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.
PBS spacetime is definitely how I got into an armchair cosmologist!
The laws of physics are also fine tuned to allow rocks.
Wasn't this about the rare earth hypothesis