Why Philosophy Is Needed for Quantum Computing
Here is an embarrassing secret about quantum mechanics: it is the most successful theory in the history of science — and after a hundred years, physicists still cannot agree on what it means. The equations predict experiments flawlessly. But ask what a superposition is, or what happens inside a quantum computer between the start and the measurement, and you will get six different answers from six physicists. Those answers are not physics. They are philosophy.
And here is the twist: the founders of quantum mechanics were all doing philosophy, and they knew it. At the 1927 Solvay Conference, Einstein and Bohr argued for days — not about equations, but about reality itself. Einstein insisted physics must describe "the very things, not simply the possibility of their occurrence." Bohr replied that a phenomenon is only defined relative to the experimental arrangement used to observe it. In 1926 Einstein had written to Max Born: "I, at any rate, am convinced that He [God] is not playing at dice." The dice — quantum randomness — offended him philosophically, not mathematically.
Schrödinger, too, was doing philosophy when he invented his famous cat in 1935. The cat — simultaneously dead and alive until observed — was not a claim about cats. It was a reductio ad absurdum, a philosophical weapon designed to show that Bohr's interpretation led to nonsense. And the label everyone uses, the "Copenhagen interpretation," was coined by Heisenberg in 1955 and papers over the fact that Bohr and Heisenberg actually disagreed with each other. The "standard view" was philosophy wearing a lab coat.
Now the punchline for this course: quantum computing itself was born from philosophy. In 1985 David Deutsch published the paper that founded the field — "Quantum theory, the Church–Turing principle and the universal quantum computer." His move was philosophical: he read the Church–Turing thesis — the definition of what is computable — not as mathematics but as a physical principle. What can be computed is decided by the laws of physics; physics is quantum; therefore the universal computer must be a quantum computer. Computation stopped being an abstraction and became a physical process. And Deutsch's motivation? He was a committed believer in the many-worlds interpretation — he thought quantum parallelism was computation spread across parallel universes. The entire field you are studying exists because one physicist took an interpretation of quantum mechanics seriously.
Philosophy has repeatedly turned into physics. In 1964 John Bell took the old Einstein–Bohr argument about "spooky action" and converted it into a testable inequality — philosophy became an experiment, and the experiments (2022 Nobel Prize) killed "local realism" for good. Wheeler's philosophical slogan "it from bit" — the idea that information is more fundamental than matter — seeded an entire research program that is still running. The pattern is reliable: today's philosophy seminar is tomorrow's laboratory.
So there is a bargain every quantum computing student makes, usually without noticing. Textbooks teach what philosopher Tim Maudlin calls the "quantum recipe" — rules that predict outcomes — instead of a proper theory, which would say what exists and what it does. The recipe lets you calculate. But the moment you ask the obvious question — what is my quantum computer actually doing between initialization and measurement? — the recipe goes silent, and the honest answer is: physics has not settled it. This module is about that unsettled territory. Not because philosophy is decoration, but because in quantum computing, the philosophy is load-bearing.
Common myth: "Philosophy of quantum mechanics is armchair speculation with no effect on real science." Deutsch's 1985 paper, Bell's 1964 theorem, and Wheeler's "it from bit" each began as philosophy and each redirected experimental physics. In this field, interpretations are research programs.
Go deeper — the math & the rigor
Let us make the "why philosophy" argument precise. Quantum mechanics has two parts, and they have very different philosophical status. The first is unitary evolution — the Schrödinger equation,
\[i\hbar\,\frac{\partial}{\partial t}|\psi\rangle = H|\psi\rangle,\]
which is deterministic, smooth, and agreed upon by everyone. The second is the Born rule,
\[P(\text{outcome } i) = |\langle i|\psi\rangle|^2,\]
which says what you will observe — probabilistically. The entire philosophy of quantum mechanics lives in the gap between these two: the equation says the wavefunction evolves deterministically into superpositions; the rule says you observe single definite outcomes with certain probabilities. What connects them — collapse? branching? ignorance? — is the measurement problem, and it is unsolved.
Deutsch's 1985 move deserves a closer look because it shows philosophy doing creative work. The classical Church–Turing thesis says every effectively computable function can be computed by a Turing machine. Deutsch's Church–Turing–Deutsch principle restates it physically: every finitely realizable physical system can be perfectly simulated by a universal computing machine operating by finite means — and since the physical world is quantum-mechanical, that universal machine must be a quantum computer. Notice what happened: "computable" stopped being a fact about mathematics and became a fact about physics. Complexity theory became a branch of physics. (As computer scientist Charlie Bennett later replied to Deutsch: physics is the universal computer that complexity theory describes.)
Deutsch's Everettianism was not incidental decoration — it was the engine. Scott Aaronson recounts the origin story: Deutsch wondered whether one could perform an interference experiment on oneself — a conscious computer placed in superposition — and followed the logic until quantum computing fell out. In The Fabric of Reality (1997) Deutsch issued his famous challenge: "To those who still cling to a single-universe world-view, I issue this challenge: explain how Shor's algorithm works." His claim: a quantum computer factoring a large number performs more computations than there are atoms in the visible universe — so where is the computation happening, if not in parallel universes?
