Alberto Cappi INAF – Osservatorio Astronomico di Bologna Observatoire de la Côte d’ Azur

1. INSAP VII - Bath THE CONCEPT OF GRAVITY IN ANTIQUITY Alberto Cappi INAF – Osservatorio Astronomico di Bologna Observatoire de la Côte d’Azur

2. The dominantphysics and cosmology in ancienttimes Aristotle (384-322 B.C.) Pre-Copernican thought about gravity had in the main been dominated by Aristotle's view that “the earth and the universe happen to have the same center; a heavy body moves also toward the center of the earth, but it does so only incidentally, because the earth has its center at the center of the universe (E. Rosen, 1971). Sublunar world isimperfect Celestialbodieshave a perfect, circularmotion and are madeof the fifthelementcalledaether (or quintessence) Voiddoesnotexist

3. Aristotle’s Universe Einstein’s Universe Aristotle’s universe (4th centuryB.C.) : spherical (finite), eternal, nothingoutside Einstein’s universe (1917): spherical (finite), eternal, nothingoutside

4. LEOPARDI AND the Finite Universe Believing in the infinity of the universeis an optical illusion: at least thisismyown opinion. I do not saythatitcanberigorouslydemonstrated in metaphysics, or thatthere are factualproofs, thatitis not infinite; but apartfrom the metaphysical arguments, I believethatanalogymakesquite plausible that the infinity of the universeis a natural illusion of imagination. When I look at the sky, someonetold me, and when I thinkthatbeyondthose visible bodies there are other and other bodies, mythoughts do not findanylimit, and the probabilitymakes me to believethatthere are stillother bodies without an end. The same, I say, happens to the child, or the ignorant, who looks aroudfrom a hightower or mountain, or is in the sea. He sees an horizon, but heknowsthatbeyondthat horizon thereisstillearth or water, without an end; and concludes, or wouldlike to conclude, that the earth or sea are infinite. Giacomo Leopardi, Lo Zibaldone, Firenze, 21 settembre 1827 Il crederel'universoinfinito, è un'illusioneottica: almeno tale è ilmioparere. […] Quandoioguardoilcielo, mi dicevauno, e pensoche al dilàdique' corpich'ioveggo, ve ne sonoaltriedaltri, ilmiopensiero non trovalimiti, e la probabilità mi conduce a crederechesempre vi sienoaltricorpipiù al dilà, edaltripiù al dilà. Lo stesso, dicoio, accade al fanciullo, o all'ignorante, cheguardaintornodaun'altatorre o montagna, o chesitrova in alto mare. Vede un orizzonte, ma sache al dilàv’èancor terra o acqua, edaltrapiù al dilà, e poi altra; e conchiude, o conchiuderebbevolentieri, che la terra o il mare fosse infinito. Giacomo Leopardi, Zibaldone, 1827

5. GRAVITY AND OTHER WORLDS AccordingtoAristotle, otherkosmoicannotexistbecause: Beingspherical, voidswouldbeleftbetweenthem b) Therewouldbeasmanycentresaskosmoi

6. Alternative Cosmologies • Two main traditions in Greek cosmology (Furley 1987): • - Closed World: permanence / teleology • Infinite Universe: evolution / mechanism (Atomists) • (materialism, S. Berryman, 2009) Soap bubbles,GerdGuenther Manchu/Ciel etEspace

7. The survivalofancient science • The goldenageof science and technology in antiquitywas the first part of the Hellenisticepoch (3rd-2nd centuryB.C.), butfewworkssurvivedamongthosewhich: • didnotsupport the (later) dominantAristotelian and Ptolemaicsystems • (D. Furley 1987) • couldnotbeunderstoodafter the declineof science in the imperialperiod • (L. Russo 1999, 2004) • Simpleworkswerecopied and saved (e.g. Pliny’sNaturalHistory or Seneca’s NaturalesQuaestiones), but no work ofHipparchussurvived (excepthiscommentstoAratus’ Phenomena). • Two main traditions in Greek cosmology: • - Closed World: permanence / teleology • Infinite Universe: evolution / mechanism • (materialism, S. Berryman, 2009) • The second picture is much less represented in surviving works • (Furley 1987)

8. EVOLUTION OF THE LYCEUM AristotleTheophrasus Strato ofLampsacus Strato wasconsidered the greatest “physicist” ofhistime. Hethoughtthat in matterthere are voids. Heprovedthatfallingbodies accelerate. HewasinvitedbyPtolemySotherinAlexandria,wherehe helpedtoorganize the Museum (the first researchinstitute fundedby a state) OneofhisdiscipleswasAristarchusofSamos, whowas the first toconceive the heliocentric system.

