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Wednesday, January 10, 2007

The Relativity of Simultaneity Posted

I have just posted a little project I've had in mind for some time, The Relativity of Simultaneity, which demonstrates how observers separated in space, even if not moving with respect to one another, may perceive events to occur in a different order; simultaneity depends upon your vantage point.

It takes light more than a second and a quarter to traverse the distance between the Earth and the Moon, which means that when you listen to recordings of Apollo Moon mission communications made on Earth, you're hearing transmissions from the Earth as they were sent, but the audio received from the Moon with a substantial delay; what the astronauts on board heard was different. In this document, I've taken a recording of the Apollo 11 Moon landing, extracted transmissions from the Earth and Moon onto separate audio tracks, and time-shifted the Earth transmissions so they occur as heard on the Moon. In addition, I have mixed in a few remarks during the landing sequence by Neil Armstrong which were captured by an on-board tape recorder in the lunar module but not transmitted to Earth. The result is an approximation of what the astronauts heard in the cabin during the descent and landing, and provides an insight into the cadence of Armstrong's first radio transmission after the touchdown.

The animation in this document was created by a Perl program invoking various Netpbm utilities, then assembled into an animated GIF with Gifsicle. In the process of making the illustration, I ran into a bug in the Netpbm program ppmdraw, present at least since version 10.35.05 (the current release for the Fedora Core 6 Linux distribution) which caused it to crash due to a reference through a pointer in a previously released buffer when the script contained two or more commands. I have fixed this problem and submitted a patch to the Netpbm developers, but if you need the patch you can get it here.

Posted at January 10, 2007 18:04