Wednesday, May 30, 2012

The Case Of The Missing Neutrinos!

Gather 'round, chill'un, while I tell you a story of explosions, flavors, and scientific inquiry.

You may recall a previous post about Hydrogen in stars.  To wit, this happens:


1
1
H
 
1
1
H
 
→ 2
1
D
 
e+ ν
e
 
0.42 MeV


Which translates roughly as two Hydrogen atoms becoming a Deuterium atom, an electron, a neutrino, and some excess energy.

There is a general principle in chemical/particle interactions of conservation.  Certain characteristics - such as spin, electrical charge, energy, momentum - must carry over from the beginning to the end.  In the early 1900's, this requirement was being defied in beta decay, which also produces a proton and an electron (or positron), resulting in what appeared to be a missing amount of energy.  Wolfgang Pauli first suggested the idea of a small (relative to other particles), electrically neutral particle, and Enrico Fermi used the idea, assuming the neutrino's existence and positing a successful model of the interaction.

Ok, lets fast forward to the 1940's.  Neutrinos (or rather, the result of other particles interacting with neutrinos) are detected in a very clever experiment.

Now let's fast forward again to the 60's, because that's where the important things happens.

As I said earlier, neutrinos are produced in some of the reactions inside stars.  Something you should know about neutrinos; as they are electrically neutral, they effectively do not interact with electromagnetic fields (there is a very tiny magnetic moment, but irrelevant for the likes of us and our blogs), meaning they can pass though matter quite unphased.  About a million billion neutrinos from the sun pass through you each second, with nary a strain on your molecular make-up (lucky you).

So!  The first of several neutrino particle detectors was built in the late 1960's.  It was placed nearly a mile down in a gold mine in South Dakota.  It was placed so deep to protect it from all the other particles and rays (mostly from the cosmos) that could interfere with the experiment.

A 100,000 gallon tank was filled with perchloroethylene, which was predicted to, on occasion, interact with neutrinos, changing a chlorine atom to a radioactive argon atom.  Of all the neutrinos to pass through this large tank (much larger than your own surface, with your million billion neutrinos per second), it was predicted that about once a day, a single neutrino will interact with a single chlorine.

Which I hope gives you a perspective on just how ridiculously delicate such a detection is.

What happened was not a reaction every day, but a reaction about every three days.  A mere third of the expected neutrino reactions were occurring.  They checked their predictions, their equipment, their calculations, their results; they could not find any issue with the experiment itself.  Eventually, other detectors were built and verified the lack of expected neutrinos.

Neutrinos were, in the Standard Model of the time, assumed to be massless.  But it was determined that if they have any mass, even the slightest bit, possibilities begin to emerge.

With mass, neutrinos would, for reasons we won't get into here, be capable of oscillating between flavors - a quantum number assigned to particles.  The idea, first proposed by Bruno Pontecorvo in 1957, that neutrinos may actually exist as a superposition of three possible states, oscillating between possibilities.  These oscillations for solar neutrinos would take place on their way out of the sun, interacting with the chaotic matter.

Now we have hope.  If the detectors were incapable of tasting every flavor, then it stands to reason that not every neutrino would be noticed.

It wasn't until 1984 that a proper detector was proposed by Herb Chen of the University of California at Irvine.  Heavy water (which has Deuterium atoms instead of Hydrogen) is an ideal candidate for interacting with all three neutrinos.  The Sudbury Neutrino Observatory, over a mile under the surface, was turned on in 1999.

In 1998 the Super-Kamiokande in Japan, using pure water and originally built to determine if proton decay exists in the universe, determined the existence of neutrino oscillation (from any neutrino source).

And finally, in 2001, more than 40 years after it was proposed, Sudbury had undeniable evidence that the solar neutrinos oscillate, and verified the originally predicted amount of neutrinos being produced.

How beautiful is that!?  A question spans continents and decades; testaments to our ingenuity are brilliantly engineered based on the theoretical propositions of people who weren't even alive anymore.  And they were right!

