Why do physicists say a proposed perpetual motion machine won't work without even looking at it?

Question

Every so often somebody posts a question about a new perpetual motion machine they have invented. They have spent time and effort on it. They have hopes that they have discovered something new and important. Yet the question is closed without even looking at the proposal. Why is this reasonable?

The purpose of this question is something we can link to minimize hurt feelings and avoid the impression that physicists are being narrow minded when such questions are closed. Intuitive answers for non-physicists would be best.

---

This question is not working out quite the way I had hoped. Most of the answers are how we should handle perpetual motion questions.

To be clear, I expect most such questions will be quickly closed without looking at details. This type of question comes up enough that I thought it would be useful to have a link we could paste in a comment to tell the OP why.

Many such posters are too convinced of their new idea to hear what we tell them. But some are open to it. Those would benefit from not being given the impression that we just blew them off. A few of the answers are good for this purpose.

Answer 1

Every so often somebody posts a question about a new perpetual motion machine they have invented. They have spent time and effort on it. They have hopes that they have discovered something new and important. Yet the question is closed without even looking at the proposal. Why is this reasonable?

First, the amount of time someone spent on anything is completely irrelevant to whether the question is on topic or not. Consider two extremes:

• someone who spends a year writing a "physics for dummies" book and posts a PDF here for us to review
• someone who takes a picture of their homework from their book and posts it for us to solve

In both cases, the post is not on topic for long-standing reasons (not a suitable replacement for peer review & homework respectively), even if you disagree with either reason.

Second, as of this post, we have 276 questions tagged as perpetual motion in total. Of these, 98 have been closed for any reason of which 45 were closed as duplicates, leaving 53 of the 276 (~20%) as closed for other reasons (probably non-mainstream or engineering?).
So given that the majority of the questions are not closed, it seems to me that at least in practice such questions are on topic, contrary to the opinion posed by OP.

This would also be in line with previous discussions on the matter: How do we deal with perpetual motion and other hoax questions? (from 2010)

Answer 2

To add to the noise, I agree with the goal of this post, but disagree with the reasoning presented.

They have spent time and effort on it. They have hopes that they have discovered something new and important. Yet the question is closed without even looking at the proposal.

Time and effort are irrelevant to PSE posts. People coming to the site don't get to see how long a certain post took to make, and there is no site policy on posts being better or worse for the site based on time and effort. Certainly, putting less time and effort into a post makes it more likely to be closed, but site policy shouldn't be relaxed because a post seems to have more effort put into it.

Additionally, there is no way to know if those who voted to close voted without even reading the question. It could be likely; answers here admit to closing based on topic (something that essentially should never be done IMO, for posts that are within the real of physics to some degree), but to generalize this seems unwarranted without the ability to actually know how much a user read before voting to close.

The purpose of this question is something we can link to minimize hurt feelings and avoid the impression that physicists are being narrow minded when such questions are closed.

Question closure is how the site works. It's not an attack on users. I agree that more experienced users should be careful about closing questions and be helpful and kind when they do so, but I am tired of seeing language like this warning people way from closing/down voting questions because it is unwelcoming, could hurt feelings, etc. If we should be wary of this, where is the line drawn then? Any question closure has the potential to hurt feelings, "waste" effort, etc. How are we to decide when it's ok to hurt feelings, and when it is not? Should we ask the OP how they would feel about their question being closed first before we close it?

---

Instead

Questions should not be closed based on their topic, but rather, based on there adherence to site policies. For example, it is not against site policy to ask for help on a homework problem, but it is against site policy to just post the question and ask for an answer, or post the question with your work for someone to check. It is fine to post a homework question as motivation for a deeper conceptual issue though. Homework questions shouldn't be closed for being homework questions, even if there is a good tendency for homework questions to go against site policy in some capacity.

The same thing is true here. Questions should not be closed just because they ask about perpetual motion machines. There is no policy saying such questions can never be asked. There can be good posts about perpetual motion, and there can also be bad ones. The topic of perpetual motion can attract bad posts, for sure, but each post should be judged on what it actually entails rather than its topic.

In summary, don't close a question just because it is a perpetual motion question, but also don't try to leave it open because it is a perpetual motion question that someone might have put effort into and be hurt / scared away. Just read the post. If it falls into a closure reason, vote to close it. If not, then leave it open. This is true of any post; the topic (as long as it is related to physics, generally speaking) doesn't matter.

Answer 3

Perpetual motion proposals always involve a misunderstanding of some physical principle. As I understand it, clearing up such misunderstandings is the entire point of a physics Q&A site. Just as any question may be ill-posed or ill-researched, so may a perpetual motion proposal. But they may also involve surprising and illuminating subtleties.

For an answer to simply say "that won't work; it violates thermodynamics" would be a disservice to the teachable moment. That could be fine first sentence, though.

Answer 4

I have nothing against perpetuum mobile problems, when phrased properly they are very educational. One of my favorites was given by the late Professor Paul Penfield, probably not well-known to physicists; see, https://ieeexplore.ieee.org/abstract/document/1447009 and the ensuing debate how to resolve it starting with https://doi.org/10.1109/PROC.1967.5560 and followed by https://doi.org/10.1109/PROC.1968.6798, etc.

Answer 5

It's always good to be polite, and I'm sure most of us don't delight in hurting people's feelings. But at the end of the day a question about a perpetual motion device is not a question about physics and has no place on this site. I vote to close such questions when I see them, and while I won't post any unkind comments I also see no need to post a detailed analysis explaining why it won't work.

I understand that the motives of the OP are (generally) sincere, but this site is simply not the place to discuss such questions.

Answer 6

I think the question is more appropriate for meta. A short answer is: a perpetual motion would violate energy conservation and/or the laws of thermodynamics.

Now, we can speak of a caveat that perpetual motion usually implies producing positive work, but literally it means simply moving without stop. In this sense, one could exploit that this by committing a fallacy of definition, and claiming that phenomena like superconductivity or persistent currents constitute a perpetual motion. But this is not what physicists mean by this term.

Answer 7

A perpetual motion machine is something that does some motion over and over. And with each repetition, it 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.

Answer 8

Here is the simplest and most intuitive answer I can think of:

Just as in banking, physics has a short set of very basic rules you must follow so your books balance at the end of the day. One of these rules is you can't withdraw more than your account contains at any moment. Perpetual motion machines are schemes in which someone invents a way of getting rich by withdrawing funds from an empty account. This might occasionally work in banking, but most times you wind up going to jail. In physics, Mother Nature enforces the rules with an iron fist, and she can't be fooled as easily as bank regulators can.

Answer 9

I think it is about the amount of effort, 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.