Lord Kelvin vs. the Aether! (1901)

The more I study the history of aether physics, the more I feel that modern physicists underappreciate both the huge influence the theory had on the development of physics and how it indirectly spurred many positive scientific discoveries, even though it is an incorrect theory. The “aether”, for those not familiar with it, was a hypothetical substance theorized in the early 1800s to be the medium in which light waves propagate, just as water waves travel through water and sound waves travel through air.  Many papers were written speculating on the nature of the aether before Einstein’s special theory of relativity (1905) argued convincingly that the aether was unnecessary.

Nevertheless, these speculations resulted in a number of interesting results.  For instance, we have noted previously that Earnshaw’s theorem (1839), an important result in electromagnetic theory, arose from an attempt to determine the forces that hold the aether together.  In 1902, Lord Rayleigh attempted to detect the aether-induced “length contraction” by measuring the birefringence of moving objects, an ingenious attempt that gave a negative result.

In the broadest sense, a “good” theory is one which raises interesting questions that may inevitably be tested by experiment.  Even if it proves to be fundamentally incorrect in the end, it has spurred numerous theoretical and experimental results.  This can be contrasted with sham “theories” such as intelligent design (the “theory” that living creatures are too complex to have developed without the aid of a creator), which has resulted in no testable predictions and exists only as a way to push religion into the classroom.

By 1900, the aether remained a vexing mystery, and perhaps the foremost scientific problem, for the physicists of the era.  It is not surprising that many famous scientists expended considerable energy to try and elucidate its properties.  In 1901, a paper appeared in the Philosophical Magazine (Ser. 6, vol. 2, 161-177) by the famous (even infamous) Lord Kelvin, entitled, “On Ether and Gravitational Matter through Infinite Space.”  It is not, in fact, an original publication; as Kelvin puts it,

This is an amplification of Lecture XVI, Baltimore, Oct. 15, 1884, now being prepared for print in a volume on Molecular Dynamics and the Wave Theory of Light, which I hope may be published within a year from the present time.

In fact, the article begins with a reprinting of material from 1854, nearly fifty years old!  This is, if nothing else, a measure of how baffling the aether was to physicists of the time — material fifty years old was still, in some sense, “state of the art”.

The 1901 paper, as a whole, summarizes Kelvin’s theoretical musings on the nature of the aether, and highlights how perplexing the topic remained before Einstein’s wonderful theory came along and shattered the aether hypothesis once and for all.

William Thomson, aka Lord Kelvin (1824-1907) is one of those curious physicists whose name is everywhere, but whose exact achievements in science are hard to pin down.  The reality is that his influence can be found in almost every aspect of 19th century physics, and often made very subtle but fundamental contributions to the foundations and methodology of physics.  An excellent biography of Thomson and his work can be found at PhysicsWorld, though it requires a (free) registration to read.

He is perhaps most known for his contributions to thermodynamics.  When Thomson approached the subject, most physicists believed that heat was a physical substance, dubbed “caloric”.  James Joule, another great of the era, was a lonely champion of the idea that heat is the result of the motion and vibrations of atoms and molecules, and the inevitable conclusion that there exists an absolute minimum of temperature (“absolute zero”).   His work was mostly ignored by the community until Thomson heard one of his talks in 1947.  Here I quote the PhysicsWorld article,

But Joule was not wrong, and Thomson – through careful thought – came to agree with him. Along the way, he connected Joule’s work with that of Carnot on heat engines. In doing so, he devised a more fundamental way of defining the absolute zero of temperature, independent of any particular material substance. It is for this reason that the fundamental unit of temperature was later called the Kelvin – the name Thomson adopted after being made a Lord in 1892. Thomson also saw the idea of conservation of energy as a great unifying principle in science, and introduced the ideas of “statical” and “dynamical” energy – or what we now call potential and kinetic energy.

