# Could Brownian Motion Be An Example Of A Perpetual Motion?

I am just wondering, is Brownian motion an example of 'perpetual motion'? This 'perpetual motion hypothesis' is restricted by the law of thermodynamics in physics which simply summarizes as: "There is no such thing as perpetual motion in the whole vast universe". But this jittery action of particles in liquid does so in any given time or, in other words, every time someone tries to observe it. So, could we conclude that brownian motion is nature's physical proof that perpetual motion is possible? Please correct me if I'm wrong I am very much willing to be taught. I'm also afraid I'm jumping into wrong conclusions. Let us discuss.

Note by Les Narvasa
6 years, 10 months ago

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A distinction needs to be made between "perpetual motion", and "perpetual motion machine". The latter produces useful work. When work was first done on classical thermodynamics theory back in the 19th century, it was theorized that perpetual motion machines would be impossible because entropy rises inexorably. But in fact, it's never been rigorously proven that entropy rises inexorably in all cases. Later physicists in the 19th and then 20th century devised thought experiments by which a perpetual motion machine could be devised, such as "Maxwell's Demon" and later refinements such as the "Brownian Rachet" However, no serious physicist disputes the fact that given a volume of gas in adiabatic condition, the Brownian motion of its molecules is perpetual.

We've had self-winding watches for some time now, where the simple motion of the arm wearing one is sufficient to wind it. We have updated versions of this idea where simple temperature changes in the air is sufficient to keep an electronic watch charged. Through nanotechnology, physicists are getting more and more clever at finding ways of wringing useful work "out of the thin air"--but so far, all of them does depend on non-equilibrium conditions. In classical thermodynamic theory, given a steady "hot reservoir", there has to also exist a "cold sink" in order to have a thermodynamic machine that can produce useful work. That is, energy has to be drawn from the hot source, work extracted from it, and waste heat has to be dumped elsewhere that's cooler. If a physicist was able to devise a machine that's "inside" of a hot reservoir in steady condition, and yet draws energy from it to produce useful work without dumping waste heat elsewhere, then, finally, the conditions are met for a true "perpetual motion machine". So far, that's not been done.

- 6 years, 10 months ago

The self winding clocks is dependent on the force exerted from the system it was in, so, obviously, it could not be perpetual, as you pointed. I think, in creating a perpetual motion machine, the problem always arises from the condition that: There would always be a system around the machine, which would 'steal' the energy from it.

- 6 years, 10 months ago

In the 19th century, early physicists didn't even have clear idea about the atomic nature of matter, so it was beyond their thinking to even imagine making machines at molecular size. So, in context of early 19th century thermodynamics, yes, perpetual motion machines were a practical impossibility. But today we're getting into nanotechnology, and so, get your bag of popcorn and watch what could happen next.

- 6 years, 10 months ago

It is not really Perpetual motion because the moving molecules are not doing any work.

Well, I am not sure but have a look at http://en.wikipedia.org/wiki/Brownian_ratchet

- 6 years, 10 months ago

Work is done (at least, for me, I believe it does). And that work is done towards other particles and molecules. Pushing and shoving against one another.

- 6 years, 10 months ago

But momentum is perfectly balanced. How is work done? On whom?

- 6 years, 10 months ago

The speed of Brownian motion fluctuates with temperature...

- 6 years, 10 months ago

could you please elaborate this for me. It does so fluctuate with temperature and temperature is always around so the motion could never halt, in any case.

- 6 years, 10 months ago

In an ideal gas, temperature doesn't fluctuate. But when you start talking about temperature at the molecular scale, then it gets to be a much more complicated subject. Which is precisely why the claim that perpetual motion machines "is impossible" may be a bit premature. In other words, if we're going to develop a true perpetual mation machine, it'll most likely work at the molecular scale. But this is a very controversal subject.

- 6 years, 10 months ago

At quantum scale, I suppose. Where everything is whizzing around and things could never be confined. This subject is quite far from the topic, I know.

- 6 years, 10 months ago

Brownian motion is caused simply by the kinetic energy of the molecules. The molecular kinetic energy is essentially the heat held by the matter. As long as the temperature of matter is above absolute zero, molecules will possess kinetic energy and continue to move randomly. So it is not perpetual. The matter gets heat energy from the environment, and it shows up as Brownian motion. One might ask, if the matter is held at a constant temperature, why must molecules move randomly about? Shouldn't they be ordered? Well, according to laws of statistical thermodynamics, there is a distribution of temperature for different molecules. Temperature of bulk matter represents sort of an average temperature of the individual molecules, which possess different kinetic energies from each other.

- 6 years, 10 months ago

So, it means to say that, if the system which exhibits brownian motion is placed in a temperature in obsolute zero or below, or perhaps, in a vacuum, the motion will stop? Am I right?

- 6 years, 9 months ago

I was thinking that the elements of nature..also like the expanding of universe,motion of stars and planets and galaxies itself are all perpetual..right?

- 6 years, 9 months ago

No one can say. At least, just yet. We are still aren't sure if the cosmos are headed to a new big bang (a theory of cyclic perpetual universe) or a big crunch (a theory that the universe will collapse on itself because of gravitational pull and utterly vanish) but majority of the physicists is more of the latter.

- 6 years, 9 months ago

The system of molecules is constantly taking thermal energy form the system..then it has to depend on sorrounding energy..then how could it be perpetual motion machine..without thermal energy they can't show brownian motion..

- 6 years, 9 months ago

I recommend that you read Jean Perrin's 1909 work, "Brownian Movement and Molecular Reality", and form your own understanding from which to make your own conclusions. It is The definitive work about the theory and its physical manifestations, and it is for this document alone that he was awarded the 1926 Nobel Prize in physics.
He mentions this issue of "perpetual motion", and provides citations to the origins of the various ideas concerning it. It's also a great read, extremely clear and well-written, as well as eye-opening (which was its entire purpose -- to clarify the theory to such an extent that it would be impossible for anyone to refute it, or the existence of molecules). It's also already been translated into English.

Since then, Brownian Motion and its underlying theory have been written in textbooks and research articles across many scientific and mathematical disciplines, yet many of the authors of these documents seem to have failed to even read this critical document (Perrin), as you'll find many, many distorted statements and explanations in these "resources" that are completely contradictory to the ideas described in the original research documents themselves. Unfortunately, it seems that the theory of BM has been thoroughly propagandized and distorted (and, if you read Perrin, you'll realize that we were doing the exact same thing to it back in the 1800s/early 1900s as well -- hence the motivation for his 1909 work). So, instead of relying on someone else's interpretation which came from another person's interpretation, etc. etc., educate yourself by reading the original documents of the time (not only Einstein, but Smoluchowki's papers are also excellent for gaining insight, Svedberg, Langevin, etc. for a depiction from the physicists representation in the early 1900s, but you can go even further to the 1800s and into the chemistry literature, and of course, Robert Brown's paper itself) and form your own understanding.

- 5 years, 3 months ago