Re: Relative to time and gravity--are your feet younger than your head?

If you observe two identical clocks -- one right next to you and one above you (such as a clock on a mountain top), the higher clock will appear to be running a little bit faster than the lower clock.

In the same way, a clock beneath you will appear to be running a little slower.

My words "above" and "beneath" can be generalized to gravitational potential -- the further into a gravitational well, the slower a clock will appear to run. Because the sun has a much deeper gravitational well than the Earth, a clock in the middle of the sun would appear to run slower than any other clock in the solar system.

What would happen if you were really tall? Yes -- your feet would appear to be aging slower (as viewed by your head!). The price you pay for this is enduring tidal forces: your feet and your head would be accelerating at different rates, and the forces would have to be compensated by strain throughout your body!

This physics actually has some important consequences. For example, the clocks on GPS satellies run a little too fast, and need to be slowed down relative to Earth-based clocks. If left uncorrected, this would result in the satellite clocks going 1 second too fast every 70 years. This might not seem like much, but it's an important effect for a precision system like GPS, where timing is everything.

For a slightly more mathematical explanation of all this, see this page.