It’s so crazy that I just happen to be in one of the rare places in our universe where
I don’t instantly asphyxiate or freeze or vaporize or dehydrate.
... Just lucky I guess.
Actually, it turns out that our very privileged perspective on the universe from Earth’s
comfortable biosphere may tell us a lot about our reality.
And perhaps resolves the Fermi Paradox.
It shouldn’t be surprising that we live on a planet that can support our existence,
in a universe that can produce such planets.
The anthropic principle tells us that we shouldn’t expect to find ourselves in some random corner
of the multiverse - there’s an observer bias.
In upcoming episodes we’ll be exploring this principle and its two main versions - the
strong and the weak anthropic principles.
The strong anthropic principle tells us that an observed universe must be able to produce
observers - and we’ll get to the implications of that soon - including the contentious idea
that this predicts the existence of universes beyond our own.
But today we’re going to focus on the weak anthropic principle, although it’s anything
It says that we must find ourselves in a part of the universe capable of supporting us.
For example, in a planetary biosphere rather than floating in the void between the galaxies.
This may seems tautological, but accounting for this observer selection bias is important
to understanding why the universe looks the way it does from our perspective.
And the weak anthropic principle is much more useful than that.
When combined with the apparent absence of alien civilizations, it may tell us that intelligent
life is incredibly rare in our universe.
To get to this, let's think about what it means to be an intelligent observer.
Your mental experience of thinking about these questions ... exists.
It's happening right now.
Your type of mental experience - your type of being - could be incredibly rare - even
unique - and only be possible in very unusual environments.
But you're having that experience - you ARE that experience.
So no matter how rare these you-supporting environments really are - by definition you’re
The weak anthropic principle places no limit on how rare those you-supporting environments
For example, if there’s only one life-bearing planet in the galaxy, or in the universe, you’re
going to be on it.
The rare earth hypothesis posits exactly this - that a range of factors made Earth
exceptionally unusual and uniquely able to produce intelligent life.
This hypothesis was inspired by some striking observations about our home planet, which
I’ll get to - but also by one other piece of evidence.
Or, rather, a lack thereof - the fact that we see no evidence for aliens.
The Fermi Paradox notes the apparent contradiction between the massive abundance of potential
opportunities for technical life to have emerged and spread through our galaxy and the apparent
lack of galactic civilizations.
Now, we’ve talked about the Fermi Paradox before, and some potential solutions.
But let’s consider the possibility that it is exactly how it seems - technological
civilizations are exceedingly rare - and maybe that’s because Earth is an exceedingly rare planet.
The solution to the Fermi paradox is often expressed in terms of one or more great filters
- extremely difficult or unlikely steps in the development from barren planet to visible
Such a filter might be after our own stage of development - climate change, nuclear obliteration, whatever
- still waiting out there to wipe us out.
But the Rare Earth hypothesis is a little more optimistic - it states that planets capable
of spawning civilizations even at our own level are very rare.
The idea was named and brought to popular attention in the book by
Peter Ward and Robert Brownlee in 2000.
It highlights a series of remarkable qualities of planet Earth that may have been needed
for life and intelligence to arise here.
Let’s take a look at them.
Actually, let’s start by something that is NOT rare about the Earth.
Earth-like planets are common.
And by Earth-like I mean rocky planets about the size of the Earth in orbit around stars very similar
to the Sun at the right distance to sustain liquid water on their surface - in the so-called
habitable or Goldilocks zone.
The Kepler mission has revealed there should be 10 billion or so in our galaxy - 40 billion
if we permit other star types.
So, billions of potential starting points for life in the Milky Way alone, even if we restrict
ourselves to boring old carbon-based water-loving planet life.
That’s billions of planets stewing for billions of years - if only one civilization had a
tiny head start on us then it could have colonized the galaxy by now.
Unless Earth has special qualities that mean true Earth-like planets are much rarer.
