- 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's try one final uncontroversial statement.

Therefore, there are countless universes.

(hums) (funky music) 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 to finding 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 is 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 sort 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.

One, we got lucky.

The 20 odd dials just happened to land in the very narrow range of numbers that allow life to form.

Or two, someone fiddled with the dials, be it God, or whoever built the simulation.

Or three, 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 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 exists.

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 wanna 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 of 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 down 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 chemistry.

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 super fuel for stars, meaning all stars in the universe would've 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 our hypothetical 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 the strengths of the strong nuclear force and gravity.

Similar balances are needed between all of the forces.

The stability of atoms in the rate of fusion in stars and in the other 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 other 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 to the power of 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 seemed tuned for life.

For example, if quarks or electrons had significantly different masses, we'd once again be in a chemistry free cosmos.

Okay, 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 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 to the power of 60 to 10 to the power of 120, larger than actually seems to be present in dark energy.

Such a universe would expand so quickly that no structure would ever form.

In 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?

Well, the odds against that are something like one in 10 to the 60, or one in 120 against.

Now, there are 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 is 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 coil 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.

Bubbled universe that's forming in a larger, exponentially expanding space-time, and in each bubble, the constants of nature, and especially the vacuum energy, taking on different values.

In fact, we covered this already.

Or there's 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 in an upcoming episode.

And in doing so, perhaps make a little more sense of our extremely fine-tuned patch of space-time.