Have you ever stopped and wondered about something incredibly simple?

Where does a thought come from?

You think, “I should call my friend.”
You remember something from childhood.
You suddenly feel hungry.
You ask yourself, “Why am I here?”

It feels as though you are creating these thoughts.

But if we start looking underneath the experience, an extraordinary chain begins to appear:

Thought → Neurons → Cells → Molecules → Atoms → Particles → Quantum Fields → Early Universe → The unknown

Following this chain takes us from neuroscience and biology to chemistry, quantum physics, particle physics and cosmology.

And eventually, we arrive at one of the deepest questions humanity has ever asked:

Where did everything come from?

Let’s travel down this chain, one layer at a time.

1. It Begins With a Thought

Right now, you might be thinking:

“What is a thought?”

A thought feels intangible. You can’t hold it in your hand. You can’t point to a physical object and say, “That’s my thought.”

Yet thoughts are associated with physical activity in your brain.

Your brain contains an enormous network of neurons. When you see something, remember something, feel something or make a decision, groups of neurons communicate with each other.

A thought isn’t sitting inside one particular neuron.

Instead, it is associated with patterns of activity across networks of neurons.

For example, imagine you see a cup of tea.

Your eyes detect light.

Your brain processes that information.

You recognize the object as a cup.

Your memory tells you what tea is.

Your previous experiences influence whether you like it.

Your body may respond to hunger, thirst or habit.

And suddenly you think:

“I want some tea.”

What feels like one simple thought is actually the result of enormous amounts of information processing.

But what exactly are the things doing this processing?

Neurons.

2. Neurons: The Communication Cells of the Brain

A neuron is a specialized biological cell designed primarily for communication.

A typical neuron has structures called:

• Dendrites, which receive signals
• Cell body, which maintains the cell and integrates information
• Axon, which carries electrical signals
• Synaptic terminals, which communicate with other cells

Neurons communicate using electrical activity along their membranes and chemical signals at synapses.

These chemical messengers include neurotransmitters such as glutamate, GABA, dopamine and serotonin.

But here’s an important point:

A neuron isn’t some mysterious “thought particle.”

A neuron is simply a living cell.

And that leads us to the next layer.

3. What Is a Cell?

A cell is the basic unit of life.

Your body contains enormous numbers of cells, including:

• Neurons
• Muscle cells
• Skin cells
• Blood cells
• Liver cells
• Bone cells

Although they perform different jobs, they are all built from the same fundamental biological ingredients.

A cell can be thought of as a tiny biological system.

It has a boundary called the cell membrane.

Inside it are molecular machines performing countless chemical reactions.

It contains DNA, which stores biological information.

It has ribosomes, which build proteins.

It has mitochondria, which help generate usable energy.

And all of this activity is ultimately based on chemistry.

So the neuron that helps participate in a thought is itself made from something much smaller.

Molecules.

4. Molecules: Chemistry Inside the Cell

A molecule is formed when atoms are connected through chemical bonds.

Water is a simple example:

H₂O

That’s two hydrogen atoms connected with one oxygen atom.

But biological molecules can be enormously complicated.

Proteins, DNA, fats, sugars and neurotransmitters are all arrangements of atoms.

For example, DNA contains an enormous amount of biological information through the sequence of its molecular building blocks.

Proteins fold into specific structures and perform specific functions.

Neurotransmitters carry chemical signals between neurons.

So when a neuron sends a signal, thousands of molecular interactions are taking place.

But what are molecules made of?

Atoms.

5. Atoms: The Building Blocks of Ordinary Matter

Atoms are the basic units from which ordinary matter is constructed.

Your body contains atoms of many elements, including:

• Hydrogen
• Carbon
• Oxygen
• Nitrogen
• Calcium
• Phosphorus
• Iron
• Sodium
• Potassium

An atom contains a nucleus surrounded by electrons.

The nucleus contains:

Protons + Neutrons

And surrounding the nucleus are electrons, described by quantum mechanics.

But now we can go deeper.

Because protons and neutrons aren’t fundamental objects either.

6. What Are Protons and Neutrons Made Of?

Protons and neutrons are made primarily from smaller particles called quarks.

Quarks are held together by the strong nuclear interaction, which is mediated by particles called gluons.

So our chain becomes:

Matter

↓

Atoms

↓

Nucleus + electrons

↓

Protons + neutrons

↓

Quarks + gluons

But electrons appear, as far as current experiments indicate, to be elementary particles.

