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THE DAILY ASTRONOMER

Monday, June 6, 2016

Rapid Fire Neutrinos

It is utterly appropriate that an article pertaining to neutrinos would precipitate a rapid fire salvo of myriad neutrino queries raining down upon us like a cloud thick swarm of descending arrows.   Well, perhaps, this description is a slight exaggeration.  However, one subscriber sent us a delicious little batch of neutrino questions and  we decided to circumvent Pandora and address them at once.    We apologize to those subscribers whose questions have been fermenting in the jar so long they're now considered vintage.     For your benefit, we will focus much of the week on Pandora, while still reserving Fridays, of course, for the weekly quiz.  (After all, this Friday we "enjoy" the next installment of "Brain of Portland!")

"Do neutrinos ever "interact" with the human body? Given that the human body is mostly vacant space, would we even know of the interactions if they happened? And when neutrinos pass through whatever they pass through, which is everything I expect, do they leave something behind or take anything with them? How fast do they travel? Do outside influences slow them down or speed them up? Do they or can they ever collide with one another? Does gravity affect them?"

-April Adams,  Topsham, Maine

Wow!

Beautiful questions.   Now, this is the perfect way to begin a new week: with five gears, eight cylinders and lame metaphors written by someone who doesn't know the first thing about car engines.     We'll address each question in order.

Do neutrinos ever "interact" with the human body? Given that the human body is mostly vacant space, would we even know of the interactions if they happened?

Neutrinos hardly ever interact with anything.   As you read these words, trillions of neutrinos are passing through your body as though it wasn't even there.  This inexorable onslaught will continue unabated for the rest of your life.   What is the chance that a neutrino will interact with a particle in the body?

Now, people have actually performed the calculation to estimate the probability of a neutrino interacting with the human body.

The precise term for this is "mean free path," the average distance a particle can travel without hitting anything.    These calculations presume that the neutrino would encounter a proton or neutron, which are substantially larger targets than electrons.   It also makes some estimations about the "size" of a neutrino, which relates directly to its energy level: the greater the energy, the larger the neutrino.       Based on these parameters, a neutrino's mean free path through human tissue is about 3000 light years!    That means that if you were 3000 light years thick, a neutrino would have a 50 percent chance of striking something.   

However, the calculation doesn't end there!

This value pertains to a single neutrino.   However, as we just mentioned, trillions of neutrinos are passing through us every second.        We therefore have an interesting statistical situation: a highly improbable chance of a neutrino collisions wedded to a staggering abundance of neutrinos.    Without delving into the math (which leaves bite marks), it turns out that, on average, a person will actually experience about 6000 neutrino interactions in their lifetimes.      If we performed another math calculation (which doesn’t' bite, but just yanks the ear to the point of annoyance)  a person will experience a neutrino collision about once every 6 - 7 days!      

Such interactions do not cause any ill effects and, unlike neutrino collisions with chlorine atoms, do not leave detectable traces.

So, you have been hit by neutrinos and you will likely be struck again, but you will never know it.   

Only through interactions would they be able to leave traces and they are also incapable, as far as we know, of capturing anything and conveying it away.      During rare encounters,  neutrinos "collide" and do not proceed along their paths. 

Astronomers are not certain anymore.   When neutrinos were first detected in 1956, the neutrino was assumed to be massless and traveling at light speed.     However, it has been established that neutrinos can "oscillate," meaning that they can spontaneously change their "flavor."  Neutrinos come in three "flavors," namely electron, tau and muon.*   Neutrinos can only oscillate if they possess some mass, although this mass even smaller than that of an electron.  However, because the neutrino is not massless, it cannot travel at light speed.  According to Special Relativity (1906, Einstein),  any massive object moving at light speed would become infinitely massive.  Neutrinos certainly aren't that.   They move at extremely high velocities that approach light speed.

Note: Some scientists once even thought that neutrinos were capable of superluminal (faster than light speed) velocities.    Physicists at the CERN laboratory in Switzerland have since debunked this notion.  (Wet blankets.)

No neutrino collisions have ever been observed.      The density is so low, despite their great numbers, that such interactions would be extremely unlikely.     Here we should point out that neutrinos have different energies.   Particle physicists believe that low energy neutrinos cannot interact at all, as the weak nuclear force through which neutrinos interact would prevent such collisions.  It is possible that two colliding neutrinos could produce an electron or a positron or a "pair production" of electrons.  Depending on the energies involved, such a collisions could, Monty, produce something completely different.

Do outside influences slow them down or speed them up? Does gravity affect them?  

This issue also remains unresolved.  As neutrinos are not massless, they would be susceptible to external gravitational influences as other massive particles would be.     They should "slow down," in theory, if they move away from massive objects due to the mutual attraction, just as they should accelerate when approaching a massive object.  The extent of these speed changes is unknown.     However, the presence of massive neutrinos in the Universe might explain "dark matter," which is believed to constitute about one quarter of the material universe.   Dark matter is not visible, hence the term "dark," but is known to exist through the gravitational influence is exerts on visible matter.  Although a single neutrino is a definite lightweight, the cumulative mass of all the neutrinos pervading the Universe could account for at least part of the material now referred to as "dark matter."

*"Flavor" is the strange word used to specify elementary particles.    The word "flavor" applies to quarks, the constituent particles comprising neutrons and protons, and also to the six leptons, which we can think of simply as those elementary particles that are unaffected by the strong nuclear force, the force that binds subatomic particles within atomic nuclei.  These lepton flavors are electron, electron neutrino,  muon, muon neutrinos, tau and tau neutrinos.    Someday we will delve more deeply into the suffocating thickets of elementary particles.

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FROM THE CATACOMBS OF INFINITE KNOWLEDGE:

Glass is a liquid that requires about one million years to decompose!

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