It is not actually necessary that “fine tuning” of certain parameters will have to exist in reality for proving the existence of God. I think light with its very peculiar properties is sufficient for that purpose.
Light originates within space and time but it goes beyond space and time. A photon coming from a star lying at a distance of one billion light-years from earth will take one billion years of earth’s time to reach the surface of the earth. During these one billion years of earth’s time it will be in a spaceless and timeless condition, because the distance between the star and the earth has become zero for it and time has also stopped. So it will be neither in space nor in time during the total period of its existence. Then it will cease to be by being absorbed by something or someone on earth.
So light originating within space and time goes beyond space and time, because space and time become non-existent for it. And we cannot claim that this is without any cause. As light is not a conscious entity, so neither can we claim here that light has the capability of deciding its own fate that it will go beyond space and time. So this must have been caused by something else. But whatever may be the cause of it, this cause cannot lie within space and time; it is impossible. Let us suppose that this cause is A and that it lies within space and time. We can now ask two questions about A:
1) Are space and time non-existent for A also?
2) Or, are they not non-existent for A?
If 2), then how can A cause space and time becoming non-existent for light when they are not non-existent for A itself? But if 1), then we will have to ask the same question about A that we were earlier asking about light: what causes space and time becoming non-existent for A, when we know very well that A lies within space and time? So we see that A cannot be the ultimate cause that makes space and time non-existent for light, because here we will have to find out again the cause that makes space and time non-existent for A itself. In this way it can be shown that there will be an infinite regress, and that there is nothing within space and time that can be this cause. So ultimately we will have to go beyond space and time in search of this cause. A cause that lies within space and time is a natural cause, but a cause that lies beyond space and time is not a natural cause; it is a supernatural cause. So the cause that makes space and time non-existent for light is a supernatural cause.
Now one can raise an objection here that it cannot be the case that light is neither in space nor in time, because we can see the star and therefore the photon must have existed in some space-time during its transition from the star to the earth. But in which space and for how long did it exist during this transition? This is because the equations of SR show that both the travel time and the travel distance have become zero for light. So if SR as a theory is correct, then light cannot, and does not, exist in any space for any time.
Some may also think that SR is a bad theory and that it requires immediate replacement, because the conclusions that can be drawn from this theory are so counter-intuitive and contradictory to our commonsensical and everyday notion of existence. Here anybody can offer a better theory if he/she thinks so and get it accepted by the peers before challenging a well-established theory of science.
About light one can also read the article “The Fundamental Nature of Light” by Dr. Sascha Vongehr in Science 2.0 (February 3rd, 2011)1