# Are all effects that change color perception examples of redshift/blueshift?

As defined by Wikipedia:

In physics (especially astrophysics), redshift happens when light seen coming from an object that is moving away is proportionally increased in wavelength, or shifted to the red end of the spectrum.

I understand the premise of the Doppler effect being attributed to redshift as well as the effects of space expansion and gravitational influence on light when speaking of redshift in astrophysics. I gather the basis of the principle as being - an increase in wavelength = a shift to red in the spectrum where alternatively a decrease = a shift to blue (blueshift). Simple concept, but is this right?

If so, my puzzlement now falls upon the seeming similarities in other theories which are used on earth in regard to colour perception, light intensity and the composite structure of photographs.

Consider the following resources: Colour Theory (see Intensity), Colour Appearance, Saturation, Bezold–Brücke shift

As defined by Wikipedia:

The Bezold–Brücke shift is a change in hue perception as intensity changes. As intensity increases, spectral colors shift more towards blue (if below 500 nm) or yellow (if above 500 nm). At lower intensities, the red/green axis dominates.

Can these occurrences, namely Bezold–Brücke shift, also be coined as redshift/blueshift given that the waves emitted by the subject being observed are at varied lengths after absorption and reflection and understanding that the main light source being the sun, can be at variable distances emitting wavelengths (varied after modification by atmosphere) when in contrast to the subject being observed and the observer?

Can the term(s) "redshift/blueshift" be used to explain the various changing spectral phases of our "individually perceived" world at any given moment?

I know this is dodgy, but if you decide to answer, please cater for a layman with relevant information, because I get the feeling that mis-interpretation runs deep in our quest for truth as a race.

The short answer to the (original) title question of "Does redshifts/blueshifts occur on Earth?" is yes, of course it does.

So why don't you notice, right?

The thing to note is that redshift from relative motion is proportional to the relative velocity divided by the speed of light.

The speed of light is 300,000 kilometers per second.

The fastest macroscopic1 objects that people accelerate go a few tens of kilometers per second.

So the ratio is of order $10^{-4}$ for the fastest human driven artifacts in existence. That is a small change.

The business with the Bezold–Brücke isn't a physics effect at all. It is---just as the quote says---a change in perception. That is, it is related to how the human visual apparatus and interpretation works. I'm cettainly not an expert in that field but it appears to just be a way of defining what is meant by "red" and "blue".

1 Of course, particle accelerators send small things to speed that approach that of light very closely, indeed. And under the right circumstances it is possible to observe very high Doppler shifts from some of these particles. But we don't generally bother because there are even more striking relativistic effect at play.

A red light is a red light, and a blue light is a blue light, regardless of the intensity. So the apparent change in hue in the Bezold–Brücke effect is not a property of the light but of human perception (e.g. due to a biological apparatus).

No. Redshift and blueshift refer very specifically to a physical change in the wavelength of a light pulse in flight - respectively an elongation and contraction of that wavelength, hence the names: red light has a long wavelength, so elongation of the wavelength will cause visible light to appear redder, and blue light a short wavelength, so contraction causes it to appear bluer (well, acutally, it cycles through the rainbow but at the far ends becomes red and blue).

There are, as far as I am aware, exactly three mechanisms by which this can occur:

1. the Doppler effect - which is due to the relative motion of source an observer - in particular a moving observer moving into an oncoming wave (or a source moving at hir) will see a blue-shifted wave because sie is able to pass by more peaks of the wave being aided in doing so by the combination of its oncoming motion and hirs together (well actually that is how it is for sound waves, but for light waves it is due to the Lorentz-FitzGerald contraction in that the onrushing observer sees the universe shrunken in the direction of hir motion so the wave is compressed and thus appears bluer, since the speed of light is invariant for all observers.).

2. the gravitational shift - which is due to the presence of a (must be strong to be readily observable) gravitational field - in particular as a photon climbs a gravitational potential difference it loses energy as its energy is converted to gravitational potential energy, thus causing its wavelength to expand by $E = \frac{hc}{\lambda}$. One can also think of it as being due to the gravitational time dilation - in particular if one imagines an antenna at the bottom of a deep gravitational well, a person standing outside will see the alternating current in the antenna reversing its direction of flow at a slower frequency, and thus the EM wave it is transmitting will have to be suitably reduced in frequency so as for the two to match as they must by the laws of electromagnetism. This is a red shift Likewise if a photon is falling down a well to an observer residing within it, the photon will pick up energy and its wavelength will shift. This is a blue shift.

3. the cosmological shift - this is due to the changing scale factor of the Universe. As the Universe expands, so too does the wavelength of photons which are embedded within the space-time (you can think of it as like scaling up a bitmap), and this causes them to acquire a red shift. Were the Universe undergoing a hypothetical contraction toward a Big Crunch singularity, a cosmological blue shift would be seen instead for the same reason - the contracting spacetime would cause the wavelengths to shrink.

In all cases it is due to a change in the wavelength. But perception is a whole different animal that belongs to neuroscience, rather than physics; and thus many other things can affect it, e.g. quirks of the brain's normal operation, the imbibing of mind-altering drugs, pathologies, etc. But these are not redshift/blueshift because those terms refer specifically and exclusively to a physical change in the observed wavelength of light - which must be from one of the three sources above. Absorption and re-emission processes are also not the same thing, that is the light disappearing from the universe followed by new light being created some, usually extremely imperceptibly, short time later - they are not a change of wavelength of light that is in flight.