# Light reaction

## Main Question or Discussion Point

Say you shoot a beam of light in a laser or flashlight. Does the photons exert a backwards force on the laser. I understand how some of Einsteins relativity works out on the train with adding velocities and all. It just makes sense that energy is being lost by not adding velocities so the energy has to go somewhere. Maybe I don't have a full understanding of relativity.

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Yes there is a backward force.
This is not directly related to relativity.
It is related to momentum conservation.
Momentum conservation is related to the absence of force of the system {laser+photon}.

Do they have equations for this?

pervect
Staff Emeritus
For a single photon of light
E = Pc = hf

where
E is energy
P is momentum
c is the speed of light
h is Planck's constant
and f is the frequency

So if you were to take a laser or flashlight and move faster then the amount of time until the battery drained would differ from someone who is not moving. I'm guess the time would be the same as time dilation would suggest.

Just remember that whan a photon is emitted there is a recoil.
Similarly when a photon is absorbed.
There are numerous experiments that illustrate that in physics laboratories.
One of these experiments is called the Mossbauer effect, was worth a Nobel price.

All these experiments are very far from the complicated batery thing that you suggest but they are quite astonishig.

russ_watters
Mentor
So if you were to take a laser or flashlight and move faster then the amount of time until the battery drained would differ from someone who is not moving.
You are always stationary in your own frame of reference and the speed of light is always the speed of light, so the time it takes to drain the batteries is always the same for the person holdiing the flashlight.