Quantum mechanics challenged, but to no avail.

I've written about light multiple times in this blog, using terms like photons, light rays, and electromagnetic radiation. You might be wondering what the deal is with all the fancy nomenclature, especially since some of these ideas seem to contradict each other. For instance, light comes in quantized packets called photons. But light is also often referred to as a ray or a wave. What gives? Well, as it turns out, light is very, very weird.

Over 100 years ago, the famous Double-Slit Experiment taught physicists that, at its core, light is both a particle and a wave. In quantum mechanical terms, the wavelike property of light means that every photon has a specific probability of being found in a given spot. When two light waves interfere (as in the double-slit experiment), these probabilities can be seen to occur in pairs: one set of positions where a photon is more likely to be found, and one set of positions where a photon is less likely to be found. Physicists at the University of Waterloo in Canada recently conducted a triple-slit experiment and were able to rule out higher-order probabilities, thus confirming a key principle of quantum mechanics.


The double-slit experiment produces a characteristic interference pattern.
Image courtesy of Timm Weitkamp.

Both the double-slit and triple-slit experiments are very simple in design. In the former, a light source is aimed at a metal sheet with two microscopic slits cut into it. The incident light then passes through the slits and lands on a blank screen. Scientists found that when they covered one slit, the light formed a pattern on the screen that was bright in the middle and faded to black at the edges. One would expect that the same pattern would result when both slits were open. If light behaved solely as a particle, half the photons would pass through one slit and half would pass through the other. The result would be a pattern that was doubly bright, but otherwise the same. Oddly enough, this is not what happened. Instead, a pattern of alternating bright and dark bands developed on the screen (see above). This was a clear indication that it was actually two light waves that passed through the slits, interfering with each other before they hit the screen and thus creating this distinctive pattern.

And the story gets weirder still. Experimentalists decided to try firing one photon at a time at the metal sheet. Surely this would force the photons to "pick" one slit over the other and prevent any wave interference from occurring. No such luck. An interference pattern still emerged, albeit more slowly as each photon took its place on the screen. Clearly this phenomenon could not be explained away by saying that light was really just a wave composed of particles. Scientists were forced to conclude that light is simultaneously a particle and a wave. The photons actually behaved as though they were waves, each photon somehow interfering with itself to form the characteristic light and dark bands shown on the screen.


Individually-fired photons produce an interference pattern over time.
Image courtesy of Ethan Hein.

In the triple-slit experiment, the Innsbruck group attempted to find evidence of third-order probabilities. According to our current understanding of the quantum world, this is impossible. The resulting light and dark bands in the double-slit experiment illustrate that such probabilities only occur in pairs. Success at Waterloo would have had drastic consequences for physics, disproving a long-held belief and necessitating a revision of quantum mechanics as we know it. Fortunately, no such result was found. Indeed, the oddities of the subatomic world live to see another day.

Spaceballs: The (Academic) Sequel.

It's almost like NASA's version of the World Cup. Astronomers using the Spitzer Space Telescope announced last week that they have observed soccer-ball shaped molecules called "buckyballs" in the planetary nebula Tc 1. First isolated in a lab at Rice University in 1985, scientists have long predicted that these molecules would one day be found in space. While previous evidence of naturally-occurring buckyballs was tenuous at best, astronomers can now claim their existence with unprecedented certainty.


Buckyballs are named for their resemblance to architect Richard Buckminster Fuller's famed geodesic domes. Image courtesy of Paul Lowry.

Buckminsterfullerenes, or buckyballs, are hollow, spherical molecules composed of pentagonal and hexagonal linkages of carbon atoms. Since their creation 25 years ago, these and other fullerenes have become increasingly popular in the world of nanotechnology due to their extreme durability and uniquely resilient chemical structure. Buckyballs are also highly aromatic, meaning that their electrons are free to move between adjacent carbon bonds rather than reacting with other, external molecules. Chemically speaking, they are more like an inert gas (such as helium or argon) than a single carbon atom. For these reasons, buckyballs and their cylindrical cousins, carbon nanotubes, continue to be the focus of much research in medicine, engineering and materials science.


The most common type of buckyball is C₆₀. This molecule consists of 60 carbon atoms arranged as shown. Image courtesy of ltamblyn.

Researchers would also like to learn more about the characteristics of naturally created buckyballs. Enter Jan Cami, an astronomer with the University of Western Ontario and the SETI Institute in California, and leader of the group that made the recent discovery. Her team was able to detect these elusive fullerenes in a carbon-rich region of nebula Tc 1. Such regions are common, as planetary nebulae result when a dying star begins to throw off its outer layers of hydrogen, helium, carbon, and other heavier elements. The astronomers actually discovered the buckyballs by accident, catching sight of their unique spectral signature while observing the nebula in infrared light. "When we saw these whopping spectral signatures, we knew immediately that we were looking at one of the most sought-after molecules," said Cami. "We are particularly excited because they have unique properties that make them important players for all sorts of physical and chemical processes going on in space." Buckyballs have already staked their claim in many exciting fields on Earth. We will have to wait and see what they have to tell us about the rest of the universe.

