M o d e l s o f A e t h e r • (In the next chapter we will learn about how Maxwell formed our understanding of waves.) • Most waves of which we have immediate experience are transmitted through some medium • water waves • sound waves • vibrating strings
M o d e l s o f A e t h e r • Question that arises: what medium transmits optical and electromagnetic effects? • Aether is a negative response to the idea of electromagnetic waves moving through a vacuum. • The debate about aether was also a debate about whether or not light should be understood as a wave
M o d e l s o f A e t h e r What is aether? • Aether is a proposed medium for electromagnetic and gravitational forces. • Mechanical gravitational aether • Luminiferous aether
D e s c a r t e s & C o r p u s c l e s • Rene Descartes (1596-1650) • All of space is a plenum (filled with matter) • An aether permeates all space • Corpuscular/projectile theory of light. • The corpsucular theory required that light travel more rapidly in a dense medium than a rarified one or a vacuum.
D e s c a r t e s & C o r p u s c l e s • Impact and pressure are the only kinds of interactions between matter. • “Action at a distance” is ridiculous. • Newton’s law of gravitation and Coulomb’s electrostatic law talk about a force between two bodies, but they give no indication how the force is transmitted
H o o k e & W a v e s • Robert Hooke (1635-1703) – English – Oxford • Opponent of Newton • Rejected corpuscular theory • Light is a vibratory motion transmitted through a medium as a series of wave fronts.
H o o k e & W a v e s • The wave theory as Hooke proposed it could not explain polarization. • By polarization, we are referring to the ‘screening effect’ that happens in common light filters like sunglasses.
N e w t o n & t h e A e t h e r • Sir Isaac Newton (1642-1727) – English – Cambridge • Newton responded to Hooke’s inability to explain polarization by rejecting the wave theory • There is some confusion about his loyalty to the idea of aether
N e w t o n & t h e A e t h e r • Newton says seemingly contradictory things about aether in various different works: • Early days – All space is filled with variable-density aether • Principia – Argues that the existence of dense aether is incompatible with Kepler’s first and third laws.
N e w t o n & t h e A e t h e r • Later he clarifies his dedication to a theory of aether • Letter to Bentley – ‘Action at a distance’ is ridiculous. • Later days – Some form of active aether exists for the transmission of gravity. It is somehow related to the idea of God’s omnipresence.
N e w t o n & t h e A e t h e r • Space is not a passive void that matter passes through. • Never defines what aether is. • Never defines light, but knows it interacts with (and is distinct from) aether. In Optiks, seems to imply a particle theory of light. • Does not necessarily support a corpuscular theory, but leaves the door open for Descartes.
H u y g e n s & W a v e T h e o r y • Christiaan Huygens (1629-1695) – Dutch – Royal Académie of France. • Disagreed with the corpuscular theory based on basic observations of light. • If two beams of light cross each other, neither beam has any observable effect on the other. There is no sign of collision between the light ‘particles,’ which should be observable as scattered light.
H u y g e n s & W a v e T h e o r y • Defended the wave theory of light. • Light waves were created by disturbances through a highly elastic, tenuous medium. • Light is a longitudinal wave • Longitudinal waves are those that undergo tiny displacements in the direction of the wave itself (compression and rarefaction) • Example: sound waves
H u y g e n s & N e w t o n • Science did not take Huygens as seriously as it should have, probably because of Newton’s opposition to Hooke’s wave theory. • Newton’s theory was so popular and was perceived to be so sufficient that most people overlooked any gaps in its explanation.
H u y g e n s & N e w t o n • Even though ‘action at a distance’ was still mysterious, people accepted it as fact. • If ‘action at a distance’ is okay, then there is no need for aether. • If there is no need for aether, there can be no waves, and thus no wave theory.
L e S a g e & K i n e t i c s • George-Louis Le Sage (1724-1803) – Geneva • Proposed a ‘corpuscular’ explanation of gravitation • Space is filled by countless tiny particles bouncing in all directions at high speeds. • A single body isolated in space would be in a sea of these particles, and would be bombarded by them equally from every direction. Since it is receiving an equal push from every direction, it will remain motionless.
L e S a g e & K i n e t i c s • If two bodies are near each other, the situation is different. • Two bodies in space shield each other from some of the particle streams on the sides facing each other. • Since there are less particles in the shielded space between the bodies, they will move into that space. • Objects are not attracted toward each other, but pushed toward each other (by net impulse)
L e S a g e & K i n e t i c s 2 major flaws in the theory: • A moving planet would receive more vigorous blows on its front side, so it should slow down • Experience vs. theory: • Experience: the strength of the gravitational pull between objects is proportional to the products of their masses • Le Sage: the strength of the gravitational pull between objects is proportional to the product of their volumes.
L e S a g e & F a t i o • Controversy surrounding whether Le Sage is the original creator of this theory • Theory is derivative of (and possibly identical to) Nicolas Fatio de Duillier’s kinetic theory of gravitation. • Le Sage was known to be obsessed with Fatio’s writings.
L e S a g e & F a t i o • Claimed that Fatio’s theory was different in that it proposed “elastic collisions,” which Le Sage said did not work as a theory of gravity. • Also claimed that he came up with the theory on his own before he read any Fatio. • Problem: Others who read Fatio’s work claimed that he never proposed “elastic collisions.”
