What is Massless: Definition and 293 Discussions

In particle physics, a massless particle is an elementary particle whose invariant mass is zero. The two known massless particles are both gauge bosons: the photon (carrier of electromagnetism) and the gluon (carrier of the strong force). However, gluons are never observed as free particles, since they are confined within hadrons. Neutrinos were originally thought to be massless. However, because neutrinos change flavor as they travel, at least two of the types of neutrinos must have mass. The discovery of this phenomenon, known as neutrino oscillation, led to Canadian scientist Arthur B. McDonald and Japanese scientist Takaaki Kajita sharing the 2015 Nobel prize in physics.

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  1. J

    I Is the Massless Photon Explanation for E=mc2 Valid?

    I have wondered how a photon could be massless, given E=mc2. I have seen explanations involving treatment of the situation through consideration of momentum. It seems to me my own explanation is valid, and I ask for comments. Actually E= m’c2 =gmc2, where g=1/sq rt{(1/[1-(v2/c2)]}. Photons...
  2. B

    I Question about massless things

    Are there any other things/objects in our universe that are massless other than Gluons and Photons? I guess energy is massless, but it's not so much a thing, more-so a property.
  3. FallenApple

    B What is the Mass of Photons According to QFT?

    So according to QFT, fundamental particles are just excitations in their underlying fields. So the photon would correspond to an oscillation of the EM field. But it takes energy to excite this field. And E=mc^2. So therefore the photon should have some mass. Energy was put into create it. That...
  4. P

    Can massless particles reach the boundary at z=0?

    Homework Statement We're working in 2-d Anti-de Sitter space with metric: \begin{eqnarray*}ds^2 = \frac{1}{z^2}(-dt^2 + dz^2)\end{eqnarray*} with 0<=z. The solution is: \begin{eqnarray*}z^2 = (t+c)^2 + B\end{eqnarray*} And we've been asked to plot this (I think its a parabola with minima at...
  5. P

    Gauge symmetry for massless Klein-Gordon field

    Homework Statement I need to gauge the symmetry: \phi \rightarrow \phi + a(x) for the Lagrangian: L=\partial_\mu\phi\partial^\mu\phi Homework EquationsThe Attempt at a Solution We did this in class for the Dirac equation with a phase transformation and I understood the method, but...
  6. S

    A Transverse polarizations of a massless spin 1 particle

    Physical polarization vectors are transverse, ##p\cdot{\epsilon}=0##, where ##p## is the momentum of a photon and ##\epsilon## is a polarization vector. Physical polarization vectors are unchanged under a gauge transformation ##\epsilon + a\cdot{p}=\epsilon##, where ##a## is some arbitrary...
  7. J

    Moment of Inertia of Massless Pulley w/String & Mass m Attached

    What is the moment of inertia of massless (ideal) pulley that has a string attached to its center and a mass m attached to that string? If this pulley were hanging from a massless rope that was attached on one side to the cieling and the other side had a tension of mg, what would be the...
  8. K

    B Since gluons are massless, shouldn't glueballs also be massl

    since gluons are massless, shouldn't glueballs also be massless? is there any possibility gluons acquire mas by interacting with other gluons, not higgs?
  9. GIM

    I QCD scale and massless limit of u & d quarks

    Hello! Could anybody help me? My wondering seems so trivial, but I can't skip it. They say that since u and d quarks are much lighter than QCD scale(~200MeV), in reality we can consider the QCD Lagrangian has an approximate global chiral symmetry with respect to these two flavors. At first, it...
  10. Ravi Singh choudhary

    What will be stress in the unbalanced force situation

    < Mentor Note -- thread moved to HH from the technical physics forums, so no HH Template is shown > Problem Statement: Case 1: Mass is m; Cross sectional area is uniform A. What will be the stress in the middle section. Case 2: Bar is massless; What will be the stress in the middle section...
  11. dschwie

    Gravity and massless particles

    Just finished reading Sean Carroll's "The Higgs Boson and Beyond". I would be grateful if someone could explain how gravity, which I understand to be a function of mass, can interact with massless particles as evidenced by the phenomenon of gravitational lensing. I understand that gravity is a...
  12. Jojo96

    Determine is box of unknown mass is sliding down a ramp?

    Homework Statement A box of textbooks at unknown mass rests on a loading ramp that makes an angle α = 16o with the horizontal. The coefficient of static friction is 0.30, and kinetic friction is 0.25. Then the box is released. (a) Will it start sliding down the ramp? (a) Find the normal force...
  13. A

    Conserved quantities for massless particles (Schwarzschild)

    If a metric admits a Killing vector field ##V ## it is possible to define conserved quantities: ## V^{\mu} u_{\mu}=const## where ## u^{\mu}## is the 4 velocity of a particle. For example, Schwarzschild metric admits a timelike Killing vector field. This means that the quantity ##g_{\mu 0}...
  14. A

    Newton's 2nd law for Massless Particles in SR

    I think I have never seen a force equation for massless particles, I wonder if such thing exist or if not why not?
  15. xpell

    Are all massless particles "born" at light speed?

