Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Johann Mahne on 24/09/2011 10:33:19
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I remember once reading about how physicists had built big traps underground and were looking for a theoretical particle but had no success for years?
What causes them to appear?
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neutrinos were predicted to exist to balance out the equations in certain very common nuclear interactions. Let me explain.
An isolated neutron is unstable and decays with a half life of about 12 minutes into a proton and an electron. So far so good the proton has a positive charge and the electron has a negative charge so the charges cancel out. BUT a neutron has one quantum of spin(angular momentum) and a proton and an electron each have one quantum of spin so the spins cannot balance out and this contravenes the law of the conservation of momentum. Furthermore if you looked at the mass energy of the neutron and the mass energy and momentum of the proton and electron you would find that some of the energy was missing. this contravenes the law of conservation of energy so something must be missing. The simplest solution is a particle that has one quantum of spin no charge and the energy to make up the deficit. This particle was called the neutrino and it was very many years before one was actually detected because it interacts so weakly with other particles. It can pass straight through a star without ever bumping into anything!
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A good explanation thank you,
If neutrinos can pass through anything then they could not have been caught in the traps that were built for them.
When were they first detected?
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Although a single Neutrino can pass thru a star without hitting any thing there are really vast numbers of them and they have a tiny chance of interacting so that in a large enough detector some will register.
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neutrinos interact very weakly with an isotope of chlorine, and the eventual result is decay to an isotope of argon. The "traps" were vats of carbon tetrachloride or similar compound put in a deep mineshaft, and they were analysed for tiny argon bubbles after a zillion or more neutrinos had passed through.
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Are neutrinos considered to be particles, although they have no mass?
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Are neutrinos considered to be particles, although they have no mass?
Well, there is some argument that neutrinos may have a tiny mass. But photons certainly have no mass, yet both photons and neutrinos are considered particles.
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Neutrinos probably do have a tiny mass because it is important that they do or they could not oscillate between the three different types electron, muon and tau. They have spin and a property (as yet not described fully) that causes them to interact via the weak interaction like the other leptons which is much stronger than gravity.
Gravitational interactions have as yet no place in particle physics because gravitation is so indescribably weak it is extremely improbable that even in neutral interactions particles pass close enough to each other to experience a gravitational interaction. Even the weak interactions of neutrinos that than pass through a star unscathed are far more probable.
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Neutrinos probably do have a tiny mass because it is important that they do or they could not oscillate between the three different types electron, muon and tau
If they do have some mass, then does this not violate relativity in that they can achieve the speed of light?
Would they have less mass than an electron?
I don't know what muon or tau are?
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Answering Johann's latest:
(1) If they do have some mass, then does this not violate relativity in that they can achieve the speed of light?
not if they do NOT QUITE achieve the speed of light. But the latest finding is that they may be exceeding the speed of light, and that certainly does violate (special) relativity.
(2) Would they have less mass than an electron?
they are much less massive than an electron
(3) I don't know what muon or tau are?
muons and tau mesons are short-lived, high energy particles that are more massive than electrons but less massive than protons or neutrons
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Answering Johann's latest:
(1) If they do have some mass, then does this not violate relativity in that they can achieve the speed of light?
not if they do NOT QUITE achieve the speed of light. But the latest finding is that they may be exceeding the speed of light, and that certainly does violate (special) relativity.
(2) Would they have less mass than an electron?
they are much less massive than an electron
(3) I don't know what muon or tau are?
muons and tau mesons are short-lived, high energy particles that are more massive than electrons but less massive than protons or neutrons
Nice answers Damocles - would only quibble with one thing - tau meson are heavier than protons/neutrons
μ- = 105 MeV/c2
p+ = 938 MeV/c2
n0 = 939 MeV/c2
τ- = 1776 MeV/c2
For Johann's further reference the combined mass of the three variants of neutrino (tau, mu and electron) must be less than 0.27 eV/c2 - ie at least 10^9 times smaller than a proton. This is only an upper bound and the actual amount could be orders of magnitude less - I don't know of the lower bound, other than the mass is greater than zero.