It is difficult to disentangle Thomson’s work on heat and the conservation of energy from that of other scientists of the time, including Clausius, Helmholtz, Joule, Liebig and Rankine. All of them can take some of the credit for the first and second laws of thermodynamics – ideas that are so important to modern science that each contributor should be held in high regard.

Emphasis mine — Thomson played a large role in establishing the formulation of physics in terms of energy!  This gives some idea of what I mean when I say that Thomson made subtle but fundamental contributions.  He did not invent the idea of conservation of energy, but was instrumental in shaping its use and emphasizing its importance in all physical problems.

There are numerous similar examples of Thomson’s influence.  Other examples include Thomson’s championing of the use of Fourier analysis to solve problems amongst British scientists, and his pivotal role in establishing a standardized set of electrical units (I’ll have much more to say about this latter role in a future post, after I’ve finished reading about 1000 pages of historical documents).  The PhysicsWorld article also suggests that it was Thomson, in correspondence with Stokes, who actually first stated the fundamental result of vector calculus known as Stokes’ theorem.  He even proposed one of the first unified theories of atomic structure, proposing that atoms are in essence swirling vortices in the aether (I’ll have to post about this again in the near future).

In the 1850s, Thomson joined with a company attempting to lay the first trans-Atlantic cable from England to North America, and became personally involved in the practical details, spending much time at sea.  The attempt succeeded in 1866, an achievement that earned Thomson a knighthood.  In 1892, due to his achievements in thermodynamics, Thomson was elevated to nobility, becoming the 1st Baron Kelvin (also known simply as “Lord Kelvin”).  In light of his many achievements — and there are many I have not covered — it is perhaps not surprising to find a website dedicated to the worship of the man with the title, “Kelvin is Lord!”  We will refer to Thomson as Lord Kelvin for the rest of this post.

While Kelvin is famous for his many achievements in physics and engineering, he is also infamous for his extreme self-confidence, bordering on if not happily crossing into arrogance.  He is well-known for making plenty of broad statements that later turned out to be untrue; for instance, from Eric Weisstein’s World of Biography,

Another example of his hubris is provided by his 1895 statement “heavier-than-air flying machines are impossible” (Australian Institute of Physics), followed by his 1896 statement, “I have not the smallest molecule of faith in aerial navigation other than ballooning…I would not care to be a member of the Aeronautical Society.”

Kelvin is most infamous for his 1862 estimate that the age of the earth is around 100 million years, later revised in 1899 to 20-40 million years; he derived this result by treating the earth as an object that is continually cooling from an initial molten state.  This estimate brought him into direct conflict with geologists and biologists, as each group required an earth around a hundred times older for its theories to be viable.   Kelvin’s estimates, which were considered quite definitive at the time, haunted Charles Darwin in particular; in a letter to Wallace dated July 12, 1871, he writes,

I feel very doubtful how far I shall succeed in answering Mivart, it is so difficult to answer objections to doubtful points, and make the discussion readable. I shall make only a selection. The worst of it is, that I cannot possibly hunt through all my references for isolated points, it would take me three weeks of intolerably hard work. I wish I had your power of arguing clearly. At present I feel sick of everything, and if I could occupy my time and forget my daily discomforts, or rather miseries, I would never publish another word. But I shall cheer up, I dare say, soon, having only just got over a bad attack. Farewell; God knows why I bother you about myself. I can say nothing more about missing-links than what I have said. I should rely much on pre-silurian times; but then comes Sir W. Thomson like an odious spectre. Farewell.

Emphasis mine!  Kelvin, however, was wrong: he assumed that the earth was continually cooling, but was unaware of the existence of radioactivity, which provides an internal heat source and makes the calculation completely invalid.

Kelvin is also known for allegedly stating in 1900 that, “There is nothing new to be discovered in physics now. All that remains is more and more precise measurement.”  This statement is especially ironic because only five years later Einstein would ignite a new era of physics by introducing the foundations of both quantum mechanics and relativity theory!   According to Wikipedia, though, this statement is always quoted without a primary source, and may be apocryphal; it certainly sounds like something Kelvin would say, however!

