Let's face it, cosmologists catch a lot of flack. It's easy to see why. These are people who routinely publish papers that claim to ever more finely constrain the size of the visible Universe, the rate of its breakneck expansion, and the distance to galaxies that lie closer and closer to the edges of both time and space. Many skeptics scoff at scientists who seem to draw such grand conclusions without being able to directly measure the unbelievable cosmic distances involved. Well, it turns out cosmologists are a creative bunch. Enter our star (ha, ha): the Type 1a Supernova. These stellar fireballs are one of the main tools astronomers use in order to make such fantastic discoveries about our Universe. But how exactly do they do it?
Read more...
Full story at Universe Today.
Showing posts with label photons. Show all posts
Showing posts with label photons. Show all posts
Thursday, January 5, 2012
Wednesday, March 9, 2011
Cosmology 101: The Present
Welcome back! Last time, we discussed the first few controversial and eventful moments following the birth of our cosmos. Looking around us today, we know that in the span of just a few billion years, the universe was transformed from that blistering amalgam of tiny elementary particles into a vast and organized expanse just teeming with large-scale structure. How does something like that happen?
Read more...
Full story at Universe Today
Read more...
Full story at Universe Today
Thursday, February 17, 2011
Cosmology 101: The Beginning
How did the universe get its start? It’s one of the most pressing questions in cosmology, and likely one that will be around for a while. Here, I’ll begin by explaining what scientists think they know about the first formative seconds of the universe’s life. More than likely, the story isn’t quite what you might think.
Read more...
Full story at Universe Today.
Read more...
Full story at Universe Today.
Wednesday, November 10, 2010
New picture of the night sky, thanks to Fermi.
Imagine a world shadowed each night by a giant cloud of mysterious, illuminated fog. From far outside our galaxy, one would observe the fog rising 25,000 light years above and below the plane of the Milky Way in two enormous plumes. In fact, this world is our own. Of course, you might be more inclined to believe me if you had the ability to see gamma rays.
The Milky Way, in visible light. Gamma rays are invisible to human eyes.
Image courtesy of erikaj121893.
The new discovery comes from a team of Harvard University scientists working with Fermi's Large Area Telescope (LAT). "This work presents a multiwavelength study of the inner Galaxy and identifies several large-scale gamma-ray features, most notably 2 large structures that we refer to as the 'Fermi bubbles'," wrote the team of astronomers in their recent paper. These Fermi bubbles could be millions of years old, but their origin is something of a mystery. Some have suggested that an ancient boom of star births may have later led to a clustered grouping of star deaths. A large number of supernovae occurring all at once could have injected enough energy into interstellar space to create the two plumes of gamma-ray emitting gas. Others, including Douglas Finkbeiner, a senior member of the team, believe that the massive black hole at the center of our galaxy gave rise to the bubbles. Many galaxies emit jets of high-energy radiation due to infalling material around a central black hole, and the Milky Way may be no exception.
The Fermi bubble structures. Image courtesy of Su et al..
It may seem odd that no one had detected these colossal structures before now. As it turns out, the Milky Way has a separate, amorphous halo of gamma-ray fog that prevented astronomers from observing the bubbles until recently. The bubbles, which are slightly more energetic than the ambient haze and have clearly-defined x-ray edges, were only apparent after subtracting this fog from LAT observations. Fermi continues to observe the universe at gamma-ray frequencies.
The Milky Way, in visible light. Gamma rays are invisible to human eyes.
Image courtesy of erikaj121893.
The new discovery comes from a team of Harvard University scientists working with Fermi's Large Area Telescope (LAT). "This work presents a multiwavelength study of the inner Galaxy and identifies several large-scale gamma-ray features, most notably 2 large structures that we refer to as the 'Fermi bubbles'," wrote the team of astronomers in their recent paper. These Fermi bubbles could be millions of years old, but their origin is something of a mystery. Some have suggested that an ancient boom of star births may have later led to a clustered grouping of star deaths. A large number of supernovae occurring all at once could have injected enough energy into interstellar space to create the two plumes of gamma-ray emitting gas. Others, including Douglas Finkbeiner, a senior member of the team, believe that the massive black hole at the center of our galaxy gave rise to the bubbles. Many galaxies emit jets of high-energy radiation due to infalling material around a central black hole, and the Milky Way may be no exception.
The Fermi bubble structures. Image courtesy of Su et al..
