What Is a Photon, Really? | The Quantum of Light Explained

Light reaches your eye in discrete packets, each delivering a fixed amount of energy at one place. But a photon is not a tiny ball, and it is not simply a wave. This video follows the idea from Young and Maxwell, through Planck and Einstein, to the experiments that actually prove light is quantized, and finally to what quantum electrodynamics says a photon is: one quantum of excitation of the electromagnetic field. Along the way: why the photoelectric effect, on its own, does not prove that photons exist (Lamb and Scully, 1969), and why single-photon anticorrelation on a beam splitter (Grangier, Roger and Aspect, 1986) and photon antibunching (Kimble, Dagenais and Mandel, 1977) do. CHAPTERS 0:00 Intro: light arrives one photon at a time 0:55 Light as a wave: Young and Maxwell 2:10 The ultraviolet catastrophe and Planck's quanta 3:45 Einstein's light quanta and the photoelectric effect 5:11 Spectral lines: photons from atoms 5:54 Photon momentum and the Compton effect 6:39 The photon's profile 7:39 One photon at a polarizer 8:18 The double slit, one photon at a time 9:41 The decisive evidence: anticorrelation and antibunching 11:20 What a photon really is: the quantum field 13:36 Photons across the universe REFERENCES J. C. Maxwell, "A Dynamical Theory of the Electromagnetic Field," Phil. Trans. R. Soc. Lond. 155, 459 (1865). https://doi.org/10.1098/rstl.1865.0008 M. Planck, "Ueber das Gesetz der Energieverteilung im Normalspectrum," Ann. Phys. 309, 553 (1901). https://doi.org/10.1002/andp.19013090310 A. Einstein, "Über einen die Erzeugung und Verwandlung des Lichtes betreffenden heuristischen Gesichtspunkt," Ann. Phys. 322, 132 (1905). https://doi.org/10.1002/andp.19053220607 G. I. Taylor, "Interference fringes with feeble light," Proc. Camb. Phil. Soc. 15, 114 (1909). R. A. Millikan, "A Direct Photoelectric Determination of Planck's h," Phys. Rev. 7, 355 (1916). https://doi.org/10.1103/PhysRev.7.355 A. H. Compton, "A Quantum Theory of the Scattering of X-rays by Light Elements," Phys. Rev. 21, 483 (1923). https://doi.org/10.1103/PhysRev.21.483 G. N. Lewis, "The Conservation of Photons," Nature 118, 874 (1926). https://doi.org/10.1038/118874a0 W. E. Lamb Jr. and M. O. Scully, "The photoelectric effect without photons," in Polarisation, Matière et Rayonnement (Presses Universitaires de France, Paris, 1969). H. J. Kimble, M. Dagenais and L. Mandel, "Photon Antibunching in Resonance Fluorescence," Phys. Rev. Lett. 39, 691 (1977). https://doi.org/10.1103/PhysRevLett.3... P. Grangier, G. Roger and A. Aspect, "Experimental Evidence for a Photon Anticorrelation Effect on a Beam Splitter: A New Light on Single-Photon Interferences," Europhys. Lett. 1, 173 (1986). https://doi.org/10.1209/0295-5075/1/4... J. N. Tinsley et al., "Direct detection of a single photon by humans," Nat. Commun. 7, 12172 (2016). https://doi.org/10.1038/ncomms12172 ATLAS Collaboration, "Evidence for light-by-light scattering in heavy-ion collisions with the ATLAS detector at the LHC," Nat. Phys. 13, 852 (2017). https://doi.org/10.1038/nphys4208 LHAASO Collaboration (Z. Cao et al.), "Ultrahigh-energy photons up to 1.4 petaelectronvolts from 12 γ-ray Galactic sources," Nature 594, 33 (2021). https://doi.org/10.1038/s41586-021-03... Particle Data Group, Review of Particle Physics: photon mass limit m less than 1×10⁻¹⁸ eV (from the solar-wind magnetic field, Ryutov 2007). https://pdg.lbl.gov NOTES ON NUMBERS Green photon (550 nm): E = hc/λ ≈ 2.25 eV. 1 mW red laser pointer (650 nm): about 3.3 × 10¹⁵ photons per second. The Sun: luminosity 3.83 × 10²⁶ W divided by a mean blackbody photon energy of about 1.34 eV gives about 1.8 × 10⁴⁵ photons per second. Cosmic microwave background: about 411 photons per cubic centimetre (T = 2.7255 K). #physics #quantum #photon #light #quantumphysics