Faster Than The Speed of Light: The Neutrino Story

Neutrinos in the News

I first talked about neutrinos in an article called All About the Particles in Physics in a paragraph called "The Enigmatic Neutrino."  In this article, I explore these mysterious particles in more detail and update you on the latest research.

In September 2011, Scientists working at the OPERA experiment in Italy announced to the scientific world they had clocked neutrinos, a type of tiny subatomic particle, traveling faster than the speed of light. This gives you an idea of the experimental set-up:

Why is this such a big deal? For many of us, faster-than-light-speed particles don't strike us as all that mind-blowing. Star Trek has been making use of faster-than-light warp drive propulsion for decades. What is it about breaking the speed of light barrier that is so important? And did they actually do it? Allow me to set up the story for you.

First, What's a Neutrino?

Neutrinos are one of the most elusive subatomic particles in the universe. They have no charge and almost no mass, and that makes them almost impossible to observe and study.

On Earth, neutrinos come from several natural sources: from the decay of thorium and uranium in the Earth, from collisions between cosmic rays and atomic nuclei in the atmosphere, from supernovae and supernova remnants, and even from the Big Bang itself. However, most of Earth's neutrino bombardment originates in the Sun as solar radiation. Every second about 65 billion solar neutrinos pass through every square cm of Earth! Billions are zooming through you right now. Because they do not interact with your atoms, you don't sustain any damage to your cells. We generally detect no evidence of their existence whatsoever, so how do we know they're even there?

Neutrinos have been observed to interact only with the weak fundamental force. That is the force associated with nuclear decay and nuclear reactions. Neutrinos were first predicted to exist in 1931 when scientist Wolfgang Pauli, shown here, noticed than some undetectable particle must be carrying off a tiny amount of energy and momentum during certain radioactive decays.

In 1956, a particle was discovered that fit the neutrino's description.

Neutrinos pass right through the Earth undetected because they only very rarely interact with ordinary matter. Only extremely energetic neutrinos can be detected at all. The most energetic neutrinos come from supernova remnants where cosmic rays are accelerated through a process called Fermi acceleration. But even these neutrinos are very hard to detect. Many detectors utilize an enormous volume of water or ice surrounded by photomultiplier tubes, all this just to detect a few neutrinos. This is what the inside of the Super-Kamiokande experiment looks like. The boat image is placed to show how enormous it is. It is 1000 m underground and contains 50,000 tons of ultrapure water, surrounded by over 11,000 photomultiplier tubes.

(copyright: Kamioka Observatory, ICRR, The University of Tokyo)

The detector works like this: Very occasionally a neutrino will interact with an electron or the nucleus in a water molecule, and the collision will create a muon (another kind of subatomic particle) or an electron in the water. When it does so, it emits a cone of light.

This cone of light moves faster than the speed of light in water, creating the optical equivalent of a sonic boom, and this is what is detected.

The photomultiplier detector sees a well-defined ring. This light emission is called Cherenkov radiation. It's the blue glow you may have seen in photographs of submerged rods in nuclear reactors such as the Reed Research reactor in Oregon, shown here.

It is important to understand that the speed of light in water is 0.75 c, or 3/4 of the speed of light in a vacuum. Cherenkov radiation propagates faster than 0.75 c BUT NOT faster than the speed of light in a vacuum. If it did, physicists would have bigger fish to fry than faster-than-light neutrinos.

The upshot of all of this is that neutrinos are very ephemeral barely-there subatomic particles, at least from our perspective.

When neutrinos were first discovered they were assumed to be massless, just like photons of light. Now, researchers believe they must have at least some mass because they oscillate between flavours as they travel. This is unique among subatomic particles. Only a neutrino can start off as one kind, say an electron neutrino for example, and be detected at the end of its journey as a another kind, say a muon or tau neutrino for example. Neutrinos come in three flavours - electron, tau and muon neutrinos.

