Science and Film logo

Reel Science: Snowpiercer‘s Perpetual Motion Machine

Korean filmmaker Bong Joon-ho’s new futuristic thriller, Snowpiercer, engages with at least two controversial propositions in science.

Set in the year 2031, the film imagines a planet where scientists have injected a “cooling substance” into the atmosphere to stem the tide of global warming. Though much debated, the practice is currently being developed; so-called “geo-engineering” plans include the idea of injecting sulphur dioxide particles into the atmosphere in order to reflect solar radiation away from the Earth.

But the film is perhaps more a story about energy efficiency. During the opening credits, we learn the disastrous side effects of the “cooling substance”—the planet has frozen over, and the only remaining human survivors populate a fast-moving elaborately constructed train that loops infinitely around the globe. The train’s sole power source is an “eternal engine.”

The notion of a self-sustaining energy system is one of the oldest energy myths in human history. Sloan Science and Film spoke with Dr. John H. Lienhard, professor emeritus of mechanical engineering and history at the University of Houston and the host of Houston Public Radio’s “The Engines of Our Ingenuity,” about perpetual motion machines, energy crises and thermal engines.

Sloan Science and Film: Can you give a little background on perpetual motion machines and why they’re not scientifically viable?

John H. Lienhard: The first perpetual motion machine that we know about was proposed by an Indian mathematician named Bhāskara in 1150. The Muslims picked it up in the next century, and then the French started playing with the same idea in the middle 13th Century. The original idea was a gravity device—an over-centered wheel where the arms stick out on one side and hang slack on the other, so it’s always out of balance and will always keep rotating. But, of course, that violates the law of conservation of angular momentum. And yet, the idea is so tasty that people have been reinventing it ever since.

As new physical phenomena have arisen, people have added all sorts of new physics: perpetual motion machines driven by out of balance hydraulics, magnetic machines, electrical machines, and so forth. It wasn’t until the late 17th Century that [Gottfried Wilhelm] Leibniz wrote about the conservation of mechanical energy, saying that overbalanced wheels wouldn’t work. And it wasn’t until the 19th Century where we had the second law of thermodynamics, which generalized that the whole system couldn’t be cooked up. So perpetual motion is clearly impossible, and yet, very smart people come along all the time out-smarting themselves with new concoctions. I get an email once a month from someone with a perpetual motion machine.

SSF: In the film, they call it an “eternal engine,” and though it’s never explained, it does have this rotating device at its core, which seems to recall that basic wheel idea. Are the latest contraptions still based on that circular machinery?

JL: It’s always hidden from view. It’s always: “We can’t show you the details or people will steal our idea.” So externally, it’s often just a box with a shaft coming out of it. Smart inventors these days don’t call them perpetual motion machines, either, because they know they’ll get scoffed at.

SSF: I read that MIT has held contests, challenging students to make perpetual motion machines. One recent winner was a rotating magnetic device, and I wonder if it was the same as that old rotating wheel.

JL: What they’re doing there is seeing how long can you keep something moving, which is really how slowly the energy will drain away. So that’s not a device that produces energy; it’s about how it will burn up its own energy at a minimum rate.

SSF: Given our current concerns about global warming and seeking alternative energy sources, it makes sense that there would be an interest in perpetual motion machines. But why do you think it’s such a recurrent myth through the centuries?

: If you look at interest in perpetual motion machines, you’ll find that they connect with energy crises. In the 13th Century, we were running out of wood. There was a huge population explosion; Europe was warming, and things were good. But we needed some new way of creating energy. And coal saved us at the 11th hour, but perpetual motion was rediscovered as a possible savior. Then things went on pretty well, and we ran into another energy crisis when Europe repopulated in the 17th century. We were running down to the water table, and we couldn’t mine any deeper. So people started getting very interested in perpetual motion machines again. If you look at scientific books published at that time, you’ll find lots of perpetual motion machines in them. Then the steam engine came along and saved us. And then in the 19th Century, people started worrying about cleaning out the British coalfields and so they started being fearful, and again, the interest rose. So it often comes and goes with energy crises.

SSF: Going back to the train in the film, what if you were to take the ice and snow from outside the train and use that as some kind of generative source? It wouldn’t be a perpetual motion machine, but…

JL: It would be a workable heat engine. Anytime you have a temperature difference you can build a heat engine. Here is a simple formula for you: the best efficiency you’ll ever get is the difference between the two temperatures divided by the higher temperature. One example of the way people have tried to exploit this is Ocean Thermal Energy Conversion (OTEC). People float a device in the Gulf Stream, for example, and they have the boiler in the warm water of the gulf stream, and the condenser in the cold water beneath, and if you look at these two temperatures, you have a 20-degree temperature difference, so you’ve got a minuscule difference. People have been working on that technology for a long time, since the latter part of the last century. With the movie, you could have something like that: with icy snow and sunlight, so presumably, you might be able to do that.

SSF: There’s a lot of talk in the film about efficiency and self-sustaining energy and keeping things in balance, and I’m wondering: What machines are most efficient and what makes them so?

JL: The best thermal engines we’ve got will give you efficiencies of 50%. The way they work is they have very high temperatures and they condense at a low temperature. Big coal-powered power plants, for example, burn the coal at a very high temperature. And then steam discharges at very high pressures and at high temperatures, and then successive turbines produce electricity, and then finally, the steam discharges into a condenser, where the cold temperatures turn it back into water, which is pumped into a boiler and it’s boiled again and it goes around and around.

SSF: So if we want to make a really efficient train, we’re back at coal?

JL: Or some sort of fuel: coal, natural gas.

SSF: Would a nuclear powered train be more efficient?

JL: Well, what is a nuclear pile? It’s something that will give you energy at as high a temperature as you’d like. So the question is how do you take your working medium, whether steam or liquid metal, being boiled? The trick is you have to boil something that can run through your engine, and at how high a temperature can you do this? A pressurized water reactor runs the steam at around 2,000 pounds per square inch, at 600 degrees or so. So it isn’t a question of a nuclear reactor, but it’s a question of how you utilize that heat; you have to use it to boil something to drive your engine.

Another form of energy utilization is the solar field. Fields of mirrors focus energy on a tower where liquid is boiled inside, and that water is used to drive the steam engine. So, again, you can get a high temperature, but the question is how do you capture that temperature?

SSF: But in all of these examples, you need an external source to bring up the heat, right?

JL: Exactly. One has to expect that if you’re watching a movie about a perpetual motion machine you have to simply make a willing suspension of disbelief. I like ghost stories, too.