What Happens When You Die?" — Feynman on Atoms, Energy, and Immortality

Here's a question that will sound a

little morbid, but stay with me. When

you die, where do you go? Now, I don't

mean heaven or hell or any of that. I'm

a physicist, and I'm asking you

something much more interesting. Where

does the stuff that is you actually go?

Because here's what's strange. You've

got about 7 billion billion billion

atoms in your body right now. That's a

seven followed by 27 zeros, seven

octillion atoms. And when you die, not

one of those atoms disappears. Not a

single one. They're all still here. So,

if nothing is lost, in what sense are

you gone? That's what we're going to

figure out today. By the end of this,

you'll understand something profound

about what you're made of, where it came

from, where it's going, and why the very

idea of you is stranger than you ever

imagined. I think you'll never look at

yourself or a blade of grass or a

distant star or even a breath of air

quite the same way again. Let's start

with something simple. Uh let's start

with a candle. You light a candle and it

burns. The wax melts, the wick glows,

and after a while, the candle is gone.

Now, you might say the candle has been

destroyed. But that's not quite right,

is it? Where did the candle go? Well,

some of it turned into light. Photons

bounced around the room and eventually

got absorbed by the walls, warming them

slightly. Some of it turned into heat,

which warmed the air and spread out

through the room. And some of it, the

carbon in the wax, combined with oxygen

in the air to make carbon dioxide. That

carbon dioxide is now floating around,

maybe drifting out the window, maybe

being breathed in by someone walking by

outside. The point is, the candle didn't

vanish. It transformed.

The atoms that made up the candle are

all still somewhere. They've just

rearranged themselves into different

configurations, different molecules,

different forms. This is one of the most

fundamental laws in all of physics.

Energy cannot be created or destroyed.

It can only change from one form to

another. We call this the first law of

thermodynamics.

And I want you to understand this isn't

just a good approximation. This isn't

just usually true. In every experiment

ever conducted, in every observation

ever made, this law has held. It is as

close to an absolute truth as we have

ever found in physics. Now, here's where

it gets interesting. You sitting there

listening to me, you are not so

different from that candle. You are in

the most literal sense burning right

now. As you sit there, your body is

taking in oxygen, combining it with the

food you ate, and releasing energy. But

let me tell you exactly how much energy,

because the numbers are extraordinary.

At any given moment, your body contains

about 250 grams of a molecule called

ATP, adenosine triphosphate.

That's the energy currency of life.

Every cell in your body uses ATP to do

work, to move, to build proteins, to

think. And here's what's remarkable.

That 250 gram of ATP represents only

about four watts of power. The

equivalent of a small LED light bulb,

that's all the ATP you have at any

moment. But wait, your body uses far

more than four watts. At rest, you're

burning about 80 to 100 watts. When

you're exercising, maybe 500 watts or

more. So, how does that work? The answer

is that your cells are recycling ATP at

a furious rate. Each ATP molecule gets

used and regenerated about a thousand

times per day. Your body produces, uses,

and regenerates roughly your entire body

weight in ATP every single day. 40 to 70

kg of ATP made fresh every 24 hours. Let

me put this another way. Over the course

of a day, a healthy person produces

about 1,200 watts of total energy.

That's enough to power 1,200 household

light bulbs simultaneously. If you could

somehow harness it all at once, of

course, you can't. The energy is

released gradually, continuously as your

cells burn through that recycled ATP.

And where does this happen? In tiny

structures inside your cells called

mitochondria. You have about 10 million

billion of them. 10 quadrillion

mitochondria, each one a microscopic

power plant. Each one burning fuel and

churning out ATP. The mitochondria have

their own DNA separate from the DNA in

your cell's nucleus. This is because

they used to be free-living bacteria. Uh

about two billion years ago, one of our

single-sellled ancestors engulfed a

bacterium but didn't digest it. Instead,

they formed a partnership. The bacterium

provided energy and the host cell

provided protection and nutrients. Over

billions of years, that bacterium became

the mitochondrian.

You are in a very real sense a colony of

organisms. Here's another way to think

about the energy numbers. The average

cell uses about 10 billion ATP molecules

per day. You have roughly 37 trillion

cells. Do the math and your body is

cycling through something like 3 * 10

the 25th ATP molecules every day. That's

300 trillion trillion molecules being

built, used, and rebuilt in a continuous

chemical symphony that never stops from

the moment you're born until the moment

you die. And consider this, your brain,

which is only about 2% of your body

weight, uses about 20% of your energy. A

single thought, a single memory being

formed requires billions of ATP

molecules. When you're thinking hard,

solving a problem, your neurons are

firing at tremendous rates. Each firing

requiring billions of sodium ions to be

pumped back across cell membranes. Each

pump cycle requiring ATP. It's estimated

that a single action potential, one

electrical signal traveling down one

neuron, requires about a billion sodium

ions to be moved, which means about a

billion ATP molecules to reset that one

neuron for its next signal. When that

symphony stops, that's death. Not

because the atoms go away, but because

the process stops, the pattern stops

being maintained. But I'm getting ahead

of myself. Let me go back to basics. And

just like the candle, you are constantly

releasing atoms into the world. Every

breath you exhale carries away carbon

dioxide, about 200 milliliters of it.

That's carbon that was part of your body

a moment ago, now floating free. Every

drop of sweat releases water and salts.

