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!