Some ideas on where the missing spin might be

Last time, we saw an overview of the proton spin crisis (whose historical basis is discussed by Jaffe): Each proton has a spin value of 1/2, but only about 25% of this value comes from its three constituent quarks. So, physicists need to look elsewhere for the building blocks of the other 75% of this spin value.

(Side note: This quest isn't entirely unlike the quest to find the missing matter and energy in the universe!)

Recent experiments have probed two possible sources: gluons and pair production.

Gluons. Remember how a proton is composed of three quarks (two up and one down)? Well, those quarks are held together by the strong force, which is mediated by gluons (similarly to how the electromagnetic force is mediated by photons). Therefore, a proton is really three quarks held together by a sea of gluons. Gluons have a spin value of 1, so it's possible for them to be oriented in such a way as to contribute to the proton's spin. Experiments at the Relativistic Heavy-Ion Collider (RHIC) at Brookhaven National Laboratory lend support to this idea.

Anti-quarks. It's also possible to temporarily break up a gluon into a quark and anti-quark. So, a proton's sea of gluons features temporary "flashes" of quark-antiquark pairs. So, these pairs might also be contributing to the proton's spin. However, other experiments at RHIC exploring this pair production showed that their spin contributes very little to the proton spin.

The Proton Spin Crisis - What protons are made of

You may not be aware of it, but there is a crisis going on in physics! Here's the brief summary:

• Each proton has a spin value of 1/2.
• Protons are made of three quarks: two ups and one down.
• The proton's spin value should, therefore, be the sum of the spin values of its quarks: For example, 1/2 + (-1/2) + 1/2 = 1/2.
• When you measure the spin of the quarks inside a proton, you don't come up with the correct total!

You can see why we call it a crisis! (Well, maybe "puzzle" is a bit less alarming and more intriguing. But, this is the internet...) This week, we're going to look at the different pieces to this puzzle, and bring you up to date on some of the latest insights into the problem.

First up: How do we know that the proton is made of these three quarks? We can't see a proton, and the nature of the strong force is such that the quarks don't exist independently!

APS's Physics provides an excellent overview of the original particle collision experiments that first revealed the proton's internal structure. The discovery required a detailed back-and-forth conversation between experimental and theoretical physicists and various mathematical models, ultimately winning the researchers the 1990 Nobel Prize in Physics.

Learning from Wikipedia

This week, we're looking at on-line resources that can help students learn physics. Today, we're going to look at a web resource that is supposedly forbidden for students.

That's right: Wikipedia.

Professors warn students about Wikipedia, and rightly so: Because anyone can edit Wikipedia, it's vulnerable to incorrect information. (But don't worry; there's a Wikipedia article about Wikipedia's accuracy.) So most professors will agree that Wikipedia is not an acceptable reference to cite in an academic paper.

However, that doesn't mean students can't learn from Wikipedia, for three reasons:

1. While there are people who will spread misinformation on Wikipedia, there are just as many who will jump in to make corrections. Caution--not outright mistrust--is the name of the game.
2. The summaries of complicated topics like physics are, generally, very well written, with helpful diagrams, animations, etc.
3. Wikipedia articles are required to cite external sources. So, when you've finished reading an article, you can check its correctness in these sources (which usually are the references you want to cite in an academic paper).

Let's take, for example, the Wikipedia article about physics' particle in a box problem. Could a malicious user come along and ruin this article? Yes! Will the misinformation remain for long, with professors and students turning to it to check equations, look for examples & animations, and refresh their understanding of the problem? No!

The description is well written, with a nice qualitative introduction, followed by a direct solution of the one-dimensional problem organized in order of increasing complexity of topic filled with links to other articles that explain the technical terms employed, capped off with a discussion of higher-dimensional problems, based on the preceding one-dimensional discussion. The text is accompanied by helpful graphics and animations. At each step along the way, a math-equipped reader can check to make sure that the content is sound and edit as required.

And at the end of the article are references--good references available on-line or at a library.

So, can you learn physics from Wikipedia? Absolutely! But more importantly, you can learn how to learn, by reading carefully and checking the information along the way.

Physics on Reddit

Reddit advertises itself as "the front page of the internet." It's an apt moniker: Reddit shows you what it's users find interesting because users can up-vote or down-vote any post. More specifically, it shows you what it's users find interesting within different subjects, called subreddits. The subreddits continue to break up fractally, such that you can find a group of people discussing just about anything. And if you find a sub-sub-sub-sub-topic that doesn't have a subreddit, you can make one.

Here's CGP Grey's explanation of Reddit and why it's an awesome place to learn new subjects:

So, for example, Reddit is a great place to learn physics, because physicists interested in discussing physics on-line tend to frequent the physics subreddit. Users post links to articles and pictures, and host "AMAs" (Ask Me Anything), in which a professional physicist hosts an open forum regarding their work. Here's a recent one regarding climate change, or one about earthquakes.

