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Emily LakdawallaMay 25, 2008

Some thoughts on relativity and simultaneity

I've been scolded by a couple of readers for making statements regarding the "current" time on Mars, or about how things will be all over on Mars, one way or another, by the time we know what happened. One reader even went so far as to send me the following quote from John Wheeler's textbook Spacetime Physics:

When the events occur at different locations along the direction of relative motion, they cannot be simultaneous in both frames. This conclusion is called the relativity of simultaneity.
It's true that by thinking through the implications of the interconnectedness of space and time, and the constancy of the speed of light, Einstein forced physicists to banish the idea of simultaneity from their models of events on the universe.

Mostly. The thing is, when you're dealing with events that happen relatively close to each other in space, and in places where the relative speed between two objects of interest is an insignificant fraction of the speed of light, you don't need relativity; Newtonian mechanics, and its approximation that time runs the same in all places, works just fine. I can guarantee you that none of the mission controllers are thinking about time dilation effects caused by the minute (when compared to the speed of light) difference between the speed of Phoenix and the speed of Earth.

Most people in mission control will be working on Earth Received Time, that is, the time measured on the spacecraft clock, plus the 15 minutes and 20 seconds it takes signals from Mars to reach Earth. So I've decided that's what time I'm going to be working on today. Measured in Earth Received Time, landing is expected at 23:53:52 UTC. Here's a direct link to Daniel Muller's countdown clock in Earth Received Time.

Go Phoenix!

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Emily Lakdawalla

Senior Editor and Planetary Evangelist for The Planetary Society
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