Rogue physicist stokes the Higgs rumor mill.

For the past week, the physics community has been buzzing about a recent blog post written by Tommaso Dorigo, a member of Fermilab's CDF team. In it, he claimed to have heard "from two different, possibly independent sources" that the infamous Higgs boson may have been detected at the Tevatron within the three-sigma statistical level of confidence (99.73%). The Tevatron is the second most powerful particle accelerator in the world (behind the LHC at CERN) and is located at Fermilab in Illinois. This discovery would be huge news, as the Higgs is one of the most elusive and sought-after entities in modern particle physics.


The Standard Model hangs on discovery of the mysterious Higgs boson.
Image courtesy of Fermilab.

Particle accelerators such as the Tevatron and the LHC were built to probe the Standard Model of particle physics. The Standard Model (see above) describes three of the four forces of nature: Electromagnetism, and the Strong and Weak forces. Gravity is left out, as is the mechanism by which objects in the universe acquire mass. This latter mystery is where the Higgs boson comes in. Although physicists know how most particles interact in order to give rise to such properties as radioactivity and light emission, no one knows where the property of mass comes from. In 1960, Peter Higgs came up with the idea of a field that permeates all of space, much like the electromagnetic field; but instead of gathering electromagnetic energy from photons in this field, particles traveling through the Higgs field would be granted mass by Higgs bosons.

At the moment, Fermilab is denying that any such discovery has been made. On July 12, scientists posted the following via the FermilabToday Twitter account: "Let's settle this: the rumors spread by one fame-seeking blogger are just rumors. That's it." Ouch. Keep in mind, however, that this is the same group who announced they had a 50% chance of detecting the Higgs particle this year. We'll just have to wait and see.

A tiny mistake.

The proton is one of nature's most famous elementary particles. Solid, stable and always positive, this subatomic celebrity is, quite literally, at the center of everything in the universe. But new research suggests that the proton may actually be smaller than previously thought. If this turns out to be true, the laws of physics will definitely need some rehabilitation.

Excepting dark matter and dark energy, the entire universe is made up of atoms. In the center of each is a small positively-charged core called a nucleus, which is made up of protons and neutrons held together by the strong force. Like a swarm of bees, electrons buzz around the nucleus in different "shells", or energy states. Hydrogen, the lightest and most abundant element, has been exploited by scientists for centuries due to its extraordinarily simple structure: one proton orbited by one electron.


Ionized clouds of hydrogen. Image courtesy of UC Astronomy Dept.

Most recently, researchers at the Max Planck Institute used our atomic minimalist to probe some choice principles of Quantum Electrodynamics (QED), a theory that merges Einstein's theory of special relativity with quantum mechanics. According to QED, an electron orbiting the hydrogen nucleus in the 2S shell will have a different energy than it would if it were orbiting in the 2P shell. This difference is called the Lamb shift, and it contradicts Paul Dirac's original prediction that the 2S and 2P shells should have the same energy. In order to learn more about the Lamb shift, the team at Max Planck replaced the electron in hydrogen with its cousin the muon, a particle that is 200 times as massive and far less stable. When researchers observed the newly created muonic hydrogen, they found that the massive muon orbited the central proton far more closely than the electron did, and was therefore far more sensitive to its size. The team's calculations assign the proton a radius of 0.84184 femtometers (0.00000000000000084184 meters), a number that is 4% smaller than its previously accepted value of 0.8768 femtometers.

4% may not seem like a whole lot, but this tiny miscalculation could have enormous implications for particle physics. If this new result turns out to be accurate, QED and the Standard Model will have to be completely rewritten. No easy task for the most relied-upon theory in modern physics. Quite frankly, scientists may have a revolution on their hands.

The universe, as seen through new eyes.

The European Space Agency has just released the most sophisticated picture of our universe taken to date. Assembled using strips of data from the Planck satellite, launched in May 2009, this image depicts both the large-scale structure of our own Milky Way galaxy and the Cosmic Background Radiation (CMB) that fills the entire universe. Here, have a look:


Image courtesy of ESA.

The center line that runs through the image is the galactic plane, sprawling across our field of view the same way it does in the sky on a clear night. The wispy, silvery-blue projections extending out of the galactic plane detail the large-scale structure of the Milky Way. But it is the top and bottom of the image, above and below these majestic plumes of gas, that show the real prize: the cosmic microwave background.

Immediately after the big bang, our fledgling universe was nothing but a hot, dense soup of charged particles. At an age of about 300,000 years, the universe had cooled enough for atoms to form, allowing light to travel freely for the very first time. Today, astronomers detect this primordial light as a nearly homogeneous 2.7K radiation that fills the entire sky. (2.7K is incredibly cold, by the way. At less than three degrees above absolute zero, detecting it in the first place is one of the modern miracles of science.) Small temperature fluctuations in the CMB are visible in this image as tiny yellow and green spots. Slightly overdense regions are hotter than the surrounding 2.7K radiation, while slightly underdense regions are cooler. It is these anisotropies that gave rise to all the structure we see in the universe today. Stars, galaxies, galaxy clusters... all of these objects were born of slightly overdense areas of radiation in the very early universe.