Let’s think about what Earth’s has got that seems critical for life and that could
If we see that even one other planet has some life-critical quality, then we know that that
quality could be relatively common.
But if we’ve only ever seen that quality on Earth then it could be hugely uncommon
- and the weak anthropic principle says it’s still not surprising for us to find ourselves
on one of the very few planets with that quality.
We’ll start by comparing planets of our solar system, because our ability to probe
extra-solar planets is still in its infancy.
Broadly, Earth has two qualities not shared by the other rocky planets in our solar system:
1) it has a very dynamic interior and 2) a very large moon.
Earth’s solid iron inner core spins suspended in a molten metal outer core, and this motion
generates a powerful magnetic field that protects Earth from dangerous space radiation and solar storms.
Above Earth’s core is a solid mantle which still flows due to its heat.
This drives plate tectonics on the surface - plates of Earth’s crust float around and
are periodic drawn back into the mantle, or subducted.
This results in shifting connections between ecosystems.
And that may have been a critical driver of evolution, promoting biodiversity.
The periodic subduction of tectonic plates recycles nutrients from the crust into the
mantle and then back into the atmosphere through volcanic activity.
Without this biogeochemical cycle, many life-critical elements may have been lost to the biosphere
So Earth’s dynamic interior seems to be life-critical in multiple ways.
By comparison, Mars is tectonically dead and Venus is at best tectonically weak - certainly
neither have protective geomagnetic fields.
We don’t know whether tectonic activity is rare in exoplanets, but it may be.
Which brings us to the moon.
Earth’s moon is ridiculously gigantic - no other rocky planet in our system has anything
Its size and also its composition and orbit suggest that it formed when a Mars-ish sized
planet collided with the Earth right after its formation.
The debris thrown up during this collision became our moon.
Now, this could be an incredibly rare scenario, even galaxy-wide.
It may also be that our moon and the event that formed it was critical to the development
That impact likely gave Earth its rapid rotation rate - with short nights essential for photosynthesis,
and also its axial tilt.
A moderate tilt could be critical if seasons are an important driver of evolution.
Too large a tilt and seasons become too extreme for life to thrive.
Earth’s tilt seems just right - perhaps even rare.
That impact may even have kickstarted Earth’s extreme tectonic activity by fragmenting Earth’s
early crust into moving plates.
And the moon’s later tidal influence may also be an important factor in enhancing ongoing
And a final possible result of our weirdly large moon is that it enabled the first appearance
It may have enabled abiogenesis.
One hypothesis for the first formation of life is that it evolved in tidal pools, with
complex chemicals and eventually proto-cells emerging as a primordial soup sloshed in these
pools, baking in the sun.
Without a large moon tides are half the size, so fewer tidal pools.
More recently, alternative hypotheses for the location of abiogenesis have gained favor
- particularly geothermal vents on the ocean floor.
OK, so Earth is weirdly dynamic and has a weirdly giant moon,
but there’s more.
Our entire planetary system is pretty weird.
We’ve only figured this out as the Kepler mission wrapped up its census of other planetary systems.
Our solar system has a huge range of planet properties - from the tiny rocky Mercury to
the gigantic gaseous Jupiter and Saturn.
In contrast, the planets of most other systems tend to be all around the same size as each other
and planets as large as Jupiter and Saturn are pretty rare - only around 10% of systems.
And yet Jupiter in particular was probably pretty important for the development of life.
That planet acts like a gigantic gravitational vacuum cleaner, absorbing a lot of the debris
left over from the formation of the solar system.
It no doubt sucked up many comets and asteroids would otherwise have hit the Earth.
If we’d had a significantly higher rate of mass-extinction-level impacts,
perhaps evolution would not have progressed so far.
Perhaps life would have been wiped out entirely.
There are a few other possible rare Earth factors - we may have an unusually hospitable atmosphere
and water content and may have been lucky in avoiding various cosmic catastrophes like gamma ray
But the final thing that may make Earth a cosmic rarity is the path taken by evolution.
