# Massless Particle Questions: Explaining Photon's Lack of Mass

• Swetha.M.L
In summary: 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.
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?

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

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

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.

DEvens
Swetha.M.L said:
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.

Last edited:
Swetha.M.L said:
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.

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.

Topolfractal said:
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.

I agree and see my error

## 1. What is a massless particle?

A massless particle is a type of elementary particle that has no rest mass. This means that it does not have any inherent mass and therefore, cannot be slowed down or at rest. The most well-known example of a massless particle is the photon, which is the fundamental particle of light.

## 2. How can a particle have no mass?

According to the Standard Model of particle physics, massless particles are described as having zero rest mass because they do not interact with the Higgs field, which is responsible for giving particles their mass. The photon, for example, is massless because it does not have the ability to interact with the Higgs field.

## 3. Does a massless particle have energy?

Yes, a massless particle like the photon has energy. In fact, according to Einstein's famous equation E=mc², energy and mass are equivalent. So, even though a massless particle does not have any rest mass, it still has energy due to its motion and interactions with other particles.

## 4. How do massless particles move at the speed of light?

Massless particles, such as photons, are able to move at the speed of light because they do not have any rest mass. According to Einstein's theory of relativity, as an object approaches the speed of light, its mass increases. Since massless particles do not have any mass, they are not subject to this limitation and are able to travel at the speed of light.

## 5. Can massless particles be affected by gravity?

Yes, massless particles can be affected by gravity. According to Einstein's theory of general relativity, gravity is not a force but rather a curvature of space and time caused by the presence of mass and energy. Since massless particles have energy, they are affected by this curvature and can be influenced by gravitational forces.

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