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Brian Koberlein
Lives in Rochester, NY
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Brian Koberlein

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Forge of Heaven

Our understanding of atoms as being made of smaller subatomic particles began with the discovery of the electron in the late 1800s. After we learned that electrons were negatively charged, and that removing electrons from an atom left it positively charged, it was thought that atoms must be held together by electromagnetic forces. There were several proposed models, but one of the most popular was known as the plum pudding model. This proposed that negatively charged electrons were held in a positively-charged atom like plums in a pudding. But in the early 1900s Ernest Rutherford scattered alpha particles off thin layers of gold foil and found that atoms consisted of dense, positively charged nucleus surrounded by electrons. It was soon found that atomic nuclei consisted of positively charged protons as well as neutrons that had no charge. While it was clear that nuclei and electrons were held together by electromagnetic forces, we had no idea what held nuclei together. What could possibly hold protons so close to each other, given the immense repulsive force due to their charges?

Since atomic nuclei also contained neutrons, it was easy to imagine them acting as some kind of “glue” which held protons together. Since neutrons have no electric charge, this couldn’t be done by electromagnetism. There must be some new force strong enough to overpower electromagnetic forces. In the 1930s, Hideki Yukawa proposed that the force between protons and neutrons could be mediated by a new type of quantum particle, just as electromagnetic forces are mediated by quantum photons. Unlike  the photon, this particle would have mass, and as a result the strength of the strong force would die off exponentially with distance. It would also be repulsive at extremely close distances. This meant the strong force would have a distance “sweet spot” to hold protons and neutrons close to each other while not causing them to collapse into a singularity. By the 1940s we began to develop the tools of particle physics, and this intermediary particle known as the pion (or pi meson) was discovered. It seemed like we were finally beginning to understand the strong force.

But over time we began to find more and more particles. In addition to the proton, neutron and pion, we found a range of others. They could be charged positively, negatively, or not at all. Some of them were heavy, like protons and neutrons (known as baryons), while some were lighter like the pion (known as mesons). Many seemed to have some similarities in behavior, but it was unclear just how they might be related. Then in the 1960s Murray Gell-Mann and George Zweig argued that all of these particles were themselves made up of more fundamental particles they called quarks. In this model quarks not only had electric charges of 1/3 or 2/3 the charge of a proton or electron, they also possessed a strong charge or “color” charge.

Although strong charges are not actually colored, the color analogy is useful because of they way they interact. With electric forces there are just positive and negative charges, and an object is electrically neutral if its charge sums to zero. With the strong force there are three types of charges: red, green and blue, and three “opposite” charges anti-red, anti-green and anti-blue (or less commonly cyan, magenta and yellow). To be strong neutral, the color charges must sum to “white.” Following the color analogy, red + green + blue adds to white, so it is neutral. So is red + anti-red, or green + anti-green. In this way, baryons are made of three quarks (one of each color or anti-color) while the lighter mesons are made of two quarks (a color anti-color pair).

From this we’ve been able to develop a model of the strong force along the lines of quantum electrodynamics (QED) for electromagnetism. Since the strong charges are “colors” it is known as quantum chromodynamics (QCD).  Like QED, quantum chromodynamics can be expressed as Feynman diagrams, but instead of charges exchanging photons it is quarks exchanging strong field quanta known as gluons. There are many similarities, but also some important differences.

In electromagnetism, photons are both massless and chargeless. This means, for example, that they can interact with electrons without changing their charge. An electron is always negatively charged before and after interacting with a photon. Gluons are massless, but possess color. Just as charge is conserved in electromagnetism, color is conserved in strong interactions. This means if a quark emits or absorbs a gluon it must change color. As a result, the specific color of an individual quark is indefinite. While we can say that the quarks of a proton have color charges that add to “white,” the fairy dance of quarks and gluons means that color charge is tossed back and forth between quarks. So quarks have color, but not a specific color at any particular time.

If that’s not strange enough, since gluons have color themselves, they interact with each other as well as the quarks. This is a dramatic difference from QED, where photons only interact with charges. Because of their interactions, gluons will tend to cluster in the region between quarks. If you could grab two quarks and try to pull them apart, the gluons would form a flux tube between them. One of the ways gluons can interact is to create a quark anti-quark pair (just as a photon can create an electron-positron pair). Given enough energy, eventually some gluons would interact to create a pair. The flux tube would then snap, and you would be left with two baryons or mesons. The overall effect of this complexity is that you can never isolate a single quark. The strong force ensures that quarks cluster into color-neutral groups of two or three (and perhaps four). It’s an effect known as color confinement.

