The inner life
Uncertainty Principle
Direct answer
Uncertainty Principle: The uncertainty principle is Werner Heisenberg's rule of 1927 that certain pairs of quantities, such as a particle's position and momentum, cannot both have sharply defined values at once.
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At a glance
- Also called
- indeterminacy principle, uncertainty relation
- Found in
- Consciousness & Spiritual Psychology
- Related
- Observer Effect, Wave Function, Complementarity, Wave-Particle Duality
The uncertainty principle is Werner Heisenberg's rule of 1927 that certain pairs of quantities, such as a particle's position and momentum, cannot both have sharply defined values at once.
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What does the uncertainty principle say?
The principle sets a floor under the product of two spreads. The more narrowly a particle's position is fixed, the wider the range of its momentum, and the reverse; the product of the two can never fall below a fixed amount set by Planck's constant. The same holds for other paired quantities.
Uncertainty Principle is often explained as clumsiness: a measurement jolts the thing measured. Heisenberg first argued that way, with an imagined microscope. The modern understanding goes deeper. The limit follows from the wave nature of matter and holds whether or not anyone measures, much as a very short musical note cannot have a single exact pitch. Because Planck's constant is tiny, the limit is unnoticeable for everyday objects. It gives no support to the saying that nothing can be known.
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What sets Uncertainty Principle apart
Its form is an inequality, which is what separates it from a general confession of ignorance: the spread in position multiplied by the spread in momentum is at least the reduced Planck constant divided by two. The quantities it governs come in fixed pairs that mathematicians call conjugate, so position pairs with momentum and not with, say, charge. And the spreads belong to the quantum state, describing a scatter of results over many identical trials; a single measurement of one quantity can be as fine as the apparatus allows.
- Published: Werner Heisenberg, Zeitschrift für Physik, 1927
- Inequality: spread in position times spread in momentum is at least half the reduced Planck constant
- Proved exactly by: Earle Kennard, 1927; generalised by Howard Robertson, 1929
- Usual German name: Unschärferelation, a relation of unsharpness
- Thought experiment: Heisenberg's gamma-ray microscope
Often mistaken for something else: The observer effect says measuring disturbs; the uncertainty principle says the paired values were never both sharp to begin with.
The story behind Uncertainty Principle
Heisenberg published the principle in 1927 while working in Copenhagen with Niels Bohr, who pressed him to rest it on the duality of wave and particle and not on disturbance alone. Earle Kennard proved the exact inequality for position and momentum the same year, and Howard Robertson generalised it in 1929. Albert Einstein tried for years to devise a way round it, and Bohr answered each attempt.
Why Uncertainty Principle matters
Uncertainty Principle is among the most quoted results of physics outside physics, and nearly always in a looser sense than the one it has.
How to read Uncertainty Principle
Physicists apply it to estimate the size of atoms and the width of spectral lines; elsewhere it is borrowed as a metaphor for the limits of knowing.
Uncertainty Principle in practice: an example
A reader who hears that uncertainty means anything is possible can recall that the principle is an inequality with a number in it.
Where readings of Uncertainty Principle go wrong
Uncertainty Principle limits pairs of physical quantities by a stated amount; it does not say that facts are a matter of opinion or that wishes shape outcomes.
Sources and editorial method
This entry summarizes the tradition in the Almanac's own words; where no source is listed, it is an editorial summary rather than a primary text, and symbolic readings are named as the tradition's, not as fact.
- Source
- The Great Almanac Dictionary
- Category
- Consciousness & Spiritual Psychology
- Aliases
- heisenberg uncertainty principle, heisenberg's uncertainty principle, uncertainty principle meaning, indeterminacy principle, uncertainty relation
- Last updated
- 2026-09-26
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Common questions
What does the uncertainty principle say?
That some pairs of properties, position and momentum above all, cannot both be precisely defined together. The spread of one times the spread of the other is never less than Planck's constant divided by four pi.
Is the uncertainty principle the same as the observer effect?
No. The observer effect is the disturbance a measurement causes. The uncertainty principle is a limit built into quantum states themselves, which would hold even for a perfectly gentle measurement.
Does the uncertainty principle apply to everyday life?
In principle to everything, in practice to nothing one can see. For a cricket ball the limit is far smaller than any instrument could detect; it counts at the scale of atoms and electrons.