    (I think) I know that massless particles can only exist traveling at c, but I find it somehow counter-intuitive (like many other real things... :D ) Would anyone please be so kind to confirm that, for instance, a gamma photon generated by the radioactive decay of a stationary isotope is already...
  16. ChickenTarm

    How do I find the normal modes of massless string w/ masses?

    Homework Statement So, a string with length L and a mass of M is given tension T. Find the frequencies of the smallest three modes of transverse motion. Then compare with a massless string with the same tension and length, but there are 3 masses of M/3 equally spaced. So this is problem #1...
  17. Neon

    Why some particles have no mass even similar sized ones?

    Photon have the same size as an electron(i think) but no mass. Gluons also. WHY
  18. S

    Massless fermions Klein tunneling

    QUOTE: "In 1929, physicist Oskar Klein[1] obtained a surprising result by applying the Dirac equation to the familiar problem of electron scattering from a potential barrier. In nonrelativistic quantum mechanics, electron tunneling into a barrier is observed, with exponential damping. However...
  19. B

    The double Atwood machine has frictionless, massless pulleys

    Homework Statement The double Atwood machine has frictionless, massless pulleys and cords. Determine (a) the acceleration of masses ma, mb, and mc, and (b) the tensions Fta and Ftc in the cords. Homework Equations F=ma The Attempt at a Solution So I drew free body diagrams for the mass A...
  20. C

    Cuts of a Feynman diagram and the massless limit

    Consider a ##j## point all massive leg one loop polygonal Feynman diagram ##P## representing some scattering process cut on a particular mass channel ##s_i##. Invoking the relevant Feynman rules and proceeding with the integration via dimensional regularisation for example gives me an expression...
  21. Madpoet626

    Faster Than Light: Nullifying Mass with E=MC2

    The main obstacle to FTL is that mass increases with velocity and in the book of my imagination there is a blank spot on how to nullify the mass of a star ship. I can't help but think the answer is hidden somewhere in Albert's equation E=MC2.
  22. Swetha.M.L

    Is Higgs boson massless or does it gain mass when interacting with water?

    Is higgs boson massless? Photon is massless , is photon get mass when interract with water?
  23. K

    Is spacetime a massless spin 2 field?

    since a massless spin 2 field when perturbatively quantized gives rise to gravitons, which couple to everything and is identical to gravitation, is spacetime itself massless spin 2 field? do virtual graviton exchange also modify time and space?
  24. O

    2 blocks connected by massless string VS. pulley system

    (Note: The title would better describe the subject if written like this: "2 blocks on a horizontal frictionless surface connected by a massless string vs. pulley system")Hello, I have a problem regarding pulling two blocks of different masses connected by a string horizontally with opposing...
  25. LarryS

    Are Neutrinos Tachyons? The Mystery of Massless Particles Explained

    Are massless particles always their own anti-particle? Thanks in advance.
  26. S

    Relativity: Massless particles and photons

    I f a particle starts moving with the velocity of light it becomes massless.But practically a massless particle has no existence.Again one of the main constituents of light is photon,then is it a massless particle?But I know electron has the least mass in this world and photon is heavier than...
  27. Swetha.M.L

    Massless Particle Questions: Explaining Photon's Lack of Mass

    may i ask you something? if there is any wrong excuse me. according to mass-energy equation mass &energy are not different but two forms of the same. photon ,graviton... are the mass less particles but photon is a form of energy. can you explain why photon is massless?
  28. sweet springs

    Four-velocity of massless particle

    is defined or not? Thanks.
  29. Spinnor

    Give mass to a massless scalar field in 1+1, Higgs like?

    Is it possible to have a free massless scalar field in 1+1 spacetime and then add another field of the right type which interacts with some adjustable strength with the massless field to give mass to the massless field? Is there a Higgs-like mechanism in 1+1 spacetime? Thanks for any help!
  30. T

    Why massless rope is required for making these two forces eq

    I am pulling a rope and that rope is pulling another body which is attached to rope. The force applied by me on the rope will be equal to the force applied by rope on that body if 1. The rope can't increase in length due to force, or 2. if the rope is massless. I understand the reason behind...
  31. Islam Hassan

    Speed of Light is a Property of Massless Particles or Space?

    The speed of light is a parameter that attaches itself to what exactly, an inertial frame of reference or a massless particle moving therein?IH
  32. andyrk

    Massless string pulled by a force

    If we take one end of a mass-less string and pull it with force F, would the string have any tension in it? Would it have any tension when we pull it with force F from both the ends?
  33. D

    Why is photon massless particle?

    i'm still wondering, i know a bit of quantum mechanics and general relativity. well... very very little, if photon is a particle, it must be "something" right? i mean, something that exist in this space and time. why is "something" that "exist" don't have a mass? what is it consist of? or...
  34. Spinnor

    Can massless states be used to build massive ones?

