How is perpetual motion impossible?

Question

I often read and hear from physicists that perpetual motion isn't possible due to the second law of thermodynamics. However, this doesn't make sense to me. The law doesn't say that the entropy of an isolated system is constantly increasing. Rather, it only asserts that entropy is non-decreasing.

So why can't we have perpetual motion, which doesn't increase entropy?

Note that my question is different from the one here because it's asking why a machine generating perpetual energy is impossible, but I'm just asking about something which is doing the exact same motion for the rest of eternity.

Answer 1

To answer this question you need to define what is meant by "perpetual motion" carefully.

In the broadest term, perpetual motion simply means that something stays in motion forever. This would indeed be possible under the current laws of physics, e.g. we would expect the Earth to continue revolving around the Sun "forever" (inverted commas because there are minor effects like the Sun's continued evolution changing its radius). In the same way, we would expect the Sun to go round the center of the Milky Way "forever".

What is impossible are perpetual motion machines that claim to do something in addition to something staying in motion forever. To quote:

A perpetual motion machine of the first kind produces work without the input of energy. It thus violates the law of conservation of energy.

A perpetual motion machine of the second kind is a machine that spontaneously converts thermal energy into mechanical work. When the thermal energy is equivalent to the work done, this does not violate the law of conservation of energy. However, it does violate the more subtle second law of thermodynamics in a cyclic process (see also entropy). The signature of a perpetual motion machine of the second kind is that there is only one heat reservoir involved, which is being spontaneously cooled without involving a transfer of heat to a cooler reservoir. This conversion of heat into useful work, without any side effect, is impossible, according to the second law of thermodynamics.

(We ignore the perpetual motion machine of the third kind because that is in theory possible, albeit only in theory.)

You are asking about a perpetual motion of the second kind, in particular, a "machine" that doesn't increase entropy (and doesn't decrease it either). As you can see from the quote above, you can certainly have such a machine,* it's just that you can't extract useful work out of it.

*An example of such a "machine" is "no machine". If you have no machine (and don't touch the system in any way) then you don't affect its entropy at all. Of course, such a "machine" doesn't actually do anything, which is the point; the laws of physics say you can't get it to do something useful.

Answer 2

The correct answer is both YES and also NO.

If you live in a perfect world, alongside spherical chickens and all that, then any device that has motion, and is perfectly frictionless, will indeed run forever.

It cannot generate any useful work/energy in the process, (which is usually assumed whenever the topic of perpetual motion is raised), as it is no more than a novelty otherwise.

In the real world, nothing known, even involving superconductivity, or deep space, is infinitely frictionless, so nothing can run infinitely.

Answer 3

Such an ideal engine exists in nature, but only in certain conditions. It is about something very tinny, like a photon of light, for example. As long as, nothing is changing it's motion state, it's still moving. And for its' speed is constant, there is no change in its own entropy.

I can see this, so called "forever movement", only in the following way: It's an electro-magnetic field, that is propagating in two different ways. Once is propagating as a wave, according to the Huygens Ondullatory Theory. Second one, it is also moving as a material body, which can have a mass.

And how is this body, that could also generate an electro-magnetic field? There must be a source of this field, like should be an electric dipole. More precisely said, only a rotating dipole, can generate such a field. Let's imagine it is more like a helicoidal, or spiral spin around an axis. And the axis is quite on the direction of propagation, this is well known.

Having two electric charges, this is not against the electric neutrality. They are moving in same direction and produce 2 opposite electric streams. But the magnetic field will also have 2 opposite components as a result. It is like a little coil, which is attracting the magnetic core inside. Here, we don't have a magnetic core, the core is the environment itself. That is, even the most advanced vide, it has electro-magnetic properties.

It would be a repulsion force, that is produced by the magnetic fields. For the electric charges cannot be separated, it is just 1 possibility. The only liberty degree, can be only to spin around the propagation axis. And this repulsion, can only be annulated, by a spatial rotation, or spin. What I had described here, it may seem like a kind of microscopic engine.

It remains just to figure out, how this movement can become a translation. At some point, an electromagnetic field can have a longitudinal component. But this is not allowed, by the System of differential equations for the Electro-magnetic Field. And any variation would be annulated by the Induction Law of Faraday. This is also consequence of the Preservation Law of electric charges.