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Pre-Lecture Homework
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6.1 Pre-Lecture Homework
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An interplanetary murder?
Suppose there are Martians. On a beautiful afternoon, Dr U emerged from the shore of a tropical (Earth) island in his wetsuit and dive gear. Something streaked across the sky and Dr U fell. It was 4:15 pm. In the investigation of the murder, it was found that at exactly 3 minutes before Dr U fell, a weapon capable of interplanetary murder was fired in Mars. It was known that Mars was 60 million km away from Earth at the time of murder. Did the Martian kill Dr U?
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Different perspectives
Get yourself a tennis ball. Also get a friend who is happy to do geeky stuffs with you. Take turns to do the following actions. While one do, the other stay still and observe.
Action 1: Stand still, throw the tennis ball vertically up into the air and catch it when it goes back down.
Action 2: Start walking at a constant pace. While walking, throw the ball up vertically and catch it when it goes back down.
Try It Yourself!
How does the motion of the ball in Action 1 and 2 differ? How does the observation of Action 2 differ for the person doing the action (the actor) and for the stationary observer?
Draw the paths of the balls of Action 2 as seen by both the actor and the observer?
Compare the distance traveled by both the actor and observer.
Also compare the time taken for the ball to go up and down.
Compare the average speed of the ball as seen by you and your friend.
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A thought experiment
Physicists like to fantasize about experiments that are simple
enough to think of but impossible to carry out due to physical constraints.
Einstein's Happiest 'Thought'
was one such gedanken (thought) experiment!
Let us begin our fantasy with something rather down to Earth - a clock. Think of a clock as a device which exhibit periodic observable phenomena. The turning of a clock needle is an example. A pendulum is another.
Now imagine a special "clock" consisting of two perfectly reflective plane mirrors facing each other, one above the other. When a photon is released in between the mirrors, in a direction parallel to the line joining the 2 planes, the photon bounces up and down, forever. The motion of the photon is periodic and thus serves as a good clock.
Take the "light clock" and do what you did in the previous experiment (tennis ball \rightarrow photon). Will your previous conclusions stay the same?