Here's one totally uncontroversial statement.
Life exists in our universe.
Okay. Let's try another one.
Therefore our universe is capable of producing and supporting life.
Okay. Let try one final uncontroversial statement.
Therefore there are countless universes.
Hmmm.
Our universe seems to operate according to a set of fundamental rules that we try to
understand and model with the equations of our laws of physics.
Those equations always include one or more fundamental constants - simple numbers that
set the scale for the equation.
We can’t determine the values of these constants from pure theory - we have to measure them
in the real universe.
These are things like the speed of light, the Planck constant, the masses of the elementary
particles, and the constants defining the relative strengths of the fundamental forces
- the so-called coupling constants.
Between the general theory of relativity and the standard model of particle physics, there
are something like 20 independent fundamental constants of nature.
20 free parameters, like movable dials that seem to be tuned to very specific numbers
in this universe.
Perhaps in other universes they could be set to other values entirely.
Or perhaps they’re connected and controlled by some master dial or dials - whose nature
awaits discovery in the long-sought theory of everything.
But whether there are few or many free parameters defining the physics of this universe, one
thing is becoming increasingly clear: it’s good that they have the values that they do.
If the constants defining our universe were different - a tiny bit different in some cases
- then our universe would be vastly different - and probably unable to produce galaxies,
or stars, or life.
We seem to live in a fine-tuned universe - a goldilocks universe - and this seems to imply
one of the following must be true: 1) we got lucky - the 20-odd dials just happened to
land in the very narrow range of numbers that allow life to form. Or 2) someone fiddled with
the dials - be it god or whoever built the simulation. Or 3) we didn’t get lucky and
there was no fiddling - it’s just that there are enough universes that somewhere the dials
had to end up with the right settings for life, and naturally that’s where we find
ourselves.
This last possibility draws on something called the anthropic principle.
In the last episode we talked about the anthropic principle as it related to where we find ourselves
in this universe - necessarily within what may be an extremely rare habitable biosphere.
We must find ourselves at a place and time in the universe capable of producing observers.
This is the so-called “weak” anthropic principle, as defined by astrophysicist
Brandon Carter in 1973.
Well, Carter also has a strong version. The universe must be such as to admit the creation of observers
within it at some stage.
By themselves, neither version of the anthropic principle explain why life-friendly planets
or universes exist; they just say that if such places do exist at all, that’s where we
must find ourselves.
The strong anthropic principle tells us that there’s an observer selection bias
that may help us understand why we live in such a finely-tuned universe.
But in order for that explanation to work, we need to propose that many NON-finely tuned
universes exist as well - in the same way that many uninhabitable planets exist.
We’re going to delve much more deeply into the implications and validity of that argument
in an upcoming episode.
Today I want to set the stage by showing you just how finely-tuned this universe seems
to be.
Let’s start with chemistry.
In our universe, quarks tend to stick together to form protons and neutrons, which stick
together and attract electrons to form atoms.
The periodic table is filled with different elements - different atom types - which interact
with each other according to their complex electron shell structures.
These chemical interactions give us different states of matter and a vast family of molecules,
which in turn are capable of forming structures of incredible complexity - including life.
There’s no other mechanism in the universe capable of similar natural complexity - at
least as far as we can tell - which is, frankly, pretty far.
But it wasn’t at all inevitable that our universe had to be capable of life-enabling
chemistry, or of complex chemistry at all.
One of the first clues to the fine-tuned nature of this universe was noticed by astrophysicist
Fred Hoyle in the early 80s.
He realized that with a tiny tweak of certain properties of the carbon nucleus, the universe
would have hardly any carbon.
The vast majority of carbon in the universe is produced in the cores of massive stars.
By happy chance, the carbon-12 nucleus formed in this process has a natural internal energy
state that allows it to quickly radiate away the excess energy it had on formation, settling
down into a stable form.
Without that convenient way to shed energy it would quickly tear itself apart again.
Carbon would no longer be abundant on the surfaces of new planets, making the emergence
carbon-based life near impossible.
Other slight changes in nuclear fine tuning would also massively reduce the amount of
oxygen that stars produce.
This all comes does to the fine-tuning of the strength of the strong nuclear force.
