TESTING NEWTON, THEN AND NOW

THE LOGIC OF EVIDENCE IN GRAVITY RESEARCH FROM NEWTON THROUGH THE 20TH CENTURY

George E. Smith

Dibner Institute for the History of Science and Technology

Philosophy Department, Tufts University

WHY NOT SIMPLY HYPOTHESIS TESTING BY MEANS OF DEDUCED PREDICTIONS?

HEMPEL’S PROVISO PROBLEM

• Deduced predictions in celestial mechanics presuppose a proviso: no other (consequential) forces are at work.

• The only evidence for this proviso is close agreement between the predictions and observation.

• But then a primary purpose of comparing deduced predictions and observation is to answer the question, Are other forces at work?

• How then is the theory of gravity tested in the process?

How did we first come to have high quality evidence in any science?

Why was orbital astronomy so much better at turning data into evidence after

Newton’s Principia than it was before?

OUTLINE

I. Introduction: the issue

II. The logic, as dictated by Newton’s Principia

III. How this logic played out after the Principia

A. “Then” – complications that obscure the logic

B. “Now” – in light of the perihelion of Mercury

IV. Concluding remarks

“GRAVITY RESEARCH” THEN AND NOW

IN CELESTIAL MECHANICS:

What are the true motions – orbital and rotational – of the planets, their satellites, and comets, and what forces govern these motions?

IN PHYSICAL GEODESY:

What is the shape of the Earth, how does the gravitational field surrounding it vary, and what distribution of density within the Earth produces this field?

CALCULATING PLANETARY ORBITS — 1680

NEWTON’S EVIDENCE PROBLEM IN THE PRINCIPIA

“By reason of the deviation of the Sun from the center of gravity, the centripetal force does not always tend to that immobile center, and hence the planets neither move exactly in ellipses nor revolve twice in the same orbit. There are as many orbits of a planet as it has revolutions, as in the motion of the Moon, and the orbit of any one planet depends on the combined motion of all the planets, not to mention the action of all these on each other. But to consider simultaneously all these causes of motion and to define these motions by exact laws admitting of easy calculation exceeds, if I am not mistaken, the force of any human mind.”

Isaac Newton, ca. December 1684

(First published by Rouse Ball in 1893)

INFERRING LAWS OF FORCE FROMPHENOMENA OF MOTION

Phenomena: Descriptions of regularities of motion that hold at least quam proxime over a finite body of observations from a limited period of timeThe planets swept out equal areas in equal times quam proxime with respect to the Sun over the period from the 1580s to the 1680s.

Propositions, deduced from the laws of motion, of the form:

“If _ _ _ quam proxime, then …… quam proxime.”

If a body sweeps out equal areas in equal times quam proxime with respect to some point, then the force governing its motion is directed quam proxime toward this point.

Conclusions: Specifications of forces (central accelerations) that hold at least quam proxime over the given finite body of observationsTherefore, the force governing the orbital motion of the planets, at least from the 1580s to the 1680s, was directed quam proxime toward the Sun.

From Evidence that is Approximate to A Law that is Taken to be Exact

Rule 3: Those qualities of bodies that cannot be intended and remitted and that belong to all bodies on which experiments can be made should be regarded as qualities of all bodies universally.

Rule 4: In experimental philosophy, propositions gathered from phenomena by induction should be regarded as either exactly or very, very nearly true notwithstanding any con-trary hypotheses, until yet other phenomena make such propositions either more exact or liable to exceptions.

This rule should be followed so that arguments based on induction may not be nullified by hypotheses.

PREREQUISITES FOR TAKING THE THEORY OF GRAVITY AS EXACT

• The theory must identify specific conditions under which the phenomena from which it was inferred would hold exactly without restriction of time – e.g.– The area rule would hold exactly in the absence of forces from other

orbiting bodies– The orbits would be perfectly stationary were it not for perturbing

forces from other orbiting bodies

• The theory must identify a specific configuration in which the macroscopic variation of gravity about a body would result from the microstructure of the body – e.g.– Gravity would vary exactly as the inverse-square around a body were

it a sphere with a spherically symmetric distribution of density

TAKING THE THEORY TO BE EXACTTHE PRIMARY IMPLICATION

Every systematic discrepancy between observation and any theoretically deduced result ought to stem from a physical source not taken into account in the theoretical deduction

– a further density variation

– a further celestial force

THE NEWTONIAN APPROACH CONTINUING EVIDENCE

• Taking the law of gravity to hold exactly was a research strategy, adopted in response to the complexity of the true planetary motions.

• Deductions of planetary motions etc. are “Newtonian” idealizations: approximations that, according to theory, would hold exactly in certain specifiable circumstances -- in particular, in the absence of further forces or density variations.

• The upshot of comparing calculated and observed orbital motions is to shift the focus of ongoing research onto systematic discrepancies, asking in a sequence of successive approximations, what further forces or density variations are at work?

• Theory thus becomes, first and foremost, not an explanation (or even a representation) of known phenomena, but an instrument in ongoing research, revealing new “second-order” phenomena that can provide a basis for continuing testing of the theory.

THE LOGIC OF THEORY TESTING

• The theory requires that every deviation from any “Newtonian” idealization be physically significant – i.e. every deviation must result from some further force or density variation.

• Basic Testing: pin down sources of the discrepancies and confirm they are robust and physically significant (within the context of the theory) while achieving progressively smaller discrepancies between (idealized) calculation and observation.

• Ramified Testing: keep incorporating previously identified physical sources of second-order phenomena into the (idealized) calculation, thereby progressively constraining the freedom to pursue physical sources for new second-order phenomena that then emerge.

• The continuing evidence lies not merely in the aggregate of the individual comparisons with observation, but also in the history of the development of the sequence of successive approximations.

NEPTUNE AS AN EXAMPLE OF “PHYSICAL SIGNIFICANCE”

seconds of arc

THE “GREAT INEQUALITY” AS A MORE TYPICAL EXAMPLE

minutes of arc

OUTLINE

I. Introduction: the issue

II. The logic, as dictated by Newton’s Principia

III. How this logic played out after the Principia

A. “Then” – complications obscuring the logic

B. “Now” – in light of the perihelion of Mercury

IV. Concluding remarks

Second-Order Phenomena Often Underdetermine Their Physical Source

Example ExampleDeviation of surface gravity from Newton’s ideal variation implies the value of (C-A)/Ma2 and hence a correction to the difference (C-A) in the Earth’s moments of inertia, and the lunar-solar precession implies the value of (C-A)/C and hence a correction to the polar moment C; these two corrected values constrain the variation (r) of density inside the Earth, but they do not suffice to determine (r) .

density

density

core-mantle boundary

RESPONDING TO UNDERDETERMINATION20TH CENTURY DETERMINATION OF (r)

ROBUSTNESS OF PHYSICAL SOURCES

Examples• Mass of Moon inferred from lunar

nutation supported by calculated tides and lunar-solar precession

• Mass of Venus inferred from a particular inequality in the motion of Mars supported by calculated perturbations of Mercury, Earth, and Mars

• Gravity fields of Jupiter and Saturn supported by variations in period of Halley’s comet

VARIETIES OF EVIDENTIAL LOGIC FOR ROBUSTNESS

• Hypothetico-deductive: confirmed deduced predictions– e.g. date of maximum for Great Inequality

• Glymour boot-strapping: confirmed deduced predictions– e.g. inferred masses of planets

• Same effect, same cause (common origin inference)– e.g. fluctuations in rotation of Earth

• Agreeing measurements– e.g. masses, moments of inertia

• From success of research predicated on claim– e.g. Uranus motion anomaly after Jupiter and Saturn effects

PROBLEMS IN ISOLATING DISCORDANCES

“The motion of the [lunar] perigee can be got [from observation] to within about 500,000th of the whole. None of the values hitherto computed from theory agrees as closely as this with the value derived from observation. The question then arises whether the discrepancy should be attributed to the fault of not having carried the approximation far enough, or is indicative of forces acting on the moon which have not yet been considered.”

G. W. Hill, 1875

Newcomb’s Discordances, 1895

• Mercury’s perihelion was 29 times probable error

• Venus’s nodes was 5 times probable error

• Mars’s perihelion was 3 times probable error

• Mercury’s eccentricity was 2 times probable error

ANOTHER EXAMPLE OF DIFFICULTY

Many professional lives have been dedicated to the long series of meridian circle (transit) observations of the stars and planets throughout the past three centuries. These observations represent some of the most accurate scientific measurements in existence before the advent of electronics. The numerous successes arising from these instruments are certainly most impressive. However, as with all measurements, there is a limit to the accuracy beyond which one cannot expect to extract valid information. There are many cases where that limit has been exceeded; Planet X has surely been such a case.

