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Wednesday
Nov102010

The Physics Of Bad 3D Movies

Imagine a beam from a high powered laser coming straight towards your face. As the beam hits your face, you will quickly discover that it - and more generally light - is electric and magnetic in origin. You will realize this because the electrons in the atoms making up your soon to be fried cells will start oscillating violently as the beam passes through them; and electrons are electrically charged and experience electric and magnetic forces. But from the oscillation of the electrons you will also realize that there is more structure to light than you may have thought: as the light passes through an electron in a certain direction, the electron will jump around and move in a plane transverse to the incoming beam; that is if the beam is coming in horizontally, its effect on the electron can be up-down and/or left-right. To be complete, the electron will also be pushed in and out along the beam for somewhat different reasons, but that motion is irrelevant to my argument. This tells you that light carries with it a two dimensional plane's worth of information: you can arrange for a laser to move the electrons only up and down, or only left and right; or a combination of both. We say light has two polarization states, or two degrees of freedom. Your eyes are not setup to see this additional information in a light beam; hence, this may come as a surprise.

However, we now have technology to mold the polarization of a light beam as we desire. This is done by letting the beam pass through certain special materials - called cryptically polarizers: the emerging beam can be made, for example, to oscillate electrons up and down only, or left and right only. If you polarize a beam left and right, then make it to pass through a polarizer oriented up-down, you can kill the beam. This has many uses, the most recent and sexy one being the emergence of 3d movies and 3d televisions.

You perceive distance and perspective in life because you have two eyes. Each eye sees a slightly different perspective of the world, and your brain puts things together for you for free and hence you perceive distance.  When you want to replicate that in a bad movie, you use a camera with two eyes - or just two cameras mimicking two eyes. You then project the video from each "eye" onto the same screen in such a way that light from one video passes through a polarizer that orients the polarization of the light say up-down; the other, left-right. Actually, one does a slightly more practical setup using so-called circular polarization, but that detail is irrelevant to the physics story. When you view the bad movie on the screen, you then put special eyeglasses: each eye has a polarizer oriented differently and picks up only the image from the corresponding video feed. So, your brain receives two different images, slightly off from each other in perspective - just so that it appears you are immersed in the movie's scene. It's a very simple idea that capitalizes on the fact that there are two degrees of freedom in light and one can package two images in the same beam; and separate things out once again with polarizers in front of your eyes. Unfortunately, all this physics will not improve the quality of some of these recent 3d movies.

The first video demonstrates the basic physics of light polarization. The second video talks about 3D TV technology.

 

Tuesday
Nov092010

Type I And Type II Personalities 

              


Here's part 2 of my post on superconductivity (see previous post for part 1). So, we have a strange material that, when cooled down enough, undergoes a dramatic change: it suddenly starts conducting electricity without loss of energy. That's a superconductor, and we talked a bit about the underlying microscopic mechanism in the previous post. There is however much more to a superconductor in terms of freaky behavior.

During your troubled childhood, you probably have had a chance to play around with magnets. You noticed that magnets can be oriented to repel or attract each other. This is the magnetic force, part of the fundamental electromagnetic force. A magnet projects around it an energy field through empty space. At points nearby the magnets, space literally store a bit of energy due to the proximity of the magnet. This energy field is connected to the repulsion or attraction that you experience when you bring two magnets close to each other. In Nature, energy is money: it is costly to generate energy and Nature would rather not do it unless It really has to… 

Now imagine you bring a magnet close to a block of some material. The magnetic energy field will in general penetrate inside materials as well. However, a material could in principle react to the proximity of a magnet by spontaneously generating a current within itself - by moving around its own electrons. This is because an electric current also generates a magnetic energy field, and it is possible for the magnetic field from the current to cancel the one from the magnet within the body of the material. That is a good thing from Nature perspective: the magnetic energy inside the material costs energy, and if it can be canceled by moving the electrons in the material, life would be improved. The problem is that generating a current in a material is also energetically costly… in general, it does not pay off and the magnet's magnetic field does penetrate readily into the material - sometimes slightly modified by the material's lame attempt to put up a fight to the intruding magnetic field.  

But a superconductor has a trick up its sleeve: it can conduct electrical current without loss of energy. So, if you get a magnet close to a superconductor, the superconductor can circulate arbitrarily large amounts of electric current within it to make sure the magnetic energy field inside of it is canceled! A superconductor repels magnetic fields! See the first picture attached to this post for a depiction. The end result is that the superconductor acts like a magnet as well in response to the proximity of the external magnet, and superconductor and magnet repel each other: hence, you get magnetic levitation (see videos in previous post). 

