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

    Klein-Gordon for a massless particle

    So I'm trying to find a solution of the Klein-Gordon equation for a massless particle. I reached the Klein-Gordon from the total energy-momentum equation. Then for a massless particle i get to this equation: $${ \partial^2 \psi \over \partial t^2 } = c^2 \nabla^2 \psi$$How do I solve for psi? I...
  2. H

    Conservation of Angular Momentum - Two ants on a massless rod

    Homework Statement This is a problem about angular momentum and torque from the physics textbook Don't Panic Volume I. Attached is a screenshot of the problem. Homework Equations L = mr2ω The Attempt at a Solution The angular momentum must be conserved to keep ω constant...
  3. N

    IR terms from massless modes in 2D sigma model

    Hi All, My first post here. As far as I've understood there are problems with massless states in two dimensions. In the problem I am working on I have a 2D (non linear) sigma model with a lot of symmetries. The worldsheet fields come as heavy and massless. My main interest is to...
  4. D

    Weightless in Free Fall: Understanding the Physics Behind a Massless State

    we are in weightless condition because there are no resultant force between Earth and me,at this instant, Earth and me are in the opposite direction moving toward each other,will Earth oso can consider weightless? when i free falling p/s : beg pardon at 1st typing massless ,typing error ==
  5. A

    3 balls connected by massless strings

    Suppose you have 3 balls, all of equal mass, M. They are connected to each other by equal-length (each length L) strings of negligible mass, such that one ball is suspended in the middle and the two balls at the end are themselves suspended from the ceiling at two points, a distance X from one...
  6. C

    Do Massless Particles Cause Gravitation?

    I know physical bodies with mass cause gravitation. But do massless particles, like photons, also cause gravitation ? I know they're affected by gravity, but would a cloud of photons bend space by itself? Thank you
  7. B

    Analyzing Rotational Motion of Massless Rod and Object

    Homework Statement A mass-less rod of length, Ls, is rotating clockwise about a point, P1 in the horizontal plane. Attached by a cable of length, Ls, to the end of the rod and at angle θ is an object of mass, M. Starting at rest and then with the rod having and angular acceleration α, describe...
  8. M

    2 springs attached by a rigid massless bar

    i am not a student i am 52 years old , and i am production engineering i like to solve some mechanics problems when i have time these problem i do not know how to solve thank you marcelo marcelo.stuhlberger@gmail.com
  9. T

    Solving Massless Pulley: Finding Accelerations, Tension & Pulley Motion

    Homework Statement m1 and m2 are connected by a massless string wrapped around a massless pulley. An external force F is applied to the pulley. m1 does not equal m2 find the acceleration of each mass, the tension in the string, and the acceleration of the pulley. F external and m1 and...
  10. B

    Renormalization massless phi4 theory in Peskin.

    In Peskin- Schroeder, pag 412: "In massless phi4 theory, the one-loop propagator correction is completely canceled by mass counterterm." So, do massless theory provides mass counterterm? How is it generated? Maybe from a bare mass...don't have any clue. I'm confused because it seems that...
  11. A

    Are There Truly Any Massless Particles?

    My question is simple: Are there any massless My question is simple: [B]Are there any massless particles? Because I've now found out that the photon, which I always thought to be massless, actually has an extremely small amount of mass. It is something along the lines of a billionth of a...
  12. E

    Internal forces for massless rods in rotation

    Lets say we have a mass attached to a rigid, massless rod that is fixed at the other end, so we have a system that can only undergo rotation. My question is, if we apply a force to the rigid rod (and not to the actual mass), how can we derive (using only F=Ma) the shear force distribution in the...
  13. P

    No problem, I appreciate your attention to detail!

    Does massless charge exist!?? We do have numerous particles with zero electric charge and non-zero mass, but do we have particles with zero mass and non-zero electric charge?
  14. C

    Showing that gauge fields become massless and massive

    Homework Statement Consider a non-abelian gauge theory of SU(N) × SU(N) gauge fields coupled to N^{2} complex scalars in the (N,N^{_}) multiplet of the gauge group. In N × N matrix notations, the vector fields form two independent traceless hermitian matrices Bμ(x) =\Sigma_{a}...
  15. M

    Massless limit of a dynamical system

    Hello All, from considering the equations (apologies for the poor notation, primes denote differentiation) G = mv' + p (v) and G' = a - v (where v = v(t), for the interested is the crack speed for a genrelized crack dynamics model, m and a are constants, p a function) one...
  16. 1

    Why light is remains massless even if travelling w/ the fastest velocity?

