Faster than a speeding photon? Precursors test whether light can be faster than light

Over the past two weeks, the biggest physics news has been the apparent observation of neutrinos (nearly undetectable subatomic particles) moving faster than the vacuum speed of light.  At first glance, this would seem to violate Einstein’s special theory of relativity, which fixes the vacuum speed of light at meters per second, and as a consequence makes it in principle impossible to travel faster than that speed.  The theoretical implications are in fact a bit more subtle, but before we worry too much about those implications the experimental results will need to be checked carefully and independently verified.

While we wait, it is worth noting that in June 0f 2011 a group of researchers performed an experiment to see if light itself could move faster than light!  In particular, the scientists used a little known optical phenomenon known as an optical precursor to see if individual photons might travel faster than while propagating in a material.  In the end, the experiment suggests that these single photons did not in fact violate Einstein’s speed limit, though the results still got a significant amount of press.

The response of many physicists to the news was a collective, “Well, duh!”  The prevailing attitude seems to have been: “What’s so interesting about proving something we already knew?”  In this post I’d like to explore that question a little bit, and explain how some uncertainty remains about the behavior of light in materials.  Along the way, we’ll introduce the fascinating phenomenon of precursors, and see how they can be used both to probe the nature of matter as well as the nature of light.

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