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ToSeek
2005-Nov-09, 05:55 PM
What Does 'Almost Nothing' Weigh? FSU Physicist Aims To Find Out (http://www.spacedaily.com/news/physics-05zq.html)


If subatomic particles had personalities, neutrinos would be the ultimate wallflowers. One of the most basic particles of matter in the universe, they've been around for 14 billion years and permeate every inch of space, but they're so inconceivably tiny that they've been called "almost nothing" and pass straight through things - for example, the Earth - without a bump.

Maddad
2005-Nov-09, 08:20 PM
Don't we have any credible models for the energy that a neutrino should have? Once you identify the range of energy that it might have, you have also identified the mass range it's allowed.

peteshimmon
2005-Nov-10, 02:06 AM
Perhaps the neutrino is Mr Uncertainty in
person:)

trinitree88
2005-Nov-12, 01:45 AM
Maddad, see the post in the String Theory thread. Neutrinos can have any wavelength, and any energy, like photons, limited only by the size of the universe....whatever that may be. Ciao. Pete.

lillieb
2005-Nov-14, 07:39 AM
Maddad, see the post in the String Theory thread. Neutrinos can have any wavelength, and any energy, like photons, limited only by the size of the universe....whatever that may be. Ciao. Pete.

Almost, but remember that mass is a form of energy. So particles with a mass have a minimum energy, and hence a *maximum* wavelength. For neutrinos that works out to around the millimeter to meter range, depending of course on what the actual mass turns out to be.

Maddad
2005-Nov-14, 06:24 PM
Yeah, that's one prediction of one model of string theory. Trouble is that it's still beyond any experimental confirmation, which makes it all speculation.

pghnative
2005-Nov-14, 07:22 PM
I thought that neutrino mass was confirmed experimentally. Or do I misunderstand what you are dismissing as speculation?

lillieb
2005-Nov-14, 08:29 PM
None of this has anything to do with string theory.

Yes, neutrino masses have been detected experimentally. I say "detected" rather than "measured" because technically what we've measured is *difference* in mass between the different neutrinos. There are three neutrinos and hence two independent mass splittings. We've measured both of those to be non-zero, and hence at most one neutrino can be massless. Note that, since we only have the splittings, we don't know what the overall scale of the masses is. There are experiments underway that will try to measure the masses more directly, and if they are sucessful they'll pin down the scale.

swansont
2005-Nov-14, 10:46 PM
Almost, but remember that mass is a form of energy. So particles with a mass have a minimum energy, and hence a *maximum* wavelength. For neutrinos that works out to around the millimeter to meter range, depending of course on what the actual mass turns out to be.


Wavelength is inversely proportonal to momentum. How are you relating wavelength to energy?

lillieb
2005-Nov-14, 11:13 PM
Doh! I was thinking about the Compton wavelength, which is set by the mass. You're right, of course.

ToSeek
2005-Nov-14, 11:32 PM
None of this has anything to do with string theory.

Yes, neutrino masses have been detected experimentally. I say "detected" rather than "measured" because technically what we've measured is *difference* in mass between the different neutrinos. There are three neutrinos and hence two independent mass splittings. We've measured both of those to be non-zero, and hence at most one neutrino can be massless. Note that, since we only have the splittings, we don't know what the overall scale of the masses is. There are experiments underway that will try to measure the masses more directly, and if they are sucessful they'll pin down the scale.

I thought the conclusion that neutrinos coming from the Sun were changing type also meant that neutrinos couldn't be massless.

lillieb
2005-Nov-15, 01:38 AM
I thought the conclusion that neutrinos coming from the Sun were changing type also meant that neutrinos couldn't be massless.

That's right. There is a technical point that the oscillation experiments only measure mass differences. What is measured is actually m_i^2 - m_j^2. Two of these mass differences are known, one from the solar neutrinos and the other from neutrinos produced when cosmic rays hit the atmosphere. So, one of the three could be massless, but the other two certainly have to have mass, and smart money is on them all having mass.

trinitree88
2005-Nov-15, 02:15 AM
re: neutrinos "the smart money is on all of them having mass".:naughty:

I disagree. That means throwing away Conservation of Electron, Muon, and Tau Family Number. Why would you throw away three Conservation Laws? It also means that the three neutrinos are indistinguishable from their anti-particles. Conservation Laws are hard won from experimental data. No particle physics experiment can make sense without inferring conservation laws..momentum, mass-energy, spin, electric charge, baryon number, electron family number, muon family number, tau family number...etc. Parity can fail, but that's not what you are talking about here. The flavor mixing in solar neutrinos is accomodated by their passage through the plasma of the sun, and the earth's atmosphere, on the way to terrestrial detectors. No mixing need occur in the relatively hard vacuum of space. No mass is needed for that scenario. I'll bet a Papa Gino's mushroom and pepper pizza on it. Ciao. Pete.

lillieb
2005-Nov-15, 02:46 AM
There are a couple points here. First, there's nothing particularly fundamental about conservation of lepton family number. There are some, like conservation of energy, that are connected to deep principles (time translation symmetry in that case). Lepton number appears to be an accidental symmetry of the Standard Model, and the lesson of recent particle physics is that, unless there is a good reason for a symmetry, it tends to be broken.

Second, there are analogous "laws" in the quark sector, conservation of up, down, strange, etc... which would be present if the quarks were massless. There doesn't seem to be any reason why something similar isn't happening in the lepton sector.

The question of whether neutrinos are identical with their anti-particles isn't addressed by the current oscillation experiments. There are models of neutrino masses where they are not identical. These should be addressed by a new generation of experiments that are specifically looking at this question. (The neutrinoless double beta decay experiments, for anyone who's interested).

Finally, there have been many analyses of the neutrino data to see if alternate models could work. Currently only models with neutrino masses seem to work. I don't know in particular if there has been an analysis of pure matter effects, though I suspect there has.

ToSeek
2005-Nov-15, 04:34 AM
That's right. There is a technical point that the oscillation experiments only measure mass differences. What is measured is actually m_i^2 - m_j^2. Two of these mass differences are known, one from the solar neutrinos and the other from neutrinos produced when cosmic rays hit the atmosphere. So, one of the three could be massless, but the other two certainly have to have mass, and smart money is on them all having mass.

Seems to me like that would be the way to bet. It doesn't make a whole lot of sense to me that a massive particle could turn into a massless one (or vice versa).

Benign Terrorist
2005-Nov-16, 08:35 PM
I thought that neutrino mass was confirmed experimentally. Or do I misunderstand what you are dismissing as speculation?We have set upper limits on the mass that a neutrion may have, and hope to lower those limits in the future. The idea is to keep lowering the maximum possible mass that they might have until you nab them.

As for strings, a theory is not theory because a theory is a hypothesis that has survived testing. Since we know of no test that could have disproved string anything, it remains speculation, not theory.