The question here is how the momentum of light transfers to a material when light goes from one medium into another medium with a different index of refraction. Supposedly there are two different theoretical description for this process, and both of them gave contradicting results. Even the experimental results till now have not been conclusive or even at odds with each other. Until now, that is.
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 travelling; 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.
You can read the rest on what they determine and which one they verified. I'm just still surprised that this problem has been hanging that long, and that I'm not aware of it. What else have I been missing? :)