The dominant science news story of the moment is the latest discovery of water on Mars, which is fortuitously timed to coincide with the release of the movie The Martian this week. A little over a month from now, the big story will be the 100th anniversary of Einstein's completion of General Relativity. These might not seem like they have much to do with each other, but in fact, Mars missions have a closer connection to relativity than you might think.

 

John Grunsfeld, associate administrator at NASA's Science Mission Directorate, speaks with... [+] colleagues at a press conference where NASA announced new findings that provide the 'strongest evidence yet' of salty liquid water currently existing on Mars. (Photo by Win McNamee/Getty Images)

John Grunsfeld, associate administrator at NASA's Science Mission Directorate, speaks with... [+]

 

 

General relativity famously involves the warping of space and time by gravity, and it was observations during a 1919 eclipse showing the bending of light that catapulted Einstein to fame. Stars near the disk of the Sun had their apparent position (relative to stars farther from the Sun) shifted slightly, as the rays that passed close to the Sun were deflected by its warping of spacetime. The measured deflection agreed nicely with Einstein's prediction, and the rest is one of the great hyperbolic headlines is history.

Of course, relativity makes lots of predictions about what should happen near a massive object like the Sun, and the bending of starlight only tests one. Another thing that ought to happen is a slight "stretching" of space-- which is why discussions of relativity almost always include stretched rubber sheets. The distance between two points in space will be slightly longer along a path that passes close to the Sun than along one that never goes near it.

 

Embedding diagrams showing the spacetime distortion in the vicinity of a massive object, and the trajectories of different orbiting objects in the resulting gravity well. Image from Wikimedia. http://commons.wikimedia.org/wiki/File:Gravity_Wells_Potential_Plus_Kinetic_Energy_-_Circle-Ellipse-Parabola-Hyperbola.png

Embedding diagrams showing the spacetime distortion in the vicinity of a massive object, and the... [+]

 

This is kind of a difficult thing to get your head around, but like everything else, it comes back to the fact that keeping the laws of physics consistent regardless of how you're moving requires the mixing of space and time. In special relativity, what one observer sees as purely a distance in space, somebody moving at constant speed relative to them will see as a mix of space and time-- the position of the two endpoints is measured at two slightly different times. This is the root of most of the "paradoxes" of relativity. The exact mix of space and time depends on the speed of the observer, and the equations of relativity tell you how to calculate that.

General relativity tells us that the exact mix of space and time for a particular measurement also depends on the presence of gravity. What an observer near the Sun sees as purely a distance in space will look, from far away, like a mix of space and time. This mixing changes the result for distance measurements.