# Can the brain detect the passage of a neutrino?

On a few occasions either in bed or sitting around a fire, with my eyes closed, I rarely but sometimes see a very quick fast flash of white and then, with my eyes still closed, the flash disappears immediately. It happens so fast that I sit up and rethink if it was even real. But I know it is real because I have had it happen to me many times in my life. I have also asked other people if it happens to them and 4/5 replied back saying that they had experienced the flash before.

Is it possible for a neutrino to pass the brain and in response produce the white flash? After all the brain is made of 73% water and neutrino detectors are predominantly water.

I tried submitting this question on biology.stackexchange and I was told that questions like these belonged on the physics.stackexhange site.

• Apollo astronauts reported similar flashes that are thought could be the result of cosmic rays. Not many cosmic rays (which are mostly high energy protons) reach the surface of the Earth but collision products from their hitting the upper atmosphere do. These products are mostly muons, detectable in a home made cloud chamber. I don't know whether these can give rise to what you see. Mar 9, 2015 at 9:47
• Hah, detecting neutrinos would be the worst superpower EVER, since many, many (Wikipedia suggests at least $6.5\times 10^{10}$ per second, per square centimeter perpendicular to the Sun) of them pass through you all the time!
– Danu
Mar 9, 2015 at 10:09
• Whatever you're seeing, it isn't caused by neutrinos. Not directly anyway.
– Mast
Mar 9, 2015 at 15:22
• @DroneScientist is correct, this has much more to do with what is occurring within the eye than anything coming from the outside affecting you. See a doctor if it persists. Mar 9, 2015 at 16:10
• How bright is this flash? How localized in your field of view? It could just as easily be an optical illusion, trick of the brain, minor hallucination, or some other non-physical reason as it could be due to uncommon physics reactions
– Jim
Mar 9, 2015 at 16:39

The cross-section for neutrino interactions is energy dependent.

For solar neutrinos at $\sim 0.4$ MeV, which would likely dominate any neutrinos likely to interact (the cosmic background neutrinos have way low energies) , the cross-sections are $\sigma \sim 10^{-48}$ m$^2$, for both leptonic processes (elastic scattering from electrons) and neutrino-nucleon interactions.

The mean free path of a neutrino will be given by $l \sim (n\sigma)^{-1}$, where $n$ is number of interacting target particles per cubic metre and $\sigma$ is the cross-section.

If your head is basically water with a density of 1000 kg/m$^3$, then there are $n_e = 3.3\times10^{29}\ m^{-3}$ of electrons and about $6 \times 10^{29} m^{-3}$ of nuclei.

Including both nucleonic and leptonic processes, the mean free path is $\sim 10^{18}\ m$.

So unless your head is 100 light years wide, there is little chance of any individual neutrino interacting with it.

This is only one part of the calculation though - we need to know how many neutrinos are passing through your head per second. The neutrino flux from the Sun is about $7\times 10^{14}$ m$^{-2}$ s$^{-1}$. If your head has an area of about 400 cm$^2$, then there are $3\times 10^{13}$ neutrinos zipping through your brain every second.

Thus is we take $x=20$ cm as the path length through your head, there is a chance $\sim x/l$ of any neutrino interacting, where $l$ was the mean free path calculated earlier.

This probability multiplied by the neutrino flux through your head indicates there are $6\times 10^{-6}$ s$^{-1}$ neutrino interactions in your head, or roughly one every two days.

Whether that would produce any perceptible effect in your brain needs to be shunted back to Biology SE. If we require it (or rather scattered electrons) to produce Cherenkov radiation in the eyeball, then this needs $>5$ MeV neutrinos and so the rate would reduce to 1 per 100 days or even lower due to the smaller number of neutrinos at these energies and the smaller volume of water in the eyeball.

EDIT:

In fact my original answer may be over-optimistic by an order of magnitude since water only acts as a good detector (via Cherenkov radiation) for neutrinos above energies of 5 MeV. Solar neutrinos are predominantly lower energy than this. My calculation ignored atmospheric neutrinos which are produced in far fewer numbers (but at higher energies $\sim 0.1-10$ GeV). The cross-section for these is 4-6 orders of magnitude higher, but I think they are produced in so much lower numbers that they don't contribute.

Conclusion It doesn't have anything to do with neutrinos. The rate would be too low, even if they could be perceived.

• Surprising result! +1 for the calculation
– Danu
Mar 9, 2015 at 10:39
• what-if.xkcd.com/73 says it's more like one every few years in your entire body. He does not show any calculations though. Still, I'm afraid that human eyes cannot see 0.4MeV=6.4e-14 joules of light energy. I think the OP should return to the biology SE, but simply ask 'Why do I sometimes see a bright flash?'. Mar 9, 2015 at 13:05
• @sanchises What you "see" is a matter of conjecture. Cherenkov radiation in the eyeball is a possibility (see edit). It is not clear to me that photons hitting the retina are required in order for the brain to perceive a flash of light. I can't explain the xkcd calculation since there isn't one. The no. of solar neutrinos is fairly uncontroversial as is their energy. The cross-section I've used also seems pretty standard. I think my number of nucleons/cubic metre is ok... So not clear why this deserves a downvote, but maybe it's not you. Mar 9, 2015 at 13:29
• @RobJeffries You got my upvote, don't worry. I was just adding a source that I figured was relevant, and which, in order of magnitude, corresponds to your answer; and I was just adding how insignificant it is energy-wise, even when it does happen. Mar 9, 2015 at 13:44
• Actually Rob and @sanchises if you read about the more modern experiments in that same article that attempt to remove the bias upwards made by excluding possible false positives, the sensitivity is about an order of magnitude greater (6 photons). I have heard similar things amongst vision researchers. Great calculation BTW. Maybe you should send this to Randall Munroe - his calculation, as you say, is uncharacteristically not there! Mar 10, 2015 at 0:24

If you are that fast in detecting light, you are seeing cosmic ray muons. They are charged and leave an ionizing track in anything they cross and Cerenkov light. in liquid, and the eye is mainly liquid.

