Saturday, February 9, 2013

XKCD On Perpetual Motion

XKCD has this right, both as an Internet phenomenon and regarding the alarming degree of scientific ignorance out there in the public.

An example relevant to thermodynamics:  About a decade ago, I was given the task of dealing with an enterprising young man who had designed and built a device that he thought would in fact be a perpetual motion machine.  It would use permanent magnets (he was very insistent that these were very powerful permanent magnets) at the end of the four spokes of a wheel; on each spoke the north pole faced outward.  It was housed in a box that had four similar magnets with the north poles facing inward.  As the wheel spun, it would first have to fight the repulsion of the magnets, then get a "kick" once the magnets passed their closest approach.  The clever part of his plan was to use movable superconducting doors to shield the magnets (via the Meissner effect) from each other on approach.  That way, he reasoned, they would not repel each other on approach, but they would repel each other when moving away, so the wheel would spin faster and faster and could be used to do work.

I hate to admit it, but at first I was not sure exactly where the flaw in his plan lay; I knew, though, that this even violates the First Law of Thermodynamics.  Eventually I realized that the doors would be repelled from a region of strong field, so closing them would use up the energy gained from the kick.  This repulsion is exactly why a magnet can be made to levitate over a superconductor; as the magnet and superconductor approach each other, the repulsion gets stronger, until (for a light, strong magnet) it equals the weight of the magnet.

Unsurprisingly, the young man did not respond well to this.  He really did think he had an idea for a perpetual motion machine, and he refused to believe otherwise.  In the end I had to tell him to go ahead and build his device, which might be a good learning experience, but not to be disappointed if it doesn't work.  I never heard from him again, so I don't know if he built it or not.

I'll give just one more example in this post.  

I grew up attending Faith Christian School, which was run by Faith Bible Church -- which church we also attended after Overstreet Bible Church, which was closer, decided to cancel church one Sunday because that day would be Christmas(!!!).  Both the churches and the school bought into a childish, "young Earth" creationism and a love-hate attitude towards science, so I wasn't very surprised when some acquaintances at Immanuel Baptist Church in Wiesbaden, Germany started a conversation with, "Being a scientist must really help your faith."  

The truth of the matter is that science has very little to do with faith one way or the other.  I suppose scientists are more likely to think critically about everything, including religious matters, but the conclusions reached by scientists vary just as widely as those reached by the population as a whole.  This is to be expected; no scientific experiment tests the existence of God, after all, but only more mundane things, such as the existence and nature of the mislabeled "God particle", the Higgs boson. A scientist might at most be better equipped to see a surprising amount of beauty in the universe, but such arguments are inevitably only suggestive, not conclusive.

Back to Wiesbaden.  It turned out my acquaintances were trying to show their comprehension of physics by pulling out the argument that the Second Law of Thermodynamics forbids the evolution of more complex forms of life from simpler forms.  As further evidence of the Second Law, they cited the fact that the rotation of the earth is slowing down over the millenia (and longer, but they would not accept longer periods of time).

Notice a few things here.

  1. The understanding of the Second Law is at the level of "cowboy philosophy" -- "As time goes on, things just get messier and messier."  My acquaintances can't be blamed too much for this; most general science textbooks describe the Second Law like a quote by Will Rogers.  What most people, my acquaintances included, do not understand is that entropy is precisely quantifiable, just like energy or temperature, and that it has units, J/K.
  2. There is an implicit belief that a ball that is not rotating is "more disordered" than the same ball when it is stationary.  This seems to be based on the everyday experience of watches and clocks winding down.  It was the same kind of observation -- that moving things tend to move unless a force on them is maintained -- that lay at the heart of Aristotle's flawed physics. Certainly everyone is taught that Aristotle's physics was wrong; it's a standard part of the attempt to ridicule the Catholic Church for the whole Galileo business, but clearly many people are not learning how Aristotle's physics is wrong.  (The reason things tend to slow down and stop is due to dissipative forces like friction and drag.)
  3. In fact, entropy has to do with how many different ways something can be re-arranged without making a noticeable difference.  If the object in discussion is a solid ball with a fixed center, the only things that can be changed are its orientation, the pole around which it rotates, and the speed of its rotation.  If these are all precisely defined, as is typical in elementary mechanics, the entropy is zero, regardless of the orientation and rotation.
  4. I explained all this, together with the real reason the earth's rotation is slowing down.  The real reason involves the tides and the transfer of both energy and angular momentum from the spinning earth to the orbiting moon.  It's actually pretty neat how this works, and it does ultimately involve the Second Law of Thermodynamics, because heat generated from the action of the tides is radiated into space and lost to the earth-moon system.  Since this did not support their idea, though, my acquaintances did not care.
  5. Finally, I pointed out that if the Second Law actually prohibited increased biological complexity, it would prevent not only evolution but also the growth and development of an individual organism.  An acorn could never grow into an oak if the Second Law meant what they thought.  I was told that the Second Law did not apply to an acorn, since "It is God's will for it to grow into an oak."  There's really not much one can say to such pigheadedness.

No comments:

Post a Comment