Why do photons travel at the speed of light without having mass?

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SUMMARY

Photons travel at the speed of light due to their lack of rest mass, as established by Einstein's theory of relativity. The equation E=mc² applies only to rest mass, which is not applicable to photons since they never exist at rest. Instead, photons possess energy described by E=hf, where h is Planck's constant and f is frequency. This distinction clarifies why photons can travel at light speed without violating the principles of relativity.

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  • Understanding of Einstein's theory of relativity
  • Familiarity with the concept of rest mass
  • Knowledge of the equation E=hf and its implications
  • Basic principles of energy and momentum in physics
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madmike159
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Einstein's theory of relativity means that the faster an object travels and the more mass it has the more it will curve space-time. This means something like a spaceship could never travel at the speed of light because it would take an infinite amount of energy to reach that speed. So obviously photons have no mass or else they wouldn’t travel at the speed of light and there would be no speed of light.

However we know from E=[tex]h[/tex]f that photons have a certain amount of energy.

So it should have a mass = E/c[tex]^{2}[/tex]

but that would mean that a photon couldn't travel at the speed of light. Does anyone know why they either don’t have mass or why they are excluded from this rule?
 
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Please read the FAQ in this section of PF.

Zz.
 
No, E= mc2 refers to "rest" mass: mass measured in a reference frame where the velocity is 0. Since photons never "rest" it does not apply to them.
 

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