Can neutrinos travel faster than light?

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Two months ago, physicists working on the OPERA (Oscillation Project with Emulsion-tRacking Apparatus) experiment at CERN announced that they may have discovered particles travelling faster than the speed of light. This implies that either there are conditions when Einstein's theory of special relativity breaks down, or that the particles took a 'short cut' through spacetime, perhaps through an extra dimension. Either way, if information can arrive faster than light, then it should also be travelling back in time, challenging our linear notion of cause and effect.

The physicists involved came to this conclusion after measuring the time taken for over 15,000 particles, known as neutrinos, to travel the 732 kilometres from CERN in Switzerland to the INFN Gran Sasso Laboratory in Italy. This gave them enough data to conclude that some of the neutrinos were travelling at about 7.5 kilometres per second faster than the speed of light in a vacuum.

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Because of the enormity of this discovery they checked their results for over six months before publishing them here and asking if anyone could see a flaw in their experiment. There were numerous replies, including Andrew Cohen and Sheldon Glashow's letter published here. This states that their results cannot be correct as the neutrinos should be slowing down, and therefore loosing energy, as they travel and a paper published last month showed that this was not the case. All previous studies have shown neutrinos to travel at or below the speed of light.

Yet no one has been able to satisfactorily explain why the results of the OPERA experiment are flawed.

The possibility of faster than light travel will not be accepted by the scientific community until their work has been reviewed by experts in their field and their results have been verified numerous times. The first step in this process was achieved last week when another, more accurate, experiment recorded the same result. Their work has been published in the Journal of High Energy Physics and can be read for free here. Further experiments are being conducted around the world. Results from the USA's Fermilab (Fermi National Accelerator Laboratory) and Japan's J-PARC (Japan Proton Accelerator Research Complex) are expected to be published in the next few months.

Time and relative dimensions in space

In 1905, Einstein showed that nothing can accelerate to the speed of light as this would require an infinite amount of energy. He came to this conclusion by combining two accepted scientific theories. The first theory states that the speed of light is measured to be the same from all reference frames, this was proven in the Michelson-Morley experiment of 1887. The second theory, Galileo's theory of relativity, states that experiments will give the same results whether you are stationary or travelling at a constant velocity. Einstein's theory of special relativity shows that both statements cannot be true unless our perception of time and space changes depending on how fast we are moving. Time and space change in such a way that objects travelling faster than the speed of light will appear to travel backwards in time. Special relativity allows for the existence of particles that travel faster than the speed of light, known as tachyons, but, unlike neutrinos, tachyons cannot slow down to light speed.

If neutrinos can travel faster than light, then the implications of this will not be known until we understand why they act this way. The theory of special relativity could remain intact if the neutrinos travelled through space and time in a wormhole or through another dimension, and proponents of string theory have already suggested this may be a possibility. If this is not the case then special relativity may need to be modified either because there is no universal speed limit or, less dramatically, because the limit needs to be changed. Whatever happens, all of the predictions of special relativity that have been experimentally verified will remain intact. Physicists will then try to discover under which conditions Einstein's theory breaks down. This is consistent with other breakthroughs in science including the leap from Newton's theory of gravity to Einstein's.

If the theory of special relativity is modified then this will affect every theory that is derived from it including Einstein's theory of general relativity, which has been used to explain how the universe evolved, and quantum field theories of gravity such as string theory. It would also show that the standard model of particle physics, that the discovery of the Higgs boson was hoped to confirm, is wrong.

If neutrinos can travel faster than light, then this contradicts our understanding of cause and effect since they will also be travelling backwards in time. Perhaps the Large Hadron Collider will one day be known as the world's first time machine. Although this may not lead to drastic technological breakthroughs (such as perpetual motion machines) since it may take too long to interpret the information.

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