From my understanding time appears to go slower on the first floor because gravity is stronger there which makes everything move slower, from particles to clock hands. But if light has no mass and can't bee slowed down by gravity then that means time is travelling at the same time on the first and second floor, but to measure light on both floors you would have to use a clock, but clocks are affected by gravity, so clocks on both floors will measure the same time a beam of light has taken to cross the rooms on both floors, is that correct? so if the rooms on both floors are 3 metres across then the time to cross them will be the same, but how can that be if the clock on the second floor is moving faster? And I don't understand how you can get an accurate measure of light because any measuring device is affected by gravity. I hoe this makes sense.