| MadSci Network: Physics |
Matthew, The photon is a massless electrically neutral spin-1 stable elementary particle. Its experimentally determined attributes are summarized in the Particle Data Group's Review of Particle Physics: http://pdg.lbl.gov/2009/listings/rpp2009-list-photon.pdf Notice that neither "size" nor "shape" are among the listed properties of the photon. "Photons, like all quantum objects, exhibit both wave-like and particle-like properties. Their dual wave–particle nature can be difficult to visualize. The photon displays clearly wave-like phenomena such as diffraction and interference on the length scale of its wavelength. ... However, experiments confirm that the photon is not a short pulse of electromagnetic radiation; it does not spread out as it propagates, nor does it divide when it encounters a beam splitter. Rather, the photon seems to be a point-like particle since it is absorbed or emitted as a whole by arbitrarily small systems, systems much smaller than its wavelength, such as an atomic nucleus (~10^-15 m across) or even the point-like electron." http://en.wikipedia.org/wiki/Photon#Wave.E2.80.93particle_duality_and_uncertainty_principles If this is troubling conceptually, remember that electrons also exhibit wave-particle duality. They interact as point-like particles but electrons can diffract and display interference like waves. http://en.wikipedia.org/wiki/Electron_diffraction One way of understanding this is David Bohm's pilot-wave theory: http://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory --Dr. Randall J. Scalise http://www.physics.smu.edu/scalise
Try the links in the MadSci Library for more information on Physics.