- The moment you started observing reality, you hopelessly collude to any conclusions you might make about it.

The anthropic principle guarantees that you are not seeing the universe in its most typical state.

When used correctly, this highly controversial idea can be extremely powerful.

So how do you correctly use the anthropic principle?

(upbeat digital music) In 1543, our perspective of the cosmos was radically disrupted.

Nicholas Copernicus presented his model for the solar system which demoted the Earth from its position at the center of the universe to just one of several planets orbiting the sun.

As our astronomy improved, we realized that our sun is a typical example out of hundreds of billions of stars in the Milky Way and that the Milky Way is an ordinary galaxy among hundreds of billions of galaxies in the observable universe.

The notion that we occupy neither a central nor a privileged position in the universe is called the Copernican principle after the guy who started it.

It's extremely important.

It allows us to study the distant universe confident that its law of physics are the same as we experience on Earth.

It allows us to understand the origin of the Earth and the Milky Way by studying the ancient light of distant galaxies.

The Copernican principle became such a powerful tool that it took centuries for us to realize that it's wrong.

Or at least flawed.

It states that we don't occupy a central, privileged position but that's only half true.

Earth certainly isn't central but it is privileged.

And not at all a typical environment.

Nor perhaps is our entire universe.

Our planet and our universe must have at least one non typical quality.

They must have been able to produce us.

To give rise to living creatures that could observe it.

This is the anthropic principle and it seems to contradict the sacred Copernican principle.

Over the last two episodes, we talked about certain observations over the uniqueness both of our planet and of the universe.

And how these feed into two versions of the anthropic principle.

Today, we're gonna bring these ideas together with the Copernican principle to see just how powerful and how misleading the anthropic principle can be.

According to the original definitions by Brandon Carter, the weak anthropic principle states that we must live in a place and a time in the universe capable of supporting observers.

In our case, a habitable biosphere.

And the strong anthropic principle which states that the universe itself must have the conditions necessary for producing environments that in turn produce observers.

That means the fundamental constants and the initial conditions of the universe must be just right to allow nice, inhabitable planets to form.

Let's just call all of this the anthropic principle.

We necessarily observe from an environment capable of producing observers.

Be that environment a planet within an universe or universe within a multiverse.

Parts of this statement is uncontroversial.

Some would even say total logical but things get interesting when you add the fact that our universe seems to have fundamental constants and initial conditions that seem extremely fine tuned to the eventual formation of life.

The anthropic principle permits a new explanation for this fine tuning besides blind luck or design.

If there are enough universes or enough regions within this universe and their properties can vary.

Then even if the vast majority are lifeless, life friendly universes can still show up.

And we shouldn't be surprise that we find ourselves in one even though Earth and a life supporting universe might be quite atypical.

This use of the anthropic principle is highly contentious.

Many scientists find it extremely unsatisfying and lazy and unscientific.

After all it doesn't tell us why the fundamental constants take on the values they do or why they may vary between universes or even why there should be multiple universes.

Some of the bad rep for the anthropic principle comes from a distortion of the idea.

In the 1986 book, "The Anthropic Cosmological Principle," John Barrow and Frank Tipler misinterpret the strong anthropic principle to mean that the evolution of observers somehow had some retro cause or influence on the initial formation of the universe.

But this commits the same flaw in reasoning that the anthropic principle may solve.

The principle is not causal.

It just tells us to account for an observer selection bias when interpreting the nature of our environment.

So our finely tuned part of the greater cosmos doesn't have to have been custom built for us.

The other common misuse is to assume the anthropic principle allows for any degree of extreme fine tuning of the properties of our environment regardless of the global distribution of properties.

Here's an example originally laid out by Roger Penrose.

The universe is most finely tuned parameter is its unthinkably low initial entropy.

The incredible density of the Big Bang was a highly ordered state.

All particles in the observable universe were packed together in a subatomic sized dot.

Everything interesting that's happened since from the formation of stars and galaxies to the evolution of life has been powered by the slow increase in entropy from that initial state.

The universe will spend the vast majority of its perhaps infinite life in a state of extreme disorder and high entropy.

Ion stars, black holes, and a mist of cold elementary particles, not very hospitable to life.

The second law of thermodynamics tells us that entropy can only increase.

Which means extreme high entropy states must be the norm in the form timeline of our universe but probably also across the multiverse if it exists.

We certainly don't observe the universe in a typical observer hostile state and so it's tempting to use the anthropic principle here.