Every time you shed a skin cell, that's

atoms leaving your body. You lose about

30,000 to 40,000 dead skin cells every

minute. every minute. You are not a

static thing. You are a process, a river

of atoms flowing through a particular

pattern we call you. Now suppose you

wanted a physicist to speak at your

funeral. I've actually thought about

this. You'd want the physicist to talk

to your grieving family about the

conservation of energy. You'd want them

to understand that your energy has not

died. Here's what I'd want that

physicist to say. No energy gets created

in the universe and none is destroyed.

All your energy, every vibration, every

bit of heat, every wave of every

particle that was your beloved, it all

remains in this world. The photons that

bounced off your face, the particles

whose paths were interrupted by your

smile, by the touch of your hair,

hundreds of trillions of them, they've

scattered off into the world like

children running in all directions.

They're still out there. They will

always be out there. According to the

law of conservation of energy, not a bit

of you is gone. You're just less

orderly. That's all. But wait, let's go

deeper. Much deeper. Let's ask where

those atoms came from in the first

place. Pick up your hand and look at it.

Let me tell you exactly what you're made

of. By mass, you are about 65% oxygen.

Surprised? Most people are, but

remember, you're mostly water, and water

is H2O, and oxygen is the heavy part.

One oxygen atom weighs 16 times as much

as one hydrogen atom. So even though

hydrogen atoms outnumber oxygen atoms 2

to1 in water oxygen dominates by weight

after oxygen comes carbon about 18% of

your mass then hydrogen at about 10%.

Then nitrogen at about 3%. Together

these four elements oxygen, carbon,

hydrogen and nitrogen make up about 96%

of your body mass. just four elements

out of the 92 naturally occurring ones.

The remaining 4% includes calcium, about

2% mostly in your bones and teeth,

phosphorus, about 1% critical for your

DNA and ATP. Then smaller amounts of

potassium, sulfur, sodium, chlorine, and

magnesium. And finally, trace elements,

iron, zinc, copper, iodine, and a few

others in tiny amounts. but absolutely

essential. The iron in your body, if you

extracted at all, would make a nail

about 3 in long. That's it. But without

that tiny bit of iron, you couldn't

carry oxygen in your blood, and you'd

die in minutes. Now, let's count atoms

instead of weighing them. By number of

atoms, you're about 63% hydrogen, 24%

oxygen, and 12% carbon. The vast

majority of atoms in your body are

hydrogen. The simplest atom, just one

proton and one electron. Here's a

striking way to think about it. There

are more atoms in your body than there

are stars in the observable universe.

The observable universe contains roughly

200 billion galaxies, each with roughly

200 billion stars. That's about 4 * 10

22nd power stars. But you have 7* 10

27th atoms. You contain about a 100,000

times more atoms than the universe has

stars. Now I want you to consider the

carbon atoms in your skin. There are

about 700 billion billion of them in

your body. 7 * 10 the 26th power. Where

did they come from? Well, you ate them.

They were in your food. Maybe in a piece

of bread or a carrot or a steak. And

where did the bread get its carbon? From

the wheat plant, which pulled carbon

dioxide out of the air during

photosynthesis.

And where did that carbon dioxide come

from? Other plants and animals that

breathed it out or decomposed or burned.

But trace it back far enough and you get

to a remarkable place. The carbon in

your hand was not always on Earth.

Wasn't even made on Earth. It was forged

in the heart of a dying star. Let me

explain this carefully because it's one

of the most beautiful things we've

discovered about the universe. In the

beginning, just after the Big Bang about

14 billion years ago, the universe had

only the simplest atoms. Let me be

precise about this because the details

matter and they're fascinating. At the

very beginning, in the first fraction of

a second, there weren't even atoms. The

universe was a soup of quarks and

gluons, so hot and dense that nothing we

would recognize as matter could exist.

The temperature was over a trillion

degrees. As the universe expanded and

cooled, quarks combined into protons and

neutrons. This happened when the

universe was about one microcond old.

Think about that. 1 millionth of a

second after the big bang, the building

blocks of all matter came into

existence. But even then, it was too hot

for protons and neutrons to stick

together. They would combine and

immediately be blasted apart by the

intense radiation. It was like trying to

build a house of cods in a hurricane. In

the first 3 minutes after the Big Bang,

the universe was a roing soup of

particles at unimaginable temperatures,

about 10 billion degrees. At 1 second

old, it was still too hot for even

simple atomic nuclei to survive. Protons

and neutrons were forming and breaking

apart constantly. Here's a key fact. At

1 second old, there were about seven

protons for every neutron. Why? Because

neutrons are slightly heavier than

protons. And in the hot conditions of

the early universe, the reactions that

converted neutrons to protons happened

slightly faster than the reverse. This

ratio would turn out to be crucial for

the chemistry of the entire universe.

But as the universe expanded, it cooled.

And when it cooled enough, about three

minutes after the big bang, something

remarkable happened, protons and

neutrons began sticking together. First

they made dutarium, heavy hydrogen, one

proton and one neutron. Then helium 3,

two protons and one neutron. Then helium

4, two protons and two neutrons. And a

tiny tiny bit of lithium 7. The timing

was everything. By about 3 minutes, the

temperature had dropped to about a

billion degrees, cool enough for nuclei

to form, but still hot enough for fusion

to happen. This window lasted only about

17 minutes. By 20 minutes after the Big

Bang, the universe had cooled too much.

The density dropped. The particles were

too far apart to collide often enough.

