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Q & A: Can a photon gain mass?

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Most recent answer: 10/02/2011
Are there any conditions where photons can gain mass?
- Joan (age 60)
st pete florida
This depends on what you mean by mass. If by mass you mean the inertial mass (the m in the momentum-velocity relation p=mv) a photon can gain some by falling in a gravitational field. However, in a vacuum the invariant (rest) mass will remain  zero.

Perhaps you're asking about photons in matter. There the pure photon can be mixed with other excitations of the charged particles, leaving more complications. If that's what you want to know about, please follow up.

Mike W.

(published on 10/02/2011)

Follow-up on this answer.