A question-led introduction to quantum physics
Quantum before symbols
States, measurement and matter
David Elliman, Neuro-Symbolic Ltd · October 2026 · 27 questions · 65 pages
Read Quantum before symbols — free PDF, 65 pagesAbout this book
The book builds one experiment slowly. The first eight questions follow a single two-route interferometer: what should happen when there are two routes, why arrows are used to calculate a chance, how one exit can be completely dark, what a delay changes, whether observation turns a wave into a particle, and whether a record can be undone. Later questions use its lessons to discuss particles, spin, entanglement and the probability rule.
No calculus is needed. You will add numbers, square them and occasionally use a right-angled triangle. Each opening asks you to make a prediction before reading the explanation, and the numbered Try questions have answers at the back. An optional final part shows how physicists compress these operations into symbols (complex numbers, kets and bras, matrices and the uncertainty formula), but it is not a prerequisite for the rest.
Established quantum physics supplies the rules used throughout. A closing case study asks what it would take for a proposed information-based theory to explain those rules; it does not present that proposal as established physics. Three companion volumes take up particles and interactions, space and gravity, and the history of the universe.
The questions
27 questions in 7 parts. Each part opens with its physical setting; each question asks for a prediction before the explanation.
One experiment, one question at a time
- What should happen when there are two routes?
- Why use arrows to calculate a chance?
- How can one exit be completely dark?
- What changes when one route is delayed?
- Does observation turn a wave into a particle?
- Can a record be undone?
- What does a quantum state tell us?
- Why can't a qubit be copied like a bit?
Particles that spread, move and stay put
- What is a particle?
- If a particle is a pattern, what moves?
- Why can't position and momentum both be sharp?
- How can a particle get through a barrier?
- Can a quantum state stay still?
Spin without a spinning ball
- Is spin really angular momentum?
- Why does an electron need two full turns?
- Why can't two electrons occupy the same state?
Correlations and interactions
- How do entangled particles know each other's results?
- Do all possible interactions collapse into one?
- Are virtual photons real photons?
- Can other particles be virtual too?
What the probability rule does
- Why do we square an amplitude to get a probability?
- Does decoherence explain why one result is recorded?
A closing case study: information and physical theory
- When does a useful description become a physical theory?
Optional: a closer look at the notation
- How can a number describe an arrow?
- What are a ket and a bra?
- What does a matrix actually do?
- What does the uncertainty formula say?
At the back: Answers to the Try questions, and a closing section of words, questions and sources.
Two sample answers
The opening of two answers, as they appear in the book. Each question is preceded by a prompt to make a prediction before reading on.
Question 5 · One experiment, one question at a time
Does observation turn a wave into a particle?
Before reading on. A device marks which route the photon takes, but nobody reads the device. Will the unmarked interference pattern survive?
The usual double-slit question contains a trap. It pictures a wave moving through the apparatus, then a person looking at it and making it become a particle. The experiment does not require that sequence. Individual detections are local clicks both with and without interference. What changes is the distribution of those clicks over many trials.
Our two-route apparatus asks the same essential question in a simpler form. Instead of a screen with many possible positions, it has two exits. Put a route marker into it. Ideally, the marker neither absorbs the photon nor changes the route probabilities. It becomes correlated with the route: one distinguishable marker state for upper, another for lower. In the quantum description, we now have a joint state of photon and marker.
The answer continues in the book.
Question 8 · One experiment, one question at a time
Why can't a qubit be copied like a bit?
Before reading on. A machine can copy the labels upper and lower perfectly. Must it therefore copy every coherent combination of upper and lower?
A bit has two distinguishable values. Once a value is recorded in a suitable physical memory, it can be copied into another memory. A qubit is a quantum system described using two basis states and their coherent combinations. For a pure state, two complex amplitudes give the components in that chosen basis. This is what two-dimensional state space means; it is not a statement that the object is a flat sheet. Saying that it has “both values” misses the relative phase and suggests that we could simply read out two classical entries.
Our route marker already copies the chosen route label. Starting with a definite upper route, it produces an upper record. Starting with a definite lower route, it produces a lower record. Starting with a coherent combination, however, it produces a correlated joint state. It does not leave two independent systems each carrying the original pure state. The change in interference shows why that distinction matters.
The answer continues in the book.
Read the book
The complete book is a free PDF with extractable text and no registration. It is best read a question at a time, making a prediction before each explanation. All four volumes are also combined into one Kindle book for 99p, if you would rather read them in your e-reader or on your phone.
