Neutrinos travel extremely quickly. We frequently use the phrase “blink of an eye” to indicate a brief period of time, but these tiny particles transit the entire planet in less time than it takes you to blink. They travel at nearly the speed of light, the fastest speed in the cosmos, making them the quickest mass-bearing particles. So, how quick are they?

The sun’s light takes eight minutes to reach Earth, as you may have heard. In reality, when you gaze up at the sun, you see it as it was eight minutes ago—before all those photons traveled 149.6 million kilometers to Earth. But odd things happen when moving at the speed of light. The voyage appears to take no time at all if you’re a photon. A photon moving at the speed of light completes its journey instantly. We travel far more slowly than photons, thus our perception of time is significantly different. According to Einstein’s theory of special relativity, time will stop progressing more slowly for an object moving at the speed of light. A photon thus experiences the entire universe’s chronology simultaneously.

Why do we talk about photons when we talk about neutrino speed? The quick response is that neutrinos practically move at a speed extremely close to that of light. By measuring the amount of time, it takes for neutrinos to travel over great distances, we attempt to gauge the speed of neutrino travel. The apparent difference between the speed of light and neutrinos, however, cannot be measured, not even in our most exact experiments. Let’s now get a little more detailed. Using Einstein’s special relativity equations, we could determine a neutrino’s speed from its energy if we knew the particle’s mass. The problem, however, is that we are unable to quantify the neutrino’s mass and do not actually know what it is. The neutrino mass has been reduced by several experiments, such as the KATRIN experiment, which recently demonstrated that the neutrino mass must be less than 0.8 eV/c2.

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Given an energy of 1 GeV and a neutrino mass of 0.7 eV/c2, this would indicate that the neutrino is moving at a speed of 0.99999999999999999995 times the speed of light. A photon would need 2.5 million years to travel from the Andromeda galaxy, which is 2.5 million light years away. The neutrino will arrive 0.0004 seconds after the photon if a photon and a 1 GeV neutrino leave from Andromeda at the same time. We now see why it is impossible to determine the difference using the speed of light. But why are we so certain that neutrinos travel at a slower rate than light? We are aware that one unique characteristic of neutrinos is their ability to alter flavor as they travel. This fact demonstrates that neutrinos do not move at the speed of light, in contrast to photons, which experience time.

The mass of neutrinos exhibits some peculiar quantum mechanical properties since it is actually a mixture of three separate mass states: mass one, mass two, and mass three. This is a case of quantum superposition. One thing to keep in mind is that waves are made up of both particles and have finite lengths. A particle can therefore be thought of as a tiny fragment of a wave or a “wave packet.” The wave packets of all three mass states are overlapping when a neutrino is created in a gas pedal because it begins life as a superposition of all three mass levels. Coherent neutrinos exist as long as the wave packets overlap. The neutrino oscillation phenomena results from the fact that any of these mass states can interact in the detector and that we are unable to distinguish between them.

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Because objects of different masses should move at different speeds, if the neutrinos propagate for a sufficiently long time, the three wave packets associated with the three different mass states will gradually scatter until they no longer overlap, which would result in what is known as decoherence. We wouldn’t observe neutrino oscillations in such scenario. Every time, we would observe the same ratio of electrons, muons, and neutrinos, but these decoherent neutrinos would have to travel a great distance—far beyond the range of any Earth-based experiment we have ever carried out. Because their energy are now too low for us to use sensitive detectors to pick them up, the only neutrinos that we are aware of that potentially experience this event are ones that have been traveling since the Big Bang.

In conclusion, it is by no means simple to measure the speed of neutrinos, and there is still a lot to learn. The OPERA experiment published a measurement in 2011 that demonstrated neutrinos were moving faster than the speed of light. However, further research revealed that neutrinos do in fact adhere to special relativity and that this was merely an equipment fault in the experiment. It’s interesting to note that ICARUS, one of the experiments that cross-checked the OPERA results and measured the same neutrinos as OPERA using a different timing scheme, still didn’t achieve any major success.

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