Now on Earth and Mars

I recently read Carlo Rovelli talking about the "extended present". He wrote that the present moment on Earth is actually 15 minutes long on Mars. Is this right? Surely if a chain of people holding hands stretched all the way from Earth to Mars, and they all (somehow) raised their hands together, that brief moment would be the same on Earth as it is on Mars and would count as the present moment. Similarly for a chain stretching across the universe ...? I understand that if you were on Earth looking along such a line with a super-powerful telescope, it might take thousands of years to see the people thousands of light years away raise their hands, but you could close your eyes and know that actually everyone raised their hands together, just as when you see a firework go off in the distance and hear the bang a few moments later - you still know that the bang happened earlier. So can't there be a present moment that extends throughout the universe? (Imagine instead of a chain of people, you had a three dimensional grid of people extending in every direction, filling every square meter of the universe, and they raised and lowered their hands once every second. That would be like a ticking clock that measures a universally agreed passage of time across the universe ... wouldn't it?
- Neil (age 50)
uk

I think what he means is this. There are a variety of ways of dividing up space-time events into sheets of "now". Each of these ways is physically ok in the sense that each such coordinate system ("reference frame") uses the same laws of physics. Now here can correspond to a range of different now-moments somewhere else, depending on which reference frame is chosen. In Special Relativity, the set of now-sheets that include this time here on Earth cover a range of moments on Mars. The time range of those Mars-now moments, measured by a clock on Mars, is 2d/c, where d is the distance to Mars and c is the speed of light. Depending on the distance, which varies a lot, that can come out to be 15 minutes.

Mike W.

(published on 05/14/2018)