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Name: Bo
Status: other
Grade: 12+
Location: MT
Country: USA
Date: Fall 2013

What is the energy source of photons traveling billions of light years?

Any photon source (nuclear reactions in a star, chemical reactions such as a fire, hot objects, etc) will create photons that last billions of years. While traveling through space there is nothing to absorb the photons.

Gregg Bradburn

Bo, The photons are not the source. The energy sources are the stars that emit the photons. Some of the photons are the light we see when we look at the stars. Some are too low (infrared, microwave) and some are too high (ultraviolet, X-ray) for our eyes to see. One light year is the distance light travels in one year. The number of light years that the photons have traveled is the distance these stars are from us. These photons just keep going until they hit something. The ones we see are the ones that hit our planet.

Dr. Ken Mellendorf

Hi Bo,

Thanks for the question. Photons are generated by atoms, molecules, ions, or nuclei losing energy and releasing electromagnetic radiation. Photons can be generated by stars, which are common sources of light.

I hope this helps. Thanks Jeff Grell


There are several answers to this question, the first being the simplest­mostly the same sources as we see near us. Stars we see in the night are simply other suns like our own that give off light. The further back you peer in time, of course, the younger the picture of the universe. Photons may have come from stars that have already lived their lives and died in a fiery explosion such as a supernova. There are other, more exotic sources, like quasars or supernovae, but the fact that a photon has travelled a billion years is not in itself unusual. If you are asking if there is a source of energy that keeps light traveling for so long, then we need a different answer. It seems intuitive that light would simply dissipate or “run down” as it travelled through long stretches of space, but it does not. It does not need an additional source of energy to boost its travel across billions of light years of space. If there is nothing to absorb the light (dust particles or such), then there is no known limit to how far light can travel. It is similar to Newton’s first law of motion which says that an object in motion will stay in motion unless acted on by an external force. In other words, if you were floating in space and threw a rock, the rock could conceivably travel billions of miles without any loss in speed. Similarly, photons, while not objects in the sense of Newton’s laws, do not lose energy if they travel in empty space. Now, just because light can travel for long distances without an energy boost along the way does not mean that the light does not become less intense as it spreads out in space. Small intensity is why telescopes need to be large and sensitive in order to see very far into the past. This reduction in intensity is not due to the light losing energy, but rather due to the light spreading in space as it travels. Less energy exists to collect in the telescope dish, and thus a bigger and more sensitive telescope is needed the farther away the light source.

Kyle J. Bunch, PhD, PE

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