Simultaneity Ain't what It Used to Be

One of the most fundamental deductions Albert Einstein made from the finite speed of light in his theory of special relativity is the relativity of simultaneity—because light takes a finite time to traverse a distance in space, it is not possible to define simultaneity with respect to a universal clock shared by all observers. In fact, purely due to their locations in space, two observers may disagree about the order in which two spatially separated events occurred. It is only because the speed of light is so great compared to distances we are familiar with in everyday life that this effect seems unfamiliar to us. Note that the relativity of simultaneity can be purely due to the finite speed of light; while it is usually discussed in conjunction with special relativity and moving observers, it can be observed in situations where none of the other relativistic effects are present. The following animation demonstrates the effect. //

... by extracting transmissions from the LM from those originating in mission control onto separate tracks with the Audacity audio editor, I was then able to time-shift transmissions originating from the Earth by the light delay of 1.2865 seconds to reproduce what Buzz Aldrin and Neil Armstrong heard through their headphones in the cabin of the Eagle lunar module on the surface in Mare Tranquillitatis. During the landing phase, an on-board tape recorder in the lunar module captured the voices of Armstrong and Aldrin even when they were not transmitting on the air to ground link. From this noisy source, I have restored the few remarks by Armstrong which were only heard within the cabin. This is, then, the lunar touchdown as heard by the astronauts who performed it.

Now it's obvious what happened to Armstrong's post-landing transmission! Right before he began the call, Duke's message, sent a second and a quarter earlier, arrived at the Moon. While, from an earthly perspective, this was spoken well before Armstrong said “Houston”, on the Moon this message “stepped on” the start of Armstrong's transmission (especially considering human reaction time), and caused him to pause before continuing with his message. Note also that on the Earth-based recording, Duke's response occurs almost immediately after the end of Armstrong's transmission, but on the Moon, the astronauts had to wait for the pokey photons to make it from the home planet to their high gain antenna on its distant satellite.