The Search for Ether in Physics

 
Search for the ether
 
HW3 Thursday. Feb 12
th
.
Quiz next Thursday, Feb 20th on relativity:
 
Short answers. 20 mins. Closed book.
 
Materials from Rohrlich pgs.34-88, Cushing Chpts. 13,16-17,
 
Sklar pgs. 25-40 & class notes.
 
Philosophical pause: 
Reductionism
(now that we have some fundamental laws)
 
Are all causal events reducible to elementary mechanical
causality?
What about the standard "ranking"
Highest level
 
Psychology
   
Biology
   
Chemistry
Deepest level
 
Physics
Is each level "reducible" to the deeper level?
What does “reducible” mean?
 
Reductionism
 
There are many flavors of reductionism, and failure to distinguish among them
leads to many arguments at cross-purposes.
1.
Very strong reductionism: All phenomena will be understood as consequences of
a small set of fundamental laws: a genuine theory of everything.
2.
Weaker reductionism: All phenomena are reducible "in principle" to a small set of
fundamental rules.
But what's the operational test of whether that's correct, if the reduction
process is hopelessly complicated?
3.
In-between reductionism: Reducible "in principle" means that phenomena at
each level must at least be 
consistent with the rules at lower levels
.
Often the lower level rules provide insight into the higher-level processes,
occasionally the lower-level rules let us predict higher level phenomena.
The question of whether the layers of causation have a final deepest layer is NOT
logically tied to the question of whether going deeper provides important insight to
the higher levels. Watch out for many pop-philosophers who conflate these two
questions.
 
Explicit Philosophy
 
Modern physics will shake up our ideas about reality. Let's prepare by a quick review of
some classical philosophical views.
Hume and Kant
: (mid to late 18
th
 century)
Hume (
Treatise on Human Nature
):  one can only learn about reality through
experience.
Causation itself is a mental construct, not inherent in phenomena themselves.
However, in pointing out that the idea of induction itself cannot be inductively
confirmed (only disconfirmed), Hume implicitly indicated a way in which we seem
to approach the world with "hard-wired" prior assumptions.
Note two problems with induction:
at the deepest level, the argument for it is circular
the categories to be used in extrapolating toward the future (grue?) are not
specified by any logical principle
Kant 
Critique of Pure Reason
, agreed with most of this, but argued that there are
two valid forms of 
a priori 
 knowledge.  One is the reasoning facility (logic) by which
we analyze our experiences.  The other is mathematics, such as geometry.
It was known, however, that as a logical system, Euclidean geometry was not
unique, but only one geometry of a larger family. Kant believed it to be the only
conceivable actual geometry of the world.
 
Views of Reality: a spectrum
 
The common man in the street
There are definite events independent of observation.  Our senses record
these events.  Theories can represent genuine causal patterns inherent in
the events. Generally, the features we use to describe things, e.g. size,
time…, are inherent in the events themselves. The world consists of
collections of 'things'.
Einstein
There's a definite real world, of which we are observers, and also parts. But
we can't count on even the deepest features to be as they seem. The world
follows simple mathematical laws.
Planck
 (Realism, not entirely naive):
The goal of physics is a unified world picture.  Laws must be independent of
the observer.  The picture must be consistent.  Simplicity is a means to get
to a true, general picture, not an end.
 
Plato
The sensed world is an ephemeral approximation to the ‘true’ world of ideal
essences. Reason tells us more about that true world than mere sensation
can provide.
Hume
 (skepticism):
Whatever we claim about reality, only senses are available.  There is no
logical basis for induction. Nevertheless we all must accept it in practice.
Mach
 (a particularly subjective version of positivism):
Sensory impression is primary.  “Substances” are patterns of impressions.
No eternal laws. (E.g., atoms aren’t real.)
Notice that Planck and Mach both recognize that we have nothing but
sense impressions and the need to organize them simply. Planck implicitly
assumes that the sense impressions come from somewhere, and have
properties that make them fit into simple patterns. Mach assumes there's
something arbitrary about the patterns we find, so that no pattern should
be expected to be stable.
 
