MATT O'DOWD: Thank you to Wix
for supporting PBS Digital Studios. We live in an unusual age, the
age when the stars still shine. We should count ourselves lucky. Nearly all of future
history will be dark. But events will still unfold
in that long-cooling blackness, and civilizations may endure. So how will the universe
and its far-future denizens spend eternity? [THEME MUSIC] In 100 trillion years, the
last star in the universe will expand, the final
atoms of hydrogen fuel and settle quietly
into a dim white dwarf before slowly fading to
black as it radiates away its remaining heat. The Era of Stars will be over. That 100 trillion years is
10,000 times the current age of the universe. And so the days of starlight
and warmth have a way to go. But even when they
are done, the universe will be young in comparison to
the long, dark ages to follow. In fact, our universe will spend
almost all of its infinite time in darkness, slowly crawling
towards maximum entropy and ultimate heat death. But that doesn't mean
that stuff won't happen. There are many fascinating ways
in which the universe can still decay to increasingly
less interesting states-- or, scientifically, many
ways for the universe to cool down, dissipate
energy, and gain entropy. Far-future civilizations
may be able to harness those mechanisms and so cling to
existence through uncountable eons-- uncountable, but
not incalculable. Life and structure
can only exist as long as the universe is not
in perfect equilibrium, what we call heat death. Today, we're going to figure
out how long before the universe reaches its final
maximum entropy, minimum interesting
state, and answer the questions, how long before
nothing ever happens again? And what will happen
to the universe as it approaches that moment? But first, let's recap
our mounting doom. In some previous
episodes, we looked at the long series
of ends of the world and of the galaxy that
are in store for us, from disasters that will almost
certainly befall the Earth, to the heating and
death of the sun, to the merger with
the Andromeda Galaxy, and, finally, to the death of
the last stars in the galaxy. This left us in a sorry state-- the merged Milky
Way-Andromeda Galaxy comprised of nothing
but stellar remnants, the ultradense neutron
stars and black holes from long-extinct massive stars,
as well as the white dwarfs left from lower-mass stars,
including the recently extinguished red dwarfs. Those white dwarfs
will fade to black in only several billion
years, far shorter than the several trillion-year
lives of those stars. And what happens to
life in that era? Civilizations may have
persisted or even started from scratch in
the Red Dwarf Era. But they'll have lost all
connection with the greater universe before it ends. Long before the last
red dwarf fades out, the accelerating
expansion of the universe will have dragged all galaxies
beyond the Virgo Supercluster outside of the
cosmic event horizon. That's the boundary of our patch
of the universe beyond which no new light can reach us. The greater universe
will fade from view, and even the cosmic
microwave background will also dim to undetectability
within the era of red dwarfs. There'll be no
evidence of a universe beyond the local
galaxy and no evidence that there was ever a Big Bang. So at this point in the
universe's future history, the Age of Stars has
passed and no starlight will ever shine again. The universe contains nothing
but cold, dark nuggets of superdense matter. We have entered
the Degenerate Age, not to be confused
with the 1970s. But neutron stars and white--
or, by now, black-- dwarfs are made of degenerate matter. This is matter that
is fully collapsed in a quantum mechanical sense. It's so densely packed that
all possible quantum states are completely filled and no
further collapse is possible, short of becoming a black hole. Believe it or not, many
of those black dwarfs will still have planetary
systems from their days as regular stars. It's hard to imagine
civilizations persisting on those
pitch-black worlds, but it's not impossible. Yet even if they did,
their days are numbered. The next destructive event is
that every planetary system in the galaxy will eventually
be disrupted by close encounters between stellar remnants. As stars randomly pass
close to each other, planets are flung
into the blackness. It'll take something like
1,000 trillion years, 10 to the power of 15
years, for, essentially, all planetary systems to
be obliterated in this way. The next calamity to befall the
combined Milky Way-Andromeda Galaxy and, in
fact, every galaxy will be its complete
dissolution. As dark star remnants rotate
through countless galactic orbits, they interact with
each other gravitationally. The remnants, mostly
black dwarfs but also the diaspora of frozen planets
and similar substellar objects, will be flung out of the galaxy. Heavier bodies, mostly
neutron stars and black holes, sink towards the center. According to an estimate by
Freeman Dyson, 90% to 99% of our galaxy's stars
will be scattered into the void in something like
10 to the power of 18 years when the universe is a million
times older than the age of the last stars' death. Around 10 times longer still,
and the entire megagalaxy will either have
dispersed or fallen into the massive black hole
at the galactic center. What happens after
the Age of Galaxies depends on a critical question. Do protons decay? That's a cool bit of physics
that deserves its own episode. But in short, protons
have the most stable composite particles. According to the standard
model of particle physics, they should last forever. But there are several beyond the
standard-model mechanisms that would allow them to decay
