Lessons From Neutrinos, Part I

Some of the particles of the Standard Model are more familiar than others. Electrons and photons, of course, everyone has heard of, and most, though not all, have heard of quarks. Many of the rest, like the W and Z boson, only appear briefly in high-energy colliders. But one Standard Model particle is much less exotic, and nevertheless leads to all manner of confusion. That particle is the neutrino.

Neutrinos are very light, much lighter than even an electron. (Until relatively recently, we thought they were completely massless!) They have no electric charge and they don’t respond to the strong nuclear force, so aside from gravity (negligible since they’re so light), the only force that affects them is the weak nuclear force. This force is, well, weak. It means neutrinos can be produced via the relatively ordinary process of radioactive beta decay, but it also means they almost never interact with anything else. Vast numbers of neutrinos pass through you every moment, with no noticeable effect. We need enormous tanks of liquid or chunks of ice to have a chance of catching neutrinos in action.

Because neutrinos are both ordinary and unfamiliar, they tend to confuse people. I’d like to take advantage of this confusion to teach some physics. Neutrinos turn out to be a handy theme to convey a couple blog posts worth of lessons about why physics works the way it does.

I’ll start on the historical side. There’s a lesson that physicists themselves learned in the early days:

Lesson 1: Don’t Throw out a Well-Justified Conservation Law

In the early 20th century, physicists were just beginning to understand radioactivity. They could tell there were a few different types: gamma decay released photons in the form of gamma rays, alpha decay shot out heavy, positively charged particles, and beta decay made “beta particles”, or electrons. For each of these, physicists could track each particle and measure its energy and momentum. Everything made sense for gamma and alpha decay…but not for beta decay. Somehow, they could add up the energy of each of the particles they could track, and find less at the end than they did at the beginning. It was as if energy was not conserved.

These were the heady early days of quantum mechanics, so people were confused enough that many thought this was the end of the story. Maybe energy just isn’t conserved? Wolfgang Pauli, though, thought differently. He proposed that there had to be another particle, one that no-one could detect, that made energy balance out. It had to be neutral, so he called it the neutron…until two years later when James Chadwick discovered the particle we call the neutron. This was much too heavy to be Pauli’s neutron, so Edoardo Amaldi joked that Pauli’s particle was a “neutrino” instead. The name stuck, and Pauli kept insisting his neutrino would turn up somewhere. It wasn’t until 1956 that neutrinos were finally detected, so for quite a while people made fun of Pauli for his quixotic quest.

Including a Faust parody with Gretchen as the neutrino

In retrospect, people should probably have known better. Conservation of energy isn’t one of those rules that come out of nowhere. It’s deeply connected to time, and to the idea that one can perform the same experiment at any time in history and find the same result. While rules like that sometimes do turn out wrong, our first expectation should be that they won’t. Nowadays, we’re confident enough in energy conservation that we plan to use it to detect other particles: it was the main way the Large Hadron Collider planned to try to detect dark matter.

As we came to our more modern understanding, physicists started writing up the Standard Model. Neutrinos were thought of as massless, like photons, traveling at the speed of light. Now, we know that neutrinos have mass…but we don’t know how much mass they have. How do we know they have mass then? To understand that, you’ll need to understand what mass actually means in physics. We’ll talk about that next week!

9 thoughts on “Lessons From Neutrinos, Part I

  1. JollyJoker

    I hope part n will explain how close we are to measuring the masses or maybe finding a nonzero lower limit for the lightest one. I have some vague memory that that wasn’t terribly far away.

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    1. 4gravitons Post author

      I’ll mention it, but I don’t know the current experimental status in much detail beyond “soon-ish”, so the main added value I could provide would be to check the Particle Data Group page.

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  2. Pingback: I think I found my lunch spot… ⋆ beyondourrights

  3. J Mark Morris

    I hope you will take this in the spirit of discovery, but I believe you can recast all of your strings as electromagnetic fields from kinetic immutable point charges as well as the structures these elements form. I find e/6 and -e/6 to fit nicely.

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    1. 4gravitons Post author

      There are zero strings mentioned in this post, so this is a spam comment. I’m giving this as a second warning, the next time you have a comment that brings up your own theories completely off-topic like this I’ll mark it as spam and delete it.

      If this is accidental, and you’re not sure whether a comment is on-topic or not, then I recommend beginning with a comment that just asks “is this post about X?” and waiting for confirmation before bringing up your own work.

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      1. J Mark Morris

        I’m not a linear thinker. The subject of the post caused me to imagine the physical implementation of neutrinos, which I know. Then I thought I recalled you were a string theorist and thought it would be fascinating to constrain string theory to the parameters of my model. Then you could solve for the neutrino and actually visualize the solution like i do. So again, your snap judgment is harsh and faulty.

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          1. J Mark Morris

            I would like some consideration of my nonlinear thinking because I’ve had to back out key pillars in GR and QM and Q**. And then replace them with a physical implementation. All of your math and theory are simply patterns. Your incorrect prior narratives have obscured the simple solution.

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