The moment you started observing reality, you hopelessly polluted any conclusions you
might make about it.
The anthropic principle guarantees that you are NOT seeing the universe in its most typical state.
But used correctly, this highly controversial idea can be extremely powerful.
So, how do you correctly use the anthropic principle?
In 1543 our perspective of the cosmos was radically disrupted.
Nicholaus Copernicus presented his model of the solar system which demoted the Earth from
its position at the center of the universe into just one of several planets orbiting
the Sun.
As our astronomy improved, we realized that our sun is a typical example out of
100s of billions of stars in the Milky Way, and that the Milky Way is an ordinary galaxy among
100s of billions of galaxies in the observable universe.
The notion that we occupy neither a central nor privileged position in the universe is
called the Copernican principle after the guy who started it all.
It’s extremely important - it allows us to study the distant universe confident that
its laws 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 can 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 of the uniqueness both of our
planet and of the universe, and how these feed into two versions of the anthropic principle.
Today we’re going to 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 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 initial conditions of the universe must be just right
to allow nice habitable planets to form.
Let’s just call it the anthropic principle: we necessarily observe from an environment
capable of producing observers; be that environment a planet within a universe or a universe within
a multiverse.
By itself, this statement is uncontroversial - some would even say tautological.
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 for 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 surprised 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 that they do
- or why they may vary between universes. Or even why their should be multiple universes.
Some of the bad rap of 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
causal 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 that the anthropic principle allows for any degree of extreme
fine tuning of the properties of our own environment, regardless of the global distribution of properties.
Here’s an example, originally laid out by Roger Penrose.
The universe’s most finely-tuned parameter is its unthinkably low initial entropy.
The incredible density at 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 - iron 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 full 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
and so it’s tempting to use the anthropic principle here.
Low-entropy regions should be 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 its probably typical.
But 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 find ourselves in a typical region
of the cosmos that is consistent with us being observers.
And by cosmos I mean the sum 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 100s of billions of galaxy-sized fluctuations than a single
fluctuation of 100s 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 entropy 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 simple 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 can 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 life may 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, although 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 bajillion 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 Bayesian 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 Bayesian 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 very careful definitions, anthropic reasoning is difficult to use well.
And that’s evident in the various bizarre predictions it can make - from Boltzmann brains,
which we covered, to the doomsday argument - the idea that the 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.
Douglass 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
— 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 rises in the sky and the air heats up and
as, gradually, the puddle gets smaller and smaller, it's still frantically hanging on
to the notion that everything's going to be alright, 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.
Today we have a very special thank you to Dr. Alex Flournoy of the Colorado School of Mines.
Dr. Flournoy, several of your students wanted to thank you for unlocking the mysteries of physics for them and
to let you know their thoughts are with you by sponsoring this video at the Big Bang level.
Now based on the very touching words they sent to us you are clearly some kind
of a hero, who’s super power is to spread their vast knowledge of physics across the
multi-verse and unleash a Star Trek-esque science utopia here on Earth.
To speed that up, we've linked to your amazing lectures on particle physics and general relativity
in the description.
So from your students and everyone here at Space Time we wish you well.
Last week we talked about how the constants of nature seem to be fine tuned for life in
our universe - and how this may imply that there are countless universes beyond our own.
Discussion in the comments was lively, to say the least.
Many people had the following objection: they say that the universe isn't really fine-tuned
for life or for observers because there could be many types of observer very different to
ourselves, that could potentially exist if the fundamental constants were different.
Well, actually, fine tuning arguments for the fundamental constants for the most part take that into
account.
We can probably assume that for an intelligent observer to emerge in any universe, that universe
must be capable of forming complex structures - whether or not it looks like life as we
know it.
So the universe needs to last a reasonable amount of time, have stable regions and energy
sources for those structures to form, and have some building blocks - whether or not
they look like atoms as we know them.
Much of the parameter space that the constants of nature could have taken eliminate one or
more of these factors.
So while there may be many small parts of that parameter space where observers can arise,
most of it - hence most universes - should be devoid of observers.
A few of you also pointed out that I missed one possibility - perhaps the dials defining
the fundamental constants were neither randomly set nor deliberately tuned.
Perhaps there's some unknown physics prinicple that demands they have exaclty the values
that they do.
Well right - that's possible - but the point is that UNLESS that principle is somehow connected
to the universe's later developing life and structure, why should it have landed on
one of the rare combinations amenable to life and structure?
It's just as easy to imagine a physical principle that gives you only one universe with an unavoidable
combination of fundamental constants that was completely devoid of life.
So I still count this option as either "getting lucky" or that the later emergence of life
was somehow retrocausal of the universe's knob-setting.
Between these options and the multiverse, I think I prefer the multiverse.
AdlockHungry makes a great point: surely if this were the Goldilocks universe there
would be life on almost every planetary body, even in this solar system.
That would certainly be true if such fertile universes where anywhere near as common as
our relatively barren one.
The self-sampling assumption says we should assume we're a typical observer - so maybe
the most typical observers are in relatively barren universes, and there are just way more
of those universes.
That may be telling us something about how the constants of nature get set - it may be
improbable to get those constants fine-tuned for life - but the more fine-tuned they are,
the more improbable.
Keith Strang correctly points out that we also have to talk about fine tuning of parameters
to get a multiverse in the first place.
Physicists are struggling to make eternal inflation work without its own fine tuning
that may be just as bad as the fine tuning needed for life in our universe.
Well that's a challenge, but inflation is still not so well understood, so it seems there's
plenty of opportunity to solve this.
If it can't be solved then we should ultimately dismiss the theory.
enotdetcelfer has an idea for a physics problem: iterate through the settings of the constants
of nature and figure out the map of possible setting that support self-propagating pattern entities.
In other words, find all possible combinations of constants that can produce observers.
Well that sounds like a great challenge question!
If anyone does this they get a free universe.
And a t-shirt.
Although if you can do this you can probably also build your own portal gun and visit all
the universes you want.