Number fourteen in the series of 'coincidences' that allow us to exist. From George Greenstein's THE SYMBIOTIC UNIVERSE.
"The second difference is that the neutron outweighs the proton. This fact is somewhat suprising, for as explained in Chapter 11 both the neutron and proton are themselves constructed of yet smaller particles known as quarks; there seems no fundamental reason why the neutron should be the more massive of the two. Furthermore, the mass difference is quite small, a mere tenth of a percent. One might think it would make no difference. But it does make a difference. Indeed, it is crucial.
"In one hand I hold a stone, in the other a hammer; whacking the hammer against the stone I break it into two parts. The splitting apart of the neutron is analogous to the splitting of the stone, and it is important to note that this stone cannot be broken into pieces BIGGER THAN IT IS. Similarly, the neutron is free to decay only because it is heavier than the proton. Indeed, it outweighs the proton and electron combined.
"But what if it did not? What if the proton outweighed the neutron?
"Within both atomic nuclei and neutron stars few differences would result. But for particles in isolation the situation would be reversed. Now it would be neutrons that were stable and protons that decayed--they would decay into neutrons. And isiolated portons are quite common in nature. They are found in hydrogen. The atomic nucleus of hydrogen consists of nothign more or less than a single proton. That element therefore depends on its existence on the stability of the proton, and this in turn depends on that microscopic difference between the proton and the neutron. If the difference were reversed, hydrogen would not exist.
"Chapter 4 discusses the properties of water, and their relevance to the requirements of life. That discussion left something out, though; The stuff has to exist. But water is H2O. If hydrogen did not exist, water would not exist.
"Speculations have occasionally been made that some form of silicon-based life form might be possible. It is an open question whether these speculations are to be taken seriously, and if so whether such beings could get along without water. Let us grant both for the sake of argument. Even so, however, the bsence of hydrogen would prove fatal to such hypothetical beings, for whatever the details of their biochemical organization, they still would require an outside source of energy--a sun. BUT THE SUN IS MADE OF HYDROGEN.
If at this very moment some mysterious process were to increase the proton mass relative to that of the neutron, little would happen for several minutes. But after that interval of time the protons in the Sun would commence decaying. Soon the Sun would be made not of hydrogen but of neutrons. And what would such an object be like?
"The central difference between such a beast and the true Sun lies in the insulating properties of the material of which they are composed. In this regard the Sun is much like a house. In constructing a house it pays to install good insulatio, for if one does not the building is unprotected against the cold of winter. The Sun, likewise, floats in the absolute zero of space; and the heat it loses must be replenished. But that reheating takes energy, fuel; and there is only so much of it available to the star. Ultimately the Sun is going to run out of fuel. The poorer the insulation, the more rapidly will this occur....andthe more rapidly will the light of the world go out.
"Neutrons, it develops, make terrible insulation--hundreds of millions of times poorer than the material of which the Sun is actually composed. If the protons in it were to decay, the Sun would commence burning fuel at a rate so enormous that within a century it would be entirely exhausted. And after that time the Sun would go out.
"Not could the Sun have survived more than a century after its formation. But one hundred years is not long enough for life to develop, let alone for evolution to proceed. In biological terms it is the merest flicker of an eyelash. Furthermore the Sun is not alone in this regard, for most stars in the sky are composed of hydrogen. none could have existed beyond that microscopic interval of time had the proton outweighed the neutron. Of course, certain other stars, the red giants such as Capella, Pollux, and Aldebaran, are composed not of hydrogen but of helium. They would hardly be affected in such a circumstance. But while red giants play a vital role in preparing the cosmos for life they do not make suitable seats for it, and for the same reason; They do not survive long enough. The same is true of every other stellar type. Only stars like the Sun, those composed of hydrogen, maintain themselves for the bast intervals of time required for the full developement of life about them; and these in turn exist only by virtue of that tiny offset--one tenth of one percent--between the masses of two subatomic particles."
Barrow and Tipler, in the COSMIC ANTHROPIC PRINCIPLE, were even more pessimistic.
"The other part of the nucleosynthesis coincidence arises because the neutron-proton mass difference is -mE(nuetron is a proton minus the mass of an electron, roughly.) In fact, this is only part of a very delicate coincidence that is crucial for the existence of a life-supporting enviornment in the present day Universe. We find that
m(Proton) -m (electron)=1.293 MeV -0.511 MeV = 0.782 MeV
Thus, since m(n)(mass of neutron) and m(p) (mass of proton) are of order 1 GeV the relation is a one part in a thousand coincidence. IF instead we found that m(P) -m(E) < or ~ 0 then we would not find the beta decay n->p + e- + v(e), occuring naturally. Rather, we would find the decay p + e- -> n + v(e). That would lead to a World in which stars and planets could not exist. These structures, if formed, would decay into netrons by pe- annihilation. Without electrostatic forces to support them, solid bodies would collapse rapidly into neutron stars (if smaller than about 3 solar masses) or black holes. Thus, the coincidence that allows protons to partake in nuclear reactions in the early universe alos prevents them decaying by weak interactions. It also, of course, prevents the 75% of the Universe which emerges from nucleosynthesis in the form of protons from simply decaying away into neutrons. If that wer eto happen no atoms would ever have formed and we would not be here to know it."
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