uncertainty principle

the principle of quantum mechanics, formulated by Heisenberg, that the accurate measurement of one of two related, observable quantities, as position and momentum or energy and time, produces uncertainties in the measurement of the other, such that the product of the uncertainties of both quantities is equal to or greater than h/2 pi, where h equals Planck's constant. Also called indeterminacy principle, Heisenberg uncertainty principle.
[1930-35]

* * *

Principle that states that the position and velocity of an object cannot both be measured exactly at the same time, and that the concepts of exact position and exact velocity together have no meaning in nature.

Articulated by Werner Heisenberg in 1927, it applies only at the small scales of atoms and subatomic particles and is not noticeable for macroscopic objects, such as moving vehicles. Any attempt to measure the velocity of a subatomic particle precisely will displace the particle in an unpredictable way, thus invalidating any simultaneous measurement of its position. This displacement is a result of the wave nature of particles (see wave-particle duality). The principle also applies to other related pairs of variables, such as energy and time.

* * *

also called  Heisenberg uncertainty principle, or indeterminacy principle 

      statement, articulated (1927) by the German physicist Werner Heisenberg (Heisenberg, Werner), that the position and the velocity of an object cannot both be measured exactly, at the same time, even in theory. The very concepts of exact position and exact velocity together, in fact, have no meaning in nature.

      Ordinary experience provides no clue of this principle. It is easy to measure both the position and the velocity of, say, an automobile, because the uncertainties implied by this principle for ordinary objects are too small to be observed. The complete rule stipulates that the product of the uncertainties in position and velocity is equal to or greater than a tiny physical quantity, or constant (h/(4π), where h is Planck's constant, or about 6.6 × 10−34 joule-second). Only for the exceedingly small masses of atoms and subatomic particles does the product of the uncertainties become significant.

      Any attempt to measure precisely the velocity of a subatomic particle, such as an electron, will knock it about in an unpredictable way, so that a simultaneous measurement of its position has no validity. This result has nothing to do with inadequacies in the measuring instruments, the technique, or the observer; it arises out of the intimate connection in nature between particles and waves in the realm of subatomic dimensions.

      Every particle has a wave associated with it; each particle actually exhibits wavelike behaviour. The particle is most likely to be found in those places where the undulations of the wave are greatest, or most intense. The more intense the undulations of the associated wave become, however, the more ill defined becomes the wavelength, which in turn determines the momentum of the particle. So a strictly localized wave has an indeterminate wavelength; its associated particle, while having a definite position, has no certain velocity. A particle wave having a well-defined wavelength, on the other hand, is spread out; the associated particle, while having a rather precise velocity, may be almost anywhere. A quite accurate measurement of one observable involves a relatively large uncertainty in the measurement of the other.

      The uncertainty principle is alternatively expressed in terms of a particle's momentum and position. The momentum of a particle is equal to the product of its mass times its velocity. Thus, the product of the uncertainties in the momentum and the position of a particle equals h/(4π) or more. The principle applies to other related (conjugate) pairs of observables, such as energy and time: the product of the uncertainty in an energy measurement and the uncertainty in the time interval during which the measurement is made also equals h/(4π) or more. The same relation holds, for an unstable atom or nucleus, between the uncertainty in the quantity of energy radiated and the uncertainty in the lifetime of the unstable system as it makes a transition to a more stable state.

* * *


Universalium. 2010.

Look at other dictionaries:

  • uncertainty principle — n. in quantum mechanics, the principle that it is impossible to measure simultaneously and exactly two related quantities, as both the position and the momentum of an electron …   English World dictionary

  • Uncertainty principle — In quantum physics, the Heisenberg uncertainty principle states that locating a particle in a small region of space makes the momentum of the particle uncertain; and conversely, that measuring the momentum of a particle precisely makes the… …   Wikipedia

  • uncertainty principle — /ʌnˈsɜtnti prɪnsəpəl/ (say un sertntee prinsuhpuhl) noun the principle that it is not possible to determine simultaneously with complete accuracy both the position and the momentum of an atomic particle, as an electron, because of the very… …   Australian English dictionary

  • uncertainty principle — noun Date: 1929 a principle in quantum mechanics: it is impossible to discern simultaneously and with high accuracy both the position and the momentum of a particle (as an electron) called also Heisenberg uncertainty principle …   New Collegiate Dictionary

  • uncertainty principle — noun Heisenberg uncertainty principle. Syn: indeterminacy principle …   Wiktionary

  • uncertainty principle — See Heisenberg uncertainty principle …   Philosophy dictionary

  • uncertainty principle — uncer′tainty prin ciple n. phs the quantum mechanical principle, formulated by Heisenberg, that measuring either of two related quantities, as position and momentum or energy and time, produces uncertainty in measurement of the other • Etymology …   From formal English to slang

  • uncertainty principle — noun Physics the principle, stated by Werner Heisenberg, that the momentum and position of a particle cannot both be precisely determined at the same time …   English new terms dictionary

  • uncertainty principle — noun (quantum theory) the theory that it is impossible to measure both energy and time (or position and momentum) completely accurately at the same time • Syn: ↑indeterminacy principle • Topics: ↑quantum theory • Hypernyms: ↑scientific theory …   Useful english dictionary

  • Heisenberg uncertainty principle — see uncertainty principle. * * * Physics. See uncertainty principle. [1965 70; named after W. K. HEISENBERG] * * * Heisenberg uncertainty principle, Heisenberg s uncertainty principle /hīˈzən bûrg(z) un sûrˈtən ti prinˈsi pl/ noun A more formal… …   Useful english dictionary

Share the article and excerpts

Direct link
Do a right-click on the link above
and select “Copy Link”

We are using cookies for the best presentation of our site. Continuing to use this site, you agree with this.