Event P is shown by a small x. At what time (in the primed frame) does P occur? A] at ct’=1 B] at ct’=2 C] at ct’=3 D] at ct’=4

Event P is shown by a small x. Where (in the primed frame) does P occur? A] at x’=2 B] at x’=2.5 C] at x’=3 D] at x’=3.3 E] at x’=3.6

In the unprimed frame, which event occurs first? Q or P A] Q B] P C] they are simultaneous

In the primed frame, which event occurs first? Q or P A] Q B] P C] they are simultaneous

OK, which event really occurs first? A] Q B] P C] neither

The line connecting two events is called a spacetime interval. If this line is rather flat, a reference frame moving sufficiently rapidly will have an x’ axis that is steeper than the line, and the events will switch their time order. If the line is steeper than 45°, then no reference frame can change the time order.

A flashbulb goes off in the exact middle of a spaceship that is traveling very fast. (The flashbulb is in the spaceship.) Does the light hit the front and back mirrors at the same time, according to an observer on the earth? A] yes B] no No. The speed of light is independent of the speed of the source or the observer, so it will hit the back mirror first. (The mirror is moving toward the source, so it reduces the distance the light must travel.)

According to an earthbound observer, a clock on the spaceship, adjacent to the back mirror, reads exactly 12 noon when the light hits the back mirror. At exactly this time (according to the earthbound observer), what does a clock on the ship, adjacent to the front mirror read? The clocks are synchronized and correct according the spaceman. A] a little before 12 noon B] exactly 12 noon C] a little after 12 noon

Clearly, spatially separated events that are simultaneous in one frame are not in another. What other bizarre things might happen? Lengths could change, clocks could run fast or slow???? Who knows??? What cannot happen: A pipe cannot contract or expand in the direction perpendicular to its motion.

Next: the light clock