Friday, January 04, 2008

Simultaneity and Apollo 11

Tom Lehrer is a retired math professor. Back in the sixties, he used to write and perform very funny satirical songs, like Wernher von Braun and So Long, Mom, I'm Off to Drop the Bomb. The Element Song is amusing and educational: it sets the names of all the elements to the music of Gilbert & Sullivan's "Modern Major General" song from the Pirates of Penzance. One of my favorite lines of Lehrer's is from the nuclear war song We Will All Go Together When We Go:

When the air becomes Uranious,
We will all go simultaneous,
Yes we'll all go together when we go.

Which leads me to the real subject of this post, simultaneity and Apollo 11.

On John Walker’s Fourmilab site, there’s an article called “The Relativity of Simultaneity.” In it he points out that because of the finite speed of light, events separated by some distance can’t easily be defined as “simultaneous.” Depending on their locations, different observers can observe the same distant events occurring in a completely different order. He illustrates this with a clever animated GIF image (shown above) and also with a real-life example, the radio transmissions from Apollo 11 on July 20, 1969!

The Moon is pretty close by astronomical standards, but it’s still about 1.3 light-seconds from Earth. This is a noticeable amount of time in human conversations, and Walker asked himself what effect this delay might have had in the radio conversations that took place during the final moments of the first Moon landing. Using audio editing software, the known Earth-Moon distance, and the documented times of the sound recordings made in Houston and on the Moon, he constructed an audio track that simulates the way Armstrong and Aldrin actually heard the conversation in the LM cockpit. He provides a stereo MP3 file (4'26" - about 4 MB) that has Houston’s transmissions on the right, transmissions from the Eagle to Earth in the center (mostly Buzz Aldrin), and Neil Armstrong’s LM-only intercom comments on the left. This recording sheds light on the pause in the middle of Neil’s first official “Tranquility Base” transmission. You can read Walker's article for more details on the problem and on what he did to create this recording. Very clever stuff.

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