Views of Reality: a spectrum
 
Views of Reality: a spectrum
 
Berkeley
 (almost solipsism):
You are only aware of your own thoughts.  External reality is an unnecessary
hypothesis. (This has become a popular academic position again. Does anyone
really believe it?) However, to account for the similarity of perceptions of different
people (rather than claim we are all simply thoughts of his) Berkeley invokes the
mind of God, in which he claims we all partake.
But: Why is it better to postulate "God's mind" than to go with the naïve
postulate of reality?  If it is meaningless to think of a reality without mind, why
does the world in our minds have so much evidence of evolution, death etc.,
just as if reality went its way independent of our minds?
 
Traditional answer to Berkeley:
If a stone isn't real, go ahead and kick it.
Russell's answer to B:
If the world is all in my imagination, why does it include the parts of
Whitehead's book which I don't understand?
 
Progress?
 
One passage of Democritus that does survive is a dialogue between
the intellect and the senses. The intellect starts out, saying: "By
convention there is sweetness, by convention bitterness, by
convention color, in reality only atoms and the void
."
 In my book,
this one line already puts Democritus shoulder-to-
shoulder with Plato, Aristotle, or any other ancient
philosopher you care to name. But the dialogue doesn't
stop there. The senses respond, saying: 
"
Foolish intellect!
Do you seek to overthrow us, while it is from us that you take your
evidence?”
Scott Aaronson
 
Some ideas people argue about
fortunately, we have a busy semester and don't have to argue about them
here, unless there's time and interest!
 
Dualism:
It is often assumed that Mind and matter are two separate categories.  That runs into
some obvious problems, in that all the minds of which we are aware are obviously
strongly affected by their material underpinnings. (alcohol…) Furthermore, minds
evolved in a seemingly continuous way from matter that seems mindless. Is there a way
to get around the apparent dualism? Can mind affect matter and 
vice versa
?  If so, in
what way are they distinct? Why not just describe mind as a particular organization of
matter?
Berkeley says everything is mind.  (no matter)
But he then has to add assumptions about the universal mind that, in effect, are
equivalent to assuming a material world.
 Russell says everything is matter. (never mind)
How does one test this claim?
defines matter as "that which obeys the laws of physics” Which laws?
We still don't have all the laws of physics! If we say that the laws of physics are the
things needed to describe how everything behaves, isn’t our argument circular?
 
Materialism and Mentalism
:
 
The crude form of materialism
: All things are made of solid constituents.  Each
constituent is described by a set of numbers (
e.g.
, position).  These are the primary
properties.  Events are relations between things.  Secondary properties are the
large-scale descriptions of collections of and interactions between the primaries.
The sophisticated form
: Russell: That which obeys the laws of physics. But since
those laws aren't fully set, matter becomes anything that fits into some coherent
laws. So the question of whether everything is "matter' ceases to be a question
about what everything is "made of" but rather a question about what types of laws
are universally obeyed by phenomena.
Implicitly, the materialist view is that the deepest laws will continue to be of a
mathematical form, i.e. in the broad class described by Galileo, and will not
revert to the more directly value-laden Aristotelian form.
A real question (non-semantic): Is there a special set of laws needed to describe
mind, or are all mental processes outcomes of the same laws that affect other
matter?
Obviously, there are mental phenomena which are easiest to describe using
special constructs, but the question is whether these are complicated
outgrowths
 of the ordinary physical laws or 
violations
 of them.
 
Does the Earth Move?
 
The Problem:
Maxwell’s equations describing electrodynamics violate Galilean invariance.
They contain an absolute (not relative) speed.
Consider the wave equation, which describes
the motion of radio and light waves
(
E 
 is the electric field):
 
That 
c
 is the speed at which the waves move, and its presence in the equation
is a 
not good for Galilean relativity
, even though the equation is pretty.
Galilean relativity says that if one person observes an object to have a
certain velocity another person (who is moving) will observe a different
velocity.
Maxwell’s equations don’t seem to accommodate this behavior for light,
because the equation doesn’t say to use a different 
c
 for different
observers.
How does one try to solve the problem?
If one is a late 19th century physicist, the natural guess is that 
c 
 is the speed
relative to the ether
, NOT to any old observer.
 
