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Brian CoxThis is NASA's Space Power Facility near Cleveland, Ohio
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Brian Coxand it is the world's biggest vacuum chamber.
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Brian CoxIt's used to test spacecraft in the conditions of outer space.
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Brian CoxAnd it does that by pumping out the 30 tonnes of air in this chamber
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Brian Coxuntil they're about 2 grams left.
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Brian CoxAnd it has got an eccentric construction which is part of its history.
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Brian CoxIt was built in the 1960s
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Brian Coxas a nuclear test facility to test nuclear propulsion systems.
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Brian CoxAnd that meant that they built it out of aluminum to make the radiation easier to deal with.
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Brian CoxAluminium is not the best thing, the strongest material to build a vacuum chamber out of.
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Brian CoxSo they built an outer concrete skin,
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Brian Coxwhich is part radiation shielding and part an external pressure vessel.
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Brian CoxSo this thing can take the force that's present on the outside
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Brian Coxwhen it's pumped out to the conditions of outer space.
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Brian CoxGalileo's experiment was simple.
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Brian CoxHe took a heavy object and a light one
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Brian Coxand dropped them at the same time to see which fell fastest.
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Brian CoxNow in this case, the feathers fell to the ground
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Brian Coxat a slower rate than the bowling ball because of air resistance.
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Brian CoxSo in order to see the true nature of gravity,
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Brian Coxwe have to remove the air.
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Brian CoxIt takes 3 hours to pump out the 800,000 cubic feet of air from the chamber,
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Brian Cox"OK, we dropped two military in the last 30 minutes."
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Brian Coxbut once it's complete there's a near perfect vacuum inside.
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Brian Cox"61-O-4 manual 10%, open station 1, go for drop, PCB 30-1 pressure set point at 240 PSI."
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Brian Cox"We are going for a drop."
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Brian Cox(Counting down) "10, 9, 8, 7, 6, 5, 4, 3, 2, 1."
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Brian Cox"Release."
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Brian Cox"They came down exactly the same."
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Brian Cox"Wow."
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Brian Cox"Exactly, exactly the same. Feathers don't move."
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Brian Cox"Nothing. Look at that.
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Brian CoxThat's just brilliant."
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Brian CoxIsaac Newton would say
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Brian Coxthat the ball and the feather fall because there's a force pulling them down - Gravity.
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Brian CoxBut Einstein imagined the scene very differently.
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Brian CoxThe happiest thought of his life was this.
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Brian CoxThe reason the bowling ball and the feather fall together is because they're not falling.
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Brian CoxThey're standing still.
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Brian CoxThere is no force acting on them at all.
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Brian CoxHe reasoned that if he couldn't see the background,
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Brian Coxthere'd be no way of knowing that the ball and the feathers were being accelerated towards the Earth,
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Brian Coxso he concluded they weren't.