At 07:59 13-11-00 -0800, Darryl wrote:
>Dan wrote:
>
> >It would be better to say that there is a natural upper
> >velocity: c, and that an object will travel this velocity if and only
> >if it
> >has no rest mass. Photons are the only known particles with zero rest
> >mass.
>
>
>I can never remember if neutrinos have non-zero rest mass. Has it been
>conclusively shown that they do? Or is just very likely that they do?
>Or is it likely that they don't?
>
>I was always under the impression that neutrinos traveled at c, has
>that been shown to be wrong? Or is our technology for observing
>neutrinos so poor that we can't say for sure?
The actual answer is that it's still an open question.
Some history: The existence of the neutrino was originally proposed in
1933 to explain a discrepancy in the decay of the neutron. It was clear
that after an average of about 12 minutes or so, a neutron will decay into
a proton and an electron. However (and I'm simplifying things a little),
if you think of a neutron that is sitting still when it decays, if only two
particles are produced, they should fly off in opposite
directions. Experiment shows that they don't, but fly off at an angle less
than 180 degrees, indicating that a third particle is produced in the decay
of the neutron. This third particle is clearly neutral (because the
positive charge of the proton and the negative charge of the electron add
up to zero). This third particle carries away a certain amount of energy
and momentum, which can be used to place an upper limit on its mass. As
you may have guessed, this particle is the neutrino. Such experiments
place an upper limit on its mass of about 1/100,000 the mass of an electron
(the lightest particle definitely known to have a non-zero mass), and are
consistent with the neutrino having a rest mass of zero. Neutrinos turned
out to be very difficult to detect because they do not interact very
strongly with ordinary matter, in fact it was not until 1956 that the
neutrino was actually detected.
By measuring the amount of solar energy that falls on a square meter of
Earth (1370 joules/second) and multiplying that times the area of a sphere
1 astronomical unit in radius, we see that the Sun must emit 3.8 x 10^26
joules per second, which, it is postulated is produced by some 600 million
tons of hydrogen being fused into helium each second in the core of the
Sun. As part of each fusion reaction, neutrinos are produced. Since they
interact so weakly with ordinary matter, the neutrinos pass through the
material of the Sun like it is transparent and arrive on Earth a few
minutes later. In fact, about 3.5 x 10^16 (35 million billion) neutrinos
from the Sun should pass through each square meter of the Earth's surface
every second. So astrophysicists suggested that detection of neutrinos
from the Sun would be a way of testing the theory that nuclear fusion was
the energy source of the Sun.
One way that neutrinos interact with matter, and so can be detected, is
that a neutrino can change a chlorine-37 nucleus into an argon-37 nucleus,
which is radioactive. An experiment was set up in the early 1970s to
attempt to detect solar neutrinos: it consisted of a tank containing
approximately 100,000 gallons (400,000 liters) of tetrachloroethylene
(C2Cl4), which is produced in significant quantities for use as a cleaning
fluid. Based on the interaction rate of neutrinos with chlorine, it was
expected that approximately one atom of argon-37 should be produce in the
tank each day. However, the observed rate of production was about
one-third of that. So either something was wrong with the models of the
Sun or with our understanding of neutrinos.
In 1936, physicists studying the interaction of cosmic rays with the
Earth's upper atmosphere discovered a new particle which was named the
muon. The muon acts very much like an electron, except that it is 207
times heavier and is unstable, with a lifetime of about 200
nanoseconds. It turns out that the muon has a neutrino associated with it
which is different from the neutrino we have been discussing, which is a
neutrino of the electron type, or an "electron neutrino." Still later,
another particle was discovered which acts like an even heavier analogue of
the electron and muon (with an even shorter lifetime): it was named the
tauon and (no surprise) there is a tauon neutrino, too.
Tying all this together:
Someone suggested that since there are three types of neutrinos, if somehow
one type of neutrino could change into another type, this could explain why
the experiment only detected 1/3 the expected number of neutrinos, because
the electron neutrinos produced in the Sun somehow changed into an even mix
of electron, muon, and tauon neutrinos before they reach Earth. Some
recent theories say that this is possible, but only if neutrinos have a
non-zero rest mass. An experiment conducted at Los Alamos in 1995
apparently showed muon neutrinos changing into electron neutrinos, which
would imply that muon neutrinos at least have mass, but more such
experiments need to be done before this is confirmed.
On the other side of the issue:
In February, 1987, a supernova was observed in the Large Magellanic Cloud,
some 160,000 light years away. Current theory suggests that nearly 99
percent of the energy produced in such an explosion would be produced in
the form of neutrinos, so approximately 5 x 10^14 (500 trillion) neutrinos
from the supernova should have passed through each square meter of Earth's
surface. Two of the experiments that had been set up to detect solar
neutrinos reported a burst of neutrinos about 3 hours before supernova
became visible. (Again, the neutrinos could escape from the core of the
exploding star while the light would be blocked until the surrounding gas
had dispersed.) So essentially the neutrinos travelled the 160,000 light
years in the same time as light, suggesting that they were travelling at
the speed of light, which would require that they be massless.
So, as I said at the start, the question is still open, with some results
being in favor of a non-zero mass and some consistent with a zero rest
mass. With luck, further experiments will answer the question one way or
another.
(BTW: I just gave this same lecture in class a couple of weeks ago. I
made the above about as brief as possible, hoping it wasn't too long for
anyone. If I simplified too much and anyone wants more details, let me know.)
-- Ronn! :)
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-- Ronn Blankenship
Instructor of Astronomy/Planetary Science
University of Montevallo
Montevallo, AL
Standard Disclaimer: Unless specifically stated
otherwise, any opinions stated herein are the personal
opinions of the author and do not represent the
official position of the University of Montevallo.
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