And the body of knowledge keeps growing.



(Super-Kamiokande)


(The current Standard Model of Elementary Particles)

Sunday, May 27, 2012

Term O' The Day

Because I just happen to like some words more than others.

Perigalacticon: The point in a star's galactic orbit nearest the galactic center.

Also, a great sci-fi surf rock band name.

Thursday, May 24, 2012

Goddamn Coffee Mug!

 I'm gonna link to Fuck Yeah Fluid Dynamics a bunch, most likely, because it's fantastic.

In this case...what a wonderful opportunity to express how neat resonance is!

So, the sum of that paper is that the stride we use while walking (a quick 'step' followed by a slow lurching over your balanced and stiff leg) resonates slightly with the 'natural frequency' of your average coffee cup.

Now, you have this walking motion (quick-then-slow), a sort of simple sinusoidal motion, matched with a fluid circulating around a small cup (try it at home, but not on carpet).  What resonance means here is that - considering both actions as simple waves - these waves hit their peaks at the same time.  So the liquid sloshing around may do so twice as quickly the step as the person walking with it; if it is under the influence of this resonance, it is not 2.5 times as quickly, it is two...or it could be three, but not 3.12...These fractional values do not resonate.  Resonance means an exacting relation between distinct waves.

Meaning the wave of our step-slow-step oscillation matches, in some integer multiple, the oscillation of our coffee cups.  And then it spills.  Like a jerk.

Making Photons Pt 2

Previously, we covered the long wavelengths of the electromagnetic spectrum, from radio waves to visible waves, from hundreds to billionths of a meter.  This is where things can start to literally get painful.  Time for a bit of the ol' ultraviolet. [I am not sorry for doing that]

UV light, as you might expect, has a higher frequency (shorter wavelength) than visible light, ranging from 10-400nm.  This takes more energy than visible light, but is created in similar fashion - excited electrons in atoms or molecules dropping states.  Because of the higher energy requirement, these drops simply must be larger than those for visible light.

Fun Fact: high-energy UV light can literally break your DNA.  FUN!

X rays!  Now these sci-fi celebrities know how to do some damage.  Ranging from .01-10 nm, these still use electrons, but in a slightly different way.  Recall how the difference in energy of excited states is equal to the energy of the photon.  In order to make x rays, first accelerate the holy hell out of some free (unbound) electrons, then smash them into nuclei [note: at this size, nothing actually ever touches anything - 'smashing' thus implies getting them really really close together until their repulsive forces push them apart].  This sudden negative acceleration produces photons, like not wearing your seat belt in a car crash and flying through the window.  This is called bremsstrahlung, German for 'braking radiation'.

X rays are capable of ionizing - completely removing - an electron from the inner shell of an atom.

Our final stop through the EM spectrum is with gamma rays, the highest-energy photons*.

*By which I mean naturally-occurring photons.  The difference between x rays and gamma rays today is a matter of source: x rays are made by the behavior of electrons.  Gamma rays are generated right in the nucleus itself, by excited or unstable protons.  We can make them right here on Earth with things like radioactive nuclei, and they're a staple of all of the most violent and powerful events in the universe.

So, have you noticed?  For radio waves we rapidly changed the current of electrons, for microwaves, we rapidly changed the magnetic field.  Infrared is generally created by vibrating molecules (coated in electrons),  with visible light created by de-exciting electrons, similarly to ultraviolet light.  X rays are produced in extreme interactions between electrons and others, or with nuclei, and gamma rays come from the heart of the atom itself.

It's all electrons! (*ahem* and protons) You need an electric charge (positive or negative) to create electromagnetic fields.  You also need charge to detect these fields.  With no charge, you become completely blind to any and all electromagnetic processes.  What a sad state of affairs that would be.

Thursday, May 17, 2012

Making Photons

You may have heard of the photoelectric effect, discovered in the late 1800's and later explained by Einstein (for which he won the Nobel).  To sum it up, the energy of a photon is defined by its wavelength.


The shorter the wavelength, the higher the energy.

