My understanding is a neutrino is an essentially massless, chargeless, subatomic particle, so small that it almost never interacts with matter and can pass through solid objects uninhibited, but we have built several SCIENCE SWIMMING POOLS water-filled detectors to detect the cherenkov radiation emitted whenever one does collide with an atom.
Somehow, I think Alex can give a better explanation though. Just a hunch

Well, you said the most important things. If I wanted to pick nits, they are not "essentially massless" - they do have mass, they are just very very light compared to other particles we know. We don't know their exact mass yet (Experiments are under way to change that, though), but it is vastly lower than all other massive particles.

The crucial issue that sets them apart from their closest relatives, the leptons (for example the electron) is that, as you say, they don't have electric charge, nor do they participate in the strong interactions. Those are the two forces (and one more effect that does not play a role here) that basically make the objects in every day life feel solid, and feel each other's impact when they meet. Since Neutrinos don't have either of those, they can simply pass through you, and through the entire earth for that matter, with only a minuscule chance to actually interact with you, i.e. crash into you rather than pass through. Why can they interact with ordinary matter at all? That's because they *do* participate in the weak force, i.e. interact with W and Z bosons. However, as long as the neutrinos don't have very large energies, the force mediated by the W and Z bosons is super short range (as discussed upthread) and so only rarely does anything at all. This is why you need BIG detectors to actually see them, because that embiggens the chances that one of the many that are passing through will actually get stuck. The coolest Neutrino Detector in every sense of the word is IceCube, which uses a kilometer cubed of Antarctic ice as detector material



One of the coolest Neutrino observations I know is this:



It is an image not taken in visible light, but from neutrinos - it is a picture of the center of our sun, and the fact that we see it in neutrino light means we directly observe that nuclear fusion is going on in the sun at that moment, because nuclear fusion produces the neutrinos.
What boggles the mind is that this picture was not taken by a telescope that actually has a line of sight to the sun - it was taken by detectors deep under ground in a water pool:



Notice the rubber boat with engineers in it

If I might. I have a question regarding the duality of light, schrodingers cat, and objective vs subjective collapsing of the wave function.

Please let me know if it is okay to ask this question.

(June 8, 2015 at 9:55 am)Anima Wrote: If I might.  I have a question regarding the duality of light, schrodingers cat, and objective vs subjective collapsing of the wave function.  

Please let me know if it is okay to ask this question.

Sorry, that's against my religion. I'm strictly a Heisenberg guy, no Schrödinger for me...

Kidding, kidding, do ask

Nicely explained once again.
I have a question regarding 'dark matter' as it's called, if you have the time. If neutrinos don't interact with the strong force but do with the weak force, would I be right in thinking 'dark matter' doesn't interect with either? I believe we can detect it indirectly by the gravitational effect it has on normal matter, but if my assumption is correct...why is this so?
Or am I talking out of my bumhole?

(June 8, 2015 at 10:06 am)Iroscato Wrote: Nicely explained once again.
I have a question regarding 'dark matter' as it's called, if you have the time. If neutrinos don't interact with the strong force but do with the weak force, would I be right in thinking 'dark matter' doesn't interect with either? I believe we can detect it indirectly by the gravitational effect it has on normal matter, but if my assumption is correct...why is this so?
Or am I talking out of my bumhole?

I'm not sure what your question is. It interacts with gravity because according to general relativity, anything that has energy bends space and interacts with gravity. As far as the weak force is concerned, the only valid answer is that noone knows whether Dark Matter interacts via the weak force. It might, but it doesn't have to. There are valid theory proposals for particle Dark Matter from either category.

For those particle dark matter candidate hypotheses that do interact via the weak force - and they are usually called WIMPs for Weakly Interacting Massive Particles, specialized underground searches are under way, for example XENON, CDMS, DAMA/LIBRA and several I've forgotten. They more or less consist of a ultrasensitive (and possibly ultracold) sensor shielded deep underground in a mine, waiting for "hits".

There is one important class of dark matter particle candidates which do not interact with the weak force - the AXIONs. They can be detected using a peculiar effect: axions can be converted to photons in a strong magnetic field. The ADMX experiment consists of a closed cavity with a resonator and a strong magnetic field. It systematically scans through the resonator frequencies, hoping to observe axion dark matter - to - photon conversions in its cavity.

Okay, my questions are as follows:

1. Once the wave function is collapsed is it collapsed for all time? So if I observe light as a particle today do all other observers from this day forth observe it as a particle as well?

2. Is the wave function only collapsed by subjective observation or may it be collapsed by objective observation? So if an inanimate object responds to the light as if it were a particle does this reaction constitute an objective "observation" of the light that collapses the wave function?

3. If the wave function is collapsed for all time upon observation and the function may be collapsed by an objective observation (as exhibited in the reaction of the object to the quantum state) of inanimate objects than may it be said all quantum states are determined upon initial interaction with another object for all time?

(June 8, 2015 at 10:22 am)Alex K Wrote:

(June 8, 2015 at 10:06 am)Iroscato Wrote: Nicely explained once again.
I have a question regarding 'dark matter' as it's called, if you have the time. If neutrinos don't interact with the strong force but do with the weak force, would I be right in thinking 'dark matter' doesn't interect with either? I believe we can detect it indirectly by the gravitational effect it has on normal matter, but if my assumption is correct...why is this so?
Or am I talking out of my bumhole?

I'm not sure what your question is. It interacts with gravity because according to general relativity, anything that has energy bends space and interacts with gravity. As far as the weak force is concerned, the only valid answer is that noone knows whether Dark Matter interacts via the weak force. It might, but it doesn't have to. There are valid theory proposals for particle Dark Matter from either category.

For those particle dark matter candidate hypotheses that do interact via the weak force - and they are usually called WIMPs for Weakly Interacting Massive Particles, specialized underground searches are under way, for example XENON, CDMS, DAMA/LIBRA and several I've forgotten. They more or less consist of a ultrasensitive (and possibly ultracold) sensor shielded deep underground in a mine, waiting for "hits".

There is one important class of dark matter particle candidates which do not interact with the weak force - the AXIONs. They can be detected using a peculiar effect: axions can be converted to photons in a strong magnetic field. The ADMX experiment consists of a closed cavity with a resonator and a strong magnetic field. It systematically scans through the resonator frequencies, hoping to observe axion dark matter - to - photon conversions in its cavity.

We're on the same wavelength in that respect then - I also have no idea what I'm asking
So you're saying that dark matter is most likely composed of multiple different kinds of as yet undiscovered particles, rather than a catchall single kind of matter? In any case it seems to be this decade's 'superpower-giving radiation' in The Flash, and one of the few holdouts in the 'science is spooky and dangerous' genre overall

TIL what a neutrino is, methinks that's enough education for this week
thanks for your mostly comprehensible explanation uncle K

(June 8, 2015 at 10:22 am)Alex K Wrote: There is one important class of dark matter particle candidates which do not interact with the weak force - the AXIONs. They can be detected using a peculiar effect: axions can be converted to photons in a strong magnetic field. The ADMX experiment consists of a closed cavity with a resonator and a strong magnetic field. It systematically scans through the resonator frequencies, hoping to observe axion dark matter - to - photon conversions in its cavity.

Would that look to the sensors like a photon just appearing out of nowhere?