So, what interesting physics topics do you see on Reddit?

What if ridiculous questions were addressed by serious physics... and stick figures?

This week, we're highlighting resources that can help students learn physics. Yesterday, we discussed HyperPhysics, an on-line concept map, making it a great complement to your physics textbook. Now imagine what if your physics textbook illustrated the concepts of physics with ridiculous yet intriguing examples, such as "What would happen if you tried to hit a baseball pitched at 90% the speed of light?" or "How hard would a puck have to be shot to be able to knock the goalie himself backwards into the net?" or "How quickly would the ocean's drain if a circular portal 10 meters in radius leading into space was created at the bottom of Challenger Deep, the deepest spot in the ocean? How would the Earth change as the water is being drained?" That's precisely what Randall Munroe, the creator of the web comic xkcd.com, sets out to do each week in his what if? blog (and now book).

Each week, what if? addresses a reader-submitted question with equal parts accuracy and ridiculousness. (Some readers may find this familiar.) Munroe explains the physics principles behind the scenario in question, shows or describes the calculations involved in determining an answer, and then extrapolates beyond the original question to ludicrous extreme cases.

Discussions like these help students learn physics in a number of ways:

1. By showing that physicists don't always take themselves/their subject/life super seriously.
2. By showing that our universe can be a weird place and physics explains that weirdness.
3. By demonstrating that, with just a few core physics concepts, one can study and make predictions regarding physical scenarios, no matter how strange.

So, what's your favorite scenario discussed on what if? What's a scenario you'd like to send in?

So you want to learn some physics...

Physics is a subject that many students find fascinating yet difficult to learn. Sometimes, all it takes is "just the right presentation" of material and something in the learner's mind clicks, and they're eager and ready to learn additional concepts.

So, it's important that physics learners have access to multiple avenues of learning (class presentation, textbook, demos, laboratory activities, student-student interaction, tutoring, office hours, simulations...). This week, we're going to look at four on-line resources that many physics learners find helpful.

First up is HyperPhysics, a web-based network of physics topics that allows students to explore connections between physics topics, read sample problems, and investigate scenarios with built-in calculators.

HyperPhysics is unique in that it's organized as a concept map, in which ideas are represented by bubbles and the relationships between them are represented by lines connecting the bubbles. You can begin exploring from the top-level bubbles on the home page:

...or by searching for a particular topic (say, collisions) and seeing how it's related to other topics and examples:





HyperPhysics is also available as an iOS app. 

So, take a look around! Search for what you're currently learning in your physics class. What new relationships do you see? What new topics had you never heard of before navigating the concept map?

What's Next in the Hunt for Dark Matter?

This past July, the US Department of Energy and National Science Foundation announced which next-generation dark matter projects they'll be supporting.

Dark matter takes a lot of equipment, people, time, and creativity to detect. It also requires being in just the right conditions. For example, the XENON experiment is buried under Italy's Gran Sasso Mountain to eliminate any stray radiation from interfering with the detector.

The winning next-generation projects are

1. The Super Cryogenic Dark Matter Search, which uses "a collection of hockey-puck-sized integrated circuits"to find Weakly Interactive Massive Particles (WIMPs), a prominent dark matter candidate.
2. The LUX-Zeplin experiment, which can find WIMPs of a wide range of masses.
3. The next iteration of the Axion Dark Matter eXperiment, which uses magnetic fields to convert axions (another dark matter candidate) into photons.

Finding Dark Matter using Gravitational Lensing

A couple weeks ago, we talked about gravitational lensing, and how it's used to find black holes. When the earth, a black hole, and a galaxy are lined up just right, a black hole bends light around itself the same way a lens bends light to form an image. 

Well, this technique can also be used to find dark matter, as described in a recent article on phys.org.

Want to learn more about dark matter? Be sure to join us at today's Society of Physics Students meeting (12:15, PENT 125) for a TED talk video about dark matter. Hope to see you there!

Dark Matter: How do we know it's there?

This week is all about dark matter - the stuff that physicists have concluded fills interstellar space that doesn't interact with light (hence the reason we can't see it). The search is on for what types of particles this matter might be made of.

But every time a new search is launched, the question arises: How do we know that dark matter is there, to begin with?

Starts with a Bang offers five reasons why physicists are certain that dark matter exists:

1. Galaxies tend to group together in clusters, and observed cluster don't have enough mass to explain this clustering.
2. Galaxies tend to spin like a top, and the rotational velocities of the stars don't match up with what you'd expect if the only mass present was the mass you could see.
3. Dark matter in the early universe left an imprint of oscillations on the Cosmic Microwave Background.
4. Galaxies can collide with each other, and the resulting distribution of stars can't be explained if the visible mass is all that's present.
5. The large-scale structure of galactic clusters has imprints of dark matter in the early universe.

