In the past few days, there have been some interesting articles in response to a NY Times article summarizing one of the discussions happening in cosmology these days.
If you’re a science geek who likes the “why” questions (why does the universe exist? why does time run in one direction only?) these articles are worth checking out. I have some thoughts about these articles, but I’ll let you read the originals before diving into specifics.
So these articles are interesting and all, but they’re kind of vague and some of them are actually misleading, so here are some thoughts on all that. (Gordon Smith, if you are out there, please read this and correct anything I get wrong.)
One of the interesting facts of the universe is that it started out in a state with very low entropy. No one knows why. Note that this is not the same thing as saying that it started very very small. Small things can have low entropy or high entropy. In fact, pound for pound, black holes have the most entropy of all. So not only was the early universe very small, it was also a very special kind of small thing with extremely low entropy.
Why is this important? Well, the second law of thermodynamics is predicated on the universe starting out in a low entropy state. Contrary to popular opinion, the second law doesn’t say that entropy increases forever. What it says is that entropy tends to get bigger until you reach a state of near maximum entropy, at which time things basically stay the same.
Example: If I take a pail of water and add a drop of red food dye, it tends to spread out. But once it has spread out evenly, it just stays that way.
The second law is important because that’s what causes our universe to be interesting. In fact, life cannot exist in a high entropy world. In order to maintain the structure and order inherent in our bodies, we need to continually consume energy and spit out heat, which is only possible when we have a low entropy world to spit that heat out into.
To summarize: the early universe was an extremely low entropy state. If you were just picking states randomly, you would be extremely unlikely to arrive at such a state. Because of this, one of the most important questions in cosmology is to ask ourselves why the early universe had such low entropy.
One hypothesis is that it happend “by chance”. That sounds like a crazy argument, but once you dive into it, the argument is not 100% crazy.
Let’s go back to the analogy of the red food dye in the pail of water. It is extremely unlikely for all of the red food dye to be concentrated in a single “drop”. But if you sat there and stared at the pail for an infinite amount of time, it would eventually happen by pure chance.
In the same way, imagine that you were lucky enough to be an observer sitting outside the entire system and you had an infinite amount of time to observe things. (Yes, I know that “time” is a concept that only makes sense within the universe itself, but bear with me.)
Maybe little universes sprout up from time to time and collapse again. And almost all of these universes are, indeed, high entropy universes. But if you have an infinite amount of time to sit around, you will eventually see a low entropy universe arise by pure chance. One estimate of the initial entropy says that the chance of randomly arriving at such a state are 1 in 10^10^123, which is to say it is very very unlikely. But given an infinite number of tries, it will eventually happen.
“So what?” you say. “Isn’t this just monkeys typing Shakespeare?” This is where the anthropic principle comes in.
Remember that life itself can’t exist in high entropy universes. So as you look at these little universes coming and going, you find that 99.9999999…% of them are extremely boring. Nothing happens inside them.
But in the few universes that have a low entropy initial state, you find that interesting stuff happens. Stars form. Life exists.
And as you look inside the tiny fraction of universes that contain intelligent life, you find that scientists are saying things like “gee… that’s funny… according to my calculations, there is a 99.99999999….% probability that the universe should a be high entropy, boring place. So what gives?”
So maybe the argument that our universe started out with low entropy “by chance” is not so crazy after all. Maybe new universes are springing up all the time, but it’s only the low entropy ones that support intelligent life.
This kind of argument relies on something called “the anthropic principle”, which says that it is ok to explain strange facts about our universe if they are necessary to support life (using the argument above). Not all scientists are 100% comfortable with the anthropic principle, but it’s not completely crazy.
With me so far? Wow. Ok, now on to the current discussion about floating brains.
A good scientist will test a hypothesis by seeing if there is any way to knock it down, so let’s try to knock it down.
The hypothesis above says that even though the initial state of the universe is extremely special, you will (a) eventually find a universe like that if you have an infinite amount of time on your hands, and (b) these are the only universes that can support intelligent life.
The problem with this line of thinking is that 10^10^123 is a HUUUUGGGGEEE number. So even though it is true that you would eventually find a universe like ours, you will also see a whole lot of other strange things.
Let’s do a back of the envelope calculation to see how likely it would be for the initial state of the universe to consist of all the particles in the solar system exactly as we see them today, and nothing else. Gee.. that’s (and I am making up this number) 10^10^80. And gee, a world like that could support life (by definition). Or just to be silly, let’s imagine that the universe only consists of your brain, thinking its thoughts, along with signals that fool it into thinking that there is a world out there to observe. Gee, the probability of a universe like that arising randomly is only 10^10^60! So it’s almost infinitely more likely than the universe we live in today.
By making this type of argument, I don’t think anyone is saying that we are actually brains floating in space. The point of this type of argument is to knock down the hypothesis that the early universe found itself in a low entropy state by “pure chance”. The anthropic argument doesn’t help here, because if you judge by entropy alone, illogical worlds that contain intelligence (like a universe consisting only of a floating brain) are WAY WAY more statistically probable than our own universe.
So all this means is that we need to find a different explanation for why the initial state of the universe had such low entropy. Any ideas? :-)