Lubricant Figure 2. Hypothetical beam target. The target consists of an inner disc-shaped central section, and a surrounding outer ection. Both inner and outer sections are made of an absorptive material or are half wave plates, and hence will extract angular momentum from an incipient circularly polarised beam. They are floated over a microscope slide on a thin layer of lubricant then the change in total angular momentum will necessarily be small and the edge effects introduced in a real experiment will be negligibl We now consider an experiment of the sort originally proposed by Khrapko 2 Let the beam fall upon a target capable of absorbing angular momentum. The target consists of a central disc capable of rotating freely, and an outer region surrounding the disc and made of the same material. For sake of argument, let them be floated a thin layer of lubricant over a microscope slide(Fig. 2). The radius of the inner portion of the target, Tt, is chosen in accordance with Figure 1(d). The value of rt is given by rt = a In(It202 (15) We now consider the interaction of the beam with the target. Integration of either the macroscopic or the microscopic angular momentum density profile unambiguously accounts for the total angular momentum density of the beam. There can therefore be no arguments about he beam carrying angular momentum in potentia. We are therefore left with two options: Either the beam transfers angular momentum in direct accordance with the profiles illustrated, or a real redistribution of angular momentum within the beam takes place on interaction with the target. If the former, then we see that under the two different models, the angular momentum transferred to the disc will be of differing sign, a result easily detected experimentally. If the latter, then we must ask what prompts this redistribution of angular momentum. Analysis shows, 45, 46 that redistribution arises as a result of the interaction between the beam and edges within the structure of the target. However, complications arise in determining what constitutes an edge. Consider our hypothetical two- part target. Provided the inner portion remains free to rotate, we may allow the distance between it and the outer portion to become infinitesimally small. But then what happens if the two portions interact, and there exists an initial static friction which must be overcome to initiate motion? In any real world situation there will always be initial friction forces which must be overcome, and therefore if angular momentum is to be transferred at all to the inner target portion, we must allow it to be transferred regardless of the presence of this initial friction. If sufficient angular momentum is transferred. then friction will be overcome and motion will be initiated Now consider an arbitrary portion of a solid object, perhaps an atomic dipole. This portion is attache to its neighbours by atomic bonds, but if these can be overcome it too will be free to rotate. These forces may be considered analogous to the friction forces described above. In performing the redistribution of angular momentum on interaction with a solid object, the edge of every single atomic dipole must be taken into accountLubricant Figure 2. Hypothetical beam target. The target consists of an inner disc-shaped central section, and a surrounding outer section. Both inner and outer sections are made of an absorptive material or are half wave plates, and hence will extract angular momentum from an incipient circularly polarised beam. They are floated over a microscope slide on a thin layer of lubricant. then the change in total angular momentum will necessarily be small and the edge effects introduced in a real experiment will be negligible. We now consider an experiment of the sort originally proposed by Khrapko.2 Let the beam fall upon a target capable of absorbing angular momentum. The target consists of a central disc capable of rotating freely, and an outer region surrounding the disc and made of the same material. For sake of argument, let them be floated on a thin layer of lubricant over a microscope slide (Fig. 2). The radius of the inner portion of the target, rt, is chosen in accordance with Figure 1(d). The value of rt is given by rt = a r ln(1 + d 2 2a 2 ). (15) We now consider the interaction of the beam with the target. Integration of either the macroscopic or the microscopic angular momentum density profile unambiguously accounts for the total angular momentum density of the beam. There can therefore be no arguments about the beam carrying angular momentum in potentia. We are therefore left with two options: Either the beam transfers angular momentum in direct accordance with the profiles illustrated, or a real redistribution of angular momentum within the beam takes place on interaction with the target. If the former, then we see that under the two different models, the angular momentum transferred to the disc will be of differing sign, a result easily detected experimentally. If the latter, then we must ask what prompts this redistribution of angular momentum. Analysis shows43, 45, 46 that redistribution arises as a result of the interaction between the beam and edges within the structure of the target. However, complications arise in determining what constitutes an edge. Consider our hypothetical two￾part target. Provided the inner portion remains free to rotate, we may allow the distance between it and the outer portion to become infinitesimally small. But then what happens if the two portions interact, and there exists an initial static friction which must be overcome to initiate motion? In any real world situation there will always be initial friction forces which must be overcome, and therefore if angular momentum is to be transferred at all to the inner target portion, we must allow it to be transferred regardless of the presence of this initial friction. If sufficient angular momentum is transferred, then friction will be overcome and motion will be initiated. Now consider an arbitrary portion of a solid object, perhaps an atomic dipole. This portion is attached to its neighbours by atomic bonds, but if these can be overcome it too will be free to rotate. These forces may be considered analogous to the friction forces described above. In performing the redistribution of angular momentum on interaction with a solid object, the edge of every single atomic dipole must be taken into account!