This discovery is linked with what was called the solar neutrino problem. For decades, scientists knew there were three different kinds of neutrinos. Nuclear fusion in the Sun creates only electron neutrinos, based on physicists' understanding of solar fusion. Yet researchers consistently got only about a third of the neutrinos they expected when they were detected on Earth. They were understandably only measuring electron neutrinos. In 2001, when scientists at the Sudbury Neutrino Observatory in Canada measured electron plus tau/muon neutrinos, they detected the missing neutrinos they expected, thus solving the solar neutrino problem. This is what the odd-looking spherical Sudbury detector looks like:

It has since been turned off but during its operation it housed 1000 tons of heavy water and 9600 photomultiplier tubes.

According to the Standard Model in physics, a well-established and well-tested model for all subatomic particles, flavour oscillations imply differences between the different neutrino masses. Neutrinos tend to change flavours when they pass through matter. The amount of neutrino flavour mixing that occurs depends on the square of their masses.

If you are unsure what flavour actually means, you are in good company. It can be explained like this: Neutrinos come in three flavours based on which particle they interact with. Electron neutrinos interact with electrons, muon neutrinos with muons and tau neutrinos with tau particles. Flavour describes the particle's overall symmetry. In most particles it is preserved. However, during some kinds of nuclear decay, this symmetry can be broken, as in the case of quark decay and neutrino oscillations. When a neutrino propagates, or travels, it is a mixture of all three flavours, all superimposed on each other at the same time. However, a neutrino can only interact as one specific flavor. Mathematically it can be shown that if a neutrino's mass was zero, it would not be able to change flavours.

This is not to say that the mass of a neutrino is known with any precision. However, cosmic behaviour puts a very narrow range on possible values for neutrino mass. It must be very small or these plentiful particles would have caused the universe to collapse in on itself long ago. In fact, the mass of a neutrino must be less than the a tiny fraction of the mass of an electron. If you are wondering, neutrinos cannot be the dark matter physicists are seeking - they do not have enough mass to account for more than about 1% of dark matter in the universe.

This sounds a bit complex but the general idea here is that most physicists believe neutrinos have mass, albeit, very small. And that simple notion adds a level of strangeness to the idea of them approaching light speed. Why is this so? Why is there a problem with mass and light speed?

Neutrino Velocity As an Open Question

If neutrinos are massless they should, according to special relativity, travel at the speed of light, just as photons do. If they have mass, they shouldn't even theoretically reach light speed.

Neutrinos are thought to have mass and they have been clocked at light speed in many experiments! For example, 10-MeV neutrinos have been clocked at light speed coming from a recent supernova called SN1987A, shown below. Actually, the supernova isn't really recent - it took place in the Large Magellanic Cloud about 160,000 years ago and the neutrinos that blasted out of it simply reached Earth in 1987, within hours of each other.

In this NASA image you can see circumstellar rings around SN1987A, with the ejecta from the supernova explosion in the middle of the inner ring.

Now, what does 10-MeV mean? Subatomic particle mass is most easily measured in electron volts. One electron volt is the amount of energy one electron gains when it undergoes a potential difference of one volt. 10-MeV is 10 million electron volts of energy. This value is the supernova neutrino's mass-energy equivalent. As Einstein discovered, mass and energy are the same thing. E = mc². This simple fact is very well documented by experimental evidence and it is one of the central foundations of the Standard Model in physics. And it is a central theme of our neutrino story.

Lets get back it now: Neutrinos have mass and can travel at light speed. How? The answer, at least for now, is in the numbers. When neutrino velocity is measured it is not 100% accurate. But physicists can get very close. This tiny wiggle room is where neutrinos can have a tiny mass and yet not violate special relativity. We don't have any detectors yet that can measure the tiny difference between neutrino speed and light speed. Neutrinos can very well travel just under light speed. We simply can't measure that difference (yet). For now, this places neutrino velocity well within the constraints of special relativity.

Light Speed is the Universe's Speed Limit

Why does special relativity put these constraints on mass and velocity?

Special relativity is all about the speed of light. Light travels at about 300,000,000 m/s in a vacuum no matter how fast the light source is moving. The speed of light is invariant. This fact came from Einstein's work using Maxwell's equations for electromagnetism. If you shine a flashlight through the front window of a spaceship traveling at 0.5 X light speed, the photons of light coming from it would still be traveling at light speed, not 1.5 X light speed. This simple fact has huge consequences. It means that space and time must, as a result, be variable. Space and time, together as one entity called spacetime, is bendable, twistable and stretchable. This bending of spacetime is described mathematically by the Lorentz transformation. Observers moving at different velocities may measure different distances, elapsed times, and masses of the same object, but they will always measure the speed of light as 300,000,000 m/s, or c.