9. De Facie quae in OrbeLunaeapparet, 923 E William Stukeley, 1752, Memoirsof Sir Isaac Newton’s life Plutarch (AD 45-120) Yet the moon is saved from falling by its very motion and the rapidity of its revolution, just as missiles placed in slings are kept from falling by being whirled around in a circle. For each thing is governed by its natural motion unless it be diverted by something else.

10. Concerning the face which appears in the Orb of the Moon Plutarchwrote the ParallelLivesand the MoralEssays(Moralia) The De Facieis a dialogueof the Moralia. Startingpoint: whatis the cause of the “face” wesee on the moon (a pretextfor a more generaldiscussion). Itcontains a lotofinterestingpoints. Amongthem, a non-Aristotelianphysics and cosmology (otherclassicalsourcessuggest the existenceofthis alternative physics). Accordingto S. Sambursky (1960) Plutarchwas “the fatherofastrophysics”… ButitisclearthatPlutarchisnotinventing a newtheory (and there are hints in otherclassicaltextsof a non-Aristotelianphysics) Russo suggeststhat the original idea was due toHipparchus (2nd centuryB.C.), whowrote a work (nowlost) on gravity (On bodiesthrust down becauseofgravity, Περὶ τῶν διὰ βαρὺτητα κὰτω φερομένων)

11. Plutarch’s Polycentricgravity […] <the downward tendency> of falling bodies proves not that the <earth> is in the centre of the cosmos but that those bodies which when thrust away from the earth fall back to her again have some affinity and cohesion with her. For as the sun attracts to itself the parts of which it consists so the earth too accepts as <her> own the stone that has properly a downward tendency, and consequently every such thing ultimately unites and coheres with her.

12. THE KOSMOS IS NOT THE WHOLE In the De Facie Plutarch makes a clear distinction between our world, in its meaning of our universe limited by the celestial sphere, in Greek the kosmos (ό κόσμος), and the sum of things, representing the totality of what exists, the whole (τὸ πάν): The sum of things is infinite […] He who asserts that the earth is in the middle not of the sum of things but of the cosmos is naïve if he supposes that the cosmos itself is not also involved in the very same difficulties. De Facie, 925 F. The distinction between our universe and the sum of things opens the possibility that something else exists outside our universe.

13. OTHER WORLDS ARE POSSIBLE Plutarch discusses the existence of other worlds in another dialogue of the Moralia, De DefectuOraculorum(The Obsolescence of Oracles): But to make more worlds than one, each separate from the other, and to delimit and distinguish the parts belonging to each to go with the whole is not preposterous. For the land and the sea and the heavens in each will be placed to accord with nature, as is fitting; and each of the worlds has its above and below and its round about and centre, not with reference to another world or the outside, but in itself and with reference to itself. 425 B-C. Thus one conclusion is left: when the centre is spoken of, it is not with reference to any place, but with reference to the bodies. 424-E For the law of reason over each world, having control over the matter assigned to each, will not allow anything to be carried away from it nor to wander about and crash into another world, nor anything from another world to crash into it, because Nature has neither unlimited and infinite magnitude nor irrational and disorganized movement.424 A-B. Link with the theoryofgravity. Worlds do notinteract. No absolutecentre. The centreof the worldsis relative. Eachonehasitsowncentre. Unthinkable in Aristotelianphysics!

