The Principle of Contingent Causation: The Impossibility of Infinite Regress.

[Angrboda]

I've come upon an interesting question in a sidestreet of some thinking about God and contingency today, and I'd just like to put it to the forum to solicit comment on it.  It occurred to me that while, even if one accepts that there had to be a first cause, the argument gives no reason why there need be only one first cause.  It would seem possible that there could be multiple first causes so long as the multiple first causes originate simultaneously, and all first causes perform the same initial causative action simultaneously.  Under these constraints, there could be an infinite number of first causes.  But this presents a problem.  According to general relativity, there is no such thing as absolute simulteneity.  Whether or not two events appear simultaneous to an observer is dependent upon their movement relative to the two things occurring and such, so that from one observer's perspective, two first causes might be simultaneous with the observer, who is also a first cause, but another first cause who may be moving with respect to the other three first causes would not see them as simultaneous.  So, while an infinite set of first causes seems possible from the perspective of classical physics, it's not clear that the idea is even coherent from a relativistic perspective.  Thinking about it just now raises another wrinkle, as if the first cause is postulated to be God, from one perspective, God would precede the effects which he causes; however, it's not clear how to define the notion that God as first cause precedes his effects as there is no such thing as an absolute frame of reference in relativity, so while God may appear to precede an effect from one observer's perspective, the causal arrows may not be as readily apparent from another in which an effect of God appears to precede God causing it.  I'm not sure what to make of all this.  It seems to open up a whole can of worms with no ready resolution.

@Nishant Xavier : What are your thoughts?

Roger Penrose advanced a form of this argument that has been called the Andromeda paradox in which he points out that two people walking past each other on the street could have very different present moments. If one of the people were walking towards the Andromeda Galaxy, then events in this galaxy might be hours or even days advanced of the events on Andromeda for the person walking in the other direction.[contradictory] If this occurs, it would have dramatic effects on our understanding of time. Penrose highlighted the consequences by discussing a potential invasion of Earth by aliens living in the Andromeda Galaxy. As Penrose put it:

Andromeda paradox

Two people pass each other on the street; and according to one of the two people, an Andromedean space fleet has already set off on its journey, while to the other, the decision as to whether or not the journey will actually take place has not yet been made. How can there still be some uncertainty as to the outcome of that decision? If to either person the decision has already been made, then surely there cannot be any uncertainty. The launching of the space fleet is an inevitability. In fact neither of the people can yet know of the launching of the space fleet. They can know only later, when telescopic observations from Earth reveal that the fleet is indeed on its way. Then they can hark back to that chance encounter, and come to the conclusion that at that time, according to one of them, the decision lay in the uncertain future, while to the other, it lay in the certain past. Was there then any uncertainty about that future? Or was the future of both people already "fixed"?
   — Roger Penrose, The Emperor's New Mind: Concerning Computers, Minds, and the Laws of Physics

The "paradox" consists of two observers who are, from their conscious perspective, in the same place and at the same instant having different sets of events in their "present moment". Notice that neither observer can actually "see" what is happening in Andromeda, because light from Andromeda (and the hypothetical alien fleet) will take 2.5 million years to reach Earth. The argument is not about what can be "seen"; it is purely about what events different observers consider to occur in the present moment.

Wikipedia || Rietdijk–Putnam argument