And so, the cosmic nuclear reactor shut

down. What was the result? a universe

that was about 75% hydrogen by mass,

about 25% helium, about 0.01%

dutyium, and about 110 billionth

lithium. And nothing else. No carbon, no

oxygen, no nitrogen, no iron, no

calcium, none of the heavy stuff that

makes up rocks and trees and people.

Here's the key point. There was a

bottleneck. There's no stable nucleus

with five nucleons or eight nucleons.

Helium 4 is very stable, but if you add

one more proton or neutron, the

resulting nucleus falls apart almost

instantly. And two helium 4 nuclei

combined would make burillium 8, which

falls apart in about 10us 16 seconds.

There was no way in those first 20

minutes of cosmic history to build

anything heavier than lithium. So where

did all the heavy elements come from?

They had to wait for stars. The first

stars formed about 200 million years

after the Big Bang when gravity pulled

together the primordial hydrogen and

helium into dense clouds that collapsed

and ignited. These first stars were

monsters 30 to 300 times the mass of our

sun, burning hot and bright and fast. A

star, you see, is a giant nuclear

reactor. In its core, under tremendous

pressure and temperature, about 15

million degrees in a star like our sun,

hydrogen atoms fuse together to form

helium. Four hydrogen nuclei smash

together and become one helium nucleus,

releasing energy in the process. That's

what makes the sun shine. That's the

power source that has kept our star

burning for 5 billion years. But in more

massive stars, the story goes further.

When the hydrogen runs out in the core,

the core contracts and heats up even

more. If it gets hot enough, about 100

million degrees, something remarkable

happens. Helium atoms start fusing into

carbon. This is called the triple alpha

process. And it's a delicate thing. Two

helium nuclei fuse to form burillium 8,

which is wildly unstable. It falls apart

in about 10 theus7 seconds. That's a

decimal point followed by 16 zeros and

then a one almost instantly. But if in

that tiny window a third helium nucleus

comes along and collides with the

burillium 8, you get carbon 12 stable

carbon. The carbon that makes up the

backbone of every organic molecule in

your body. Here's something remarkable

about this process. The reaction rate

depends on temperature to the 40th

power. The 40th power. That means if you

double the temperature, the reaction

rate increases by about a trillion

trillion trillion times. This is why the

triple alpha process only happens at

very high temperatures above 100 million

degrees and why it's so sensitive to

conditions. It's like a finely tuned

mechanism. The physicist Fred Hy

actually predicted that this reaction

must exist based purely on the

observation that carbon exists in the

universe. He said in effect, if carbon

exists, there must be some way to make

it. There must be a resonance, an energy

level in the carbon nucleus at exactly

the right energy to make this reaction

possible. And when experimenters looked,

they found exactly the resonance he

predicted at 7.65

million electron volts above the ground

state. This is sometimes cited as

evidence for the anthropic principle,

the idea that the universe must have the

properties that allow for the existence

of observers like us. If this resonance

didn't exist, if the energy levels were

even slightly different, carbon wouldn't

form efficiently in stars, and we

wouldn't be here to wonder about it.

Nature had to be precisely tuned for

carbon to exist. And here we are made of

it. Now, the story doesn't stop at

carbon. In massive stars, much more

massive than our sun, the fusion

continues. Carbon fuses with helium to

form oxygen. Oxygen fuses to form neon.

Neon to magnesium, magnesium to silicon,

silicon to iron. But iron is the end of

the line for normal fusion. Here's why.

When you fuse lighter elements together,

you release energy. The products weigh

slightly less than the ingredients. And

that missing mass becomes energy

according to Einstein's famous equation.

But iron is at the bottom of the binding

energy curve. Fusing iron doesn't

release energy. It absorbs it. It's like

trying to roll a ball up a hill instead

of down. So when a massive star builds

up an iron core, something dramatic

happens. The core can no longer generate

energy to hold itself up against

gravity. In less than a second, an iron

core the size of the Earth and with the

mass of the sun collapses into a ball of

neutrons just a few kilometers across.

This gravitational collapse releases an

enormous amount of energy, more than a

100 times what our sun will radiate over

its entire 10 billionyear lifetime. The

outer layers of the star are blown off

in a catastrophic explosion. This is a

supernova, one of the most violent

events in the universe. For a few weeks,

a single exploding star can outshine an

entire galaxy of a hundred billion

stars. And in that explosion, in those

brief catastrophic seconds, something

magical happens. The conditions become

so extreme, so hot, so dense with

neutrons that elements heavier than iron

can form. This is called the R process.

R for rapid. Neutron capture happens so

fast, millions of captures per second,

that the nuclei can build up faster than

radioactive decay can tear them down.

Think about what's happening. You have a

nuclear fire raging at temperatures

exceeding a billion degrees. Neutrons

are flying everywhere. An iron nucleus

absorbs a neutron, then another, then

another. In seconds, it's captured

dozens of neutrons and become wildly

unstable. But before it can decay, it

captures more. The nuclei are racing up

the periodic table, becoming gold,

platinum, uranium. In 2017,

astronomers detected gravitational waves

from two neutron stars colliding, and

they pointed their telescopes at the

source. They saw the glow of newly

formed heavy elements, including gold,

being forged in real time. We watched

the universe make gold. The kilonova, as

it's called, produced about 10 Earth

masses of gold in seconds. 10 Earth

masses. That's where much of the gold in

your wedding ring came from. Neutron

star collisions. So, some of your heavy

elements came from supernovas and some

came from colliding neutron stars.

