The order in which you observe them to occur depends on the relative motion. If two events occur at different locations (say, flashing a torch) then, depending on how you are travelling relative to each of those events, you may see them occurring at different times (for instance, if you are accelerating relative to one then you will see it occur later, as if time is slowed). ![]() One practical aspect of relativity is that the concept of simultaneity is frame-dependent. Critically, it breaks (special) relativity, which states there’s an absolute speed-limit – the speed at which massless particles travel – that doesn’t depend on relative motion. If a particle is able to travel faster than c, a few odd things happen. Presuming for now all the possible sources of error are accounted for, what would this result mean? Time-travel seems to be the go-to topic when faster-than-light particles are mentioned, but don’t hold out hope for a TARDIS just yet. The OPERA scientists made use of the more precise GPS system and a cesium atomic clock to ensure their timing and positions were as accurate as possible. This does require the GPS antenna to be above ground, though, so one also needs to take into account the timing for signals to travel along wires to the underground experiments. At the very top range is “ carrier phase tracking”, which can beat one-centimetre accuracy. More sophisticated methods are used for proper surveying, such as differential GPS (10-centimetres accuracy). Traditional civilian grade GPS has an accuracy of about 15 metres. ![]() This means knowing those two positions – and the geodesic distance between them – to within three metres out of 730,000 metres. There’s the issue of knowing the exact positions of the source and detector to within the quoted uncertainty – keeping in mind that in the extra 60 nano-seconds the neutrinos are supposedly travelling they will cover a total of 18 metres. But on the face of it, it seems the OPERA team has been very careful. The peer-review process is usually quite efficient at eliminating likely sources of error, and in this case there are plenty of possibilities. The scientists concerned have released the findings to the scientific community in the hope that, if something has been overlooked, it will be picked up by their peers. If accurate, this would be a six standard-deviation result – enough to convince physicists that something is genuinely awry. ![]() What they claim to have found, though, is neutrinos arriving 60 nano-seconds (0.00000006 seconds) early. Using GPS timing and position data, the OPERA team claim to know the distance between the point at which neutrinos are emitted from the LHC and the point at which they are detected in Italy to a precision that allows them to predict the time the neutrinos should arrive to within ten nano-seconds (a nanosecond being a billionth of a second).
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