    I got confused.. XD "Under Einstein's theory of relativity, an object accelerated close to the speed of light gains a tremendous amount of mass.That is why no amount of energy can suffice to accelerate any object, even an elementary particle, to the speed of light - it becomes infinitely...
  17. K

    Torque and Massless beam with Two Objects

    Torque and Massless beam with Two Objects! Homework Statement http://capa.mcgill.ca/res/mcgill/dcmcgill/oldproblems/mcgilllib/Dik/graphics/torque.jpg A massless beam is supported only at one point, called the pivot point, as shown in the diagram. A block with mass m1 sits at the left end...
  18. X

    Lagrangian Problem. Two masses on a massless circle

    Homework Statement Two equal masses are glued to a massless hoop of radius R that is free to rotate about its center in a vertical plane. The angle between the masses is 2*theta. Find the frequency of small oscillations.Homework Equations \frac{d}{dt} \frac{∂L}{∂\dot{q}}=\frac{∂L}{∂q} The...
  19. M

    How Is Tension Calculated in a Massless String System?

    Homework Statement A mass m1 = 5.8 kg rests on a frictionless table and connected by a massless string to another mass m2 = 5.2 kg. A force of magnitude F = 30 N pulls m1 to the left a distance d = 0.87 m. 2)How much work is done by the normal force on m1 and m2? Homework Equations...
  20. K

    What Makes Photons, Bosons, and Gluons Massless?

    According to Einstein's equation E=moc2, any object which has mass has energy, and, conversely, anything which has energy must also have mass. Then, why are photons, bosons and gluons said to be massless?
  21. S

    Solving for Tension with a massless cord

    Homework Statement One 2.0kg paint bucket is hanging by a massless cord from another 2.0 kg bucket, also hanging by a massless cord. a) If the buckets are at rest, what is the tension of each cord? b) If the two buckets are pulled upward with an acceleration of 1.30m/s2 by the upper cord...
  22. J

    Do Massless Particles Travel at Light Speed?

    do all mass less particles travel at the speed of light?
  23. B

    Are photons massless or practically massless

    Title says it all. Also, do photons in a vacuum travel in a straight line? I find that kind of strange. Also, are gluons massless or practically massless? How are gluons detected?
  24. K

    How to couple the angular momenta of a massive particle and a massless one

    Say, I want to couple the angular momenta of a electron spin angular momentum, and a photon's momentum. I guess in terms of representations it's: SU(2) \otimes [U(1) \oplus U(1)] = [SU(2) \otimes U(1)] \oplus [SU(2) \otimes U(1)] But I'm not at all certain if this is correct, one thing for...
  25. S

    Two blocks tied with massless rope sliding down an incline

    Homework Statement On 20 degree incline, there are two blocks tied together with a rope, sliding down. the block on the top has mass of 2.0 kg (m1) and u_k1(coefficient of kinetic friction for block 1) = .30 the block on the bottom of the incline has mass of 3.0 kg (m2) and u_k2 = .20 What is...
  26. atyy

    Force on a massless classical particle

    Can a massless classical particle experience a nonzero Newton's second law force? Dickfore produced a very interesting formula in https://www.physicsforums.com/showpost.php?p=3333233&postcount=52 . Is this generally accepted? Are there other expressions that work? Or are all acceptable...
  27. N

    Momentum of massless particle after decay

    Homework Statement I must be overlooking something very simple: "A particle of mass M decays from rest into two particles. One particle has mass m and the other particle is massless. The momentum of the massless particle is..." Homework Equations energy² = mass² c^4 + p² c² momentum is...
  28. K

    What can make a string massless?

    What makes some strings have mass, and others none? (eg. graviton vs. electron)
  29. A

    Can massless charged particles exist in nature?

    Is it possible to have a massless charged particle in nature?
  30. B

    Massless particle sends electron flying?

    massless particle sends electron flying? Hi, I'm an 11 year old boy and I was listening to a lecture on my ipod by proffesor Steven Pollock, who I have been litening to for a while and I would be really surprised if he's giving me false information. More likely I just misunderstood him. But he...
  31. J

    Photon vs. Z-boson - one massive, one massless, mix of same things?

    Apparently before we apply the Higgs mechanism we have a set of massless bosons, the photon, the B, and the W1, W2 & W3 bosons. I'm reading that the photon and Z boson that we are familiar with can be expressed as a combination of a mixture of some of these massless states: \gamma =...
  32. P

    On why massless particles move at the speed of light

    On "why massless particles move at the speed of light" It has come up a few times whether you can derive that massless particles must go the speed of light, strictly using SR. Bcrowell proposed a way that some argued against. I have a different tack for consideration. I recently derived...
  33. bcrowell

    Universality of Doppler shift for energy of any massless field

    In "Does the inertia of a body depend upon its energy content?," http://fourmilab.ch/etexts/einstein/E_mc2/www/ , Einstein invokes a result from his 1905 SR paper, which is that the Doppler shift of a light wave's frequency D(v)=\sqrt{(1-v)/(1+v)} is also the factor by which its energy changes...
  34. tom.stoer

