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Re-testing super luminal neutrinos

If you missed it, the news is that the CERN experiment which observed neutrinos travelling faster than speed of light, is being done again. The goal is to eliminate one kind of possible error from the experiment.

I will try to explain what the error is, very briefly. What happened in the initial run was this. They had beamed 1020 protons (that is one followed by 20 zeroes, a 1oo billion billion), that collided into atoms to produce neutrinos. The number of neutrinos eventually detected was 16,000. These neutrinos, the scientists observed, reached the detector, 60 nano seconds (a nano second is 1 billionth of a second) ahead of the estimated time (the estimate was based on the speed of light).

The crucial point was that these 1020 protons were not released at the same instant. They were released as one single pulse, that spanned more than 1000 nano seconds. The detection of the neutrinos was also a spread. So what was measured was the average release time of the 1020 protons and the average detection time of the 16,000 neutrinos. But, we don`t know whether the detected 16,000 neutrinos were from protons that were released early in the 1000 nano seconds pulse or in the later part of the pulse, or were they too spread out in an average manner. If the source protons of the detected neutrons were spread out evenly in the initial 1000 nano seconds pulse, then the observations are correct. This is important since the neutrinos were detected only 60 nano seconds ahead of their scheduled arrival, which is very small compared to the duration of the initial pulse. Thus, it is possible that due to some unknown, all the 16000 neutrinos observed could have been from the later part of the initial pulse, thus invalidating the average starting time of these neutrinos.

This is what is being checked now. What the scientists are planning now, is to send multiple pulses of 1 to 2 nano seconds duration, instead of one single pulse of 1000 nano seconds. Moreover, in between these pulses, there will be a gap of 500 nano seconds. So there will not be any confusion on which is the source pulse of the detected neutrinos. Since the pulse duration of 1 to 2 nano seconds, is far less than the supposed early arrival period of the neutrinos (which is 60 nano seconds), this test should verify if the the results observed were indeed true.

I learnt a good deal about the experiment from these blog posts by Ethan Seagal, who explains the intricacies with a lots of pictures. His posts usually have a lot of pictures which are very helpful in visualising what he says. His post on this re-run have some very good pictures explaining this pulse duration problem and the possible bias of it. Have a look.

It could turn out that in this re-run, neutrinos arrive on time and not 60 nano-seconds ahead, as was previously thought. In that case, the whole thing can be scrapped, and we can get back to our work. But if this re-run confirms the earlier finding, there is still a possibility of a systematic error, which can be detected only if other teams of scientists replicate the observations. This re-test will take a few weeks. So it wont be long before the results are out.

Categories: Physics Tags: , ,
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  1. November 20, 2011 at 19:23

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