermodynamics which Einstein said are the only rules of physics which would never be overthrown. The possibility of perpetual motion rests on the assumption that a system can have an efficiency of 1, otherwise the losses will slow it. The laws of thermodynamics state that this is impossible as a system always loses energy. Your point about looking at it struck me as rather funny, as the act of observing a system means it is losing energy, sound, light etc. Possibly the universe could be considered the only feasable perpetual motion machine as it has nothing to lose energy to.

Answer 11

If spinning forever could be classed perpetual motion then place a spinning object in deep space. It would reach a thermal equilibrium probably close to absolute zero. Also it would only vanishingly rarely receive any collisions with stray particles. Any random collisions that did occur would probably balance out over time so it would continue to spin. However in the infinite reaches of time the chances of random collisions totally cancelling out would also be zero, so it it could at some time slow and possibly stop spinning. Entropy dooms the universe and all of us in it. Sleep tight.

Answer 12

In light of what's been said and to better understand the issue, I'm mentioning some paths in the opposite direction, i.e. paths that suggest do not a priori ruling out perpetual motion machines, as I think it's missing in the threads so far. This allows to assess the plausibility of these paths, and thus increase or decrease our confidence in their impossibility. The key lies in the two laws of thermodynamics, so I'm specifically focusing on them (on the 2nd mostly).

1) 1st law

Energy is not conserved in our universe in accelerated expansion. This has been explained here in other threads, and in press. Would that suffice to a machine of the 1st kind? I don't know. The violation of any of the two laws are necessary conditions to get a perpetual motion machine, but not sufficient.

2) 2nd law

The universal validity of the 2nd law is harder to challenge but several paths suggest that it cannot be a priori discarded:

2A) Literature challenging the second law in exotic scenarios:

1. There is research of cosmological scenarios in which the second law, or its generalized version, is violated, e.g. in the following scenarios: bouncing universes, f(T) gravity*, repulsive dark energy during inflation, phantom dark energy. And... dark matter haloes? Below some references:

   • Ferreira, P.C., Pavón, D. Thermodynamics of nonsingular bouncing universes. Eur. Phys. J. C 76, 37 (2016). https://doi.org/10.1140/epjc/s10052-016-3886-2 Abstract: Homogeneous and isotropic, nonsingular, bouncing world models are designed to evade the initial singularity at the beginning of the cosmic expansion. Here, we study the thermodynamics of the subset of these models governed by general relativity. Considering the entropy of matter and radiation and considering the entropy of the apparent horizon to be proportional to its area, we argue that these models do not respect the generalized second law of thermodynamics, also away from the bounce.

   • Generalized second law of thermodynamics in f(T) gravity* Journal of Cosmology and Astroparticle Physics. K. Karami, A. Abdolmaleki 2012/04/10 7, 4, 1475-7516 https://dx.doi.org/10.1088/1475-7516/2012/04/007 Abstract: We investigate the validity of the generalized second law (GSL) of gravitational thermodynamics in the framework of f(T) modified teleparallel gravity. We consider a spatially flat FRW universe containing only the pressureless matter. The boundary of the universe is assumed to be enclosed by the Hubble horizon. For two viable f(T) models containing f(T) = T+μ1{(−T)}n and f(T) = T−μ2T(1−eβT0/T), we first calculate the effective equation of state and deceleration parameters. Then, {we investigate the null and strong energy conditions and conclude that a sudden future singularity appears in both models. Furthermore, using a cosmographic analysis we check the viability of two models. Finally, we examine the validity of the GSL and find that for both models it} is satisfied from the early times to the present epoch. But in the future, the GSL is violated for the special ranges of the torsion scalar T.

   • Grøn, Ø. (2012). Entropy and Gravity. Entropy, 14(12), 2456-2477. https://doi.org/10.3390/e14122456

   • Germán Izquierdo, Diego Pavón, The generalized second law in phantom dominated universes in the presence of black holes, Physics Letters B, Vol 639, 1, 2006 https://doi.org/10.1016/j.physletb.2006.05.082. Abstract: This Letter considers the generalized second law of gravitational thermodynamics in two scenarios featuring a phantom dominated expansion plus a black hole. The law is violated in both scenarios.