According to special relativity, as an object with mass approaches light speed, its mass becomes infinite, from the reference point of an observer at rest. To accelerate an object past light speed would require infinite force. Force = mass x acceleration. The mass we are talking about here is relative. Hopefully that will become clear in a moment as we return to the idea of mass as mass-energy. For a particle at rest, its mass-energy is equivalent to the rest mass, but particles tend to be in motion, so mass-energy takes into account the momentum of the particle as a whole. How is mass relative then? If you were traveling right alongside an SN1987A neutrino you could theoretically measure its mass and it would be some very, very small value (some researchers estimate it to be a few eV. That's its rest mass. But if you could measure its energy as it struck a detector on Earth (two detectors on Earth, considered to be at rest, did so) its mass-energy would be considerably higher, about 10-MeV, because it's velocity is close to c. Even 10 million eV is relatively small for a particle's momentum and that's because neutrinos have almost zero mass and because their velocity is very close to BUT NOT c (or the mass would be infinite, right?). The table below compares mass to velocity:

Some people call this mass relativistic mass. It's perhaps more accurate to call it momentum. Physicists measure these mass differences as momentum differences (momentum = mass x velocity) when they smash particles together in accelerators.

You could put the mass-c (c stands for speed of light) situation in other ways if you like. To accelerate a non-zero rest mass to c requires infinite time with a finite acceleration or an infinite acceleration over a finite time. However you put it, accelerating a non-zero rest mass to c requires infinite energy. This is a speed limit placed on all objects with mass, including neutrinos. If neutrinos were clocked traveling faster than c, then special relativity would have to be rewritten and particle physics as we know it would have to revisited.

Even particles without mass such as photons CANNOT travel faster than light speed, according to special relativity. However, general relativity does not specifically inhibit faster-than-light travel (even for objects with mass), as long as the rules of special relativity aren't broken. We'll expand on this very cool notion (and other tricks) next.

So, How About Faster Than Light Speed?

Last September, physicists at the OPERA experiment in Italy detected neutrinos sent from CERN in Switzerland, a 731 km journey. They claimed the neutrinos arrived 60 nanoseconds faster than if they traveled at light speed. This result is in the red writing in the white rectangle in the first graphic of this article. The news sent the scientific world into a spin. This is where things get very interesting and where our story really begins.

Particles With Mass Traveling Faster Than the Speed of Light?

This tentative finding had huge implications, of which the physicists involved were well aware. Here are just three examples of the implications of faster-then-light travel:

First, it violates special relativity, for all the reasons above. To be wrong about the postulates of special relativity would mean that we still don't understand the basic behaviours of subatomic particles, even though mountains of experimental evidence backs up the Standard Model.

Second, it brings up the possibility of time travel. How? Well, consider two particles traveling close to but under light speed. If they are separated by space and not travelling parallel to one another, any events linked to these particles could be viewed as happening in different orders depending on where observers are located. By the same reasoning, faster than light travel could be seen as traveling backward through time if viewed from some other equally valid frame of reference. This is the basis for the concept that objects travelling faster than light speed also travel backward through time. Could neutrinos be time travelers?

Third, is there something special about neutrinos we just don't yet get? They are the most recently discovered particle and, being difficult to study, they are still elusive. As you have seen, physicists still don't know their rest mass with any precision. They also don't know their magnetic moment, another important descriptor for subatomic particles. Based on the nuclear reactor results, however, physicists can say with some certainty that neutrinos have a half integer spin, like neutrons and protons do. They also carry energy and linear momentum (that's how they were first detected).

Maybe neutrinos, and only neutrinos, can break some rules of special relativity. Having one type of particle that violates special relativity again brings the whole theory of special relativity into question, or at the very least it would require some tweaking to accommodate them.

