Saturday, July 28, 2012

Star Trek: Neutrinos

Star Trek the Next Generation (1987-1994) (Picture)
Season 2, Episode 8
A Matter of Honor

In an episode where Riker becomes first officer of a Klingon ship, there was a brief mention of neutrinos. A beam of neutrinos was used to remove organisms from the hulls of ships.

You may have heard of neutrinos, cute and mysterious little particles, but what are they and where do they come from? Could they actually be used to rid a ship of organisms?

What is a neutrino, exactly?

Neutrinos are very tiny neutral particles that until recently were believed to be massless. They were first observed in β decaying isotopes, which emit neutrinos during the decay.

Neutrinos are hard to "see" because they are very weakly interacting. They can penetrate almost anything without there being any sort of interaction. They commonly pass all the way through the earth without any noticeable interaction.

This is why neutrino detectors are very large. The image below shows the Super-Kamiokande detector in Japan. It can hold 50,000 tons of ultra-pure water when in operation. The small golden dots on the detector are photomultipliers. Since they interact so infrequently, a large detector is needed to give as much of a chance as possible to "see" a few neutrinos.

Neutrino physicists tend to have to deal with very small data sets because neutrinos are so difficult to detect.

Super-Kamiokande, a neutrino detector in Japan (Source)

Neutrinos, in addition to being small and hard to detect, travel near the speed of light. Recently is was actually believed (due to the OPERA experiment) that they traveled faster than the speed of light! It seems now that a faulty cable or two were to blame for the superluminal measurements.

These fascinating little particles come in three "flavors": electron, muon, and tau.

Where do neutrinos come from?

Some of the many places neutrinos in our galaxy come from are: nuclear reactors, the sun, supernovae, and the Big Bang.

In nuclear reactors, typically Uranium-235 is fissioned (split) by neutrons. The products of such a reaction are incredible varied. All elements smaller than Uranium-235 have some probability of being produced in the reaction. Many of these products are radioactive, and some of them reach stable modes via β decay. Neutrinos are emitted along with a positron or an electron during the decay.

β- and β+ decay (Source)

In the sun, you may know that hydrogen "burns" to produce energy and helium. During this process, hydrogen nuclei (protons) fuse together. In this case, four protons fuse to create a helium nucleus. However, a helium nucleus has by definition only two protons. In order to achieve this, two of the protons must "convert" to neutrons. This occurs via β+ decay, in which a proton converts to a neutron and emits a positron and an electron neutrino.

The same types of nuclear processes occurred during the Big Bang and during supernovae: when heavier elements are created via fusion it is often necessary to convert protons to neutrons, and occasionally the opposite is true. In the case of a neutron converting to a proton, an electron and an anti-neutrino are emitted in a process called β- decay.

Why do physicists seem to think it is impossible for neutrinos to travel faster than light?

Light has been said to be a sort of cosmic speed limit. When Albert Einstein and others developed the theory of relativity, the central principle was that the speed of light is constant in all frames and nothing with mass can travel as fast or faster than the speed of light.

Since neutrinos have mass, the existence of superluminal neutrinos would violate of Einstein's theory of relativity. As it stands, the theory holds that it would take an infinite amount of energy to accelerate anything with mass to the speed of light.

Neutrino beams on the Enterprise

Unfortunately, I'm going to have to be a killjoy. I don't see how it would be possible to use a neutrino beam to rid a ship's hull of an organism. As we've seen already, neutrinos are very weakly interacting. The vast majority of the neutrinos would pass right through the organisms and the entire ship. It would probably make more sense to use a laser to rid the ship of the pesky organisms.

Bonus: neutrino oscillations

There is an obvious question here: how do we know neutrinos have mass? If it turns out they don't have mass, then maybe there is no violation of relativity. And maybe there is some new physics in all this.

As it turns out, neutrinos have a special way of telling us they have mass: they "oscillate."

What this means is that they change between their three forms. Imagine a chameleon. This chameleon lives in a three sided room. One wall is green, another red, and the last one is blue. Depending on where in the room the chameleon goes it will choose to blend in with the red, green, or blue wall color. Neutrinos act somewhat like this chameleon; as they travel through space near the speed of light, they switch between their three flavors. Electron. Muon. Tau. Muon. Tau. Electron.

A diagram showing that neutrinos oscillate between their three forms (Source)

It turns out that this property, the ability to oscillate between three forms, requires mass. And thus we know neutrinos have mass, and therefore observing superluminal neutrinos would be a contradiction of relativity.

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