into positrons, neutrinos, and gamma ray photons. This decay has
never been detected. But failed attempts
to spot it tell us it must take at least 10 to the
power of 34 years for a proton to have a 50%
chance of decaying. Theory says that if
protons decay at all, this half-life could be up to
10 to the power of 37 years. So in something like 10 to the
power of 39 or 10 to the power 40 years, all protons in
the observable universe will be gone. That means all of the planets,
black dwarfs, space dust, everything. The universe will contain
only photons, electrons, and black holes. And so we would enter
the Black Hole Era. If protons decay,
black holes would be the only mass of bodies
left in the universe after 10 to the power 40 years. Some will be the remnant black
holes of individual stars that were flung from
galaxies long ago. But most of the
black hole mass will be in supermassive black holes. These monsters once
grew in the cores of the now-forgotten galaxies. Now they are all that's
left of those galaxies. Some have grown to masses
of up to 100 trillion suns, having swallowed good-sized
bites from entire galaxy clusters. But all black holes
evaporate over time via Hawking radiation, something
we've discussed in detail. They slowly leak away their
mass as a cool heat glow of random particles for the
most part faint radio light. The small black holes,
say, around 10 times the mass of the sun, completely
evaporate in around 10 to the power of 67 years. The largest supermassive black
holes that might ever form will take a little longer,
up to 10 to the power of 106, or a million googol, years. During the long
Black Hole Era, there is still energy to be had for
an enterprising, super-advanced civilization. Black holes themselves
can be used as engines through Hawking
radiation, as we've also discussed previously, but
also via other mechanisms that deserve episodes of their own. If life manages to master
this energy source, then its future history
could be as ridiculously long as the Black Hole Era. By the way, dark matter
will probably also be long gone by now. Even though we don't
know exactly what it is, dark matter
particles will likely either annihilate themselves
as they collide with each other or be captured by
dense stellar remnants during the Degenerate
Age and, ultimately, end up in black holes. But even black holes must end. Occasional flashes of gamma
rays will light up the darkness as black holes reach
that last explosive stage of their evaporation. And after that, just particles
and light, now not even bound gravitationally. The last stuff in the universe
will become more and more diffuse and dim as the
accelerating expansion of space continues and the
infinitely long progression to absolute heat death
is all that remains. Now, before we finish, we have
to take a step back and ask, what if protons don't decay? In that case, there will be
structure in the universe for a very, very long time. Black holes evaporate. But smaller stellar remnants,
ancient planets, asteroids, et cetera, could persist. The fate of these depends
on quantum mechanisms. Over infinite time,
nothing is truly stable. By a process called
quantum tunneling, everything eventually reaches
the lowest possible energy state. For the remaining
matter in the universe, quantum tunneling allows the
elements lighter than iron to fuse together, while elements
heavier than iron decay. In the end of this scenario,
every atom in the universe must fuse or decay into
iron, the most stable element on the periodic table. Black dwarfs decay
into iron stars-- still degenerate
and insanely dense, but now perfect balls of iron. This will take something like
10 to the power of 1,500 years. But even iron stars
can't last forever. The same process of
quantum tunneling eventually transport a star's
material toward its center. Iron stars evolve
into neutron stars. That would take an unthinkable
10 to the power of 10 to the power of 76 years. But we may not have to wait
that long for oblivion. Quantum tunneling may actually
bring on the Black Hole Era much earlier. It depends on how small
black holes can really be. If small, stable black
holes are possible, then quantum tunneling
should allow small regions within larger bodies to
collapse into black holes, which would then consume the rest
of the surrounding body. This possibility was also
pointed out by Freeman Dyson. He figures that the most
likely minimum black hole mass is the Planck mass
of 20 micrograms. If that's the case,
then all matter larger than a dust grain will collapse
into a black hole in around 10 to the power of 10 to
the power of 26 years, then promptly evaporate
by Hawking radiation. This same process will nail
all of the neutron stars too. Again, that's if
protons don't decay. If they do, then matter
is kaput much earlier. So after somewhere between
a million googol and 10 to the power of 10 to
the power of something ridiculous, the
universe will be nothing but an increasingly diffuse
void of elementary particles with maybe a bit of
dust if you're lucky. At this point,
there's really no hope for extracting useful
energy from the universe. It approaches heat death in
which all energy is perfectly distributed, entropy
has peaked, and there's nothing for any
future civilization to cling to, assuming they don't
have technology so advanced that they arrest the expansion
of the universe itself or develop a portal gun-- not likely. The end of the universe will
probably be this eternally expanding, cooling nothingness. Or it could be more dramatic. Dark energy may tear space
to shreds in the Big Rip. Vacuum decay may
drop the universe to an even lower energy state,
wiping out the laws of physics as we know them. Or quantum fluctuations
