質量のない粒子：なぜ光速で移動しなければならないのか

Massless Particles: Why They Must Travel at Light Speed Welcome to Science Reader! What we do here Tuesday , 5 May 2026 The New Intelligence The Science of Thought The World We Discover Topics Glossary Home Science Glossary Massless Particles: Why They Must Travel at Light Speed Massless Particles: Why They Must Travel at Light Speed Massless particles have zero invariant mass and must travel at the speed of light in vacuum. The photon is the only confirmed example. Tormod Guldvog March 23, 2026 904 Physics and Mathematics Share Science Glossary · Explore this series › Tormod Guldvog Editor & Duowriter March 23, 2026 Key Takeaways Photons are the only confirmed massless particles. Massless particles must travel at exactly the speed of light. Neutrinos were reclassified as massive after 1998. ✓ Fact-checked · See verification details AI assistant: Research, Drafting, QA A massless particle is an elementary particle with zero invariant mass that travels at the speed of light in vacuum. The photon, carrier of electromagnetic force, is the only confirmed massless particle in nature. Why It Matters Key figure 299,792,458 m/s Speed of all massless particles in vacuum The behavior of massless particles underpins two pillars of modern physics: electromagnetism and the strong nuclear force. Photons carry every electromagnetic interaction, from radio transmission to gamma radiation. Gluons bind quarks into protons and neutrons, holding atomic nuclei together. Without these two massless force carriers, matter as we know it could not exist. The distinction between massive and massless particles also shaped one of the biggest surprises in recent physics. The Standard Model originally treated neutrinos as massless. In 1998, the Super-Kamiokande experiment in Japan detected neutrino oscillations, proving that neutrinos switch between flavors during flight. That switching is only possible if neutrinos carry mass. Takaaki Kajita and Arthur McDonald shared the 2015 Nobel Prize in Physics for the discovery, which forced revisions to the Standard Model. Understanding which particles are massless also connects to open questions in gravitational physics. The hypothetical graviton , proposed as the carrier of gravity, would need to be massless to produce gravity's infinite range. No experiment has detected one. How It Works Einstein's special relativity provides the framework. The full energy-momentum relation for any particle is E² = (mc²)² + (pc)², where m is rest mass and p is momentum. For a massless particle, m = 0, and the equation simplifies to E = pc. Energy depends entirely on momentum. This creates an unusual constraint. A massless particle at rest would have both zero mass and zero momentum, giving it zero energy. A particle with no energy does not exist. Massless particles therefore have no choice: they must always move, and they must always move at exactly the speed of light, 299,792,458 meters per second. Key figure 1998 Year neutrinos were proven to have mass That speed is the same in every reference frame, a consequence of special relativity that Albert Einstein established in 1905. Massless particles also experience no proper time. From the photon's frame, emission and absorption happen simultaneously, regardless of the distance traveled. The Higgs mechanism explains why some force carriers have mass and others do not. W and Z bosons, which mediate the weak nuclear force, interact with the Higgs field and acquire masses of roughly 80 and 91 GeV respectively. Photons and gluons do not couple to the Higgs field, so they remain massless. Key Context Albert Einstein's 1905 paper on the photoelectric effect established that light behaves as discrete packets of energy, later named photons by the chemist Gilbert Lewis in 1926. James Clerk Maxwell's equations had predicted in the 1860s that electromagnetic waves travel at the speed of light, but the particle interpretation took four more decades. Gluons occupy a peculiar position among massless particles. Although the Standard Model predicts they have zero mass, gluons can never be observed in isolation. The strong force increases with distance (a property called color confinement), so pulling a gluon away from a quark generates enough energy to create new quark-gluon pairs instead. Their masslessness remains theoretical, supported by particle collision data but never directly measured. FAQ Are photons the only massless particles? Photons are the only experimentally confirmed massless