Dark matter, the article concludes, offers an explanation for all these observations, while alternatives (such as modifying our understanding of gravity) cannot explain more than one.

Finding Water Vapor on an Exoplanet

One of the greatest discoveries about an exoplanet was the recent confirmation of water vapor on HAT-P-11b. This discovery is the result of combined observations from three different space telescopes. The data from these observations are processed using transmission spectroscopy, which you might be familiar with in an intro physics or chemistry class: The energy levels of a given compound permit the transmission of only specific wavelengths of light; by examining the wavelengths that transmit through a material, we can determine what the material is made of.

Finding Earth-Like Exoplanets

In the abundance of exoplanets we've found so far, we've had our fingers crossed that we would find worlds with the two main characteristics that make our Earth unique: Being just the right distance from the sun and being just the right size.

A planet's distance from its sun determines the temperature of the planet, based on the radiation it receives from its star, and that temperature determines whether liquid water can exist on the planet in sufficient abundance to support life. The freezing and boiling points of water, therefore, determine the habitable zone for a star.

A planet's size helps determine what type of planet it is, and Earth-sized planets are more likely to be rocky, like our Earth.

Huffington Post keeps a running list of stories about Earth-like exoplanets, including...

• Kepler-186f, which orbits just at the edge of its star's habitable zone and is about 1.44 times the mass of Earth.
• "Super-Earth" Gliese-832c, which might have the right combination of characteristics to harbor Earth-like life.
• Kepler-62f, which may be covered entirely by a single ocean.

Wild Worlds

In our hunt for exoplanets, one of our primary hopes is to find earth-like planets where we might find life (though, of course, alien life might look nothing like we do) and maybe someday live.

But most of the exoplanets we've found so far look nothing like earth; most are a lot more like Jupiter. And some are even wilder than that.

Discovery.com has put together a list of the most horrific planets found so far, including a planet with one side encased in perpetual darkness, a planet being melted by its star, and a planet so close to its star that we need to come up with a new planetary evolution model to explain how it came to be.

Exoplanets - We've found A LOT!

This week, we're looking at humanity's search for planets outside of our solar system, which we call exoplanets. While we can see stars outside of our solar system ("exostars?") easily because they emit light, seeing planets orbiting those stars is rather difficult. The first observation of an exoplanet orbiting a star wasn't made until 1995.

However, since then, we've found over 1700 exoplanets, with even more waiting to be found by NASA's ongoing Kepler mission. The Kepler mission uses the transit method of looking for a dimming of a star's light as a planet passes between the star and Kepler's view.

There are several great visual guides to learn about the exoplanets we've found; here are a few:

Some important planets discovered by Kepler: http://pbs.twimg.com/media/BujAPsrIEAAo2JG.jpg:large

xkcd's 786 planets to scale: http://xkcd.com/1071/ and interactive version: http://visual.ly/exoplanets-interactive-version-xkcd-1071?view=true

xkcd's set of habitable-zone planets within 60 light-years of earth: http://xkcd.com/1298/

Video of Kepler's candidates: http://www.universetoday.com/83200/video-visualization-of-kepler-exoplanet-data/ 

Black Holes and Gravitational Lensing

We end our week's discussion of black holes by looking at one of the ways we can "see" black holes - Gravitational lensing. This is the process where a black hole bends light around itself (light that doesn't get caught in the event horizon, of course!) the same way a lens bends light to form an image. When the earth, a black hole, and a galaxy are lined up just right, we can see amazingly distorted images that help us figure out where a black hole is and how strong its gravitational force is!

Massive Black Holes at the Centers of Galaxies

It's thought that many galaxies have massive black holes at their center, including our own. There's also now evidence that these massive black holes "grow up" with their host galaxy, affecting each other's size and shape. Volonteri & Ciotti present the details here.

Studying Black Holes

We saw last time that black holes are defined in principle as objects so massive that light cannot escape from them. There's also lots of properties we study about black holes, including their mass, their spin (which can be quite fast), and the size of their event horizon (the point of no return, where the escape speed equals the speed of light). Narayan reviews these properties for several observed black hole candidates.

Black Holes: What are they and how do we know they're there?

Black holes are one of the most popular scientific topics, and many of their properties are straightforward to understand with an intro-level understanding of physics. Starts With a Bang has an excellent article describing the basics of what a black hole is and how we look for them--even though, by definition, we can't see them directly.

If you add enough matter to a star, Siegel writes, the gravity would be so strong that "not even light would be able to escape. As Hawking (and others before him, going all the way back to John Michell in the 18th Century) have noted, this would create a black hole in space, where matter (and other forms of energy) could fall in, but nothing — no matter, no light, no nothing — could get out."

But what does this concept of "escaping gravity" mean? If you wanted to "escape" the earth's gravity, for example, how would you know you had accomplished it?

The answer lies in thinking about energy.

