Ripropongo qui un articolo scritto qualche anno fa da me in inglese sul tema dei viaggi nel tempo. Naturalmente non è aggiornato. Lo scienziato S. Hawking, le cui teorie sono in parte qui riportate, ha spesso cambiato idea in tema di viaggi nel tempo. Ultimamente ha anche rivisto le sue teorie sui buchi neri, tanto che alcuni giornali, esagerando come al solito, hanno titolato che i “buchi neri non esistono” oppure che “non sono neri”. Ad ogni modo tempo fa, sosteneva che i buchi neri, contrariamente a quello che si pensava, non possono essere utilizzati per viaggiare nel tempo. Ultimamente ha rivisto queste sue teorie… poco male perché per quello che mi riguarda non esiste una “verità assoluta” e come dimostra lo stesso Hawking tutto può essere rimesso continuamente in discussione. Dopo tutto ci sono sempre delle cose che non tornano e proprio in questo consiste il fascino della ricerca scientifica. Quando poi si traspongono teorie scientifiche nella narrativa tutto acquista un gusto particolare perché quello che non si può dire o fare in un laboratorio di ricerca lo si può fare nelle pagine di un romanzo o di un libro di racconti e basterà miscelare poi il tutto come in un shaker da cocktail per fare nascere nuove idee magari risolutive in entrambi i casi. Che nessuno me ne voglia… Per intanto mi fa piacere mettere alla fine di questo post un filmato su delle particelle che esistono solo in teoria. Forse il video spazia fra scienza e fantascienza, ma comunque propone idee intriganti. Tiriamo fuori allora in questa sede come dal cilindro di un prestigiatore una inesistente particella. Signori e Signore ecco a voi i…”tachioni”!!!!!! Attenzione il tachione è una particella ipotetica, superliminale (viaggia a una velocità superiore a quella della luce). Con queste particelle si possono fare cose strabilianti. Qui pongo solo una domanda: sapete quanto tempo ci vuole per andare da un estremo all’altro della nostra galassia? Un centinaio di migliaia di anni. Insomma meglio che rimaniamo a casa nostra tanto…ma con i tachioni beh eh…
Are time travels really possible?
A colour, a taste a flavour or a song and our memory allow us to go back in Time, in that particular moment of our life that we would like so much to experience again. It is not enough for the protagonist of The Time Machine (2004), a movie based on the book of H.G.Wells written in 1895. He builds a machine to travel back in Time, with the aim of changing the past to save from a murder the women he loves. Unfortunately (or fortunately) he cannot change it and the result is to assist again to his beloved woman’s death.
The first episode (1988) of a popular saga by Robert Zemeckis, Back to the Future, deals with the paradoxes arising when travelling back to the past. In the movie, the protagonist Marthy Mc Fly (interpreted by Michael J. Fox) accidentally goes back in Time when visiting an old friend, a scientist who is building a Time machine. In the past, Marthy has to confront himself with a weird situation. He meets his future mother who falls in love with him, with the consequence that his own life is at risk.
Time paradoxes are of great interest for Physics. Travelling back in Time implies the possibility to break the cause-effect relationship. As Back to the Future originally showed, if we go back in Time and we change the past, the future will change according to it. In this sense, the effect comes before the cause with consequences that can be dangerous. Scientist Stephen Hawking, known to the public for his best seller A Brief History of Time, in a public lecture on “Space and Time Warps” explains the solution probably at the basis of Back to the Future. The solution is based on the sum over histories concept introduced by the physicist Richard Feynman. According to Quantum Theory, the Universe doesn’t have just a unique single history. Instead, the Universe has every single possible history, each with its own probability. Alternatively, Hawking proposes the Chronology Protection Conjecture: the laws of physics conspire to prevent time travels on a macroscopic scale.