So how do you make a photon?  Why, you accelerate a charge!  This acceleration (I have a post on inertia in the making that will help illuminate what it really means to accelerate something) comes at a cost of momentum, and this cost is taken out in the form of a photon.  This means that the energy (wavelength) of a photon is dependent on the strength of the acceleration.  What do I mean?  Well lets take a cue from that image up top and start on radio waves.

Radio waves are the longest wavelength, least energetic photons we can make, ranging from 1mm to 100km (note; this does not mean the photon is that length, merely the wavelength).  As you might expect, the acceleration to create these is relatively gentle.  One way is to generate a current through a wire.  Starting this current accelerates the electrons in the wire, emitting a wave, and then stopping the current accelerates them in the other direction (deceleration is not a thing), emitting another wave.  The energy of the waves are dictated by the strength of the current.

Microwaves!  We know them, love them, and still can't figure out why parts of our burritos are cold (another post, perhaps).  They actually fall under the category of radio waves, so they are made in a similar way.  Your microwave in particular actually uses a magnetron (not a decepticon) to rapidly change the magnetic fields of streams of electrons (another future post, on the relationship between electricity and magnetism).  In fact, it does so at such a rate that the microwaves emitted have a wavelength of 122mm, which makes them perfect for spinning the water molecules in your food.  This spinning bumps them against other molecules, transferring temperature (kinetic energy), and heating your food.

Infrared waves are created a little differently.  More energetic than radio waves at ~1-300um (millionths of a meter), these are created from vibrations of molecules.  For this reason (temperature is the kinetic energy of molecules), it is often considered heat radiation.  It's what you see through infrared cameras, and it is often used in astronomy as the infrared light travels through interstellar dust easier than visible light.

Now onto the tiny band we're all so delightfully familiar with.  Every lovely color you've ever seen fits in a tiny little spectrum between 400-700nm (billionths of a meter), with blue/violet at the lower wavelength and red at the higher(hence 'ultraviolet' and 'infrared').

There are a few ways to make visible light.  One way is similar to infrared light, when something is so very hot, the wavelengths begin to creep in the red visible region (like a stove burner coil).  A much more common way is to excite an atom's electrons, putting them in a higher, more energetic orbit.  When this electron falls back into its ground state, it emits a photon.  Fortunately for us, the energy difference between many excitation states falls generally within the energy of visible light.

You may notice that the energy needed to create these is getting larger.  The 'size' of the vibration or acceleration is getting smaller (first on the order of antennae, then on the order of molecules, then on the order of electron orbits).  You may even notice that this means there's a relationship between the wavelength of the light and the 'wavelength' of the vibration.

Good!  Contemplate that for a while, as we head into the higher wavelengths in tomorrow's post.

Wednesday, May 16, 2012

Hydrogen, the Light-Bringer

Ahhh, the Hydrogen atom.

[not to scale]

One proton (positively charged), one electron (negatively charged), locked together in a quantum mechanically determined orbit.  Of all the items in the world, few do we understand as well as the Hydrogen atom.  It is the most common element, as 74% of the light matter in the world.

Now see, you know that thing, that particle-wave duality some folks like to bring up when talking about how batshit crazy the universe is?  So imagine those two dots up there not as dots, but as little standing waves, just sitting there, waving about in a constrained area.  Now apply some pressure.  Imagine that pressure gets these waves all excited, and they start waving about a little bigger - the more pressure, the more excited, the bigger the wave.

Now imagine two of these.  Actually, imagine trillions upon trillions, an ocean of H+.  That's a lot of mass, and a lot of mass means a lot of gravity, which means a lot of pressure.  Get enough of these little fellas together, and those waves start getting big.  In fact, the wave of the proton starts to get so big, it reaches outside of the electron radius.  But all these atoms are right next to each other, so that means it's reaching into the other atom.

Generally, the electron's negative charge (both from it's own atom and the surrounding atoms) repels the proton, and keeps it inside.  But this wave is so big, the proton slips right past this 'impenetrable' barrier, and hops right into the other atom.

