The opening scene for Contact is quite possibly the best opening scene in a movie, ever. (When you watch this clip, it’s recommended that you change the resolution to 720p HD and turn up the volume.)
This is a wonderful illustration of the principle that distance (and motion) is equivalent to time. The speed of any signal, whether light or sound or carrier pigeon, is always finite. It takes time for a signal to travel the distance between the source and the receiver, so this means the signal is always telling us something about the past. Back in the days when people communicated with each other using letters, it took a week or two to receive them, depending on how far they had to travel; when the recipients read those letters, they were always reading about something that was recent to the sender, but already one or two weeks in the past for the recipient. Similarly, when we look at the star Betelgeuse, the light that we see has taken time to travel the distance between Betelgeuse and the Earth, and so we are seeing it as it was in the past (at a distance of 400 trillion miles and with light traveling at a speed of 186,000 miles per hour, we are seeing Betelgeuse as it was 640 years in the past). Conveniently, such vast distances are often expressed in units of light-years (the distance light travels in a year), which also tell us at what point in cosmic history we are observing something.
In the opening scene of Contact, the further away we travel from Earth, the older the radio transmissions become. We first hear contemporary (for 1997) music, then the soundtrack gradually shifts to music and news from further and further in our past. Once we get beyond a certain point we hear static, then silence.
But sounds don’t travel through space, I hear some of you saying. True. However, radio signals are light waves, not sound waves. The radio waves, which carry information, are transmitted in all directions and are picked up by a receiver with an antenna (say, an AM/FM radio) that converts the radio signal into the sound you hear. So, in principle, anyone who might be not too far out in space could pick up our terrestrial radio and television signals and learn all kinds of interesting things about the inhabitants of Earth.
Perhaps you are wondering from how far out in space someone could receive intelligent signals from Earth. The scale of the Contact sequence isn’t quite right—it was fudged a bit for creative/dramatic purposes. The truth is, if aliens are zipping past Pluto at this moment, they would receive transmissions from Earth that are from only five and a half hours in the past. Little Green Men near our closest stellar neighbor, Alpha Centauri, would receive transmissions from the year 2009. Civilizations on a planet orbiting the star Alpha Mensae (33 light-years away), however, might catch old episodes of Dallas. Someone passing close to the star 51 Pegasi (50 light-years away) might be aware that someone named John F. Kennedy has been assassinated in a place called Dallas. An alien near the star Regulus (77 light-years away) could be watching images of Hitler opening the Olympic Games in Berlin. Further still, near the star Eta Herculis (112 light-years away), curious beings might just now be detecting Marconi’s radio transmissions. Beyond this distance, the Earth is silent. Considering that our Milky Way galaxy is more than 100,000 light-years across, we’ve barely announced our existence to the neighbors down the street. Not that this is entirely lamentable. Personally, I find it comforting that so little of the universe is aware of the Kardashians.