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The Little Book of String Theory by Steven S. Gubser

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Chapter THREE

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GRAVITY AND BLACK HOLES

ONE FINE SUMMER DAY SOME YEARS AGO, I DROVE WITH MY father up to Grotto Wall, a popular climbing crag near Aspen, Colorado. We aimed to climb a classic moderate route called Twin Cracks. After we finished it without incident, I trotted out another idea: aid-climbing a harder route called Cryogenics. Aid-climbing means putting pieces of hardware into the rock that support your weight, instead of holding on with your hands and feet. You tie yourself to a rope, and you clip the rope to all the hardware you place, so that if the piece you’re standing on pulls out, the ones below it will catch your fall.

Cryogenics ...

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