Outside a sealed room sit three light switches, all currently off. Inside hangs a single incandescent lightbulb. You may manipulate the switches as much as you like, but you can enter the room only once. How do you determine with certainty which switch controls the bulb?
Answer
1. Flip switch one on. Leave it on for five minutes to allow the bulb to heat up.
2. Flip switch one off and flip switch two on.
3. Step into the room immediately.
– If the bulb is lit, switch two controls it.
– If the bulb is unlit but warm to the touch, switch one controls it.
– If the bulb is unlit and cold, switch three controls it.
This uses time and thermal energy as physical variables alongside light itself.
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