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Transcript
Models
of Aether
Models of Aether
• (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
Models of Aether
• 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
Models of Aether
What is aether?
• Aether is a proposed medium for
electromagnetic and gravitational forces.
• Mechanical gravitational aether
• Luminiferous aether
Descartes & Corpuscles
•
•
•
•
•
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.
Descartes & Corpuscles
• 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
Hooke & Waves
• 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.
Hooke & Waves
• 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.
Newton & the Aether
• 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
Newton & the Aether
• Newton says seemingly contradictory things
about aether in various different works:
– Early days – All space is filled with variabledensity aether
– Principia – Argues that the existence of dense
aether is incompatible with Kepler’s first and
third laws.
Newton & the Aether
• 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.
Newton & the Aether
• 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.
Huygens & Wave Theory
• 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.
Huygens & Wave Theory
• 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
Lesage & Corpuscular Theory
• Text.
Lesage & Corpuscular Theory
Huygens & Newton
• 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.
Huygens & Newton
• 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.
Le Sage & Kineti cs
• 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.
Le Sage & Kineti cs
Le Sage & Kineti cs
• 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)
Le Sage & Kineti cs
Le Sage & Kineti cs
2 major flaws in the theory:
1. A moving planet would receive more vigorous blows on its
front side, so it should slow down
2. 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.
Le Sage & Fatio
• 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.
Le Sage & Fatio
• 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.”
Bradley & Stellar Aberration
• 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
Bradley & Stellar Aberration
Bradley & Stellar Aberration
Bradley & Stellar Aberration
Euler & Mass
• 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
Young & Interference
• 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.
Young & Interference
• Explained Newton’s interference rings.
Young & Interference
Young & Interference
• 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
Foucault & Fizeau
• 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
Foucault & Fizeau
• According to aether theories
• http://courses.physics.northwestern.edu/P
hyx103/lightH2O.pdf
• (Dr. David Taylor)
Foucault & Fizeau
The Elastic Solid Aether
• 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.
MacCullagh & Elastic Solids
• 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
MacCullagh & Elastic Solids
• 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
Riemann & a Unified Theory
• 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
Boussinesq & Restriction
• 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
Faraday & Electromagnetism
• 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
Faraday & Electromagnetism
• 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).
Faraday & Electromagnetism
• 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)
Faraday & Electromagnetism
• 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.’
Thompson‘s Model
• 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
Thompson‘s Model
Thompson‘s Model
• This discovery was important because it
related two different kinds of theory,
1.
2.
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.
Thompson‘s Model
• 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
Thompson‘s Model
• Proposed a rotary model of magnetic fields because
they rotate the plane of polarization of a light wave
• Re-proposed the MacCullagh model – a mechanical
aether that only resists rotation
• His model of aether was composed of tetrahedons
made of four spheres connected by rigid rods.
• Oppositely rotating flywheels resist changes in
orientation by the law of inertia
Thompson‘s Model
Thompson’s model
of a unit of aether
Maxwell‘s Model
• James Clerk Maxwell (1831-1879) – English –
Cambridge
• Was inspired by Faraday and Thompson
• Agreed with Thompson that the B field was a
rotation of the aether
• Considered the E field to be the velocity of a
non-compressible fluid, and this fluid had sinks
and sources like a body of water
Maxwell‘s Model
Maxwell‘s Model
• Maxwell’s model:
– a sea of small particles that act as charges and ‘idle
wheels’
– the current through an electric wire produces vortices
– concentric rings connecting the vortices are the magnetic
field lines
– the rotating vortices set the small particles in motion
– the particles are tied to the vortices, so instead of breaking
loose, they communicate their motion to a second layer of
vortices, etc
Maxwell‘s Model
Maxwell‘s Model
Maxwell‘s Model
• Small particles would be displaced every time
the current through the wire stops or starts
• Displacement in the small particles = electric
field
• Vortices = magnetic field
• This means that a wire with constant current
only produces a magnetic field, while a current
that starts and stops produces both
*Stress from the displacement of the equilibrium in this model = electrostatic force
Maxwell‘s Model
• Maxwell had some problems with aether
– Permanent magnets would have vortices for an indefinite
amount of time
– All known fluids that have vortices are also viscuous
• Nevertheless, he fiercely and continuously defended
its existence
– He wanted to do away with the ‘mystical’ concept of action at
a distance
– also hinted that aether could have something to do with
consciousness, like a physical replacement for spiritual
substance
Aether Today
• Einstein’s theory of relativity removes
the need for a substantial aether
according to the general thinking of
physicists today
– no need for a universal frame of reference
– space-time serves as unifier
• Einstein’s theory of relativity removes
the need for a substantial aether
according to the general thinking of
physicists today