# Chapter 2: Relativity — The Greatest Illusion

## Endnotes

• 1
• Quote: The ideal isolated bubble would be a thick-walled, windowless spaceship far out in interstellar space, gliding gently with its rocket engine switched off.
• Endnote: Exactly what constitutes “isolated” is a complex issue if we look at it closely. But informally, an isolated bubble should shield anyone inside from all information about objects outside; otherwise, those objects might create effects inside the bubble that would obscure the relativity principle.
• Discussion (coming soon)
• 2
• Quote: Our planet rotates and roams the heavens, but our motion is nearly steady. That makes it nearly undetectable, thanks to Galileo’s principle.
• Endnote: As pointed out by the nineteenth-century French physicist Léon Foucault, the Earth’s rotation, the least steady of our motions, is reflected in the motion of a tall pendulum. Many science museums around the world have such a “Foucault pendulum” on exhibit.
• Discussion
• 8
• Quote: For the wings to generate enough lift for flight, a plane needs a minimum airspeed. If it starts its takeoff roll into a headwind, then the air rushes over the wings faster than the wheels move over the ground—the airspeed is higher than the ground speed—and so it can take off when its ground speed is still rather low. If it takes off into a tail- wind, the situation is reversed, and so a much higher ground speed is needed to reach the required airspeed for liftoff. To get to that higher ground speed requires much more runway, and so there’s much less margin if anything goes wrong. The same goes for landing: when flying into the wind, the plane can stay afloat with a much lower ground speed and therefore needs less run- way to come to a stop.
• Endnote: Another relative speed of note is the plane’s speed relative to the Sun, which determines how quickly the aircraft passes through time zones and how quickly the Sun appears to cross the sky. Flying east, a plane is carried along with the Earth’s rotating atmosphere, so it moves rapidly across the sunlit half of the Earth; flying west, counter to the Earth’s rotation, it can delay sunset for many hours.
• Discussion: