the unseen world and other essays-第4章

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nergy　of　position　at　first　existing　should　have　been　retained　in　some　form　or　other。　But　nearly　all　of　it　has　been　lost；　and　only　an　insignificant　fraction　remains　with　which　to　endow　a　new　system。　In　order　to　reproduce；　in　future　ages；　anything　like　that　cosmical　development　which　is　now　going　on　in　the　solar　system；　aid　must　be　sought　from　without。　We　must　endeavour　to　frame　some　valid　hypothesis　as　to　the　relation　of　our　solar　system　to　other　systems。

Thus　far　our　view　has　been　confined　to　the　career　of　a　single　star；our　sun；with　the　tiny；　easily…cooling　balls　which　it　has　cast　off　in　the　course　of　its　development。　Thus　far；　too；　our　inferences　have　been　very　secure；　for　we　have　been　dealing　with　a　circumscribed　group　of　phenomena；　the　beginning　and　end　of　which　have　been　brought　pretty　well　within　the　compass　of　our　imagination。　It　is　quite　another　thing　to　deal　with　the　actual　or　probable　career　of　the　stars　in　general；　inasmuch　as　we　do　not　even　know　how　many　stars　there　are；　which　form　parts　of　a　common　system；　or　what。　are　their　precise　dynamic　relations　to　one　another。　Nevertheless　we　have　knowledge　of　a　few　facts　which　may　support　some　cautious　inferences。　All　the　stars　which　we　can　see　are　undoubtedly　bound　together　by　relations　of　gravitation。　No　doubt　our　sun　attracts　all　the　other　stars　within　our　ken；　and　is　reciprocally　attracted　by　them。　The　stars；　too；　lie　mostly　in　or　around　one　great　plane；　as　is　the　case　with　the　members　of　the　solar　system。　Moreover；　the　stars　are　shown　by　the　spectroscope　to　consist　of　chemical　elements　identical　with　those　which　are　found　in　the　solar　system。　Such　facts　as　these　make　it　probable　that　the　career　of　other　stars；　when　adequately　inquired　into；　would　be　found　to　be　like　that　of　our　own　sun。　Observation　daily　enhances　this　probability；　for　our　study　of　the　sidereal　universe　is　continually　showing　us　stars　in　all　stages　of　development。　We　find　irregular　nebulae；　for　example；　we　find　spiral　and　spheroidal　nebulae；　we　find　stars　which　have　got　beyond　the　nebulous　stage；　but　are　still　at　a　whiter　heat　than　our　sun；　and　we　also　find　many　stars　which　yield　the　same　sort　of　spectrum　as　our　sun。　The　inference　seems　forced　upon　us　that　the　same　process　of　concentration　which　has　gone　on　in　the　case　of　our　solar　nebula　has　been　going　on　in　the　case　of　other　nebulae。　The　history　of　the　sun　is　but　a　type　of　the　history　of　stars　in　general。　And　when　we　consider　that　all　other　visible　stars　and　nebulae　are　cooling　and　contracting　bodies；　like　our　sun；　to　what　other　conclusion　could　we　very　well　come？　When　we　look　at　Sirius；　for　instance；　we　do　not　see　him　surrounded　by　planets；　for　at　such　a　distance　no　planet　could　be　visible；　even　Sirius　himself；　though　fourteen　times　larger　than　our　sun；　appearing　only　as　a　〃twinkling　little　star。〃　But　a　comparative　survey　of　the　heavens　assures　us　that　Sirius　can　hardly　have　arrived　at　his　present　stage　of　concentration　without　detaching；　planet…forming　rings；　for　there　is　no　reason　for　supposing　that　mechanical　laws　out　there　are　at　all　different　from　what　they　are　in　our　own　system。　And　the　same　kind　of　inference　must　apply　to　all　the　matured　stars　which　we　see　in　the　heavens。

When　we　duly　take　all　these　things　into　the　account；　the　case　of　our　solar　system　will　appear　as　only　one　of　a　thousand　cases　of　evolution　and　dissolution　with　which　the　heavens　furnish　us。　Other　stars；　like　our　sun；　have　undoubtedly　started　as　vaporous　masses；　and　have　thrown　off　planets　in　contracting。　The　inference　may　seem　a　bold　one；　but　it　after　all　involves　no　other　assumption　than　that　of　the　continuity　of　natural　phenomena。　It　is　not　likely；　therefore；　that　the　solar　system　will　forever　be　left　to　itself。　Stars　which　strongly　gravitate　toward　each　other；　while　moving　through　a　perennially　resisting　medium；　must　in　time　be　drawn　together。　The　collision　of　our　extinct　sun　with　one　of　the　Pleiades；　after　this　manner；　would　very　likely　suffice　to　generate　even　a　grander　nebula　than　the　one　with　which　we　started。　Possibly　the　entire　galactic　system　may；　in　an　inconceivably　remote　future；　remodel　itself　in　this　way；　and　possibly　the　nebula　from　which　our　own　group　of　planets　has　been　formed　may　have　owed　its　origin　to　the　disintegration　of　systems　which　had　accomplished　their　career　in　the　depths　of　the　bygone　eternity。