You probably learned at some point in school that energy primarily comes in two forms: kinetic energy (energy associated with movement) and potential energy (energy associated with where you are). These concepts help you determine, for example, how hard you would need to roll a ball if you wanted the ball to make it over a hill. The higher the hill, the more kinetic energy you'd have to give it at the beginning.

This relationship is determined by a law called the conservation of energy: The total amount of energy in the universe has to remain the same. In the case of "escaping gravity," that means you need enough kinetic energy when you launch from the earth to overcome to amount of potential energy you have at launch (the size of the hill). "Having enough kinetic energy" means having a fast enough speed, and it's actually pretty straightforward to calculate this escape speed.

So, when we say that "light can't escape a black hole," what we means is that the escape speed from a black hole is higher than the speed of light!

Spin-polarized states in graphene

Today we look at an article about creating spin-polarized electron states in graphene. You might be familiar with the concept of polarization in the context of waves. There, the term refers to which direction a wave is oscillating:

Image credit: http://www.photonics.com/images/WebExclusive/Omega%20Optical/Fig-3.jpg

Spin-polarization means much the same thing: You have a stream of electrons whose spins are aligned in a common direction. (Another term for "spin-polarized" is "spin-helical.") Since spintronics technology requires the manipulation of individual electron spins, you can imagine how important it is to set up spin-polarized electron states in graphene!

Graphene: More uses!

Graphene proves to have amazing uses. This article describes how graphene can be used as a tunneling barrier--a "wall" through which electrons can tunnel through at a specified rate.

Tunneling is the quantum mechanical process by which a particle shoots through a region of potential energy that classical mechanics says should be inaccessible to it because of conservation of energy. For example, suppose you kick a soccer ball (mass 0.4 kg) with a speed of 12 meters per second toward a hill that rises 10 meters high. Its kinetic energy would be 1/2*(0.4 kg)*(12 m/s)^2 = 29 joules. Since the soccer ball only has 29 joules of energy to climb with, once it reaches a maximum height of (29 J)/(0.4 kg * 9.8 m/s^2) = 7.4 meters, it would turn around. You'd have to kick the ball faster to make it over the 10-meter-high hill.

However, if you repeat the same experiment with an electron, quantum mechanics says the electron can still end up on the other side, even though it doesn't have "enough" energy to do so!

This process, called tunneling, is demonstrated beautifully by the simulation below:

Click to Run

Graphene - a technical overview

Today, we examine the technical details of graphene more deeply, through a helpful review article by Geim. This article makes a few references to crystal structure and effective mass:

• Crystal (AKA lattice) structure refers to the regularly repeating pattern of atoms in solid materials. Sodium chloride (NaCl, table salt), for example, has a cubic structure with Na and Cl atoms alternating at the corner and center of each cube. The shape of a lattice is often (including in Geim's article) noted using Miller indices.
• Effective mass refers to how an electron's motion is affected by its surroundings. If a single electron were on its own, its effective mass is its "normal" mass of 9.11x10^-31 kg. However, the presence of the lattice of atoms and the other electrons cause the electron to behave (i.e., respond to forces) as if it had a different mass.

Introducing graphene

This week, we take a look at one of the greatest developments in physics over the last decade: Graphene.

The graphite in your pencil is made of carbon atoms arranged in a repeating hexagon pattern called a lattice. The layers of this lattice are very loosely bound, which is why it makes such a great writing implement: The layers shed off as you drag the pencil across paper.

Graphene is what you get if you remove a single layer of graphite, producing a purely two-dimensional material.

CNN has a great interview with the physicists who discovered graphene, along with a great series of infographics that describe some of the amazing properties of this this wonder material and explain what it's useful for.

Quantum weirdness in quantum computing

We've seen this week how weird things can get in quantum mechanics, and how useful that weirdness is. Today, we conclude this series by looking at how quantum weirdness is used in quantum computing.

Recently, researchers that University of Tokyo developed techniques for manipulating light between a particle-like state and a wave-like state, one of the greatest experimental goals of quantum mechanics:

Image credit: http://cdn.phys.org/newman/gfx/news/2014/11-experimental.jpg

The summary article linked above (full article here) describes the applications of this technology to qubits, which are the basis for quantum computers. Your (classical) computer operates by storing information in binary code: everything breaks down to a 1 or a 0, called bits. A quantum bit has the added property that the physical information storage is so small (like, an electron spin) that the rules of quantum mechanics apply, and the quantum bit (or "qubit"--see what physicists did there?) exists as a 1 and a 0 simultaneously. This property allows quantum computers to perform calculations with greatly reduced times; imagine, for example, a chess program that can sample all possible moves at the same time (instead of one at a time, which a classical computer must do).

Quantum mechanics in drug design

We've seen that quantum mechanics produces some weird effects, namely...

• Energy comes in discrete lumps (instead of being smooth).
• Position is governed by probability (instead of being well-defined).

