# Momentum transfer dependent on distance?

1. Aug 11, 2009

### jaketodd

Does the amount of momentum transfered by an EM wave to a particle differ by how far away the particle is from the EM wave emitter? The more spread out the EM wave is, the less momentum transfered? I'm not talking about a collision between a photon and a particle; I'm talking about the momentum transfered to a particle by the wave nature of an EM wave.

Thanks!

2. Aug 11, 2009

### kote

Waves don't have momentum, particles do. So you can think of the problem as having a shotgun-like source spraying photons everywhere, each with their own momentum. The total momentum transferred will be proportional to the number of photons colliding with your particle. It will be proportional to the intensity of the light where you're measuring.

3. Aug 11, 2009

### jaketodd

Actually waves do have momentum. If it were a multitude of particles then you would get particles all over the place from one EM wave emission in the double slit experiment, for example. An EM wave is a wave and then collapses to a particle. And that wave transfers momentum to everything it touches (even the things that the wave don't manifest as a particle on). So would someone else please answer my question? I do appreciate your message though, Kote. I agree that the intensity of the wave would be proportional to the momentum transfered, but I'm wondering if distance and how much the wave has spread out makes a difference in how much momentum is transfered to the particle.

4. Aug 11, 2009

### kote

There is no momentum involved in the double slit experiment. That experiment measures waves, not particles. Interference is a property of waves; momentum is a property of particles :). A photon is neither a wave nor a particle persistently, but in certain interactions it will manifest the properties of either a wave or a particle. A particle paradigm can be used to explain the momentum transfer interaction accurately because momentum is a property of particles.*

What you said is exactly correct. The distance is proportional to the intensity (and momentum) of the light from a normal light bulb type source by an inverse square relationship. If you have a laser then the distance will not change the momentum at all since the intensity is the same at any distance. All of the particles in a laser are flying in the same direction and don't spread out.

*Now, if you want to know the intensity and related momentum on the other side of a 2 slit setup, you've got a little more work to do. That would involve a step where you consider the light to be a wave and then a step where you consider light's particle properties.

5. Aug 12, 2009

### jaketodd

In the double slit experiment, there is a tiny amount of momentum transfered to the wall in between the slits and everywhere the wave touches. It is a wave until it changed into a particle on the detector array behind the slits. If it were a bunch of particles or a wave and a bunch of particles at the same time, then you would get significant momentum transfer to everything it touches.