MadSci Network: Astronomy

Re: Why does looking at the most distant Galaxies assume looking back in time?

Area: Astronomy
Posted By: Mark Friedman, Undergrad, Biology
Date: Sun Feb 16 22:03:20 1997
Message ID: 855380635.As

MadSci Network: Astronomy

In order to answer your question, we must discuss a basic property of light. The only way that we can visualize an object is when light reflects off its surface and subsequently enters the human eye.

For now, let's imagine light as a type of wave that moves through space at extremely fast speeds. As fast as light moves, it still takes time for it to cross extremely large distances. For example, light coming from the Sun takes approximately 8 minutes to reach Earth.

Lets think about this for a moment. That means that when you look up at the Sun, the light you are seeing left the sun 8 minutes ago. You are not seeing what exists at this precise moment but what occurred in the past. Now, lets look at a more extreme example. Distant stars are millions of times further away than the sun. This means that quite possibly, it could take 100 million years for light from a particular star to reach us. The light that we see when looking at a star is not the light that was immediately released, but what was released 100 million years ago. In essence, we are looking millions of years into the past.

As one more example, think about this: If at this very moment, a star entered supernova and faded away, we would not be able to visualize this for millions of years. When we finally saw the star disappear, we would be getting a glimpse of an action that occurred long ago. Hopefully this helps.

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