- the laws of nature seem to be fine-tuned to allow biological life.
Anthropic reasoning goes this way: if the laws would not be fine-tuned, then there would exist no observer who would be wondering the fine-tuning.
Let us analyze what exactly is involved in the reasoning. What are the assumptions?
If there can only exist one universe, then the anthropic principle does not explain anything
Suppose that laws of nature dictate that exactly one universe must exist. Not zero or 2.
Why are the laws of nature fine-tuned to allow biological life and sentient observers in that one universe?
The anthropic principle in this case does not explain anything. It could well be that the only universe is not suitable for life.
The existence of observers in our universe is an a posteriori observed fact. There is nothing a priori which requires observers.
Religious assumptions: the one universe must be fine-tuned for life
Let us assume that laws of nature dictate that exactly one universe must exist.
There might be a law of nature which requires that observers must exist in that one universe, and that I must be born as a creature which as an adult will become an observer. This would explain my a posteriori observation.
These assumptions resemble a religion: the laws of nature must be fine-tuned for life. It is like the creation myth in the Bible.
John D. Barrow and Frank J. Tipler formulated a hypothetical law of nature: the universe must be constructed in such a way that intelligent observers will arise.
A multiverse and the anthropic principle
Let us then assume that there exists a vast number of universes, with different laws of nature. Some models in "string theory" have this assumption: the multiverse.
Then there might exist a very large number of universes suitable for life. Many universes will contain observers.
Let us assume the following:
1. My "soul" is predestined to be born inside a creature which will be sentient and become an observer as an adult.
2. There are many universes with such creatures.
Then it is not surprising at all that I was born into a universe which contains observers.
Note that we need assumption 1, too, in addition to 2. If I would be predestined to be born either as a human, a rock, or an electron, then 2 would not explain why I happen to be an observer. Why am I not an electron?
Why do I find myself being one of the first intelligent observers on Earth?
There have been a huge number of vertebrates on Earth in the past 530 million years. Only about 10 billion humans have, so far, been aware of other galaxies besides the Milky Way.
Can an anthropic principle explain why I am among the first such observers on Earth?
If Earth could not harbor life, there would exist no such observers. This is the standard anthropic argument.
But there is no reason why I should be among the first. Why not the 10²⁰'th such observer?
We come back to the doomsday argument.
Observers could exist in the universe, but I could be a fish?
The a posteriori observation which I have made is that intelligent observers exist in a universe which contains billions of galaxies.
If I were a fish living in the year 2025, I would not have made such observation, even though it is true currently.
What do anthropic principles say about this?
They say that life must exist in our universe. But do they say anything about why I am not a fish?
An analogous question: suppose that on some exoplanet there exists an observer who is far superior to humans. A man is like a fish to this superobserver. Why was I born as a human (= fish), and not as the superobserver?
Weak anthropic principles basically say that for an observer of a type A to exist, the universe must be such that A can exist. That is close to a tautology. Obviously, it cannot answer to more complex questions, like "why was I not born as a fish?"
A human fetus may not be much more intelligent than an adult fish. If my "soul" would choose its birthplace by random from creatures which have some rudimentary intelligence, it is extremely unlikely that I would be born as a human fetus, which as an adult will be one of the first 10 billion humans to know about about galaxies.
We need stronger principles than the weak anthropic principles, to explain why "I" am among the first intelligent observers on Earth
Let us look at so-called strong anthropic principles.
Barrow and Tipler (1986) proposed at least the following variants:
1. laws of nature require that there must exists exactly one universe, and that universe must contain observers;
2. laws of nature require that any existing universe must contain observers (e.g., in quantum mechanics, an observer is required to make the wave function to collapse);
3. laws of nature require that there must exist many universes (and some must contain observers).
All these imply that there exists at least one universe with observers. But they do not explain why I was not born as a fish. They do not explain why I was able to observe that this universe contains observers. Furthermore, they do not explain why I am among the ~ 10 billion first intelligent observers on Earth.
The Copernican principle
The Copernican principle can be stated like this: the physical location of the Solar system is "typical" in the universe. It is not the center of any important cosmological structure. The principle is strictly in contradiction with the Ptolemaic model of the Solar system where Earth is the center of everything.
Empirical observations strongly support the Copernican principle.
What about the Copernican principle in the time dimension?
The doomsday argument is a Copernican principle with respect to time.
Do we exist at a "typical" time in the history of universe? No. The universe is expected to be very much suitable for life for ar least 1,000 billion years. We are living in a "young" universe.
The Copernican principle does not seem to hold on the surface of Earth. I am not a fish.
Conclusions
"Weak" anthropic principles are almost tautologies.
"Strong" anthropic principles contain a very brave hypothetical law of nature: a universe must necessarily produce "observers" at some point of time.
The Copernican principle is true for the spatial location of the Solar system, but it is not true with respect to the time dimension. Furthermore, on Earth the spatial Copernican principle does not hold at all: I was born a human and not a fish, even though there are more instances of a fish than a human.
Can we conclude that there must exist a mysterious law of nature "outside our universe", which places us to the current epoch and into conscious observers called humans?
Yes, that is the natural conclusion. Note that even if we would be placed at a random epoch, also that would a constitute a law of nature: you can expect to exist at a random epoch.
Ordinary laws of physics do not say anything about where and when we can expect to exist as an observer. Laws of quantum physics do talk about a collapse of a wave function caused by an "observer". Usually, people assume that the "observer" can be any large object which causes the wave function to "decohere". That does not require that I am the observer.
We can talk about natural laws of the placement of the subject. The placement seems to be non-random.
Suppose that you buy a new computer game and choose the character you are going to play in the game. The game is the "universe" and the character is the "observer". Obviously, the character will probably not be a random character in the game.
The hypothesis that our universe is a computer game, explains why we find ourselves living during a very special epoch as very special observers. The hypothesis implies strong anthropic principles: a computer game always contains "observers" if it has players.
What other hypothesis could explain our special position?
The doomsday argument is refuted because we are not born as random humans. The big oversight in the doomsday argument is that it assumes that we somehow know the prior probabilities of how we would be placed as observers. We do not know.
Suppose then that we would find ourselves as random observers. Why would we be random? What hypothesis could explain that?
In Plato's allegory of the cave, people have spent their entire life chained to the wall of a cave. They see shadows projected on the opposite wall. They do not see the real world, only shadows. However, through philosophy, one can learn about the real world.
Plato's allegory is somewhat similar to the computer game hypothesis. We are living inside a computer game. But through mathematics and logic we can learn something of the real world which exists outside the game.
When I started this blog in 2013, I, unfortunately, named it "metaphysical thoughts". The blog had been about physics, not metaphysics. The current blog post and the previous one can honestly be called "metaphysics". We finally have some content which fits the name of this blog!
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