Anybody who has read my origin story knows the prominent role the second law of thermodynamics played in moving me from agnosticism to theism. But for those who haven't, here's the TLDR.
I learned about the second law of thermodynamics in the Navy's nuclear power school. After I got out of the navy, I went to the University of Texas at Austin. One day, while out walking, I was thinking about the first and second laws of thermodynamics, and it occured to me that they couldn't both be true. I stewed over that for a few days or weeks (don't remember exactly how long) until I came to the conclusion that the universe must've had a beginning, which means there must be a creator. That was the end of my agnosticism.
Since that time, I was introduced to Aristotle, Aquinas, and Christian apologetics. I discovered additional reasons to think the universe had a beginning. (As a side note, Aristotle didn't argue for a temporal beginning, but I did misunderstand him to be making that argument.)
Also, since that time I gained a greater understanding of entropy and the second law of thermodynamics. In the process, I came to realize the second law does not necessarily imply a beginning after all. It still suggested a beginning, but I stopped relying on it so heavily because it didn't strike me as being a strong enough reason to think there was a beginning.
But since that time, I've come back around to thinking the second law does give us strong reason to think the universe had a beginning. It isn't certain, but it's very close to it.
Why I originally thought the Second Law of Thermodynamics implied the beginning of the universe and therefore God
Here's the reason I originally thought the second law implied a beginning. If the second law is true, then entropy increases in every process. That means no matter what happens in the universe, the net entropy will go up. This is true even if entropy goes down in some localized place. In power school, we were taught that entropy was the amount of energy in a system that's no longer available to do work. As entropy goes up, less and less energy is available to do work, i.e. to bring about change. Unless you get rid of some of that useless energy or add some new energy to the system, eventually the system will wind down and nothing else will happen.
If the first law is true, then energy is neither created nor destroyed. That means whatever exists has always existed and always will exist. But if the whole universe has always existed, then it should've already reached maximum entropy. There shouldn't be any stars or galaxies. There shouldn't be any life. The universe should be in a state of thermodynamic equilibrium. It should just be a big homogeneous, uninteresting, diffuse something or other. Cosmologists call it the heat death of the universe, which I learned later.
Yet here we are with plenty of order, complexity, and activity. The universe is far from thermodynamic equilibrium. That means it can't be the case that the universe has existed for infinite time. The universe must've begun a finite time ago. And since the universe had a beginning, it had to have had a cause. That cause could not itself be physical or it, too, would be subject to the second law, and it would also have to have had a beginning. The more I thought about it, the more it looked like God, so that's what I thought it was.
That was my original reason for becoming convinced that there was a God. From there, Christianity was an easy sell because it's what I knew, I already had an affinity for it, and I never really stopping calling myself a Christian even while I was agnostic.
Why I shied away from appealing to the second law to argue for a beginning of the universe
I later gained a better understanding of entropy and the second law of thermodynamics that weakened my case for a beginning of the universe.
The reason entropy increases with every process is because that's the most likely thing to happen. The second law of thermodynamics is not a fundamental law. It's an emergent law. It's a consequence of statistical probability.
Entropy is colloquially called a measure of disorder, spreading out, equilibrium, etc. These are all decent descriptions because they capture what most of us observe when we observe entropy increasing. If you put a hot cup of coffee in a cold box, heat will flow from the hot to the cold until the temperature of everything is the same. If you put a bunch of folded clothes into a dryer and turn it on, the clothes will become unfolded. If you drop a basketball, it will bounce repeatedly until it eventually comes to rest. If you throw a bunch of chemicals in a jar, they'll react until they settle down and stop reacting. These are all examples of entropy increasing. The second law says entropy increases in every process, so it's always increasing. The second law of thermodynamics is the reason there can never be a perpetual motion machine. If you want to keep the thing running, you have to add new energy to it. Otherwise, it will run down and eventually stop.
That is a decent enough layman understanding of entropy. But there is a more robust, precise, and scientific definition. Entropy is a measure of how many micro states correspond to the same macro state. The equation for entropy looks like this:
\[ \normalsize S = k_B \ln \Omega \]
Where. . .
- \( S \) = entropy
- \( k_B \) = Boltzmann’s constant
- \( \Omega \) = number of microstates consistent with the same macrostate
By "macro state," I mean a view of the system from afar, zoomed out, so to speak. It's a coarse description of the whole system. By "micro state," I mean the zoomed in view. It's a more fine or granular look at the system. It's a detailed look at the parts.
Here's a couple of examples. Consider a cylinder that contains a gas under pressure. You can consider the pressure in the container as the macro state. The exact position and motion of each molecule of gas in the container is the micro state. There are countless configurations the individual molecules could be in that would result in the pressure of the container being the same. So this is a high entropy system.
Now, consider a computer screen that uses only three colours to generate any image. The image on the screen currently is all blue on the right half and all green on the left half. If each pixel can only be red, green, or blue, then there aren't very many micro states that can produce the same macro state. So this is a low entropy situation.