This rather lengthy introduction to Kelvin serves to paint a picture of a supremely confident scientist who focused his energies on understanding the foundational principles of physics.  It is not a big stretch to imagine that the mysterious and completely unquantified mechanical aether would irritate and intrigue him.

Let’s turn now to his 1901 paper on the properties of the aether.  Overall, it is a fascinating attempt to deduce properties of the aetherial matter based solely on a general understanding of the properties of wave motion; from there, it wanders off on interesting tangents.  The title of the beginning section reads, “Note on the Possible Density of the Luminiferous Medium, and on the Mechanical Value of a Cubic Mile of Sunlight.”  Here we are immediately led to the following 1892 footnote:

The brain-wasting perversity of the insular inertia which still condemns British Engineers to reckonings of miles and yards and feet and inches and grains and pounds and ounces and acres is curiously illustrated by the title and numerical results of this article as originally published.

I have shown this quote to pretty much all of my colleagues, and I never get tired of reading it!  As we have noted, Kelvin was an active proponent in the development of a standardized system of units, and it is no surprise that the use of miles in the U.K. would really irritate him.  I can hardly imagine criticizing it in print in the manner that he does, however!  An extra footnote explains that the current version of the article is written with metric units.

Let’s get to the substance of the paper!  Lord Kelvin attempts to determine the density of the aether by broad analogy with other types of waves.  Let’s look briefly at the simplest example possible, namely a monochromatic (single-frequency) wave on an infinite string, traveling to the right:

If we look at a single “atom” of this string (for instance, the leftmost point of the picture), it moves up and down, undergoing simple harmonic motion (SHM), like a pendulum clock.  It can also be seen from the animation that the “atom” has its top speed v when it passes through the zero of wave amplitude.   From the mathematics of SHM, the energy E of such a vibrating atom of mass m is given by

\displaystyle E=\frac{1}{2}mv^2.

The energy of the atom is constant as it moves; at the extreme ends of its motion, the energy is purely potential energy, while at the zero of amplitude the energy is purely kinetic.

Dealing with waves on a string, it is more natural to talk about the mass of the string per unit length \lambda; the energy per unit length \epsilon of the string is then

\displaystyle \epsilon = \frac{1}{2} \lambda v^2.

The energy contained in a length of string L is then E = \epsilon L.

In Lord Kelvin’s time, the aether was assumed to be a material medium whose vibrations corresponded to observed light waves.  Imagining a monochromatic light wave passing through the aether with mass per unit volume (density) \rho, he reasoned that the energy per unit volume would be of the form

\displaystyle \epsilon = \frac{1}{2} \rho v^2,

where v is the maximum velocity attained by the aetherial matter during its oscillations.

The amount of solar energy falling on the earth’s surface was already known from experiment, which meant that one could readily determine \epsilon.  If one could estimate the maximum velocity v of the aetherial matter, one could solve the above equation for the density of the aether!

At this point, Lord Kelvin needed to make what amounts to an educated guess.  From the theory of SHM, the following relation holds:

\displaystyle \frac{v}{c} = 2\pi \frac{A}{\lambda},

where c is the speed of light, A is the maximum displacement of the vibrating aetherial matter, and \lambda is the wavelength of the light wave.  It seemed reasonable to assume that the maximum displacement A was much smaller than the wavelength, presumably because no effects associated with this displacement had ever been observed; this in turn meant that the maximum velocity was much smaller than the speed of light c, which Kelvin wrote as

\displaystyle v = \frac{1}{n} c,

where n is a large positive number.

