I'm an A level student looking into the fine tuning of various constants. Physicists explain the extensive effects that would happen if these constants were to be changed/different and hence, how this affects the probability of life existing. What I fail to understand is why, if these constants were to be different, life wouldn't adapt to these changes. If gravity was stronger, then wouldn't the general muscle mass/stability of life be greater through evolution in order to withstand a greater force? Or am I looking at it from the wrong perspective? Some clarification on this would be appreciated.

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    $\begingroup$ It's more fundamental than that: if certain constants were different, it could prevent stars and planets from forming, much less allow liquid water to exist, and then allow for organic chemistry as we know it. $\endgroup$ – Dmitry Brant Mar 27 '19 at 20:22
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    $\begingroup$ Check out UFTmachine.com (non-commercial) to simulate the tuning-in of 6 constants of nature. PC or Mac, no tablets. $\endgroup$ – Kovalick Aug 25 '19 at 4:43

The variation you are talking about here would still be considered relatively 'fine-tuned', in the following sense:

If the strength of gravity was stronger by such an amount such that the processes that govern the formation of stars, planets, complex molecules, and life were relatively unchanged (in that they still take place in a recognizable fashion), then the strength of gravity must be quite similar to what we observe. If this were the case, yes, there is no reason that life might not develop to be a bit tougher.

However, such a difference would have to be very small indeed. Arguments about fine-tuning are based on the observation that even relatively small changes to certain constants would be enough to drastically change the make-up of the universe.

For example, Paul Davies notes that if the strong force were 2% stronger than it is, hydrogen would fuse to form diprotons as opposed to helium as it would be energetically favorable. This would drastically alter structure formation in the early universe, leading to a today where planets do not even exist, let alone weak or strong animals on them. I should note here that the 2% figure quoted by Davies may not be accurate, but this is the idea at play here.

In short, the problems from fine-tuning start to occur far before life would ever develop in the first place.

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    $\begingroup$ also look at the triple $\alpha$ process (en.wikipedia.org/wiki/Triple-alpha_process) which appears terribly fine tuned, and is the only way to make lots of carbon and oxygen, which are life's favorite elements. $\endgroup$ – JEB Mar 27 '19 at 22:16
  • $\begingroup$ The proton-proton chain does start by making a diproton (aka $^2_2\mathrm{He}$), which by the weak force can turn into a deuteron, but normally the diproton just falls apart instead. According to Ben's answer here the number of times a solar core proton makes a diproton before it suceeds in making a deuteron is on the order of $10^{23}$. So stars would have very short lifespans if the diproton were stable. $\endgroup$ – PM 2Ring Mar 28 '19 at 8:13

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