You should be able to see from these two examples why entropy can be thought of as a measure of disorder, randomness, homogeneity, spread-outness, equilibrium, etc. When things are pretty evenly distributed, there are countless ways the zoomed in details could be different while the zoomed out picture would be the same. But when things are very ordered, there are fewer ways the zoomed in picture could be different while keeping the big picture the same.
You should also be able to see where the second law of thermodynamics comes from. It comes from the fact that in any given system, there are vastly more configurations that are just random noise than configurations that have structure and order. Consider a cookie sheet full of little pieces of alphabet cereal. There are vastly more configurations that sheet of cereal could take that don't form words and sentences than there are configurations that make words and sentences. With that being the case, if you were to start with a sheet of alphabet cereal that has maybe a couple of sentences while the rest of the bits are spread around randomly, and you shook that sheet, it is more probable that the final state would have fewer words and sentences than that it would have more. Whenever there's a change in a system, it's always more probable that the next state of that system will be closer to equilibrium than farther from equilibrium. The probability of a less ordered state is so great that it appears to be a law that entropy always increases when things change.
However, you should notice that given this understanding of entropy, the second law of thermodynamics is not absolute. It's just a generalization. The reason entropy increases in every process is because higher entropy states are vastly more probable than low entropy states. So every time something in the universe changes, the next state of the universe is vastly more likely to be a higher entropy state than a lower entropy state. That, in turn, means that while lower entropy states are highly improbable, they are not impossible.
Suppose, then, that the universe has been around forever. We should expect that it would be, on average, in thermodynamic equilibrium. It always has been. However, given enough time, there should be occasional random fluctuations of low entropy. Smaller fluctuations will be more frequent than large fluctuations because the bigger the fluctuation, the less probable. But given enough time, even the most unlikely low entropy fluctuations are bound to happen. Given infinite time, even a fluctuation of low entropy as large as our universe was in the beginning of the big bang is inevitable. That seems, on the surface, to undermine the argument for an absolute beginning from the second law of thermodynamics. A universe like ours, with an extremely low initial entropy, is practicaly inevitable, even if the universe is infinitely old.
With that in mind, I shied away from appealing to the second law of thermodynamics to try to prove that the universe had a beginning.
Why the second law of thermodynamics turns out to be a good argument for the beginning of the universe after all
Remember that small fluctuations of low entropy happen more frequently than large fluctuations of low entropy. The reason is because small fluctuations (i.e. fluctuations that deviate from equilibrium by a small amount) are more probable than large fluctuations (fluctuations that deviate from equilibrium by a large amount). If our universe began as a random fluctuation of low entropy, then it would have to have been an unimaginably rare and improbable event. If the universe is infinitely big and/or infinitely old, much smaller fluctuations of low entropy would be more common.
Universes that contain only a single small cluster of galaxies would be more numerous than universes like ours. Universes that have only one galaxy would be more numerous still. Universes with one solar system would vastly outnumber universes like ours that have many solar systems in many galaxies.
A random low entropy fluctuation that produced a single brain, just momentariliy, configured in such a way as to generate the sensation of observing a universe like ours is far more probable than an actual universe like ours populated with billions of conscious observers and trillions of stars and galaxies. That means that if we are to explain our existence as being the result of a random low entropy fluctuation, then it is overwhelmingly more probable that we are just brains that fluctuated into existence a moment ago and are about to disintegrate than that we are actually in a 93 billion light year sized or bigger universe. There would be more of these types of brains (called Boltzmann Brains by the experts) than real observers in real universes, so we are more likely to be Boltzmann brains than real people.
As Sean Carroll has explained, the problem with being a Boltzmann brain is that it calls all your beliefs into question. Since all of your alledged knowledge, perceptions, and experiences were spontaneously created in a random fluctuation, they have no connection to what reality is actually like. If you embrace the idea that you're a Boltmann brain, then you lose all justification for anything you believe. That includes whatever reason you allegedly have for thinking you are a Boltzmann brain. As Sean Carroll puts it, Boltzmann brains are cognitively unstable.
To be rational people, we must reject any cosmological model that makes it more probable than not that we are Boltzmann brains. To be honest people, we have to reject these models. After all, none of us honestly believe we are Boltzmann brains. People may toy with the idea, but if they claim to actually believe it, they're probably not being honest with themselves.
The second law of thermodynamics turns out to be a good reason to think the universe has a finite past after all. The only way to escape this conclusion is to embrace a model of the universe that makes it probable that you are a Boltzmann brain, which means you have to embrace a model of the universe that destroys all justification for believing anything, including that model. Denying that the universe has a finite past on the basis that the second law of thermodynamics isn't absolute and that given enough time, there can be random fluctuations of low entropy that results in structured universes and conscious observers turns out to be a self-defeating objection. With that objection out of the way, the second law of thermodynamics makes it very likely that the universe had a beginning.
The argument for a beginning falls short of certainty because it's still at least possible that we are Boltzmann brains, even if we don't know it. But let's just be reasonable.
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