And now we have entered the world of:

Quantum Physics

7. What Is Quantum Physics?

Quantum physics is the framework scientists use to describe nature at microscopic scales.

Atoms, electrons, photons and quarks don’t behave exactly like everyday objects.

At our everyday scale, we might imagine a ball as having a precise position and velocity.

Quantum systems behave differently.

Their states are described mathematically using quantum mechanics, which often gives probabilities for the outcomes of measurements.

One of the remarkable features of quantum physics is that matter can exhibit both particle-like and wave-like behavior.

There is also a fundamental uncertainty associated with certain pairs of physical quantities, such as position and momentum.

Quantum physics sounds strange because our everyday intuition was developed for the macroscopic world—not for electrons and quantum fields.

But quantum mechanics isn’t merely philosophical speculation.

It has been tested incredibly precisely and underlies much of modern technology.

Lasers, semiconductors, MRI technology and countless other technologies depend on quantum physics.

But quantum physics has another level.

Quantum fields.

8. What Is a Quantum Field?

This is where our everyday picture of particles changes dramatically.

Instead of imagining the universe as simply being filled with tiny balls called particles, modern quantum field theory describes nature in terms of fields.

There are different quantum fields associated with different types of fundamental particles.

For example:

• Electron field
• Quark fields
• Electromagnetic field
• Higgs field
• And others

A useful analogy is an ocean.

Imagine the ocean stretching across an enormous area.

The ocean itself is the underlying medium.

A wave is an excitation of that ocean.

Similarly, in quantum field theory, a particle can be understood as an excitation of an underlying quantum field.

An electron can be viewed as an excitation of the electron field.

A photon is an excitation of the electromagnetic field.

Quarks are excitations of quark fields.

This gives us a deeper picture:

Quantum fields → particle excitations → atoms → molecules → cells → neurons → brains

But now we encounter an extraordinary question.

9. Where Did the Quantum Fields Come From?

This is where physics begins moving from particle physics into cosmology.

Our current understanding tells us that the universe has been expanding for approximately 13.8 billion years.

If we trace that expansion backward, the universe becomes progressively hotter and denser.

The early universe was radically different from the universe we see today.

There were no planets.

No stars.

No humans.

No cells.

No atoms like the ones surrounding us today.

The universe was an extremely hot, dense environment in which fundamental particles and fields played a central role.

This is described by the Big Bang model.

But there’s an important misconception to clear up.

10. The Big Bang Wasn’t Simply an Explosion

The Big Bang wasn’t necessarily an explosion in an already existing empty space.

Instead, the universe itself was expanding.

As the universe expanded, it cooled.

You can think of the broad sequence like this:

Extremely hot, dense early universe

↓

Expansion

↓

Cooling

↓

Formation of fundamental particles and nuclei

↓

Formation of atoms

↓

Formation of stars

↓

Formation of galaxies

↓

Stars manufacture heavier elements

↓

Planets form

↓

Chemistry becomes increasingly complex

↓

Life eventually emerges

↓

Evolution

↓

Brains

↓

Neurons

↓

Thought

So there is a remarkable connection between cosmology and your consciousness.

The universe became increasingly structured over billions of years until matter eventually became organized into a biological system capable of thinking about the universe.

11. Where Did the Atoms in Your Body Come From?

This is one of the most beautiful parts of the story.

Some of the simplest elements, particularly much of the hydrogen in the universe, originated in the very early universe.

But many of the heavier elements required for life were produced through processes involving stars.

Stars are essentially enormous nuclear furnaces.

Inside stars, nuclear reactions can create heavier elements.

When massive stars die, they can eject elements into space.

Other extreme cosmic events can also create and distribute heavy elements.

Eventually, this material becomes part of new stars, planets and other objects.

The Earth formed from material that had already been processed by earlier generations of stars.

And eventually, some of those atoms became part of your body.

The carbon in your cells.

The oxygen you use.

The calcium in your bones.

The iron in your blood.

The phosphorus in your DNA.

These atoms have a cosmic history that predates the existence of Earth—and certainly predates you.

In that sense, your body is connected to the history of the cosmos.

12. But Then We Reach a Problem: Gravity

At this point, we have two extraordinarily successful descriptions of nature.

The first is quantum physics / quantum field theory, which describes microscopic particles and fields.

The second is Einstein’s general theory of relativity, which describes gravity and the large-scale structure of spacetime.

General relativity tells us something profound:

Gravity isn’t simply a conventional force pulling objects together.