Either way, we're talking about the most

extreme events in the cosmos. And then

all of this material, everything the

star spent millions of years building in

its core, everything the supernova or

neutron star collision created in

seconds, it gets flung out into space at

10,000 km/s.

It mixes with interstellar gas clouds

and eventually those clouds collapse

under their own gravity to form new

stars, new planets, new everything. Our

sun is a third generation star. The dust

cloud that formed the solar system was

already enriched with heavy elements

from earlier supernovas. The earth

condensed from that dust about 4 and a

half billion years ago. And you, every

atom of carbon and oxygen and nitrogen

and calcium and iron in your body, you

are made of star stuff. This isn't

poetry. It's fact. The iron in your

blood, the same iron that carries oxygen

from your lungs to your tissues, was

forged in a star that exploded before

the sun was born. The calcium in your

bones, came from the dying breath of a

red giant. The phosphorus in your DNA,

the element that forms the backbone of

the molecule that carries your genetic

code, was created in a supernova.

Speaking of DNA, let me tell you how

much information is stored in your body.

A single human cell contains about three

billion base pairs of DNA. If you

stretched out all the DNA in one cell,

it would be about 2 meters long. Now you

have roughly 37 trillion cells. And most

of them have a complete copy of your

genome. If you stretched out all the DNA

in your body, end to end, it would reach

from Earth to the sun and back about 600

times. That's over a 100red billion km

of DNA. And that DNA is constantly being

copied, checked, repaired, and read.

Every second, your cells are reading

millions of genetic instructions,

building proteins, maintaining the

patterns that make you you. I like to

put it this way. I am a universe of

atoms. And I am also just an atom in the

universe. Now, let me tell you something

that might surprise you. Those atoms in

your body, most of them weren't there a

year ago. Studies using radioactive

tracers have shown that about 98% of the

atoms in your body are replaced every

year. This was first discovered in the

1,950

seconds at Oakidge National Laboratory,

where scientists used radioactive

isotopes to track atoms moving through

the body. The water in your body turns

over in about 2 weeks. Half the water

molecules in you right now weren't there

16 days ago. Think about that. You

drink, you sweat, you urinate, you

breathe out water vapor. The water flows

through you like a river. The sodium and

potassium in your cells, they cycle in

and out constantly through ion channels,

millions of times per second in each

cell. The calcium and phosphorus in your

bones, even your bones, the hardest

tissues in your body, they get replaced

every year or so as the tiny crystals in

your skeleton dissolve and reform. Your

bones are not static structures. They're

constantly being demolished and rebuilt

by specialized cells called osteoclass

and osteoblasts.

Different tissues replace themselves at

different rates. And the numbers are

fascinating. The cells lining your

stomach last only about 5 days. They're

constantly being destroyed by stomach

acid and constantly being replaced. That

means your stomach lining is completely

new every week. Your taste buds last

about 10 days. Your red blood cells live

about 4 months before they're broken

down in your spleen and recycled. Your

white blood cells, the soldiers of your

immune system, live anywhere from a few

hours to several years depending on the

type. Your skin cells last two to three

weeks. Your liver regenerates itself

every year or two. Some cells though are

with you for life. Most of the neurons

in your brain, the cells that hold your

memories and make you who you are, were

born when you were very young and will

be with you until you die. Some neurons

in your cerebral cortex are as old as

you are. The cells of your eye lens

formed before you were born will never

be replaced. And certain cells in your

heart muscle, the cardiammyioytes, are

remarkably long lived. They turn over

very slowly, only about 1% per year. So

here's a strange thought. The atoms that

made up your body when you were 10 years

old are almost entirely gone, scattered

to the winds, literally. Some of them

are in other people now. Some are in

trees. Some are in the ocean. Some have

been breathed in by a stranger on the

other side of the world. The atoms that

were you at age 10 are not the atoms

that are you today. And yet you feel

continuous. You remember being 10. You

feel like the same person. So what is it

that persists if not the atoms

themselves? It's the pattern, the

organization, uh the information. Think

of it like a whirlpool in a river. The

water flowing through the whirlpool is

constantly changing. New water comes in,

old water flows out. But the whirlpool

itself, the pattern, the shape that

persists, the whirlpool is not a thing

made of specific water molecules. It's a

dynamic pattern that water molecules

temporarily participate in. You are like

that. A pattern that persists while the

material flows through. Or think of a

flame. The flame of a candle is not made

of any particular molecules. The

molecules are constantly changing. Fuel

coming in, combustion products going

out. But the flame persists as a

pattern, a self- sustaining process as

long as conditions allow. You are in

some sense a very complicated flame.

Now, wait. Someone might say, "If our

atoms are constantly being replaced and

we're still the same person, then what

actually changes when we die?" Uh,

that's a good question. Let me try to

answer it carefully. When you're alive,

you're what physicists call an open

system. You're constantly exchanging

matter and energy with your environment.

Food comes in, waste goes out, heat

radiates away, new atoms replace old

ones. But there's a pattern that

maintains itself. Your cells keep

working together in a coordinated way.

Your heart keeps beating. Your brain

keeps firing. The pattern sustains

itself by consuming energy and

maintaining organization against the

natural tendency toward disorder. This

tendency toward disorder is called

entropy and it's described by the second

law of thermodynamics.

The second law says that in any isolated

system, entropy tends to increase.

