I preface this by admitting that when it comes to astrophysics, physics, and pretty much all hard sciences I’m a dolt. The big questions and answers leave me drooling.
I say this to warn you in case you find that your answer does not penetrate my thick skull.
But the question is in the title…does light exist forever? I find it really unfathomable that we are seeing light now that was generated millions of years ago (when we look at faraway stars). FIrst of all, do light waves count as “matter”? And if so, are ya telling me that this matter didn’t scatter into different particles of whatever the hell it is over millions of years, but hung together and kept traveling all that time? Will it keep going? Where will it end up?
I have to stop now, I’m getting a really major headache.
The Law of Conservation of Matter & Energy states clearly that no form of matter or energy can ever be destroyed. It can be moved, dispersed, altered in for or frequency, & matter can be turned into energy or energy into matter–but neither can ever be destroyed.
So, light goes on forever, unless it turns into something else, and then it goes on forever, but not as light.
Sometimes a photon will hit an atom, be absorbed (giving its energy to the atom for a while), and be re-emitted. Then you would probably not say it’s the same photon.
But space is incredibly empty. In between planets, there are quite a few atoms–around five per cubic centimeter. But then, think how small an atom is. . . that’s a lot of empty space.
Interstellar space is emptier: about one atom per cubic centimeter.
The space between galaxies is even emptier: about one atom per cubic meter.
And a photon has to have just the right energy to be absorbed by an atom. If it’s the wrong wavelenth, it’ll just float right on through, like the atom isn’t even there. That’s how light gets through the Earths atmosphere to us on the ground.
There are photons that have been traveling without hitting a single thing since 300,000 years ago–that’s the cosmic microwave background. We are constantly bathed in these low-energy photons which have traveled billions of lightyears. For that matter, the light of galaxies billions of lightyears away is constantly beaming down on us, too feeble to detect, drowned out by the light of the Sun by day and the stars by night.
(Now you gone and done it, got me waxin’ poetical . . .)
However…
Just as there is some (ie, nonzero) probability of a particle pair emerging from “empty space” via a “vacuum fluctuation,” isn’t there some nonzero probability of photon absorption “into” space itself? And as long as the probability is indeed nonzero–given enough time, IT WILL HAPPEN. Correct?
Granted, the universe may not exist long enough outside its black hole mausoleum for this to happen even once.
Light lives a very long time. Photons are considered “stable” in many tables (energetic ones can decay into an electron positron pair, however). They certainly don’t decay significantly over the lifespan of the universe.
The microwave background photons are much older than 300,000 years. Hell, you can see photons from the Andromeda galaxy with the naked eye, and they are millions of years old. The microwave background is about as old as the universe as a whole, the estimated age of which has been about 15 or 20 billion years through many refinements of cosmology over the last decades. Note I am using the American sense of “million” (10^6) and “billion” (10^9).
Light isn’t “matter” according to the physical definition of matter as stuff that has rest mass. Photons have zero rest mass - if you stop one it disappears into some other form of energy.
Because the walls of the room and everything else in the room, absorbs this enegery and retains it as heat. I suppose if you had an etire mirrored room you could do it until the light hits the original emitter since that couldn’t be mirrored