Search for the ether HW3 - PowerPoint Presentation

Slide1

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.

Slide2

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?

Slide3

ReductionismThere are many flavors of reductionism, and failure to distinguish among them leads to many arguments at cross-purposes.

Very strong reductionism: All phenomena will be understood as consequences of a small set of fundamental laws: a genuine theory of everything.

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?

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.

Slide4

Explicit PhilosophyModern 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.

Slide5

Views of Reality: a spectrumThe 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.

The ontological spectrum

Idealism

Realism

Berkeley Mach Plato Planck Einstein

MIS

postmodernists? Hume

Slide6

PlatoThe 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

Slide7

Views of Reality: a spectrumBerkeley (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?

Slide8

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

Slide9

Some ideas people argue aboutfortunately, 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?

Slide10

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.

Slide11

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.

Slide12

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.

Slide13

How to look for the etherIf 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)

Slide14

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

Slide15

More key searches for the etherEther 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.

Slide16

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)

Glass rod

Light

source

Half-mirror

detector

Velocity of apparatus

relative to ether

mirrors

Slide17

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!

Slide18

The Searches don’t workSomething 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?

Slide19

Lorentz fixes a lotH. 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.

Slide20

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.

Slide21

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

Slide22

M-M resultsIf 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.

Slide23

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 .

Slide24

Ether effectsThe 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:

Slide25

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.

Slide26

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?

Slide27

Stellar Aberration RevisitedAlthough 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!

NOW

NOW

THEN

THEN

Slide28

Einstein's approachInitially 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?

Slide29

Einstein’s two Postulates1. “

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