Union University
Union University Department of Physics

The Science Guys

Science Guys > June 2000

June 2000

Why do astronomers say that when we look at the sky, we are looking backward in time?

The Cosmic Background Explorer satellite (COBE) was thrust into space atop a pillar of fire. Then the booster, Atlas rocket 189, ejected COBE and the satellite’s two solar panels unfurled like the wings on an insect. After a series of tests, COBE’s cryocooled infrared and microwave sensors were deployed to snap photos of the universe. The millions of dollars spent on the COBE project were intended to accomplish one thing - to take baby pictures of the Universe, to look back in time, and see the universe as it was a very long time ago. How is it possible to see back in time?

To answer this question we must turn to a relationship of science that involves the basic concepts of space (distance) and time, and ties them together through a term called speed. In science, speed is equal to the distance traveled divided by the time traveled. Using this relationship we can ask how long something takes to travel a certain distance. If we know the speed of the object, then the time can be calculated by dividing the distance by that speed.

When observing through a telescope, we are seeing light that left some place in the past. Light travels at a specific speed in space (186 000 miles per second). If we know the distance to the point of origin, we can calculate the time it takes that light to get to Earth. For example, the Sun is 93 million miles away. When we look at sunlight, the light that we receive was emitted from the Sun a few minutes ago. (Actually 93 million miles divided by 186 000 mps is 500 seconds or a little over 8 minutes.) Thus, we are actually perceiving what happened on the Sun 8 minutes ago.

Perhaps as a more obvious example, assume three friends send you a local newspaper every day; one lives in Memphis, one in Denver, and one in San Francisco. The papers all travel by trucks going 60 mph. When you receive the papers every day, each copy will be a few days old. The paper from San Francisco will be older than the paper from Memphis since it has taken longer to get here. On any given day you can read yesterday's news in the paper you received from Memphis but the news in the San Francisco paper is five days old. This is the way it is with light. Light from nearby stars take less time to travel to Earth than light from distant stars.

After the Sun, the next closest star, Proxima Centauri, is 25 trillion miles from Earth. Light from that star takes 4.3 years to reach us. Thus, when we observe this star we are seeing it as it was 4.3 years ago. We do not know what is actually happening there now. Astronomers actually use the time it takes light to travel as a distance measurement - a light year, the distance light travels in one year (about 6 trillion miles).

When astronomers view an object 3000 light years away, they are looking at light that left that celestial object 3000 years ago. Hence they are looking at history or back in time. We are constantly trying to develop bigger and better telescopes to see farther away (farther back in time), closer to the beginning of the universe, that point in time when God said fiat lux (’let there be light’).