This puts two protons in one atom, which is called Deuterium [EDIT: what actually happens is the reaction releases a positron, removing the positive charge from the system and turning the proton into a neutron; H/T Ian], sometimes 'heavy hydrogen'.  To oversimplify the process (get them deets here), all this excitement overcoming barriers results in Helium, every child's  favorite inhalant.


That cluster of neutrons (no electric charge) and protons in the middle there tends to keep itself in line.  What this means is that it is easier, energetically, to hold a Helium nucleus together than a Hydrogen atom.  This is significant.

So we have all these trillion trillion trillions of trillions of Hydrogen , pressuring each other, turning into Helium, and Helium is easier to hold together.  But we know energy is always conserved!  If the energy holding the Hydrogen isn't needed anymore, where did it go?

It was...released.




That energy is the nuclear explosion we all know and love and rely on.  This is how stars are born.


Have a wonderful day!

Tuesday, May 15, 2012

What's All This About?

Hello out there, you beautiful literates.  Welcome to Collected Knowledge, which is...probably not the first blog of it's name, but I'm not much at naming things.

So!  What's this one all about?

Well, my name is JD and I've just graduated with a BS in physics.  Which is neat!  It confers at least a slight bit of authority - I may not know something, but I come equipped with a tool kit that allows me to figure out a reasonably approximate explanation.  For years, one of my favorite bits of joy through a day comes when I get to explain something to someone else; how stars form, what a ring species is, the dimensionality of Pollock paintings, why you shouldn't be scared of black holes, why cancer is such a damnably difficult problem, and just what the hell is a point particle anyway?

I love how much we know.  How much we've deduced and derived and modeled.  Paraphrasing Einstein, I am blown away that the world is even comprehensible at all, let alone how far we've progressed in that comprehension in the few hundred years we've really gone at it.

Our understanding of the universe is a wonderful thing, but "our" in this context can be disappointing.  As a species, we have explored many nooks and crannies of the world, a practice that isn't slowing (as a species - as far as nations go, some sadly are falling behind...), a practice that has turned out the greatest amount of understanding of any explanatory model we've yet considered.

But on an individual level, scientific literacy...well, it's not great, is it?  One might even call it downright atrocious.

To some extent, who could blame anyone?  The sad, homogenized, blunted, stuffy science and math nearly everyone (in America) learns throughout school is about as alienating and unpleasant as it gets.  It's enough to breed a sort of anti-intellectualism, resentment towards the smug academic elite.

Or maybe its just enough to make you not give a shit.  Ever again.

But maybe you've also noticed a growing surge of scientific cheerleading not seen since the days of the space race.  Science and technology has finally become powerful enough to perpetually insert their influence into our lives and minds.  You're reading this on some sort of computing system, ones and zeroes (however that works) transferred and downloaded (whatever that means) through space, bouncing off satellites, onto your eyeballs.  Even if these things are beyond you, you can hold reverence and respect for the efforts it took to create them.  Finally, the population cannot choose to ignore the power of our manipulation and understanding of the universe around us - and we are embracing it.  There are scientists out there with fans these days.  Fans!

So, in a sense, that's what I want to do with this blog.  I want to facilitate that embrace.  I'm going to find things - the path of the Earth-Moon system around the sun, quantum tunneling, the Mandelbrot set, duck vaginae, the salinity of the Amazonian delta, anything - and explain, and expand, and hopefully even entertain.  I want to enhance the perception you have of the world around you.  I want to tell you something that helps you understand something else.  The famous samurai Musashi said "from one thing, know ten thousand things".  Never in my life have I come across any more beautifully apt description of the scientific endeavor.  The models we create to explain some phenomena, if they've any inherent value to them, always, always, always find themselves explaining other phenomena while they're at it.

And what a lovely thing that is!  So if you don't mind, I'd like to give you some really quality building blocks for the model in your head.

Thanks for reading,
JD

Please, don't be afraid to email questions, comments, and corrections.  That only makes this easier for me.