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. L

    Does a massless string cause a normal force when attached to a wall?

    I'm almost certain that if it was a "steel rod" or something heavy like that, the normal (the force written in green) would exist. But does it exist for a "massless ideal string"? I mean, there is tension in the string of course, but would that cause the normal perpendicular to the wall?
  2. L

    I Are photons definitely massless?

    Recently saw this video. "Why No One Knows If Photons Really Are Massless: What if they Aren't?" Arvin Ash He says photons need not be massless, but they must be quite light nonetheless. He separates speed of light from speed of causality. Is it true that we can't know its mass below a...
  3. S

    I Propagator of massless Weyl field

    I have this Lagrangian for a free massless left Weyl spinor, so it’s just the kinetic term, that can be written embedding the field into a larger Dirac spinor and then taking the left projector in this way: $$i \bar{\psi} \cancel{\partial} P_L \psi$$ Srednicki says that the momentum space...
  4. H

    Sign hanging from massless beam attached to hinge supported by string

    Attempted creating equations for zeros of torque and components of forces in x and y as seen in picture. Got lost with having only variables and the d & 2L for the length of the beam. Not sure how to do the question with two points of contact between the beam and the sign. Is the center center...
  5. B

    Two Particles Connected by Massless Rod: Dynamics Analysis

    Two point-like particles of mass m. The particles are rigidly connected to each other with a mass-less rod of length L. The particles are initially at rest in such a way that one particle is at the origin and the other is at the point (0, L). A point-like particle of mass M collides with a...
  6. paulimerci

    Net torque about an axis through point A in a massless rod

    The net torque about an axis through point A is given by, If I take the axis of rotation perpendicular to the paper and the solution I arrive would be the following below Net torque = 30 cos45 x 1.5 - 10 cos30X 3 = 5.829Nm ( counterclockwise) But the book gives an answer...
  7. A

    Inertia from four masses connected by massless rigid rods

    I found xcm = 6.2cm, ycm = 4.9cm, but am unsure of how to tackle the inertia part of the problem.
  8. kasnay

    Hanging mass on a massless pulley

    I have a conceptual question about this problem. I can write the 3.5 kg block equation as Fnet(block 1)=(Force of tension)-(Force of friction)=m1a I can write the 2.8 kg block as Fnet(block 2)=(Force of tension)-(Force of gravity2)=m2a My question is this If I set the forces of...
  9. tomdodd4598

    I Equations of Motion for Massless Particle in Potential

    The Lagrangian for a massless particle in a potential, using the ##(-,+,+,+)## metric signature, is $$L = \frac{\dot{x}_\mu \dot{x}^\mu}{2e} - V,$$ where ##\dot{x}^\mu := \frac{dx^\mu}{d\lambda}## is the velocity, ##\lambda## is some worldline parameter, ##e## is the auxiliary einbein and...
  10. B

    B (ELI5) why can’t massless particles travel infinitely fast?

    I feel like if something is massless it should be able to travel infinitely fast with any amount of energy. When you have something with mass, you would need an infinite amount of energy to push it infinitely fast, but if the thing you’re pushing is massless, you should be able to push it with a...
  11. spacecadet11

    I General physics question -- How can massless photons have momentum?

    P=mv *momentum equals mass X velocity. Light particles or "photons" are said to be "massless". And yet they have momentum. How is that possible? (p.s. I used to know the answer)
  12. Lunct

    B Gravitational Force acting on a massless body

    It's a well known fact that acceleration due to gravity is independent of the mass of the accelerating body, and only depends on the mass of the body it is accelerating towards and the distance from it. One can prove this mathematically very easily. F=GMm/r^2 (equation 1) but also F=ma...
  13. P

    A Klein paradox in the massless case

    I have a question about the Klein paradox in the massless case, for a potential step of height ##V_0## (this is exactly the situation described by Wikipedia). I don't have a problem to understand the "paradox", and I think the Wikipedia's illustration is quite telling. My question is : what...
  14. R

    Boundary conditions for 2 ropes fixed to a massless ring with a damper

    Hi, I'm not quite sure if I'm correct. I need to find the boundary conditions for 2 ropes ##T_1 \mu_1, T_2 \mu_2## fixed at ##x=0## to a massless ring with a massless damper of force ##F_d - -bv_y## Here what I think, since the ring and the damper is massless ##\sum F_y = 0##. Thus, ##-T_1...
  15. P

    A Massless electrons stable if the Universe approaches a de Sitter Universe?