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Careless of me Imatfaal; I stand corrected on the tau meson. I will add to your answer though that the electron mass in your units must be about 0.5 MeV/c^2 , which is at least 2 million times that of a neutrino
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Thanks for all the really good replies.
Would the momentum of a neutrino simply be mv, as it's a particle?
Would this momentum have any effect if it collided with say a neutron, or is a neutron simply to massive?
Also from Imatfaal :
How does volts/c² relate to mass?
If e=mc²; or e/c²=m then volts would equate directly to energy, which is not true.
Or is an electron volt actually energy?
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electron volt is an energy unit: E = Q*V; it is an electron charge = 1.6E-19 coulomb times 1 volt potential difference. So an electron volt is
1.6E-19 Joule,
or 1.6E-19 * 6.022E23 = approx 100 kJ per mol
(I leave it to you to look up exact values if you want a more exact conversion).
And the momentum of a neutrino would be simply mv, BUT the mass quoted is a rest mass m0; mass increases at near light speed according to m=m0/√(1-v^2/c^2)
When the speed v gets very close to the speed of light c, the mass can become very many times the rest mass, because the correction is very close to a division by zero.
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m=m0/√(1-v^2/c^2)
Thanks Damocles,
I forgot about the relativity effects.
So with this massive amount of momentum how do neutrinos go through stars unimpeded, or do they simply miss particles because of their small size?
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BUT a neutron has one quantum of spin(angular momentum) and a proton and an electron each have one quantum of spin so the spins cannot balance out and this contravenes the law of the conservation of momentum
I don't understand what a quantum of spin actually is, and how this relates to angular momentum.
If a proton and an electron can each have one quanta of spin, does this mean that the proton will spin much slower than an electron due to it's much larger mass, and that a neutrino will spin even faster than an electron?
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Spin in subatomic particles is a rather difficult concept. It is not strictly like electrons and protons are little globes spinning away but is more related to underlying symmetries in the particles. However the dimensions of Planck's constant are the same as the dimensions of angular momentum. Also an electron and a proton (and many other subatomic particles) as well as having an electric charge has a magnetic field and this implies that the electric charge must in effect be moving in circles in some way.
As for the speed with which this is happening it is not known or measurable so you just have to consider it as a quantum number
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How does volts/c² relate to mass?
If e=mc²; or e/c²=m then volts would equate directly to energy, which is not true.
Or is an electron volt actually energy?
Just to be explicit - Damocles cleared up why an electron volt is a unit of energy - in most forms of particle physics c - the speed of light - is set to one. in these cases the mass (e/c^2) is equal to the energy - thus the masses of the subatomic particles are usually given in units of eV or MeV or even GeV. If you are a stickler and/or a struggler (like me) you need to realise and make clear that the units you are using are (energy)/[(speed of light) squared] - and thus the mass is given in eV/c^2 ie per the mass energy equivalence equation
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I hate to agree with everybody above but SSurfers comments on spin are spot on. This is a cut-off point where you can no longer think classically - the electron cannot be seen as a little globe spinning on its axis; if it was the surface would be travelling at a velocity above c. Spin is an intrinsic property - which is testable,predictable, and closely connect to angular momentum ; but it is not a little particle spinning like a top
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neutrinos interact very weakly with an isotope of chlorine, and the eventual result is decay to an isotope of argon. The "traps" were vats of carbon tetrachloride or similar compound put in a deep mineshaft, and they were analysed for tiny argon bubbles after a zillion or more neutrinos had passed through.
Did this method work?
How does CERN detect neutrinos nowadays?
I remember seeing pics of tracks of particle collisions. I'm not sure if they were in some type of metal plate?
The paths were all looked very strange.Were the properties of neutrinos discovered in this way as well?
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Hi Johann
In reply to your latest:
Neutrinos are usually detected and characterized via the discrepancy in momentum in the patterns of those tracks of collisions you refer to. The neutrinos themselves never produce tracks; they are always the "leftover" that is required to balance the picture.
The carbon tetrachloride method did and does work, though the first few times it was tried produced inconclusive results.