In a 2007 Drug Discovery Today article, Raha et al discuss how these properties are vital in drug design. Read over their article and post in the comments below an answer to the question: What is one example of how they use the weirdness of quantum mechanics (discrete energy and/or probabilistic position) in drug design?

Quantum Weirdness - Why don't we see it?

Last time, we looked at some of the weird properties of quantum mechanics, leaving us with the lingering question of, "Why don't we experience these weird properties in the everyday world?" For example...

• Why doesn't my energy come in discrete levels?
• Why is my position so easy to measure, and not spread out over the entire universe?

In 1991 issue of Physics Today, Zurek outlines an answer. As you look over his analysis, here are some of the concepts he discusses:

• The state |ψ> is what we, last time, referred to as the probability density of the particle's position. (Technically, you use |ψ> to calculate the probability density, and it can be rewritten as the probability density of any measurable quantity, but the simple explanation suffices for now.) The pointy shape | > that ψ is encased in is just a symbol that denotes what type of quantity it is (an infinite-dimensional vector of complex components, which is a member of a set called Hilbert space). Just think of it as a function--a very, very special function! (Long description here.)
• Spin-1/2 is related to what we discussed in our series about spintronics, that protons and electrons (and, therefore, atoms) have an inherent property whose equations look a lot like the particles are spinning. Because spin is a quantum mechanical property, it comes in discrete lumps, and for protons and electrons, the spin (as measured along a selected axis) can take on two values (in units of Planck's constant): +1/2 and -1/2. These values lead to the colloquial terms "spin-up" and "spin-down," and electron or proton spin is a "simple" problem to study, since the state |ψ> need only specify two numbers: α (the probability of the particle being spin-up) and β (the probability of the particle being spin-down).

The mathematics in Zurek's article can get a bit cumbersome, so if you're new to quantum mechanics, focus on his commentary! Have a question about a step he takes or what a symbol means? Post it in the comments below!

Quantum Weirdness: Why bother?

This week, we take a look at some of the strange behaviors in the universe that arise because of quantum mechanics. If you've studied physics for more than a semester, or have watched any physics documentaries on TV, you've probably heard of quantum mechanics and its two types of weird behavior, which apply in the world of very small particles or very cold temperatures:

• Physics properties that we, in the everyday world, think of as smoothly varying (most notably, energy) occur in discrete lumps (or "quanta"--hence the name "quantum mechanics"):

  

  Image credit: http://hyperphysics.phy-astr.gsu.edu/hbase/quantum/imgqua/hosc9.gif
• Physics properties that we, in the everyday world, think of as well-defined and localized (most notably, position and momentum) are actually spread out or "fuzzy:"

  

  Image credit: http://upload.wikimedia.org/wikipedia/commons/9/90/QuantumHarmonicOscillatorAnimation.gif

I emphasize that quantum mechanics is different from the "everyday world" because you and I, as beings made of many many particles at very high temperatures, do not notice these effects in our experiences. This raises the question of why we should bother studying quantum mechanics at all, if it doesn't relate to "the real world."

The Royal Society has a wonderful brief answer to this question, pointing out that studying quantum mechanics gives us...

• A better understanding of chemistry.
• A basis for working with radioactivity.
• The laser!
• The physical mechanism by which our eyes work.
• Digital cameras.
• Scanning tunneling microscopes.
• Encryption (coming soon!).
• Quantum computing (coming soon!).

This week, we'll look more deeply at the technological applications of quantum mechanics to continue to answer this question.

Spintronics - recent developments

Let's wrap up our discussion of spintronics with a look at some recent developments in the field: creating spin-valve devices in graphene.

Graphene is a relatively new wonder-material that we'll discuss later this semester. Graphene comes in sheets made of single layers of carbon atoms: 

Image credit: http://upload.wikimedia.org/wikipedia/commons/thumb/9/9e/Graphen.jpg/800px-Graphen.jpg

Graphene is extremely strong and extremely conductive of both electric current and heat.

A spin-valve device is multiple conducting materials stacked in layers whose combined resistivity changes drastically depending on whether their magnetizations are parallel or antiparallel. (Sound familiar?) In other words, this device permits or prevents current passing through with a simple switch of the magnetic field, just like a faucet permits or prevents water passing through with the turn of a handle.

In http://arxiv.org/pdf/1407.1439.pdf, Fu et al discuss the creation of spin-valve devices with graphene using chemical vapor deposition, which assembles nanoscopic devices one layer at a time. Have a look to see spin valves in action!

Spintronics - Making it work!

This week, we're learning about spintronics - manipulating the spins of individual electrons to store information. Unfortunately, electron spins tend to "reset" (lose the spin orientation that we set up) after only a hundred picoseconds (a picosecond = 10^-12 seconds), which is too short to be read by our computer processors. (Remember: period = 1/frequency! So, for example, a 1 gigahertz processor would need the spins to have a lifetime of at least 1/(10^9 Hz) = 10^-9 seconds.) 