THE MANY SOURCES OF DISCREPANCIES

In observations:

1. Simple error – “bad data”

2. Limits of precision

3. Systematic bias in instruments

4. Inadequate corrections for known sources of systematic error, incl.

5. Imprecise fundamental constants

6. Not yet identified sources of systematic error

In theoretical calculations:

1. Undetected calculation errors

2. Imprecise orbital elements

3. Imprecise planetary masses

4. Insufficiently converged infinite-series calculations

5. Need for higher-order terms

6. Forces not taken into account

7. Gravitation theory wrong

“The ultimate goal of celestial mechanics is to resolve the great question whether Newton’s law by itself accounts for all astronomical phenomena; the sole means of doing so is to make observations as precise as possible and then to compare them with the results of calculation. The calculation can only be approximate….”

Poincaré, 1892

“SECULAR” MOTION OF THE MOON

18th Century:Acceleration in motion of Moon announced by Halley (1693)

A physical source identified by Laplace (1787):

19th Century:Adams finds that Laplace has accounted for only half of the “secular” motion (1854)

A further physical source: earth is slowing from tidal friction Owing to perturbations

from gravity toward the planets, eccentricity of Earth’s orbit changing.

EXAMPLE OF SPECTACULAR SUCCESS SPENCER JONES (1939)

• Residual discrepancies in the motions of Mercury, Venus, and Earth correlate with unaccounted-for discrepancy in lunar motion

• Common cause => Earth’s rotation irregular (in more ways than one)• Expose a still further systematic observation error, requiring correction:

– 1950: replace sidereal time with “ephemeris time”

This form of evidence can be very strong

• It is evidence aimed at the question of the physical exactness of the theory

• The sequence of successive approximations leads to new second-order phenomena of progressively smaller magnitude

• New second-order phenomena presuppose not only the theory of gravity, but also previously identified physical sources of earlier second-order phenomena, thereby constraining the freedom to respond to these new phenomena

• Theory becomes entrenched from its sustained success in exposing increasingly subtle details of the physical world without having to backtrack and reject earlier discoveries

OUTLINE

I. Introduction: the issue

II. The logic, as dictated by Newton’s Principia

III. How this logic played out after the Principia

A. “Then” – complications obscuring the logic

B. “Now” – in light of the perihelion of Mercury

IV. Concluding remarks

INEXACTNESS EXPOSED:THE PERIHELION OF MERCURY

“The secular variations already given are derived from these same values of the masses, the centennial motion of the perihelion being increased by the quantity

Dt = 43.″37

In order to represent the observed motion. This quantity is the product of the centennial mean motion by the factor

0.000 000 0806”

PERIHELION OF MERCURY: CURRENT

FROM NEWTONIAN TO EINSTEINIAN GRAVITY

Discrepancy between Newtonian calculation and observation:

43´´.37 ± 2.1 ==> 43´´.11 ± 0.45

Increment from the Einsteinian calculation:

43´´. ==> 42´´.98

Newtonian gravity is the static, weak-field limit of Einsteinian!

A limit-case idealization

The orbital equation becomes, where μ = G(M+m), u = 1/r:

CONTINUITY OF EVIDENCE ACROSS THE CONCEPTUAL DIVIDE

• 43´´ per century was a Newtonian second-order phenomenon

• From limit-case reasoning, evidence for Newtonian gravity carried over, with minor qualifications, to Einsteinian

• Earlier evidential reasoning for Newtonian gravity, even though requiring some qualifications, was not nullified

• Although curvature of space, gravitational time dilation, and non-linear effects of mass on the metric were not previously unidentified forces or density variations, they were still physically intelligible in a Newtonian context

• Previously identified physical sources of Newtonian second-order phenomena remained intact in Einsteinian

“… though the world does not change with a change of paradigm, the scientist afterward works in a different world…. I am convinced that we must learn to make sense of statements that at least resemble these.

Thomas S. Kuhn, SSR, p. 121

The continuity of evidence across the conceptual divide between Newtonian and Einsteinian gravity highlights an extremely important sense in which the scientist afterward works in the same world.

PRIMARY CONCLUSIONS

• The most important evidence in classical gravitational research came from the complexities of the actual motions and of the gravitational fields surrounding bodies.

• This evidence consisted of success in pinning down physical sources of deviations from “Newtonian” idealizations, in a sequence of increasingly precise successive approximations.

• This evidence carried forward, continuously, across the tran-sition from Newtonian to Einsteinian gravity and remains an important source of continuing evidence today.

FIVE CORRELATIVE POINTS

• In response to a particular evidential problem– May not carry over to other areas of research

• Theory primarily an instrument in ongoing research– Not primarily serving to explain

• Two stages of evidence: initial, continuing– Initial stage focuses on promise of the theory

• Importance of getting evidential logic right– Hypothetico-deductive understates force of evidence

• Historical development itself a source of evidence– Cumulative development versus back-tracking