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Albert Einstein asserted that the speed of light is a fundamental constant and the maximum speed anything can travel. Photograph: Philippe Halsman/AFP/Getty

Tentative new evidence suggests Einstein’s rule that nothing can travel faster than the speed of light may be wrong. What are the implications for our understanding of the world?

What has been discovered?

A fundamental subatomic particle, the neutrino, seems to be capable of travelling faster than the speed of light (that is, the speed of a photon through a vacuum).

Why do physicists believe nothing can go faster than light speed?

At the turn of the 20th century, Albert Einstein used earlier work by the physicist James Clerk Maxwell to show that the speed of light, c, is a fundamental constant and that it is also the maximum speed that anything can travel. In practice, the only things that do travel at this speed are photons (particles of light) moving through a vacuum.

Einstein encapsulated c in his special theory of relativity, which says that the laws of physics are the same regardless of who is observing or experiencing them. To accommodate the invariance of the speed of light, Einstein had to modify Newton’s laws of motion so that time and space became stretchy concepts – as an object moves faster, its size contracts and the time it experiences slows down. Special relativity also leads to Einstein’s most famous equation, E = mc² (where E is energy and m is mass), which shows that energy and matter are equivalent.

Where on the scale of amazing/surprising is this finding?

If the Gran Sasso results are proved correct, scientists would have reason to believe that the current formulation of special relativity is wrong. This is troubling, since the theory has been tested countless times in experiments and has not been disproved. It is a cornerstone of our understanding of the universe.

The speed limit of light is also the basis of cause and effect: effects always follow causes. If that does not always hold, the basic laws of physics might have to be rewritten.

What exactly did the physicists do?

Scientists at the Opera (Oscillation Project with Emulsion-tRacking Apparatus) experiment in Gran Sasso, Italy, found that beams of neutrinos sent to its detectors from Cern, 730km away in Geneva, arrived earlier than they should have.

Special relativity also leads to Einstein's most famous equation, E = mc2

The trip would take a beam of light around 2.4 milliseconds to complete, but after running the experiment for three years and timing the arrival of 15,000 neutrinos, the scientists discovered that the particles arrived at Gran Sasso 60 billionths of a second earlier, with an error margin of plus or minus 10 billionths of a second.

Since the speed of light in a vaccum is 299,792,458 metres per second, the neutrinos were apparently travelling at 299,798,454 metres per second.

What are neutrinos?

Neutrinos are electrically neutral particles that have a tiny (but non-zero) mass. They interact very weakly with normal matter, making them almost impossible to detect. Tens of billions of neutrinos pass through your fingertip every second. They are created in certain types of radioactive decay, during collisions between atoms and cosmic rays and during nuclear reactions such as those that occur at the heart of the Sun.

Are there any theories that might explain the result?

If the result is proved correct – and that is still a big if – you have to go into some relatively uncharted areas of theoretical physics to start explaining it. One idea is that the neutrinos are able to access some new, hidden dimension of space, which means they can take shortcuts. Joe Lykken of Fermilab told the New York Times: “Special relativity only holds in flat space, so if there is a warped fifth dimension, it is possible that on other slices of it, the speed of light is different.”

Alan Kostelecky, an expert in the possibility of faster-than-light processes at Indiana University, put forward an idea in 1985 predicting that neutrinos could travel faster than the speed of light by interacting with an unknown field that lurks in the vacuum. “With this kind of background, it is not necessarily the case that the limiting speed in nature is the speed of light,” he told the Guardian. “It might actually be the speed of neutrinos and light goes more slowly.”

Does this mean that time travel is possible?

Don’t hold your breath, we won’t be routinely jumping into the past in DeLoreans any time soon. If particles could travel faster than light, special relativity suggests that travelling backwards through time is a possibility, but how anyone harnesses that ability to do anything useful is way beyond the reach of any technology or material we have today.

Physicists have postulated a hypothetical particle, known as a tachyon, that can travel faster than light and that can therefore move backwards in time. But they also think that tachyons, if they exist, would have no way of interacting with normal matter.

By Alok Jha, science correspondent guardian.co.uk,

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