# How do we exactly use neutrinos to probe the core of the Sun (if they can only interact weakly)?

Neutrinos are interacting with ordinary matter weakly. They pass through matter mostly unimpeded.

Thus, neutrinos typically pass through normal matter unimpeded and undetected. Neutrinos' low mass and neutral charge mean they interact exceedingly weakly with other particles and fields. This feature of weak interaction interests scientists because it means neutrinos can be used to probe environments that other radiation (such as light or radio waves) cannot penetrate. Using neutrinos as a probe was first proposed in the mid-20th century as a way to detect conditions at the core of the Sun. The solar core cannot be imaged directly because electromagnetic radiation (such as light) is diffused by the great amount and density of matter surrounding the core. On the other hand, neutrinos pass through the Sun with few interactions.

https://en.wikipedia.org/wiki/Neutrino

Based on this, neutrinos interact with matter they pass through, just very weakly, and with few interactions.

This could mean they lose some energy as they pass through matter and interact with it weakly with few interactions.

I am curius as how we exactly can probe the core of the Sun when neutrinos only interact weakly. Do they lose any energy through this process or how do we use them?

Question:

1. How do we exactly use neutrinos to probe the core of the Sun (if they can only interact weakly)?
• There's some relevant info about neutrino reactions at en.wikipedia.org/wiki/Neutrino_detector#Theory Mar 27, 2020 at 18:56
• Strategy for the self-answerer: compare to Compton scattering, but use neutral-current cross sections rather than electromagnetic cross sections.
– rob
Mar 27, 2020 at 18:58
• Why the downvote? Mar 27, 2020 at 19:21
• it might be the weak force Since the only two forces that neutrinos feel are the weak force and gravity, there is no reason to say “might”. Mar 27, 2020 at 19:42
• There appears to be a semantics problem here, in that weakly interacting doesn't mean the interaction is kinematically weak (which would be hard to detect), it mean the probability of a perfectly "violent" collision is very, very, small.
– JEB
Mar 27, 2020 at 20:00

The neutrinos are produced by nuclear reactions in the core of the Sun. About 2% of the mass lost in the conversion of hydrogen to helium ends up in neutrinos (almost entirely in the form of kinetic energy, since neutrinos have a negligible mass here).

The neutrinos interact very weakly with matter and therefore they basically all escape from the Sun. The neutrino flux from the Sun can be measured and compared with the predictions of theoretical models, hence probing the conditions (temperature and density) in the core.

Note that your neutrino detector detects a very, very small fraction of the neutrinos incident upon it; but so long as you know what that fraction is, then there is no problem estimating the neutrino flux from the Sun.

• thank you so much. Can you please help with the expression "interact very weakly with matter". Do I understand correctly that this means they interact only via the weak force and very rarely? Mar 27, 2020 at 21:31
• @ÁrpádSzendrei it takes a light year of lead to stop a neutrino. They only feel the weak force, and it is called that for a reason. Mar 27, 2020 at 22:37

To enlarge slightly on Rob's answer, for a neutrino to interact with (for example) a proton requires it to strike the proton essentially "head on" which greatly reduces the chance that any neutrino will interact with a given chunk of matter, even if traveling vast distances through solid rock. But if you collect enough matter together that reacts with neutrinos in a particular way and wait long enough, you can make a neutrino detector out of it even though the vast majority of neutrinos that pass through it do not register on it.

This means two things: you can easily filter out false signals by putting your neutrino detector a couple of miles underground, and you can increase your chances of catching a few by making the detector really big. So the most sensitive neutrino detectors we have are big and placed deep underground.

• thank you. Can you please elaborate on "to strike the proton essentially "head on" which greatly reduces the chance" is this because most of the normal matter around us is just empty space? So the neutrino will pass through empty space inbetween the quarks? Mar 27, 2020 at 22:43
• it passes through the mostly-empty space of an atom, and if it doesn't hit the nucleus, nothing happens. even if it enters the nucleus, it can zip through without interacting with a quark- unless its trajectory coincides with where the quark just happens to be in that particular billionth-of-a-second. I'm not sure how much "empty space" there is inside a proton however! Mar 28, 2020 at 3:07
• The cross-section for neutrino interaction with a proton is ten orders of magnitude or more smaller than the cross-sectional area of a proton. Mar 28, 2020 at 16:53
• thank you Rob! I knew it was small but I had no idea it was that small. Mar 28, 2020 at 17:40