 
Why look for the ether?
 
By analogy with the behavior of other waves (
e.g.
, sound and water waves) it was
natural to expect light waves (“light” means any electromagnetic wave) to be
carried by a medium.  The ether might transmit other long distance effects, such as
gravity, as well.
It offered the possibility of a resolution of the Newton-Leibniz (
i.e.
, the
substantivalist-relationist) debate.  If the ether exists, then it (i.e. relations among
its parts) becomes a candidate for Newton’s absolute space. 
(Sklar, 
Space, Time, &
Spacetime
, p. 196)
If the ether takes over the role of absolute space, there is now just one reference
frame in which you can use the simple laws of physics (Maxwell's equations.)
Galilean relativity would be out the window.
A careful experiment could either verify or falsify Maxwell's equations in the
observer's frame, and thus say in a meaningful way if that frame is moving.
 
How to look for the ether
 
If you are moving through a medium, the observed speed of the
wave will vary with direction.
The apparent direction of a source will vary with the observer’s
velocity (aberration, see Rohrlich, p. 53).
These effects are not large.  The largest speed you have easy access
to is the speed of the Earth in its orbit, about 30 km/s, which is about
10
-4 
c. (rotating around some unknown average velocity)
 
Searches for the ether:
Aberration
 
If the telescope (mounted on the earth) moves through the ether, you have to tilt
the scope a little so that the rear end is in the right place when the light gets to it.
As the Earth goes around the sun, the apparent direction of a star changes by ±0.3
minutes of arc.  This is only x10 smaller than Tycho could see by eye, and is easily
measured with a telescope.
Conclusion:  The Earth changes its motion through the ether periodically, just as it's
supposed to if it orbits a Sun which is not accelerating.
(Proof of Copernicus’ theory?) (Proof of ether idea?)
But since we don't independently know which is the "true" position of the stars, we
don't know when our telescope is pointed straight at the stars and when it's tilted.
We've 
measured
 the Earth's velocity
 changes
, i.e. 
acceleration
, but 
not its velocity
relative to the ether.
 
 
Aberration had been
seen in 1674,
(Hooke), described
in 1728 by Bradley
 
More key searches for the ether
 
Ether drag.  
It was known that light moves more slowly through materials which
have an index of refraction.  So, filling a telescope with water (Fresnel) should
have a calculable effect on aberration. (You would have to tilt the telescope a
little more, to allow for the longer time-of-flight.)
It didn’t.
Conclusion:  The ether is partially dragged along with the moving material:
 
 
 
where c' is speed of light in the matter.
 
More key searches for the ether
At the detector, there are interference stripes between light that went around
clockwise and counterclockwise. The position of the stripes is a very sensitive
function of the time difference between those two trips, which go in opposite
directions through the moving rod.
The observation is that the fringes DO NOT SHIFT regardless of the Earth's
motion. That again requires that the 
ether
 be 
partially dragged 
along with the
glass, by the same amount that Fresnel claimed.
 
 
Hoek's
Experiment
(1868)
More key searches for the ether
 
 
Fizeau's experimen
t was like Hoek's, except that the rod was
replaced with a tube containing water. When the water was
flowing
, the fringes 
did
 shift, in the amount predicted by the
partial-ether-drag picture
But we still haven't managed to measure the Earth's speed- we
just measured the 
change in velocity of the water (relative to
apparatus)
 when it is flowing, but that's nothing new!
 
The Searches don’t work
 
Something is frustrating: we have all sorts of experiments that fit a theory that
says that Maxwell's equations only work in a special frame- but somehow we
can't quite measure our motion with respect to that frame, can’t even tell if
we’re at rest in that frame.
Even worse, there was a major paradox
:
Because the index of refraction of the water (or glass) 
varies with color
,
the speed of light in these materials varies with the color, so the inferred
speed of the ether depended on the color of the light.
How could this be? Are there a whole collection of different ethers, for
the uncountably infinite possible frequencies of light?
 