It’s taken us decades to understand the complexities of the strong force. Even now the calculations are so complex it takes powerful supercomputers to solve most of the equations. There’s still much to learn, but we now know that Yukawa was pretty close to the mark back in the 40s. When quarks are close together in a proton or neutron, for example, they tend to interact directly through the gluons. But the distance between a proton and neutron in the nucleus of an atom are more widely separated, and so they tend to interact when gluons produce a meson. Yukawa’s pion model is a good approximation, and there really is a distance “sweet spot” between protons and neutrons. Rather than simply being a jumble of quarks, atomic nuclei are distinct clusters of protons and neutrons that can combine into stable arrangements. This means not only that atoms can be stable, but that their nuclei can combine to create larger stable nuclei. At high temperatures and densities, hydrogen can become helium, helium can become carbon, etc. This type of temperature and density exists within the cores of stars, and through the strong force they fuse elements to produce heat and light.

When you look up at the night sky, the stars you see are driven by strong force interactions. In that way the strong force truly is the forge of heaven.

Tomorrow: The weak force takes us on an even stranger path. One that leads us toward the origin of life itself.
The strong force is a complex interaction that holds nuclei of atoms together. It is also the force that lets stars create the elements we see around us.
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Cradle to Grave

Gravity is perhaps the best known of the four fundamental forces. It’s also the one that’s easiest to understand. At a basic level, gravity is simply the mutual attraction between any two masses. It’s the force that lets the Sun hold the planets in their orbits, and the force that holds the Earth to you. The force is always attractive, and the strength of the force between two masses depends inversely on the square of their distances, making it an inverse square force. But gravity’s simplicity is just a veneer that hides a deeply subtle and complex phenomenon.

When Newton proposed his model of universal gravity, one criticism of the model was how gravity could act at a distance. How does the Moon “detect” the presence of Earth and “know” to be pulled in Earth’s direction? A few ideas were proposed, but never really panned out. Since Newton’s model was so incredibly accurate, the action-at-a-distance problem was largely swept under the rug. Regardless of how masses detected each other, Newton’s model let us calculate their motion. Another difficulty came to be known as the 3-body problem. Calculating the gravitational motion of any two masses was straight forward, but the motion of three or more masses was impossible to calculate exactly. The motion could be approximated to great precision, and was even used to discover Neptune, but an exact, general solution for three masses would never be found. Newton’s idea was simple, but it’s application was complex.

In the early 1900s, we found that gravity wasn’t a force at all. In Einstein’s model, gravity isn’t a force, but rather a warping of spacetime. Basically, mass tells space how to bend, and space tells mass how to move. General relativity isn’t just a mathematical trick to calculate the correct forces between objects, it makes unique predictions about the behavior of light and matter, which are different from the predictions of gravity as a force. Space really is curved, and as a result objects are deflected from a straight path in a way that looks like a force.

But despite its simple approximation as a force, and its beautifully subtle description as a property of spacetime, we’ve come to realize over the past century that we still don’t know what gravity actually is. That’s because both Newton’s and Einstein’s models of gravity are classical in nature. We now know that objects have quantum properties, with particle-like and wave-like behaviors.  When we try to apply quantum theory to gravity, things become complicated and confusing. In most quantum theory, quantum objects exist within a background framework of space and time. Since gravity is a property of spacetime itself, fully quantizing gravity would require a quantization of space and time. There are several models that attempt this, but none of them have yet achieved a fully quantum model.

Usually our current understanding of gravity is just fine. We can accurately describe the motions of stars and planets. Seemingly odd predictions such as black holes and the big bang have been confirmed by observation. Every experimental and observational test of general relativity has validated its accuracy. Large objects with strong gravity can be described just fine by classical gravity. For small objects with weak gravity we our approximate quantum gravity is good enough. The problem comes when we want to describe small objects with strong gravity, such as the earliest moments of the big bang.

