Heisenberg Uncertainty principle

Meaning: This is a built-in limit of quantum mechanics. Making the uncertainty in a particle's position smaller makes the uncertainty in its momentum larger, and the reverse. It follows from the wave nature of matter (p = h/λ): a wave confined to a small region must combine many wavelengths, so its momentum is spread out.

Equation: Δx · Δp ≥ h/(4π), where Δx is the uncertainty in position (m), Δp is the uncertainty in momentum (kg·m/s), and h is Planck's constant, 6.63 × 10⁻³⁴ J·s. Because p = mv, Δp = mΔv.

On the MCAT: A passage may ask why the Bohr model's fixed orbits fail. Electrons are described by orbitals, which are probability distributions, because an exact path would need exact position and momentum together. Another common item compares an electron with a baseball confined to the same Δx. The electron's tiny mass gives a large Δv, while the baseball's Δv is negligible. Confining an electron to a smaller region increases its momentum uncertainty.

Common confusion: Students often blame clumsy instruments or a photon bumping the electron. The limit is a basic property of wave-particle behavior, not a flaw in measuring. It applies to large objects too, but the effect is far too small to notice.