Things tend to become more disordered,

more mixed up, more uniform. Heat flows

from hot to cold, never the reverse. A

drop of ink spreads through a glass of

water. It never spontaneously

concentrates. A glass can fall off a

table and shatter. The pieces never

spontaneously reassemble and jump back

up. Why? Because there are vastly more

disordered arrangements than ordered

ones. Think of a deck of cards. There's

only one way to arrange it in perfect

order, ace through king, in each suit.

But there are about 8* 10 67th ways to

arrange it. If you shuffle the deck

randomly, what are the odds you'll get

perfect order? Essentially zero. Not

because physics forbids it, but because

the disordered arrangements

overwhelmingly outnumber the ordered

one. The same is true of atoms. The

atoms in your body could in principle

spontaneously rearrange themselves into

random gas. Physics doesn't forbid it,

but the probability is so astronomically

low that it will never happen in the

lifetime of the universe. Life fights

against this tendency toward disorder.

Every living thing is a temporary pocket

of low entropy, a temporary island of

order in a universe that trends toward

disorder. But it takes work. It takes

energy to maintain that order. You have

to keep eating, keep breathing, keep

burning fuel to stay organized. The

physicist Irwin Schrodinger in his

famous book, What is Life? pointed out

that living things feed on negative

entropy. They take in ordered energy

richch molecules like glucose and they

excrete disordered waste like carbon

dioxide and water. The price of

maintaining your low entropy body is

increasing the entropy of your

surroundings even more. Life doesn't

violate the second law. It just shifts

entropy from here to there, from inside

to outside. While overall entropy still

increases. When you die, that fight

ends. The pattern stops sustaining

itself. The coordination breaks down.

Your cells stop working together and the

second law takes over. What happens next

is remarkably fast. Without oxygen, your

cells begin to die within minutes. The

enzymes that were busy building things

start breaking things down instead.

Bacteria that live peacefully in your

gut, held in check by your immune

system, begin to multiply and spread.

Within hours, they're consuming you from

the inside. Decomposition releases

carbon dioxide back into the atmosphere.

It releases nitrogen compounds into the

soil. It releases water. Over weeks and

months, the complex organic molecules

that made up your body are broken down

into simpler and simpler compounds. The

atoms that were organized into you

disperse. Some of your atoms become part

of the soil. Some get taken up by plant

roots. Some are eaten by insects and

bacteria. Some evaporate as gases and

drift away on the wind. Your carbon

atoms might end up in a blade of grass.

That grass might be eaten by a cow. That

cow might be eaten by someone in a

restaurant across the world. Your atoms,

the ones that were briefly part of you,

become briefly part of other living

things. This is the carbon cycle. About

90% of all the carbon dioxide returned

to the atmosphere each year comes from

decomposition, from the breakdown of

dead organisms. Without decomposition,

carbon would get locked up in dead

bodies, and the whole cycle would grind

to a halt. Decomposition is not just the

end of life. It's what makes new life

possible. Here's a remarkable thing to

consider. The carbon atoms in your body

have been around for billions of years.

Some of them were probably once part of

a dinosaur. Think about that. A 100

million years ago, a dinosaur breathed

in, ate plants, incorporated carbon into

its bones and muscles, then it died,

decomposed, and that carbon went back

into the cycle. Plants pulled it from

the air. Animals ate the plants.

Microbes broke down the dead. And so on

for a 100 million years, atoms cycling

through countless living things until

some of that carbon ended up in the food

you ate last week. And now it's part of

you. Here's another way to think about

it. Consider a glass of water. The water

molecules in that glass have been around

for billions of years, cycling through

the hydraological cycle. Some of those

molecules evaporated from a prehistoric

ocean, fell as rain on a Jurassic

forest, were drunk by a dinosaur,

excreted, evaporated again, fell again,

flowed through rivers and underground

aquifers for millions of years, and

ended up in your glass. You're drinking

the same water made of the same oxygen

and hydrogen atoms that dinosaurs drank.

And it goes the other way, too. Some of

the atoms in you right now will millions

of years from now be part of creatures

that don't exist yet. You are connected

materially, atomically to the deep past

and the deep future. But here's

something even more immediate. Every

breath you take connects you to everyone

who has ever lived. Let me show you a

calculation that physics students

sometimes do called Caesar's last

breath. When Julius Caesar was

assassinated in 44 BC, he exhaled a

final breath. That breath contained

about 25 seextilian molecules. That's a

25 followed by 21 zeros. Now, that seems

like a lot, but the Earth's atmosphere

contains about 10 to the 44th molecules.

So Caesar's last breath was just a tiny

fraction of the whole atmosphere. But

here's the thing. Over the past 2,000

years, those molecules have mixed

throughout the entire atmosphere.

They've spread around the globe.

Nitrogen, which makes up most of the

air, is remarkably stable. A nitrogen

molecule, two atoms bound together by a

triple bond, can persist for millions,

maybe billions of years. It doesn't

react with much. So those molecules from

Caesar's last breath are still out there

spread uniformly through the atmosphere.

And when you do the math, when you

compare the tiny fraction that was

Caesar's breath to the enormous number

of molecules in your breath, the numbers

almost exactly cancel out. The result,

on average, every breath you take

contains about one molecule from

Caesar's dying gasp. Not metaphorically,

mathematically, statistically. And it's

not just Caesar. Every breath you take

contains molecules breathed by

Cleopatra, by Genghask Khan, by Leonardo

da Vinci, by your great great

grandmother. The air we breathe connects

us to everyone who has ever exhaled. Let

me pause here and check that I've been

clear because this is important and I

want to make sure a bright 12-year-old

could follow along. Here's what we've

established. Everything is made of

atoms. The universe started with only

hydrogen, helium, and a trace of

lithium. All made in the first 3 minutes

after the big bang. Everything else,

every carbon and oxygen and iron atom

was made later inside stars. Stars fuse

light elements into heavy ones.