Without a complete theory of quantum gravity, we won’t fully understand the earliest moment of the universe. We know from observation that the early observable universe was both very small and very dense. From general relativity this would imply that the universe began as a singularity. Most cosmologists don’t think the universe actually began as a singularity, but without quantum gravity we aren’t exactly sure. Even if we put the quantum aspects of gravity aside, there is still a part of gravity we don’t understand. Within general relativity it is possible to have a cosmological constant. Adding this constant to Einstein’s equations causes the universe to expand through dark energy, just as we observe. While general relativity allows for a cosmological constant, it doesn’t require one. The cosmological constant agrees with what we observe, but there are other proposed models for dark energy that agree as well (at least for now). If dark energy is really due to the cosmological constant, then the constant must be very close to zero, at about 10-122. Why would a constant be so incredibly close to zero? Why does it even exist when general relativity doesn’t require it?

We don’t know, and without that understanding, both the origin and fate of the universe remain mysteries.

Tomorrow: Electromagnetism was the first unified theory, combining the forces of magnets and charges. The result gave us a new understanding of light, and led us down a path toward a theory of everything.
We often speak of gravity as a force. More accurately it is a feature of spacetime. Even more accurately, we don't know what it is.
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Damn good post +Brian Koberlein​- your article was a great read, and a great lead-in to what I would call one of the more successful threads I've had the pleasure of scrolling through!
I can't tell you- all of you, how relieved and happy I am to be writing this two days after being posted and I find everyone remained civil! I'm so pleased, to be honest my faith was wavering a bit but the first post of Brian's I read in quite a few days and what do I see?, even +CynicalCell​ has curbed his crass curmudgeonry! Not only that but he and Spilotro are chumming it up like they were on the HS football team together or somethin lol! This is neat I like this- I hope this mood or whatever it is persists..
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Snap To

Quantum theory is often viewed as a strange and mysterious model where objects behave in illogical ways. While it’s true that quantum objects behave in ways that are counterintuitive, we actually understand the behavior quite well. In fact, many of these strange behaviors are used in modern astronomy. Take, for example, the quantization of magnetic moments.

Most atoms have a small magnetic field. This magnetic field can be approximated as a small magnet, just as the Earth’s magnetic field is sometimes treated as a magnetic. The strength of that imaginary magnet is given by the magnetic moment of the atom. With this in mind, the orientation of an atom’s magnetic field can be represented by the orientation of the magnet.

Suppose, then, that we were to toss atoms through an inhomogeneous magnetic field. Individually the atoms have no particular orientation, so we would expect that the orientation of their magnetic moments are entirely random. As a result, some of the atoms would be more strongly attracted toward the north direction of the magnetic field, while others would be more attracted to the south, and everywhere in between. If the atoms really did act like tiny magnets, we would expect to see the beam of atoms spread out evenly by the magnetic field. In fact, what we see is that the atoms either move toward the north or south, and that’s it. Instead of spreading out evenly, the atoms lock into specific orientations. This experiment is named the Stern-Gerlach experiment, after the physicists who first performed it in 1922, and it demonstrates one of the basic aspects of quantum theory. When you try to measure the state of a quantum system, the results you get are often snapped to discrete results. It would be like measuring the height of a random collection of people, and finding they are all exactly either 5 ft or 6 ft tall.

As strange as this is, we actually use a similar effect to measure the strength of magnetic fields in the Sun. Since electrons also have magnetic moments, strong magnetic fields can cause their energy levels in an atom to shift. As a result, the emission lines an atom gives off can be shifted by magnetic fields. Emission lines can even be split slightly, which is known as the Zeeman effect. We see this effect near sunspots, which is how we know that sunspots are cooled by magnetic dampening.

That’s part of the real power of astrophysics. Once we understand a phenomena, however strange, we can use it has a tool to study the stars.
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No, Thank You
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The People in Your Neighborhood

Name a living scientist.

If you’re like many people, the first name that came to you was Stephen Hawking. Perhaps Jane Goodall or Neil Tyson. It almost certainly wasn’t May-Britt Moser, a neuroscientist who won the Nobel prize in medicine last year. Or Peter Higgs, Nobel laureate in physics. Or Rajini Rao, research scientist at Johns Hopkins University. Or me.

