According to the uncertainty principle, the position and momentum of a subatomic particle cannot be measured simultaneously with an accuracy greater than that set by Planck's constant. This is because in any measurement a particle must interact with at least one photon, or quantum of energy, which acts both like a particle and like a wave, and disturbs it in an unpredictable and uncontrollable manner. An accurate measurement of the position of an orbiting electron by means of a microscope, for example, requires the use of light of short wavelengths, with the result that a large but unpredictable momentum is transferred to the electron. An accurate measurement of the electron's motnentum. on the other hand, requires light quanta of very low momentum (and therefore long wavelength). which leads to a large angle of diffraction in the lens and a poor definition of the position.
According to the conventional interpretation of quantum physics, however. not only is it impossible for us to measure a particle's position and momentum simultaneously with equal precision, a particle does not possess well- defined properties when it is not interacting with a measuring instrument. Furthermore, the uncertainty principle implies that a particle can never be at rest, but is subject to constant fluctuations even when no measurement is taking place, and these fluctuations are assumed to have no causes at all. In other words, the quantum world is believed to be characterized by absolute indeterminism, intrinsic ambiguity, and irreducible lawlessness. As the late physicist David Bohm ( 1984, p. 87) put it: it is assumed that in any particular experiment, the precise result that will be obtained is completely arbitrary in the sense that it has no relationship whatever to anything else that exists in the world or that ever has existed.