How does weak gravity affect the origin of life?

Answer

It is true that we would not be here if gravity had been a hundred times stronger. A planet like Earth would have had a much thicker atmosphere with a blistering greenhouse effect, and the greater gravity might never have allowed for mobile large creatures. Each time ‘would’, because with such a high gravity stars like the Sun would have burned up very quickly, with too little time to allow interesting evolution in a planetary system. Again ‘would’, because the universe itself would probably have collapsed again shortly after the Big Bang, long before stars could form.

Yet that is not the whole story, because it assumes that the other forces were the same as they are now. Had gravity been the same, and the other forces a hundred times weaker, the problem would have been no different. So the discussion is about why the relative strengths of the different forces are what they are, it’s not about gravity alone.

Physics tries to describe a whole of phenomena from a limited number of natural laws, and that succeeds quite well by the way. In that picture there are four fundamental forces, each with their own way in which they work, namely gravity, electromagnetism, the weak and strong nuclear forces. They each have their own strength, which is expressed by fundamental constants, which we can measure, but for which we have no real explanation that determines why those constants are as great as they are and nothing else. In other words, we cannot find any logical reasoning to tell us why the relative strength of those four forces is the one we can determine through experiments and observations. This also means that the physics research question is not finished, we cannot claim that we ‘understand it all’.

If one does not understand why something is the way it is, it can be a useful way to ask what the effects would be if it had been a little different, in this case if the ratios of the strengths of the interactions had been different. . If gravitation had been noticeably stronger than electromagnetism, then, as mentioned above, stars would have been short-lived in a universe that never really had the time to make them. If the gravitation had been a bit weaker, it might not have been possible to make stars, because there would have been too little gravity to contract clouds of gas in the expanding universe. But it doesn’t end there: if the strong nuclear forces had been a little weaker than electromagnetism, atoms heavier than hydrogen could never have formed; if the weak nuclear forces had been even weaker, the first nuclear reaction to form the energy cycle in a star like the Sun could not have happened. In either case, there would never have been stars as an energy source for chemistry on any planets around them!

Notice that the word “life” had not yet been used in this answer. Consciously, because it smells too much of putting ourselves at the center of our reasoning about the universe, and we’ve had bad experiences with that in history. At the same time, of course, it is a trivial truth that we cannot live in a universe in which we cannot exist. But we don’t have to set ourselves the standard here either: in a universe with different natural constants, other complex structures, which are, however, much less complex than living beings, cannot arise either.

The path taken today to solve this problem is the search for a theory that unifies all four interactions in just one fundamental interaction. The four interactions are then manifestations of the same unique fundamental interaction, the symmetry of which has spontaneously disappeared. A bit like water that freezes becomes ice, with locally different fault lines. If one estimates in how many ways symmetry can be broken within such theories, one arrives at insanely large numbers: so there would be many possibilities of what the world could look like. If it turns out that in that large family a world like ours is one of the few that can grow very large and last a long time, then we might be one step further… On the other hand, it could also turn out that completely different worlds are possible. that can make complex evolution possible, but completely different from ours.

Conclusion: there is still a lot of room for wonder and open curiosity in physics. And if the history of our science has taught us anything, it has often been the new fresh outlook of young people that has brought about conceptual innovation.