may spawn new universes from the void. We'll explore these extreme
futures of spacetime time in the near future
of "Space Time." Before we get to
comments, two things. First, we want to
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can click on the link in the description below. Next, if anyone is interested in
hearing two serious experts go into more detail about the
far future of the universe, well, you're in luck. Two cosmologist
friends of mine get into these and
other deep physics stuff on their new channel
"Alas Lewis and Barnes--" link in the description. OK, onto your comments
for last week's episode on the most accurate
prediction in all of science, the
anomalous magnetic moment of the electron. To start with, a
few of you asked how we know that the
g-factor of the electron should have been equal to 1
in the classical case and 2 in the quantum case. And Gareth Dean answered you. So I'm going to read his
answer because the dude knows his stuff. "Initially, the g-factor
was a fudge when it was assumed to be exactly 2. People went, there's
probably a reason for that-- but couldn't go much deeper. It's a simple
enough calculation. If you have your mass and charge
distributed exactly the same, then g equals 1. If, by contrast, you have
an infinitely thin shell of charge surrounding a
mass, your g equals 5/3. In the macroscopic world,
you can build an object with any g you want. Classically, we assumed
the electron was a ball of charged
stuff, hence g equals 1. Dirac's theory, however,
predicts exactly twice that value, g equals 2. This, in essence, is
because his equation has the charge and mass
distributed differently. QED further tweaks
this by showing the charge can smear by
something called the vacuum polarization. It's not that the
moment should be 1. That's the classical
theory, which is wrong-- and also suggested that
electrons should spin faster than light. QED predicts it should
be a bit above 2, and that's what we see. So QED is, if not gospel
truth, the most right thing we have for
describing electrons." OK, nice knowledge bomb
there, Gareth Dean. Epsilon Jay asks why
electrons are thought of as infinitesimal points. Well, the answer
is, essentially, that as far as quantum
mechanics is concerned, size is a property of
composite particles, things that are made up of multiple
elementary particles. It's the size of that bundle
of elementary particles. But elementary
particles themselves don't have size in this sense. All they have is their
quantum wave function, which tells the probability
of the particle's location, momentum, spin,
direction, et cetera. Now, we can think of a
quantum wave function as having a size because it
can be spread out over space. But that spatial sprint
really just tells us the probability of finding
the electron, say, here or here or here. If we know with 100% certainty
the position of an electron, then the size of its quantum
wave function becomes zero. In practice, the Heisenberg
uncertainty principle makes this impossible. But really, in
principle, there's no minimum precision
with which we can know the
electron's location, so there's no minimum size. ComputersHowtos says
that sometimes it feels like we're trying
to fit maths randomly to the observations
when we come up with stuff like
virtual particles and that it's so weird
that that actually works. Well, you know, sometimes
that's the case. That's exactly
what we try to do. We try to pull
theory out or our-- out of nowhere to try to
match to our observations, and we keep those
theories that work. But the key here is that we
don't keep the theories if they only fit one observation. We only keep them if they fit
observation after observation after observation and that they
predict brand new things that were never suspected. And then we go on to
experimentally verify those things. The key is that our
accepted models of reality are consistent in ways that
random chance couldn't possibly allow. Also, we don't pull
items out of nowhere. We tend to make clever
guesses and then tweak them incrementally. Still, I agree it's weird
we can even do this much. HELLDAD compliments
my Lehman T-shirt that I wore in that
previous episode and asks how the financial
firm is doing these days. Well, first, that was
a Lehman College shirt. It's part of the City
University of New York and is widely considered to
be the Hogwarts of New York City, at least in its
grand, "ye olde" elegance. It's also where I'm a professor
of physics and arithmancy-- sorry, astronomy. As for the little
financial firm, yeah, I heard they
were doing pretty well under the wave of banking
deregulation of the '80s and '90s. Apparently, that didn't work
out so well for anyone-- Slytherins, right?
In this video, Matt says that the virgo super cluster is the biggest structure which will remain bound together in the far future.
Kurzgesagt, in this video, say that the local group is the biggest gravity bound structure that will resist expansion.
I'm no astronomer but I think these are different? If so, then which is it?
Just asking in case I have to make some traveling arrangements in the future. Woulnd't want to book an hotel I can never reach.
The drinking word is "heat death".
Thanks for the existential panic attack, Matt.
i Don't think there will be end of universe because like just us if any intelligent life exists then they (and us humans too) will eventually be able to create stars, they will possess the technology to stop the heat death of universe (at least in a very tiny tiny space). they will wield the immeasurable power. they may even have robots set up to explore the universe, gather resources, spread life and intelligence. create habitable planets, starts etc. The life will keep the universe alive.