Supernovas and neutron star collisions

create the heaviest elements. These

atoms get scattered into space, form new

solar systems, and eventually become

part of planets and people. The atoms in

your body came from stars that exploded

billions of years ago. While you're

alive, your atoms are constantly being

replaced about 98% every year. But the

pattern that is you persists when you

die. The pattern stops being maintained

and your atoms scatter back into the

world to become part of other things.

Your energy doesn't disappear either. It

just spreads out and becomes less

organized. And every breath you take

connects you materially to every person

who has ever lived. Got it? Good. Now,

let's tackle a common misconception.

Some people hear this and say, "Well, if

my atoms go on existing after I die,

then in some sense, I'm immortal." But

this isn't quite right. And it's

important to understand why. The atoms

that make up you have no memory of being

you. A carbon atom that was in your

brain doesn't carry around some essence

of your thoughts. It's just a carbon

atom. Six protons, six neutrons, six

electrons behaving exactly like every

other carbon one 2 atom in the universe.

What made it part of you was its

position, its relationship to all the

other atoms. The pattern it was

participating in. Once that pattern

dissolves, the atom is just an atom

again. So the immortality of your atoms

is not the same as your immortality.

Your atoms will persist, but you, the

pattern, the organization, the

information, the particular arrangement

that thinks and feels and remembers,

that's what ends at death. Is this sad?

I don't know. I find it fascinating and

in a way beautiful. Let me give you a

thought experiment. Suppose I told you

that in 5 years, every atom in your body

will have been replaced. Would you feel

like you're going to die in 5 years? Of

course not. You feel like you're going

to keep living just with different atoms

carrying on the pattern. The specific

atoms don't matter. What matters is the

continuity of the pattern. When the

pattern stops being maintained, that's

death. But the atoms themselves are

indifferent to the whole thing. Now,

here's something truly remarkable. How

long do atoms themselves last? A carbon

atom in your body is made of protons,

neutrons, and electrons. The electrons

can be knocked off and replaced. That

happens all the time in chemistry. The

neutrons, if they're outside a nucleus,

decay in about 15 minutes into a proton,

an electron, and an anti-utrino. But

inside a nucleus, bound together with

protons by the strong nuclear force,

neutrons are stable. And protons,

protons appear to be extraordinarily

stable. Scientists have been looking for

proton decay for over 40 years. They've

built enormous detectors, tens of

thousands of tons of ultra pure water in

deep underground mines, isolated from

cosmic rays, watching patiently for a

single proton to disintegrate. And

they've never seen it happen, not once.

The current experiments tell us that the

average proton must live longer than 10^

the 34th years. That's a 1 followed by

34 zeros. For comparison, the universe

is only about 10 to the 10th years old,

14 billion years. The lower limit on

proton lifetime is a 100 trillion

trillion times longer than the current

age of the universe. Now, some theories

in physics predict that protons might

eventually decay. Grand unified theories

suggest that protons could have a

halfife of maybe 10 the 36 years or even

longer. But even if that's true, it's so

rare we've never observed it. What does

this mean? It means the atoms that make

up your body are for all practical

purposes immortal. They've already

existed for billions of years. They will

continue to exist for billions more,

maybe trillions, maybe forever. They

were here before the Earth formed.

They'll be here long after the sun burns

out and becomes a white dwarf. They are

far more permanent than any building,

any mountain, any star. And you, for one

brief moment in cosmic time, got to

borrow some of them. You got to organize

them into something that could think

about where they came from. Something

that could look up at the stars and

realize that the atoms in its eyes were

once inside those very stars. Now, let

me tell you about the far future because

it's relevant to understanding what

happens to energy and matter in the long

run. Remember how I said entropy always

increases? The second law of

thermodynamics says the universe is

heading toward a state of maximum

disorder. What does that mean? taken to

its logical conclusion. Physicists call

it the heat death of the universe

doesn't mean things get hot. Quite the

opposite. It means things get cold and

uniform and still. But before we get to

that ultimate fate, let's talk about

what happens closer to home. In about 5

billion years, the sun will exhaust the

hydrogen in its core. Without hydrogen

fusion to hold it up against gravity,

the core will contract and heat up. This

will cause the outer layers of the sun

to expand enormously. The sun will

become a red giant, swelling to perhaps

200 times its current size. Its surface

will reach out past the orbit of

Mercury, past Venus, and possibly engulf

the Earth. Even if Earth survives, it

will be sterilized. The oceans will boil

away. The atmosphere will be stripped

off. Everything alive will be gone long

before the sun itself dies. Eventually,

the sun will shed its outer layers in a

beautiful planetary nebula, and its core

will collapse into a white dwarf, a

dense ball of carbon and oxygen about

the size of the Earth, but with the mass

of the sun. This white dwarf will slowly

cool over billions of years, eventually

becoming a cold, dark cinder. But that's

just our sun.

What about the universe as a whole?

