# How can a photon exist if it has no mass?

• ScienceNerd36

#### ScienceNerd36

How can a photon exist if it has no mass? I've always found this concept hard to grasp. (Rhyming not intended)

We're not in classical physics any more. Photons are quintessentially relativistic and quantum-mechanical. The general relation for energy, mass and momentum is

$$E^2 = (mc^2)^2 + (pc)^2$$

(This uses what is often called "rest mass", not "relativistic mass".) An object can have zero mass but still have nonzero energy and momentum.

Note that even in classical electrodynamics, electromagnetic waves carry energy and momentum without having mass.

Why does everything that exists have to have mass? Is there an a priori reason??

Hi ScienceNerd36!

(erm … rhyming not achieved)

mass is energy, energy is mass

a photon does have mass

(it has no rest-mass, but why should that matter when a photon is never at rest? )

tiny-tim said:
Hi ScienceNerd36!

(erm … rhyming not achieved)

mass is energy, energy is mass

a photon does have mass

(it has no rest-mass, but why should that matter when a photon is never at rest? )

oohh now you have started this war again Timmy, prepare for battle

## 1. How can a photon exist if it has no mass?

According to the theory of relativity, mass and energy are equivalent. Therefore, a photon, which is a form of energy, does not need mass to exist.

## 2. If a photon has no mass, how does it travel at the speed of light?

The speed of light is considered to be the maximum speed at which anything can travel. Since a photon has no mass, it does not need to accelerate and can travel at the speed of light naturally.

## 3. Does the lack of mass in a photon affect its behavior?

Yes, the lack of mass in a photon affects its behavior in several ways. For example, it allows a photon to move at the speed of light, travel in a straight line, and be unaffected by gravity.

## 4. Can a photon be affected by external forces if it has no mass?

No, since a photon has no mass, it cannot be affected by external forces such as gravity or friction. However, it can still interact with other particles through electromagnetic forces.

## 5. How do we know that a photon has no mass?

Extensive research and experiments, such as the Michelson-Morley experiment and the theory of relativity, have consistently shown that a photon has no mass. Additionally, the behavior and properties of a photon, such as traveling at the speed of light and being massless, further support this conclusion.