When Alfven's discovery made waves

Physics

Waves are nature’s fascinating phenomena. Tidal waves, ripples on the water surface in lakes, sound waves carrying speech, musical notes and noise, light waves transmitted from the sun and stars are all well known common phenomena. Waves carry information useful to unravel many mysteries of nature. They are dynamic and carry energy in a physical system. It was therefore a fundamental discovery when, in 1942, Hannes Alfvén (1908-1995), a Swedish physicist, showed that a new type of low frequency wave which cannot exist naturally in the Earth’s atmosphere, can be present ubiquitously in space.

The discovery of these waves, known as Alfvén waves, led to a new understanding of the plasma universe and, in particular, of space in our neighbourhood. Plasma is a fully or partially ionised, electrically conductive gas, a mixture of negatively charged electrons and positively or negatively charged ions, atoms with stripped of or added electrons; the gas is pervasive. The trajectories of these particles (unpaired electron spins) are governed by magnetic fields.

Hannes Olof Gösta Alfvén, born on May 30, 1908 in the small town of Norrköping, Sweden, in a highly intellectual family. His early interest in astronomy and electronics can be seen in all of his important discoveries made in later years; he got a fresh perspective by approaching astrophysical problems from an electromagnetic point of view. Actually it was in the 1930s, that Alfvén suggested that sunspots are the result of the sun’s magnetic field becoming temporarily “frozen” into the solar plasma.

A well-known English scientist, reviewing Alfvén’s classic work ‘Cosmic Electrodynamics’, published in 1950, referred to him as “an electrical engineer in Stockholm”. Rather, I have to reword that to say “… an electrical engineer of space who understood how the electrical and magnetic fields in the cosmos interact with a conducting fluid - plasma”.

Wave dynamics

Alfvén argued that when a conducting fluid moves across a magnetic field, an electromagnetic force appears in the conductor.  Currents drawn by this electromagnetic force will then flow in the fluid. Two processes happen then. First, the magnetic field due to the current in the fluid will modify the original magnetic field that created it. Second, the current flows in the presence of a modified magnetic field. The fluid then experiences a mechanical force (of electromagnetic origin). Alfvén formulated this interaction between electromagnetic fields and fluid motion in a simple and clear mathematical form. Using his mathematical model he happened on a new type of waves which he named ‘Electromagnetic- hydrodynamic waves’ and published his results in a letter to Nature, the prestigious scientific journal, in 1942.

Stretched string analogy

The remarkable property of these waves is that they choose to travel only in the direction of the magnetic field lines of force embedded in the fluid. As the conductivity of the naturally occurring fluids is very high, Alfvén originated the concept that the lines of force are almost frozen-in in the fluid. The magnetic field and the fluid move together. This concept made Alfvén draw an analogy with waves along stretched strings (like the strings of a sitar or tanpura, to use an Indian analogy). Intuitively, we can “feel” that the frequency of vibration of a string is inversely proportional to its thickness and directly its tension and, in a given medium, frequency controls the wave velocity. For Alfvén waves, magnetic field tension along the lines of force and the mass density of the fluid which is glued to the magnetic lines of force, give rise to a transverse wave along the magnetic strings.

Scientists did not immediately accept Alfvén’s discovery. Some critics maintained that because these waves involved electrodynamics of fluids, Maxwell (James Clark Maxwell who made fundamental contributions to the study of electrodynamics in the 19th century)would have discovered them, had they existed.

Fermi’s ‘of course’ did it

The breakthrough came in 1948, after a seminar delivered by Alfvén in Chicago, physicist Enrico Fermi, nodded his head and said, “Of course! Such waves could exist”. The prestige of Fermi, the famous nuclear physicist, was such that “…the next day every physicist nodded his head and said ‘of course!’”. These waves are not present in our close environment and it took several years to verify their existence in the laboratory. This contributed to skepticism about the reality of the waves.

Now, with progress in space technology, we have numerous satellite and spacecraft observations pointing to the omnipresence of these waves. That is understandable because magnetic fields pervade the plasma universe and the excitation of Alfvén waves is inevitable. They have proved to be an invaluable concept in plasma research in its application to controlled fusion, in design of high-energy “atom smashers”, and in understanding of various phenomena in Nature as manifestations of the presence of Alfvén waves. Alfvén was awarded the Nobel Prize for his discovery in 1970.

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