Lorentz fixes a lot
 
H. A. Lorentz
   resolved all but one of the problems above.
if 
the ether were entirely stationary
, the propagation of light would still be
affected by the motion of the electrons with which it interacts in the material
it's travelling through.
He 
derived from Maxwell's equations
 how big that effect would be.  It gave
exactly the Fresnel effect, and thus explained ALL of the experiments above.
The medium in which light propagates is actually
(ether + electrons, etc.) so "partial ether drag" becomes just
"
fixed ether + moving electrons
."
All those experimental results follow naturally from Maxwell's
electromagnetism, plus the quantitative theory of how light interacts with
the electrons in materials.
We don't need separate ethers for each color, just need the simple fact that the
electrons scatter an amount of light that depends on color.
 
But are we moving?
 
So none of these experiments have done anything to measure our absolute motion
through the ether,
although aberration at least seems to have shown 
changes
 in that motion.
Why is it so hard to think of an experiment to measure that absolute motion?
Is there a serious experiment to measure the absolute motion of the Earth?
We need light just propagating in a vacuum, not any of these messy
complications due to interactions with moving media. And we need a round-
trip, so that we can compare timing of two signals at the same place.
 
Michelson-Morley
 
So the time taken on the two round-trip paths is different, depending on which
way lines up with the motion through the ether. The fractional change in the
time is about v
2
/c
2
, or about 10
-8
 for the earth's orbital speed.  But that's
comparable to one wavelength of light, if the path L is a few tens of meters.
Velocity of apparatus wrt ether
 
 
 
 
Light paths
in Ether Frame
 
Light paths
in Lab Frame
 
vt
 
M-M
results
 
If the apparatus is moving through the ether, the interference pattern will shift left
or right.  This is a very sensitive method, since the wavelength of light is 5*10
-7
 m.
The experiment was supposed to be sensitive enough to detect even the rotation
of the Earth (300 m/s) as well as the orbital motion.  
It didn't.
Possible explanations:
Complete ether drag
: local ether is always at rest with respect to local  matter
(
incompatible with aberration
).
Speed of light is determined by the source.
Ruled out by using the Sun as the interferometer’s light source.
The apparatus shrinks in one direction as it moves through the ether.
 
Lorentz-Fitzgerald Contraction
 (
1892)
 
In order for the third explanation to work, the contraction must exactly cancel the
expected effect.  In technical terms, “a conspiracy.”  The size of the effect is tiny:
For v = 30 km/s the factor is ~0.999999995 (
i.e.
, 1 – 5*10
-9
).
Did this make sense?  Maybe, because materials are held together by chemical
(electrical) forces, so the same thing that affects light might affect materials as well.
However, there was no quantitative theory that predicted the contraction.
To maintain a consistent picture, clocks which are moving through the ether must
also run slowly by the same factor (time dilation). Lorentz also found that it seemed
necessary for masses to change as they moved through the ether.
These two effects are part of what is called the Lorentz transformation, a set of
rules for how things change in moving reference frames.
 
Warning. 
The   interpretation   of   these   effects   will   soon change .
 
 
Ether effects
 
The ether started out as an almost meaningless hypothesis, "the
stuff in which light is a wave" or "the stuff which is always at
rest in the frame in which Maxwell's equations work". Now to
explain experiments, we find that the ether has all sorts of
effects on things moving through it
Shrinks rods
Slows clocks
Changes masses.
That sounds like a great confirmation of the ether's reality, until
you notice that the net effect of all these changes together is
that:
 
The motion through the ether is undetectable!
 
Maxwell's equations seem to work in ANY inertial frame!
Lorentz griped that nature was conspiring against us.
similar to some other conspiracy theories, in which every absence of a
detectable evidence is taken as proof of how deep the conspiracy goes
 
“Almost” Relativity?
In 1904, Poincaré suggested that it might be 
impossible
 to measure one’s
speed through the ether. He proposed that "A complete conspiracy is itself
a law of nature."  He asked, “What must be true if one’s speed through the
ether is to be unobservable?”  He was able to show that the mass of an
object (the “m” in “momentum = mv”) would increase as an object’s speed
increased.  Also, the speed of light would be the maximum possible speed.
These conclusions may sound familiar to those of you familiar with Special
Relativity.
However, there was still an underlying assumption, left over from the first
impression made by Maxwell's equations, and perhaps from our
Aristotelian instinct,  that one reference frame was "right", however hidden
it might be.
 
will the ether frame reveal itself?
 