Fortunately, in 2012, IBM announced they had successfully synchronized the spins of clusters of electrons, increasing the spin lifetime by a factor of 30.

Here's an example of their data (with time increasing as you move up the vertical axis), showing that the spins remain coherent for just over a nanosecond - enough time to be useful to a 1 gigahertz processor!

http://www.computerworld.com/common/images/site/features/2012/06/Spintronics%20photo1.jpeg

Spintronics - some important concepts

Last time, we introduced the concept of spintronics - manipulating spins of individual electrons for applications in memory storage and quantum computing. Today, we take a more detailed look at the physics concepts and material properties involved in spintronics. Science published a great review article about this topic in 2001, available at http://www.dtic.mil/cgi-bin/GetTRDoc?AD=ADA516289. 

Here are some concepts discussed in the article that you might like some additional resources to learn about:

Giant Magnetoresistance (GMR): Magnetoresistance (giant or small) is when a material's electrical resistance increases when the current runs parallel to an applied magnetic field. Magnetoresistance can therefore be used to interface with magnetic storage devices, but ordinary-sized magnetoresistance isn't strong enough to do the job. GMR, on the other hand, employs quantum mechanical concepts to introduce a large change in resistance, such that the resistance change can be used to read information stored in magnetic memory.

Ferromagnets: In elementary school, you might have learned to call these "permanent magnets"--materials that retain their magnetization even when there's no external magnetic field to keep all the spins in the same direction. But ferromagnets do have a weakness; if you heat them beyond their Curie temperature, they'll lose their magnetic ordering! So, when we design magnetic storage devices, it's important to know how hot they can get!

Semiconductors: On first pass, a semiconducting material is a pretty straightforward concept: It has a resistivity somewhere between the high resistivity of an insulator (letting no electrons through) and the low resistivity of a metal (letting all the electrons through). The reason a semiconductor behaves this way, though, is that its electronic band structure (the configuration of quantum states that the electrons are allowed to be in) has a small gap that can be easily manipulated:

Image credit:
 http://upload.wikimedia.org/wikipedia/commons/thumb/0/0b/Band_gap_comparison.svg/350px-Band_gap_comparison.svg.png

If you found the article above interesting, check out these even deeper (i.e., lengthier) reviews at http://arxiv.org/pdf/cond-mat/0405528.pdf, http://arxiv.org/ftp/arxiv/papers/0711/0711.1461.pdf, and http://arxiv.org/pdf/0801.0145v1.pdf. 

Spintronics! Learn from the leaders of the field

This week, we'll take a look at the emerging field of spintronics, which offers many technological revolutions through manipulating the spins of electrons.

Spin is a fundamental property of matter, just like mass or charge. In the equations, this property behaves as if the particles were spinning like a top, so that's why we call it spin. Spin has the important property that it produces a magnetic field. In most materials, electrons' (and protons') spins are randomly oriented, so they all tend to cancel out. If you align those spins, though, you could create a powerful magnet. If you align local clusters of the spins, you can use encode information!

Spintronics is likely already being used in your computer's hard drive and refining the technology can lead to novel applications like quantum computers. 

In 2009, Physics World shared a series of video interviews with some of the leaders in the field. Have a look at these this week, and over the next few days we'll take a look at some of the breakthroughs and applications of spintronics!

Galactic doomsday is nigh!

And now, the topic you've waited all week for: The collision of our Milky Way (MW) galaxy with the Andromeda Galaxy (AG). For a while, we've known that MW and AG were moving toward each other; that's pretty easy to expect from the gravitational force between them, and the component toward us of AG's velocity has been pretty easy to measure. But until recently we did not know the perpendicular components of AG's velocity, which determines the difference between a collision and a miss.

Once these measurements were obtained, van der Marel et al (http://iopscience.iop.org/0004-637X/753/1/9/pdf/0004-637X_753_1_9.pdf) used two complementary methods to predict the future motion of AG relative to MW based on appropriate initial conditions (just like we discussed on Monday).

Below is a sample of their results for the Milky Way (labeled MW), Andromeda (labeled M31), and the Triangulum galaxy (labeled M33):

Image credit: http://iopscience.iop.org/0004-637X/753/1/9/pdf/0004-637X_753_1_9.pdf

Image credit: http://iopscience.iop.org/0004-637X/753/1/9/pdf/0004-637X_753_1_9.pdf

The red curve on the second graph is perhaps the most important, as it shows the separation between the Milky Way and Andromeda as a function of time (with t = 0 indicating today). As you can see, in about 4 billion years, we're in for a collision!

Here are more dramatic demonstrations: A video of the simulation and images of what it will look like from earth!

