The opening scene for Contact

How distance is time in the universe

The universe as it was in the year 702 AD. This is star cluster NGC 3532 as imaged by ESO’s 2.2-m telescope at the La Silla Observatory, Chile.

The opening scene for Contact is quite possibly the best opening scene in a movie, ever.

This is a magnificent illustration of the principle that distance is equivalent to time.

The speed of any signal, whether carried by light waves, sound waves, 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.

When people primarily communicated with each other over long distances using letters, it took days to weeks to receive them, depending on how far they had to travel. When recipients read those letters, they were always reading about events that were recent for the sender, but days to 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’re seeing the star 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, that means we’re always 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.

This is the creative genius of 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, TV shows, 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. Radio waves, which carry information, are transmitted in all directions and picked up by a receiver with an antenna, like 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 doesn’t really tell us, because 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.

Going further out, Little Green Men near our closest stellar neighbor, Alpha Centauri, would receive transmissions from the year 2019.

Civilizations on a planet orbiting the star Alpha Mensae (33 light-years away), however, might think we’re all watching episodes of Cheers or The Golden Girls.

Someone passing close to the star 51 Pegasi (50 light-years away) might be aware that someone named Elvis Presley gave a concert in a place called Hawaii.

An alien near the star p Velorum (87 light-years away) could be watching images of Hitler opening the Olympic Games in Berlin.

Further still, near the binary star system HD 89744 (126 light-years away), curious beings might just now be detecting Marconi’s first 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.

This is an updated and edited repost of an article originally published on my old blog.