Video: the Arrow of Time

We’re halfway through our discussion of entropy, which is a perfect place to take a quick break for a video. It may not be very exciting, and it may not be very clearly explained, but it’s related to our current topic and may at least ignite your curiosity a bit. We’ve all thought about time, but the technical description of time’s biggest mystery, from a physics perspective, is pretty intriguing.

As we’ll soon see, entropy provides one of our clearest indicators for the direction of time’s arrow. Stay tuned for more!

5.6 Entropy, Part 1: What Is It?

Critical Questions:

  • What is entropy?

In order to gain a full appreciation for energy, the last idea you’ll need to understand is something called entropy. It’s kind of a tough one, but I guarantee it’ll be worth it if you persevere to the end of this chapter!

The reason I’ve saved it for last is that many people find entropy quite difficult to understand. There are two reasons for this: one is that it’s another purely abstract concept; the other is that there are about twenty different ways to define entropy, and they all seem very different from each other.

Luckily for us, however, there is one easy way to understand entropy that we can explain with a simple example.

Take a large cardboard box, a can of red paint, and a can of blue paint.1 We’re going to paint the inside of the box in these two colours: red for the left-hand side, and blue for the right.

Get a load of these suckers.

Once the paint has dried, grab 8 ping pong balls again2 and put them into the box.


  1. Note: the actual colours are not important. Let your aesthetic sense be your guide.
  2. This is no coincidence. I will not hide the fact that I like ping pong balls.

New 3D Panorama from Mars

This lovely 3D panorama, which was made by photographer Andrew Bodrov, is a stitched-together collection of photos taken by the Mars Curiosity rover. It’s a desolate image, but still immersive and a bit magical.

Click the image to go to the 3D panorama.

From a related Wired article:

“The best way to enjoy it is to go into fullscreen mode and slowly soak up the scenery — from the distant high edges of the crater to the enormous and looming Mount Sharp, the rover’s eventual destination.”

Don’t forget to pan up and check out that dismal-looking sun, too, which as of this posting is only managing to heat the surface to a few degrees above freezing, with a daily low of about -70 °C (-94 °C).

5.5 Heat and Temperature

Critical Questions:

  • What is the difference between heat and temperature?
  • What happens when water freezes or boils?

It may seem out of place to start talking about heat and temperature in the middle of a chapter about energy, but in fact, there are many connections between these ideas. And in order to understand the ways in which we can and can’t use energy, we have to know something about heat.

Heat Movie Poster
1995, starring Al Pacino.

The first thing science teachers do when teaching this topic to young people is try desperately to communicate the idea that heat and temperature are, in fact, two very different things. I never really understood why this seemed so important to them, but nevertheless, they’re right.

Heat, in physics, is a quantity of energy that is transferred from one object to another. When you hold your hand over a fire and your hands get hotter, heat has been transferred from the air into your skin.

Temperature, on the other hand, is a measurable property of any object. Specifically, it is a measure of how quickly the particles in that object are vibrating.

You see, it’s fine to picture solid matter as being made up of all of these atoms and molecules and whatnot, but these particles do not sit around waiting for stuff to happen. Even in a seemingly still, solid object, the particles that make up matter are constantly vibrating and bumping into each other.1 And when heat is transferred into an object, its particles vibrate more than before.


  1. This is called Brownian motion, named after the Scottish botanist Robert Brown, who noticed this behaviour when looking through a microscope at grains of pollen.

Brian Cox Introduces You To Quantum

Here it is: a lifeline to get you through this Pop Physics drought. Professor Brian Cox, host of the excellent BBC series Wonders of the Universe, delivering a fun and insightful introductory Quantum Physics lecture. Complete with celebrities like Simon Pegg, a million-pound diamond, and a breathless northern English accent.

Professor Brian Cox: A Night With The Stars, on YouTube
Part 1
Part 2
Part 3
Part 4

A Real Perpetual Motion Machine, in the Wild!

Somewhat coincidentally, I just came across this video of a supposed perpetual motion machine / free energy device via reddit. It’s a coincidence because my last post was all about perpetual motion and why it’s impossible. It may seem like a bit of an archaic problem to tackle, but as the popularity of this video shows, these things still pop up from time to time in real life. This one uses magnets, as most self-respecting perpetual motion con artists do these days.

My favourite part is in the video description: “This technology has been suppressed because it is a threat to the profits of the energy corporations.” Not sure how those tricky energy corporations haven’t managed to get this video taken off YouTube yet, but I suppose it’s only a matter of time.

So why doesn’t it work? (more…)

5.4 Perpetual Motion Machines

Critical Question:

  • What are perpetual motion machines, and why are they impossible?

There was an episode of The Simpsons in which Lisa made a perpetual motion machine, which angered Homer because “it just keeps going faster and faster.” Later, he called her into the room and yelled, “In this house, we obey the laws of thermodynamics!”

homer perpetual motion machine
Man, what won't Homer say next?

He had every right to be angry, but Lisa is not alone in her fascination with the idea of a machine that never stops. As we’ve already seen, motion requires energy, and energy isn’t always easy to come by. But if we had a machine that could keep moving forever without assistance, the possibilities would be endless.

Alas, this is one of those cases that really is too good to be true.


5.3 Conservation of Energy

Critical Questions:

  • What is energy?
  • What do all of the different types of energy have in common?
  • How do we use food to move ourselves around?

Now that we’ve described a bunch of different types of energy (in the previous post), we can talk about how they behave. Following on Mr. Feynman’s comments, each of these types of energy has a very specific mathematical formula, so we can calculate exactly how much of each type of energy we have in a given situation.

The most interesting thing about energy is this: it cannot be created or destroyed; it can only change forms.

I’ve already hinted at this idea when talking about the examples from the previous post. Any kind of potential energy can turn into kinetic energy when the object in question is free to move. The simplest example involves one of those wind-up toys you probably had as a kid, or at least you probably saw them in old cartoons.

caterpillar wind-up toy
I like this one because it's a happy caterpillar.

To make these toys work, you turn a little crank; this tightens a spring inside the toy, which means that you have increased the toy’s elastic potential energy. When you let go of the crank, the spring turns some gears, which cause the toy to spin, or walk, or drive away. Whatever it does, it’ll probably move somehow – in other words, it’s gained kinetic energy. But meanwhile, the spring is no longer wound up, so it’s lost all its potential energy.