Why is it that when scientists look at very distant galaxies through powerful telescopes they see the galaxies as they were millions or billions of year ago?

Answer 1

Because that's how long it has taken the light from those galaxies to reach us.

You don't 'see' something until the light from it reaches you. If the thing is on the moon,

it takes about 1.27 seconds. It it's on the sun, it takes about 81/3 minutes.

If it's on the nearest star outside our solar system, it takes about 4.2 years. If it's

in the nearest galaxy outside of ours, it takes about 2.5 million years. And if it's in

the farthest galaxy that our present technology is able to detect, it takes about

14 billion years.

To get here at the speed of light.

Here's another way to start understanding it. You get a letter in the mail from

an old friend. In the letter, he writes "Today we had a terrible storm here. The

wind was howling, the thunder and lightning went on for hours, trees were blowing

by in the sky, and by the time it was over, we had almost 30 inches of rain."

When did the storm happen ? When you opened the envelope, took out the paper,

and read the letter, it said "Today". But you know very well that the storm didn't

happen "today" because it took some time for the letter to reach you.

And that's the whole idea. Light takes time to move from place to place. It's

awesomely fast ... like 7 or 8 times around the Earth in one second ... but it does

take time to cover a distance. You can never see anything at exactly the same

time as it happens. You have to wait until the light from it reaches you.

Answer 2

This phenomenon is due to the finite speed of light. Light from distant galaxies takes time to reach us, and the farther away a galaxy is, the longer it takes for its light to reach Earth. So when we observe faraway galaxies, we are seeing them as they were in the past, billions of years ago, because that's how long it took for their light to travel across the vast distances of space to reach us.