    Hi, All cosmological models with a non-zero cosmological constant will approach a de Sitter universe in the far future. In theory this can means that the most basic group of particle physics should be the de Sitter rather than Poincaré. Mass is a Casmir operator of the Poincare but not of the...
  16. DaniV

    The 1-loop anomalous dimension of massless quark field

    I tried as first step to find Z_q the renormalization parameter, to do so I did the same procedure to find the renormalization parameter of the gauge field of the gluon A^a_\mu when a is representation index a \in {1,2,...,N^2-1} such that A^{a{(R)}}_{\mu}=\frac{1}{\sqrt{Z_A}}A^{a}_{\mu}...
  17. ergospherical

    A Massless Particle Action under Conformal Killing Vector Transformation

    For a massless particle let\begin{align*} S[x,e] = \dfrac{1}{2} \int d\lambda e^{-1} \dot{x}^{\mu} \dot{x}^{\nu} g_{\mu \nu}(x) \end{align*}Let ##\xi## be a conformal Killing vector of ##ds^2##, then under a transformation ##x^{\mu} \rightarrow x^{\mu} + \alpha \xi^{\mu}## and ##e \rightarrow e...
  18. U

    I Why is Scalar Massless Wave Equation Conformally Invariant?

    It can be shown mathematically that the scalar massless wave equation is conformally invariant. However, doing so is rather tedious and muted in terms of physical understanding. As such, is there a physically intuitive explanation as to why the scalar massless wave equation is conformally invariant?
  19. lomidrevo

    I Polarization mode symmetries of massless particles

    I am just reading Carroll's textbook on GR, where at the end of chapter 7.4 Gravitational Wave Solutions he discuss how rotational symmetries in polarization modes are related to spin of massless particles. He then explains that we could expect associated spin-2 particles to gravity - gravitons...
  20. John100861

    David Morin classical mechanics Problem 2.6: Disk held up by a massless string

    The first part is easy, we have 2T= Mg T= 0.5 Mg Now for the second part where I'm having trouble understanding Morin's solution: I take the normal force on a small circle arc to be N, we know that the y component of the normal force must be balance with Mg for the whole disk, therefore Ny =...
  21. T

    Spring: does it have mass or is it massless?

    I can easily do the second problem if only I knew the answer to the first. I am just not sure how I would go about figuring out if the spring has mass or not. And if it does, how would I calculate that mass?
  22. Ranku

    I Energy of massless particles before electroweak symmetry breaking

    Would it be correct to represent the energy of massless particles before electroweak symmetry breaking as ##E = cp##, just as we do with photons post-symmetry breaking?
  23. Ranku

    I Energy momentum of massless particles

    Before electroweak symmetry breaking, there were massless particles. Can these massless particles be seen in terms of energy momentum relation ##E = ##c##p##?
  24. UnPetitGarcon

    The acceleration of a massless pulley in a double Atwood machine

    So there is a textbook physics question in which it asks us to calculate the acceleration of pulley B(which is massless). This exact question was posted and asked previously in this thread. However, it didn't discuss my doubt. To be exact, the question I have troubles with is (b)...
  25. Shivam

    A smooth massless wedge is pushed by a horizontal Force P....

    Answers- 1,3,4 My attempt, the wedge being massless, there shoul not be any force acting on as it will then have infinite acceleration, so by that i really can't think of how force is applied on pully.
  26. D

    How does a massless photon impart force?

    I've been watching a lot of physics videos lately and a couple of said that photons are massless. What I don't understand is how a massless photon can impart force? Like the ideas of having a laser propel a deep space probe.
  27. C

    B Are massless particles truly massless?

    Most of the mass of matter comes from energy of strong force interactions between quarks. However the quarks still have intrinsic mass. Other particles have no intrinsic mass but still have energy. So according to mass-energy equivalence, these particles should still have effectively mass, to my...
  28. W

    I Is this proof regarding massless strings correct?

    I am reading the Tipler and Mosca textbook and am on the part about massless strings. I understand that in real life a string has more tension at the top than the bottom because the top part has to support a greater mass of rope. However, in other examples such as pulling a sled with a rope I...
  29. A

    I Alternative approach to analyzing a massless string

    Consider a massless string which can rotate about a fixed pulley (first picture). The coefficient of static friction is μ. Assuming that the motion is impending, the goal is to find the equation that describes the variation in tension of the string. ( T2/T1 = eμΦ where Φ is the subtended angle.)...
  30. L

    Two pendulums connected with a massless rope

    Homework Statement Two pendulums of same mass and length that oscillate in same horizontal plane are connected with maseless horizontal rope. What is dependence of amplitude of pendulums as a function of time? Homework Equations For harmonic oscilation x=x_0\sin(\omega t+\varphi_0) Kinetic...
  31. J

    Massless theories can be conformally invariant

    Homework Statement The exercise needs us to first show that ##P^2## (with ##P_\mu=i\partial_\mu##) is not a Casimir invariant of the Conformal group. From this, it wants us to deduce that only massless theories could be conformally invariant. Homework Equations The Attempt at a Solution I...
  32. M

    How Black Holes Absorb Light Despite Photons Being Massless

    We know light made up of photons which is massless, but why it can absorbed by black hole? Is it becuz the Einstein's relativity about every object can curve time space
  33. S

    I Massless Electron Compton Scattering: Energy of Outgoing Photon

    Hello! I found this problem where we are asked what happens to the energy of the outgoing photon in a Compton interaction, if the mass of the electron goes to zero and what is the physical intuition of it. So the formula is this: $$\lambda - \lambda_0 = \frac{h}{m_0 c}(1-cos \theta)$$ So when...
  34. AlvisPrabhu

    How are Photons massless? How can a particle have no Mass?