B r a d l e y & S t e l l a r A b e r r a t i o n • James Bradley (1693-1762) – English – Oxford • 1728 – Discovered stellar aberration • Stellar aberration – The effect of an apparent motion in the heavens because of the relative motion of the observer on a moving Earth
E u l e r & M a s s • Leonhard Euler (1707-1783) – Swiss – Imperial Russian Academy, Berlin Academy • Favored the wave theory because: • Objects do not lose any detectable mass from emitting light • If light is corpuscular, it would have some mass, and this mass would have to come from the object emitting it
Y o u n g & I n t e r f e r e n c e • Thomas Young (1773-1829) – English – Royal Institute • First scientist to bring the ideas of constructive and destructive wave interference into the discussion of light. • Used the analogy of water waves on the surface of a lake. • Won many victories for the wave theory by being able to explain specific phenomena.
Y o u n g & I n t e r f e r e n c e • Explained Newton’s interference rings.
Y o u n g & I n t e r f e r e n c e • Augustin-Jean Fresnel (1788-1872) and Dominique Arago (1786-1853) • showed that two beams of light polarized at right angles do not produce interference • Young • light is a transverse wave, thus two different light waves will have linearly independent directions of polarization perpendicular to the direction of their propagation, and thus will be able to cause interference
F o u c a u l t & F i z e a u • Jean Bernard Léon Foucault (1819-1868) • Armand-Hippolyte Fizeau (1819-1896) • 1850 – directly measured the speed of light in water and air • Cushing: Light is faster through air than through water – which is a victory for wave theory
F o u c a u l t & F i z e a u • According to aether theories • http://courses.physics.northwestern.edu/Phyx103/lightH2O.pdf • (Dr. David Taylor)
T h e E l a s t i c S o l i d A e t h e r • Proceeding from Focault & Pizeau’s theory, a whole era of scientists devoted themselves to the elastic solid theory of aether • Claude-Louis Navier (1785-1836) and Augutin-Louis Cauchy (1789-1857) worked on mathematical equations of motion for an elastic medium • Siméon Denis Poisson (1781-1840) – solved the equations, and showed that an elastic solid able to be compressed and distorted must have both transverse and longitudinal waves.
M a c C u l l a g h & E l a s t i c S o l i d s • James MacCullagh (1809-1847) – Irish – Trinity College Dublin • Proposed new kind of elastic solid aether • Its potential energy depended on the rotation of its elements, and not compression and distortion • In such an aether, only transverse waves were propagated since there is no compression allowed for longitudinal waves
M a c C u l l a g h & E l a s t i c S o l i d s • v= √(μ/ρ) • ρ = density of the medium • μ = constant resistance of the aether to rotation • A mathematically consistent way to solve the problem of an elastic medium that would only propagate transverse waves • Sadly, MacCullagh’s work was largely ignored because there was no plausible mechanical model for such an aether
R i e m a n n & a U n i f i e d T h e o r y • Georg Friedrich Bernhard Riemann (1826-1866) – German – Georg-August University of Göttingen • Proposed an aether that resisted both rotation and compression • the rotation transferred optics and magnetism • the compression transferred gravity and electrostatic effects • First unified theory of optics and electromagentism • Unfortunately, he never pursued this theory
B o u s s i n e s q & R e s t r i c t i o n • Joseph Valentin Boussinesq (1842-1929) – French – University of Lille • Limited the use of aether to only optics • Aether is uniform everywhere (even inside material bodies) • Optical effects came from the interaction of the aether’s particles with ordinary matter
F a r a d a y & E l e c t r o m a g n e t i s m • Michael Faraday (1791-1867) – England – Royal Institute • Inquired deeply into electromagnetism • The more he studied the lines of magnetic forces, the more he came to think of them as literal, physical lines of force
F a r a d a y & E l e c t r o m a g n e t i s m • Faraday’s concept of a physical line of magnetic force developed into an idea of aether. It appears to me possible... that magnetic action may be communicated to a distance by the action of the intervening particles... (Faraday 1952).
F a r a d a y & E l e c t r o m a g n e t i s m • Aether lent its usefulness to magnetism as well as light, so that an electromagnetic aether was possible. ...it is not at all unlikely that, if there be an ether, it should have other uses than simply the conveyance of radiations (Faraday 1952)
F a r a d a y & E l e c t r o m a g n e t i s m • Faraday broke from the mainstream views of ‘action at a distance.’ • He said the transmission of magnetism is ‘one of continuous transmission at a finite speed.’
T h o m p s o n ‘ s M o d e l • William “Lord Kelvin” Thompson (1824-1907) – English – Cambridge, University of Glasgow • Cambridge had fallen behind since the time of Newton, and Thompson was part of the new ‘Cambridge school’ scientific revolution led by George Green. • 1841 – as student at Cambridge, showed a mathematical equivalence between electrostatic and similar thermal lines of force
T h o m p s o n ‘ s M o d e l • This discovery was important because it related two different kinds of theory, • electrostatic – formulated as action at a distance • steady heat flow – formulated as transfer from one particle to another in a medium in a coherent manner.
T h o m p s o n ‘ s M o d e l • Thompson devoted himself to analogies between forces. • He looked at the analogy between the electric field and an elastic solid under strain. • He thought the E field was analogous to the displacements from an aether’s equilibrium