    Two spin d.o.f. for massless gauge bosons

    It is well know that massless spin-1 gauge bosons have two spin states s³=+1 and s³=-1. There are two independent approaches how this can be shown: 1) via the representations of the Lorentz group for p²=0 2) via fixing / eliminating unphysical gauge d.o.f., e.g. via elimination of the...
  35. O

    Understanding de Broglie formula for massless particles

    I have taken down in my notes that for massless particles the formula by DeBrogle becomes E = pc, where p is momentum and c is the speed of light. But what I don't understand is how you can calculate momentum without mass? I thought momentum was mass times velocity? The specific example I...
  36. I

    Block collides with massless spring

    1. A 2 kg block collides with a massless spring of spring constant 109 N/m attached to a wall. The speed of the block was observed to be 1.5 m/s at the moment of collision. The acceleration of gravity is 9.8 m/s2 How far does the spring compress if the sur- face on which the mass moves is...
  37. L

    Why do massless particles have only two degrees of freedom?

    Why do massless particles have only two degrees of freedom, i.e. the two helicity states. Why do massless spin-j particles do not have 2j+1 degrees of freedom like the massive particles? thanks
  38. Z

    Can someone please explain to me why massless particles follow null geodesics?

    Anything that is massless must follow null geodesics. Why is this?
  39. Z

    Relativistic energy/momentum, massless particles

    Homework Statement A pion at rest decays into a muon and an antineutrino. The mass of the antineutrino is zero, find the energies and momenta of the muon and antineutrino. Mass of the pion is 139.57 MeV/c^2 and the mass of the muon is 105.66 MeV/c^2 Homework Equations pion -> muon +...
  40. R

    Photon in a massless mirrored box

    Suppose we have a photon bouncing around in a massless mirrored box - does this system have inertial and/or gravitational mass? To be more realistic, if we have a massive black body cavity at some temperature above absolute zero, will the inertial and/or gravitational mass be equal to the mass...
  41. T

    Massless Dirac equation and graphene

    I am reading about the electron flow in graphene and the article said this "This behavior is not described by the traditional mathematics (Schrodinger equation) but by the mass-less Dirac equation" What does this mean and what is the massless Dirac equation... the whole paragraph is...
  42. W

    Vertical massless spring

    Homework Statement A 4.50 kg weight hangs from a vertical massless ideal spring. When set in vertical motion, the weight obeys the equation y(t)=(8.50 cm)sin[(3.85 rad/s)t-1.40]. What is the maximum speed of the weight? What is the spring constant? Homework Equations Hooke's law: F = -kx...
  43. F

    Are Proto-Photons the Only Matter at the Outer Limits of the Universe?

    Photons were trapped within the early stages of the expanding Universe. When the early universe expanded to a critical size (or cooled to a critcal temperature) photons escaped their dense enclosure. Today, these proto-photons must be still moving and located at the furthest limits of the...
  44. S

    What does it mean when a problem says massless pulley?

    What does it mean when a problem says "massless pulley?" I am just curios about what it means, does it mean that the pulley doesn't bear any friction that works against a whole pulley system?
  45. bcrowell

    Massless Klein-Gordon equation not conformally invariant?

    massless Klein-Gordon equation not conformally invariant?? Wald discusses conformal transformations in appendix D. He shows that the source-free Maxwell's equations in four dimensions are conformally invariant, and this makes sense to me, since with photons all you can do is measure the...
  46. I

    Massless pulley and acceleration

    Homework Statement In the following situation (see attachment) both pulleys and the string have zero mass. If m_{2} > m_{1} we would expect a net force and therefore an acceleration in the direction of the net force. But if pulley 2, which is moveable, has no mass the sum of the forces...
  47. A

    Acceleration of masses connected with string, non massless pulley

    Homework Statement Homework Equations \SigmaF = ma \Sigma\tau=I\alpha ? The Attempt at a Solution m1 = 1 Kg m2 = 8 Kg \theta = 33 degrees mpulley = 7 Kg rpulley = .11 m \mu = 0.27 g = 9.8 m/s2 My attempt, I'm falling behind in class due to an illness and I can figure this...
  48. T

    Ineracting massless particles

    Are there any reactions in that require the interaction of massless particles or am I right in thinking this is impossible? Thanks
  49. C

    BRS: Static Axisymmetric Gravitationless Massless Scalar Field Solutions

    BRS: Static Axisymmetric "Gravitationless" Massless Scalar Field Solutions This thread is an (easy and amusing) companion to a previous BRS, "The Weyl Vacuums" www.physicsforums.com/showthread.php?t=378662 I. The Family of "Gravitationless" Solutions I will describe a family of...
  50. G

    Electric Field and a Charged Cork Ball on a Massless String

    Homework Statement A charged cork ball of mass 1g is suspended on a light string in the presence of a uniform electric field. When E = (3i + 5j) * 10^5 N/C, the ball is in equilibrium at \theta = 37 degrees. Find the charge on the ball and the tension on the string. ==================== |\...
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