# E=mc^2 Confusion (1 Viewer)

### Users Who Are Viewing This Thread (Users: 0, Guests: 1)

#### Prasanna Suman

"I beleive E=mc^2 works." But!
Photon (Packet of energy as described by Einstein in 1905, "Photoelectric effect") is mass less particle. So, the relation suggests that light should not have energy, though it has.

Is this wrong, if yes why?

#### Tomaz Kristan

It is nothing wrong. If a photon had a "nonzero resting mass", its "c speed traveling mass" would be infinite.

But it isn't.

#### Jheriko

$E={m_0}{c^2}$ where $m_0$ is the rest mass. Since energy and mass are equivalent, when a particle moves it gains mass, even if its rest mass is zero, like a photon.

For an object in motion there is, where p is the momentum, ${E^2}={{m_0}^2}{c^4}+{p^2}{c^2}$, so the photon's energy (with it's zero rest mass) is $E=pc$

Last edited:

#### lightarrow

Jheriko said:
$E={m_0}{c^2}$ where $m_0$ is the rest mass.
No. $E={m}{c^2}$ where m is the relativistic mass.

You can understand that E is not ${m_0}{c^2}$, for a moving particle, from the correct equation you wrote then:

${E^2}={{m_0}^2}{c^4}+{p^2}{c^2}$

If it were $E={m_0}{c^2}$, substituting this value in the previous equation, you would have:

${{m_0}^2}{c^4} ={ {m_0}^2}{c^4} +{p^2}{c^2}$

that is: p = 0, which implies v = 0, so the particle is necessarily at rest.

Last edited:

### The Physics Forums Way

We Value Quality
• Topics based on mainstream science
• Proper English grammar and spelling
We Value Civility
• Positive and compassionate attitudes
• Patience while debating
We Value Productivity
• Disciplined to remain on-topic
• Recognition of own weaknesses
• Solo and co-op problem solving