This is known as gravitational time

dilation. So, let's again imagine you're

sitting here at some radius R away from

the black hole and I'm sitting out here

way off at infinity just watching you.

And we're static relative to each other.

You're just suspended here by your

pulley system and the question is how

fast does your watch go relative to

mine? I observe your wrist watch to be

running slow. You observe my wrist watch

to be running

>> [music]

>> fast. In other words, if I slowly lower

you down towards the black hole, you

hang out some finite distance [music]

away from the black hole for what feels

to you like a year and then raise you

back up a long way away from the black

hole, you will return to a world [music]

that has aged a lot more than you have.

Time passes slower down here than it

does up here. This is [music] a fact

that has by now been extremely well

observed experimentally. In the 1950s,

in the Harvard physics department, they

put two [music] atomic clocks at two

different heights in the building and

noticed that the one that was higher was

running faster than the one that was

[music] slower. This is an effect that

is now considerably within the precision

of for example GPS. It just has to

subtract that effect, otherwise [music]

everything would drift all over the

place. GPS clocks that are sitting on

the Earth's surface are running slow

compared to [music] the atomic clocks

that are in in orbit sending out the

signal and you have to subtract off that

difference in order to get an accurate

accurate reading.