by Prasad Ganti

Life on Earth came into existence because of some favorable factors. Earth is located in our solar system at the right distance from the Sun where we get the light and heat for the sustenance of life, where water exists in liquid form and the atmosphere is thick enough to protect us from radiation and trap heat to keep us warm. It is thin enough so as not to crush us.

Unlike other planets like Venus that has a thick atmosphere of carbon dioxide and traps so much heat that it is a runaway greenhouse. On the other hand, Mars has almost no atmosphere. It is pretty cold and frigid. The outer planets from Jupiter onwards do not have any land or solid surface worth talking about. They are just gaseous giants where it is difficult for life to imagine.

No other planet in our solar system is conducive for the existence of life barring Mars or some satellites like Europa (of Jupiter) or Titan (of Saturn). In effect, the Earth is supposed to be in a Goldilocks zone in our solar system. When we look for alien life elsewhere, these considerations come into the picture. Now let us expand this concept to the level of our Universe.

There are some fundamental constants of our Universe that make it what it is. Other Universes may have different values for these constants, which could make them different than ours. These constants are described in the book called “Just Six Numbers” by Martin Rees. We will talk about some important ones of those constants.

Gravity is one of the four fundamental forces which operates at a macro level across the Universe. Then we have the strong nuclear force which works at the micro level. At the outset one might get a feeling that Gravity is very strong indeed. In a way it is. It sculpts the stars, keeps the planets in orbit, keeps the solar systems within Galaxies and so on. It is also the basis of the universal monsters called Black holes. It can bend and fold the fabric of space. It can cause time to pass by very slowly or speed up like the press of a fast forward button.

And since the nuclear force operates at such a micro level, we do not feel it. It keeps the protons and neutrons bound together to the nucleus, which is no mean feat. But it would be surprising for us to know that the ratio of the nuclear force to gravitational force is a number called “N” whose value is 10**36 (10 raised to the power of 36 or 10 followed by 36 zeroes). We see that the Gravity pales in comparison to the Nuclear force. But the impact of the nuclear force diminishes very rapidly with distance. And gravity comes into force when huge masses are involved.

What happens if this number N is any different from what it is? If this number is any smaller, the Universe would be much smaller because Gravity would crush anything as large. The stellar lifetimes would be much shorter. Conditions for complex evolution of life would be less favorable. Life forms, as we know it, need very long timelines to appear and evolve. On the other hand, a weaker gravity may not help in the creation of structures like stars of galaxies. Again chances of life appearing would be slim.

Like our Earth in our solar system, our Universe also seems to be in a Goldilocks zone where the constants are very favorable for life to exist for someone to write this article and for someone else to read it! Are there other Universes with different ratio of gravity to nuclear force? We have 3 spatial and 1 time dimension. Are there other Universes with different number of dimensions? We keep learning more as observations are made using more sophisticated instruments and more complex theories are conceived.