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From maxwell's equations, it occurred to me that photons are stable. Decrease in electric field creates magnetic field and vice versa and somehow there is a harmony that allows photon to exist as long as it travels with c. Therefore I wouldn't think photons would decay as they travel.

However, I know that radiation decays with 1/r from the source and a photon is a form of radiation. Does that mean photons can travel in space with c, but they also have to decay? I kind of imagined them like harmonic sine waves.

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    $\begingroup$ Please ask the 2nd part as separate question - it is a good question and deserves its own answer. $\endgroup$ – Martin Beckett Sep 19 '15 at 18:31
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No - assuming they don't hit anything they don't decay.

The distance dependant "decay" is the drop in the number of photons per volume as the volume gets bigger - it's not a decay of individual photons. It's the same as a crowd dispersing as it leaves a subway exit - nobody is disappearing.

Photons can lose energy as they collide with gas or dust in space but can also gain energy if they interact with a high energy particle - so the wavelength of light from a distant object can change.

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    $\begingroup$ Photons also change their wavelength over time as the space around them expands. $\endgroup$ – John Dvorak Sep 20 '15 at 1:23
  • $\begingroup$ @dick that's an interesting idea. Both the expansion of the universe and a traditional Doppler shift expand all wavelengths by the same factor. Perhaps you could derive from the expansion that the edge of the visible universe is moving away from you at just below the speed of light, just enough so that the gamma radiation to be red-shifted to microwave wavelengths. I'm not sure if the math works out well enough. Also, the Wikipedia article on cosmic microwave background does not suggest Doppler effect as the primary mechanism. In fact, the observable universe is equally big in all directions.. $\endgroup$ – John Dvorak Sep 20 '15 at 6:17

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