Wednesday, June 30, 2010

One of these things is never like the other.

Orville Wright once said, “If we worked on the assumption that what is accepted as true really is true, then there would be little hope for advance.” Presumably, this was the rationale behind a recent experiment conducted by a team of particle physicists at the University of California, Berkeley. Scientists operate on the assumption that all of the known subatomic particles fall into two distinct categories, bosons and fermions, whose respective properties are unique. If even one boson was to be observed to be acting under the laws that govern fermions, the whole of the Standard Model of particle physics would collapse. Fortunately, the UC Berkeley physicists have concluded that bosons - specifically, photons - never act like fermions. Ever.


Two photons of different wavelengths.
Image courtesy of NASA/Sonoma State University/Aurore Simonnet.


From the forces of nature to the basic constituents of matter, the Standard Model explains everything we think we know about our world. For instance, matter arises from different configurations of two groups of fermions: quarks and leptons. Quarks are the particles that make up protons and neutrons, while leptons are stand-alone particles such as the electron. The Standard Model also tells us that there are four basic forces: the strong and weak nuclear forces, electromagnetism, and gravity. These forces operate by exchanging bosons such as the gluon, the W and Z particles, and the photon.

Bosons and fermions behave according to the laws of quantum field theory, the physics behind the Standard Model. Each has a property called "spin," which defines its intrinsic angular momentum in terms of a number. Bosons are only allowed to have spins that are integers, e.g., 0, 1, 2, 3. Fermions can only have half-integer spins, e.g., 1/2, 3/2, 5/2. In quantum field theory, this is called the Spin-Statistics Theorem. Another difference between the two lies in their configuration; namely, that no two fermions can occupy the same quantum state. This is why the electrons in an atom arrange themselves in separate shells around the nucleus. Bosons, however, can share a single quantum state. This is the phenomenon behind laser beams and Bose-Einstein condensation.

The idea behind the experiment at UC Berkeley was to try and violate the Spin-Statistics Theorem and thus disprove the distinctness of bosons and fermions. The scientists fired two lasers at a group of barium atoms in order to see whether any electrons in the barium would be excited to a higher energy level. The transition they were looking for occurs when two photons are absorbed at once, and is forbidden to bosons (like photons) by the Spin-Statistics Theorem. According to Damon English, a post-doc with the Berkeley group, if photons acted like fermions, the transition would "go like gang-busters." But it didn't. Not one two-photon absorption was observed. The researchers thus concluded that our understanding of the Spin-Statistics Theorem is sound. "Photons are bosons," claimed English, "at least within our experimental sensitivity."

Saturday, June 26, 2010

Was there once life on Venus?

Of our two planetary neighbors, Mars was always the favored child, making headlines year after year as scientists alternately bolstered and debunked claims of its habitability. Meanwhile, Venus sat idly by - gaseous and fuming, nothing but a bright "star" in the early morning and evening skies. But all that might be about to change. For the last four and a half years, the Venus Express satellite has been gathering data that suggests that, at one time, the gassy planet may have supported liquid water.


Image courtesy of NASA.


Today, Venus is an oppressive and stormy place. With an ambient temperature hot enough to melt lead, surface pressure comparable to the depths of Earth's oceans, and an atmosphere composed mainly of carbon dioxide and sulfuric acid, Venus is an inconceivably inhospitable place for life to thrive. However, this was not always the case. The Venus Express satellite has detected large amounts of hydrogen and oxygen escaping into space from the Venusian atmosphere; more specifically, two molecules of hydrogen for every one molecule of oxygen. Sound familiar? It should. Liquid water, or H2O, shares this exact ratio and evaporates into its constituent gases when struck by UV light from the sun. Current research implies that most of this water was probably atmospheric, but it does not rule out the possibility of ancient oceans on the Venus' surface. Water is thought to be a crucial ingredient for the evolution of carbon-based life forms, making the existence of standing water on Venus at some point in history appealing to astrobiologists studying the origin of life in the solar system.

Friday, June 25, 2010

Physiology of a Suntan, or Why I Am So Red.


Image courtesy of Vox Efx.


As I lay out in the blazing 100-degree sun today, I started wondering exactly what was happening to my skin on the microscopic level. Welcome to my nerdy daydreams. As it turns out, my mental image of thousands of tiny cellular defenders banding together in formation to fire missiles at hoards of incoming free radicals isn't quite right.

