Scientists Solve Century-Old Optics Mystery

evan_arrrr! writes "From the article: Since the early 20th century physicists have known that light carries momentum, but the way this momentum changes as light passes through different media is much less clear. Two rival theories of the time predicted precisely the opposite effect for light incident on a dielectric: one suggesting it pushes the surface in the direction light is traveling; the other suggesting it drags the surface backwards towards the source of light. After 100 years of conflicting experimental results, a team of experimentalists from China believe they have finally found a resolution."

Comments

No physics background here

[fprintf]

Does this article help explain how those little lightbulb things with the rotating black/white cards work? I always loved those as a kid... in fact I was shocked to find them at Home Depot the other day in a demonstration of why LowE glass can be a good thing. They had two of them, but the one behind the low E glass was barely rotating when exposed to a lightbulb while the other behind regular glass was whizzing around.

Re:No physics background here

[Hatta]

Both zero and infinite mass work in all of the equations.

Really? Momentum is p=mv. If the mass of a photon is infinite, then its momentum is too. Since momentum is conserved in a collision, when that photon collides with an object it transfers infinite momentum to that object. If the object is of finite mass, then p/m=v=infinity.

So why don't I recoil with infinite velocity when I'm hit by a photon?

Re:No physics background here

[blueg3]

I don't mean to imply that you're flat-out wrong, only that you're treading in dangerous territory with your understanding.

I should clarify that photons don't simply have "no rest mass". They have a rest mass, and it is exactly zero. (If I recall correctly, particles with zero rest mass are conveniently constrained to move at v=c, and particles with nonzero rest mass are always capable of moving at v=0 in some frame.)

Photons certainly do have mass, since they have energy, and the two are the same. In fact, if you had a perfect-mirror box, you could put five pounds of light in a box.

Most fundamental particles (quarks and leptons, for example) have some nonzero rest mass that appears to be a fundamental quantity. Everywhere else, "mass" is a euphemism for energy that isn't carefully-accounted for. The "rest mass" of a brick is made up of the fundamental rest masses of its constituent particles, plus lots of binding energy (between quarks, between nuclear particles, between electrons, et cetera). In the same way, we call kinetic energy, abstracted away in a statistical fashion, "heat". At a fine-grained level, there is no heat. Likewise, at a fine-grained level, there is only fundamental rest masses and energy. (And energy is the same as mass.)

So your five pounds of light in a box could suddenly become accounted away as box with a rest mass of five pounds (plus mass of empty box).

The danger is allowing any relativistic quantity to touch a non-relativistic equation or view of the world. It's sure to lead you to very wrong ideas. (See, for example, "zero mass is the same as infinite mass", elsewhere in this discussion.)