Some point to various cosmological models involving an eternal universe. Whatever one might make of the mathematical merits of these models, the nature of science renders it incapable of ever demonstrating the universe to be eternal, even if the universe is eternal. Science is an empirical discipline based on what can be observed and quantified. For science to prove that the universe is eternal, it would have to do so empirically. But this is impossible. An eternal past cannot be observed or quantified.
Consider a staircase. Suppose there exists a staircase that extends far into space beyond what we are able to observe. Some speculate that the staircase is infinite in size, while others contend that it is enormous in size, but still finite. How would one go about testing whether the staircase was infinite or just really big? Remember, we are talking about science, so we are limited to empirical methods of inquiry. One way to test the possibility is for a scientist to start walking the staircase, counting each step along the way: one, two, three, four...1000...1,000,000...1,000,000,000.... Could our scientist conclude after traversing the 1,000,000,000th step that since the staircase continues on beyond his observational horizon it must go on infinitely? No. For all he knows, it may end 100,000 steps ahead and if he keeps walking/counting for one more week he would finally reach the end of the staircase. Because he wants to make a scientific-and hence, empirical-assessment of the staircase's size, our dedicated scientist keeps walking and counting. Could he, one hundred years and 100 trillion trillion trillion trillion steps later, conclude that the staircase is infinite? No. For all he knows it could end one trillion steps from where he stands. So he traverses one trillion more steps, but there's still no end in sight. Is he justified at this point in concluding that the staircase is infinite? No, not empirically. For all he knows, it could end one trillion steps from where he stands.
The same is true of the universe. No matter how far back in time scientists are able to see by peering through a telescope, they can never know whether the universe continues on infinitely into the past, or just a little past their present observational horizon. Even if they could see 1 trillion years back in time, the empirical nature of science prohibits them from making any conclusions about what-if anything-is beyond that point. For all they know, the regress could keep going on forever, or it could terminate at 1.1 trillion years in the past.
Science can only speak to what it has observed and since it is impossible to observe an infinite number of past moments, science is incapable of verifying that the universe has existed from eternity past. And if science is incapable of verifying that the universe is eternal, scientific objections against the second premise of the KCA are dead in the water. Only philosophy is equipped to answer questions about the existence of infinites. If it can be demonstrated philosophically that the infinite is incapable of being instantiated in reality, then it can be demonstrated that the universe is not past-eternal. While I think philosophy has demonstrated this quite clearly, even if I am mistaken and premise two is actually false, it could only be demonstrated to be so philosophically, not scientifically.
While scientific discoveries can support rational or philosophical arguments, they cannot inveigh against them.
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