It may seem odd that no one had detected these colossal structures before now. As it turns out, the Milky Way has a separate, amorphous halo of gamma-ray fog that prevented astronomers from observing the bubbles until recently. The bubbles, which are slightly more energetic than the ambient haze and have clearly-defined x-ray edges, were only apparent after subtracting this fog from LAT observations. Fermi continues to observe the universe at gamma-ray frequencies.
Saturday, August 7, 2010
Waking sun throws a tantrum, produces dazzling light shows.
On August 1st, NASA scientists watched as ten billion tons of blazing plasma and electromagnetic radiation were thrown off from our sun without warning, sent hurtling through space toward Earth at over a million miles per hour. You might be wondering why we aren't dead yet.
A coronal mass ejection screams toward Earth's magnetosphere.
Image courtesy of NASA.
Actually, events like this are fairly normal in our stellar neighborhood. Every eleven years or so, the sun completes a cycle of waxing and waning electromagnetic activity. During its more active phases, such furious discharges of material (coronal mass ejections, or CMEs) are common. The last peak of solar activity was in 2001. Now, nine years later, it seems that our star is once again getting ready to pull out the big guns. Between now and 2013, scientists predict that CMEs like the one that occurred last week will become more frequent. But far from signaling our imminent doom, these violent outbursts will actually treat us to a demonstration of one of nature's most amazing spectacles: the aurora.
The Aurora Borealis in Alaska. Image courtesy of Joshua Strang.
Aurorae occur when solar material is spontaneously thrown toward Earth, as in a CME. Upon reaching the magnetosphere, highly energetic solar particles are carried groundward, where they collide with and excite nitrogen and oxygen atoms in the ionosphere. The atoms cannot remain excited, so they release excess energy in the form of a photon. The amount of energy needed to excite an atom determines what color photon is emitted when it makes the transition back to its ground state - in this case, ionized oxygen emits green light while ionized nitrogen emits blue or red light. The flickering lights that spread across the sky during an aurora are simply large-scale representations of the transitions that occur when atmospheric particles are excited by the solar wind.
Atoms in the ionosphere interacting with the solar wind.
Image courtesy of NASA.
Sure enough, the CME on August 1st provided audiences across North America and Europe with spectacular visions of the lights a few days later. Although visibility is usually limited to areas surrounding the poles, some American observers this time around reported light shows as far south as Massachusetts! As solar maximum draws nigh, the after-effects of the sun's raging temper should become more and more apparent. Keep your eyes peeled. Depending on where you live, you might be glad you did.
A coronal mass ejection screams toward Earth's magnetosphere.
Image courtesy of NASA.
Actually, events like this are fairly normal in our stellar neighborhood. Every eleven years or so, the sun completes a cycle of waxing and waning electromagnetic activity. During its more active phases, such furious discharges of material (coronal mass ejections, or CMEs) are common. The last peak of solar activity was in 2001. Now, nine years later, it seems that our star is once again getting ready to pull out the big guns. Between now and 2013, scientists predict that CMEs like the one that occurred last week will become more frequent. But far from signaling our imminent doom, these violent outbursts will actually treat us to a demonstration of one of nature's most amazing spectacles: the aurora.
The Aurora Borealis in Alaska. Image courtesy of Joshua Strang.
Aurorae occur when solar material is spontaneously thrown toward Earth, as in a CME. Upon reaching the magnetosphere, highly energetic solar particles are carried groundward, where they collide with and excite nitrogen and oxygen atoms in the ionosphere. The atoms cannot remain excited, so they release excess energy in the form of a photon. The amount of energy needed to excite an atom determines what color photon is emitted when it makes the transition back to its ground state - in this case, ionized oxygen emits green light while ionized nitrogen emits blue or red light. The flickering lights that spread across the sky during an aurora are simply large-scale representations of the transitions that occur when atmospheric particles are excited by the solar wind.
Atoms in the ionosphere interacting with the solar wind.
Image courtesy of NASA.
Sure enough, the CME on August 1st provided audiences across North America and Europe with spectacular visions of the lights a few days later. Although visibility is usually limited to areas surrounding the poles, some American observers this time around reported light shows as far south as Massachusetts! As solar maximum draws nigh, the after-effects of the sun's raging temper should become more and more apparent. Keep your eyes peeled. Depending on where you live, you might be glad you did.
Tuesday, July 27, 2010
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.
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.
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."
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."
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.
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