Credit: Phys. Rev. Lett. 101, 243601 (2008)
Although it sounds like the fight my spouse and I have about what happened to the blankets last night, an old pushing-versus-pulling debate is flaring up because of some work (including video evidence – see below) by Chinese researchers recently published in the recent issue of the Physical Review Letters that seems to go a long way toward resolving the controversy. Or perhaps not.
For a century, scientists have argued over whether light gains or loses momentum as it passes from glass to air. Around 1908, mathematician Hermann Minkowski went toe-to-toe with physicist Max Abraham about the issue. Minkowski postulated that there is an inverse relationship between the momentum of light and it’s speed. Abraham disagreed, arguing that the momentum of light goes down with decreasing speed. Abraham’s position has had an intuitive feel to it because in ordinary life and non-quantum physical observations, momentum does indeed decrease with speed. Minkowski based his quantum mechanics view on the notion that a photon’s momentum increases up as the light’s wavelength decreases, something that happens when light enters air after passing from another material.
So here are the latest developments: Weilong She and others Zhongshan University in Guangzhou conceived an experiment using a .5-micron silica fiber that was 1.5 millimeters long, down which they would shine 270-millisecond laser pulses at a wavelength of 650 nanometers. Abraham’s position would be vindicated if the fiber shows any recoil. However, if Minkowski’s position is correct, the fiber will remain straight down. “When I began this experiment, I was really unsure which one is correct,” said Weilong recall. In fact, Weilong’s fiber showed a recoil motion of a slight, 30-nanometer bend with each pulse. A second experiment with non-pulsed light and a longer fiber provided similar results. Weilong also found the size of the recoil to be consistent with theoretical estimates. See Weilong’s video of the recoiling fiber here.