Risu nodded. "That's it exactly. Very well done, Mr Miner." He leaned against his desk. "To briefly recap: According to calculations done before Einstein even entered the scientific scene, the speed of light seems to be a constant of nature. No matter how fast you are, light will always recede from you at the speed of light. This led, once Einstein took the calculations at face value, to demolish the notion of absolute time. In Einstein's universe, time slows down the faster one moves - this is because, if it did not, light would have to travel faster than itself. Our thought experiment with the Hogwarts Express has shown this.

"But now... what about absolute space? Galileo has made us rethink the idea of absolute space already - since there is no way to say with certainty whether something is standing still or in motion, there is no such thing as absolute space against which we can measure the movement of something. You can only stand still or move relative to something. But could space be just as strange as time? Is there a way that relativity makes space stretch and compact just as it does time?"

Risu grinned. "As you can probably guess, the answer is yes. If Einstein's equations are correct, then space itself becomes malleable with relativity. Specifically, the faster you move, the smaller space becomes. We will now investigate how that is..." He checked to see that he had everyone's attention. "In the 1990s, the Brookhaven National Laboratory in the state of New York in the United States provided a test of this hypothesis. The scientists there built a machine that allowed them to send particles around in a circle."

He drew an imaginary circle in the air with a finger. "They used a particle called the muon and shot it around the ring at 99 percent the speed of light. You don't need to know what a muon is, by the way," Risu added quickly, "it's not important for our discussion. What you need to know is that the muon is very unstable, living only for 2.2 microseconds before it decays into something else. The scientists calculated that if space stayed the same, the muon would be able to whiz around the ring 15 times before it decayed. When they did the experiment, however, the muon managed 400 laps before it finally decayed!"

Risu paused. "2.2 microseconds is too short to complete 400 rounds. Think about it for a moment. If you were able to speed alongside the muon, you and it would be still relative to each other and your time would be the same. You would only have 2.2 microseconds to circle the machine 400 times. Not possible." He shook his head and looked at the students seriously. "No, what happens is that space shrinks, reducing the distance the muon has to travel. It literally shrinks. This is not some physical illusion. Objects really do get smaller when they move faster. Similarly, time does tick slower when you get close to the speed of light. These are both real effects."

He paused again. "Before we continue, has everyone understood this experiment? Are there any questions?"