Tuesday, December 13, 2011

TIME TRAVEL: Big Bang Theories, Space-Time and the Expanding Universe - Neutrino Misbehavior Suggested 50 Years Ago, Can Neutrinos Travel Into the Past?!

Can neutrinos travel into the past? That's the title of a New Scientist news article from 1963. It reports on a fascinating paper: "Neutrinos and the Arrow of Time in Cosmology", which suggests an experiment involving a neutrino emitter and detector – unimaginable at the time.

The author of the 1962 paper was Jayant Narlikar, a Cambridge physicist who, along with Fred Hoyle, Thomas Gold and Hermann Bondi, believed there was no start to the universe – no "big bang", as Hoyle so memorably (and mockingly) put it. Space-time in an expanding universe changes over time, so the full quantum description of a neutrino in that type of universe is different in the past and the future. The paper analyses how neutrinos would behave in both expanding and unchanging "steady state" universes. Narlikar found that in a universe that is expanding after a big bang event, neutrinos would turn up at a detector before they were emitted. "Only future-going neutrinos were possible in the steady state cosmology while the ever-expanding big bang models gave neutrinos travelling into the past," Narlikar told me. If you see firm evidence of neutrinos arriving at the detector before they are sent, that can't happen in a steady state cosmology, so the big bang has to be right. Or equivalently, no faster-than-light neutrinos, no big bang.

New Scientist declared that the paper, "when stripped of its mathematics, reads like high-class science-fiction". The writer points out that "no practical details are given so now it is presumably up to the experimental physicists." The experimental physicists took half a century, but we can now measure the comings and goings of neutrinos – as shown by all the fuss surrounding the faster-than-light neutrinos supposedly seen in Italy's Gran Sasso lab. In fact, faster-than-light neutrinos can be interpreted as travelling into the past, as this Guardian Q&A describes. According to Narlikar's 1962 paper, the Gran Sasso results could be seen as tentatively offering support for the big bang theory – if we could find a way to test that they are indeed travelling backwards in time. "I have not been able to relate the idea to the 'faster than light' neutrinos since there are no causality checks to decide if they are travelling in the past," Narlikar told me. Narlikar's idea may not solve the mystery of faster-than-light neutrinos, and it may not even shed much light on it – the details of the analysis might be wrong, or be filled with old-fashioned ideas. But I thought it was fascinating that a 50-year-old New Scientist story touched on a topic that is all over the headlines today.

The other interesting thing is that, apart from a few die-hards, no one now doubts the big bang theory is correct anyway. Almost everyone doubts the Gran Sasso results. Welcome to science: it's not about cast-iron, dictatorial results that leave no room for doubt. Instead, we weigh the merits of each piece of evidence. The widespread acceptance of the big bang is due to the cosmic microwave background radiation, discovered not long after Narlikar's paper was published. Science built on the discovery of this radiation, the first light to travel through space after the big bang, has allowed us to reconstruct the entire history of the universe. But Narlikar, Hoyle and others never accepted that these observations consigned the steady state universe to the dustbin of physics. Narlikar still says the widespread acceptance of the big bang is the result of "prejudice". Equally, if the Gran Sasso results are disproved to the satisfaction of most, some will always refuse to accept the consensus. And, paradoxically, that is good news: to keep it on track, to stave off error, science requires well-qualified contrarian challengers. - New Scientist.

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