Yet another voice in the neutrino OPERA

I feel just a little prophetic, given that only a week ago I wrote,

“... it is also useful to simply measure as many new things as possible. History is littered with moments where we didn't bother measuring something because we knew what the result would be, only to get a big surprise whenever someone finally did measure it. So nowadays if something is measurable, then someone, somewhere, is trying to measure it.”

Well, true to the claim, someone out there had the ridiculous idea that measuring the speed of neutrinos was a good idea, go figure. And, lo and behold, it seems they've found something! Maybe? Who knows!

Well, what sort of blog writer, who is a scientist (physicist even), would I be if I didn't join in with the neutrino cacophony? So, what follows is my attempt to say something interesting that hasn't already been said a million times already. I will focus on why this is such a surprising result and why this means nobody believes it.

This disbelief has nothing to do with the quality of the experiment. The measurement was recorded with impeccable accuracy and has been extremely carefully analysed. Neither is the disbelief because the result "proves Einstein wrong". It even has very little to do with the prospect of over-turning special relativity.

The disbelief is due to the fact that this observation brings into question the holy principle of causality – that is, the seemingly incontrovertible fact that cause must come before effect and not after

And, as my former PhD supervisor, said in the Guardian today:

“If we do not have causality, we are buggered."

For people living under rocks, what has happened?

Just briefly, in case anyone reading this isn't aware of what I'm talking about, an experimental group, OPERA, based in Italy, have been detecting neutrinos (a type of fundamental particle) that were generated at CERN (not in the LHC), in Geneva. OPERA have also calculated the speed of these neutrinos. What has made this front page news on the websites of the BBC, Guardian, Daily Telegraph, Nature, New Scientist, etc is that these neutrinos seem to be travelling faster than the speed of light. This is problematic and if true would be arguably the biggest discovery for 100 years, or more. The original article is here.

So why doesn't anyone believe it? (Would you believe the impossible?)

I think it is safe to make the following two claims about this result:

1. Almost no physicist, including those at OPERA, believes (yet) that these neutrinos are actually travelling faster than the speed of light.
2. Absolutely nobody can see any fault in OPERA's methodology, yet.
   
   

It might seem that this is an incredibly odd viewpoint for physicists to hold. How can you be completely convinced that something is wrong, despite the fact that the method used to arrive at the result appears to be 100% correct? It isn't an odd viewpoint though at all. I'm going to try to explain why through analogy.

Suppose you were a juror in a murder trial. Now suppose an expert witness explains how they can determine the time of death of the victim based on the body temperature of the victim's body when it was found. They explain how this has been used in many different trials and has always been accurate to within ten minutes. Based on this evidence, you decided that their method must work well and should be trusted.

If the expert witness was to then conclude that the victim died 30 minutes after they were last seen alive, you would probably choose to believe their testimony. It is trustworthy and the result also makes sense. However, suppose the expert witness instead concluded that the victim died six hours after the body was found. Then, despite the fact you previously thought the method was entirely sound and you have found no particular flaw in the methodology you would probably have serious doubts about the testimony. The point is, that while you wouldn't know exactly what went wrong in the analysis, you would presume that something had to have gone wrong because the outcome was too unbelievable.

The same thing is happening with these neutrinos. There doesn't seem to be any flaw in the methods OPERA have used, but the result is just so unbelievable that it is far more likely that something has gone wrong than that the result is correct, even though nobody has any idea what.

This concept was summed up by Carl Sagan (apparently) with the phrase “extraordinary claims require extraordinary evidence”. OPERA have really, really good evidence, but neutrinos travelling faster than the speed of light is extraordinary and they don't have extraordinary evidence.

Why is faster than light such a big deal?

One of the consequences of special relativity is something called “the relativity of simultaneity”. This is an effect that causes two people (observers) moving relative to each other to not consider the same set of events to be simultaneous. More simply, what this means is that if one of the observers sees two events happen at two different places at exactly the same time, the other one will see them happen at different times. It is important to realise that this isn't an illusion but a physical fact. Events that happen at exactly the same time for one observer do not happen at the same time for another. This is counter-intuitive, but it is the way the world works.

Also important to realise is that, although the relativity of simultaneity was first predicted by special relativity, it has subsequently been observed many, many times in many different circumstances. Therefore, even if special relativity turns out to be wrong for some reason, the relativity of simultaneity will still be true.

So, why is relativity of simultaneity a problem if neutrinos are travelling faster than light? The problematic situation arises when you consider two separate events, A and B, that happen at locations far enough apart that a light ray could not have passed from one event to the other. Because of relativity of simultaneity there will be potential observers that will see event A happen before event B, and other potential observers who see event B happen before event A. Note that the events need to be separated like this for this statement about simultaneity to be true.

If neutrino beams can travel faster than light, there is an immediate problem. A neutrino beam that is emitted at event A could actually be received at event B, and will, according to some observers, arrive before it left!?!

Even worse, what if someone at event B decides that whenever he receives a neutrino beam from event A, that he will send a neutrino beam of his own back to the location of event A? Because of the relativity of simultaneity, according to some observers, this neutrino beam from the location of event B to the location of event A will arrive at A before A has sent her beam. But what if A then chooses because of this not to send her beam at all!? Well, then B will never receive the neutrino beam and so will never send his beam. But A is there, looking at B's beam, so B must send his beam. But he can't, but he must, but he can't, but he must.

And causality has evaporated in a puff of faster than light neutrinos.

However, if instead nothing can travel faster than light, then no information can ever be sent between these events, A and B. And thus causality survives.

Now, one last time for emphasis, while this effect of relativity of simultaneity was initially predicted by special relativity, it has now been seen and measured. It is real. It is fact. So while faster than light neutrinos would indeed cause special relativity to need to be corrected, relativity of simultaneity would survive. That is why this neutrino observation is so very, very shocking. An observation that simply overturned special relativity would certainly be interesting and would generate a lot of interest. However, overturning causality would result in a paradigm shift of philosophical proportions.

So forget the neutrinos proving Einstein wrong and forget them overturning modern physics. These neutrinos seem to have a problem with cause and effect itself.

I couldn't have said it better myself...

So that is why nobody believes these results. OPERA themselves have done a stellar job and if they'd measured anything else with this much care and precision people would be jumping on this result. But yeah...

“If we do not have causality, we are buggered."

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