Are colors real or just a deception of our brain? © agsandrew/ thinkstock

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Is a tree really green - even if nobody sees it? And when we see colors, do we share this experience with others, or does each individual perceive something different? Mankind has been preoccupied with these questions for thousands of years - and science has given and gives different answers to them.

This is the second part of this article series. If you have missed the first, don't forget to catch up on it:

• How real are colors: All illusion? - Part 1

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Part 2: From the waves to the stimulus. Physics and color perception.

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Is our perception of the senses really only semblance - or is the tree green after all? Can we give our perception of colour objectivity, and if so, in what way? Let us begin with the observation that neither physics nor neurobiology can reduce colors to physical processes, i. e. explain them satisfactorily.

Physics and Perception

Physics has excluded the sensation of colours from its section of the world. What remains are reflections of light rays of different wavelengths at interfaces - but these are not colors. Let's assume that a subject on a meadow would see a green tree: Even a comprehensive physical examination and description of all that is happening outside and inside her body would not allow any statement about her color perception.

Without our experience of color, science would have no reason to suspect its existence - just as little as one could not predict its foul odor from fluctuations in air pressure as a result of hearing sounds or from the structure of hydrogen sulphide. From a scientific point of view, only the necessary prerequisites for qualitative sensory perception can be stated, but they cannot be explained by themselves.

Correlations instead of reality?

Some people will probably read this in a head-shaking way: Isn't it possible to physically measure colours, to produce them by appropriate mixing of light waves of different frequencies or to produce them in chemical processes? Yes and no, because what we measure or produce is only correlated with the colors we perceive - light waves as such are not colored, and even the chemist sees colors as indicators of elementary processes that can be represented by reaction equations but are not colored themselves.

There is no doubt that we need the light waves that irritate the retina so that we can see something, or the sound waves that cause our eardrum to vibrate so that we can hear sounds. But we see no light waves and hear no sound waves, but colours and tones. The fact that these measurable waves themselves are not colored or loud is no reason to deny the reality of colors and tones, as neuro-constructivists do. The waves are just the transmission media for our perception.

Certainly, in the purely physically describable world there are no colours, only their correlates. However, after all, we know a wealth of other characteristics of reality, which also fall through the rather rough raster of physical descriptions - such as the fertility of grapevine, the hierarchy of a pack of wolves, the constitution of the USA or the German export surplus in 2017 - should all this not mean anything, just because physics has nothing to say about it?

Source: Prof. Dr. Dr. Thomas Fuchs, Universität Heidelberg: Ist der Baum Grün?

Part 3: From the stimulus to the subjective sensation