SciTech

Information has a finite speed

Credit: Tami Tedesco/ Credit: Tami Tedesco/

Light is the fastest thing in our universe. Its speed is a universal constant that no object with mass can attain or surpass. We know these truths thanks to Einstein and his brilliant theories of special and general relativity, published more than a hundred years ago.
There is one byproduct of Einstein’s theory that is surprising, perplexing, and unintuitive. The speed of light isn’t a cap solely on objects traveling in space, but also a limit on the propagation of information.

What is information? Is it the words in a textbook, the content of a coded message, a friend’s secret, a song, or ones and zeros in a computer? The answer is yes — those are all instances of information. Information to a physicist is anything that tells observers about the universe surround- ing them.

Let’s consider you as an observer. You may be reading this article on a laptop, on a phone, or in print. Your eyes see its black text thanks to light — the same light that has a nite speed. Now think about the world you see around you: plants, people, the sky, your hands. All that information is delivered to your eyes by light. So, your ability to receive information about what surrounds you is limited by the speed of light.

This becomes more obvious when distances become unimaginably large. The Sun is 93 million miles away from the Earth, so its light takes eight minutes to reach us. When we look up, we see the Sun as it was eight minutes ago. So, our information here is eight minutes late.
Our closest neighboring galaxy, Andromeda, is 2.5 million light-years away, where one light-year is the distance light travels in a year — six trillion miles. So Andromeda is 2.5 million times six trillion miles away — that’s really, really far. Since it takes light 2.5 million years to reach us from Andromeda, we see it as it was 2.5 million years ago, when humans didn’t event exist. Now, our information is lagging behind by millions of years.

This effect increases as we look farther out into the universe. In fact, cosmologists look out far enough to view the universe as it was billions of years ago, close to the time of the Big Bang.
You’re probably wondering how the speed of information relates to, say, a song playing. Sound waves travel at a mere 770 miles per hour, meaning it would take an observer an hour to hear the chords of a song playing 770 miles away. Obviously, as the distance increases, so does the lag time for the sound.

This phenomenon can be extended to anything you can think of. Imagine you and a friend are playing a game of tug of war with a rope one light-year long. You’re tugging with all your might, and so is your opponent. The speed of information tells us that, if the rope were to snap exactly down the middle, you and your opponent wouldn’t notice for half a year. You could still tug with all your weight and not fall, for six months.

And that’s because of the nature of tension, the force in a rope when it’s taut. Tension is the result of the object’s particles experiencing strain and relaying that stress to neighboring particles via bonds. Bonds are made from electrostatic interaction between electrons, and electromagnetism propagates at the speed of light.

No matter what, we can’t learn about an event faster than it takes light to travel. Nothing we observe or feel is truly instantaneous, even when it’s right in front of us.