But Nature has more elaborate tricks up its sleeve… Imagine you ramp up the strength of the external magnet so that it generates more and more magnetic energy fields around it. A superconductor involves a microscopically delicate balance of forces that binds pairs of electrons together (see previous post for the details). If enough magnetic energy field from an external magnet is present, this delicate balance can be disturbed as the electrons feel an external magnetic force. So, it is possible  to destroy the superconducting state of a superconductor by putting it in the vicinity of a very strong magnet. But things are much much more interesting than this… When one attempts to destroy a superconducting state with a strong magnet, one finds two very different types of behaviors - depending on the detailed properties of the superconducting material involved. We call these two different types of superconductors Type I and Type II.

Imagine a region of strong magnetic energy field penetrating a superconductor. And imagine that this region of magnetic field penetration is in the shape of a rough cylindrical blob: inside, there's magnetic field and the superconductor is no more, outside there is no magnetic field and the superconductor is still putting up a good fight. The interface between these two regions - the boundary of the blob - is a place of tension and can pack energy. There are two possibilities however. Possibility 1 or Type I: the energy in the interface costs additional net energy, and such regions would rather coalesce and merge to reduce the size of the boundary - imagine droplets of water or mercury merging together to form larger and larger blobs of water or mercury. If you are science fiction fan, imagine instead the liquid metal in the Terminator movies, or the Klingon blood in empty space in Star Trek… The end result is that the region where the magnetic energy field penetrates into the superconductor grows in size quickly and overtakes the whole body of the superconductor; or I should say the late superconductor which is now no more.

Something amazing happens however in Type II superconductors. The contribution of the interface to the total energy balance of the situation is negative! The blob of magnetic energy field penetrating the superconductor would rather maximize the size of the interface… As a results, such blobs fragment into smaller ones - instead of combining as in the Type I case. You then generate a dramatic cascade of magnetic energy regions disintegrating into smaller and smaller regions and the superconductor gets threaded with a lattice of cylindrical vortices of magnetic field (see accompanying actual picture of vortices from the 2003 Noble prize announcement for superconductivity). In between these vortices, the material is still superconducting; within the vortex, it's not. But why don't these regions of magnetic energy fields, these vortices, collapse further - perhaps disappear all together? The reason is a delicate one: there is a minimum amount of magnetic energy field in a vortex that you cannot eliminate without having to rearrange the energy distribution in the entire body of the material; and this costs loads of energy and would rather be avoided. There is then a global aspect of these magnetic vortices that is sensitive to the material's entire state; as if things are held together by a larger order beyond the physics of the local neighborhood of a vortex. What is quite amazing about this is that these vortices behave very much like particles: you can scatter them off each other, they can combine and decay, there are forces between them, there are even anti-vortices around… check out the videos accompanying this post for some entertainment value. The first video is about dynamics of vortices - actual video of vortices in action! The second video is a simulation; focus on the top graph only since that depicts magnetic energy. The lattice of magnetic vortices in a Type II superconductor can also "pin" a levitating magnet as seen in the videos of my previous post. It's like you are putting magnetic nails through a floating superconductor - pinning its orientation in mid-air… beautiful, amazing, elegant, and intricate physics across the board… that's superconductors for you in a nutshell - actually in two nutshells.

Monday
Nov082010

How To Mutate A Physicist

The deep universe is littered with violent astrophysical events: exploding stars (supernovae), colliding galaxies, merging black holes tearing the fabric of space-time, crazy fast spinning pulsars, movies by Quentin Tarantino… these events can create powerful shockwaves that kick intergalactic or stellar gas particles to immense speeds and energies. These particles - often simply protons (i.e. Hydrogen atom nuclei), but sometimes also X-rays and atoms like Carbon - attain unimaginably large energies. They are all around us, so energetic that they pass through our bodies and much of the Earth's rock layers. We call them collectively and cryptically "cosmic rays".

Cosmic rays - unlike the neutrinos of my previous post - do occasionally interact with organic matter (i.e. biological cells) and play an important role in the evolution of life on Earth: they can help mutate DNA when they penetrate a cell, and mutations are a key ingredient in the process of evolution. What is quite astounding about cosmic rays is their extremely high energies. Physicists often measure energies in "electron Volts" - some useful unit of energy related to the more familiar ones such as Joules or Calories. To put things in perspective, the largest particle accelerator built by the pitiful human race generates particles with energies around several trillions of electron Volts. Cosmic ray particles have been detected with energies around a million times more… but not much more than that. Lower energy cosmic rays - in the thousands of trillions of electron Volts range - can be detected directly through detectors raised to the edge of the atmosphere with balloons; higher energy ones are seen indirectly through showers of particles they generate when they collide with the Earth's atmosphere. Much is not known or understood about these mysterious visitors from outer space. But in the past few decades, we have started tracking them down actively and aggressively.

The first video features the late celebrated science narrator Carl Sagan and gives a great overview of the subject. The second video is part 1 of a 3 part series that goes into more details.

Sunday
Nov072010

WIMPs And MACHOs

Only 4% of the stuff in our universe is directly visible. Of the remaining 96%, 70% is a mysterious anti-gravitating substance called Dark Energy (see post 1 and post 2 for more). That leaves 26% of less crazy but still exotic stuff called Dark Matter.

Dark Matter cannot be seen, but its presence can be deduced from its gravitational pull on other visible stuff in the universe. It is now believed that each galaxy has a spherical halo of dark matter, typically larger than its size. Recently, using the Chandra X-Ray Observatory, a most dramatic evidence of galactic dark matter was observed - perhaps bringing us the closest we've been to actually seeing this exotic stuff. Two galaxies underwent a graceful collision, each dragging with it its dark matter halo (see post on galaxy collisions). See attached first video for a simulation. The red stuff in the video is visible matter/gas; the blue, dark matter. As the stars and gas in the galaxies skimmed past each other and slowed down, the dark matter whizzed by, past the colliding visible stuff: dark matter interacts weakly hence can travel further during a collision. This effect could be seen in the motion of the visible gas in the galaxies as the dark matter pulled on it and slowed the collision - like the effect of molasses or air drag! 

Currently, there are several candidates for what dark matter may be, from the least exotic to the most lunatic. On the least crazy side, dark matter may be made of MACHO's… yes, MACHO stands for MAssive Compact Halo Object: dead stars - rather small brown dwarves, black holes, neutron stars - littering the universe. We can't see them, but they pull on things gravitationally. Another more exotic category are WIMP's… WIMP stands for Weakly Interacting Massive Particles: subatomic particles that we have not yet discovered in our labs that (1) interact weakly with the rest of known particles - and are hence difficult to detect; and (2) that have potentially large mass, perhaps large enough that it is too energetically costly to create them in the lab. WIMPs can come in different flavors: lame neutrinos (see post on neutrinos), and/or particles needed to have supersymmetry in Nature (see post on supersymmetry), and/or particles that arise from having a universe that has more than three spatial dimensions (see post on higher dimensions). Beside MACHOs and WIMPs, there are other possibilities as well, the most interesting of which are axions - light particles that have an intimate relation to the nuclear force. In short, we have no clue what dark matter is at this point in time. But the thrill of the chase is the way physicists earn their paychecks after all…

The second video accompanying this post is a general discussion of what is known about dark matter so far.

Friday
Nov052010

The Trouble With Small Things

One of the most enigmatic particles of the subatomic world is the so-called neutrino. The neutrino comes in three flavors and plays a central role in the physics of one of the four fundamental forces of Nature - the Weak force. It is prevalent in many radioactive processes and exists copiously all around us. For example, our Sun spits large amounts of neutrinos at us. As you are reading this, neutrinos are passing through your flesh and bones like knife through butter… if you feel a tingling, it's either neutrinos or the broccoli you ate yesterday. 

Two interesting things about neutrinos set them apart from the other particles of the subatomic world. First, they are extremely light. In fact, they were thought to be massless for a long time. Only recently was it discovered that they have tiny masses. Tiny numbers in physics are psychologically troubling. Usually, when something is extremely different from everything else around it - like very very light neutrinos compared to protons and electrons - you would worry that something deep is at work, something that you need to understand better. The extreme lightness of neutrinos is such a philosophical puzzle: it doesn't point to an actual problem in our description of the physics, but it suggests that our description may be incomplete or missing a deep principle at work. 

The second unusual attribute of neutrinos has to do with the weakness of their interaction with the rest of the matter in the universe. Neutrinos travel near the speed of light and just go through virtually everything in the universe. To neutrinos, we and the Earth are mostly transparent. It is hence very difficult to detect neutrinos. Physicists have to build large underground tanks filled with water and live like hermits in caves hundreds of meters below the surface to detect neutrinos… at least that is the official excuse for this lifestyle. 

Neutrinos also play an important role in understanding the cosmological history of the universe. They may for example be the source of the mysterious dark matter that pervades galaxies. And they are crucial for understanding how our Sun powers itself. The first video accompanying this post is a short one and talks about the basics of neutrinos - particularly the ones coming from the Sun. The second video is rather lengthy but very well done; and it goes into the physics of radioactivity and neutrinos in great detail - for those of you trying to understand that tingling you feel now and then...

Thursday
Nov042010

Synchronized Swimming

Three key conceptual ingredients underly the phenomenon of superconductivity, each one more beautiful than the other. When certain materials are cooled down to very low temperatures, it is found that - at some critical temperature - the material is able to conduct electricity without any loss of energy! Hence the term "super"conductivity. The importance of practical applications of this cannot be overstated. Its conceptual physics underpinnings are however even more dramatic. Here are the three key ideas involved in this remarkable phenomenon.