One other issue needed to be addressed before an estimate could be made, however; a real light wave is not monochromatic, and consists of many, many different frequencies of oscillation.  Lord Kelvin addressed this as follows:

The mechanical value of the disturbance kept up by a number of coexisting series of waves of different periods, polarized in the same plane, is the sum of the mechanical values due to each homogeneous series separately, and the greatest velocity that can possibly be acquired by any vibrating particle is the sum of the separate velocities due to the different series.  Exactly the same remark applies to coexistent series of circularly polarized waves of different periods.  Hence the mechanical value is certainly less than half the mass multiplied into the square of the greatest velocity acquired by a particle, when the disturbance consists in the superposition of different series of plane polarized waves; and we may conclude, for every kind of radiation or light or heat except a series of homogeneous circularly polarized waves, that the mechanical value of the disturbance kept up in any space is less than the product of the mass into the square of the greatest velocity acquired by a vibrating particle in the varying phases of its motion.  How much less in such a complex radiation as that of sunlight and heat we cannot tell, because we do not know how much the velocity of a particle may mount up, perhaps even to a considerable value in comparison with the velocity of propagation, at some instant by the superposition of different motions chancing to agree; but we may be sure that the product of the mass into the square an ordinary maximum velocity, or of the mean of a great many successive maximum velocities of a vibrating particle, cannot exceed in any great ratio the true mechanical value of the disturbance.

In effect, this long passage amounts to dropping the factor of “1/2” from the expression for the energy!  Because the velocity itself is being only crudely estimated, this extra factor contributes little to the estimate.  The only real concern addressed by the above passage is that the energy cannot be greater than mv^2.

We can now calculate Lord Kelvin’s numerical estimate for the density of the aether.  We begin with the tabulated value of his time for the mechanical value of the energy in a cubic kilometer of sunlight near the earth’s surface, 412 meter-kilograms.  The units here require some explanation; the “mechanical value” refers to the energy divided by the gravitational constant g = 9.81 \, \mbox{m}/\mbox{s}^2.  This convention evidently arises from the view of Kelvin’s time that all physical laws arose from mechanical processes, and also from the expression of energy in British units as “foot-pounds”; in the British system, a “pound” represents a force as well as a mass.  Lord Kelvin evidently thought that the maximum velocity of the aetherial material would occur near the surface of the sun, so he converted this number to the mechanical value near the sun’s surface:

The mechanical value of sunlight in any space near the sun’s surface must be greater than in an equal space at the earth’s distance, in the ratio of the square of the earth’s distance to the square of the sun’s radius, that is, in the ratio of 46,000 to 1 nearly.

This gives us the mechanical value of a cubic kilometer of sunlight near the surface of the sun as 1.9\times 10^7 m-kg.

If we write M as the total mass of the aether in a cubic kilometer of sunlight, we then have the expression,

\displaystyle (1.9\times 10^7\mbox{ m-kg}) = \frac{M}{g}v^2=\frac{M}{g} \frac{c^2}{n^2}.

Solving for M, we have

\displaystyle M = \frac{(1.9\times 10^7\mbox{ m-kg})g}{c^2} n^2=2\times 10^{-9}\mbox{ kg }n^2.

This is an estimate of the mass of the aether in a cubic kilometer of space.  Lord Kelvin suggested n = 50, which sets the mass of the aether in a cubic kilometer as

\displaystyle M \approx 5\times 10^{-6}\mbox{ kg}.

It should be appreciated that this is an incredibly small number!  To put it in perspective, we note that it follows that the hypothetical mass of the aether contained in a cubic centimeter of space is

\displaystyle m \approx 5\times 10^{-21} \mbox{ kg}.

The mass of a proton is 1.67\times 10^{-27}\mbox{ kg}.  This means that the mass of a cubic millimeter of aether is comparable to the mass of only a million protons.  By comparison, a mole of hydrogen gas, 6.022 \times 10^{23} protons, has a volume of roughly 14.4 cubic centimeters.

By Lord Kelvin’s estimate, the hypothetical aether is orders of magnitude less dense than hydrogen gas, which is a real problem because the aether was also assumed to be a solid material; liquids and gases cannot support transverse waves.