   • Finally, in this forum in other threads there's been the open debate on whether dark matter could represent a case of entropy decrease: How does dark matter collapse?: Entropy considerations

2. A comprehensive survey of several challenges is this book:

• Čápek, V., & Sheehan, D. P. (2005). Challenges to the second law of thermodynamics: Theory and experiment (Fundamental Theories of Physics, Vol. 146). Springer. https://doi.org/10.1007/b105046

3. Certain cosmological models, such as T. Gold's symmetric universe, or R. Penrose's Conformal Cyclic Cosmology, consider moments of entropy decrease. Penrose talks of a "reset" at each epoch, due to a drastic reduction of the available phases space; I at least call this a 'decrease' (although I've seen reluctance to put it in these terms). Again, even if these models are just hypothetical models but allowed by the laws of physics, then this would attenuate the status of the 2nd law.

2B) the physical possibility of a Maxwell demon.

The most detailed analysis for the physical possibility of a Maxwell demon is the book and several papers of the philosophers Hemmo and Shenker. They insist on the conceptual relevance of such thought experiment, basically to not a priori rule out perpetual motion machines.

• Shenker, Orly & Hemmo, Meir (2012). The Road to Maxwell’s Demon. Cambridge University Press.

For other philosophical analysis skeptic of the universal validity of the second law, see also:

• Robertson, Katie (2019). Stars and steam engines: To what extent do thermodynamics and statistical mechanics apply to self-gravitating systems? Synthese 196 (5):1783-1808.

• Callender, Craig (2011). Hot and Heavy Matters in the Foundations of Statistical Mechanics. Foundations of Physics 41 (6):960-981.

2C) Poincaré's recurrence theorem

This result states that a certain physical system will, eventually, return to its initial state. It might never happen in our actual universe, but perhaps it suffices that it's physically possible, in order to attenuate the status of the 2nd law.

Answer 13

This is a fascinatingly difficult question, which is why it is so common to have the perpetual motion dismissed out of hat. I think it's best to start with the heretical admission: one cannot know a perpetual motion machine can't work. That level of certainty is, in my opinion, hubris. The universe is bigger than us

So I need a different language to capture rational belief. The Baysean interpretation of probability is a good one. It's is described as a betting game. If you are 80% confident of something, you should be willing to bet 4:1 on the truth of that something.

In this sense, I redefine the idea of "knowing perpetual motion is impossible" as you would be willing to bet some pretty extreme odds (like putting up a \$1 million to win \$1). This definition is useful because it easily adapts to the amount of effort it takes to entertain a question.

It also gives some credence to the handling of impossible things, like the EM drive. The EM drive defies the conservation of momentum. Most scientists would bet against it, but it has earned a sort of mild neglect, even permitting some attempts and on orbit testing.

The next fine detail is that you call it a "proposed" perpetual motion machine. It's not a machine, it's a set of words describing the machine. And this is where the idea of "immediately" dismissing a perpetual motion machine starts to come into play. The laws of thermodynamics forbid perpetual motion machines, full stop. It can't be done. So any perpetual motion machine must contain something that is not part of the world of thermodynamics. If a machine offers nothing by electronics, magnetism, and other commonplace concepts, all of those are well understood to obey the laws of thermodynamics.

So for a perpetual motion machine to avoid immediate dismissal, there must be some argument for why thermodynamics does not apply

Many get dismissed right here. They just don't have any reason to believe it. If all you have is a gas engine, electrolysis, and a battery, you haven't left the well trod roads of thermodynamics.

Others claim gestalt effects. They took get dismissed, but for different reasons. If your argument is "all of these parts are thermodynamic, but put them together and they aren't," that's gestalt thinking. And you know what? I can't prove they're wrong. However, modern science is founded on a theory that all emergent behaviors are explained by their components. So maybe the machine does work. But it doesn't do so by science. It does so by some other force of nature.

The final common dismissal I know of is one of incomprehensible theories. Vortex Math and the Rodin coil fits into this. They have a theory for why their device produces infinite energy, but have not been able to put it in a language that lets other scientists work with the theory. Maybe they work, but it's hard to believe the explanation.