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Does light travel till infinity?

Date: Thu Apr 23 12:53:12 1998
Posted by s gill
Grade level: grad (non-science)
School: No school entered.
City: chd State/Province: PANJAB
Country: INDIA


we hear of telescopes receiving light/em waves from billions of light years away . does this imply that electro magnetic waves keep travelling till infinity?is there any attenuation at all?

All things being equal, light should travel forever.

However, things usually aren't all equal! The Universe is not empty, but instead is filled with dust, gas, stars and other inconvenient obstacles so that light sometimes has a hard time poking its way through. Worse, we believe the Universe itself is expanding. This means light from very far away has to struggle to overcome that expansion to get to us, which means the light loses energy (like you have to use energy to pedal a bicycle up a hill). When light loses energy its color shifts to the red, which is what we mean by "redshift". As you get farther away, the light has to use up more energy to get to us, and so eventually it loses so much energy we can no longer detect it. It's still there, it has simply lost so much energy that it has become too feeble to detect.

There are some people that think that light does "leak" energy after a while. This theory is usually called "tired light", and is used to explain the redshift of light without needing a Big Bang scenario. Most proponents of tired light don't like the Big Bang theory for their own reasons, but of course tired light has many problems of its own. The overwhelming majority of astronomers think that the Big Bang theory is the correct one. It currently explains what we see better than any others.

©2008 Phil Plait. All Rights Reserved.

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