Giuliano Preparata Preparata Giuliano Dai Quark Dai Breve Storia Breve Storia Dentro La Materia Di U Lungo Viaggio Di U Lungo Ai Cristalli Ai Cristalli

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Giuliano Preparata Preparata Giuliano Dai Quark Dai Breve Storia Breve Storia Dentro La Materia Di U Lungo Viaggio Di U Lungo Ai Cristalli Ai Cristalli Giuliano Preparata Dai quark Breve storia dentro la materia di u lungo viaggio ai cristalli Bollati B oringhieri S cienze `i Giuliano Preparata Dai quark ai cristalli Breve storia di un lungo viaggio dentro la materia Bollati Boringhieri Prima edizione giugno zoo2 Indice © 2002 Bollati Boringhieri editore s.r.i., Torino, corso Vittorio Emanuele II, 86 I diritti di memorizzazione elettronica, di riproduzione e di adattamento totale o parziale con qualsiasi mezzo (compresi i microfilm e le copie fotostatiche) sono riservati Stampato in Italia dalla Stampatre di Torino ISBN 88-339-1392-9 Schema grafico della copertina di Pietro Palladino e Giulio Palmieri Stampato su carta Palatina delle Cartiere Miliani Fabriano vrr Prefazione di G. Boaretto, E. Del Giudice, G. Galli e C. Medail Dai quark ai cristalli 3 o. Perché questo libro? S 1. Farò il fisico 10 2. Lo spin dei bosoni 23 3. Quark ad Arcetri 43 4. Princeton e dintorni 64 5. Il cono di luce 81 6. Ma che cosa sono i quark? 95 7. You are old, Father Feynman 110 8. Il vuoto è tutto 128 9. Ricostruire la materia 138 I o. Sorella acqua 150 z z . Il sole in provetta 168 I 2. Materia super VI INDICE 18o 13. Magie della coerenza Prefazione 196 14. Ritorno ai principi 209 15. Un ponte verso la biologia 217 Elenco delle abbreviazioni 221 Appendice Bibliografia degli scritti di Giuliano Preparata 245 Indice dei nomi Questo è un libro inconsueto. È il libro in cui un fisico teorico, Giuliano Preparata, scomparso prematuramente due anni fa, rac- conta il suo percorso nella scienza fisica, partendo dagli studi della giovinezza, quando fu uno dei maggiori protagonisti della costru- zione del modello standard delle interazioni subnucleari, per arriva- re alle tematiche della dinamica della vita e dell'emergenza di una psiche dalla vita biomolecolare. Nel corso di questo cammino lo scienziato si è sempre più spogliato della sua astrattezza e si è avvici- nato alla vita reale e concreta. Ma — viene da chiedersi — perché que- sto viaggio è stato fatto, tra tanti scienziati, proprio da Giuliano? Si può tentare di formulare un'ipotesi. Questo «lungo viaggio dentro la materia» non è stato concepito per un desiderio di fuga, per la necessità di sottrarsi ai dolori della vita reale e trovare rifu- gio negli spazi astratti e freddi delle «supersimmetrie», delle «su- perstringhe», dai quali non potrà mai venire nulla di drammatico, tranne forse una noia mortale. È stato al contrario il viaggio di chi cercava nella materia, al di sotto delle collisioni e degli urti, una condizione di risonanza che ponesse in fase i diversi individui e rendesse lo stato di ognuno premessa e conseguenza dello stato altrui. E questo obbiettivo è stato perseguito con la passione di chi a tale scopo ha dedicato l'intera vita. Passione: ecco una parola che, secondo un pregiudizio diffuso, dovrebbe suonare come una bestemmia per uno scienziato, al quale invece dovrebbe convenire piuttosto il distaccato rigore. Ma forse proprio questo pregiudizio è l'espressione di quella mentalità che finora non ha saputo far compiere alla scienza il salto oltre il fosso PREFAZIONE VIII PREFAZIONE IX profondo che divide l'inorganico dal vivente. Un pensiero non poteva scomporsi in corpuscoli microscopici, le molecole, formate separato dalla passione può penetrare i misteri che l'approccio posi- a loro volta da atomi. Gli atomi vengono successivamente scissi in tivistico, fratello gemello dell'oscurantismo irrazionalistico, non rie- nuclei ed elettroni (questi ultimi almeno per ora indivisibili); a loro sce a illuminare. Ecco allora che il viaggio faticoso attraverso i volta i nuclei si rivelano costituiti da protoni e neutroni (entrambi quark e i nuclei genera l'esperienza e la chiaroveggenza necessarie detti anche nucleoni). Questi, poi, nei moderni acceleratori di par- per comprendere la «memoria dell'acqua» o l'omeopatia, e la fusio- ticelle, come quelli operanti a Stanford, in California, o al CERN ne fredda o l'azione di deboli campi magnetici su organismi vi- di Ginevra, fanno sgorgare dal loro seno nuove particelle, chiamate venti, lungi dall'essere fenomeni «complessi», se osservati dall'an- mesoni. I nucleoni possono trasformarsi in particelle più pesanti, golo giusto rivelano una semplicità commovente e sconvolgente. gli iperoni. L'insieme dei nucleoni e iperoni (chiamati complessi- Il libro di Giuliano Preparata può essere letto come percorso ini- vamente barioni) e dei mesoni forma i cosiddetti adroni, uno zoo ziatico e cioè come esperienza evolutiva di chi condivide l'amore di particelle composte di corpuscoli ancora più elementari, i quark. per la verità, intesa come disvelamento e non solo come descri- A fianco dei quark, e in corrispondenza univoca con essi, esiste zione. E tale passione non può che essere collettiva perché il velo un'altra famiglia di particelle elementari, i leptoni, di cui fanno copre l'intera verità e quindi lo scienziato deve aiutare tutti, anche parte gli elettroni e i