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

    Do Massless Objects Travel at the Speed of Light Relative to Each Other?

    Objects with mass always travel less than c with respect to other objects with mass. Objects without mass always travel at c with respect to objects with mass. But what speed do objects without mass travel with respect to each other? Do two massless photons travel at c with respect to the...
  2. T

    Helicity is different form Spin for massless particle(photon)?

    As we know photon's helicity are \pm1. Helicity is the projection of the spin S onto the direction of momentum, p, which is considered as Sz. What about Sx and Sy? They are both ZERO?
  3. K

    Massless particles that can generate forces?

    Hello, I am a game developer working on a science fiction video game. The game is intended to be vaguely scientifically plausible and I'd love some advice from someone with a greater knowledge of physics than me! In the game, you have a futuristic device that creates cubes of "energy"...
  4. KiNGGeexD

    Two point charges connected by a massless rope

    Question: Two positive charges q(a) and q(b) and masses m(a) and m(b) are at rest, held together by a massless string of length d. Now the string is cut, and the particles fly off in opposite directions. How fast are each going when they are far apart.My attempt: From this the first thing I...
  5. J

    Massless 3D Density of States

    Homework Statement Calculate the single particle density of states for massless particles with dispersion E=h_bar ck for a 3D cube of volume V Homework Equations E=pc, p=E/c, dp=dE/c, d^3p = 4pi*p^2 dp k=sqrt(k_x^2+k_y^2+k_z^2) k_j = 2pi/L l_j (j=x,y,z) The Attempt at a Solution I...
  6. D

    Massless Particles and the Universal Speed Limit

    I was hoping someone could help me with a simple physics question I've been mulling over, I've failed to find an answer using online queries, I suspect my line of inquiry is flawed in some way. I would be grateful if someone could confirm if I've understood this correctly: Particles without...
  7. L

    Why massless particle can only have two helicity states?

    why massless particle, such as photon, can only have two helicity states? Photon's helicity is 1,-1. Helicity zero is forbidden. why?
  8. S

    Why is a massless spin-2 automatically a graviton

    The Wikipedia page on the graviton http://en.wikipedia.org/wiki/Graviton contains the following sentence: "Additionally, it can be shown that any massless spin-2 field would give rise to a force indistinguishable from gravitation, because a massless spin-2 field must couple to (interact...
  9. N

    Circular motion and energy of a massless spring

    Homework Statement A massless spring of constant k is fixed on the left side of a level track. A block of mass m is pressed against the spring and compresses it a distance d, as shown in the figure. The block (initially at rest) is then released and travels toward a circular...
  10. vanhees71

    Massless charged particle in electrostatic field

    Does anyone here know a source, where the equation of motion for a massless charged particle (which of course does not exist in Nature as far as we know) in a homogeneous electric field is actually solved? I googled, and there are some papers about this, but the equations are never actually...
  11. C

    Time Dilation: Current vs Massless Universe

    Disclaimer: I have no formal education in physics. I watched a TedTalk http://www.tedxamsterdam.com/video-wubbo-ockels-on-time-and-gravity/ and it got me thinking. Dangerous, I know. As I understand both velocity and gravity have an effect on our perception of time. How much does our...
  12. M

    Quantum corrections to massless fermionic field

    Hello, in QED the corrections to electron propagator change the bare electron mass from m_0 to m=m_0+δm=m_0+∑({\not}p=m) (Peskin, formula 7.27). This is the consequence of the result, that the propagator changes from i/({\not}p-m_0) to i/({\not}p-m_0-∑({\not}p))). This part is written very...
  13. C

    Pinned massless stick with a concentrated mass at the other end.

    Homework Statement A massless stick is pinned at point A. And it has a concentrated ball mass of 1kg attached to it at point B. Given the initial values: Initial angular position θ=45◦ Initial angular velocity ω=0 Initial angular acceleration α=0 Show that because of the setup of the...
  14. Superposed_Cat

    Why are heilicty and chirality equivalent for massless particles?

    Salutations, question: ~wikipedia. I'm assuming that's because (correct me if I'm wrong) those particles would travel at c and you could not overtake them so their chirality and helicity are equal.But if a photon was coming towards you then zoomed past you surely the chirality would change...
  15. T

    How do we know that a photon is massless?

    How do we know a "photon" is massless? Do we have any experimental proof? Photon has momentum, it even bends in gravity, doesn't that mean photon has mass? A radioactive material will go on emitting radiations until it balances, a way in which it reduces it's mass. So, isn't photon the...
  16. soothsayer

    How can massless strings be the building blocks of matter?