Matter and energy influence the geometry of spacetime.

Objects then move through that curved spacetime.

This framework successfully describes planets, stars, black holes and the evolution of the universe on large scales.

Quantum theory successfully describes the microscopic world.

Both theories work extraordinarily well.

But there’s a problem.

13. Quantum Gravity: Where Our Theories Meet

Imagine the earliest universe.

It was extremely hot and dense.

Gravity mattered enormously.

Quantum effects mattered enormously.

So we need a theory that can describe:

Quantum physics + gravity

together.

That’s the problem known as quantum gravity.

We currently don’t have a single experimentally confirmed theory that completely solves it.

Physicists have developed different approaches, including string theory and loop quantum gravity, among others.

But none has yet been experimentally established as the final answer.

This means that when we try to push our understanding all the way back toward the earliest conceivable moments of the universe, our existing theories eventually reach their limits.

14. What Happened at the Beginning?

This is where we have to distinguish between what science knows and what it doesn’t.

We have strong evidence for the universe’s expansion.

We have strong evidence that the early universe was extremely hot and dense.

We have observations of the cosmic microwave background.

We have evidence for the formation of light elements.

We have models describing how the universe evolved.

But asking:

“What happened at the absolute beginning?”

is much harder.

If we extrapolate classical general relativity backward far enough, we encounter what is called a singularity in the mathematical description.

But this doesn’t necessarily mean that nature literally contained an infinitely dense point.

It may instead indicate that our theory has reached its limit.

We likely need a quantum theory of gravity to understand what really happened under those extreme conditions.

15. And Then Comes the Biggest Question

Suppose one day scientists develop a complete theory of quantum gravity.

We might then understand the earliest physical state of the universe much better.

But another question could remain:

Why does that physical reality exist at all?

Why are there quantum fields?

Why are there laws of physics?

Why is there a universe?

Why is there something rather than nothing?

Science has not established a definitive answer to these ultimate questions.

There are hypotheses and philosophical ideas, but they should not be confused with experimentally confirmed scientific facts.

And that’s an important boundary.

16. The Complete Journey

Let’s put everything together.

You begin with something apparently simple:

“I am having a thought.”

But underneath that thought:

Thought

↓

Neural activity

↓

Neurons

↓

Biological cells

↓

Molecules

↓

Chemical bonds

↓

Atoms

↓

Electrons, protons and neutrons

↓

Quarks and fundamental particles

↓

Quantum fields

↓

Fundamental interactions

↓

Early universe

↓

Expansion and cooling

↓

Formation of nuclei and atoms

↓

Stars and galaxies

↓

Heavy elements

↓

Planets

↓

Complex chemistry

↓

Life

↓

Evolution

↓

Brains

↓

Neurons

↓

Thought

In other words, the story is almost circular.

The universe produced the matter and conditions from which stars, planets and life eventually emerged.

Life produced brains.

Brains produced conscious experiences and thoughts.

And now those thoughts are being used to investigate the universe that produced the brain.

17. The Humbling Part

There’s something almost poetic about this.

The universe existed for billions of years before human beings appeared.

Stars were born and died.

Galaxies formed.

Planets emerged.

Chemistry became increasingly complex.

Life appeared.

Evolution experimented for billions of years.

Eventually, nervous systems became increasingly sophisticated.

And eventually, one species developed brains capable of asking:

“What is a thought?”

Then:

“What is a neuron?”

Then:

“What is a cell?”

Then:

“What are molecules?”

Then:

“What are atoms?”

Then:

“What are particles?”

Then:

“What are quantum fields?”

And finally:

“Where did the universe itself come from?”

The remarkable thing is that the atoms asking the question are themselves products of the universe.

You are, quite literally, part of the universe trying to understand itself.

18. Where Science Currently Ends

Our understanding currently gives us an extraordinarily successful description of the universe:

Conscious experience

→ neuroscience

Neurons

→ biology

Cells

→ chemistry

Molecules

→ atomic physics

Atoms

→ quantum mechanics

Fundamental particles

→ quantum field theory

Space, time and gravity

→ general relativity

Extremely early universe

→ cosmology

Quantum behavior of gravity

→ an unsolved problem

Ultimate origin of reality

→ unknown

That final word is important:

Unknown.

Not “impossible.”

Not “magic.”

Not “nothing.”

Simply unknown.

And perhaps one of the most exciting things about science is that “unknown” doesn’t mean we have reached the end.

It means there is still something to discover.