Can precise clocks really measure time running differently at your head than at your feet?
An emailer cites a NASA scientist on Joe Rogan claiming clocks can measure time passing differently over a two-foot height, and asks whether it is real given that time dilation comes from relativity.
Answer
On occasion you will hear physicists use special relativity to say that time is flexible. If you take a cesium clock, which keeps time by the regular decay rate of the cesium atom, up a high mountain like Everest, it will not keep the same time as one on the ground, because gravity is stronger on the ground and slows the decay rate. That part is a real, measurable effect. The difference is in the interpretation. Special relativity does not say the time is the same and merely measured differently because of gravity; it says the time actually changes, running slower on the ground and faster on the mountain, because there is no absolute time. Isaac Newton, by contrast, made space and time absolute; your watch might read differently, but time itself did not change. Einstein had to make time flexible, part of spacetime, and the reason he had to is the same reason behind all of it: to keep the earth moving around the sun. A motionless earth gives you absolute space and time; to keep the earth moving you have to make them flexible. So when the physicist on Joe Rogan says the time at your head is different from the time at your feet, all she is really saying is that special relativity measures time differently because of gravity, and calls that difference real time.