(1) The quantum world - i.e. the real world - looks very different from the world we are used to (see this and this previous posts for more). In reality, all matter is to be described by waves of probabilities - waves that behave very much like those on the surface of the ocean. What's waving is not water, but the likelihood that a particle of matter is here or there. And these probability profiles propagate, combine, and scatter much like water waves do. When one has a large number of constituents in a system, the wavelets of each particle add up incoherently and the quantum effects are washed away (no pun intended) at large macroscopic distances. Occasionally, one can however arrange to sync the wavelets of matter so that they reinforce each other and form a substantial probability tsunami that brings the freaky quantum world into our own safe and naive realm. This happens typically when you cool down a system of particles to low enough temperatures. However, the effect is most dramatic when the particles are bosons instead of fermions (see post on bosons and fermions). The result is what is known as a Bose condensate: a collection of quantum wavelets in sync and adding up to a macroscopic probability profile. This is for example the case for photons in a laser, electrons in superconductivity, and molecules in superfluidity (see post on superfluidity for a bit more).

(2) A typical metal can be visualized as follows: a lattice of atomic nuclei spread around in a regular pattern - forming the scaffolding of the material; and waves of electrons propagating through the lattice randomly. When a piece of metal is hooked up to a battery, the electrons are forced into motion in a certain direction, like water through a pipe. However, electrons are attracted by the nuclei and repel each other through the electromagnetic force. The result is that the flow of electrons is not a smooth one; it is interrupted by various obstacles - other electrons and nuclei in the material. This is a source of energy loss for electrical energy flow. Hence, current in a metal typically involves "resistance", a mechanism for loss of energy. When certain metals are cooled down to very low temperatures, there is the possibility for the electrons to form a coherent Bose condensate - generating an electrical flow more akin to soldiers walking in step and reinforcing the efficiency of the flow. But the electrons are not bosons, they are fermions… To realize the quantum syncing of electron wavelets as needed, we would want bosonic particles. Nature comes up with an amazing solution to this puzzle: if you bind two electrons together - as in an purely "electron atom" - the result is a boson. From my previous post about bosons and fermions, you may remember that a boson is associated with integer spin, and a fermion with half-integer spin. Electrons have spin 1/2; two bound together necessary generate integer spin, and hence a bosonic entity. So, pairs of bound electrons - known as Cooper pairs - can potentially form a Bose condensate at low enough temperatures, which then can carry electrical energy flow through the material at very high efficiency, all the way to 0% energy loss: a quantum effect involving the syncing of probability waves and manifesting itself through a dramatic macroscopic effect…

(3) But there is a problem with this story: electrons are attracted to the metal nuclei but repel each other electromagnetically… How the heck can you then bind two together? Once again, Nature kicks in with an amazingly intricate solution. At low enough temperatures, in certain materials the lattice of atomic nuclei deforms under the influence of the electrons flowing through it. It does so in a very special way. The accompanying figure shows a cartoon of the setup. The deformed nuclei mediate a pull between the two electrons, hence binding them through a precarious balance of forces! The mechanism is ingenuous, counter-intuitive, and delicate: all ingredients of Nature working at its best.

With all these factors colluding together, certain materials can transfer electrical energy or current with no energy loss - at low enough temperatures. But this is not the end of this amazing story. Superconductors exhibit other dramatic effects having to do with their interactions with magnets. For this, we will need to talk about two types of superconductors - enigmatically called type I and type II. But we'll need another post just to address this subject. Meanwhile, have also a look at the videos in my previous post to see superconductors in action.

Wednesday
Nov032010

The Deserted Island

If I were to be secluded on a deserted island - which actually would not change much my daily routine - and was asked for five physics topics to take with me, one would be superconductivity. Superconductivity is the phenomenon of current flow with zero resistance in certain substances cooled down to low temperatures. Several reasons why the topic would be on my short list on the (hopefully tropical) island: (1) It involves a delicate microscopic mechanism that you would never be able to guess at unless you were both a genius and intoxicated at the same time; (2) The physics involved is one of the most profound techniques Nature uses to achieve a change of state - for example, it is the same one that is also responsible for the Higgs particle (see post on the "god particle") giving mass to all matter; (3) It involves a phase transition - a phenomenon whereby a state of matter undergoes dramatic change that reflects intricate microscopic dynamics; (4) It is one of the few places in Nature where the crazy effects of quantum mechanics - usually tucked into the microscopic world - peak out directly into our macroscopic world.

The first video demonstrate superconductivity, along with the magnetic pinning phenomenon in so-called Type II superconductors. The second video talks about a current practical application of this physics. All this is in preparation to a post coming up tomorrow on the actual detailed physics of superconductivity. In the meantime, enjoy.

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