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.