It is said that a neutrino is so uninterested in matter that it can fly through a hunk of lead a light year in length without a scratch. Can it fly through a neutron star? What about a black hole?
I think it would be unlikely that a neutrino could pass through a neutron star, given that the cross section for collisions would be huge. In normal matter such as your light-year of lead, there is a great deal of space so the neutrino is likely to miss all the nuclei of the atoms. But, a neutron star is one big nucleus, so I think it would be likely that it would interact at some point.
Let’s compare a light-year of lead to a neutron star:
[ul][li]Density of lead: 11340 kg / m[sup]3[/sup][/li][li]Nasa gives the following properties of a typical neutron star: 1.4 solar masses, radius 5 miles.[/li][li]My physics text gives the mass of the sun as 2 x 10[sup]30[/sup] kg[/li][li]Volume of a sphere of radius 5 miles: 4/3 <font face=symbol>p</font>r[sup]3[/sup], where r = 5 miles x 1.609 x 10[sup]3[/sup] m/mile; Volume = 2.2 x 10[sup]12[/sup] m[sup]3[/sup][/li][li]Density of neutron star = 1.4 x 2 x 10[sup]30[/sup] kg / (2.2 x 10[sup]12[/sup] m[sup]3[/sup]) = 1.27 x 10[sup]18[/sup] kg / m[sup]3[/sup][/li][li]Neutron stars are 1.1 x 10[sup]14[/sup] times more dense than lead.[/li]
[li]speed of light c = 3 x 10[sup]8[/sup] m/s[/li][li]Seconds in a year ~= <font face=symbol>p</font> x 10[sup]7[/sup][/li][li]1 light-year = 3 x 10[sup]8[/sup]m/s x <font face=symbol>p</font> x 10[sup]7[/sup] s = 9.4 x 10[sup]15[/sup] m.[/li]
[li]Since neutron stars are 1.1 x 10[sup]14[/sup] more dense, a neutrino will encounter the same amount of material in 1 light-year of lead in approximately 85 meters of neutron star.[/ul][/li]
Based on that comparison alone, I think you could say it’s a safe bet the neutrino will interact and thus be absorbed. However, the different composition of a neutron star (1 giant nucleus) versus a light-year of lead (many small nuclei separated by large amounts of space) would increase the likelyhood of interaction even further.
As for black holes, nothing can escape them. The neutrino would lose all of its energy before it escaped from the black hole.
Yeah, Douglips sounds right on the money.
“What contemptible scoundrel has stolen the cork to my lunch?” --W.C. Fields
Even though neutrinos are thought to be massless (or nearly so – the jury is still out on that one) they do have energy. And energy is also subject to gravitational attraction. So, as stated above, they would not pass through a black hole, but would be absorbed.
“pluto … a seriously demented but oddly addictive presence here.” – TVeblen
Indications from the big neutrino detector in Japan (I forget the name, something like Kamikonde) are that neutrinos can interconvert, which for some arcane reason requires them to have different masses.
If this is true, then at least two of the three kinds must have nonzero mass.
The neutrino detector is Kamiokande, for Kamioka NDE (neutrino detector (experiment?))
I believe Kamioka is a place-name in Japan.
The mass of a neutrino is irrelevant to whether it escapes a black hole. Even massless photons cannot escape.
I’m just a housewife and stumbled upon this in Google so bare with me. Could someone please explain: if a neutrino doesn’t have any mass and it’s neither charged, then what is it if its something at all?? How can i picture it in terms of a needle in a footbal field? Thanks guys and please be nice.
First off, neutrinos do have mass, just not very much of it, their rest mass is at most a million times smaller than that of an electron, but it is still there. Second, the one thing which actually does have zero mass and neutral charge is a photon which clearly does exist.
Neutrinos were first introduced to explain missing mass/energy in beta decay. Basically the things that came out of beta decay reaction (where a neutron turns into a proton and an electron) had slightly less energy than they should have. Therefore it was hypothesized that something else was generated that no one could detect and the actual result was (proton + electron + ???). The mysterious carrier of this energy was called a neutrino.
Although they are very small and non-charged they do interact with the “weak force”, but this force only acts at very close range, so the Neutrino has to “get lucky” an hit just right to have any effect on matter. For the most part they just zip around at near light speed minding their own business and enjoying the energy they got from the beta decay.
Neutrinos do (probably) have mass, just a very small one (note that that point has become more certain over the lifetime of this thread). They do not have electric charge, but they do interact through the Weak Interaction (which is involved in some radioactive decays) and Gravity.
Yeah, as far as I am aware, neutrinos must have mass because they experience time. We know they experience time because neutrinos can change or oscillate between one type or another. I’m pretty sure someone won the nobel prize in physics for demonstrating this, so I don’t think there’s any doubt about it.
Massless particles (like photons) do not experience time whatsoever, so if a neutrino were truly massless, they couldn’t “change” from one type into another.
The mass is very, very small though, compared to any other massive particles we know of. I think.
But as another poster pointed out, photons are an example of a particle that do not have mass and do not have charge. But they definitely have energy, and an associated momentum. So that’s what photons are. Packets of energy, essentially.
It’s worth noting that this applies only to low-energy neutrinos such as those that the Sun produces most commonly. However, high-energy neutrinos have a much higher interaction probability (as discussed here http://www.phys.hawaii.edu/~jgl/nuastron.html
" First the interaction probability for neutrinos goes up with energy. For the largest present underground detector, Super-Kamiokande, only about one in a trillion neutrinos of the typical energy (about 1 GeV, or the equivalent to the proton rest mass) interact when passing through the detector and can be studied. This goes up almost in proportion to the energy of the neutrino however. In fact above about 1 PeV, the earth is opaque to neutrinos and one must look for neutrinos only coming downwards. "
And to definitively answer the OP: Neutrinos do pass through neutron stars mostly unscathed (perhaps a tenth of a percent of them would interact, but not much more than that), and this is in fact one of the major ways in which neutron stars cool off. They do not pass through black holes, though, any more than does anything else.
Then there is dark matter which does not experience the electromagnetic force, the weak force, or the strong force. It still has mass so it does interact gravitationally with ordinary matter, but otherwise nothing is known about it. Perhaps there are other forces it experiences that ordinary matter doesn’t. It could be almost like a parallel universe.
Thank you for your explanations. They were useful in helping me better understand.
I didn’t mean to kill the thread…lol
If photons have no mass, then how come they are attracted to the black hole?
It’d been dead for 12 years before you showed up, so you shouldn’t worry.
Photons are a gauge boson, that is, it is the force carrier particle of the electromagnetic force.
According to General Relativity, spacetime is warped or curved by the presence of matter, and EM radiation follows these warped paths (or “geodesics”) of spacetime.
Black holes warp spacetime so much, that once anything crosses its event horizon, these “geodesics” all converge to the singularity at its center.
I noticed but still, it felt awkward 
Nitpick: The nonbaryonic dark matter certainly doesn’t experience the electromagnetic force, and presumably doesn’t experience the strong force, but most proposed dark matter candidates are still subject to the weak force. Now, we don’t know for sure which, if any, of the proposed candidates are real, so we can’t be certain, but it seems safe to say that the real dark matter (whatever it is) is subject to the weak force.
Braaanes.