Sorry if this has been covered. I did a search and couldnt find it.
Once light travels from the sun to the earth, does that ray of light live forever? Just speeding around the universe at 186,000 miles a second?
Or does it eventually diminsh, wear out and die?
Regards
I suppose eventually they lose so much energy that they just stop existing. (IIRC, Heisenberg’s uncertainty principle can also be expressed as a function of energy and time.)
It wears out and dies, and darned if it always doesn’t happen right after the warranty expires.
Seriously… when a photon of light encounters matter, it comes to an end. It is absorbed and radiated as either a different photon of light, or some other type of energy. There’s another active thread on this right now with some nice analogies, but I can’t be arsed to dig it up.
There’s no such thing as a ray of light. The waves in the electromagnetic field from a point generating source have lower amplitudes farther from the source, but it’s the mode of vibration in spacetime of the field as a whole which is a photon. Nothing really dissipates or diminishes.
The wavelength increases as the universe is expanding, so that would indicate a loss of energy, maybe more like a lose of entropy, as your not suppose to be able to loose energy.
Um, entropy only goes up. You know, one of those other laws of thermodynamics than conservation of energy?
The wavelength gives the energy density, and the expansion increases the volume, thus conserving energy.
Do photons come in different sizes?
I see the sun as a giant nuclear reactor in the sky, shotting down light beams to earth (photons)
Do these photons vary in size depending on the pulse? Or are they always the same size?
Thank you
You may be interested in this thread: What happens to photons?
The “size” of a photon is about its wavelength. But photons are not hard, with a well-defined edge. Photons are squishy. 
From what I understand, photons don’t really have a property that could correctly be called “size”.
Back to the OP, we know we can see the farthest galaxies, quasars, and other early universe objects. Those photons have been traveling effectively as far as photons can go in our observable universe. They have not faded out or lost energy (despite the efforts of some who wish to believe that the redshift is caused by something other than distance and relative speed).
So, unlike diamonds, photons are forever.