You may have seen similar images before. Two previous missions, COBE in 1996 and WMAP in 2003, have already measured the CMB; however, Planck will map this radiation with unprecedented accuracy. Astronomers are hoping that the current mission will allow them to glimpse evidence of inflation, the accelerated expansion the universe is believed to have undergone very early in its development. The stunning image above is only the first of many to come, and is probably the most crude. Researchers will have to eliminate the "noise" caused by the Milky Way if they are to properly map the CMB across the entire sky. Luckily, Planck's refined observations between now and 2012 should allow them to do just that.

For more on the CMB, check out this paper written by an incredibly beautiful, witty and smart young cosmologist.

Nature's symphony heard for the first time, thanks to the LHC.

Today, I stumbled upon the Coolest Website in the Universe. Don't get me wrong, I have come across some great sites in my time (exhibits A, B, and C). But this one takes the cake.

Since January, a group of physicists, engineers, and musicians have been getting paid to convert data collected at the Large Hadron Collider into music. This isn't just some artist's rendering of the cosmic symphony of particle physics. This is actual science. Using a process called data sonification, the group is creating nonspeech audio that correlates with both real and simulated results from CERN's Atlas detector. Atlas is one of the six projects at the LHC that smashes particles together in order to probe the mysteries of the subatomic world.


A simulated collision of the elusive Higgs boson. Photo courtesy of CERN.

The LHCsounds group has been using information collected by Atlas' calorimeter, a device that measures the energies of particles that collide with the detector. Each collision is assigned a note depending on what kind of particle was involved, where the particle struck the calorimeter, and its energy. Different particles (e.g., protons, electrons, photons) sound like different musical instruments that play more loudly at higher energies. Particles that strike the calorimeter closer to an observer are assigned a higher pitch than those that do so further away. Using this formula, LHCsounds has created quite the arrangement. See for yourself here. Make sure to check out the simulated HiggsJetSimple, as well as CalorimeterEndcapLayers (proton collision) and Top Quark Jet, which were both created from real data at 7 TeV!

Processes like data sonification have the potential to revolutionize the way scientific data is analyzed. But the LHCsounds group claims that this isn't their main goal. Instead, their aim is to share the intrinsic beauty of physics with those outside of the scientific community. I'd say they are doing a pretty good job.

In argument over cosmic fallout, supernovae come out on top.

4th of July fireworks ain't got nothin on type Ia supernovae. These exploding stars exhibit their catastrophic brilliance in a systematic fashion, allowing scientists to use them as "standard candles" to measure properties of the universe at large. But for such a uniform piece of cosmic machinery, researchers have long wondered why the ejecta of some type Ia supernovae appear to expand at a different rate from that of others. Now, astronomers from the Dark Cosmology group at the Neils Bohr Institute believe they have found the answer.

Type Ia supernovae are born when a small, dense star called a white dwarf accretes material from a companion star. When the tiny dwarf reaches a mass of 1.4 times that of our sun, it can no longer support itself. High density and pressure in the star's core allow carbon to be fused into oxygen in a runaway process and the star explodes into a supernova. Since the white dwarf's mass limit is always 1.4 solar masses, peak luminosity is the same for all type Ia supernovae. By comparing the supernova's known luminosity with its apparent brightness from our position in space, astronomers can easily and accurately calculate its distance from us. Researchers (myself included!) have used the brightness and distance of type Ia supernovae to determine how quickly the universe is expanding, and what proportion of dark energy is causing it to do so.


Double supernova remnants DEM L316. The smaller remnant on the left is believed to be that of a Type Ia supernova. Image courtesy of NASA.

Since stellar explosions of this kind are all presumed to ignite in the same way, researchers had difficulty understanding why the ejected material from some type Ia supernovae appeared to decelerate at different rates. This discrepancy in so-called velocity gradients led some scientists to believe that these objects were not truly "standard candles." But according to new research from Dark Cosmology, white dwarf enthusiasts can rest easily. As it turns out, the team claims, the observed discrepancies between supernovae can be chalked up to our earthly perspective.

It has long been assumed that type Ia supernovae are triggered in the center of a white dwarf, leading to a symmetrical explosion and a homogeneous ejection of material in all directions. But if a supernova ignites at, say, the outer edge of a star, opposite sides of the explosion will progress at different rates. According to Giorgos Leloudas, a member of the team, "What we could see was that the varying natures of the supernovae could be explained by an asymmetric explosion, where the ignition takes place away from the centre." Whether astronomers observe a high-velocity or low-velocity gradient in the star's ejecta depends on which side of the supernova faces Earth. This conclusion reaffirms the role of type Ia supernovae as the universe's standard candles and, in turn, bolsters the case for dark energy. The team's research is published in the July 1 issue of Nature.