particles. Gluons are predicted to be massless by the Standard Model, but because they cannot exist as free particles due to color confinement, their zero mass has not been directly measured. The hypothetical graviton would also be massless if it exists. How can massless particles carry momentum without having mass? In special relativity, momentum does not require mass. The relation E = pc shows that a massless particle's momentum equals its energy divided by the speed of light. A photon's momentum is real and measurable: solar sails use radiation pressure from photon momentum to propel spacecraft. Why were neutrinos once considered massless? The original Standard Model assumed neutrinos had zero mass because no experiment had detected neutrino mass. The 1998 Super-Kamiokande discovery of neutrino oscillations proved otherwise. Oscillation between neutrino flavors requires mass differences between neutrino types, meaning at least two of the three neutrino types must have nonzero mass. What is the difference between massless particles and dark matter? They are opposite in almost every way. Massless particles travel at the speed of light and interact through known forces like electromagnetism and the strong force. Dark matter has mass, moves at non-relativistic speeds, and interacts gravitationally but not electromagnetically. Dark matter constitutes roughly 27% of the universe's energy content; massless particles carry energy but contribute negligibly to its total mass. Related Reading Standard Model: Particles, Forces, and What It Cannot Explain Bosons: The Particles That Carry Nature's Forces The Graviton Mystery: Why Physics' Most Important Particle Remains Hidden Sources Primary Research: Are there any massless particles? (Dr. Christopher Baird, West Texas A&M University) Additional Context: Massless particles can't be stopped (Symmetry Magazine, Fermilab/SLAC) Massless Particles Traveling at the Speed of Light (University of Illinois Physics) Super-Kamiokande neutrino mass evidence (IceCube Neutrino Observatory) The Standard Model (CERN) Tormod Guldvog Editor & Duowriter Tormod Guldvog is the editor of Science Reader, covering the space where human and artificial intelligence meet. With a background in technology and science communication, he writes about how AI is changing scientific discovery — and what that means for us. More about Science Reader Cite this article Tormod Guldvog. (2026, March 23). Massless Particles: Why They Must Travel at Light Speed . Science Reader. https://sciencereader.com/glossary/massless-particles/ Fact Check: Claim-by-Claim Verification Verified All 10 claims verified against authoritative physics sources. No corrections needed. 1 Supported Photon is the only confirmed massless particle Confirmed by West Texas A&M Physics and CERN Standard Model page . 2 Supported Gluons bind quarks into protons and neutrons Standard Model physics confirmed by CERN. 3 Supported Super-Kamiokande detected neutrino oscillations in 1998 Confirmed by IceCube Neutrino Observatory historical record. 4 Supported Kajita and McDonald shared 2015 Nobel Prize in Physics Nobel Foundation records confirm. 5 Supported For massless particles E = pc Standard special relativity, confirmed by University of Illinois Physics . 6 Supported W and Z bosons have masses of roughly 80 and 91 GeV Particle Data Group: W ~80.4 GeV, Z ~91.2 GeV. 7 Supported Gilbert Lewis coined "photon" in 1926 Historical record: Lewis 1926 letter to Nature. 8 Supported Dark matter constitutes roughly 27% of the universe's energy Planck 2018 results: ~26.8% dark matter. Sources used for verification Are there any massless particles? - wtamu.edu Massless particles can't be stopped - symmetrymagazine.org Massless Particles and Speed of Light - illinois.edu Super-Kamiokande neutrino mass - icecube.wisc.edu The Standard Model - cern Glossary Term Higgs boson neutrino particle physics quantum field theory special relativity standard model physics Share Related Articles AI and computer science Science Explorer Is AI Making You Dumber? Not If You Challenge It Cognitive debt is the cost of letting AI think for you. New research shows the difference between healthy and harmful AI use comes down to one habit. April 20, 2026 Space and astronomy Science Explorer Artemis II Flew on AI, but Came Home on Engineering The Artemis II mission flew on autonomous AI systems, but the crew's survival depended on engineers solving a heat shield flaw by hand. 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