(For previous posts in this series, see here.)

Neutrinos are very elusive particles that are produced in nuclear reactions. They interact hardly at all with anything, which enables them to penetrate anything easily. In any given second, tens of billions of neutrinos are coming from the Sun and passing though each square centimeter of our bodies and the Earth without doing anything, and heading off into the vast empty reaches of space on the other side. As a result of its extremely low interactivity with matter, it is hard to measure their properties, even basic ones like mass, because measurement involves getting the measured object to interact with the detector so that we know something about it. The existence of neutrinos was first postulated in 1930 as a theoretical device to explain missing energy in certain nuclear reactions but its elusive nature meant that it took until 1956 for direct experimental detection of their existence.

While the fact that neutrinos interact hardly at all with matter makes them hard to detect and discern their properties, this same elusiveness make them attractive candidates for measuring speed. This is because once produced they ignore everything in their path and travel in a straight line with constant speed so that measuring the distance traveled and the time taken does give you the speed. Even light is not as good for this purpose because both its speed and its trajectory are affected by the matter it passes through, as we all experience when we see how distorted things look when seen through glass prisms or bowls of water. Even slight changes in the density of the atmosphere can affect the path of light, which is the reason why we see mirages. So if you use light, the path taken by it in going from one point to another may not correspond to the straight geometric line distance connecting the two points that can be calculated once we know the coordinates of the two points, and so calculating the distance traveled by the light is not simple. But in the case of neutrinos, the path taken is dead straight and thus the geometric straight-line distance between two points will be the actual distance traveled by the neutrinos.

Another advantage is that the speed of neutrinos, unlike that of light, is unaffected by the medium it travels through. When light passes through glass or water, its speed is reduced which is the cause of the distortions we observe. As another example, take the light coming from the Sun. This light is produced as a result of nuclear reactions that produce both photons (particles of light) and neutrinos, among other things. But because the Sun is such a dense gas, it slows down light considerably and the photons produced at the core of the Sun can take as much as tens of thousands of years merely to reach the surface of the Sun, a distance of roughly 700,000 kilometers. Once there, it can travel freely in the vacuum of space to cover the remaining150 million kilometers to the Earth (over 200 times the radius of the Sun) in just over eight minutes. Neutrinos that are also produced in the core, however, travel almost as fast within the Sun as they do in the vacuum in space because matter is almost invisible to them. So if a neutrino and a light photon are produced in the same reaction in the core of the Sun, the neutrino will reach us long before the photon does.

Supposing the CERN-Gran Sasso experimental result holds up and the neutrinos are in fact traveling faster than the speed of light. Does this mean that Einstein’s theory of relativity is completely overthrown? No. Einstein’s theory does not rule out particles traveling faster than the speed of light. Such particles, known as tachyons, have always been allowed by the theory but we have never confirmed their existence so far. There have, however, been various false alarms in the past, which is part of the reason for the skepticism about the present claim.

What Einstein’s theory says is that if a particle has zero mass, then it travels at exactly the speed of light but if it has non-zero mass, then its speed can approach the speed of light but cannot attain it. Particles can approach the speed of light ‘from below’ (these are the normal particles we have experience with that always have speeds less than that of light,) or ‘from above’ (they always have speeds greater than that of light, and these are called tachyons that we have never shown to definitively exist), but neither can cross the barrier of the speed of light to the other side. So the existence of faster-than-light particles would not overturn Einstein’s theory of relativity completely since that theory always allowed for their existence, but would still be a momentous discovery because it would be a completely new phenomenon.

So does this mean that the existence of tachyons can be easily absorbed into existing knowledge? Not quite. The problem with the existence of tachyons is what it does to something known as ‘Einstein causality’, which is something that is connected to the theory of relativity, but is in addition to it. What this says is that if two events are causally connected, (i.e., one event causes another) then the cause must precede the effect. Going back to the commonly used bloodthirsty example, if person A fires a gun and the bullet enters person B, Einstein causality says that the firing of the gun by A must occur before the bullet enters person B because one caused the other. This seems eminently reasonable but we have to bear in mind that it is an assumption that is based on experience and, like all such assumptions, is subject to empirical scrutiny. If faster-than-light particles exist, the theory of relativity says that Einstein causality can be violated. i.e., effects can precede causes. It is this possibility, sometimes referred to as ‘going backwards in time’, that boggles the mind.

So how does the existence of tachyons violate Einstein causality? In the first post in this series, I gave an example where there seemed to be a situation of going backwards in time but said that this was not really so, because that was an illusion that arose due to the fact that we were dependent on when light from an event reached the observer.

To better understand what constitutes violations of Einstein causality, we have to get into the subtleties of what we mean by measuring distance and time, and this lies at the heart of the theory of special relativity. What Einstein did was make our understanding of how to measure distances and time more precise and operational, and in doing so altered our fundamental understanding of those two seemingly mundane concepts.

Next: Measuring time and space