Chapter 10 - Propulsive Implication of Photon Momentum in Media

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Chapter 10 - Propulsive Implication of Photon Momentum in Media

Notes by David A Roffman on Chapter 10 of


Chapter by Michael R. LaPoint,

Project Manager, Science Research and Technologies Project Office,

NASA Marshall Space Flight Center, Huntsville, Alabama 

    There has been a major debate in science for about 100 years over the issue of momentum density of electromagnetic field propagation through an index of refraction greater than unity.  Now, there have been two theories for how to handle the situation, and they are direct opposites. 

     One alternative is the Minkowski equation and the other is the Abraham equation.  For the first equation, momentum density increases when passing through a greater index of refraction (by a factor of n2).  As for the second equation, the opposite happens; momentum density decreases (by a factor of 1/n).  It is important to know that both are attempted derivatives of the Maxwell Field Equations, which pertained to a vacuum. 

    Attempts to prove either theory have ranged from thought process to experimentation.  In 1935, Halpern showed that symmetric energy (provided prerequisites), will not satisfy Minkowski’s equation.  So, Halpern was an Abraham fan.  Balasz used a thought experiment to buttress Abraham.  He proposed two enclosures in uniform motion without external forces.  Both enclosures have a dielectric rod, and one has a slower electromagnetic wave passing though the enclosure.  The wave moving through the vacuum will be moving faster than the one in the rod.  To keep center of mass and momentum, the Abraham (slowing down) tensor must be used.

    While thought experiments are nice, they are not real physical experiments.  The first physical experiment took place in 1954 with Jones and Richards.  Although it wasn’t well known, it did demonstrate one thing.  For this experiment, many metallic reflectors in air and in dielectric media were used to test radiation pressure.  This experiment backed up Minkowski  But the results were ignored.

    Another experiment was carried out in 1973 by Ashkin and Dziedzic.  This one used laser light to measure the radiation pressure on an air-water interface.  A net outward force was detected, thus giving more credit to Minkowski.   The same year a follow up was done.  Gordon published a report on pseudomomentum.  He summarized that Abraham was correct for nondispersive dielectric media, while Minkowski was correct for determining the radiation pressure of an object in a media.  Using time varying voltage, Walker helped to prove that Abraham was right.

    Later experiments by Brevik and Gibson (used a photon experiment) helped to prove that both view points were correct.  For high frequencies, Minkowski’s tensor should be used.  But for low frequencies, Abraham’s tensor is a good idea. 

Other researchers have proposed taking the average of the two tensors.  The Bose-Einstein condensate experiment utilized rubidium in a gas to check the tensor theories.  Based on the refraction and momentum, Minkowski’s tensor seemed correct.  To summarize this cyclical debate, there is still no answer about who was right (what tensor is right).  The book mentions on page 356 that a narrow band optical pulse of 600-fs duration ans 1-MW/cm2 peak power, incident upon a multilayer photonic bangap structure  with mass of 10-5 grams may produce accelerations up to 108 m/s2.  In optics, an ultrashort pulse whose time duration is on the order of the femtosecond (which is 10 − 15 seconds.  There is an indication that the short interaction time limits displacement and velocity, but these experiments could be used to test electromagnetic phenomena and momentum transfer to large objects (like a spaceship). Using a relativistic acceleration formula it is obvious that the speed of light cannot be reached or surpassed.

    While all of the previous discussion is nice, propulsion is what really matters.  Here the chapter starts with Slepian’s Electromagnetic Space Ship.  He used an oscillating magnetic field for propulsion.  He asks two questions: Is there an unbalanced force acting on the material system of the spacecraft, and can the unbalanced force be used to propel the ship?  (answers - yes to 1, and no to 2).  This was not to be taken seriously, as it was published with a rebuke by him a month later.  It was designed only to provoke thought.  The idea is nonsense, as the thruster would go forward and back an equal distance (no net movement).  However, Corum et al. say that unidirectional motion is possible.  This claim has been shown to be false, but the time derivative of electromagnetic density might alter space, causing some acceleration (this was shown by the US Air Force Academy).

    Brito tried to use the principles that this chapter discusses to create a propulsion device (Electromagnetic Inertia Manipulation Propulsion), but it failed.  However, there was some force observed that may not have been due to error.  Another idea is Feigel’s Hypothesis, that is, Zero Point vacuum fluctuations move dielectric objects in crossed electric and magnetic fields.

     The European Space Agency’s companion, van Tiggelen et al. believe the Feigel hypothesis to be measurable and quite real.  They calculate that a small magneto-electric object inside an isotropic (and monochromatic radiation field) could move if external fields were switched on.  Also, for a field intensity of 10kW/cm2, a velocity of approximately 10-5 cm/s could happen if an object’s composition was FeGaO3.  This can be verified by a 10 micro gram crystal of the same substance as the previous object mounted at the end of a piezo-resistive cantilever (but there is no more data in this area). 

    The real question is how to use the two tensors developed, and how to apply them to space travel.  This is still a mystery, and many more tests are needed.  A breakthrough here will not only finish a hundred year battle in physics, but may open up new worlds for us to explore.  Note p = h/λ, where “p” is the momentum, “h” is Planck’s constant, and “λ” is wavelength.  This applies to photons and particles with mass.