# What is Massless particles: Definition and 52 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. ### 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...
2. ### 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...
3. ### 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?
4. ### 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##?
5. ### 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...
6. ### The attractive force of photons on other photons & matter

Summary: massless particles (or at least photons) are attracted to other photons and to matter, but which is most attractive, and why ... Summary: massless particles (or at least photons) are attracted to other photons and to matter, but which is most attractive, and why ... I am really...
7. ### A Charged particles mass before symmetry breaking

How could electrically charged particles be massless before the symmetry breaking? Wouldn't the energy stored in the electric field contribute to particles mass?
8. ### 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...
9. ### 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?
10. ### 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...
11. ### 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. ### What is frequency from a photon's perspective?

If a photon does not experience time, how can it change? Without change, how can it have frequency?
13. ### 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. ### 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. ### 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. ### 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...
17. ### 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?
18. ### 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
19. ### 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...
20. ### 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.
21. 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"...
22. ### 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...
23. ### 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...
24. ### 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...
25. ### 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...
26. ### 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
27. ### Do Massless Particles Travel at Light Speed?

do all mass less particles travel at the speed of light?
28. ### 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...
29. ### 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...
30. ### 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
31. ### Can someone please explain to me why massless particles follow null geodesics?

Anything that is massless must follow null geodesics. Why is this?
32. ### 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 +...
33. ### Do Massless Particles Interact in Particle Physics?

Are there any reactions in that require the interaction of massless particles or am I right in thinking this is impossible? Thanks
34. ### How to detect nteraction between two massless particles?

What equipment one can use to detect interaction between two massless particles, such a photons? Is that even possible? If it is possible, I guess the only thing that one will be able to measure is the difference in energy between particles.
35. ### Massless Particles and the algebra involved

Homework Statement The positive pion decays into a muon and a neutrino. The pion has rest mass m=140 MeV/c^2, the muon has m=106 MeV/c^2 while the neutrino is about m=0 (assume it is). Assuming the original pion was at rest, use conservation of energy and momentum to show that the speed of the...
36. ### Massless Particles: Speed of Light & Gravitons

Do all massless particles travel away from you at the speed of light regardless of you speed . i know light does , does this include gravitons .
37. ### Massless Particles: Carrying Momentum?

In my lecture this morning i was told that charged particles interact by the exchange of virtual photons and that they serve as carriers of momentum and force. But it says that a photon is a boson with spin number 1, massless and zero charge. The question is basically how can a massless...
38. ### Massless Particles: Gauge Bosons, Photons & Gluons

in what way are gauge bosons ,photons and gluons massless? please explain the significance of the massless nature,i mean every particle is supposed to have mass?
39. ### The Pauli-Lubanski vector for massless particles

Does anyone know how to prove that W^2 = 0 for massless particles, where W is the Pauli-Lubanski vector W^\mu = \frac{1}{2} \epsilon^{\mu \nu \rho \sigma} M_{\rho \sigma} P_\nu ?? The books I have either give no proof, or they rely on the expression obtained in the massive case...
40. ### Massless Particles: At Rest & Why?

Can we find massless particles at rest? and why?
41. ### Does massless particles experience gravity

From Newtonian theory massless particles wouldn't experience force of gravity. Then from General relativity prospective gravity is ripples in space time due to a massive body so other objects move in the straight path in this curved space-time hence this objects appears to move in a...
42. ### Can Witten's Trick Apply to Any Number of Extra Dimensions?

I have remembered Witten had a trick to see massless particles in 4+n dimensions as if it were massive particles in 4 dimensions. Does the trick works for any number of extra dimensions? Or only for 4+n=10?
43. ### Massless Particles: Can Photons Travel Faster than Light?

Hey, well I've just been having a discussion with someone this minute, about traveling at the speed of light. We said that for something to travel at the speed of light, the partcile will need to be massless, but when I looked it up, I found that a particle has to have mass? Does a photon have a...
44. ### Why massless particles must travel at c

Homework Statement give a logical argument for why a particle with m0 = 0 with nonzero energy has to move with a speed equal to c. Homework Equations The Attempt at a Solution I attempted using the momentum equation but I always get a multiplication by 0 which doesn't make sense...
45. ### Finding the energy and momenta of massless particles

Homework Statement A pion spontaneously decays into a muon and an antineutrino according to pion^1- => muon^1- +antineutrino.. Current experimental evidence indicates that the mass m of the antineutrino is no greater than about 190 keV and may , in fact, be zero. Assuming that the pion decays...
46. ### Is the momentum of a massive particle traveling at the speed of light zero?

I know this isn't a deep discussion and the thread will be short but this question has been bothering me for a while and I hope the moderators won't terminate it. Are photons the only massless particles that travel at the speed of light? Diagrams of the standard model do not reveal the...
47. ### Do all massless particles travel at the speed of light?

I know photons have no mass but are there other particles that have no mass and if so can they travel at the speed of light? Thanks RAD
48. ### Forces on massless particles

do massless particles not feel forces - other than gravity? The only massless particles I know are photons and neutrinos. Photons feel gravity, but other than that, once a photon is created, is its energy fixed? Laura
49. ### Do massless particles always have momentum in the direction of travel?

I'm not sure this is the right forum, but I wanted to ask the following (possibly bizarre) question: Is it implicitly assumed that zero rest mass particles have momentum in the same direction as they travel?
50. ### Tunnel-effect possible for massless particles?

For the tunnel-effect, the relation between the transmissioncoefficent T and length of the barriere L is: T=e^{-2KL} in which: K=\sqrt{\frac{2m(U_0-E)}{\frac{h}{2\pi}^2}} Now, the tunnel-effect is possible for light right? But light means m=0, meaning K=0, meaningT=1! I must be...