[LMB] I never thought I'd see THIS come out of CERN

Nicholas D. Rosen ndrosen at erols.com
Tue Sep 27 04:15:04 BST 2011

With some caveats about possible measurement errors and 
so forth, physicists have announced that they seem to have 
observed neutrinos traveling faster than light.  This is very 
significant if true, and not just because the speed of light was 
taken to be an arbitrary speed limit.  If it be possible to transmit 
information faster than light, then a large part of modern physics 
comes crashing down, or at least will be in need of major 

According to special relativity, if two events at stars 10 light-years 
apart happen simultaneously in one reference frame, event A can 
happen 6 years before event B, and 8 light-years away in space, as 
measured by an observer in another, equally valid reference frame.  
Time and space are relative, although the four-vector is invariant.  
Therefore, if information can be transmitted instantly between A and 
B, or even faster than light, paradoxes arise; it becomes possible to 
send messages back in time. 

There have been questions raised about whether quantum weirdness, 
like the http://en.wikipedia.org/wiki/EPR_paradox  
Einstein-Podolsky-Rosen Paradox (that Rosen was no relation of mine), 
involves information moving faster than light.  The phase velocity of 
a wave (or particle, since at the quantum level, waves and particles 
cannot really be distinguished) may exceed the speed of light, but 
that doesn't matter, since no information moves at the speed of the 
phase velocity.  It's the group velocity that matters, and the group 
velocity cannot exceed the speed of light in vacuum, except for 
circumstances where it can, and then it turns out that no information 
is transmitted that quickly.

I seem to remember reading in a popular science article years ago 
of some evidence suggesting that neutrinos could be tachyons 
(faster than light particles).  Now we may have further evidence that 
they are, and even if they only crossed a distance of hundreds of 
kilometers 60 nanoseconds faster than light (beating photons in vacuum 
by 18 meters, if I did the math right), they could in principle be used to 
send messages back in time.

Or could they?  Neutrinos are hard to detect, they seem to exceed the 
speed of light only very slightly, and processing information would pose 
practical difficulties.

Perhaps some kind of "cosmic censorship" is operating, so that tachyons 
can't actually give rise to paradoxes, although it would seem that they 
could.  Read the science supplement in next week's newspapers, or next 

Nicholas D. Rosen
ndrosen at erols.com

More information about the Lois-Bujold mailing list