    I did look it up , but couldn't find anything that explains this in a simple manner.
  35. H

    A Question about coefficients of massless quantum fields

    From Chapter 5.9 Weinberg's QFT Vol 1, massless fields are defined as: \psi_l(x)=(2\pi)^{-3/2}\int d^{3}p\sum_{\sigma}[k a(p,\sigma)u_l(p,\sigma)e^{ipx}+\lambda a^{c\dagger}(p,\sigma)v_l(p,\sigma)e^{-ipx}] With coefficients defined by the conditions: u_{\bar{l}}(p,\sigma) =\sqrt{|k|/p^0}...
  36. Suyash Singh

    What is the formula for calculating force using a spring balance?

    Homework Statement Homework Equations only basic school level physics will be needed in this question. The Attempt at a Solution The mass m1 pulls spring from first side and m2 pulls spring from the other side therefore reading should be m1-m2. But my answer is wrong. I can't understand what...
  37. Johns24

    Normal force of a block with a massless rope

    Homework Statement Mass of block: 19kg Applied force: 225N at 16 degrees to accelerate block from rest Coefficient of static: 0.55 Coefficient of kinetic: 0.30Homework Equations Unknown The Attempt at a Solution I have solved for parallel tension and perpendicular tension Tperp =...
  38. W

    A Why is a glueball not massless, if it's a Goldstone boson?

    Scalar glueballs in QCD appear as a result of violation of global conformal (scale) symmetry - the energy-momentum tensor has a nonzero trace. According to the Goldstone theorem, this (violation of global symmetry) corresponds to the appearance of scalar massless bosons. Why, then, are the...
  39. pellman

    I 4-momentum of a massless particle

    The 4-momentum of a massless particle traveling in the z direction is (k, 0, 0, k). What is the significance of the value of k? It does not determine the speed since they always travel at light speed. If one particle has momentum (k, 0, 0, k) and another has (j, 0, 0, j) with j not equal to...
  40. binbagsss

    QFT T-duality, Massless vector states

    Homework Statement Part C) Please: Homework Equations above,below The Attempt at a Solution so I think I understand the background of these expressions well enough, very briefly, changing the manifold from ## R^n ## to a cylindrical one- ##R^{(n-1)}^{+1}## we need to cater for winding...
  41. N

    I Magnetic moment of a massless charged Fermion?

    The spin magnetic moment of a charged, spin-1/2 particle is $$g \frac e {2m} \frac \hbar 2$$ where g is the g-factor (2 for any particle in tree-level approximation, 2.00231930436182 for electron), e is charge m is mass ##\frac \hbar 2## is spin But with zero mass this expression does not make...
  42. K

    I Exploring Mass & Constrained Massless Particles

    Poorly phrased but here goes - I'm trying to understand some of the SpaceTime videos on youtube, specifically the massless mirrored box and how the mass (ie resistance to acceleration) is a function of the change in momentum of the contained (constrained) photons. It makes sense but raises a...
  43. S

    A 'Massless' Electrons in Graphene

    So I've read that electrons traveling inside a sheet of graphene are said to travel "masslessly". I'm interpreting this as meaning "zero apparent mass" and not zero actual mass. Presumably, the graphene doesn't somehow weigh less than the sum of its constituent electrons and nuclei. But given...
  44. H

    Frictionless Disk Supported by Massless String

    In the following diagram, a frictionless disc is supported by a massless string. This problem was given by the author of a book, and a solution was given to some questions that were asked about this diagram. One thing the author said in one of the solutions, was that the tension in the string...
  45. ohwilleke

    I Geometry of GR v. Spin-2 Massless Graviton Interpretation

    In classical general relativity, gravity is simply a curvature of space-time. But, a quantum field theory for a massless spin-2 graviton has as its classical limit, general relativity. My question is about the topology of space-time in the hypothetical quantum field theory of a massless spin-2...
  46. J

    String attached to a massless ring in both ends

    First, sorry if there's grammar mistakes,english is not my native language. 1. Homework Statement Find the normal modes of a string of length L with a massles ring ,free to move on the y-axis ,attached to each end. Homework Equations General wave solution: u(x,t)=A·e(kx-wt)i+B·e(-kx-wt)i...
  47. shubham_ju

    Will a Massless Pulley with Friction Rotate with Unequal Masses on Either Side?

    a massless pulley with friction between its surface and the string passing over it .If string is having two unequal masses connected to it on both side of pulley then will the pulley rotate?
  48. Chris Miller

    B Gravitational Force and the Endless Supply of Massless Particles

    If gravitational force is caused by a particle (tensor boson) which is massless and so travels at c, why doesn't matter ever exhaust, or even seem to reduce, its supply of these particles?