“The principle of physical relativity is an experimental fact ... and as such it
is susceptible to constant revision.” “The principle of relativity thus does
not appear to have the rigorous validity which one was tempted to
attribute to it.”
Poincaré,quoted by Holton, p. 205.
The situation was unsatisfying from a philosophical point of view to
Lorentz and Poincaré  and others:
“...surely this course of inventing special hypotheses for each new experimental
result is somewhat artificial.  It would be more satisfactory if it were possible to
show by means of certain fundamental assumptions ...”
H.A. Lorentz, quoted by Holton, 
Thematic Origins of Scientific Thought
, p.229.
Maybe the principle of relativity should be taken as a postulate, not just
a contingent fact. (At least tentatively.)
Will that give us anything beyond the description of the phenomena in terms of
Lorentz contraction, time dilation, etc.?
And what will become of the stellar aberration observations, which seemed to
show a direct ether effect?
 
Stellar Aberration Revisited
 
Although none of the experiments managed to find an absolute velocity, remember that
stellar aberration found evidence of CHANGES in our velocity "with respect to the ether."
We can't just abandon the ether hypothesis unless we can still account for stellar aberration.
Relativistic interpretation
: treat the Earth as stationary NOW.  In this frame, the star is
moving. The star's velocity is opposite to the conventional velocity of the Earth in a Sun
frame. You have to point your telescope back toward where the star was when it emitted the
light. That requires a tilt of v/c.
 
 
 
 
 
In 6 months, you use a different frame (the Earth has changed its motion) and in the new frame,
the star is moving the other way, so you point your scope back the other way.
We assume that the acceleration is small enough that it has no 
direct 
effects.
The net observed effect is identical to the "stationary ether" prediction
-
but without the hypothesis of a particular special reference frame!
 
 
Einstein's approach
 
Initially motivated by Mach’s conception of a completely relationist
universe. (For Mach, even acceleration was relative.)  Precursors to special
relativity retained the notion of absolute space in the guise of the ether. For
Poincaré the ether had become as undetectable as Newton’s absolute
space. This was unsatisfying to Einstein.
Why does one use one equation to describe a conductor moving past a
magnet and another to describe a magnet moving past a conductor?
“It is known that Maxwell’s electrodynamics – as usually understood at the
present time – when applied to moving bodies, leads to asymmetries which do
not appear to be inherent in the phenomena.” Einstein, “On the
Electrodynamics of Moving Bodies, 
Annalen der Physik
, 
17
 (1905).
Einstein:
"The phenomena of electrodynamics as well as of mechanics possess no
properties corresponding to the idea of absolute rest.  The same laws ... will be
valid for all frames of reference."
That postulate (relativity) sounds familiar, but how can we combine it with
Maxwell's equations?
 
Einstein’s two Postulates
 
1.  “If, relative to K, K’ is a uniformly moving coordinate system devoid of rotation, then
natural phenomena run their course with respect to K’ according to the same general
laws as with respect to K.  This statement is called the 
principle of relativity
 (in the
restricted sense).”  Einstein, 
Relativity
, p. 16.
This principle applies to 
all
 phenomena (including electrical and optical), not merely
mechanical.
 
So, what to do about Maxwell’s equations?
We accept them:
 
2.
 
“... experience in this domain leads conclusively to a theory of
electromagnetic phenomena, of which the law of constancy of the velocity of light in
vacuo is a necessary consequence.” p. 23.
The insistence that these two "apparently incompatible" principles are consistent is
the new idea.
Let's see more carefully why these two ideas seem inconsistent- the worst thing
you can do is to simply accept them without drawing the necessary consequences.
NEXT TIME
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Maxwell's equations in 19th-century physics led to a quest for the elusive ether, a hypothetical medium that could resolve issues of relativity and explain light waves. Physicists sought to detect the ether through experiments like aberration, attempting to observe changes in light direction due to Earth's motion. The existence of ether would challenge Galilean relativity and introduce a fixed reference frame for physics laws. This search for ether provides insights into the historical evolution of physics theories and experimental practices.