Here's the timeline as best we

understand it. In about a 100 billion

years, the expansion of the universe

will have carried all the distant

galaxies beyond our cosmic horizon. The

sky will go dark except for the stars in

our own local group. Eventually, the

Milky Way and Andromeda will merge into

one giant galaxy, and that's all we'll

have. In about a trillion years, the

last new stars will form. The gas clouds

needed to make new stars will be

exhausted. The lights will start going

out. In a 100red trillion years, the

last red dwarf stars, tiny and misily,

burning their hydrogen slowly, will

finally exhaust their fuel and fade to

black. The universe will be dark. But it

gets worse. The stellar remnants, the

white dwarfs and neutron stars, they'll

still be around for a while. But over

time scales of 10 to the 37th years, if

protons do eventually decay, even these

will slowly evaporate. Atom by atom, the

matter in the universe will dissolve.

And the black holes, they evaporate,

too. Stephven Hawking showed that black

holes slowly radiate energy and shrink.

A black hole with the mass of the sun

would take about 10 to the 67th years to

evaporate. A super massive black hole at

the center of a galaxy would last maybe

10 to the 100th years. But eventually

even they're gone. What's left? Just

photons, nutrinos, and maybe some

electrons and posetrons spread

incredibly thin across an

incomprehensibly vast cold empty space.

No temperature differences. No energy

gradients, no possibility of doing work,

maximum entropy, heat death. The

universe will be just a few degrees

above absolute zero. Actually, not even

that. As the universe continues to

expand, the temperature will asmtoically

approach zero, but never quite reach it.

In this state, nothing interesting can

happen. No stars, no chemistry, no life,

no change. Just a vast, cold, dark,

expanding, nothing forever. This is what

the second law of thermodynamics

predicts as the ultimate fate of the

universe. Everything you love,

everything humanity has ever built,

every star and galaxy and planet will

eventually be dispersed into a uniform,

featureless void. Is this depressing?

Maybe. But here's another way to think

about it. Right now, the universe is

young. It's only 14 billion years old.

The era of stars, the era in which

complex structures can exist, in which

life can flourish. This era will last

for roughly 10 to the 14th years. That's

a 100red trillion years. We're at the

very beginning. If the history of the

universe were a thousandpage book and

the Stelliferous era, the era of stars,

were the whole book, we would be on the

first word of the first sentence of the

first page. Stars have been shining for

about 14 billion years, and they'll keep

shining for another 100 trillion. We've

seen 0.01%

of the age of stars. And the universe

didn't have to be this way. The laws of

physics could have been different. There

could have been no stable matter, no

stars, no planets, no life. But instead,

we got a universe that makes carbon in

stellar furnaces that spreads it through

space in spectacular explosions that

allows it to clump together into planets

where chemistry can become biology and

biology can become consciousness. We are

unimaginably lucky to exist at all. And

we are unimaginably lucky to exist now

in this brief window of cosmic history

when the universe is interesting. This

brings me to the big picture. Why does

any of this matter? What does it change

about how we see the world? I think it

changes a lot when I look at a leaf. I

don't just see a leaf. I see carbon

atoms that were pulled from the air that

came from decomposed plants and animals

that were exhaled by creatures living

and dead for millions of years that

ultimately trace back to ancient

supernovas. Every leaf is a temporary

assembly of atoms that have been on epic

journeys through space and time. When I

breathe, I'm taking in atoms that have

been through countless other lungs,

human and animal, over millions of

years. Right now, as you listen to this,

you are breathing in atoms that were

once part of dinosaurs, ancient forests,

Roman emperors, medieval peasants, and

every other breathing creature that has

ever lived. We are all connected not in

some vague spiritual sense, but

materially, atomically, demonstrabably.

And when I think about death, I don't

see it as an ending in the sense of

annihilation. Nothing is annihilated.

Everything is transformed. The pattern

that is me will dissolve. Yes, the

organization will disperse. The low

entropy island that I represent will

melt back into the high entropy ocean of

the universe. But the atoms will go on

to be part of new patterns. Some of them

will end up in other people. Some will

end up in trees or birds or fish. Some

will be breathed in by children not yet

born. Some will eventually find their

way back into new stars where they'll

participate in fusion reactions and

maybe be scattered again by new

supernovas to become part of new planets

and new life forms in the distant

future. There's a kind of comfort in

that, I think. Not the comfort of

personal survival, because that's not

what this is, but the comfort of being

part of something vast and ongoing, of

knowing that you're made of the same

stuff as everything else, and that stuff

will keep existing and keep rearranging

itself into new wonders long after

you're gone. The universe is not wasting

material. Nothing is thrown away. Every

atom that has ever been part of a living

thing will be part of living things

again. I once stood at the seashore and

started to think about this. The waves,

mountains of molecules, each one

stupidly minding its own business,

trillions of them apart, yet forming

white surf in unison, ages and ages

before any eyes could see. Year after

year, thunderously pounding the shore as

now. For whom? For what? On a dead

planet with no life to entertain. And

then life appeared. Deep in the sea,

molecules began to copy themselves. They

made others like themselves. And a new

dance started, growing in size and

complexity. Living things, masses of

atoms, DNA, protein, dancing a pattern

ever more intricate, out of the cradle

onto the dry land. Here it is standing.

Atoms with consciousness, matter with

curiosity. That's what you are. You are

matter that has organized itself to the

point where it can wonder about itself.

You are the universe asking questions

about its own nature. You are atoms

contemplating atoms. And when you die,

the universe doesn't lose any atoms. It

just loses one particular way of asking

questions. But here's the remarkable

thing. The questions you asked, the

thoughts you thought, they had effects.

