I think I have to go with Scott on this one. It is very, very, very likely that we are alone.
Many people have a tendency to think that the universe is so big, certainly there is other life out there. But, this is based on an ignorance of how big the universe must be in order to satisfy that claim. Also, people have a tendency to believe that, given suitable conditions for life, life will likely arise. This is based, somewhat, on the observation that, once you have life it always finds a way – and this is true. But critical factor there is “once you have life” and, there lies the rub. There is no theory, whatsoever, that can explain how life evolves from inert material.
Let’s consider both of these questions: How does life form from non-living material and how large must the universe be to put produce a living organism?
I’ll begin this discussion by saying that we are restricting ourselves in this discussion to the laws of physics as they are currently understood. That being said, there *may* be some force beyond our present understanding of Physics that strongly influences the processes that I will describe. That “force” is off-limits in this discussion, however, because it is beyond our present understanding of Physics. So,….
The most basic building blocks of life, as we know it, are amino acids and nucleotides. Chains of amino acids form proteins, which is the material that most a of a cell is made of. Chains of nucleotides form nucleic acids such as RNA and DNA.
The simplest known living thing is a mycoplasma hominis H39. It is comprised of about 600 different kinds of proteins, each with an average length of about 400 amino acids. Meanwhile, the theoretical limit for the simplest living thing has been calculated to need 239 proteins with average lengths of 445 amino acids.
There are 20 different types of amino acids found in all biological systems. Although amino acids can occur with a left or right-handed symmetry, *all* biologically produced amino acids feature left-handed symmetry. Meanwhile, amino acids produced by random chemical processes will have equal numbers of both left and right symmetries. This means that there are actually 40 different types of amino acids when formed by random, chemical processes.
The probability of randomly forming the correct 445 amino acid sequence for a given protein is 1/40^425 = 8.25×10^712 against.
That’s some pretty big odds, but some people will say, “Yeah, but the universe is so big and so old, I don’t see a problem.” So let’s take a look at how big and old the universe is.
We believe there are about 2×10^11 galaxies in the universe and that each of those galaxies may contain an average of 2×10^11 stars. The average star has a mass of about 2×10^30 kg and each kg contains about 10^27 atoms. So we can estimate the total number of atoms in the universe to be around 10^(11+11+30+27) is around 10^80 atoms.
Now, instead of saying that some planet has liquid water so it might have life, let’s go several steps further. Let’s say that every atom in the entire universe is an amino acid. Let’s also say that every amino acid in the universe can combine with any other to form a chain of amino acids 425 units long and that the universe can try these combinations a thousand times per second and it has been trying these combinations every microsecond since the beginning of time 13.8 billion years ago.
If that were true, then an entire universe of amino acids doing nothing but trying to make one specific protein since the beginning of time could have tried ((10^80)/425)x4.25×10^20 = 10^98 combinations. That tells us that, as big and old as the universe is, it is still about 10^614 times too small or too young to have produced just one particular protein by random process. (That’s a million, million, million,…one hundred times, too small.) We’re not even talking about life yet. We’re just talking about one average length protein. In order to make life, we would also need to make 238 other specific proteins, in relatively the same place and same time. The odds of that are 10^(712×239)=10^(170,168) against.
To imaging these odds, imagine a lottery where everyone in the world has a ticket. Your odds of winning are about 7.7 billion against. You don’t win, but your neighbor does. It’s amazing! But next week there’s another world lottery and your damned neighbor wins it again. How many weeks in a row could your neighbor be the one and only person in the world to win the weekly world lottery before you start to say “Impossible!” or suspect that the process is rigged? Would you believe every week for 325 years? Would you accept that as possible and consistent with random chance? I wouldn’t.
Clearly, random processes cannot account for the origin of life. Therefore, some people argue that the amino acids don’t combine randomly because, maybe the four types of nucleotides arranged themselves into chains which would act as templets for the amino acids sequence. There are only four varieties of nucleotides compared to the 40 types of amino acids, so the odds would be better. But that only reduces the odds to 10^(170,165) instead of 10^(170,168). Also, where do nucleotides come from?
I may be wrong, but as far as I’ve been able to determine, even our high-tech bio-engineering labs cannot make nucleotides from scratch. They must start with biologically produced nucleotides and use in vitro or in vivo methods to alter them. So how can we expect nucleotides to have just randomly formed?
If we must rely on random processes and the laws of physics as we currently understand them, the probabilitiy of alien life out there is, essentially, zero. However, if there is some other “super-natural” force out there – something beyond our present understanding of physics, then and only then, it is possible that the universe is teaming with life. With God, all things are possible.
3 replies on “Life in the Universe – or lack thereof”
Your last sentence sums it all up nicely. “With God, all things are possible.” I recently read Douglas Axe’s book, Undeniable: How biology confirms our intuition that life is designed. Your post reminded me of some of his arguments. Thanks.
Ok, I am commenting just to say, wow that was a lot of math. And I wanted to ask if in this discussion are we assuming that life is going to look like we have if hear or are we open to the idea that life could happen in very different environments. Now I’m not implying that there would be intelligence beyond animal life. I of course want a super intelligent alien pet, just saying.
We can’t assume that life will look anything like Earth life, but we have to start somewhere. There are some theories that a silicon based life that processes ammonia and fluorine is possible. But those molecules would be very fragile and unable to withstand temperatures higher than -200C (-328F). So they would not be very practical. Meanwhile, something really different, like an interstellar, intelligent cloud (as in The Black Cloud by Fred Hoyle) is too far removed from anything in our experience that we can’t really speak intelligently of it. So I have restricted this analysis to planetary life and the rules of chemistry seem to suggest that carbon-based is most likely. But that could be totally wrong.
Meanwhile, the reason I still keep this perspective is because this is the perspective that other astronomers always use when they try to sell you on the idea that there might be life on this planet or that because of the so-called habitable zone or presence of water.
Of course, nobody knows the answers. I can only say, with a fair amount of certainty, that life as we know it is impossible unless there is some other force at work which is beyond our present comprehension. If so, there may be plenty of life out there.