Solar Neutrino Problem

Not understanding the actions of neutrinos seems to be a common theme for scientists. As discussed in last weeks ADYK, claims that neutrinos travel faster than the speed of light are currently stumping scientific theory, but neutrinos have always been mysterious particles...  

 

We discussed last week that neutrinos are neutral subatomic particles that are created in large quantities inside the sun. Astronomers in the 1940's hypothesized this, and decided to build detectors on Earth that would measure the amount of incoming solar neutrinos here on Earth. Based on the then current theories of solar fusion, astronomers predicted how many neutrinos they would expect to see in their detectors. Long story short, they measured only one third of the expected number of neutrinos, and the solar neutrino problem was born. Astronomers spent the next 50+ years trying to figure out where their theories went wrong. It was originally thought that neutrinos were massless particles, like light, and therefore existed in one form only. This turned out to be the problem, as was hypothesized by particle physicists in the 1960's and 1970's. If neutrinos had a tiny bit of mass, then quantum theory says that they have the ability to switch between three different "flavors" of neutrinos (electron, muon, tau), each with slightly different properties. If this were true, electron neutrinos would be created and released by the sun, then on their travels towards Earth, probability suggests that 33% would switch to tau neutrinos, and 33% would switch to the muon neutrinos. The detectors in the 1940's were only sensitive to electron neutrinos, and thus never detected the other two types (aka the other 66% of the missing neutrinos!). This sounded like a fantastic solution, but astronomers and physicist had to wait until the years 1962 and 2000 for the first detection of the muon and tau neutrino, respectively, thus confirming the theory. It was a long wait, but proving that neutrinos come in three flavors allowed scientists to refine the standard model of particle physics, and astronomers to really understand what was happening in the interior of stars.

Neutrinos Break the Speed Limit!

 It's by far the hottest news in physics and astronomy right now: neutrinos were found to move faster than the speed of light!

What are neutrinos and where do they come from?

Neutrinos are a subatomic particles. They are essentially the building blocks of atomic particles such as protons and neutrons which are inside atoms. Neutrinos come in different types (or flavors as particle physicists call it) and are often the result of nuclear reactions or radioactive decay of an atom. They are created in stars and supernova explosions, and we can also create them here on Earth using particle accelerator labs. Neutrinos are electrically neutral which means that they don't interact with things often and can therefore travel far distances and through thick objects without ever being bothered. As an example, the sun creates so many neutrinos that every square cm of our body is being hit with 65 billion neutrinos every second! That's right every second! And we never feel anything.

How do you measure the speed of neutrinos?

The first task at hand is to build a device that can detect neutrinos. It's very hard to stop a neutrino, but particle physicists have figured out a way to detect there presence. I'm not going to go into the details here, so you'll just have to take my word for it. The next task is measuring their speed, and that theory is simple. Velocity, or speed in a given direction, can be calculated by taking the distance traveled and dividing by the time it took to move that distance. It's the same concept as driving a car. The distance from point A to point B, divided by the time I took to get there, gives me my average traveling speed in miles per hour. Now imagine a very long underground vacuum tube like the one particle physicists have at CERN. Neutrinos are created at point A at a given time, travel down the tube, and are detected at point B some amount of time later. We know how long the tube is, and we know the departure and arrival time of the neutrinos, so we can calculate a speed!

Faster than light?

Assuming that neutrinos travel at the speed of light (which is the current theory), particle physicists knew how long it should take for the neutrinos to get form point A to point B. When they looked at the timestamps given by the computers, the neutrinos actually arrived at point B ~60 nanoseconds too early! (That's 0.000000060 seconds) This implies that they traveled faster than the speed of light! (By a very tiny amount, but still measurable). The team that conducted the experiment has been checking for any possible errors that they may have made, and have yet to find any.

If no mistakes were made, what does this mean?

Attempting to interpret this result has some pretty cool implications in the world of physics. What does it mean if a neutrino travels faster than the speed of light? Well it sort of means that the neutrino traveled backwards in time! Our sense of time is defined around the speed of light, so if something moves faster than light, it can be observed before it even occurs. Very weird to think about! Another option is that the neutrinos took "short cuts" through higher dimensions during their travels. In this sense, the neutrinos didn't travel faster than light, they just took a shorter path from point A to B that we as humans can not perceive. The final option is that certain parts of the theory of relativity are incorrect and objects can move faster than the speed of light, without traveling back in time or entering different dimensions. Which one of these solutions is right? Well we will have to wait for the next Einstein to come along and figure it out!