Okay, while we’re doing threads based on subatomic particles, let’s try this one. What I remember about them is that they’re given off in radioactive breakdowns (actually antineutrinos are what are usually emitted, so far as I can tell), will go through something like eight light years of solid lead plate without noticing it’s there, and are thought to be the reason that supernovas collapse after exploding. (They carry off too much of the energy that keeps the star “inflated.”)
Now some years ago they did an experiment with a telescope made of chloride in a mile-deep mine, to keep everything but the neutrinos out. Supposedly neutrino impacts would convert chlorine atoms to argon, which could be detectable. That way they’d observe solar neutrinos coming from the core of the sun.
What they found out is that only a third of the neutrinos theory predicted were observed. This caused a bit of consternation.
Then a few years later somebody suggested that there were three kinds of neutrinos, corresponding to electrons, muons, and pions and emitted alongside them in atomic reactions. Somebody else came up with the idea that that somehow explained the solar neutrino deficiency.
Then about a year ago, it was suggested that neutrinos had mass. (Previously, they’d been considered massless, much like photons.) And that was supposed to be the explanation, or part of it, for the missing mass question relative to whether the universe is closed or open.
Does anybody have any information, comments, or WAGs on any of these three neutrino-related questions?
There was a decent summary of the latest experiments on neutrinos within the last couple of months of Scientific American. Briefly, there are indeed three kinds of neutrinos (electron, muon, and tau), corresponding to the three generations of leptons (again, electron, muon, and tau) and quarks (up-down, charm-strange, and top-bottom). Recent experiments seem to indicate a very small mass for the neutrino (I’m remembering something less than 1 eV, compared to 500 thousand eV for an electron). What this implies is that the neutrino can oscillate in flight between being one type (electron) and another (muon). Think of the neutrino as being in a mixed quantum state, where the probability of measuring it in one state or another is time varying (this cannot happen if the neutrino is massless). Since the detectors only detect one kind of neutrino, they miss all the ones that are currently most probably in the other state. In reality, I should point out that I have stated the reasoning backwards–it is the evidence for the oscillations that has been used to show that the neutrinos have mass. At any rate, this at least partially explains the relative (vs theory) dearth of neutrino flux from the Sun.
Which makes it clear that I screwed up–the oscillation is between muon and (possibly) tau neutrinos, not muon and electron as I stated. Thanks for the link.
In the astrophysics course I took 'way back in 1989, the prof. said exactly the opposite. He said that it was the enormous flux of neutrinos emitted by the collapsing core that ignited the outer layers of the star and caused the supernova.
So we have neutrinos to thank for all that heavy-element enrichment we benefit from.
The truth, as always, is more complicated than that.
I seem to recall one explanation of the low neutrino count being that the sun has ‘gone out’, and is not fusing nearly as much as we thought, and is currently producing energy through gravitational collapse. It’s been years since I read this, but I think the theory went something along the lines that the reaction would stop, the sun would start to collapse, pressures would build, the reaction would start, energy from the reaction would force the sun to expand, etc.
If a photon is generated at the center of the sun it takes years for that photon to make its way to the surface. So if the reactions had stopped we wouldn’t know about it through photon measurement for a long time, but neutrino flux would drop almost immediately.
It takes a (gamma ray) photon generated in the sun’s core a million years to reach its surface, in fact.
But unless we measured a change in the flux of neutrinos coming from the sun, I wouldn’t put much stock in such a doom-and-gloom prediction. Even when the missing neutrinos were first reported, a flaw in the standard model of neutrinos was the first suspect.
(Okay, the second suspect. The first suspect was a flaw in the neutrino detector. But subsequent experiments have ruled that out.)
The truth, as always, is more complicated than that.
The article also implied that, because a neutron could shift between being an electron, muon, or tau neutrino as just one of its quantum states, this made the neutrino different from all other massive particles.
Wouldn’t the way other massive particles besides neutrinos (e.g. pions or muons) decay into something else also qualify as a change in that particle’s quantum state?
Quick-N-Dirty Aviation: Trading altitude for airspeed since 1992.
The neutrinos don’t decay into something else, though. They are oscillating continuously back and forth between the different types. In a decay, the products are free, and don’t periodically recombine into the original particle, then back to the decay products. That’s the difference, I think, between the two processes.
tracer,
Have I missed something? Has it been shown that neutrinos have mass?
If so, this would have major implications for cosmology. I’ve heard that even an extremely tiny mass would be enough to close the universe.
Our current estimates of the mass of the Universe (based on all the “non-dark” matter we can detect) put it at about 1/10 the value necessary to close the Universe.
According to the SciAm article, if neutrinos have the mass they think it might, this would double our estimate for the mass of the Universe. This is certainly a big leap, but is still only 1/5 of the mass required for a closed Universe.
Quick-N-Dirty Aviation: Trading altitude for airspeed since 1992.
