MATT O'DOWD: Thanks to brilliant.org for supporting
PBS Digital Studios.
Feeling a little chilly this winter?
Never fear.
In several hundred million years,
the brightening sun will turn the poles
into tropical paradises.
And then evaporate the oceans and extinguish all life.
The sun is slowly burning through its fuel.
Hydrogen is fused into helium in the sun's core,
producing energy that keeps it shining
and keeps the earth warm and hospitable to life.
But that fuel will run out, after which
the sun will swell into a red giant and flash fry the earth.
In fact, that frying, well slow roasting,
will begin much earlier.
See, the sun is getting brighter even now.
This has complex, and for the most part terrible,
implications for life.
The end of the world will come sooner than you think.
All stars brighten as they age and they deplete their fuel.
Now that sounds counterintuitive.
Why does less fuel mean a brighter star?
Well, the sun is able to remain as a giant ball
of fiery hydrogen due to a delicate balance
between the outer flow of energy produced by fusion in the core
and the gravitational crush of its immense mass.
The rate of fusion of hydrogen into helium
increases with the temperature and density of the core.
So when the sun was born, it's initial gravitational collapse
from a giant gas cloud was halted
as soon as the core became dense enough, dense enough for fusion
to produce exactly the energy to balance
that gravitational crush.
At that point, the sun had plenty of hydrogen fuel.
But over time that hydrogen has been diluted.
As the sun's fuel gets diluted, you
might expect fusion to slow down.
But as soon as that starts to happen,
the drop in energy production disrupts the delicate balance
between outward pressure and gravity.
The core collapses slightly under the immense weight
above it.
That increases the density and temperature
of the core which increases fusion rate,
once again re-establishing equilibrium
between gravitational crush and energy production.
But with the same mass packed into a smaller volume,
the core is under more gravitational pressure
than before.
That means it needs to increase its energy output
to resist the increased crush.
So over time, the sun's core shrinks and heats up,
brightening the entire star.
This increased energy output actually
causes its outer layers to expand slightly
even as the core shrinks.
This all takes billions of years.
It's been calculated that the sun should currently
be increasing in energy output by close to 1%
every 100 million years.
But that rate will increase.
Now that's way too small to have any effect on human time
scales.
But it's huge on geological time scales.
Now the sun was around 30% dimmer and 10% smaller
when it first formed, but it will be about 2/3
again as bright and a third again as large before it
finishes burning it store of hydrogen fuel in around 5
billion years.
But how much more energy will be too much?
And what does this mean for the Earth?
Well, after a brief boom in the Antarctic beach front property
market, the oceans will evaporate.
But that's not the first disaster
to result from the sun's brightening.
In around 600 million years, most photosynthesis
will shut down, killing the majority of plant life.
And this is due to CO2 leaching from the atmosphere.
Normally we associate higher temperatures
with increased CO2.
CO2 warms the surface due to its greenhouse effect
and warm oceans are less able to absorb
carbon so more CO2 is released.
This dangerous feedback cycle dominates
on the time scales of the glaciation cycle
and of anthropogenic climate change.
However, over hundreds of millions of years,
CO2 will actually be lost due to rising temperature.
See, the rate of weathering of silicate rocks
increases with temperature and that process sucks in CO2
to produce carbonate minerals.
In around 600 million years, CO2 levels
will be at 50 parts per million compared to the current 400
parts per million.
And this will be too low for photosynthesis
by C3 carbon fixation.
The vast majority of plant species
rely on C3 photosynthesis including all trees.
So these will all die.
Plants using the more efficient C4 carbon fixation, including
many of the grasses, will hold out for a bit
but not for long with the ever increasing temperatures
dropping CO2 and the evaporating oceans.
About them oceans, currently Earth
is comfortably inside the solar system's Goldilocks
or habitable zone.
This is the distance from the sun
where sustained liquid water is possible.
The brightening of the sun means the habitable zone
has shifted outwards since the formation of the solar system.
In fact, in the beginning when the sun was
at 70% of its current brightness,
Earth would have been outside the outer edge
of the habitable zone, at least given its current atmosphere.
Its entire surface should've been frozen solid.
Yet the geological record tells us
that there was plenty of water back then
and life appeared quickly.
Earth was highly habitable.
This discrepancy between the astrophysical prediction
and the geological evidence is called the faint young sun
paradox, and was pointed out by Carl Sagan and George Mullen
in the early '70s.
But planet surface temperature and the location
of the habitable zone depends on the planet's atmosphere
as well as the star's brighteners.
If the early Earth had a much stronger greenhouse
effect due to high carbon dioxide or methane content,
then the increased heat retention
may have compensated for the fainter sun.
There are a few plausible scenarios
that give the right levels of greenhouse gases.
However, there is not yet a consensus on the exact solution
to this paradox.
Whatever the solution, life has found a way
to exist on Earth for at least 3 and 1/2 billion years.
But life doesn't have such a long stretch ahead of it.
The Goldilocks zone will expand beyond Earth's orbit.
And depending on how our atmosphere evolves,
our beautiful blue orb may become a desert planet
relatively soon.
Now the oceans don't actually need to boil for this
to happen.
As atmospheric water vapor increases
H2O molecules in the stratosphere
will be broken into hydrogen and oxygen
and the light hydrogen atoms will be lost to space.
This is happening already but will massively increase
as solar radiation rises.
Exactly when Earth loses all the oceans
depends on complex climate models.
For example, the increase in the greenhouse effect
due to increased atmospheric water vapor
will speed things up.