  • Physics
  • Ether
  • Relativity
  • Maxwells Equations
  • Experimental Physics

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  1. Search for the ether HW3 Thursday. Feb 12th. Quiz next Thursday, Feb 20th on relativity: Short answers. 20 mins. Closed book. Materials from Rohrlich pgs.34-88, Cushing Chpts. 13,16-17, Sklar pgs. 25-40 & class notes.

  2. Does the Earth Move? The Problem: Maxwell s equations describing electrodynamics violate Galilean invariance. They contain an absolute (not relative) speed. Consider the wave equation, which describes the motion of radio and light waves (E is the electric field): 2E x2= 2E t2 1 c2 That c is the speed at which the waves move, and its presence in the equation is a not good for Galilean relativity, even though the equation is pretty. Galilean relativity says that if one person observes an object to have a certain velocity another person (who is moving) will observe a different velocity. Maxwell s equations don t seem to accommodate this behavior for light, because the equation doesn t say to use a different c for different observers. How does one try to solve the problem? If one is a late 19th century physicist, the natural guess is that c is the speed relative to the ether, NOT to any old observer.

  3. Why look for the ether? By analogy with the behavior of other waves (e.g., sound and water waves) it was natural to expect light waves ( light means any electromagnetic wave) to be carried by a medium. The ether might transmit other long distance effects, such as gravity, as well. It offered the possibility of a resolution of the Newton-Leibniz (i.e., the substantivalist-relationist) debate. If the ether exists, then it (i.e. relations among its parts) becomes a candidate for Newton s absolute space. (Sklar, Space, Time, & Spacetime, p. 196) If the ether takes over the role of absolute space, there is now just one reference frame in which you can use the simple laws of physics (Maxwell's equations.) Galilean relativity would be out the window. A careful experiment could either verify or falsify Maxwell's equations in the observer's frame, and thus say in a meaningful way if that frame is moving.

  4. How to look for the ether If you are moving through a medium, the observed speed of the wave will vary with direction. The apparent direction of a source will vary with the observer s velocity (aberration, see Rohrlich, p. 53). These effects are not large. The largest speed you have easy access to is the speed of the Earth in its orbit, about 30 km/s, which is about 10-4 c. (rotating around some unknown average velocity)

  5. Searches for the ether: Aberration Rotation is required to let the eyepiece move into the path of the light. v Aberration had been seen in 1674, (Hooke), described in 1728 by Bradley If the telescope (mounted on the earth) moves through the ether, you have to tilt the scope a little so that the rear end is in the right place when the light gets to it. As the Earth goes around the sun, the apparent direction of a star changes by 0.3 minutes of arc. This is only x10 smaller than Tycho could see by eye, and is easily measured with a telescope. Conclusion: The Earth changes its motion through the ether periodically, just as it's supposed to if it orbits a Sun which is not accelerating. (Proof of Copernicus theory?) (Proof of ether idea?) But since we don't independently know which is the "true" position of the stars, we don't know when our telescope is pointed straight at the stars and when it's tilted. We've measured the Earth's velocity changes, i.e. acceleration, but not its velocity relative to the ether.

  6. More key searches for the ether Ether drag. It was known that light moves more slowly through materials which have an index of refraction. So, filling a telescope with water (Fresnel) should have a calculable effect on aberration. (You would have to tilt the telescope a little more, to allow for the longer time-of-flight.) It didn t. Conclusion: The ether is partially dragged along with the moving material: vether=vmatter(1 c'2 c2) where c' is speed of light in the matter.

  7. More key searches for the ether mirrors Hoek's Experiment (1868) Glass rod Light source Half- mirror Velocity of apparatus relative to ether detector At the detector, there are interference stripes between light that went around clockwise and counterclockwise. The position of the stripes is a very sensitive function of the time difference between those two trips, which go in opposite directions through the moving rod. The observation is that the fringes DO NOT SHIFT regardless of the Earth's motion. That again requires that the ether be partially dragged along with the glass, by the same amount that Fresnel claimed.

