After watching Susskind's lectures, I think I've finally understood one problem that has bothered me since undergraduate days almost 50 years ago. When studying special relativity, we learned about 'time dilation', where things moving faster than us have clocks that _appear_ to be going slower. We also learned about getting on a star cruiser after fathering a child & later coming home younger than your child (the so-called "twin paradox"). The question for me was 'what is the cause of the time dilation?' -- is it the speed itself, or is it the acceleration? It appears that the time dilation is related to the acceleration, so -- in effect -- all time dilation is caused by acceleration, whether it be 'real' acceleration due to rocket thrust, or to gravitational acceleration due to some mass in the neighborhood. I.e., the father/child paradox requires the acceleration to get up to speed, as well as the deceleration necessary to get back to meet with the child.