The Arrow of Time Reversal
If not with a Time machine, there could be another way to live backward our life. Time and Space originated with the beginning of the Universe according to the most accredited theory of the Big Bang. One of the hypothesis about the end of the Universe is the Big Crunch. According to this theory the Universe, that is now expanding, will re-collapse again. As Stephen Hawking pointed out in a public lecture dedicated to the “Beginning of Time”, one speculation associated with the Big Crunch hypothesis is that the collapse would be the time reverse of the expansion. As a consequence the arrow of Time would point the other way in the contracting phase and people would get younger, as the Universe gets smaller, eventually disappearing back into the womb. Isn’t that charming? Unfortunately, this is not the case as Hawing reveals. In the contracting phase the Universe will get more and more lumpy and irregular, as it gets smaller, and disorder will increase. “This means – explains Hawking – that the arrow of Time will not reverse. People will continue to get older, even after the Universe has begun to contract”.
As we have seen, travelling to the past implies many difficulties. Eventually travelling to the future is less complicated.
A shortcut to other worlds
Travelling to the future is clearly possible. Einstein’s Special Theory of Relativity has shown that Time is relative to the observer and depends on the speed of the medium. When approaching the speed of light (300.000 km/sec.) Time slows down, nearly stops. Particles can travel at that speed. As concerns us, human beings, we are far more complicated.
A similar type of time travel is permitted by General Relativity. Time in a neutron star with respect to an external observer would considerably pass more slowly. This is due to the effect of gravity. Celestial bodies with huge mass and density slow down Time at the point that in a black hole, Time would stop. Black holes are bodies with gravity so intense that even for light it is impossible to escape. Gravity is at the origin of really weird concepts related to the space-time curvature. We are familiar with some of them by watching movies or TV serial. Believe it or not they are not the product of human imagination.
In Back to the Future or in The Time Machine, we do not see how the machine is able to travel in Time. In a more sophisticated movie, Contact, we can see with the help of imagination what happen when entering a wormhole. A book written by Carl Sagan, who in turn consulted the physicist Kip Thorne, inspired that movie. Wormholes are shortcut from one point of the Universe to another. They are like holes created by a worm travelling over the surface of an apple. If the worm burrows a wormhole directly through the apple, instead of staying in the apple’s surface, distances shorten considerably. The surface is the Universe by analogy. Physicists speculate that black holes are able to warp the Space-time to the point of creating a wormhole that in theory allows Time-travels in any direction either in the past or in the future, even in other universes to allow the solution of time paradoxes. When passing from theory into practice – as for instance building a spaceship able to really do this experience or creating an artificial wormhole in a Lab – things get far more complicated and Time travels almost impossible. Or not? There are physicists convinced that is really possible to build a Time machine. Other physicists are instead sceptic. Among them, Stephen Hawking has changed his mind different times, maybe because, as all of us, he is a Time-travelling dreamer. Nevertheless since he is a physicist he cannot ignore signals of new possible evidences. In a recent article about the black hole information loss problem he writes: “ I am sorry to disappoint science-fiction fans but if information is preserved, there is no possibility of using black holes to travel to other universes”.
An “artistic” reality prefigured by science
Newton was convinced that Time was absolute. Einstein’s physics demonstrated the relativity of Time and Space. This charming reality is difficult to understand. In 1931 the Spanish painter Salvador Dali painted a surrealistic picture, interpreting one of the mystery of our life. Despite the progress of Physics, our relation with Time remains a mystery. Dali called the painting “The Persistence of Memory”.
“Soft” clocks represent Time and one may wonder if he was inspired by Einstein’s theories. In any case, Memory had an important and eventually positive role in that painting that was reinterpreted by Dali in a different way years later (The Disintegration of the Persistence of Memory, 1954). Both this paintings show that despite Physics, our relation with Time depends on us. After all, clocks are a human invention, used to represent Time. On the contrary, the Universe does not need clocks, nor it needs to be travelled. It is as to say that Time does not exist if we cut off the observers, the human beings.