Lord Kelvin’s paper continues with other discussions of the aether’s properties, each of which conflicts with the intuition of how a mechanical solid should behave.  He makes an estimate of the rigidity of the aether, and calculates that a tremendous amount of force would be required to displace it even slightly.  He concludes,

We shall find ourselves forced to consider the necessity of some hypothesis for the free motion of ponderable bodies through ether, disturbing it only by condensation and rarefactions, with no incompatibility in respect to joint occupation of the same space by the two substances.

In other words, Lord Kelvin was forced to conclude that ordinary matter can pass through the “solid” aether without disturbing it.  This is quite a strange conclusion to arrive at for a theory that tries to explain electromagnetic waves purely in terms of mechanical interactions!

Going further, Lord Kelvin points out that gravity presents unique problems for the aether.  If the aether is effected by gravity, it must be drawn towards large gravitational bodies, and other aether.  It is not difficult to reach the conclusion that either the aether must be very inhomogeneous, forming “clumps” everywhere just like ordinary matter forms stars and planets, or it must not be affected by gravity!  Because we have just made an estimate of the mass density of the aether, however, this suggests that the aether must have inertial mass but not gravitational mass.  This is again quite a strange conclusion to reach for a theory that tries to explain light by mechanics!

Lord Kelvin’s paper wanders off after this into a discussion of the motion of stars and the relation to gravity.  Kelvin was apparently well-known to diverge on strange tangents during his lectures, a habit that irritated his British audiences but endeared him to his American audiences (according to the PhysicsWorld article, again).  There are some interesting thoughts in the latter part of the paper, which I will endeavor to return to in a future post.

The one intellectual leap that Lord Kelvin fails to make, however, is to doubt the existence of the aether itself.  As we have noted, Kelvin’s conclusions make for a very strange material: it is a solid, but incredibly less dense than hydrogen gas; it has inertial mass, but not gravitational mass; it is a massive solid that does not interact with ordinary matter.  With hindsight, these conclusions scream out against the existence of the aether, and it is telling that such a brilliant scientist such as Lord Kelvin did not realize that something was wrong.  The notion of the aether was so ingrained in the minds of the physicists of the time that they never even considered questioning it.

Kelvin’s musings do illustrate nicely that the evidence was piling up against the aether; in only a few short years, in 1905, Einstein would, in essence, find the smoking gun against it.

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13 Responses to Lord Kelvin vs. the Aether! (1901)

  1. Peter Morgan says:

    This is very nice. Thanks.

    I find the intensity of your “these conclusions scream out against the existence of the aether” somewhat anachronistic. We have become accustomed to empiricism, but at the time of Kelvin that philosophical approach was arguably only beginning to be applied to science. Mach and Duhem, and Poincar\’e’s conventionalism, were coming things, but most (English) physicists constructed models. Maxwell famously tried, of course, to construct vortex models for electromagnetism, models which we now ignore because they go so far beyond the empirically useful mathematics of Maxwell’s equations considered in relative isolation. To a model builder, however, it shouldn’t be important that extremely different properties are needed to get the right behavior to match with experiment. I can imagine Thompson feeling confirmed rather than disheartened that everything about the aether had to be different from ordinary matter to match with experience.

    Once one throws empiricism into the mix, however –and even more if one introduces post-positivism–, we notice that many different models of a theory, or even models of different types of theories, may be able to give as good an account (that is, to use the buzzword, experiment underdetermines models). Insofar as we cannot as empiricists commit ourselves to hypotheses, we abstract from the models to what seems characteristic of all the models we can create, instead of committing to one of the models. Einstein’s construction of SR in terms of empirical principles does not rule out aether models, it only requires that the dynamical equations of any aether model must be Lorentz invariant (which I take to be a retrospective way of putting it), a condition that is satisfied by Maxwell’s equations. But holding Kelvin to this kind of standard, when he can clearly be taken to be the rearguard for 19th century model building, seems not to appreciate his mindset.