But we may delay the inevitable by offsetting that greenhouse
rise with the loss of CO2 or the decrease
in atmospheric pressure due to nitrogen loss.
The increase in cloud cover may reflect
some of the extra radiation.
However, most models agree that runaway ocean loss
begins around a billion years from now or perhaps
a little earlier.
Whenever it happens, the end result is the same.
The surface of the earth becomes Arrakis, a single vast desert.
That doesn't necessarily mean the end of life.
Currently there are several oceans
worth of water locked in minerals within Earth's mantle.
And this will continue to be released to the crust.
Life will surely adapt and cling to these last wet patches
on or below the surface.
But ultimately life will vanish in the reverse order
that it arrived.
After the initial extinction wave
from the loss of much of Earth's plant life,
other complex multicellular organisms
will succumb to heat, perhaps even
before the oceans have gone.
Single celled extremophiles will last a while
longer being much more able to adapt
as surface temperature approaches
the boiling point of water and atmospheric CO2 plummets.
Complex eukaryotic cells will give out next.
But simple prokaryotic life, the very first life to emerge,
will hold out until CO2 flatlines,
perhaps in less than 2 billion years time.
Nice work, prokaryotes.
You were the first to arrive and you will be the last to leave.
Don't let the door smack you in the flagellum on your way out.
Right.
So that's all very cheery, isn't it?
So how out some bright sides to distract us
from the inevitable end of all life?
Well, Mars will warm up a bit.
It's currently on the outer edge of the habitable zone.
And so its ice reserves should thaw as the sun brightens.
Unfortunately, it has neither the mass
nor the atmosphere needed to retain the released water.
It'll be lost to space just like its initial atmosphere was.
But perhaps we'll have advanced to the point
of being able to replenish the Martian atmosphere by then.
But if we're talking about large scale geoengineering then
I suppose we could also try to save the Earth.
Now we're not going to stop the sun
from brightening but we might try to block
some of the extra light.
Giant beach umbrella?
Actually seeding the upper atmosphere
with reflective nanoparticles might
do the trick, assuming humans or our descendants are around
in another several hundred million years,
this tick should be well within our capability.
Then perhaps we can hold out until the sun eventually
exhausts its fuel and becomes a red giant expanding
to consume the only home we've ever
known in all of space time.
Listening to me yap on about space and physics
may be fun and all, but that's not enough
if you really want to learn this stuff.
To learn, you need to do.
To really gain intuition about our often very unintuitive
universe, you need to start solving problems in physics,
math, and astronomy.
That's why brilliant.org's superbly
crafted courses may be the perfect next step for you.
They've built problem based courses
a huge range of subjects, including a lot of math
but also physics, astronomy, and computer science.
In no time at all you'll be coding gravity simulations
in Python and calculating the radiation emitted
by black holes.
To support spacetime and learn more about brilliant,
go to brilliant.org/spacetime and sign up for free.
And also the first 200 people to go to that link
will get 20% off the annual premium subscription.
Last week we talked about how slicing a piece out
of space time caused different observers to disagree
on the nature of the vacuum.
You guys had all the good questions.
Existenceisillusion asked about the nature
of the thermal particles produced in horizon radiation.
Are they photons or what?
Actually, all quantum fields are affected by the presence
of a space time horizon.
The vacuum state of all fields are
redefined in the vicinity of a black hole
or for an accelerating observer.
That means the thermal bath of particles
should include all particles and their energy distribution
should be the Planck distribution,
the same distribution as black body radiation from heat.
Actually, now that I think about it,
that might mean that you would only
see particles whose rest masses are
less than the allowed energy.
I'm going to have to look into that.
Of course, you would always see photons
because photons are massless.
Those photons would have a perfect black body spectrum.
Empty space would look hot.
[INAUDIBLE] makes a good point about the pop sci explanation
of Hawking radiation.
If the event horizon swallows one half of a virtual matter
anti-matter pair, then how does the black hole lose mass?
After all, both matter and anti-matter have positive mass.
So this is a good example of why these pop sci
explanations can be more misleading than helpful.
In fact, as far as I know, there is no derivation
of Hawking radiation that invokes the splitting
of particle-antiparticle pairs.
So it's not clear how meaningful that picture even is.
Very soon we'll be talking about the exact line of reasoning
that Stephen Hawking used to hypothesize his radiation.
But in short, it involves calculating
the state of the vacuum before the black hole formed
and then again after it formed.
In order for the two states to be consistent,
the after vacuum state has an excess
of positive energy modes.
These are interpreted as radiation from the black hole.
And conservation of energy demands that the black hole
give up mass.
Bananazohar says of the last episode, this is Gabe level.
Nice work.
You hear that, Gabe?
Someone remembers you.
Yeah, they do.
And thanks for the shout out, guys.
But if you want to see some really nice Gabe level work,
you should check out our sister channel PBS "Infinite Series."
It's a math channel when we're covering topics
from topology to cryptography to number theory.
I co-host it with the very talented Tai-Danae Bradley.
In particular, you should tune in to our next episode
where I have a really hard challenge problem concerning
the golden ratio.
I think it's up to "Space Time" standards
and you guys will enjoy it.
So check out the link to the channel down in the description
and also in the end credits.
Hope to see you guys there.
"Infinite Series" is pretty awesome and pretty hardcore.
But come on, "Space Time."
I know you went back on "Space Time," Gabe?
Collab?
I mean, it sounds like you want to invite yourself
over to "Infinite Series" to hang out where we have
the good challenge questions.
I think that's perfectly fine, Matt.
I'll talk to Ty.
We'll make it happen.
Let's talk.