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Name:  Chris B.
Status:  student
Age:  15
Location: N/A
Country: N/A
Date: 2000-2001


Question:
For light to have force it must have mass (force=mass*acceleration). Is the force false? And if it does have force what is it (the force)?


Replies:
Newton's law F=ma applies for object of significant size (larger than a few molecules) and less than maybe one-tenth of the speed of light. It is an approximation that works very well in non-extreme situations. Light is not a particle in the strictest sense. It behaves as a bundle of waves. Its energy works out to be proportional to its frequency. Its momentum works out to be proportional to its wavelength. Light only exerts enough force to affect individual particles. To analyze the universe on the level of individual particles, quantum mechanics is required.

The best way to describe force of light on a particle is absorption and then emission of the light. When the light is absorbed, so are the light's energy and momentum. When the light is emitted, so are the energy and momentum. If the light is emitted in a direction different from the original direction, there is a change of momentum in the particle. This momentum change is seen as a change of the particle's motion.

Kenneth Mellendorf


It is not necessary for light to have a mass in order to have a force. It must have momentum. Now let us see how this works without it getting too complicated -- which it is. Given: The speed of light in a vacuum is a constant, c (m/sec). The energy of a photon of light, call it W(joules), is W = h*(nu), where h = Planck's constant (joule*sec) and (nu) is the frequency of the light in (1/sec).

The momentum of photon, call it p(kg*cm/sec) = W(joules)/ c(cm/sec). Recall that energy in joules is in units (kg*m^2/sec^2), so W/c =p has units(kg*m^2/sec^2)/(m/sec) or (kg*m/sec) which is the units of momentum! And the force (pressure*area) acts in the direction of propagation of the light.

It all works out the same whether you treat light classically (Maxwell's equations) or quantum mechanically, but it is the momentum that is the critical variable, and in the case of light you can't separate it into mass*speed, it is "lumped" together.

Vince Calder



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