    Sorry; this, as a blog comment, is not as coherent as I would like. I’m engaged in what I partially see as a model-building approach (in the modern style of Morgan & Morrison’s “Models as Mediators”, perhaps) to reconsidering quantum field theory, so you hit a few of my concerns. If a class of models that could be called aether models might be made to work (be empirically adequate in some sense), we should not commit to any individual model, but we might commit to whatever abstract properties are shared by all the models we have successfully made work. When we successfully construct two (or more) very different types of models that work, the process of abstraction from the class of models that work becomes much harder to do, until we find a way to construct a mathematical formalism that contains both (or all) the types of models as special cases. But again, it is anachronistic to hold Thompson to this kind of standard unless one insists that he should have read and agreed with Mach and Duhem, say, which, however, is hardly likely given the relatively vague nature of their attempts to formulate an effective empiricism (that would come to a temporary fruition in the positivism of the 1920s-50s, when the post-positivist rot began). [I’ve said it three times. Time to stop.]

  2. Dr. Dave says:

    I would be careful with calling Einstein’s 1095 paper on SR a “smoking gun” against the aether, as the idea died an awfully slow death, and was still being discussed as late as the 1920s. In fact, Einstein himself said as much in an address delivered in 1920…

    “The special theory of relativity forbids us to assume the ether to consist of particles observable through time, but the hypothesis of ether in itself is not in conflict with the special theory of relativity. Only we must be on our guard against ascribing a state of motion to the ether.”

    (Albert Einstein in address delivered on May 5th, 1920, at the University of Leyden)

  3. JuanBobsDad says:

    I must disagree about the attribution that “Einstein’s special theory of relativity (1905) argued convincingly that the aether was unnecessary.” If it did indeed do that it was only because the nail in the aether’s coffin was the Michaelson-Morley experiment.

    • Dr. Dave says:

      @JuanBob… the MM experiment was performed in 1887. People were still writing books about the aether in 1920. It was far from the nail in the coffin. The idea that the MM experiment killed the notion of an aether is an ahistorical, post-hoc interpretation. This is a point I try VERY HARD to make the students in my relativity courses understand, by, for example – having them read the entry on Aether from the 1911 Encyclopedia Britannica. Aether did not die out with either Michelson & Morley OR Einstein!

      • Peter Morgan says:

        Dr. Dave, you might find Harvey Brown’s “Physical Relativity. Space-time structure from a dynamical perspective” (Oxford University Press), 2005, of interest. This was a winner of the Lakatos award in 2006, which is “given annually for an outstanding contribution to the philosophy of science, widely interpreted, in the form of a book published in English during the previous six years” (according to the award web-site at http://www.lse.ac.uk/collections/philosophyLogicAndScientificMethod/lakatos/Default.htm). I understand Harvey to take a somewhat realist view of physical theory, which plays out in his approach to SR and the aether. In general, however, there is a whole class of fine-grained models that would appear to coarse-grained experiments as SR does, so it is difficult to make a strong claim for any particular structure that one might suggest for the aether. The first half-dozen pages of Amazon’s LookInside gives a moderately good idea of his reasoning.

        It’s not clear to me, however, whether you are making a historical distinction mostly as a matter of History of Physics, or also seriously considering the issues as a matter of Philosophy of Physics.

      • Dr. Dave says:

        I’m making a mostly-historical point concerning the common textbook assertion that MichelsonMorley + Einstein = unequivocal abolishment of the very notion of aether, which is certainly not how the process played itself out historically.

      • Dr Phil says:

        ‘Aether did not die out with either Michelson & Morley OR Einstein!’

        Yes, these days we refer to it as ‘dark matter’.

