Sep 23, 2011

Hold your neutrinos... err... horses

If you haven't seen the latest greatest in science recently, take a look here:

(pick up at least v2 for the updated measurement last week).

Oh, okay, so you want a "plain English" summary, then you should check (first and foremost) the CERN website:

In short: A recent measurement of the velocity of neutrinos indicates that they are 0.0025% faster than the speed of light in vacuum as currently measured. The way this is done is by firing a beam of neutrinos toward a detector, while you know the start and stop times very accurately from GPS time information. It is a measure of the absolute speed, not a relative speed to the speed of a light beam that is collinear.

I spent the afternoon when this was released at Harvard to meet some friends for lunch and chat some physics, and the conversation quickly turned to the OPERA measurement (thanks to Matt Strassler, Matt Schwartz, David Krohn, Jesse Thaler, and everyone hanging around on the 4th floor of Lyman). We bandied about many ideas of how this can be explained. Almost none of us were really willing to give up on the speed of "information" (I won't call it the speed of light anymore). The latest results conflict heavily with existing
data on supernovae, and there are lots of arguments as to why this
theoretically cannot happen to begin with. It's literally impossible
to keep up with the maelstrom of activity over the last few months,
the paper itself has over 100 citations already:

Some light reading if you're interested, I won't even pretend that
there is consensus:

A few friends asked me to comment on the "but what if it's true" scenario. There are a number of possibilities. First, let me
define the terms.

* Let's take "c0" to be the speed limit as predicted by general relativity.
* Let's take "c" to be the speed of light.
* Let's take "cnu" to be the speed of neutrinos.

Until recently there was no evidence that c was different from c0, nor
that cnu was faster than c (the latter of which has much evidence that
it's the same, for instance supernovae data).

The possible scenarios for what could be happening if the results really are true:

1. There really is no "speed limit" and relativity is therefore only
an effective theory (like Newtonian gravity), superseded by another
more accurate one.

2. Light does not travel at the "speed limit" because of some new
physics that interacts with photons that doesn't interact with
neutrinos (i.e. c < c0, and cnu = c0).

3. "Vice-versa" from (2): Neutrinos can travel faster than the "speed
limit" because of some other new physics that only affects neutrinos.
(i.e c < cnu), but it says nothing about the true speed limit (c0).

Some examples for (2) and (3) could involve extra dimensions, some
possible weirdness with the electromagnetic interaction which has
another broken symmetry with the weak interaction (aside from the
Higgs mechanism), etc.

This was a long post, but I hope you found it useful. Ultimately the "jury is still out" as the scientific community attempts to puzzle through this one. I'll try to maintain a somewhat up-to-date commentary (at least "up-to-date" on geological time scales! :) ).

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