The Speed of Light Can Be Broken - With Some Very Important Caveats

There are special cases where one could think of the speed of light barrier as being broken yet not breaking the rules of special relativity. Here are a few examples:

1. The uncertainty principle implies that an individual photon, for example, can briefly surpass c in a vacuum. This is the basis behind quantum electrodynamics. And it indeed allows particles, but only virtual particles, to travel backward in time. Here's the catch - virtual particles are detectable only as exchanges of force. Unlike real particles, they do not exist in any quantifiable or "capturable" way. We can detect neutrinos on a detector. That makes them real particles, even if they are tricksy little elusive things.

2. A vacuum has energy, so it is also possible that if the vacuum energy were lowered then light could travel faster through such a vacuum, called a Cassimir vacuum. This is how the theory works: Even a perfect vacuum has energy because it is full of virtual particles popping in and out of existence all the time, and these particles have some, very small, energy associated with their transformation. As a photon of light travels through a vacuum, it interacts with virtual particles. It is absorbed by them, and this gives rise to a positron-electron pair of particles. The pair just as quickly annihilates and gives rise once again to a photon. The time the photon spends as an electron-positron pair slows its velocity down to c. One the other hand, a photon traveling between two Cassimir plates won't be so encumbered because there is not enough space between the plates to allow for the wavelengths of many virtual particles to fit in. There are less virtual particles to slow light down so it should travel faster than c. The closer the plates, the higher the speed of light should be. The effect, however, is predicted to be extremely small and there is of yet no experimental apparatus sensitive enough to measure it.

3. Because spacetime is malleable, there is the possibility that spacetime itself could be accelerated so that an object within that accelerated region of spacetime could be observed to be traveling faster than light speed, even though in its frame of reference it is not violating special relativity. This argument has been brought up as a possible solution to the problem of faster-than-light-speed cosmic expansion, which according to several lines of evidence, occurred soon after the Big Bang. Here, special relativity is not violated because the expansion of spacetime itself exceeds c, not an object moving through it. NASA is even working on the theory behind a rocket that can warp space-time and therefore travel faster than light speed, at least from our vantage point here on Earth.

4. There is an attempt to modify special relativity into what is called doubly special relativity. In this case, Planck length is also invariant. Planck length is the smallest possible unit of length. It is derived from c and two mathematical constants: Planck constant and the gravitational constant. It does not change regardless of velocity (unlike any larger length measurement!). This tweaking of special relativity was motivated by recent work toward a theory of quantum gravity. Basic to this work is the idea that there is an ultimate minimum length, energy or even volume possible, under which quantum fuzziness obscures any possible meaningful measurement. A consequence of this is that it makes the speed of light variable, where photon speed varies with energy. This idea has been criticized because the Lorentz transformation, the mathematical description of spacetime, is not an observable phenomenon, so it should not be held to some standard of observable measurement. Perhaps more importantly, photons of widely varied energies from recent gamma ray bursts have arrived at almost exactly the same time at detectors on Earth, discounting the idea.

Physicists are not entirely out of their minds when they consider that neutrinos could travel faster than light speed, under certain conditions, which do not violate special relativity. But I think most would agree that such conditions would be extremely unlikely in the case of the neutrinos traveling from Switzerland to Italy in the OPERA experiment. They weren't in a Cassimir vacuum nor were they going through warped space-time, for example.

Lesson #1: Always Verify Results

Perhaps not unexpectedly, the OPERA results have been recently discounted. Neutrinos officially DO NOT travel faster than light speed. At this I let out a sigh of relief mixed with a tiny bit of disappointment, and even a small laugh. After all, the initial OPERA results created a delightful buzz in physics that we may not soon enjoy again. A few days ago, two leaders of the OPERA team resigned after a vote of no confidence.  Their resignations came after two major blows to their earlier findings. First, their timing of neutrinos is now linked to a faulty cable connection. And second, an independent research group, using the same OPERA equipment, was unable to replicate the faster-than-light results.

The OPERA physicists stated with their original findings that they hoped public release would foster further inquiry and debate. And they were open to possible sources of error in the experiment. But the results they obtained were just so mind-blowing! With perfect hindsight, it seems obvious that they released their findings prematurely.

Does that really close the books on the neutrino story? Perhaps, but all the controversy, all the excitement, betrays a wonderful passion that is alive and well in science doesn't it? And discoveries like this one challenge our understanding of the energy and motion of things right to the core, always a good thing.