    To clarify, I'm not confused as to how massless strings can add up to make massive objects, I understand why that happens. My question has more to do with the speed at which strings move. If strings are massless, they must travel at the speed of light, correct? If so, then first off, what is the...
  17. I

    Is Light Massless? Evidence & Experiments Explained

    I read an article a while back, I can't seem to find it now, that stated that photons can deteriorate into smaller particles, and that photons actually have mass. Is this a senseless article, or is there scientific evidence to this happening. The article said that after a trillion years or so...
  18. T

    Angular acceleration of a massless pulley

    Consider a massless string going over a massless frictionless pulley with masses M1 and M2 at the end of the string. The tension in the string would be same. The torque equation of the pulley says (T1-T2)=Iα .Now L.H.S is zero since T1=T2 and also I=0 . So,we have a condition 0=(0)(α)...
  19. andyrk

    Is Tension Constant in a Massless String?

    Why is tension in a massless string same throughout its length?
  20. J

    Should photons be considered massless?

    Hello all, I am asking this question in the context of general relativity. In general relativity the stress-energy tensor is related to the spacetime metric through the Einstein field equations. The production of a curved spacetime is what creates what we call gravitation. For example a...
  21. A

    Force on a massless inextensible string passing around a pulley

    I just can't get why for a massless inextensible string passing around a frictionless pulley we easily take T1=T2 (in fig.).My profs. never did explain this & jumped on further & when i ask them they won't answer saying it was very basic and other students say it's obvious. pls. help also pls...
  22. andyrk

    Massless and Frictionless Pulley

    Can a massless and frictionless pulley rotate if a string goes over it connecting two blocks of unequal weight? I think it shouldn't rotate and the string just skids/slides over the top of the pulley. Is it correct?
  23. N

    What are the equations for a massless particle's worldline?

    Hello, I've been trying for a while to calculate the paths of massless particles around a Schwarzschild black hole, and basically I've failed, but I really need the answer for writing a simulator. Knowing the initial conditions of the particles, I need to know the equations for...
  24. N

    Two blocks and a massless horizontal spring question?

    [b]1. Two masses of m1 and m2 is connected by a massless undeformed spring rest on a horizontal plane.Find the minimum constant force Fmin that has to be applied on m2 so that other block get shifted if D is the coefficient of friction between blocks and surface [b]2. Homework Equations...
  25. Saitama

    Finding momentum of a massless particle

    Homework Statement This question is a part of a problem I am doing. I am stuck at finding momentum of the massless particles. The two particles move around a stationary point (evenly spaced from the particles). The potential energy between the particles is a function of distance r between the...
  26. C

    Regular empirical evidence of curved space or massless photons?

    NOT including the prediction capabilities of the particular math equations of GR or SR. In particular, hard evidence such as, or close to; here's an electron, because we measured it directly, or saw it in an electron microscope. Or here's a cell under a microscope. Or this is a brain scan/MRI...
  27. S

    Circular Motion, Massless String - Swinging Rock

    1. Homework Statement : A rock of mass m is attached to a string of negligible mass and length L. The rock is released from rest from a horizontal position. When the rock is at point P, the string attached to the rock makes an angle θ with the horizontal. In terms of the quantities, m, L, θ and...
  28. Y

    How Do You Solve a Frictionless Pulley System Problem?

    Homework Statement A light rope is attached to a block with mass 4.00 kg that rests on a frictionless, horizontal surface. The horizontal rope passes over a frictionless, massless pulley, and a block with mass m is suspended from the other end. When the blocks are released, the tension in...
  29. Y

    Finding Tension of a Massless Rope

    Homework Statement An adventurous archaeologist crosses between two rock cliffs by slowly going hand-over-hand along a rope stretched between the cliffs. He stops to rest at the middle of the rope. The rope will break if the tension in it exceeds 2.25*10^4N, and our hero's mass is 86.9kg...
  30. Y

    Calculating Tension For A Massless String

    Homework Statement A gymnast of mass 59.0kg hangs from a vertical rope attached to the ceiling. You can ignore the weight of the rope and assume that the rope does not stretch. Use 9.81 m/s^2 for the value for the acceleration of gravity. Calculate the Tension, T, in the rope if the gymnast...
  31. B

    What Forces Act on a Ball Attached to a Massless Rod in a Vertical Loop?

    Homework Statement A ball of mass m is attached to a massless rod and is suspended from a frictionless pivot. It is moving in a vertical circle or Radius R such that it has speed v1 at the bottom. The ball is in a vacuum. Find an expression for the force exerted by the rod at the top of the...
  32. arivero

    Mass of the proton with massless quarks?

    A usual lore from chiral perturbation theory is that the mass of the pion is proportional to the sum of the up and down masses, and then it is going to be zero when such masses are zero. Now, for the proton, I notice the following remark from Chris Quigg Why is it different of the pion?
  33. T

    At what angle does a ball on a massless pendulum reach equilibrium?

    Homework Statement A rod of neligeable mass is released from the horizontal position. As a ball at the end of the rod falls, it reaches a point at which the tension, T, in the rod equals the ball's weight. At what angle from the verticle does this occur. I am not getting the same answer...
  34. naima

    Is Hestenes massless electron beyond standard model?

