If light has no weight, how can it push objects?

[ZapperZ]

Can anyone link to a write-up of an experiment that proves absorbtion of photons results in a momentum exchange to the object absorbing the photons?

D.S. Weiss et al., "Precision measurement of the photon recoil of an atom using atomic interferometry" Phys. Rev. Lett. 70, 2706 (1993).

G.K. Campbell et al., "Photon Recoil Momentum in Dispersive Media" Phys. Rev. Lett. 94, 170403 (2005).

Zz.

 

[cmb]

You are asking us to write a book. Unless you are a junior in an undergrad physics program taking the introductory condensed matter course, any quantum explanation of reflection is going to be hand-waving at best. I gave the hand-waving, semiclassical description above.

Ultimately it must be an electric-magnetic interaction. One might therefore interpret this as some sort of magnetic repulsion of two distant bodies, for which the photon is 'only' a mediation.

 

[D H]

Ultimately it must be an electric-magnetic interaction. One might therefore interpret this as some sort of magnetic repulsion of two distant bodies, for which the photon is 'only' a mediation.

Ultimately, it is quantum electrodynamics that explains the interaction of photons and matter, and that is way, way beyond the scope of this thread.

 

[dlr]

In order to get specular reflection you need structures which are significantly larger than a wavelength and are smooth over that scale. That simply doesn't happen at the molecular level, so a specular reflection implies an interaction of a photon with a continuum of molecules.

OK, but am I'm right in saying the underlying physical mechanism is the interaction of an electromagnetic field with charged particle(s) (ie, electrons)?

And, if I understand you correctly, you are objecting that the magnetic field of the photon is so large compared to the size of electrons and molecules that the interaction is between a photon's magnetic field and 1000's or 10's of thousands of electrons -- a sea of electrons (metals) or a lattice of electrons (a crystal) or a sea of molecules (water), instead of with just one electron. Which makes sense.

And then what happens next, after the charged particles get the magnetic "push" from the photon's magnetic field, they move of course, which gives a little push to the protons/neutrons of all the nearby molecules? This effect ultimately being caused by the effect that keeps electrons from getting too close to a proton/neutron?

 

[Dickfore]

the magnetic field of the photon

What is this? I don't even

 

[fizzle]

It's easier to see from a classical em viewpoint. You have a positive charge sitting stationary in space. A step function em wave approaches from the left with the electric field always pointing down your computer screen and magnetic field always pointing into the screen. The charge accelerates downward due to the E field, so its velocity increases. That downward velocity interacts with the em wave's magnetic field via v X B, resulting in a force to the right, which accelerates the charge to the right. Voila, the em wave has imparted downstream velocity/momentum to the charge.

One interesting special case is when the charge is initially traveling with or against the em wave. In that case, v X B becomes -v/c * E and the resulting total initial force on the charge is (1 - v/c)*E. Looks a lot like a "Doppler Shift" of the electric field strength, eh? You'll note in the extreme case of v = c (i.e. when the charge is moving with the wave at the speed of light) there is no net force on it.

That's what classical em says ... not that I believe it (or QM). I don't have any better answer so I basically keep my mouth shut!

 

[dlr]

The charge accelerates downward due to the E field, so its velocity increases. That downward velocity interacts with the em wave's magnetic field via v X B, resulting in a force to the right, which accelerates the charge to the right. Voila, the em wave has imparted downstream velocity/momentum to the charge.

Yes, the force is perpendicular to the direction of travel of the light ray. So then, why does the solar sail move directly away from the light rays hitting it?

 

[Dale]

Yes, the force is perpendicular to the direction of travel of the light ray.

You mis read the post. Fizzle showed that you do get momentum imparted in the direction of the light ray.

Here is a more general derivation:
http://farside.ph.utexas.edu/teaching/em/lectures/node90.html
http://farside.ph.utexas.edu/teaching/em/lectures/node91.html

An EM field has momentum, that momentum is either conserved in the field or it is imparted to matter.

 

[fizzle]

Yes, the force is perpendicular to the direction of travel of the light ray. So then, why does the solar sail move directly away from the light rays hitting it?

I was showing above that a simple, constant step wave transfers downstream momentum to a charge. For light, which is a circularly polarized (CP) em wave, you don't get the down-the-screen motion because the E field rotates; so the charge rotates in a circle while being accelerated downstream by v x B. The overall path is a helix in the direction of the wave.

BTW, if you do a full relativistic/classical em analysis of a CP wave interacting with a charge you can derive the Compton Scattering equations - as Compton noted in his original paper. The problem is that the transfer a "photon" of energy to the charge requires an unrealistically high E field (10^16 V/m or more) in the CP wave, sort of a Compton catastrophe. This is the root cause of QM and its associated weirdness (e.g. a photon is a particle+wave, wave function collapse, zero-point energy with its enormous but "hidden" fields, etc.). No one was able to figure out how a realistic CP em wave could transfer so much energy to a single charge.

"speculation deleted"

Integral