As we all know, sunburns and suntans are caused by the sun's ultraviolet radiation (UVR). There are two types of UVR that can penetrate the earth's atmosphere: UVA and UVB. UVA rays are the most prevalent, no matter the season or time of day, and are responsible for wrinkles, sunspots, mottled, droopy skin, and other signs of aging.


Skin cells under a microscope. Image courtesy of euthman.


UVB rays, on the other hand, are responsible for sunburns and suntans. Upon contact with UVB light, skin cells called melanocytes begin producing melanin, a pigment that darkens the skin and absorbs harmful radiation before DNA damage can occur. UVB rays prompt the secretion of MSH, or Melanocyte Stimulating Hormone, from the pituitary gland. MSH binds to a receptor on the surface of melanocytes, triggering melanogenesis through the release of a chemical called cAMP. The more cAMP that is present, the greater the amount of melanin that is synthesized. Once melanin is released from the melanocytes, it is transferred to surface skin cells called keratinocytes that darken and give skin its tanned appearance. In fair-skinned individuals, MSH cannot bind to its receptor as effectively, so less melanin can be produced. With less melanin, UVB rays can easily strike melanocytes and keratinocytes, causing nuclear damage and inciting an inflammatory response, which we call a sunburn. Generally speaking, Caucasians have less melanin than other, darker-skinned populations and so tend to have higher incidences of sunburn and skin cancer.

Take it from me, the fair-skinned hypocrite who forgot to reapply her sunscreen today: sun damage takes its toll over time and can even be fatal. Take action to protect your skin, and keep those nasty photons away from your melanocytes.

Sunday, June 20, 2010

A star is born: Infant star causes a stir in the astonomical community.

Call it slapping hydrogen gas on its proverbial butt. A team of American and German astronomers has announced its observation of the youngest known stellar object ever, according to a paper published in the most recent issue of The Astrophysical Journal. The fledgling star bears the poetic name L1448-IRS2E and was observed developing in the Perseus star-forming region, 800 million light years away. Stellar objects of this age are notoriously difficult to observe because they are not yet true stars and do not give off much light. The team of astronomers discovered L1448-IRS2E by detecting radiation emitted by dust surrounding the object.


A group of young stars in the Perseus constellation. Image courtesy of NASA.


Stars form out of molecular clouds when an overdense area of hydrogen begins to collapse under the influence of gravity. As the clump of gas becomes more massive, it begins to draw in gas and dust from the surrounding area. This "prestellar" phase lasts until the object forms a core that is dense and hot enough to fuse hydrogen into helium. It can then be called a protostar. Due to high-velocity streams of gas being ejected from its center, L1448-IRS2E is believed to have passed the prestellar phase; however, it is not emitting enough light to truly be called a protostar.

The object was originally discovered using Nasa's Spitzer Space Telescope and the Submillimeter Array in Hawaii. The team plans to continue observing with the newly launched Herschel telescope and hopes that its research will shed some light on the mechanics of early stellar evolution.

Saturday, June 19, 2010

Exotic form of matter falls 33 stories and lives to tell the tale.

General relativity is to quantum mechanics as oil is to water. Since the 1930s, physicists have been trying, unsuccessfully, to unite the two disciplines in an effort to explain the physics of both the very large and the very small. Now, almost a century later, scientists are beginning to take drastic measures. For instance, a team at the University of Hanover in Germany recently hauled a very expensive piece of technology down a 110-meter long shaft just to see what would happen.

The idea behind their experiment was to create a strange state of matter called a Bose-Einstein condensate (BEC) and to test its behavior during free fall. First created in 1995 at the University of Colorado at Boulder, a BEC is an extremely cold collection of atoms that magnifies the strange world of subatomic physics. How? When certain gases are cooled to near absolute zero, some of the atoms in the gas are able to occupy the same quantum state. They coalesce to form a visible super-atom, allowing scientists to observe quantum fluctuations on a macroscopic scale - in this case, during 4.7 seconds of free fall. According to Einstein's Equivalence Principle, the conditions experienced during free fall are identical to those felt in an environment without gravity. Experiments such as the one conducted by the Hanover team may shed light on whether quantum mechanical systems obey the same rule. If they do not, it could indicate new hope for the marriage of quantum mechanics and gravity.


A Bose-Einstein condensate created from rubidium atoms. The three diagrams show material gradually condensing in the blue and white regions. Image courtesy of NIST.


180 drops later, the experiment concluded without a hitch. The team next plans to split the BEC and repeat the experiment, sending each half along a different trajectory. Any differences in the motion of the two halves during free-fall would indicate an exception to the Equivalence Principle at the subatomic level and might help to spawn a new theory of quantum gravity.