Taffy Photons and
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In many cosmology texts the authors confront a puzzle. Since 1965 astronomers have known that the sky glows with an almost perfectly uniform radiation like the radiation that would emanate from a perfectly black body at a temperature of 2.725 Kelvin. But that radiation must have come from a Universe-filling plasma that had cooled to a temperature of about 3000 Kelvin. How did the radiation become so much colder?
Many scientists see a possible answer and an explanation of that answer in one of the fundamental properties of light, the fact that it has a wavelength. Light sprawls, however minutely, over space. And that space, we know, expands continuously. Thus some cosmologists have inferred that as light crosses the vast depths of space, the expansion of that space stretches its wavelengths, making it cooler.
Can light stretch as it crosses expanding space? We might feel a temptation to answer yes, but then several facts from the quantum theory stymie us. We know that light comes in packets, that we call photons, that possess definite momenta and energies. So an affirmative answer to the above question imputes to the Universe violations of the conservation laws pertaining to linear momentum and energy. Further it gives us a violation of the second postulate of Relativity (by requiring a part of a light ray to move faster or slower than another part). The idea that expanding space stretches the wavelength of light (and carries light x-1 steps away from us for every x steps it moves toward us) reflects the Šther, which Einstein dismissed as irrelevant, sneaking subtly back into physics.
We seem to have a metaphor here that presents light as a pattern printed upon a stretching fabric. Among the wondrosities that awe us in this image we find the idea that light can behave like taffy; that is, that it can stretch. Well, certainly it can stretch in a gravitational field: the gravitational redshift gives us a key piece of evidence favoring General Relativity as a true description of Reality. But expanding space differs from gravity in not connecting to a massive body that can manifest energy and momentum in order to satisfy the conservation laws as light loses energy while climbing out of the gravitational field.
In order for that taffy metaphor to serve as an accurate representation of Reality we must in some way understand light to be attached or bonded to space and not merely passing through it. But space has no fabric that would support an attachment. An inertial frame is a ghost, not a substance.
And what can we say about "tired light"? What does that phrase mean? It could only mean that light, in its long voyage, has done work upon something. Could empty space exert a kind of friction upon light passing through it? No, empty space contains nothing upon which light can do work. Even the graininess attributed to the quantum vacuum, a kind of Štherial roughness, won't impede the flow of light. And light cannot do work upon space itself, because space cannot manifest energy and, thus, any work that light does upon space vanishes from Reality in violation of the conservation law pertaining to energy. Light simply does not do any work as it travels through cosmic space, so it can't get "tired".
So if expanding space doesn't stretch photons and light doesn't get tired as it flies, then how can we account for the observation that the cosmic background radiation appears over one thousand times colder than we expect? Let's answer that question by looking at the correct analysis of the cosmic redshift.
Imagine flying away from the sun at a speed close to the speed of light relative to the sun. Now, in your mind's observatory, look back at the sun and measure its temperature. You can only do that by measuring the temperature of the radiation and you find that the radiation from the sun has a temperature substantially lower than the 6000 Kelvin that your astronomy textbook gives as the temperature of the solar photosphere. But the light that you observe has not had time to stretch or to grow tired as it crosses intergalactic space. Indeed, the light has done nothing at all except propagate through space: the "cooling" that you infer from your observation comes from your having put yourself in an inertial frame in which the sun's light is already cold and is so because of the Doppler shift of the wavelengths relative to the wavelengths the light manifests in the sun's inertial frame.
We actually tripped ourselves up with the wording of our question "How did the radiation become so much colder?" In the stating of that question we have tacitly assumed that the radiation did, indeed, begin at a high temperature and evolve to a lower temperature. We need to use a question that carries fewer implicit assumptions (or makes them more explicit), such as, "Why does the cosmic background radiation appear so cold when we calculate that it must have had a higher temperature when it decoupled from matter?" Not using the comparative form of the word "cold" removes the hidden assumption that the light has undergone some change.
Astrophysicists might still answer that question with some light-weakening phenomenon, but I believe that "It must be some kind of illusion" might also more likely have occurred to them. And that is the answer that I present: it's all an illusion caused by the relativistic expansion of space. Cold photons passing us now are still hot relative to the galaxies (which are receding from us) that they passed eons ago. Likewise, photons that were hot when they passed our galaxy eons ago appear colder to observers in more distant galaxies. The energy in the radiation, measured relative to Earth, has not changed over time. And of course the more distant the point of decoupling lies from Earth, the faster it recedes from us. That fact means that the light coming out of more distant regions of space is even colder than the light from nearer regions and it comes to us later both because it originated so far away and because time dilation delayed its release.
illustrates the fact that we have not fully incorporated the concepts of
Relativity into our scientific culture and into our ways of thinking. Those
concepts and their mathematical elaborations go so contrary to our intuitions
that even people, such as astrophysicists, who know them intellectually
sometimes seem not to understand them at all. But we can hope that, eventually,
our scientific Rationalism will correct that error and thereby solve the problem
that it creates for our science.
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