A dunking bird toy, December 28, 2011.
| Photo Credit: RobinLeicester (CC BY-SA 3.0)

Nothing lasts forever. This is good life advice, and it also happens to be an important feature of our physical universe.

Natural philosophers considered and discarded the idea of ‘perpetual motion’ a long time ago. The basic premise of perpetual motion is that it should be possible to operate a system without supplying power to it. We know from daily experience that this can’t be true: for example, your phone’s battery will drop to zero as you use it without charging.

In physicists’ parlance, perpetual motion violates the law of conservation of energy. The first and second laws of thermodynamics also stipulate that anything that offers power must also liberate heat. If there was an infinite power supply, there would also have to be an infinite heat liberator. There isn’t.

But this simple explanation hasn’t stopped some people from wondering whether perpetual motion machines might exist.

A simple example is the dunking bird toy. It uses a temperature differential between the bird’s top and bottom to move back and forth. Very simply speaking, when it moves forth, its beak dips into a glass of water that flows to the bird’s bottom, and the weight causes the bird to move back.

The dunking bird toy can work like this for a long time, but it can’t move perpetually: it will stop when the temperature differential between the bird’s top and bottom vanishes.

Karthik Vinod is an intern with The Hindu.