# Fermilab

## Inquiring Minds

 Questions About Physics

How do you define momentum for a photon with no mass? What is symmetry?

You asked:

Photons are said to have no mass but how is that. The momentum equation is m=velocity*mass so therefore the photon would have no momentum and therefore could not repel the two negatively charged electrons. Was my information right?

-What exactly is symmetry laws and gauge symmetry?

Dear David:

Thank you for your questions.

No. The equation momentum=velocity*mass only applies for massive particles. For massless particles it is given by momentum=constant*frequency. So photons do carry momentum. The constant in the equation is a constant of nature that is related to the speed of light.

-What exactly is symmetry laws and gauge symmetry?

Symmetry laws refers to the framework that argues that the outcome of a physics experiment should be invariant under certain symmetry considerations. For example, if the outcome of an experiment does not depend on whether a particle travels from right to left or left to right, then the physics equations should reflect this mirror symmetry.

Geometrical symmetries occur frequently. (Symmetry of a snowflake, etc.) But symmetry can also appear as a more abstract concept. In many cases, the laws of nature contain deep mathematical symmetries that reflect the physical nature of the universe. Among all abstract mathematical descriptions, a certain class of symmetry is called gauge symmetry.

Example of a simple gauge symmetry: In electromagnetism, the motion of charged particles depends on voltage DIFFERENCES, not the absolute value of the voltage. So one can gauge the whole setup, increasing the voltage everywhere by 100 volts, and still the physics is the same. This is an example for a gauge symmetry.

Check the pages at http://www.emmynoether.com/ to learn more about symmetry.

Best wishes,

Kurt Riesselmann

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 last modified 12/6/2000   physicsquestions@fnal.gov

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