Massless Particle Questions: Explaining Photon's Lack of Mass

[Swetha.M.L]

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?

 

[jtbell]

Mass has energy associated with it via E=mc², but not all energy has mass associated with it via m = E/c².

 

[Simon Bridge]

Not everything has all forms of energy at the same time. Photons have kinetic energy but not mass-energy.

 

[wabbit]

You're probably thinking of the relation $E=mc^2$ - but this applies only in a frame where the particle is at rest, and photons are never at rest since their velocity is $c$ in any frame.

The general equation is $E^2=m^2c^4+p^2c^2$ where $p$ is the momentum of the particle, and for a massless particle this reduces to $E=pc$. In other words photons do not have mass but they have momentum, hence (kinetic) energy, given by that formula.

 

[PeterDonis]

can you explain why photon is massless?

It depends on what you would consider an "explanation". Experimentally, photons are known to be massless to a very high accuracy. Theoretically, photons are modeled as massless because that's the model that best matches experiments.

 

[DaveC426913]

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?

Are you asking how a photon - which is energy - can be on one side of the mass-energy formula, yet not have any mass?

Isn't it kind of like asking how ice and steam are two forms of the same thing, yet only one of them will break your knuckles if you take a swing at it?

i.e. the very nature of the "two" in "two forms of one thing" is that they have different properties.

 

[Topolfractal]

Well there are several reasons why a photon is considered is considered massless .
1. Yes through experimentation.
2. Special Relativity implies it as shown below.
- E = m(c^2)/ ( sqrt ( 1-(v^2/c^2))
Multiplying both sides by sqrt(1-(v^2/c^2))
E(1-(v^2/c^2))= m( c^2)
Okay when v= c the E side equals (0)
So therefore the speed of light not being zero implies the mass is zero.
Also this implies it does have no rest energy and therefore it's energy is entirely kinetic in nature.

 

[PeterDonis]

2. Special Relativity implies it as shown below.
- E = m(c^2)/ ( sqrt ( 1-(v^2/c^2))
Multiplying both sides by sqrt(1-(v^2/c^2))
E(1-(v^2/c^2))= m( c^2)
Okay when v= c the E side equals (0)

This is not a valid argument, because your formula for $E$ is only valid for an object with $v < c$.

Also, it's obvious that there's a problem when your formula says $E = 0$ for $v = c$, since photons do not have zero energy. Saying that $m = 0$ makes the equation work out is irrelevant, because the equation still says $E = 0$. But photons have $E > 0$ with $m = 0$; your argument does not explain how that can be.

 

[Topolfractal]

I agree and see my error