Holding on to radiation
(appeared in Sep 2015)

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Electric charges with no field or magnets that have no poles would be invisible, says S.Ananthanarayanan.

It is well known that electric charges that are speeded up or slowed down would radiate electric and magnetic waves. But still, there are electrically charged electrons going round in atoms and they never lose energy and slow down. The quantum theory has it that electrons have special orbits where they do not radiate, and electrons jump from one orbit to another, in steps of energy, when they absorb or give out a packet of energy, in the form of a photon, or a particle of light. But why there should be such favoured orbits has not been clearly answered.

An international team, Andrey E. Miroshnichenko, Andrey B. Evlyukhin, Ye Feng Yu, Reuben M. Bakker, Arkadi Chipouline, Arseniy I. Kuznetsov, Boris Luk’yanchuk, Boris N. Chichkov and Yuri S. Kivshar, from the Acton in Australia, Hannover, Moscow, Singapore, Darmstatd, near Frankfurt, and St Pertersburg report in the journal, Nature Communications, that they have succeeded in creating an arrangement that contains electrical energy but shows no radiation at a distance. Such an arrangement would throw light (sic) on the nature of atoms and even the nature of dark matter, which has been found to dominate the universe but does not interact except in the form of gravity, says a release from the Australian National University.

Among the earliest of the discoveries in modern science, along with the laws of motion, the gas laws and thermodynamics and the wave theory of light were the effects of electric charges or the motion of electric charges, which is a current, at a distance. Lines of electric field radiate from a single charge, to repel other similar charges, and to attract opposite charges. Moving the charges leads to magnetic effects and it was shown that the action of magnets is actually the result of moving charges within the atoms of the material of the magnet. The whole picture was elegantly written down in a set of mathematical equations, which connected the magnetic effect of moving charges and also the current that changing a magnetic field could generate. And the theory showed that moving a charge back and forth or in a circle would create a magnetic wave, which would give rise to an electric wave, which would, in turn, create a magnetic wave and so on, to give rise to radiation in the form of heat or light, or even X Rays and more energetic radiation.

Given this well established theory, amply realised in devices like the electric bulb, the electric motor, the telephone and wireless telegraphy, the newly discovered structure of the atom, as a positively charged centre surrounded by orbiting negative charges presented a mystery. As the orbiting electrons were in motion in a circle, should they not radiate electromagnetic waves and gradually crash into the opposite charge in the nucleus? But it was known that the atom had a positively charged centre and the surrounding electrons had to be in motion, for the atom to exist. And then there was the pattern in the energy of light that atoms emitted, which could be explained neatly by proposing that there were special orbits.

What the international team writing in Nature Communications has done is to create a geometric arrangement of conductors and charges in such a way that the wave of energy generated by one part exactly cancels out the wave generated by another part. Waves consist of alternating motion or the compression of particles or, in the case of e/m waves, of alternating electric and magnetic effects. If the stages of alternation of two waves, at a spot, are in opposition, like a rising ocean wave at the seaside meeting a falling wave returning from the beach, the combined effect is that there is no motion, or no electric or magnetic effect.

Electromagnetic waves, like radio waves, or light waves, when the waves are very rapid, can be generated by up and down motion of charged particles, which happens in a radio station antenna, or when charges are moved in circles, as in a coil. The nature of the waves generated becomes complex when the path of the charges assumes special shapes. One such is the toroid, or the doughnut shape of a hollow long coiled spring which is itself wrapped round in a ring. Electric charges flowing through such a path would have one motion along the coils of the spring and also motion in the circle of the ring. A straight coil creates a field like a bar magnet. A toroid would then create a field in a circle and the arrangement would show a field along the axis of the toroid, called the electric dipole and also due to the movement in circle along the toroid, called the toroid dipole. It is possible to consider dimensions of the arrangement where two alternating effects at a distance consist of waves that are in opposition and hence cancel out. The arrangement would be an instance of an ‘anapole’, or an assembly ‘without a pole’

In general, such an arrangement, which represents a state that takes energy to set up, but has no external field, is not feasible as it violates principles of symmetry. In working out the principles of these arrangements the authors of the paper found that an alternate way of working out the fields showed that there were dimensions at which the toroid dipole effect became important, and could almost cancel the electric dipole field. “We realised that these toroidal components were not just a correction but could be a very significant factor,” says Dr Miroshnichenko if the Australian National University. The implication is that the anapole at these dimensions should not scatter incident light and become invisible!

As for the question of whether it was possible to experimentally observe the anapole mode, the team worked it out that a silicon nanodisk measuring 50 nanometers high with diameter 200 to 400 nm would satisfy the requirements. They hence grew silicon crystals of 160 to 310 nm on a quartz substrate and carried out a series of experiments of scattering of light. It was found that for diameter more than 200nm, there was a dip in the intensity of scattering about the wavelength 550nm (green-yellow), moving to the red side and becoming more pronounced as the diameter of the disk was increased.

The effect is essentially one of the incident radiation exciting two different and interfering dipoles in the anapole. Radiationless current configurations could be investigated as underlying the stable orbits of electrons in atoms. The same idea also supports a mode that has been proposed to account for ‘dark matter’, a form of non-radiating and non-scattering matter that form 85% of the mass of the universe.


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