Momentum Measurement
Momentum is a property of motion that is classical physics is a vector (directional) quantity that in closed systems is conserved during collisions. In Newtonian physics momentum is measured as the product of the mass and component velocity of a body. For massless particles (e.g., photons) moving at the speed of light (v = c) the momentum (p) is equal to Planck's constant divided by the wavelength.
The first formal definitions and measurement of momentum date to the writing of French philosopher René Descartes (1596-1650). Descartes intended momentum to a quantifiable and measurable concept related to what he termed the "amount of motion".
Measurement of momentum often concentrates on rates of change in the momentum of bodies. In accord with the law of inertia, a body with no net force acting upon it experiences no change in momentum and therefore measurement of momentum reflect that momentum is conserved. Whenever a net force is applied to a body the change in momentum is proportional to the force applied but the conservation of momentum dictates that the momentum of the agent applying force to the body must correspondingly decrease so that the measured momentum of the combined systems remains unchanged.
Modern device used to measure momentum of subatomic particles often employ tracking devices located in strong magnetic fields. The paths of particles moving through these fields reveals their charge and momentum. The direction of deflection reveals the particles change and the momenta of particles can be calculated from the fact that the paths of particles with greater momentum deviate less than those of lesser momentum (i.e., those particles with higher momentum tend to travel along straighter or less bent paths).
Quantum theory dictates that the measurement of certain pairs of properties of particles, including position and momentum are limited by the Heisenberg uncertainty principle first advanced by German physicist Werner Heisenberg. In essence, although it is possible to measure either position or momentum they pair can not be measured simultaneously. The more exact the determination of position, the more uncertain becomes the measurement of momentum.
Although the uncertainty principle isn't relevant to the measurement of momentum of large objects, it places severe constraints on measurements of momentum of subatomic particles. Accordingly, quantum theory places a limitation on the experimental measurement of momentum. The more accuracy required in the determination of position, the less the accuracy possible with regard to the determination of momentum. For example, in attempting to make an accurate determination of the position of an electron it is necessary to bombard the electron with photons. In doing so the collisions between the photons and the electron alter the momentum of the electron and therefore introduce uncertainty in the measurement of the momentum of the electron.
Moreover, there are important philosophical ramifications to the measurement of momentum, In the Copenhagen interpretation of quantum mechanics, reality is dependent upon the observer's measurement. Essentially, the Copenhagen interpretation dictates that in the measurement of momentum or position of a two particle system, the measurement of momentum or position of one particle gives reality (a quantifiable value) to the momentum or position of the second particle. In this theoretical interpretation conflicting realities result when there is an attempt to measure the momentum of one particle and the position of the other. Because time-ordering of the measurements is dependent upon on the inertial frame varying reference frames yield differing realities and give rise to a problem in nonlocality problem related to the instantaneous propagation of information related to the measurement across and real space.
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