Verdict out on relativity questioning experiment

 

If relativity is wrong, it is hard to explain how GPS works successfully, writes CORMAC O'RAIFEARTAIGH

IN SEPTEMBER 2011, a team of scientists at Gran Sasso in Italy announced that they had observed tiny particles of matter travelling slightly faster than the speed of light. The finding made headlines around the world as it seems to contradict Einstein’s theory of relativity. So was the experiment an anomaly or was Einstein wrong?

Q: What’s the big deal?

A: Relativity predicts that the speed of light in vacuum represents a fundamental speed limit for any object. The theory suggests that as a body approaches this speed, its mass (the amount of matter in it) will begin to increase, slowing it down. This dependence of mass on motion leads to the hypothesis that mass is simply a form of energy, ie E = mc2 (c is the speed of light in vacuum).

Q: But that’s just a theory, right?

A: Thousands of experiments have demonstrated the effect. The smallest particles of matter, accelerated to extremely high energy in giant particle accelerators, never quite reach the speed of light. Instead, their mass is observed to increase. Other relativistic effects are also observed: brand new particles are routinely created out of pure energy, just as predicted by E = mc2.

Q: So what happened at Gran Sasso?

A: The OPERA team at Gran Sasso was conducting an experiment where a beam of neutrinos – the lightest known particles – travel from the CERN facility in Switzerland to a detector in an underground lab 730km away in Gran Sasso. Neutrinos are expected to travel almost at the speed of light but time-of-flight measurements suggested they arrived 60 nanoseconds (60 billionths of a second) early.

Q: Is the result a simple mistake?

A: Not simple, no. This is a highly experienced team and they checked and rechecked their measurements over many months.

Q: So is relativity wrong?

A: Not likely. Most physicists expect there is a systematic error hidden in the experiment. The distance measurements seem straightforward, so attention is focusing on the time-of-flight measurements. The synchronisation of the measuring clocks in CERN and Gran Sasso is critical. Another problem is that it is not possible to monitor individual neutrinos since they are created when heavier particles (protons) decay. In the experiment, one compares the shape of a proton pulse leaving CERN with the shape of the neutrino pulse arriving at the detector at Gran Sasso. But a recent re-run of the experiment with a much shorter proton pulse did not remove the effect.

Q: Any other reasons for scepticism?

A: The constancy of the speed of light is a fundamental tenet of a second theory of relativity. This theory, general relativity, holds that space and time are affected by gravity. General relativity predicts many spectacular phenomena that have been observed, from the bending of light by massive bodies to the existence of black holes.

Q: What’s all this about relativity and GPS?

A: Relativity has also found an important technological application in GPS navigation. Here, the synchronisation of terrestrial and satellite clocks is critical. The clocks on GPS satellites have long incorporated a relativistic correction factor that takes account of the motion of the satellites relative to the Earth, and of the fact that they are in a weaker gravitational field. If relativity is wrong, it is hard to explain how GPS works successfully.

Q: Reasons for scepticism besides relativity?

A: Yes. One instance where light and neutrinos are emitted almost simultaneously is when a certain type of star explodes in a supernova. If neutrinos can really travel faster than light in vacuum, physicists should detect neutrinos from a distant supernova many years before the corresponding light arrives. This has never been observed.

Q: Suppose the result stands?

A: It’s hard to believe relativity breaks down at these energies, given the amount of supporting evidence. If no error is found in the timing of the OPERA experiment, perhaps there is something odd about neutrinos. One possibility is that the particles have found a short-cut through a higher dimension. Such extra dimensions are predicted by many modern theories of physics – but they are not expected to be manifest at these energies.

Q: So a sceptical verdict?

A: Yes. This result will be an anomaly unless it’s repeated elsewhere, like at the US’s Fermilab or Japan’s Super-Kamiokande.

Dr Cormac O’Raifeartaigh lectures in physics at Waterford Institute of Technology and writes the science blog ANTIMATTER