When we observe the world surrounding us, it is not immediately obvious that light takes a finite amount of time to travel through space. Because our immediate environment is so small, travel times are virtually instantaneous. Light can cover 300,000 kilometers per second, a speed which translates to crossing a large classroom in one ten-millionth of a second, a football field in a third of a thousandth of a second, and going once around the entire world in two-hundredths of a second.

We must track signals from as far away as the Moon to find travel delays of order even a second. Once we start to explore the greater Universe, however, the light travel times quickly begin to mount up – 8.3 minutes between the Earth and the Sun, 28,000 years to get to the center of the Milky Way, and 2.5 million years to Andromeda, the nearest large galaxy. When we calculate the time it takes for light to reach us from distant galaxies, we quickly find ourselves working in units of billions of years, an appreciable fraction of the entire age of the Universe.

The result of this is that when we observe the most distant galaxies, we see them not as they are today, but as they were billions of years in the the past. It takes that long for the light emitted by the stars within those galaxies to travel across the Universe and reach us.

If the light from distant galaxies takes billions of years to reach us here on Earth, how long do you think that it takes for the light from our Milky Way galaxy to reach those distant galaxies?