There is a good web page at University of Wisconsin
http://icecube.wisc.edu/info/neutrinos (http://icecube.wisc.edu/info/neutrinos)
that provides reliable summary answers to a lot of your questions.
I would suggest that you check the CERN website for the detail of how they are investigating neutrinos at present.
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Neutrinos probably do have a tiny mass because it is important that they do or they could not oscillate between the three different types electron, muon and tau.
I will add to your answer though that the electron mass in your units must be about 0.5 MeV/c^2 , which is at least 2 million times that of a neutrino
If a neutrino has such a low mass, how could it transform to a much heavier particle?
Is a neutrino considered to be a "elementary particle"?
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Neutrinos probably do have a tiny mass because it is important that they do or they could not oscillate between the three different types electron, muon and tau.
I will add to your answer though that the electron mass in your units must be about 0.5 MeV/c^2 , which is at least 2 million times that of a neutrino
If a neutrino has such a low mass, how could it transform to a much heavier particle?
Is a neutrino considered to be a "elementary particle"?
The transformation (more properly thought of as an oscillation) is not from neutrino to electron or other heavy particle - it is between the three flavours of neutrino the electron neutrino νe the mu neutrino νμ and the tau neutrino ντ. It was calculated through quantum mechanics that a νe has a non-zero probability of being a νμby the time it arrives somewhere else (and so on for the other flavour changes). The masses of the three flavours of neutrino are indeed different and very complicated as they are in fact mass expectation values and we can only at present measure the difference between the different flavour masses.
The three flavours of neutrino are indeed considered at elementary have a look at the diagram at bottom this page at Stanford (http://www2.slac.stanford.edu/vvc/theory/fundamental.html) - you will see the neutrinos shown above the big brother leptons from which the flavours take their name.
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Hi Imatfaal,
Thanks for the link. Also to Damocles.
Which neutrino "flavours" are in action at CERN. Is it right to assume that more massive types are detected at lower velocities?
Under what conditions are the three different types produced?
SoulSurfer was talking about the decay of a neutron into a proton electron and neutrino. What flavour occurs in this case?
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Johann - this is the quantum mechanical world, and nothing is simple. There is not really any point talking about a particle as simply a tau neutrino; it is a particle with a probability of being a ντ, but it also has a finite probability of being a νe or a νμ. But when you detect a neutrino you either get a ντ, a νe, or a νμ - but before that it is a superposition of all three.
When you detect a neutrino using a huge amount of water (super-k) you are are basically on the lookout for what is called Cherenkov radiation. Cherenkov radiation is the light given off when a charged particle travels through a medium (not a vacuum) faster than the local speed of light! Now this has nothing to do with the recent Gran Sasso claim - but rather the fact that light travels through media other than the vacuum at below c. Highly energetic particles can travel through this medium quicker than light (but still less that c).
When a neutrino actually interacts with something in these huge water baths (vanishingly small probability) it produces a charged particle which is really moving quickly - so much so it will be going quicker than light does in water. This charged particle will give off cherenkov radiation which is detected by photomultipliers arranged around the edges.
If the neutrino that interacts is currently a mu-neutrino you will get a high speed muon formed, an electron-neutrino will form an electron, a tau-neutrino... The tau and the muon (the big leptons not the neutrinos) decay rapidly into unique showers of other particles (most of which are high-speed) the electron bashes into a water molecule. The particle showers of the tau and the muon and the single spark of the electron produce very different patterns of cherenkov radiation - so the detector will know which flavour the neutrino was when it interacted.
in beta- decay a neutron -> a proton an election and a neutrino. In this case the flavour is electron but it is in fact an anti-neutrino. as neutrinos have no charge it is possible they are their own anti-particle but the quantum chromodynamics only really works out if the particle emitted is an antimatter version. in positron emission (ie PET scan) a proton -> nuetron a positron and a electron-neutrino.
hope that is not too confusing
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Hi Imatfaal,
Thanks for the link. Also to Damocles.
Which neutrino "flavours" are in action at CERN.