They changed other patterns. They

rippled outward. Every conversation

you've ever had changed someone else's

brain slightly. Every book you've read,

every idea you've shared, every moment

of kindness or cruelty left traces in

the world. Information propagates.

Ideas outlast the brains that conceive

them. The patterns you created in other

minds, the ways you influence the world,

those can persist far longer than any

particular arrangement of carbon and

oxygen. Perhaps the most durable part of

you isn't your atoms at all. It's the

patterns you created in the minds of

others. I want to leave you with one

last thought. We've talked about how

atoms cycle through living things. How

energy transforms but never disappears.

How you're connected to stars and

dinosaurs and everyone who has ever

breathed. But there's something even

stranger that modern physics hints at,

though we don't fully understand it yet.

The atoms in your body, they're not

really separate things at all. At the

deepest level, quantum field theory

tells us that particles are exitations

of underlying fields. Let me explain

what that means. Think of a field as

something that has a value at every

point in space. Like temperature in a

room or the height of waves on a pond.

Now, quantum mechanics says these fields

can't sit still. They fluctuate. They

vibrate. And when they vibrate in

certain ways, we see what we call

particles. The electron in your eye and

the electron in a distant star are

fundamentally the same kind of ripple in

the same underlying electron field.

There's one electron field that fills

the entire universe. And what we call

individual electrons are just places

where that field is vibrating in a

particular way. As the physicist David

Tong puts it, every particle in your

body, indeed, every particle in the

universe is a tiny ripple of the

underlying field molded into a uh

particle by the machinery of quantum

mechanics. This is why every electron in

the universe has exactly the same mass,

exactly the same charge, exactly the

same properties. They're not separate

objects that happen to be identical.

They're excitations of the same field.

Like two ripples on the same pond,

aren't two different ponds. They're two

disturbances in the same water.

Similarly, for quarks, for photons, for

all the fundamental particles, they're

all ripples and fields that permeate all

of space and time. So the separateness

we see, the boundary between you and me

and the air and the ground, that's a

kind of useful approximation created by

the particular way matter is arranged.

At the deepest level, it's all one

thing. Even empty space isn't really

empty. The quantum fields are always

there, always fluctuating. If you could

somehow remove all particles from a box,

you wouldn't have nothing. You'd have

the vacuum, which in quantum field

theory is a sthing bubbling sea of

virtual particles constantly appearing

and disappearing. Field fluctuations

that briefly create particle

antiparticle pairs before they

annihilate each other. This isn't just

theory. We can measure the effects of

these vacuum fluctuations. The Casemir

effect, for example, is a measurable

force between two metal plates placed

very close together. The vacuum

fluctuations between the plates are

restricted, while outside the plates

they're not. And this imbalance creates

a tiny but measurable force pushing the

plates together. We've measured this

force. The vacuum is real and it's busy.

The lamb shift discovered in 1947 showed

that the energy levels of hydrogen atoms

are slightly different from what we'd

expect without vacuum fluctuations. The

electron in a hydrogen atom is

constantly being jostled by virtual

particles appearing and disappearing

around it. And this shifts its energy

levels by a tiny amount. This shift was

one of the great triumphs of quantum

electronamics matching experiment to 11

decimal places. So, even nothing isn't

nothing. Even the emptiest parts of

intergalactic space, billions of light

years from the nearest galaxy, are still

full of quantum fields, still full of

fluctuations, still full of virtual

particles coming into existence and

vanishing again. Nothingness itself is

complicated in physics. Uh, I don't know

exactly what to make of all that. I

don't think anyone does yet. But I find

it suggestive. It hints that the unity

we feel when we realize we're made of

star stuff, that unity might go even

deeper than atoms. It might go all the

way down to the fundamental fabric of

reality. So when you ask what happens

when you die, here's the answer physics

gives us. Your atoms scatter and become

part of other things. Your energy

disperses and becomes less organized.

The pattern that was you dissolves, but

nothing is lost. Nothing is destroyed.

It's all still here. It's all still part

of the same vast ancient ongoing cosmic

process that has been running for 14

billion years and will continue for

billions or trillions more. You were

never really separate from it anyway.

The fields you're made of extend to

infinity. The atoms you're made of have

journeyed through stars. The energy you

embody has transformed countless times

and will transform countless more. You

are a temporary eddy in a river of

matter and energy that has been flowing

since the beginning of time. I find this

view of things deeply satisfying. It

connects me to everything. It makes me

feel like a participant in something

much larger than myself. The universe

isn't just something I look at. I am

part of it. I am it looking at itself.

Consider the hydrogen atoms in the water

you drank this morning have been around

since the first 3 minutes after the Big

Bang. They are among the oldest things

in existence. And now 14 billion years

later, some of those primordial atoms

are arranged in such a way that they can

contemplate their own origin. They can

wonder about the big bang that created

them. They can calculate the conditions

in which they formed. They can

understand the fusion reactions that

will eventually transform them into

heavier elements. That's extraordinary.

That's what you are. Ancient matter

organized by evolution and chemistry

into a pattern that can understand

itself. And yes, the pattern that is me

will end someday. But the atoms will go

on, the energy will go on, the effects I

had on the world will go on. And that's

enough. It has to be enough because it's

what's true. Now, I'd love to hear from

you, knowing that every breath you take

probably contains at least one molecule

from Caesar's dying gasp, from

Cleopatra's whisper, from a dinosaur's

roar a 100 million years ago. Whose

atoms do you think you're carrying?

Whose breath are you sharing? Leave your

answer in the comments below.