Thursday, April 5, 2012

No "Faster Than Light" Neutrinos


Most people see science as purporting itself to be infallible and they can twist this perception for many reasons (e.g. "See, they didn't see what they thought they saw so science cannot be trusted.").  The truth is that science is a process.  It must be reproducible by others.  Sometimes, an experiment comes around that seems to defy the current understanding of science and people are quick to jump and accuse science of being unreliable.  Really, when results like this come to light, it is the duty of other scientists to scrutinize the results: to try to reproduce them and, if they cannot, try to find where the errors in the original experiment occurred.  Most of the time, radical findings are disproved.  When they are not, this is an exciting time for science to learn more about the world around us!  We scientists often spend as much time trying to disprove things as we spend trying to prove them.  Truly revolutionary results often exploit a subtlety in a theory (which in science means a highly tested and verified description of how something works and NOT a hypothesis or guess as it is sometimes used in everyday language) or law that opens the way to a deeper understanding.  Science is not created or invented by scientists - the Universe has its properties and we simply pursue the discovery of them so we can understand better how it works.


While at the APS April Meeting this past week, there was a lot of excitement (see the talk abstracts in this session) about the "faster than the speed of light" neutrinos that the OPERA collaboration claimed to have observed.  There was extra excitement since there was a final resolution at the beginning of the meeting along with a little drama.  There were even talks on how to use this new coverage as a great outreach opportunity to illustrate science as a process (don't think of the scientific method that you were taught in school - science almost never follows that prescription but it is a good starting point).  I've had many people bring this up to me when I talk about how gravitational waves are expected to travel at the speed of light but could travel slower - never faster.  Then there is usually someone who asks about the new neutrino results and this is when I get to talk about how science is a process.  So, I've decided that I would dedicate today's blog post to the subject matter.  Spoiler alert: there are NO "faster than light" neutrinos!  If you are interested in a very good discussion of these results, disproof, and aftermath, read more about it here.

***  What is a neutrino?  ***

A neutrino is a virtually massless particle that interacts so weakly with matter that it can travel right through any matter with only a few (of billions and billions) interacting with matter.  The neutrino has never been directly detected but we know when one interacts with matter because it produces other subatomic particles or radiation.  Every second, about 10,000,000,000,000 (that's 10 trillion) neutrinos from our Sun pass through every square foot when the Sun is directly overhead.  Those neutrinos pass right through you and, since they so rarely interact with anything, you don't notice a thing. 

Because neutrinos are virtually massless (I say virtually because there is evidence they they do indeed have mass, but it is so small that it hasn't been accurately measured) they can travel at or so near the speed of light that we haven't measured evidence of them traveling slower.  This agrees with special relativity: only massless particles can travel the speed of light and massive particles can only travel slower (there are theoretical particles called tachyons that can only travel as slow as the speed of light and travel faster otherwise - these have never been observed).

***  What is the OPERA experiment?  ***

The OPERA experiment used a beam of neutrinos created at CERN on the Franco-Swiss border to send to the OPERA detector in Gran Sasso, Italy.  That's right, the beam of neutrinos was shot right through the intervening earth between these 2 sites.  Since the distance is known to high precision, the time it takes the neutrinos to arrive at OPERA is directly related to their speed.  It appeared that they were measuring their arrival about 60 nanoseconds (0.00000006 seconds) before they should have if they traveled at the speed of light. 

***  What did we know about the speed of neutrinos before OPERA?  ***

There have been many experiments that have observed neutrinos traveling at the speed of light.  These experiments have been both Earth-sourced (where we create and then detect the resulting neutrinos) and Universe-sourced.  A spectacular example of using neutrinos from space was the detection of neutrinos that preceded the supernova 1987a.  They arrived 3 hours before the light from the stellar explosion did.  This is what is expected because neutrinos are created when the matter in the star collapses before the supernova explosion.  If neutrinos traveled as fast as the OPERA collaboration claimed to have observed them traveling, then after traveling the more than 160,000 light years to Earth they would have arrived 4 years before the accompanying light we observed.

***  Should OPERA have published their result?  ***

So, was the OPERA collaboration wrong to publish their observations?  Absolutely not (in my opinion at least)!  Nowhere in their paper did they claim that they have found a fault with the current understanding of the physics - they simply couldn't disprove their own observations so they opened their experiment up to the scrutiny of the scientific community.  They even recognize the controversial results and their desire for scrutiny of their experiment in their paper (which can be read in full here):
"Despite the large significance of the measurement reported here and the stability of the analysis, the potentially great impact of the result motivates the continuation of our studies in order to investigate possible still unknown systematic effects that could explain the observed anomaly. We deliberately do not attempt any theoretical or phenomenological interpretation of the results. "

In the end, it was found that a loose fiber optic cable and an error in their timing produced the superluminal (fancy way of saying 'faster than the speed of light') observations.  THERE IS NO EVIDENCE TO SUPPORT THAT NEUTRINOS CAN TRAVEL FASTER THAN THE SPEED OF LIGHT.  Also, the ICARUS experiment (located in Gran Sasso with OPERA) independently reproduced the experiment and found no faster than light neutrinos.

The heads of the collaboration resigned their post on March 30 (just a few days ago) after a vote of no confidence.  There were scientists in the collaboration who felt the publication of the results was premature, and that not everything that was done was good experimental procedure.  It seems that the resignations were the result of their rush to publish the paper, more than what they published.