  8. More key searches for the ether Fizeau's experiment was like Hoek's, except that the rod was replaced with a tube containing water. When the water was flowing, the fringes did shift, in the amount predicted by the partial-ether-drag picture But we still haven't managed to measure the Earth's speed- we just measured the change in velocity of the water (relative to apparatus) when it is flowing, but that's nothing new!

  9. The Searches dont work Something is frustrating: we have all sorts of experiments that fit a theory that says that Maxwell's equations only work in a special frame- but somehow we can't quite measure our motion with respect to that frame, can t even tell if we re at rest in that frame. Even worse, there was a major paradox: Because the index of refraction of the water (or glass) varies with color, the speed of light in these materials varies with the color, so the inferred speed of the ether depended on the color of the light. How could this be? Are there a whole collection of different ethers, for the uncountably infinite possible frequencies of light?

  10. Lorentz fixes a lot H. A. Lorentz resolved all but one of the problems above. if the ether were entirely stationary, the propagation of light would still be affected by the motion of the electrons with which it interacts in the material it's travelling through. He derived from Maxwell's equations how big that effect would be. It gave exactly the Fresnel effect, and thus explained ALL of the experiments above. The medium in which light propagates is actually (ether + electrons, etc.) so "partial ether drag" becomes just "fixed ether + moving electrons." All those experimental results follow naturally from Maxwell's electromagnetism, plus the quantitative theory of how light interacts with the electrons in materials. We don't need separate ethers for each color, just need the simple fact that the electrons scatter an amount of light that depends on color.

  11. But are we moving? So none of these experiments have done anything to measure our absolute motion through the ether, although aberration at least seems to have shown changes in that motion. Why is it so hard to think of an experiment to measure that absolute motion? Is there a serious experiment to measure the absolute motion of the Earth? We need light just propagating in a vacuum, not any of these messy complications due to interactions with moving media. And we need a round- trip, so that we can compare timing of two signals at the same place.

  12. Michelson-Morley Velocity of apparatus wrt ether Light paths in Ether Frame 2 2 2 2 2 = + c t v t L Light paths in Lab Frame 1 L = t vt c 2 v 1 ( ) 2 c L L = + = t + ( 2 ) ( 2 ) c v c v 1 L 2 c v 1 2 c So the time taken on the two round-trip paths is different, depending on which way lines up with the motion through the ether. The fractional change in the time is about v2/c2, or about 10-8 for the earth's orbital speed. But that's comparable to one wavelength of light, if the path L is a few tens of meters.

  13. M-M results If the apparatus is moving through the ether, the interference pattern will shift left or right. This is a very sensitive method, since the wavelength of light is 5*10-7 m. The experiment was supposed to be sensitive enough to detect even the rotation of the Earth (300 m/s) as well as the orbital motion. It didn't. Possible explanations: Complete ether drag: local ether is always at rest with respect to local matter (incompatible with aberration). Speed of light is determined by the source. Ruled out by using the Sun as the interferometer s light source. The apparatus shrinks in one direction as it moves through the ether.

  14. Lorentz-Fitzgerald Contraction (1892) In order for the third explanation to work, the contraction must exactly cancel the expected effect. In technical terms, a conspiracy. The size of the effect is tiny: c() 2 v L L 1 For v = 30 km/s the factor is ~0.999999995 (i.e., 1 5*10-9). Did this make sense? Maybe, because materials are held together by chemical (electrical) forces, so the same thing that affects light might affect materials as well. However, there was no quantitative theory that predicted the contraction. To maintain a consistent picture, clocks which are moving through the ether must also run slowly by the same factor (time dilation). Lorentz also found that it seemed necessary for masses to change as they moved through the ether. These two effects are part of what is called the Lorentz transformation, a set of rules for how things change in moving reference frames. Warning. The interpretation of these effects will soon change .

