Black hole time dilation is when the closer an object is to a black hole, the slower time passes relative to someone far away. This effect can also be seen to a much lesser degree on Earth. Physicists at NIST have tested this effect using clocks separated at different altitudes and proved the effect is real even at small scales. Black holes are much more extreme, if one astronaut spent one hour near the black hole, the other astronaut that watched from further away may experience years passing by.