Does Light Lose Energy as it Travels?
Most recent answer: 10/13/2013
- Zacki (age 23)
Mumbai, India
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?
- angling kusumo (age 36)
indonesia
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)