# Relativity: Massless particles and photons

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1. Jun 14, 2015

### sunmoon

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 it.So I am totally confused.

2. Jun 14, 2015

### Staff: Mentor

It does not.
Particles are either massless, then they always travel at the speed of light, or they are not, then they never do that.
"Existence" can have philosophical issues, but I think light clearly exists.
It is not a constituent. Light can be described with the concept of photons.
It does not. Neutrinos are lighter. And massless particles are even lighter of course, this includes the photon.

3. Jun 16, 2015

### Suraj Nehra

Hi there you have miss understood the equation if a particle travell with speed of light then its mass would become infinity remember the equation

M'= Mo/(√1- V2/C2)

4. Jun 17, 2015

### ChrisVer

That is still misleading... Keep in mind that no massive particle can travel with the speed of light...
If you take the limit of the velocity going to c, then you have that the relativistic mass (M' you wrote) goes to infinity, and not the rest (invariant) mass, which we call the mass of a particle.
The relativistic mass is more like the total energy of your particle (contains also the kinetic energy) and that's why the energy is written as $E= m_0 \gamma (= m_{rel})$.

5. Jun 17, 2015

### ZapperZ

Staff Emeritus
You have an error in cause-and-effect here. A particle doesn't become massless when it "starts moving with the velocity of light". It is either massless, or it isn't! If it is massless, then it will move at c. It doesn't become massless when it reaches c.

Says who? What natural law are you using here to claim that there are no massless particle, especially when the Standard Model of elementary particle says that there is? Be very careful, on this forum, about espousing your personal beliefs, especially when it is glaringly wrong! Read the PF Rules!

It is not the "main constituents", it is the ONLY constituent! Light is not made up of anything else.

Where did you get that a photon is "heavier" than an electron? It is difficult to answer your question when you are already asking us to explain it based on your faulty understanding. A photon is NOT heavier than an electron, as far as having a mass is concerned. A photon is massless. Period!

Maybe you need to start reexamining one conjecture at a time! Every single sentence that you posted here is faulty! Start from the beginning, and you might learn something without asking if you start reading our FAQ first, because I have a strong feeling a lot of your wrong understanding can be corrected by at least a couple of our FAQs.

Zz.

6. Jun 17, 2015

### ChrisVer

Says the person who have contributed in writing many parts of those FAQs I can feel you there...