  15. Ether effects The ether started out as an almost meaningless hypothesis, "the stuff in which light is a wave" or "the stuff which is always at rest in the frame in which Maxwell's equations work". Now to explain experiments, we find that the ether has all sorts of effects on things moving through it Shrinks rods Slows clocks Changes masses. That sounds like a great confirmation of the ether's reality, until you notice that the net effect of all these changes together is that:

  16. The motion through the ether is undetectable! Maxwell's equations seem to work in ANY inertial frame! Lorentz griped that nature was conspiring against us. similar to some other conspiracy theories, in which every absence of a detectable evidence is taken as proof of how deep the conspiracy goes Almost Relativity? In 1904, Poincar suggested that it might be impossible to measure one s speed through the ether. He proposed that "A complete conspiracy is itself a law of nature." He asked, What must be true if one s speed through the ether is to be unobservable? He was able to show that the mass of an object (the m in momentum = mv ) would increase as an object s speed increased. Also, the speed of light would be the maximum possible speed. These conclusions may sound familiar to those of you familiar with Special Relativity. However, there was still an underlying assumption, left over from the first impression made by Maxwell's equations, and perhaps from our Aristotelian instinct, that one reference frame was "right", however hidden it might be.

  17. will the ether frame reveal itself? The principle of physical relativity is an experimental fact ... and as such it is susceptible to constant revision. The principle of relativity thus does not appear to have the rigorous validity which one was tempted to attribute to it. Poincar ,quoted by Holton, p. 205. The situation was unsatisfying from a philosophical point of view to Lorentz and Poincar and others: ...surely this course of inventing special hypotheses for each new experimental result is somewhat artificial. It would be more satisfactory if it were possible to show by means of certain fundamental assumptions ... H.A. Lorentz, quoted by Holton, Thematic Origins of Scientific Thought, p.229. Maybe the principle of relativity should be taken as a postulate, not just a contingent fact. (At least tentatively.) Will that give us anything beyond the description of the phenomena in terms of Lorentz contraction, time dilation, etc.? And what will become of the stellar aberration observations, which seemed to show a direct ether effect?

  18. Stellar Aberration Revisited Although none of the experiments managed to find an absolute velocity, remember that stellar aberration found evidence of CHANGES in our velocity "with respect to the ether." We can't just abandon the ether hypothesis unless we can still account for stellar aberration. Relativistic interpretation: treat the Earth as stationary NOW. In this frame, the star is moving. The star's velocity is opposite to the conventional velocity of the Earth in a Sun frame. You have to point your telescope back toward where the star was when it emitted the light. That requires a tilt of v/c. NOW THEN THEN NOW In 6 months, you use a different frame (the Earth has changed its motion) and in the new frame, the star is moving the other way, so you point your scope back the other way. We assume that the acceleration is small enough that it has no direct effects. The net observed effect is identical to the "stationary ether" prediction- but without the hypothesis of a particular special reference frame!

  19. Einstein's approach Initially motivated by Mach s conception of a completely relationist universe. (For Mach, even acceleration was relative.) Precursors to special relativity retained the notion of absolute space in the guise of the ether. For Poincar the ether had become as undetectable as Newton s absolute space. This was unsatisfying to Einstein. Why does one use one equation to describe a conductor moving past a magnet and another to describe a magnet moving past a conductor? It is known that Maxwell s electrodynamics as usually understood at the present time when applied to moving bodies, leads to asymmetries which do not appear to be inherent in the phenomena. Einstein, On the Electrodynamics of Moving Bodies, Annalen der Physik, 17 (1905). Einstein: "The phenomena of electrodynamics as well as of mechanics possess no properties corresponding to the idea of absolute rest. The same laws ... will be valid for all frames of reference." That postulate (relativity) sounds familiar, but how can we combine it with Maxwell's equations?

  20. Einsteins two Postulates 1. If, relative to K, K is a uniformly moving coordinate system devoid of rotation, then natural phenomena run their course with respect to K according to the same general laws as with respect to K. This statement is called the principle of relativity (in the restricted sense). Einstein, Relativity, p. 16. This principle applies to all phenomena (including electrical and optical), not merely mechanical. So, what to do about Maxwell s equations? We accept them: 2. electromagnetic phenomena, of which the law of constancy of the velocity of light in vacuo is a necessary consequence. p. 23. The insistence that these two "apparently incompatible" principles are consistent is the new idea. ... experience in this domain leads conclusively to a theory of Let's see more carefully why these two ideas seem inconsistent- the worst thing you can do is to simply accept them without drawing the necessary consequences. NEXT TIME

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