A redshift occurs whenever a light source moves away from an observer.
i.e, The observer sees the light source in the hue of RED if that light source appears to be moving away.
A blueshift occurs whenever a light source moves toward an observer.
i.e, The observer sees the light source in the hue of BLUE if that light source appears to be approaching.
In order for a light source to "move away," a combination of movements interrelating with one another is necessary. Any given light source which travels at the same speed we are and moving in the same direction would then necessarily be in a position neither moving away nor approaching closer—resulting in our purple here and now, perhaps.
How many possible combinations are there in terms of celestial bodies' movement to and from each other? If the universe were as chaotically spread out as it appears to be, we might conclude that any and all possible configurations exist; since we know that galaxies and solar systems actually operate in an orderly manner compressing more or less into a pancake shape and spiraling around a supermassive so-called black hole, we can conclude that the possible configurations of celestial bodies moving toward and away from each other represent a limited set.
If something shifts red—okay, the relation of our movement to each other is such that this light is moving away—which could either mean we are both headed in the same exact direction only we are the slower of the two—or it could mean we are moving in opposite directions. Whichever the case—we are moving apart from each other. Redshift.
Concerning humans and the so called possibility of extraterrestrials. Radio waves. Do they only appear longer to each other? Will what we each experience here and now as "visible light"—by the time it reaches another living solar system—be perceived there as radio waves? Why, certainly. All we must do, is understand that space in no way should be perceived as being "very large," as tradition has unfortunately cemented. Putting it in the most direct and simple terms, there is no such thing as space. Quite naturally, for we all exist solely in time. Time and matter. Now we're getting warmer.
IF we "zoom in" at the stars with a telescope more powerful than any we've heretofore glimpsed through—a question to an astronomer might be—will their radio waves gradually shorten with inverse proportion to our zooming to the point the telescope effectively "tunes in" to the visible light bandwidth as it normally appears in relation to the examined solar system? Is it that even our eyesight itself when operating cannot exceed the speed of light—regardless of how much we "enhance" it through powerful telescopes ? If the latter were to be the case—(and I'm starting to think it is)—then that would mean that by a fundamental property of nature itself (nothing can exceed the speed of light) we as sentient beings may be (fundamentally) incapable of receiving anything but radio waves from those solar systems out there that are redshifting farther and farther away from us. We must not lose sight that we are the observers, here. (Which is why I'm currently obsessed by the idea we might be able to process radio waves by somehow reverting them to their original shorter wavelength through some sort of oscilloscope or something, in order that we might conjure up a visible light image—aka, a "cosmic photograph," if you will—of the system we're receiving the radio waves from.)
So under which circumstances does blueshifting occur? [Answer: Andromeda] I'd have to guess...extremely temporary ones wherein distant solar systems on an overall path of moving farther away from us might appear for a microfraction of a millisecond to be momentarily approaching as said star system whips in its clockwise rotation about its galaxy riding on its spiral arm while we ourselves might be spiralling momentarily towards it until the moment later when this synchronization no longer produces the micro-flash of blueshift.
From what I've been led to understand about the nature of our galaxy and universe and relativity, all stars should be redshifting away from us. Except for one thing—the stars in our own galaxy that we can see represent only the slightest microfraction of the total in our galaxy. The stars we're following along with ("chasing") which are in direct proximity to us on our spiral arm must neither be redshifting nor blueshifting as we're perpetually bound together in this arm of the galaxy spiraling about our core. Welcome to the purple shift.
Redshifting and blueshifting must only be a phenomenon particular to powerful telescope observations. How could anything blueshift at all in a universe where everything is allegedly expanding apart from one another ? You'd think everything would be redshifted.
Redshift, then, must be the normal setting for most everything we see in the night skies; and Blueshift merely a strange exception due to momentary synchronized alignments with other portions of our galaxy which flicker into the blue each time they momentarily appear closer.
Of the 300 billion stars in our Milky Way—we can only see six or seven thousand of them with the naked eye, approximately. And we can only see up to three thousand of those at any given time.
- Only the merest fraction of about .000003% of our galaxy is represented by the visible stars in our night skies.
- Putting that in another context: it means that 99.999997% of our Milky Way galaxy is not visible to the human eye.
It's remarkable to think that all those stars we see at night barely even scratch the surface of our remaining galaxy. We can barely see past the skin of our own atmosphere—and hardly beneath the skin of our galaxy. We really need to think about this more while hoping for a proper sense of perspective to begin developing.
moving out of gravitational fields.
In the theory of general relativity, there is time dilation
within a gravitational well.
Redshifting indicates a slide towards the radio wave end of the spectrum (red) while blueshifting indicates a slide toward the gamma ray end of the spectrum (blue).