    I think I have read that massless fermionic states can be used (as a basis?) to build up massive fermionic states. Is that true? If so, could you please give me a very short outline of how this is done, or maybe better, a mathematical statement that "says" the same thing? Thanks for any help!
  35. B

    Acceleration of massless pulley and blocks

    Homework Statement Consider a situation I'm curious why is that the massless pulley's acceleration is just the avarage of acceleration of m1 and that of m2? ab=(a1+a2)/2 Homework Equations Forces acting on pulley are Tc-2Ta=0 (because pulley is massless, force can't give it acceleration)...
  36. G

    Massless Particles: Force Bearing & Propagation Speeds

    It looks to me like all the massless particles so far discovered may be force bearing particles, there being 3 of them. Am I right? If so can it be said that forces propagate at the speed of light whilst what we might think of a tangible particles cannot? I did post this question on another...
  37. Spinnor

    Massless complex field+assumption(s)=massive field. Doable?

    Suppose we have a massless complex field in 3+1 spacetime where E^2 = P^2. Suppose that the only excitations that are possible are those that in some rest frame consist of an excitation of a pair of states p1 and p2 such that p1 = -p2 and ιp1ι = ιp2ι = mc^2 = (+or-)E, and the pair of states p1...
  38. Symmetry777

    Can a Massless Photon create Mass?

    Can a Massless Photon create Mass? Is Electrostatic Induction a form of “Mass Transfer through the process of Absorption”? A new star is created. The star generates massive Magnetic Fields. What happens to the hydrogen particles, Nebula, when they are influenced by the Magnetic Field? If the...
  39. Bendelson

    Massless photons? Is everything light?

    Are photons massless? They have energy and can be affected by gravity, so don't they have mass? I've heard plenty of people say that photons are massless, does this just mean that they have negligible mass? Also, if only light/photons can move at the speed of light, then wouldn't everything be...
  40. S

    Lagrange mechanics: Pendulum attached to a massless support

    Homework Statement A simple pendulum of length ##b## and bob with mass ##m## is attached to a massless support moving vertically upward with constant acceleration ##a##. Determine (a) the equations of motion and (b) the period for small oscillations. 2. Formulas ##U = mgh## ##T = (1/2)mv^2...
  41. jerromyjon

    Does Relativity Support the Masslessness of Photons?

    From what I understand Einstein was the person who decided length contraction would indeed occur in the vector of velocity, more so as you approach c. If it were possible to accelerate a particle to c, would the length would become 0? If the length were to become zero, would the notion of mass...
  42. K

    Ward Identity in Schwartz's QFT Book: Massless Photon Assumption or Not?

    I was reading Schwartz's qft book. I saw the proof of ward identity taking pair annihilation as an example. he claimed he didn't assume that photon is massless in this derivation. but i have confusion with this statement. gauge invariance is a fact related to massless particles. now he has...
  43. M

    Equation of motion for a massless spring system

    Homework Statement Homework Equations f_spring = k(x_near - x_far) The Attempt at a Solution a. FBD: The force goes through the nodes, and the sum of the forces must be 0 because the nodes are massless. Therefore, kx_1 = x(t) So x_1(t) = x(t)/k b. FBD: For this system, the parallel...
  44. G

    Vibration Massless spring static equilibrium

    Let us imagine that there is a Massless spring with fixed at one end. Unloaded length = L Spring constant: = k Mass = 0The spring is at static Equilibrium Force: F Displacement: d F = k d, elongated length: L+d If I remove F, what will happen ? 1. Spring return to...
  45. C

    How to describe the spin state of a massless particle?

    For a massive particle, say, an electron, we can use a spin vector ##\vec{S}## to denote its spin state, and the corresponding covariant four vector ##S^\mu##. And we have ##P \cdot S = 0## in the particle rest frame, so it holds in any frame for it is an Lorentz scalor. But, what if the...
  46. LarryS

    Spin Direction for Massless Particles

    Mass-less particles travel at the speed of light. If such a particle has an intrinsic spin, the DIRECTION of the spin angular momentum vector must be in the same direction as the linear motion of the particle. Why exactly is that? Thanks in advance.
  47. J

    What-If: Massless, Charged Particle

    Please note that I neither hypothesize the existence of such particle, nor know any evidence of it. This is purely a what-if scenario. So, my questions are: 1)Is there any law or whatsoever that forbid the existence of a massless particle that possesses charge? 2)If such particle exist, how...
  48. J

    Differentiation between massless Bosons

    Three Bosons: the Photon, the Gluon, and the Graviton are all massless Bosons; they all travel at the speed of light and therefore have no interaction with the Higgs field. How is it possible to differentiate between these three particles being that the Photon is a Majorana Fermion and therefore...
  49. LarryS

    Massless Fermions: Physics & Existence

    Does modern physics allow for the existence of massless fermions?
  50. G

    Ignoring SSB in massless phi^4 theory

    If a theory is spontaneously broken, you usually make calculations by expanding the field about the ground-state expectation value \nu , so that you define a new field \rho such that \phi=\nu+\rho such that \langle \rho \rangle =0 . What if you just calculate perturbation theory with \phi...
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