neutrini, particelle strane ed elusive, prive di i suoi più tenaci oppositori, a guardare nel cannocchiale di Galileo. massa e carica elettrica. Il percorso raccontato nel libro non riguarda, dunque, solo nuove Negli anni settanta, anche con il contributo di Giuliano Prepa- leggi scientifiche, ma anche una nuova etica che vede la realtà come rata, a questo enorme insieme di dati fu dato ordine e fu elaborata complesso di forze interagenti e nessi di idee individuali, un'etica una teoria, il modello standard delle particelle elementari, fondata in cui suscitare l'altrui attenzione è necessario per pervenire allo sui risultati delle due grandi rivoluzioni scientifiche dell'inizio dei stato di risonanza essenziale alla ricerca. La grande lezione morale secolo: la rivoluzione quantistica e quella relativistica. Il modello ed epistemologica del testo nasce dalla presa di coscienza che la standard descrive, finora senza errori, tutta la fenomenologia pro- natura tende al collettivo e che il collettivo tende all'umanità e non dotta dai numerosi esperimenti condotti con gli acceleratori di all'individualità: in altre parole, alla base del senso del collettivo particelle. Resta però un mistero: i quark, la cui esistenza è stata vi sono amore, passione dell'essere umano per l'essere umano e provata esplorando l'interno dei nucleoni con quelle particelle pic- soprattutto il superamento della paura dell'altro. Il processo di colissime che sono gli elettroni. Si ottengono così vere e proprie ricerca diviene allora il processo di disvelamento del senso del no- «radiografie» dei nucleoni, grazie alle quali si osserva che i quark stro vivere. possono esistere esclusivamente in collettività, mai da soli! Non A questo processo di ricerca è chiamato anche il lettore, il quale possono conoscere l'esperienza dell'individuo isolato, ma soltanto a una prima lettura potrà forse non comprendere la metà dei con- la vita in interazione. cetti esposti, ma se si abbandona al discorso dell'autore subisce un Si potrebbe supporre che gli aggregati dei quark siano oggetti tale fascino che non avrà pace finché non riuscirà a entrarvi, por- estremamente rigidi legati da forze di intensità incredibile. Questi tandovi il carico dei propri sogni. Uno scrittore che ha avuto modo aggregati possono tuttavia subire ogni sorta di trasformazioni in- di leggere il manoscritto ha commentato acutamente: «Questo terne, possono prodursi nuovi aggregati di quark che si separano testo non può lasciare in ogni caso il lettore indifferente: ne farà o dall'aggregato originario per viaggiare in modo indipendente. Ricor- un sostenitore entusiasta o un avversario accanito». rendo a una immagine metaforica, si pensi allo scontro frontale, a Ma a quale viaggio ci invita Giuliano Preparata? grandissima velocità, tra una Ferrari e una Maserati; l'esperienza Nel xx secolo la realtà fisica è stata smontata in pezzi sempre più macroscopica induce a immaginare un gran numero di rottami. piccoli. All'inizio del secolo si è visto che la materia macroscopica Invece, se si applicassero al mondo delle auto le leggi del mondo X PREFAZIONE PREFAZIONE XI delle particelle, dall'urto della Ferrari e della Maserati emergereb- è la scienza del macroscopico a guadagnare la possibilità di genera- bero una Volvo e una Mercedes (più un gran numero di monopat- re nuove configurazioni prima impensate; basti pensare ai materiali tini e go-kart) oppure una Lancia e un pullmino e così via. Fu pro- artificiali (il nylon, il kevlar, i polimeri) che la chimica industriale prio questa grande «creatività» degli urti adronici a convincere ha potuto produrre sulla base della conoscenza delle proprietà degli Geoffrey Chew — che negli anni sessanta era il guru della West atomi. Coast e che fu il maestro di Fritjof Capra, il guru della New Age — Alla fine del xix secolo lo schema teorico di Maxwell e Boltz- che ogni adrone fosse composto da tutti gli altri adroni, una con- mann aveva permesso di dedurre l'intera termodinamica dai moti cezione con reminiscenze del pensiero indiano. degli atomi descritti dalla fisica classica. Ma vi fu allora un ulte- Il mistero dell'inseparabilità dei quark e della peculiare dinamica riore progresso della termodinamica col terzo principio formulato dei suoi aggregati, che fa sì che si presentino sempre e comunque da Nernst, che prevede l'annullamento dell'entropia di un sistema come adroni, spinse Preparata a considerare questo il vero pro- fisico al tendere della sua temperatura allo zero assoluto. Non fu blema della fisica contemporanea cui dopo dieci anni di lavoro possibile «ridurre» questo principio a nessuna descrizione in ter- trovò la soluzione descritta nel libro. Da allora egli cominciò ad mini di moti classici degli atomi. allontanarsi dalla maggior parte dei suoi colleghi. La storia di que- Si aprivano allora due alternative. Alla luce dell'abituale prassi sta separazione ha un'origine molto lontana. dell'epistemologia contemporanea, si sarebbe potuto considerare Nell'Ottocento Maxwell e Boltzmann furono gli artefici all'in- fallito l'ideale riduzionistico, affermare che il macroscopico e il terno della fisica di un enorme balzo in avanti concettuale, il ridu- microscopico sono retti da leggi diverse e lasciare meccanica clas- zionismo scientifico.
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