The Search for Ether in Physics

Search for the ether

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HW3 Thursday. Feb 12

th

.

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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)

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Are all causal events reducible to elementary mechanical

causality?

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What about the standard "ranking"

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Highest level

Psychology

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Biology

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Chemistry

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Deepest level

Physics

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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.

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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

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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.

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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

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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?

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Traditional answer to Berkeley:

–

If a stone isn't real, go ahead and kick it.

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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?

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“

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!

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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?

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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.

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 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

:

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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.

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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.

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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

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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

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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

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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.

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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)

•

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

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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.

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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.

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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