14. GOD(S) AND UNIVERSE(S): A THEOLOGICAL PROBLEM This Zeus of yours too, is it not true that, while in his own nature he is single, a great and continuous fire, at present he is slackened and subdued and transformed, having become and continuing to become everything in the course of his mutations? (De Facie, 926 D) Then again, who could feel alarm at the other notions of the Stoics, who ask how there shall continue to be one Destiny and one Providence, and how there shall not be many supreme gods bearing the name of Zeus or Zen, if there are more worlds than one? For, in the first place, if it is preposterous that there should be many supreme gods bearing this name, then surely these persons' ideas will be far more preposterous; for they make an infinite number of suns and moons and Apollos and Artemises and Poseidons in the infinite cycle of worlds. But the second point is this: what is the need that there be many gods bearing the name of Zeus, if there be more worlds than one, and that there should not be in each world, god possessing sense and reason, such as the one who among us bears the name of Lord and Father of all?Or again, what shall prevent all worlds from being subject to the Destiny and Providence of Zeus, and what shall prevent his overseeing and directing them all in turn and supplying them all with first principles, material sources, and schemes of all that is being carried out? (De Defectu, 425 E-F)

15. NICOLAUS COPERNICUS (1473-1543) Formy part, I thinkthatgravityisnothingbut a certainnaturalstrivingwithwhich partshavebeenendowed [...] so thatbyassembling in the formof a spherethey may join together in theirunity and wholeness. Thistendencymaybebelievedto bepresentalso in the sun, the moon, and the otherbrightplanets, so thatitmakes themkeepthatroundnesswhichthey display. (De Revolutionibus, I, 9) Copernicus's theory of gravity postulated a separate process of gravitational cohesion for individual heavenly bodies, not only the earth but also the sun, moon, and planets, each of which maintained its spherical shape through the operation of this tendency. […] Copernicus made germinal contributions to what later developed into the concepts of universal gravitation and inertia (Edward Rosen)

16. WILLIAM GILBERT (1544-1603) De Mundo (1651) It is evident that all the heavenly bodies, set as if in a destined place, are there formed unto spheres, that they tend to their own centres and that around them there is a confluence of all their parts.

17. Johannes KEPLER (1571-1630) New Astronomy (Astronomia Nova, 1609) Gravityis a mutualcorporealtendencyofkindredbodiesto unite or join together. The moonis a body akinto the earth. Kepler translatedPlutarch’s De Facieinto Latin.

18. PLUTARCH AND NEWTON In the prefaceto David Gregory’s AstronomiaePhysicaeetGeometricaeElementa, published in 1702, thereis a listofancientauthors and quotationswhichshould show that the ancientsknew the lawofgravity. From the papersof Gregory weknow thatthislistwasprovidedtohimby Newton himself.

19. The OtherUniversesofPoe If such clusters of clusters exist, however - and they do - it is abundantly clear that, having had no part in our origin, they have no portion in our laws. They neither attract us, nor we them. Their material - their spirit is not ours - is not that which obtains in any part of our Universe. They could not impress our senses or our souls. Among them and us - considering all, for the moment, collectively - there are no influences in common. Each exists, apart and independently, in the bosom of its proper and particular God. (Eureka, 1848) Some Wordswith a Mummy, 1845

20. PLUTARCH’s COSMOLOGY IN THE XIX CENTURY In the past the De DefectuOraculorum and the De Facie were not obscure works known by a small group of specialists, as probably they are today: in the first half of the 19th century they were still quoted in famous works related to science. For example, De DefectuOraculorum is mentioned in Lyell's Principles of Geology when discussing the Egyptian cosmogony; the De Facie is mentioned by von Humboldt in the first volume of Cosmos and by Whewell in the first volume of his History of the Inductive Sciences.

21. Conclusions DuringHellenisticepochalternativestoAristotelianphysics and cosmologyweredeveloped. Mostof the originalworkswerelost, and theywereonlypartiallytransmitted. The De Faciewas a vehiclefor some ideas on gravitywhichwerepresented in thoselostworks. Offeringan alternative toAristotle’s physics, itwas a usefulreferenceforCopernicansbefore the Principiaof Newton. Itis a niceexampleof a literary work whichhasbeen a source ofinspirationfor science.