Why do photons have momentum if dp/dt is zero?

[coki2000]

Hi PF,

I have a question about light pressure: Let's take a laser beam. Each photon in the laser beam has a momentum p=h/λ.

If photons have pressure, dp/dt must be non-zero but, it is zero.

Where do I make mistake?

Thank you for answers.

 

[Chopin]

What makes you think dp/dt is zero?

Do you just mean that the photon has a constant momentum as the beam travels through space? If so, then the answer is: the change in momentum does not happen when the photon is just hurtling through space, it happens when the photon hits a piece of matter and interacts with it. Even a plain old billiard ball rolling across a table has a constant momentum--it's when it hits the other balls that its momentum changes.

 

[coki2000]

What makes you think dp/dt is zero?

Do you just mean that the photon has a constant momentum as the beam travels through space? If so, then the answer is: the change in momentum does not happen when the photon is just hurtling through space, it happens when the photon hits a piece of matter and interacts with it. Even a plain old billiard ball rolling across a table has a constant momentum--it's when it hits the other balls that its momentum changes.

Oh, okey. Thank you for your answer :) I understand.

 

[andrien]

pressure of light is owed to it's magnetic field.when light will fall on a charged particle ,the electric field of light will give a motion in $$\vec{E}$$ direction and then the magnetic field will act .it is easily shown by the curl of (v×B) that the force is in direction of propagation.(it does not depend on the sign of charge)