What it says
The product of the uncertainty in position, Δx, and in momentum, Δp, can never be smaller than ħ/2. Pin down position sharply and momentum spreads out, and vice versa. It's often explained as light disturbing the particle when you look, but the deeper reason is that particles behave like waves, and no wave has both a sharp location and a sharp wavelength. It's a property of nature, not a limit of instruments.
Consequences
It explains why electrons don't spiral into the nucleus, why particles can tunnel through barriers, and why even a vacuum has zero-point energy. Along with the photoelectric effect, it upended classical determinism; Einstein famously objected that "God does not play dice." For everyday objects, ħ is so tiny the effect is unnoticeable.
FAQ
- Does it mean observers change reality?
- No—it means particles don't have exact position and momentum simultaneously, apart from measurement disturbance.
- Does it apply to a baseball?
- In principle, but ħ is so small it's negligible.
Sources reviewed: 2026-10-26
References
- Heisenberg, W. (1927). Über den anschaulichen Inhalt der quantentheoretischen Kinematik und Mechanik. Zeitschrift für Physik, 43, 172–198.
- Griffiths, D. J., & Schroeter, D. F. (2018). Introduction to Quantum Mechanics (3rd ed.). Cambridge University Press.