# Photon's Mass and Speed of Light [Confused]

• Atran
In summary, there are differing opinions on whether a photon has mass. Some argue that a black hole's mass increases when it consumes photons, indicating that photons have mass. However, the mass of a photon is typically measured by dividing its energy by c^2, and this measured mass is only for a moving photon with constant speed. If a photon does have mass, it would exert force on other particles, but this is typically attributed to its momentum rather than its mass. The general formula E^2=p^2c^2+m^2c^4 can be used to calculate the mass of a photon, but it reduces to E=pc for the photon specifically.

#### Atran

Hello,

When you read, you see that there are different opinions on the question, does a photon have mass?

From what I think, I know that a black hole's mass increases when it consumes photons, and everybody knows that a photon has energy and momentum. Since energy can be trasformed to mass, therefore a photon has mass.
The mass of photon is measured by dividing its energy by c2, that measured mass is the mass of a moving photon with its constant speed, the same speed in all reference frames.
If a moving photon has mass, then it must exert force on other particles. The exertion of force causes acceleration and therefore change in speed, and this violates the idea of constant light speed.

Can you correct me?
Thanks...

Atran said:
When you read, you see that there are different opinions on the question, does a photon have mass?

I was not aware that there was different opinions… The photon has no mass. Take a look on the Particle Data Group site for a lower limit of this mass determined experimentally
Oh and by the way : the formula E=mc^2 can be applied to pretty much anything but the photon E=mc^2 is a particular case of the more general formula E^2=p^2c^2+m^2c^4 which reduces to E=pc for the photon
And you're right : a photon exert a force on particle but via its momentum p, not its mass.

## What is the mass of a photon?

A photon, being a fundamental particle of light, has no rest mass. This means that it has no mass when it is at rest. However, it does have a relativistic mass, which is determined by its energy and speed.

## Is the speed of light the same as the speed of a photon?

Yes, the speed of light in a vacuum is the same as the speed of a photon. This is because a photon is a particle of light, and light travels at a constant speed of approximately 299,792,458 meters per second in a vacuum.

## Can a photon have a different speed in different mediums?

Yes, the speed of a photon can vary depending on the medium it is traveling through. For example, light travels slower in water or glass than it does in a vacuum. This is due to the interactions between the photons and the particles of the medium.

## Does a photon have a maximum speed?

According to the theory of relativity, the speed of light is the maximum speed that any particle can travel in the universe. Therefore, a photon, being a particle of light, also has a maximum speed of approximately 299,792,458 meters per second in a vacuum.

## How does the mass of a photon affect its speed?

Since a photon has no rest mass, its speed is not affected by its mass. However, as the energy of a photon increases, its relativistic mass also increases, which can affect its speed slightly.