    Hi all I just read Hestenes paper He has a massless electron which rotates about a fixed point at the speed of light . (look at eqns 8 9 10 and fig 1) I wonder if this is in the standard model or beyond? What is the lagragian of this "zitter" model?
  35. B

    Massless beam held against a wall

    Homework Statement Just took my finals and there was one problem that bugged me, I have done these types of problems multiple times and understand it well but this particular problem seems like there isn't enough information. There is a massless beam being held against a wall by a massless...
  36. S

    How to understand why massless particles must travel at c?

    First of all, i need to say that i have read various threads posted by other users in physicsforum who have asked the same question. However, I still don't have a clear picture of why massless particles must travel at c. Personally, I have came up with an explanation which is rather simple...
  37. Islam Hassan

    Higgs Field: Perfectly Static Particle is Massless?

    Like the question says: if a particle is perfectly static with respect to the Higgs field, can we still define mass or is mass then irrelevant? On another note, (and if I am not mistaken) why does mass not vary with a particle's speed with respect to the Higgs field? IH
  38. S

    A Good Book on Massive vs Massless Particles

    Hello, would you be able to recommend a book on introduction to relativity? A one that does a thorough comparison on the physics of massive vs. massless particles If the book is a popular read (low on math), that would still be quite okay. I am familiar with "Why does E=mc2" by...
  39. phosgene

    Momentum of a massless particle?

    Homework Statement Consider a neutron which decays at rest. Calculate the momentum of the electron in MeV/c when the proton is produced at rest. (You may assume that the anti-neutrino has zero mass). Homework Equations Relativistic momentum = \gamma(mv) The Attempt at a Solution...
  40. R

    Two Blocks Connected by a Massless String Over a Pulley

    A 6.0kg box is on a frictionless 35° slope and is connected via a massless string over a massless, frictionless pulley to a hanging 2.0kg weight. What is the tension in the string if the 6.0kg box is held in place, so that it cannot move? m1= 6.0kg m2= 2.0kg θ=35° So, for Box 1 to be...
  41. G

    How to Calculate Tension in a Massless Wire Correctly?

    So I'm just supposed to calculate the tension in the wire (see attachment) and get to the formula shown. What I've done so far is to draw free body diagrams of all the boxes, and then I've just tried to set up some equations and manipulate them. However, I can't seem to end up with the...
  42. M

    Statics - Show a massless stick feels forces along the stick

    Homework Statement A stick is connected to other partrs of a static system by hinges at its ends. Show that a massless stick feels forces, at the hinges, that are directed along the stick. Then show if the stick is massive, the forces do not need to point along the stick. Homework...
  43. M

    Wave going to a massless ring on a pole

    Hello, I was reading my textbook and watching MIT Opencourseware and noticed a discrepancy between the two. There was a string attached to a massless ring on a pole without friction, on the MIT website it said the ring will go twice the amplitude of the wave, but in the textbook it said it...
  44. coktail

    Are massless objects immune to time dilation?

    Imagine there is a train moving at .9c with a person on the train and an observer on a platform watching the train go by. The person on the train walks forward while shining a flashlight. To the observer on the platform, the person on the train is time dilated and walks forward in "slow...
  45. K

    [Math Verification] Massless Pulley - Acceleration of Block A & B?

    Homework Statement http://img152.imageshack.us/img152/8286/imag0103l.jpg *I'm aware my free body diagram is off and that normal force should be in the left quadrant. Homework Equations F=m*aThe Attempt at a Solution Would I be correct in saying.. Fg of A = mg * sin (20) = 2kg * 9.8 * sin...
  46. P

    Two marbles colliding elastically on massless strings

    Homework Statement Two marble spheres of masses 30 and 20 grams, respectively, are suspended from the ceiling by massless strings. The lighter sphere is pulled aside, as shown in the diagram, through an angle of 75° and let go. It swings and collides elastically with the other sphere at the...
  47. A

    Does the bending of light by gravity imply that photons have mass?

    Because light bends due to gravity, as can be seen in Gravitational Lensing, if gravity responds to mass and light is bent by gravity, does that not imply that the photon is NOT 100% massless?? That the photon indeed has a very tiny amount of mass?? Because if light bends due to gravity, then...
  48. J

    Can a photon be a particle without mass?

    Is it possible to explain how a photon can be a particle yet have no mass? I think I need a scientific clarification either of what a particle is or what mass is.
  49. M

    Wigner's little group, massless case

    Hello, I'm reading Weinberg's vol.1 on Quantum Theory of Fields and stuck on the following problem. In the massless case Wigner's little group is the group of Lorentz transformations that keep the vector (0,0,1,1) invariant. (I'm going with Wigner's notations, where the vector is denoted...
  50. P

    Free, massless boson propagaor

    Homework Statement Cosider a single, free, massless boson with action S=\int\mathcal{L}=\frac{1}{2\pi}\int\partial X \overline{\partial}X in two dimensions \overline{\partial}X(z,\overline{z}) = \partial_{\overline{z}} X(z,\overline{z}) Show, that the propagator \langle...
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