Johann
Didn't notice this bit. CERN/OPERA Gran Sasso was set up to look at the oscillations of neutrinos that we were talking about above. CERN was able to use a proton beam striking a graphite wall to produce a beam of mesons that with a little filtering and magnetic channelling would end up decaying and producing a beam of almost solely mu-neutrinos. This beam of νμ was directed at Gran Sasso ~730km away. Gran Sasso was set up to detect ντ however - and it was hoped that accurate measurements of the change and probabilities of oscillation could be made. I haven't read up on the Gran Sasso detectors (lotsa lead and photo-multipliers) so I cannot say if they were only detecting ντ - but I presume they must have been able to either measure only ντ or differentiate ντ from νμ and νe for their experiment to work
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I hate to agree with everybody above but SSurfers comments on spin are spot on. This is a cut-off point where you can no longer think classically - the electron cannot be seen as a little globe spinning on its axis; if it was the surface would be travelling at a velocity above c. Spin is an intrinsic property - which is testable,predictable, and closely connect to angular momentum ; but it is not a little particle spinning like a top
Stephen Hawkins says that if a particle has for example a spin of 0 then it always "looks the same" from all directions
If it has a spin of 2, it "looks the same" for every 180 degrees of rotation.
Is he talking about the electrostatic or magnetic fields that "look the same", or something else?
How is this spin related to angular momentum?
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I hate to agree with everybody above but SSurfers comments on spin are spot on. This is a cut-off point where you can no longer think classically - the electron cannot be seen as a little globe spinning on its axis; if it was the surface would be travelling at a velocity above c. Spin is an intrinsic property - which is testable,predictable, and closely connect to angular momentum ; but it is not a little particle spinning like a top
Stephen Hawkins says that if a particle has for example a spin of 0 then it always "looks the same" from all directions
If it has a spin of 2, it "looks the same" for every 180 degrees of rotation.
Is he talking about the electrostatic or magnetic fields that "look the same", or something else?
How is this spin related to angular momentum?
Stephen Hawking is a great scientist and mathematician - and I wouldn't and couldn't criticise his work; but his popularizations can be a bit odd. He will quite happily give a great and accessible explanation for a incredibly difficult concept (which I will lap up and feel I understand) and then a few paragraphs later say that the explanation was of""heuristic value only" ie not really true.
I do not know if this is one of those occasions but I suspect it is. Spin is an intrinsic and basic property of fundamental particles - and like the other ones it is very hard to get to the nitty-gritty of what it actually is!
A good start is Wikipedia's page Spin(physics) (http://en.wikipedia.org/wiki/Spin_%28physics%29)
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Spin and current loops are interlinked magnetically. But how can 3D current loop spin or frequency be linked into the standard model? CliveS
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I listened to this Podcast yesterday, perhaps it may be of use to the OP and others.
http://www.bbc.co.uk/programmes/b0106tjc
Melvyn Bragg and his guests discuss the neutrino.
In 1930 the physicist Wolfgang Pauli proposed the existence of an as-yet undiscovered subatomic particle. He also bet his colleagues a case of champagne that it would never be detected. He lost his bet when in 1956 the particle, now known as the neutrino, was first observed in an American nuclear reactor.
Neutrinos are some of the most mysterious particles in the Universe. The Sun produces trillions of them every second, and they constantly bombard the Earth and everything on it. Neutrinos can pass through solid rock, and even stars, at almost the speed of light without being impeded, and are almost impossible to detect. Today, experiments involving neutrinos are providing insights into the nature of matter, the contents of the Universe and the processes deep inside stars.
With:
Frank Close
Professor of Physics at Exeter College at the University of Oxford
Susan Cartwright
Senior Lecturer in Particle Physics and Astrophysics at the University of Sheffield
David Wark
Professor of Particle Physics at Imperial College, London, and the Rutherford Appleton Laboratory.
Producer: Thomas Morris.
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Thanks for all the excellent links.
This is the re formatted link of Imatfaal
http://en.wikipedia.org/wiki/Spin_%28physics%29 (http://en.wikipedia.org/wiki/Spin_%28physics%29)
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Thanks Johann - I had a curly instead of a square bracket
Have modified the original