By Dr. Marc Rayman
Asked by our friends at the Children's Museum of South Carolina in Myrtle Beach.
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Well, here's something to think about. When you hear a plane, do you ever have trouble spotting it in the sky? If you look up where the sound seems to be coming from, the plane is no longer there. That is because it takes time for the sound waves to reach you, and by the time they do, the plane has moved on. So, in a sense, you are "hearing back in time," because the sound you hear is the sound of the plane several seconds ago, not the sound of the plane now. When you do spot the plane, you are actually seeing it as it was a tiny fraction of a second ago because just as sound waves take time to travel, so do light waves. Now, nothing can travel faster than light, but as fast as it is, light still needs time to get anywhere, whether it's across a room, across the span of atmosphere beneath an airplane, or across a big part of the universe.
The farther away an object is, the longer its light takes to reach us. When you look across the room, you see something as it was a few billionths of a second ago, and when you look at the moon, you see the light that left it a little more than 1 second ago. If our star, the Sun, were to suddenly burn out, we wouldn't even know it for more than 8 minutes,because the last bit of light that left it would take that long to travel to Earth! Don't worry, though. The Sun is going to last for another 5 billion years or so! But what about all the more distant stars? It takes much longer for their light to reach us. When you look at the stars at night, you are seeing light that may have taken 20 or 30 or even a few hundred years to reach your eyes. You are looking back in time. The distances in the universe are so vast that scientists use the term "light year" to measure them. A light year is the distance light travels in one year, or 10 trillion kilometers. That's 1 followed by 13 zeros!
The Hubble Space Telescope can see objects even more distant than your eyes can. When it takes a picture of a galaxy 100 million light years away, we are seeing the galaxy as it looked 100 million years ago. At the time that light left that galaxy, dinosaurs still roamed Earth and humans would not appear for many millions of years!Because distant galaxies appear to us as they were millions or even billions of years ago, we can study how they change over time.
By looking at distant galaxies, we see what they looked like when the universe was much younger, as galaxies were first forming. As we look at closer and closer galaxies we see how they change as they age, just as looking at babies, children, teenagers, and then adults can show how we humans change as we age.
Find out about some of the fascinating types of galaxies in the universe and make your own glittering galactic mobile.