Number fifteen in the series of 'coincidences' that allow intelligent life to form in this universe.
From George Greenstein's THE SYMBIOTIC UNIVERSE.
"...a quite extraordinary matching between the properties of the Sun and those of plants. Failing that match, photosynthesis could not take place."(For those of you reading this, this will NOT be carbon-centric or earth-centric. Keep reading.)
"Chlorophyll is the molecule that accomplishes photosynthesis. This molecule is universal in lants, and it is what gives leaves, grasses and the like their distinctive green color. The mechanism of photosynthesis is iniaited by the absorbtion of sunlight by a clorophyll molecule. But in order for this to occur, the light must be of the right color. Light of the wrong color won't do the trick.
"A good analogy is that of a television set. In order for the set to receive a given channel it must be tuned to that channel; tune it differently and reception will not occur. It is the same with photosynthesis, the Sun functioning as the transmitter in the analogy and the clorophyll molecule as the receiving TV set. IF the molecule and the Sun are not tuned to each other--tuned in the sense of color--photosynthesis will not occur.
"As it turns out, the Sun's color is just right. Light from the Sun is readily absorbed by chlorophyll, so initiating the photosynthetic process. On the other hand, the Sun's color is related to its temperature: Things heated moderately glow a dull red, but if heated yet more glow a brillant yellow. Thus the matching under consideration is between the temperature of the Sun on the one hand and the molecular structure of chlorophyll on the other. Without that matching, life could not exist upon the Earth.
"Nor could life exist anywhere else. It is not a matter of seeking out the correct niche--some star of the right temperature. As emphasized above, only stars made of hydrogen are suitable seats for life, and it turns out that all such stars have roughly similar clors. Fomr the coolest to the hotest, the variation is not that great. Thus starlight impinging upon all planets, wherever they may be in the universe, but a roughly similar color. Either all stars provide good niches or more of them do.
"One might think that a certain adaption has been at work here; the adaption of plant life to the properties of sunlight. After all, if the Sun were a diffferent temperature, could not some other molecule, turned to absorb light of a different color, take the place of chlorophyll? REMARKABLY ENOUGH, THE ANSWER IS NO, for within broad limits all molecules absorb light of similar colors. The absorbtion of light is accomplished by the excitation of electorns in molecules to higher energy states, and the general scale of energy required to do this is the same no matter what molecule you are discussing. Furthermore, light is composed of photons, packets of energy, and photons of the wrong energy simply cannot be absorbed. The energy carried by these photons, though, depends upon the color of the light. In terms of the above analogy, it is simply not possible to build a TV set capable of receiving signals differing wildly in frequency from the normal. If some station, more enterprising than wise, elected to transmit at such a radically differing frequency, no sets would be capable of receiving its signal. It would be outside their range.
"Similarly, if stars were slightly different in their temperatures no great differences would arise. But the light from radically cooler stars could not be absorbed by molecules of any sort. Photosynthesis could not occur--not the photosynthesis we know, employing chlorophyll, NOR ANY CONCEIVABLE FORM OF IT EMPLOYING ELECTRON EXCITATION IN MOLECULES OF ANY DESCRIPTION. A planet huddled up close to such a hypothetical star, say, the distance of Mercury from our sun, could easily be warm enough to support life, and it might well possess large amounts of water. That planet would have oceans, rain clouds, warm and balmy breezes...but no plants, and therefore, no animals. Alternatively, if stars were far hotter their emission would pack such a wallop as literally to tear molecules apart, dissociate them. A high temperature star would emit not visible lifght but ultraviolet, known to be dangerous; still hotter ones would emit X rays. Planets far from such stars, say the distance of Pluto from our sun, could hold oceans, rian clouds, warm and balmy breezes...but down upon their ravaged surfaces would pour a flood of sterlizing radiation. As things stand in reality, there is a good fit between the physics of stars and that of molecules. Failing this fit, however, life would be impossible."
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