When　the　problem　is　extended　to　these　huge　dimensions；　the　prospect　of　an　ultimate　cessation　of　cosmical　work　is　indefinitely　postponed；　but　at　the　same　time　it　becomes　impossible　for　us　to　deal　very　securely　with　the　questions　we　have　raised。　The　magnitudes　and　periods　we　have　introduced　are　so　nearly　infinite　as　to　baffle　speculation　itself：　One　point；　however；　we　seem　dimly　to　discern。　Supposing　the　stellar　universe　not　to　be　absolutely　infinite　in　extent；　we　may　hold　that　the　day　of　doom；　so　often　postponed；　must　come　at　last。　The　concentration　of　matter　and　dissipation　of　energy；　so　often　checked；　must　in　the　end　prevail；　so　that；　as　the　final　outcome　of　things；　the　entire　universe　will　be　reduced　to　a　single　enormous　ball；　dead　and　frozen；　solid　and　black；　its　potential　energy　of　motion　having　been　all　transformed　into　heat　and　radiated　away。　Such　a　conclusion　has　been　suggested　by　Sir　William　Thomson；　and　it　is　quite　forcibly　stated　by　the　authors　of　〃The　Unseen　Universe。〃　They　remind　us　that　〃if　there　be　any　one　form　of　energy　less　readily　or　less　completely　transformable　than　the　others；　and　if　transformations　constantly　go　on；　more　and　more　of　the　whole　energy　of　the　universe　will　inevitably　sink　into　this　lower　grade　as　time　advances。〃　Now　radiant　heat；　as　we　have　seen；　is　such　a　lower　grade　of　energy。　〃At　each　transformation　of　heat…energy　into　work；　a　large　portion　is　degraded；　while　only　a　small　portion　is　transformed　into　work。　So　that　while　it　is　very　easy　to　change　all　of　our　mechanical　or　useful　energy　into　heat；　it　is　only　possible　to　transform　a　portion　of　this　heat…energy　back　again　into　work。　After　each　change；　too；　the　heat　becomes　more　and　more　dissipated　or　degraded；　and　less　and　less　available　for　any　future　transformation。　In　other　words；〃　our　authors　continue；　〃the　tendency　of　heat　is　towards　equalization；　heat　is　par　excellence　the　communist　of　our　universe；　and　it　will　no　doubt　ultimately　bring　the　system　to　an　end。　。。。。　It　is　absolutely　certain　that　life；　so　far　as　it　is　physical；　depends　essentially　upon　transformations　of　energy；　it　is　also　absolutely　certain　that　age　after　age　the　possibility　of　such　transformations　is　becoming　less　and　less；　and；　so　far　as　we　yet　know；　the　final　state　of　the　present　universe　must　be　an　aggregation　（into　one　mass）　of　all　the　matter　it　contains；　i。　e。　the　potential　energy　gone；　and　a　practically　useless　state　of　kinetic　energy；　i。　e。　uniform　temperature　throughout　that　mass。〃　Thus　our　authors　conclude　that　the　visible　universe　began　in　time　and　will　in　time　come　to　an　end；　and　they　add　that　under　the　physical　conditions　of　such　a　universe　〃immortality　is　impossible。〃

Concerning　the　latter　inference　we　shall　by　and　by　have　something　to　say。　Meanwhile　this　whole　speculation　as　to　the　final　cessation　of　cosmical　work　seems　to　meas　it　does　to　my　friend；　Professor　Clifford＇3＇by　no　means　trustworthy。　The　conditions　of　the　problem　so　far　transcend　our　grasp　that　any　such　speculation　must　remain　an　unverifiable　guess。　I　do　not　go　with　Professor　Clifford　in　doubting　whether　the　laws　of　mechanics　are　absolutely　the　same　throughout　eternity；　I　cannot　quite　reconcile　such　a　doubt　with　faith　in　the　principle　of　continuity。　But　it　does　seem　to　me　needful；　before　we　conclude　that　radiated　energy　is　absolutely　and　forever　wasted；　that　we　should　find　out　what　becomes　of　it。　What　we　call　radiant　heat　is　simply　transverse　wave…motion；　propagated　with　enormous　velocity　through　an　ocean　of　subtle　ethereal　matter　which　bathes　the　atoms　of　all　visible　or　palpable　bodies　and　fills　the　whole　of　space；　extending　beyond　the　remotest　star　which　the　telescope　can　reach。　Whether　there　are　any　bounds　at　all　to　this　ethereal　ocean；　or　whether　it　is　as　infinite　as　space　itself；　we　cannot　surmise。　If　it　be　limited；　the　possible　dispersion　of　radiant　energy　is　limited　by　its　extent。　Heat　and　light　cannot　travel　through　emp