Perhaps life is extremely common - or at least extremely simple life is common.
Perhaps the great filter is one or more extremely improbable steps that happened in the evolutionary
transition from single-cellular life to complex life, or to intelligence.
Just one example of this: the evolution of the eukaryote cell.
This seems to have been a freak evolutionary incident, in which two much simpler cell types
fused - one absorbing the other, perhaps in a failed attempt at dinner.
The absorbed cell became mitochondria, an energy power house that allowed the new chimerical
cell to massively increase its complexity, ultimately leading to the first multicellular
There are many factors that shaped Earth’s formation and development - what if
the Cambrian explosion had never happened, or the asteroid never wiped out the dinosaurs, or an extra
asteroid wiped out our ancestors?
There are lots of ways that it seems Earth got lucky.
The question raised by the rare earth hypothesis is just how lucky were we?
Many of Earth’s life-critical qualities or development steps have not been seen elsewhere,
nor do they seem to have happened more than once - at least yet.
The weak anthropic principle allows that these singular events were phenomenally unlikely
- we simply can’t assign them larger probabilities until we get more evidence - which ideally
means seeing them happen more than once.
It’s very possible that a combination of extremely unlikely factors means it’s extremely
rare for planets to spawn intelligence.
The Fermi paradox surely has a solution, and that solution may be that the galaxy is as
empty as it looks.
We find ourselves in the only place we could be: gazing out from our rare earth into the
untamed, unpopulated reaches of spacetime.
We skipped comment responses last episode, so today we're covering two episodes -
loop quantum gravity and time travel.
A few of you wondered if there's a connection between the loops of loop quantum gravity
and the closed strings of string theory.
The answer is not really.
The strings of string theory have a somewhat physical interpretation -
the fact that they can hold energy and vibrate and exist in space.
But the loops of LQG aren't really physical at all.
Which brings us to the most common question - what actually ARE the loops of loop quantum gravity?
Well, crudely, they're a mathematical way to describe the geometry of space - which
means they aren't in space, they sort of ARE space.
Or at least familar 3-D space emerges in an abstract way when we think about a network
of these loops.
But that doesn't mean the loops are physical, they're just a way to parameterize the quantum-scale
geometry of space.
Where we transition from the abstract to the physical is not at all clear, and, honestly,
hurts my head.
Dig into spin networks and spin foam if you want a headache too.
Serenity Receiver asked about the experiment to test Loop Quantum Gravity
So LQG predicts that light of different wavelengths travels at very slightly different speeds.
And this was NOT observed in the light from a distant gamma ray burst, which presents a
challenge for the theory.
So why does LQG predict different speeds of light?
Well, if space is quantized on tiny scales, then we expect the very shortest wavelengths of
light to be slightly perturbed by these quantum cells of space - sort of like traveling through
cracked glass - they interact with the edges and slow down.
Wavelengths longer than this quantum scale can ignore this fragmentation and so travel
at normal speed.
There were some good questions on the Typler cylinder - that's the infintiely long cylinder
that you can travel around to end up back where you started - in both time and space.
Some One asks if a circle in 2 spatial dimensions would allow for closed time like curves.
I saw this in at least one paper.
Time travel is easy in flatland.
Troy Henry asked if a torus would serve as an infinitely long cylinder
- well the answer is, sadly, no.
The cylinder has to literally go on forever in both directions.
At the beginning of the time travel episode I invited future time travelers to show up
on set. None of you did.
But several of you got onto the comments to say I forgot to post the address of the studio one year from now.
But I have to ask - why did you wait until after that episode to tell me that?
Surely as time travelers you could have reminded me in the loop quantum gravity comments the week before.
Or maybe Stephen Hawking's chronology projection conjecture prohibits anyone from remembering
to post addresses to time traveler parties.
But Guy Frost has a better explanation - YouTube won't survive into the far future.
I guess when it goes offline you can blame that invitation.