Galactic cannibalism in action, one drop at a time

Recent images from the Hubble Space Telescope show what may be an amazing manifestation of galactic cannibalism in action:

Image credit: http://imgsrc.hubblesite.org/hu/db/images/hs-2014-26-a-web_print.jpg

We saw earlier this week that interacting galaxies will often have streams of gas and stars between them, but this example shows a delicate series of "super star clusters" in a corkscrew pattern evenly spaced 3000 light years apart. "We have two monsters playing tug-of-war with a necklace," says Grant Tremblay of the European Southern Observatory in Garching, Germany, "and its ultimate fate is an interesting question in the context of the formation of stellar superclusters and the merger-driven growth of a galaxy's stellar component."

Is our our Milky Way a galactic cannibal?

We saw last time that neighboring galaxies can "eat" each other through collisions and tidal forces, a process called galactic cannibalism. It is an amazing if violent process that seems to happen frequently, leading to the question, is our own Milky Way galaxy a galactic cannibal?

The short answer is yes. The motion of stars in our galaxy's outer halo suggests that the Milky Way has been absorbing smaller galaxies over the years. (There's also our impending collision with the Andromeda Galaxy, which we'll discuss later this week!)

If it makes you feel uneasy living in a universe of carnivorous galaxies, take heart: It looks like the earliest galaxies peacefully consumed their own gases to produce new stars at a remarkable pace.

Galactic Cannibalism: How do we know it's happening?

Welcome to our first week of articles for the fall 2014 semester! This week's topic is galactic cannibalism, which is, in fact, as cool as it sounds.

You're probably familiar with the concept of a galaxy (a collection of stars, gas, and dark matter that generally swirl around in space). To look at pictures of galaxies and think of their sizes and ages, you might conclude that galaxies are immovable and unchanging constructs. After all, what force in the universe could be strong enough to move or mangle a galaxy?

The answer, it turns out, is other galaxies.

Because galaxies are so massive, the gravitational forces between them are strong enough to overcome the vast distance between them and create a notable acceleration. So, galaxies often hurl toward each other on a collision course. Even our own Milky Way Galaxy will collide with the Andromeda Galaxy in about 4.5 billion years. (We'll read more about our impending doom later!) 

But that's not all: Because galaxies aren't rigid objects, but a collection of individual stars and gas clouds, galaxies on a collision course also distort each other.

Here's how it works: Suppose you're on earth as the Andromeda Galaxy approaches the Milky Way, and let's suppose the Milky Way is oriented such that Andromeda is approaching toward the Alpha Quadrant as depicted in 

Image credit: http://www.freewebs.com/captaingestl/milkyway.gif

Now, let's suppose you have a Borg friend (it's 4.5 billion years in the future; let's assume we've made peace with the Borg by then) sitting in the Delta Quadrant (on the opposite side of the galaxy from you). If you compare the force that each of you feels from Andromeda, your force will be much stronger than his, because the gravitational force decreases the farther away you are from the source. If you experience a greater force than your Borg friend, it means you (and, consequently, the earth and the sun) will be pulled toward Andromeda faster than your Borg friend, and the distance between you will increase! Andromeda will literally stretch the Milky way and tear pieces of it apart! (By the way, this is the same thing that happens on earth when the waters of the ocean accelerate toward the sun, producing tides!) Fear not, though! We'll be doing the same thing to Andromeda.

So, when one galaxy collides with another, the result can be quite violent! We call this process galactic cannibalism, and we can see it taking place when we see streams of gas and stars between galaxies, like in the images below:

Image credit: http://sci.esa.int/hubble/42637-merger-stages-of-interacting-galaxies/

One great example of galactic cannibalism that we've been observing is between the Large Magellanic Cloud and the Small Magellanic Cloud, as described by Connors et al in http://arxiv.org/pdf/astro-ph/0402187.pdf. This work uses a program called GCD+ (http://mnras.oxfordjournals.org/content/340/3/908.full.pdf+html) to model the formation and evolution of galaxies. This program takes into account a number of processes that happen during a galaxy's lifetime, including "self-gravity, hydrodynamics, radiative cooling, [and] star formation." 

In order to run the program, Connors et al fed a host of information into GCD+ from observations of the Magellanic Clouds (including their sizes, masses, and orbit characteristics). However, even all this information is not enough to predict the Clouds' behavior, so Connors et al had to run the program many times, each time feeding in slightly different values for parameters like the clouds' halo-to-disk mass ratios and velocity dispersion. 

How did they know when they had arrived at the best set of parameter values? By comparing the results of their calculations with the stunning visual result of the Clouds' interaction: The Magellanic Stream, just like we talked about above! 

Image credit: http://arxiv.org/pdf/astro-ph/0402187.pdf 

Here's a comparison of the neutral hydrogen flux in the Magellanic Stream as observed (on the left) and modeled by GCD+ (on the right). As you can see, the program produces a pretty good match, identifying the general shape and regions of greatest flux.

