I am reading Stephen Hawking "A Brief History of Time" recently, and I cannot understand perfectly this little fragment:
(...) Another prediction of general relativity is that time should appear to slower near a massive body like the earth. This is because there is a relation between the energy of light and its frequency (that is, the number of waves of light per second): the greater the energy, the higher frequency (...)
How is that light influence time? He writes also:
As light travels upward in the earth’s gravitational field, it loses energy, and so its frequency goes down. (This means that the length of time between one wave crest and the next goes up.) To someone high up, it would appear that everything down below was making longer to happen.
And that I can understand, it just a matter of perspective. But what's most confusing for me is this:
This prediction was tested in 1962, using a pair of very accurate clocks mounted at the top and bottom of a water tower. The clock at the bottom, which was nearer the earth, was found to run slower, in exact agreement with general relativity
How is that related to light? How light affects situation when clock at the bottom was found to run slower? I know basics of general relativity, I am just confused why did he mentioned "relation between the energy of light and the frequency" in that. I'am probably missing something obvious here, that's why I am waiting for someone to help me.