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).