If it were half a percent higher or lower, carbon-based life wouldn’t be a thing.
The balance between the fundamental forces in general seems to be just right for complex chemisty.
If the strong force were a bit stronger, protons would be able to bind to each other to form
a diproton - a neutron-free version of helium which is unstable in our universe.
But if it were stable, this stuff would be like superfuel for stars - meaning all stars
in the universe would have burned out before life ever got a chance.
On the other hand, if the strong force were a bit weaker then deuterium - heavy hydrogen
- becomes unstable, eliminating one of the key steps in the fusion process inside stars
- so stars like our sun wouldn’t burn at all.
Now, this all assumes that the universe has the same strength of gravity as our real one.
With weaker gravity, diproton-burning stars could last longer, and with stronger gravity, fusion
could skip the deuterium step.
But the fine-tuning is still there - we need the right balance between between the strengths
of the strong nuclear force and gravity.
Similar balances are needed between all of the forces.
The stability of atoms and the rate of fusion in stars and in the early universe depends
on the balance between electromagnetism and the strong force.
Mess with these much in either direction and the universe remains a mist of subatomic particles.
Even the weak nuclear force is important; it regulates the conversion of protons into
neutrons, and it seems to have about the right strength to ensure that there are about the
same number of protons as neutrons in the early universe.
Without neutrons, no elements heavier than hydrogen would be possible.
The strengths of the fundamental forces are set by the so-called coupling constants - for
example, the fine structure constant for electromagnetism and the gravitational constant for gravity.
They span a vast range of values nearly 10^40 from the strong nuclear force to gravity,
and there’s no apparent pattern in their values. As far as we know, they could have
taken on any values in that range.
Another set of free parameters are the masses of the elementary particles.
These are determined by the interaction of those particles with the Higgs field - but
again, there’s no apparent pattern and we don’t know why they take on the exact values
that they do.
But they also seem tuned for life.
For example, if quarks or electrons had significantly different masses we’d once again be in a
chemistry-free cosmos.
OK, so it seems that the balance of the strengths of the forces and the masses of the elementary particles, is just right for things like stars
and complex matter to form in our universe.
But an even more finely-tuned parameter appears to be the cosmological constant.
It represents the action of dark energy.
The existence of dark energy was discovered in the 90s as astronomers tried to measure
the expansion rate of the universe, only to find that expansion was accelerating.
This didn’t come as a complete surprise to everyone.
Physicists knew that empty space itself could have its own energy.
The quantum fields, which fill all of space and whose oscillations produce the familiar
particles of matter or radiation, were expected to have a so-called zero-point energy.
They should interact with themselves even when there are no particles around, resulting
in a quantum buzz of energy everywhere in the universe that would accelerate its expansion.
The problem is, when we try to calculate the strength of this vacuum energy from our theory
we get an enormous number - 10^60 to 10^120 larger than actually seems to be present in
dark energy.
Such a universe would expand so quickly that no structure would ever form.
If fact, that’s true if the strength of vacuum energy were only a little larger than
is observed.
So there must be factors which bring that number down to nearly zero.
One possibility is that the zero-point energies of unknown quantum fields cancel out the known
contributions.
But to cancel out 60 or 120 orders of magnitude almost perfectly, but not completely perfectly?
The odds would be something like 1 in 10^60 or 10^120 against.
Now there are other theoretical ways to get
the vacuum energy to take on different values in different regions of the universe - or
in different universes - and I’ll come back to those.
But even so, odds are stacked enormously against a universe that doesn’t blow itself up before
life has a chance.
We don’t know why the dials of our universe are set the way they are.
Leaving aside the blind luck or intentional knob-fiddling for now, let’s get back to
the anthropic principle.
If there are many universes, and the physics of these universes can vary, then maybe we’re
just in one of the few universes with the right physics for life - just like we’re
on one of the few planets with the conditions suitable for life.
But that iss a big if.
Can we justify the idea of a multiverse with changing fundamental constants?
Well it turns out that multiple rather well regarded ideas do support this.
For example we have string theory: this candidate theory of everything suggests that the values
of the fundamental constants, including the vacuum energy, are set by the particular configuration
of the extra coiled dimensions of that theory.