  4. Geez, I go away for a few hours and controversy erupts! 🙂 Of course, this is what I like to see on the blog. A few thoughts:

    Peter wrote: “I find the intensity of your “these conclusions scream out against the existence of the aether” somewhat anachronistic. We have become accustomed to empiricism, but at the time of Kelvin that philosophical approach was arguably only beginning to be applied to science.”

    Well, that’s why I preceded my statement by, “in hindsight”! Hopefully I didn’t come across as critical of Kelvin’s views; I endeavor to avoid judging scientists by modern standards. Regardless of the prevalence (or lack) of empiricism of the time, it is very telling that the increasingly ad hoc aether theory did not make raise more eyebrows than it did.

    Dr. Dave wrote: “I would be careful with calling Einstein’s 1095 paper on SR a “smoking gun” against the aether, as the idea died an awfully slow death, and was still being discussed as late as the 1920s. In fact, Einstein himself said as much in an address delivered in 1920…”

    Of course, the history is a little more complicated than I can put at the end of a blog post — it always is! I’ve pointed out in previous posts that there were many challenges to relativity theory, such as Dayton Miller as late as the 1930s, and even Whittaker’s 1950 dissing of the significance of Einstein’s results.

    As a first approximation, though, I feel pretty comfortable in saying that Einstein’s special relativity really shifted the ‘center of gravity’ away from aether research to relativity research.

    As far as compatibility of aether with relativity, I think I’ve stated better in other posts that relativity didn’t “disprove” the aether as much as it made it completely unnecessary. The biggest “evidence” in favor of the aether, aside from the analogy with other types of waves, was the “aether drag”. Once it became clear that this naturally arose in relativity, there wasn’t really much reason to keep the aether around. Also, the hypothetical aether that Einstein spoke of was a far cry from the mechanical aether of 19th century physics, and essentially incompatible with it. From Einstein’s Leiden lecture:

    Recapitulating, we may say that according to the general theory of relativity space is endowed with physical qualities; in this sense, therefore, there exists an ether. According to the general theory of relativity space without ether is unthinkable; for in such space there not only would be no propagation of light, but also no possibility of existence for standards of space and time (measuring-rods and clocks), nor therefore any space-time intervals in the physical sense. But this ether may not be thought of as endowed with the quality characteristic of ponderable media, as consisting of parts which may be tracked through time. The idea of motion may not be applied to it.

    Who knows, though — maybe future research will demonstrate that we are all “vortices in the aether” after all!

    JuanBobsDad wrote: “I must disagree about the attribution that “Einstein’s special theory of relativity (1905) argued convincingly that the aether was unnecessary.” If it did indeed do that it was only because the nail in the aether’s coffin was the Michaelson-Morley experiment.”

    That’s the funny thing, though, as Dr. Dave pointed out: Michelson-Morley didn’t by itself kill the aether hypothesis! Scientists such as Lorentz, Abraham and Minkowski worked very hard to introduce new properties to the aether that took into account the null result of MM, and were superficially quite successful at it.

    That era between MM and 1905 is one of the most interesting scientific periods, in my opinion, but it is rather tricky to determine exactly what the scientific community as a whole thought of the developments. The period right after the 1905 paper is another one that is hard to figure out. It seems that scientific consensus moved ‘relatively’ (tee hee) rapidly to accepting the new theory, but I’m still a little hazy as to how quickly and what the deciding factors were. I think it is pretty clear, though, that Einstein’s results were the turning point in discarding the aether.

