Neutrinos and Magnetic Moments
In the answers in your archives it is stated that
neutrinos do not have magnetic moments. Yet the information I find
on-line says that because they have mass they also have a magnetic
moment, like the neutron. I know new discoveries are made all the
time, so which is correct? I am now thoroughly confused.
Neutrinos are a wonderful subject within particle physics because we are actively
learning more about their properties, both experimentally and theoretically. Our
understanding of their properties has changed as better observations are made and
better explanations/descriptions offered. Neutrino mass is a great example of this.
In fact, to my outsider's knowledge of particle physics it seems like neutrinos have
probably be the area of most progress within particle physics in the past couple of
decades. (It is worth saying that I study condensed matter physics myself and thus
should not be treated as an expert on particles which are far from my area of
The long standing thought on the matter was that neutrinos have no observable mass and
could be treated as massless. This was correct within the data available for many
decades. Attempts to both directly and indirectly observe or measure the mass of a
neutrino all came back with "no measurable mass." This did not mean zero mass, but
just that it seemed to always be zero to within our ability to detect. However,
recently (1999 I think) neutrinos have been observed to change their type (or flavor
as it is commonly called) with time. I do not want to go into the details of this,
but in order for this to occur, there must be a difference between the mass of the
different neutrino flavors. A difference in mass requires that the masses are not
all 0. It is furthermore quite expected that if some of the flavors have mass, then
they all must. It makes little sense to have some with mass and others without (though
perhaps some of the people that actually study neutrinos could disagree with that
statement and point out why it would in fact make sense).
Anyhow, we are able to observe the neutrinos changing flavor and this allows us to measure
a mass difference between the neutrinos. If you then follow that conclusion that neutrinos
must all have mass, then it is possible for them to have a magnetic moment! I believe the
magnetic moment would be linearly proportional to mass of the particle. The estimated
value of this magnetic moment is very small and below our ability to currently detect.
However, perhaps at some point in the future scientists will figure out a means to test
it. The history of our understanding of neutrino properties offers a great deal of
insight into how science works and how ideas evolve over time. In the strict sense that
people used to say "neutrinos are massless" were wrong. However the longer version of
saying "we think they are massless, cannot measure any mass, and for all our relevant
calculations and data they appear massless, therefore we will treat them as massless,"
was entirely correct. It is only with more modern experiments that we have been able to
begin to unravel this mystery. The initial postulate for the existence of neutrinos was
another great example of science, but that is taking us too far away from your question.
So 15 years ago the short answer was, "Neutrinos have no mass and no magnetic moment, at
least within our ability to measure." Now we are able to observe that neutrinos must have
some mass, albeit we have not measured it directly, only the difference in the masses and
that the mass of the lightest neutrino is in fact quite small. Since they have mass, then
they should have magnetic moments too! That answer may continue to evolve (I hope so!) as
we learn more.
Michael S. Pierce
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Update: June 2012