So, what did they learn from this simulation? 1. That there must have been an "encounter" (That's a polite way of hinting at galactic cannibalism!) between our galaxy and the Magellanic Clouds about 1.5 billion years ago and 2. The Large and Small Magellanic Clouds interacted with each other strongly about 200 million years ago.

We'll continue to look at galactic cannibalism this week by looking at how galactic cannibalism gave our galaxy its shape, looking at recent examples of how we can see this process now, and taking a deeper look into our future collision with Andromeda!

JU Physics alum begins teaching at Georgia Southern University

Congratulations to JU Physics alum Ashley August, who recently graduated with her Master of Science in Teaching at the University of Florida and is beginning a new job teaching physics at Georgia Southern University!

Many in the JU community will remember August as a member of JU's volleyball team and an exciting presence in the classroom. Now, she is looking forward to sharing that excitement with over 170 students at GSU this fall.

August says that she first became interested in teaching when she was a student in second grade and her teacher directed the class in a common exercise: Students exchanging papers to grade. "I don't know what happened," August says, "I just got so excited. I love grading papers." She later found an interest in physics, and found the two passions combine in a call to teach physics.

We asked August what she learned in her MST program that she plans to put to use at GSU. She says she learned that "if I didn't know something, which happened more often than not, I found that I was capable of learning on my own. I knew that before, but this was at a whole new level." She describes this new confidence as "quite a valuable lesson" that she plans to pass on to her students.

August also learned a great deal about teaching based on students' learning needs. "Everybody comes from a different background [with] different skills," she says. "It can affect a student... I think at GSU, it's going to be my job to be observant of my students and be willing to accommodate the learning style that each of them has."

We also asked August what she learned at JU that has been most helpful to her and that she plans to pass on to her students. She says she learned that one does not have to teach the same way every time: "There are no limits in the classroom... Being adventurous is a good thing because students see that it's new as well and get excited."

Through her interactions with JU professors outside the classroom, August also learned that "Teachers are people too... When you had us in office hours, it was like you still had that authority, but you became almost like a student and were welcoming and kind and wanted us to ask questions." From this experience, August says she wants students to understand that there are things that happen outside the classroom that affect one's academic performance.

Finally, August learned at JU that, when teaching, it's important to be oneself. "I need to be okay with learning what my teaching style is... and try to have some fun!" She says that, in doing so, teachers will "inspire and motivate in any way that we can."

We thank Ashley for talking with us about this next adventure and wish her the best!

Fall 2014 blog series!

Welcome to the fall 2014 semester! Whether you are a new or returning student, a faculty member, or a friend of the JU community, Phys dot JU exists to help connect you with the exciting world of physics.
Each week this fall semester¹, this blog will feature a series of posts about a given topic that is actively being researched by physicists today in the various subfields of physics.

In spite of the picture we often paint in our introductory courses, physicists do not spend their days calculating the acceleration of blocks down ramps. From elementary particles and condensed matter to black holes and galaxies, physicists study any number of complex systems built out of basic particles and interactions. (We use those ramps to help our students first learn about basic particles and interactions, but the problems quickly expand to encompass the entire universe!)

However, it's difficult for young physicists to learn about all the possible avenues of physics research, and so it's easy to feel like you've missed something and it's difficult to identify your research interests! So, each of these weekly series will highlight a currently active problem in physics research in an accessible manner by...

• Providing links to free-access articles. Each week, we'll include a popular article about the week's topic, an article that overviews the foundations of the week's topic, an article describing practical applications of the week's topic, and a recently published article highlighting new findings about the week's topic.
• Summarizing important key concepts and terms employed in each article, along with links to helpful explanations. Even if you never understand or even read the actual article, these explanations will be helpful and interesting!

Each of these posts will be shared via our Twitter and Facebook accounts, so follow and/or like us to see our updates.

So, how can you make the most of these series?

1. Stay updated! Follow or Like  us to learn the week's topics and follow the links to each day's post!
2. Read each day's post! If you come across a term or concept you don't understand, click on the associated links! If you still don't understand, search for the term on Google; if you find a more helpful explanation, share it in the comments!
3. Read each day's article! You likely won't understand the entire article, and that's okay! Journal articles are not meant to be memorized and recited, but to be treasure troves of knowledge where you gain one or two gems each visit.
4. Discuss the article and topic with your classmates and professors! Ask questions of them, or in the comments of each post.
5. If you find a topic particularly interesting, search for other references about it! You may have just stumbled upon a topic that carries you through a lifetime of learning and research.
6. Finally, share what you've learned with someone else: A friend, a family member, an English professor... You'll be surprised at how much you've learned, and they'll b eager to hear it!

The series begin next Monday, September 1, with galactic cannibalism!

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1 BTW - Did you know JU classes are on break all week for Thanksgiving this year?!^↩