Wednesday, July 1, 2020

Why the Past Cannot be Infinite

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Relevance to Theism



A finite past bears relevance to the kalam cosmological argument (KCA) which goes like this:
  1. Anything that begins to exist has a cause.
  2. The universe begins to exist.
  3. Therefore, the universe has a cause.
Further arguments are given to show that the cause of the universe is (among other things) a transcendent personal cause. If we have adequate grounds for thinking the universe has a transcendent personal cause, this gives at least some evidence for the truth of theism. I’ve justified premise (1) in my anything that begins to exist has a cause article. In this article I’ll argue for premise (2) by arguing for a finite past.

Would a finite past mean that not even God is sempiternal (i.e. having existed for a beginningless, infinite duration)? Yes it would. One idea is that God is timeless sans creation. Note that if spacetime itself began to exist and our spacetime universe had a cause, that cause would have to transcend space and time. Whether you want to call this spacetime-transcending cause supernatural or not, such a cause would have to be something beyond the physical laws as we know them today. The fact that there is some sort of (at least) de facto supernatural cause beyond space and time creating the universe would seem to make atheism less plausible.

An Infinite Traversal



Some philosophy lingo: a potential infinite is a collection that grows towards infinity without limit but never actually gets there. For example, if you started counting “one, two, three…” at a rate of one number per second and continued indefinitely, the number you’re at would grow larger and larger without limit but you’d never actually arrive at “infinity.” An actual infinite is a collection that really is infinite, such as the set of all positive whole numbers.

Traversing an actual infinite region at a finite rate seems impossible. Suppose for example there were a road that starts at a particular location and is infinitely long. Someone named Jill Walker starts at the beginning of the road and walks at a rate of one meter per second. Will she ever traverse an infinite region? She will not; the distance (and time!) she traverses is a potential infinite only. What if she were given infinite time? The problem is that traversing an actual infinite amount of time can never happen. Even if she is given unlimited time she will never traverse an actual infinite amount of time or an actual infinite distance; both will be a potential infinite only.

A similar problem occurs with a beginningless past: for a beginningless, infinite past to exist an actual infinite amount of time would need to be traversed, which is impossible, and thus we never would have arrived at the present moment. Another way to look at it: imagine if we viewed a universe with an infinite past and rewound it, traversing it at the same rate as time normally goes but backwards. Could we traverse the entirety of the infinite past? The infinite past would be impossible to completely traverse even given unlimited time. Similarly, going the other direction would be impossible because it requires an infinite traversal and we never would have arrived at the present moment (or at any moment, since any moment in the infinite past has an infinite amount of time before it).

The Eternal Society Paradox



There are also various paradoxes one can make with an infinite past, an example of which is the Eternal Society paradox. Roughly (in the paper the Eternal Society paradox was published), an Eternal Society is a society that has existed for a beginningless, infinite duration of time and has the abilities of ordinary human beings in each year of its existence; e.g. in each year people in the society can flip coins, write books, sing songs, and pass on information possessed in the current year to the next year. Because of the society’s extremely modest abilities, it seems like an Eternal Society would be possible if an infinite past were possible (note that by “possible” in this article I’ll be referring to metaphysical possibility, as opposed to e.g. physical possibility).

Now imagine the Eternal Society has the following Annual Coin Flipping Tradition: each year they flip a coin and if it comes up heads, they all get together to do a particular chant but only if they have never done the chant before. If the coin does not come up heads they do not do the chant for that year.

The coin flips are probabilistically independent events, so any particular infinite permutation of coin flips is equally unlikely but also equally possible. Consider scenario S1 in which the coin came up heads for the first time last year. The Eternal Society gets together to do the chant for the first time. This seems like it would be possible if an infinite past were possible (an eternal society with the ability of ordinary humans, by which I mean the society has the ability of ordinary humans in each year of its existence, could surely do something like this), but this scenario is provably not possible.

Again, the coin flips are probabilistically independent events, so if scenario S1 were possible, then another scenario, that we can call scenario S2, would be possible: the coin came up heads each year of the infinite past. If the coin came up heads each year, did the Eternal Society ever do the chant? They would have had to have done the chant some year, because they would have done the chant last year if they hadn’t done it yet (since the coin came up heads last year). And yet any year you point to, there is a prior year in which they would have done the chant if they had not done the chant before. So they had to have done the chant (since the coin came up heads last year), yet they could not have done the chant (there is no year they could have done it), and so this scenario creates a logical contradiction.

Although scenario S1 is not directly self-contradictory, scenario S1 is impossible because it implies the possibility of a logical contradiction. The Eternal Society argument against an infinite past goes like this:
  1. If an infinite past were possible, an Eternal Society would be possible.
  2. If an Eternal Society were possible, then scenario S1 would be possible.
  3. If S1 would be possible, then S2 would be possible.
  4. S2 is not possible.
  5. Therefore, an infinite past is not possible.
One could deny premise (4) particularly since that seems to be the most vulnerable premise, but as the Eternal Society paradox paper says, “Surely there is something metaphysically suspicious about an infinite past if an eternal society with the abilities of ordinary humans can actualize a logical contradiction.” The idea that an infinite past is possible but an Eternal Society is not possible strikes me as overly ad hoc due to the Eternal Society’s extremely modest abilities (the abilities of ordinary humans in each year of its existence).

Conclusion



While there is also scientific evidence favoring a finite past, philosophical arguments seem to provide a strong case for temporal finitism (the view that the past is finite). For a beginningless, infinite past to exist an actual infinite amount of time would need to be traversed, which is impossible, and thus we never would have arrived at the present moment. Moreover, the Eternal Society paradox shows that an eternal society with the abilities of ordinary humans would have been able to create a logical contradiction, which strongly suggests that an infinite past is metaphysically impossible.