14 Fine tuning

The Stan­dard Model has about twenty-​​six freely adjustable para­me­ters. Cer­tain com­bi­na­tions of these para­me­ters appear to be remark­ably fine-​​tuned for life.[1–3] As Stephen Hawking wrote,[4]

The laws of sci­ence, as we know them at present, con­tain many fun­da­mental num­bers.… The remark­able fact is that the values of these num­bers seem to have been very finely adjusted to make pos­sible the devel­op­ment of life.

Example 1: Super­nova explo­sions only occur if a cer­tain numer­ical rela­tion involving the dimen­sion­less cou­pling con­stants of the weak and grav­i­ta­tional inter­ac­tions and the proton-​​electron mass ratio are approx­i­mately sat­is­fied. If the weak cou­pling con­stant were too large, the neu­trinos released by the col­lapse of the core of a star could not reach the stellar enve­lope before losing most of their energy. And if it were too small, the neu­trinos would escape with most of their energy. In either case, the star would fail to explode.

Example 2: Stars rely on two mech­a­nisms for trans­porting energy from their cores to their sur­faces: radi­a­tion and con­vec­tion. Astro­nom­ical obser­va­tions indi­cate that only stars which are at least par­tially con­vec­tive have planets. On the other hand, only radia­tive stars explode. So stars of both types are needed, and this calls for a del­i­cate bal­ance between the dimen­sion­less cou­pling con­stants of elec­tro­mag­netism and gravity and the proton-​​electron mass ratio. If gravity were slightly stronger, only radia­tive stars would exist, and if it were slightly weaker, only con­vec­tive stars would exist.

Example 3: The syn­thesis of carbon is a two-​​step process. The first step is the for­ma­tion of an 8Be nucleus out of two 4He nuclei (alpha par­ti­cles), the second the for­ma­tion of a 12C nucleus out of the 8Be nucleus and another 4He nucleus. The prob­a­bility of this process would be extremely small, were it not for two “coin­ci­dences”: the 8Be ground state has almost exactly the energy of two alpha par­ti­cles, and 8Be+4He has almost exactly the energy of an excited state of 12C. In other words, the 8Be ground state “res­onates” with a system com­prising two alpha par­ti­cles, and the excited 12C state res­onates with a sys­tems com­prising 8Be and 4He. The exis­tence of the second res­o­nance was pre­dicted by Fred Hoyle before its actual obser­va­tion, based on the observed abun­dance of carbon in the Uni­verse and the neces­sity for it to be formed in stars. The energy at which this res­o­nance occurs depends sen­si­tively on the inter­play between the strong and the weak nuclear inter­ac­tions. If the strong force were slightly stronger or slightly weaker — by just 1% in either direc­tion — then the binding ener­gies of the nuclei would be dif­ferent, and the req­ui­site res­o­nance would not exist. In that case, there would be no carbon or any heavier ele­ments any­where in the Uni­verse. “I do not believe,” Hoyle con­cluded,[5] “that any sci­en­tist who exam­ined the evi­dence would fail to draw the infer­ence that the laws of nuclear physics have been delib­er­ately designed with regard to the con­se­quences they pro­duce inside the stars.”

It is self-​​evident that the actual fea­tures of the Uni­verse impose con­straints on the laws by which it is governed.

If the uni­verse con­tains (carbon-​​based) life, and if it is gov­erned by gen­eral rel­a­tivity and the Stan­dard Model, then the adjustable para­me­ters of these the­o­ries must be so con­strained as to allow for the evo­lu­tion of life. This truism is known as the “weak anthropic prin­ciple.” What is nev­er­the­less remark­able is the number of con­straints that have been uncov­ered and, in con­se­quence, the extent to which those para­me­ters are fine-​​tuned for life.

But there is no need to invoke life. It suf­fices to have objects that have spa­tial extent, that are com­posed of a finite number of objects without spa­tial extent, and that nei­ther explode nor col­lapse as soon as they are cre­ated. If such objects are to exists, we must have quantum mechanics, spe­cial rel­a­tivity, gen­eral rel­a­tivity, and the Stan­dard Model — the latter two at least as effec­tive the­o­ries — and the adjustable para­me­ters of these the­o­ries must be so con­strained as to allow for the exis­tence of such objects. Quantum mechanics pre­sup­poses macro­scopic objects, including objects that can func­tion as outcome-​​indicating devices, and it seems all but cer­tain that the exis­tence of such devices calls for ele­ments whose exis­tence depends on stellar nucle­osyn­thesis and super­nova explo­sions. If so, it calls for some of the same fine tuning that has been shown to be nec­es­sary for life.


1. [↑] Barrow, J., and Tipler, F. (1986).The Anthropic Cos­mo­log­ical Prin­ciple,Oxford Uni­ver­sity Press.

2. [↑] Gribbin, J., and Rees, M. (1989). Cosmic Coin­ci­dences, Bantam Books.

3. [↑] Davies, P.C.W. (2007). Cosmic Jackpot: Why Our Uni­verse Is Just Right for Life, Houghton Mifflin.

4. [↑] Hawking, S. (1988). A Brief His­tory of Time, Bantam, p. 125.

5. [↑] Hoyle, F. (1959). Reli­gion and the Sci­en­tists, SCM Press.