# Q & A: Does light lose energy as it travels?

Q:
How can a photon travel for billion of years with the same speed, without losing enery? What is the source of energy for photons?
- Zacki (age 23)
Mumbai, India
A:

Hi Zacki,

Let me turn your question around: in a vacuum, why would a photon lose energy? Even a baseball in space won't lose much energy, since there isn't any air resistance, friction, etc. The baseball will interact with radiation pressure, however, so it might lose energy slowly over thousands of years.

Photons, however, don't interact strongly with anything except charged particles. When they travel through empty space, one might expect that there is no mechanism by which they can lose energy.

Actually, however, there is one way that photons do lose energy as they travel through space. Because the universe is expanding, the photon's wavelength increases very slightly over time, and in so doing loses a bit of energy.

For the record, the source of a photon's energy is the "flashlight." For example, accelerating charges, hot objects, and particle decays can all lose energy by radiating photons. These photons are simply packets of electromagnetic energy.

David

(published on 04/16/2013)

## Follow-Up #1: why is light speed constant?

Q:
after reading this thread http://van.physics.illinois.edu/qa/listing.php?id=22284 i want to ask something that cross my mind about the speed of light. sorry if this is a dumb question. is it true that the constant value of light speed was a mere agreement to create some kind of unchangeable godlike unit that 'literally' govern all. for example like the 'black hole accelerating light' case, we know something change. but we don't truly know what change. the speed or the distance. but again, since we assume/decided that c always constant 'no matter what', then we assume it must the distance that change. so back to the question. is it true that basically, c is constant because we said so? or maybe i should say, is c a mathematical constant that we need to complete the equation? (i a mean mathematical constant as in phi) once again, sorry. be easy on my stupidity. ^ ^
- angling kusumo (age 36)
indonesia
A:

It's true that we have some choices about what sorts of coordinates to use, so we don't absolutely have to say that the speed of light is constant. Still, there are some natural choices. Say that you make a whole lot of meter-sticks in some factory, all just the same. Put little mirrors on each end. Then you make a whole batch of identical clocks, and put one one each meter-stick. Then these  timer-sticks get distributed around to different places and set in various state of motion. Each one can measure a speed of light by seeing how many clock ticks happen as the light bounces back an forth one meter-stick length. They all get the same result. That's what we mean by the constancy of the speed of light. It's a physical fact, regardless of how you express it.

Mike W.

(published on 10/13/2013)