Countless such configurations are possible, with any given universe - or perhaps every
section of the same universe - having different properties.
And there are other proposals for how the fundamental constants might vary over space
or between universes.
But how to get enough universes to make sure that our extremely improbable configuration
of dials becomes a sure thing?
Again, there are multiple options.
Andre Linde’s eternal inflation is perhaps the most popular - bubble universes forming
in a larger exponentially expanding spacetime, and in each bubble the constants of nature
- and especially the vacuum energy - taking on different values.
In fact we covered this already.
Or Lee Smolin’s idea that universes are born when black holes form, with each new
universe having slightly different fundamental constants to its parents.
Now we’ll come back to that one.
Coupled with some popular, albeit speculative ideas about multiverses, the strong anthropic
principle seems to make sense of the incredible fine tuning of our own universe.
But does that fine tuning actually predict the multiverse?
Well this is a highly controversial point, but despite popular opinion it’s not untestable.
And we will test it - next time, and so perhaps make a little more sense of our extremely
fine-tuned patch of space time.
In the last episode we discussed the Rare Earth Hypothesis - the idea that planets supporting
intelligent life may be extremely rare, and how this relates to the weak anthropic principle.
The comment section got rowdy.
Of course some of you mentioned some "rare earth" qualities that we didn't get to.
Spider Juice points out that the formation of the moon due to a giant impact would also
have increased the amount of iron in Earth's core, which may help explain the strength
of our protective magnetic field.
The scenario is that some mars-sized planet plowed into early earth, causing a lot of
Earth's rocky crust to be thrown into orbit before congealing into the moon.
Meanwhile, Earth's iron core would have grown as it absorved the iron core of the smaller
planet.
The result: more iron, less rock, more magnetic field.
Inkblack wanted us to mention the grand tack hypothesis.
This is the idea that Jupiter may have wandered into the inner solar system during the solar system's formation,
disrupting the planets that were forming there before moving back out again to its current location.
Now, the current inner planets are therefore much smaller than they would have been because
they had to form from the leftover material.
If life requires smaller rocky planets rather than super-earths then this could definitely
be a rare-earth property.
Quite a few ifs here, but it's a good thought.
Bill Williams disagrees with a lot of these 'rare earth' type scenarios because the factors
we talk about are needed for OUR evolutionary tree, but other types of life are no doubt
possible.
This is a valuable point to bring up, and several eminent scientists feel the same.
My counterpoint is that, although we don't know if any one rare quality of Earth is absolutely necessary
for life, there do seem to be quite a lot of qualities that are very non-standard for planets, even
as we learn more and more about other planetary systems.
That's also true of properties of our universe - yeah you might be able to get different
types of complexity and even life by tweaking the fundamental constants in different ways,
but there seem to be way WAY more combinations of properties that can't possibily give you
a life-friendly universe.
Fine tuning of the universe is hard to deny - the fine tuning of our planet is less clear,
but the weak anthropic principle makes it a totally plausible possibility.
Roman 213 points out that endosymbiosis - the process by which we think mitochondria got
inside the first eukarote cell - actually did happen multiple times for different types
of cells, and therefore doesn't qualify as a great filter.
That's a very nice point, Roman, although this particular point was only intended as an example of evolutionary great
filters - and I certainly didn't come up with this one myself.
It's also worth pointing out that this particular evolutionary event, the first evolution of the eukaryote cell, took at least half of life's
entire evolutionary history to happen.
That seems rare enough.
Many of you said you found the episode depressing - as in, sad to think we might be all alone
in the galaxy. Well I know what you mean, but honestly, I find it kind of inspiring.
I mean, what an awesome responsibility - to be among the first intelligent civilizations
in our galaxy.
If that's actually the case. But whether or not it's the case, being a rare potential progenitor
of galactic civilization, seems like such a huge responsibility that maybe we should start acting as though the stakes
really are that high and not screw it all up.
On a related note, several of you commented on how the grey in my beard is so much more
noticable now that we've moved to 4k.
Actually these just came in while I was writing the rare earth episode, as soon as I started
thinking about how we're totally screwing it all up.