They are the most numerous energetic particles arriving at sea level, with a flux of about 1 muon per square centimeter per minute. This can be compared to a solar neutrino flux of about 5 x 10^6 per square centimeter per second.

Even though there are a lot more neutrinos they do not generate photons to first order so as to be detectable in bubble and spark etc chambers, and therefore not even to the eye.

The easy creation of cloud chambers showing muon tracks is recorded on several YouTube videos .

With such a chamber, you could have your eye under the cup and have a friend check for coincidence with one of the tracks coming in, to verify the sharpness of your light detection. :)

Edit after googling:

It is proposed that the primary cosmic radiation is responsible for the light flashes observed by astronauts in translunar flight. Cherenkov radiation may be an important or even the dominant mechanism. An alternative mechanism is the direct excitation of the retina by cosmic ray particles.

And then I remembered a story told me by an oldtimer physicist at those early times of high energy physics experiments where physicists controlled the beams: he would center the beam to his detector by the cerenkov light in his eye. Possibly no connection was made with radiation and cancer at those times, and the beam fluxes were not as strong as the beams we currently have. (just recalled that I asked about it and he did the centering with a very weak beam.)

The retina excitation part cannot hold for one off cosmic muons. One would not see a flash, just a point would be excited by the ionization which only travels microns.

• I have not treated the brain hypothesis because it is extremely improbable that a muon hitting randomly the brain tissue will coherently create a flash in the receptors or the area that interprets light from the eye. It would need a biophysical model . Mar 9, 2015 at 10:12
• (1/2) I'd believe that a cosmic ray could induce an action potential event in a neuron that wouldn't otherwise have fired, but the data carried by the optic nerve has already been transformed and encoded to the point where I doubt a single extra spike would be perceived as a flash in a coherent location.
– zwol
Mar 9, 2015 at 16:35
• (2/2) It is much more plausible to me that a cosmic ray would directly trigger a cluster of photoreceptor cells in the retina, which would be perceived as a flash. I am not a physicist but I have studied neurobiology in some detail.
– zwol
Mar 9, 2015 at 16:38
• @zwol The muon is a point particle, its electromagnetic field does not spread further than a few microns. It cannot hit a cluster of cells except if it hits them tangentially, the probability much smaller than the cerenkov radiation while crossing the fluid of the eye. Mar 9, 2015 at 18:22
• At sea level, aren't cosmic rays ~90% neutrons, which would induce a particle shower if they hit something?
– zwol
Mar 9, 2015 at 18:26

You cannot "see" a flash if the eye receives no stimulus even though the brain is directly hit by a neutrino and it would be impossible to detect it because: 1) There is no receptor of this sort in the brain. 2) A neutrino has an extremely tiny mass. I can assure you that if the human body was capable of detecting an individual subatomic particle colliding with us, there would be no practical use of bubble chambers and other complex equipment of this sort.

Even if a neutrino were to hit a rod or a cone in your retina, the stimulus wouldn't nearly be strong enough to generate a receptor potential, much less trigger an action potential (synapses would ensure that any weak impulse is filtered out and not sent to the brain). To even see anything, we need more than one photon to hit our retina. (Quite a few actually)

• Actually I beg to differ. Since your brain controls what you see I am sure you do not need to see things with only your eyes. A good example is Daniel Tammet whos' memories part of brain connected with the seeing part. Now he literally sees shapes, landscapes and numbers that he describes looks like "when you get a camera and turn it on slow shutter speed and write something on a wall." He doesn't physically see these shapes but his brain does. Mar 9, 2015 at 10:37
• Well yes that's true. But the brain will certainly not detect a colliding neutrino. A person can "see" things if the brain imagines those. But you won't see landscapes because of something small colliding with your brain. Mar 9, 2015 at 10:42
• It takes about 50 photons (total energy perhaps 100eV) to cause a response. Solar neutrinos have energies of 0.1-10 MeV. If these collide causing inverse beta decay, a relativistic electron can produce Cherenkov radiation in the eyeball. Direct stimulation of the optic nerve is also a possibility en.wikipedia.org/wiki/Cosmic_ray_visual_phenomena ; universetoday.com/94714/seeing-cosmic-rays-in-space Mar 9, 2015 at 17:15
• @RobJeffries Your so pro at physics. May 18, 2015 at 1:19

This definitely is not a neutrino. Neutrinos are hard to detect because they are light, quick, and have no charge, making them usually pass through matter. We build giant machines to detect single neutrinos. The chances of this happening extremely low.