gilesgoat wrote:you mean that for 3 millions of years that photon did not loose any energy at all
Yup, pretty much.
After all, when you say "losing energy", what's the photon losing energy to? Where would that energy be going?
Conservation of energy. In order to "lose energy", that energy needs to be going somewhere or converting to something.
But it's in the hard vacuum of space - so where would that energy be lost? Where would it be going?
This is a thing on the International Space Station. They have these big infrared radiators on the station, to radiate heat out from the station into space. Because the problem is that humans are warm-blooded - and, also, our electronic equipment generates heat - but they are surrounded by the hard vacuum of space. By convection / conduction, there's nowhere for this heat to go. So they have to convert it into infrared heat - photons - and then dump it, en masse, from big radiators. Otherwise, the temperature would just go up and up and up, eventually cooking the astronauts and cosmonauts on board.
It's one of those mad things that you ordinarily don't think about. Because, down here on Earth, we're surrounded by atmosphere and objects, which convect / conduct the heat away - often annoyingly so, in deep Winter, when you're trying to keep the house nice and warm - so we're used to the idea that there's always energy loss. Heat loss, photon loss over distance, a thrown object (like a baseball) eventually slowing down and coming to rest, etc. but that's because we're on a planet, in an atmosphere. So there's always friction and absorption and so forth going on.
Indeed, that was the measure of Newton's genius, in his First Law, that he told us that an object in motion stays in motion - or an object at rest stays at rest - unless an external force acts upon it. And, in space, there generally is no external force there. So, yup, if you threw a baseball in space, then it just keeps going. Indefinitely.
(Although, the way I like to reinterpret the First Law is just that an object in motion stays in motion - and we don't need the bit about "an object at rest stays at rest", if you just consider "at rest" to mean a motion of zero. An object with a motion of zero stays with a motion of zero, unless an external force acts upon it. Or, more generically, motion is preserved (because in the absence of anything external to come along and change things, the energy is conserved, as it has nowhere to go). Ah, this is a more Einsteinian way to think of it, where Einstein points out that there's really no such thing as "at rest" anyway - at rest compared to what? You might feel you're "at rest" right now, sitting in your chair, but you are, of course, actually on a spinning planet, orbiting the Sun, which is orbiting the galaxy, which is moving through the universe. Hence the name "Relativity" because, well, it's all relative, innit?)
Which is so unintutive to our experience here on Earth, where things always eventually slow down. Friction, air resistance, gravity. But his amazing insight - particularly for how many centuries ago Newton was predicting this - was that, absent all that crap we have down here on Earth, that baseball really would just keep going. Never losing any kinetic energy... not until it comes into contact with something or is perturbed by the gravity well of some object along its path.
(Worth noting that plenty of photons won't actually make it. There is the odd stray hydrogen atom or two floating around in the void, that a photon might encounter. But, of course, by definition, we never see those here on Earth. A variation of the Anthropic Principle - we only see the photons that make it, because if they don't make it, then we never see them.)
"Words ought to be a little wild - for they are assaults of thought on the unthinking."
- John Maynard Keynes