Low entropy regions should be a vastly less common than high entropy regions but if they exist we're gonna find ourselves in one but guess what, that statement is wrong.

Or at least incomplete.

The anthropic principle appears to fail here.

But to understand why we need to go all the way back to the Copernican principle.

In fact, we need to bring these two seemingly conflicting principles together.

And in doing so, we'll end up with a much more powerful version of the anthropic principle.

One that will even make testable predictions.

By the Copernican principle, we are most likely to observe a very typical environment.

This is just a statement of probability.

There are more typical environments than non typical ones so pick a random environment and it's probably typical.

But the anthropic principle tells us we must account for our status as observers when we interpret our environment including the probability of being in it.

So let's formulate a refined anthropic principle.

We should expect to find ourselves in a typical region of the cosmos that is consistent with us being observers.

And by cosmos, I mean some total of reality be it universe or multiverse that allows us to be in a rare observer friendly environment but tells us that we should be in the most typical of such environments.

Our perspective on the cosmos might be a rare one but it should be no more rare than is necessary to explain our existence.

This refined anthropic principle gets really interesting when applied to our low entropy big bang.

If low entropy regions happened just by chance fluctuations from a high entropy state then the lower the entropy, the less probability of that region forming.

For example, a fluctuation the size of a galaxy is insanely more likely than one the size of our observable universe.

It's much easier to produce hundreds of billions of galaxy size fluctuations than a single fluctuation of hundreds of billions of galaxies.

You surely don't need more than one galaxy to spawn a life bearing planet.

So there should be many more observers in small entry fluctuations than in large ones.

Our refined anthropic principle appears to fail here but actually it doesn't.

We've just witnessed the potential power of this principle.

Under the assumption that our universe resulted from a single random fluctuation in an otherwise high entropy cosmos, the anthropic principle predicts that we should be in the smallest such fluctuation that could produce us.

Apparently we aren't and so we could probably rule out a simple random entropy fluctuation as a sufficient explanation for our big bang.

Or at the very least, we need extra physics.

For example, we need the right initial density fluctuations to evolve and expand for the right amount of time so that live would always be in big universes.

If we then assume that the starting conditions for our universe were typical, that can tell us something about the physics of how universes are born.

For the anthropic principle to be useful and not misleading, we need to be careful.

Philosopher Nick Bostrom has made valiant attempts to clean up what he calls anthropic reasoning in his book "Anthropic Bias."

In it he defines, though doesn't invent, the self-sampling assumption which states that all other things equal, an observer should reason as if they are randomly selected from the set of all actually existent observers, past, present, and future, in their reference class.

If there are a bazillion observers in the entire cosmos, you should consider yourself randomly selected from them.

That means you're most likely a common type of observer and in a common environment in which observers can exist.

The anthropic principle and the self-sampling assumption encourage Bayes' Theorem thinking.

We should take into account what we already know, our priors, before assessing any probabilities.

The prior is that we are an observer but proper Bayes' Theorem thinking requires careful definition of priors.

For example, Bostrom talks about observers in their reference class but how do we know what our reference class is?

Carbon based sentient life?

All conscious entities?

Anything capable of thinking about the anthropic principle?

Even with careful definitions, anthropic reasoning is difficult to use well and that's evident in the bizarre predictions it can make from Boltzmann brains which we covered to the Doomsday arguments to the idea that self-sampling assumption predicts that the end of the world is nigh.

Which we'll cover soon, hopefully soon enough.

The anthropic principle in its proper form is without question an important thing to take into account whenever we observe the universe.

It's one possible explanation for why our planet and our universe appear to fit us so well.

Even if they weren't intentionally made for us.

Douglas Adams put it best.

If you imagine a puddle waking up one morning and thinking, this is an interesting world I find myself in, an interesting hole I find myself in, it fits me rather neatly, doesn't it?

In fact it fits me staggeringly well.

Must have been made to have me in it.

This is such a powerful idea that as the sun shines in the sky and the air heats up and as gradually the puddle gets smaller and smaller, it still frantically hanging onto the notion that everything's going to be all right because this world was meant to have him in it, was built to have him in it.

So the moment he disappears catches him rather by surprise.

I think this may be something we need to be on the watch out for.

Used well, the anthropic principle gives us a deep perspective on our place in the cosmos.

And can also be a powerful, albeit slippery, scientific tool.

But misused, it can lead to unscientific conclusions as can any tool in science.

We'll come back to some of the wilder predictions of both good and bad anthropic reasoning real soon and talk about what you can know about your universe given your privileged status as a typical conscious observer of space time.

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