  5. IronMonkey says:

    While the mechanistic description of aether (as nicely laid out here) has been convincingly ruled out by the M-M experiment among others; Paul Dirac – much like Einstein – has pointed out that relativity theory does not necessarily rule out the existence of a physical aether, it just shows that its velocity may not be measured with this theory alone [http://www.jstor.org/pss/20945]. Further along, Dirac suggests that by introducing Quantum mechanics – especially Heisenberg’s indeterminacy principle – in the discussion allows one to realize that aether (i.e. “that something which does not allow a perfect vacuum”) may well exist. However a full mathematical theory of aether compatible with physical experiments has yet to be found…

  6. Peter Morgan says:

    Skullsinthestars, I guess I just think empiricism was the coming thing. I suppose that even amongst people who built concrete models there will have been a feeling that one couldn’t tell which model was right. The idea that one could abstract away from concrete models to common features of all the models one could construct was already well established by the success of thermodynamics, and an empiricist approach was being elaborated on in an abstract way within Philosophy of Science with reasonable if not perfect clarity, by Mach, Duhem, and Poincar\’e, at least, so when Einstein so successfully introduced a few empirical principles that would be satisfied by any model, and got out something worthwhile, it must have seemed good to enough people.

    I note that your only mention of Minkowski above is in connection with his pre-relativity concrete model-building, but Minkowski must have adopted the SR point of view pretty quickly to have been able to introduce a lucid 4-dimensional perspective so promptly, in 1907. That was a significant mathematical introduction, of course.

    I note also that Wikipedia has it that “Planck was among the few who immediately recognized the significance of the special theory of relativity. Thanks to his influence this theory was soon widely accepted in Germany.” I’m not sure how historically accurate it is that there were only a “few who immediately recognized the significance” of SR, and I wish it said what it means by “immediately” — a week, six months, five years? I suppose that people who were unhappy with the empiricist turn will nonetheless have found it quite quickly impossible to ignore. Such a debate once started, and with weight on both sides, will go where it will. The question is, perhaps, why did weighty physicists such as Planck come in quickly on the side of SR, what did they find attractive about the general approach?

    To return to the philosophical question, SR can be understood as a classification of the properties of concrete (aether) models that could match experiment. In modern terms, the speed of light is invariant for inertial observers is to say that the dynamics of any model will have the same Lorentz symmetry as Maxwell’s equations. There may be other requirements that will have to be satisfied by the dynamics of all models, but Lorentz invariance will be one of them. However many such empirical principles we introduce, there will certainly be a plethora of models that experiment does not differentiate between — all those models will be aether models that satisfy the empirical constraints. Aether models are not ruled out, but we will not be able to empirically justify an ontological commitment to any of them. As an aside, I would put the thesis of “Ontic Structural Realism”, without endorsement, to be an ontological commitment to the empirical principles that we find effective, without being ontologically committed to possible models (but see the SEP article http://plato.stanford.edu/entries/structural-realism/ for details according to James Ladyman).

    I look forward to what more you have to say, since it will be apparent that I am an opinionated dilettante in History of Physics.

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  8. George (Gunars Berzins) says:


    I am a retired communications engineer, and after lifelong work in connection with the transmission and reception of electromagnetic radiation, have no doubt whatever that the aether exists. Of course, all work in radio engineering is based on Maxwell’s equations and on the assumption that the aether exists, the photon theory of radiation not being of any use in, for example, the design of antennas. What makes me so sure?

    First, incident radiation does not behave like a shower of particles, mainly because the dipole antenna intercepts far more energy than what would hit the antenna structure along straight-line trajectories. And, furthermore, after the wavefront has passed, there is no ‘hole’ left in it where the antenna had extracted energy. Any ‘hole’ soon gets ‘filled in’ , just as in the case of sound waves. And, of course, there is the double slit experiment, where interference patterns arise even when the target is being hit be extremely weak radiation, photon-by-photon. The interference patterns are still there, but what is interfering with what? Purely mathematical ‘probability waves’? No, purely mathematical structures certanly cannot produce phycical effects! But what, in that case, does the aether consists of? I have some ideas on this subject, and would be happy to explain them should anyone here be interested.



  9. munty13 says:

    Hi. Great post. Hope you don’t mind, but I’ve produced a copy of this essay in my own blog. It does a good job in high-lighting some of my frustrations with aether theory. Thanks for sharing.

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