Quote: You mean that to see real red you need somebody who sees 7671 angstrom?
This raises a point that's been largely ignored in this thread: where does the reality of color lie? I would argue that color perception is largely subjective and, as a result, it doesn't make much sense to argue that the red one person sees is less "real" than the red another person sees. When someone says, "There goes a red balloon," they're saying the balloon looks red to them. That's a subjective judgment and it's very different from saying, "There goes a balloon that is brightest at 650 nanometers."
In truth, color is not synonymous with wavelength and wavelength is not synonymous with color. While it is true that, under "normal daylight conditions," humans tend to see an object that does a really good job of reflecting 650 nm wavelength radiation as red, it is also true that not everybody will see red when looking at that object. And under different conditions, many humans would see that same object as a different color. One could even imagine a different life form that would perceive that same 650 nm radiation as a smell, a taste, a sound or as having some other sensory quality that we humans can't even begin to imagine.
The question of how an object interacts with electromagnetic radiation is very different from the question of how a human being will perceive that object. The spectral profile of a balloon can be objectively measured. It's equally possible to measure the spectral sensitivity of an individual person or a group of persons. The science of visual perception allows us to make reasonable predictions about what most people will see under specific circumstances. But real world experience also teaches that no two people are exactly alike. If you measure the spectral sensitivity of a group of people under various lighting conditions, no two will exactly match. In fact, as you increase the test population, you increase the number of people who will have significantly different spectral sensitivity profiles than the majority of subjects.
How do you characterize the results of that test? If 20 people look at a balloon and say, "That balloon's red," but two test subjects look at the same balloon under the same conditions and say, "That balloon's purple," do you conclude that the purple people are wrong...or lying...or seeing something that's less real? I would argue, no. The perception of purple is just as real as the perception of red. And with further testing, you may be unveil a physical explanation for why the minority saw a different color than the majority. That said, the minority's experience is no less real. Nor is it wrong. It's just different.
The recent trend of this thread has been something of an apples and oranges debate. On one hand, there's the debate about human visual performance as objectively tested under various lighting conditions. Scientific research in this area informs our understanding of color perception but it does not tell the whole story. On another hand, there are countless reliable reports by observers who have seen a broad range of colors when looking at celestial objects. These subjective reports also inform our understanding of color perception while also not telling the whole story.
To answer the question, "Is is possible to see nebulae with color?" all you have to do is ask an observer if he's ever seen color when looking at nebulae. Google "colorful nebulae" and in a fraction of a second you'll get about 220,000 responses in the affirmative. The fact is, observers see color in nebulae. Accepting this fact does not, however, tell us how or why people see color in nebulae. That's a different question; it's the question that's been at the focus of most of the recent discussion in this thread. Unfortunately, that very interesting discussion is being muddied to some extent by the debate about whether some observed colors are more or less "real" than others. That's a debate that never gets resolved in online forums. Google "green stars" if you want to get a sense of the vast bandwidths and rivers of time that have been expended in the effort to count the angels on the head of that pin.
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Re: Is it possible to see the nebula with colour?
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