Aaronson — who admires Deutsch enormously — pushes back, and the pushback is itself philosophical. His correction: "A quantum computer is NOT like a massively-parallel classical computer! Exponentially-many basis states, but you only get to observe one of them. Any hope for a speedup rides on the magic of quantum interference." On his view, even the simplest quantum speedup, Deutsch–Jozsa (1992), works not by trying all answers but by arranging interference so that wrong answers cancel and the right global property of the function survives into a single measurement. Aaronson's slogan for the whole field: quantum computing is "probability theory with minus signs." Same equations, opposite stories about what they mean.
This is why the module exists. A student who learns only the recipe can run the circuits but cannot answer what the circuits are. The six interpretations in the next lesson all reproduce every experimental result — including every quantum computation ever run — yet tell incompatible stories about reality. Choosing between them (or refusing to choose) is philosophy. And as Bell showed, today's philosophical disagreement is tomorrow's Nobel Prize — provided someone finds the question that can be tested.
Further reading: Scott Aaronson, Quantum Computing Since Democritus (2013) — the philosophical implications of computing, from a skeptic of many-worlds; David Deutsch, The Fabric of Reality (1997) — the Everettian case, including the Shor challenge; Adam Becker, What Is Real? (2018) — how Copenhagen orthodoxy sidelined the alternatives; Philip Ball, Beyond Weird (2018) — the best single-volume survey of interpretations and experiments.
Key takeaways
- Quantum mechanics predicts perfectly but explains nothing agreed-upon: the Schrödinger equation (deterministic, agreed) and the Born rule (probabilistic, disputed) leave a gap — the measurement problem — that is philosophy.
- The founders were doing philosophy: Bohr vs Einstein at Solvay 1927, Einstein's 1926 'God does not play dice' letter, Schrödinger's cat (1935) as an anti-Copenhagen reductio, and the 'Copenhagen interpretation' label itself (Heisenberg, 1955) papering over real disagreements.
- Quantum computing was born from philosophy: Deutsch's 1985 paper read the Church–Turing thesis as a physical principle and was driven by his many-worlds convictions — including the challenge 'explain how Shor's algorithm works' without parallel universes.
- Philosophy keeps becoming physics: Bell (1964) turned the EPR debate into a testable inequality; Wheeler's 'it from bit' seeded the reconstruction program.
- Aaronson's counterweight: a quantum computer is not a massively-parallel classical machine — speedup rides on interference ('probability theory with minus signs'), and supremacy experiments add nothing new to the interpretation debate.
Check your understanding
Q1.What was Deutsch's philosophical move in his 1985 paper that founded quantum computing?
The Church–Turing–Deutsch principle made computability a fact about physics rather than mathematics — and Deutsch's Everettianism supplied the motivation.
Q2.What was Schrödinger's cat (1935) intended to be?
Schrödinger designed the cat to look absurd — it was a weapon against Copenhagen completeness, not an endorsement of dead-and-alive cats.
Q3.According to Aaronson, where does quantum speedup actually come from?
Aaronson's correction: exponentially many amplitudes exist, but you observe one outcome — the speedup rides on interference sculpting the distribution, not on parallel classical computation.
References
The books, papers, and articles this lesson drew on — with a note on what each one was used for.
- David Deutsch, “Quantum theory, the Church–Turing principle and the universal quantum computer,” Proceedings of the Royal Society of London A 400, 97–117 (1985). · source ↗
Used for: The founding paper of quantum computing — the source of Deutsch's physical Church–Turing principle and his Everettian motivation. - David Deutsch, The Fabric of Reality (Allen Lane, 1997).
Used for: The Everettian case for quantum computing, including the challenge: “explain how Shor's algorithm works” without parallel universes. - Scott Aaronson, Quantum Computing Since Democritus (Cambridge University Press, 2013).
Used for: The counterweight — quantum computing as “probability theory with minus signs”; speedup rides on interference, and supremacy experiments add nothing new to the interpretation debate. - John Bell, Speakable and Unspeakable in Quantum Mechanics (Cambridge University Press, 1987).
Used for: Collects Bell's 1964 theorem paper — the moment the Einstein–Bohr argument became a testable inequality. - Tim Maudlin, Philosophy of Physics: Quantum Theory (Princeton University Press, 2019).
Used for: The distinction between a predictive “quantum recipe” and a proper theory — the framing of this lesson's bargain. - Adam Becker, What Is Real? The Unfinished Quest for the Meaning of Quantum Physics (Basic Books, 2018).
Used for: The history of how Copenhagen orthodoxy sidelined alternative interpretations for decades. - “The flawed multiverse,” Physics World (review of Deutsch’s The Beginning of Infinity). · source ↗
Used for: The preferred-basis critique of many-worlds and Deutsch's multiverse account of quantum computation.
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