Therein lies a central problem in the way we view scientists. If you think of a teacher, plumber, or engineer, you likely think of someone you know personally, or at least casually. They are people who live in your neighborhood, send their kids to the same schools your children attend, and shop at your favorite grocery store. But a scientist? They’re some internationally known genius whom you might have seen give a talk once. What’s worse, only 1/3 of Americans can name a living scientist.

It’s not like scientists are particularly rare in the world. In the U.S. alone there are about 200,000 PhDs in the life and physical sciences. Not to mention all the non-doctorate scientists working across the country, or those in the social sciences. If you look up the closest university in your area and check out their science departments, you’ll see lots of scientists live and work in your neighborhood.

Science can be a passion, but it’s also a job. It’s what we do day in and day out, just as teachers teach and plumbers plumb. Outside our jobs we have and raise children, wash dishes, do laundry, and shop for groceries, just like everyone else. We have hobbies and interests beyond our jobs and we worry about getting that promotion, or caring for elderly parents. And like most everyone, we aren’t household names.

So the next time you’re at a sports game or the grocery store, look around. A few of your neighbors are likely to be scientists.
There are lots of scientists in the world, so why do we only think of the famous ones as scientists?
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Jerry Coyne!
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Missed It By That Much

Back in December I wrote about a star heading our way. It’s nothing to worry about, but it demonstrated that stars can come relatively close to our solar system on long time scales. Now new research finds that one close approach occurred just 70,000 years ago.

The star is known as Scholz’s star, and is a small red dwarf star with a brown dwarf companion. Currently it is about 20 light years from the Sun, but 70,000 years ago it was only 52,000 AU away, or about 0.8 light years. That might not seem particularly close, but the outer edge of the Oort cloud likely extends farther than that, so the star could have gravitationally perturbed Oort cloud objects, sending some toward the inner solar system.

Some of you might look at that 70,000 BC date and wonder if this might have anything to do with the “bottleneck  theory” of human evolution, where the human population supposedly winnowed to about 10,000 individuals. The answer is no for two reasons. The first is that genetic evidence for a 70,000 BC bottleneck is not particularly strong, the second is that any cometary bodies perturbed by the close encounter would take time to reach us. In this case, the trip from the outer Oort cloud to the inner solar system is about 2 million years. So there might be a slight uptick in comets in a couple million years, but the team estimates that this won’t be significant.

The work also puts doubt on the upcoming close encounter of HIP 85605 I wrote about in December. It seems the distance of HIP 85605 was underestimated by a factor of 10, which means it won’t come nearly as close as we thought. If that’s the case, then Scholz’s star marks the closest stellar approach known. The number and distance of these kinds of close encounters are likely to change a bit as we continue to get better data on close faint stars. For example, when the Gaia spacecraft becomes active we’ll have position and motion data of more than a billion stars.

Paper: Eric E. Mamajek et al. The Closest Known Flyby of a Star to the Solar System ApJ 800 L17 (2015)
About 70,000 years ago Scholz's star came within 0.8 light years of the Sun.
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John K
Scary, yet true.
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Light in Motion

Often in astronomy we’ll show false color images of celestial objects to enhance the visual appeal, or to show how an object appears at different wavelengths. While it might look like the above image is a similar multi-wavelength view, it’s actually much more sophisticated. Rather than simply being a few at different wavelengths, the four images of the Milky Way are each focused on the origin of the light in question, and each tells a particular story about our galaxy.

The red image in the upper left is light that follows a thermal spectrum (what we call blackbody radiation). In this case the source of the light has a temperature of about 20 K. This is the temperature of much of the interstellar dust in our galaxy, so this image shows the distribution of dust in the Milky Way. The central band is the plane of our galaxy, and is where most of the dust lies, though it’s clear that most of the sky is somewhat dusty. This is the main reason why efforts to observe effects of inflation in the cosmic microwave background have been so problematic.

The yellow image in the upper right is light emitted at a specific radio wavelength emitted by carbon monoxide. Wherever you see that wavelength, you know carbon monoxide is there. Carbon monoxide is abundant in stellar nurseries, and where stars are actively forming. Carbon monoxide is much less abundant than hydrogen in these regions, but light from hydrogen is much more dim.

The green image in the lower left is a bit more complicated. It shows thermal bremsstrahlung light, formed which particles collide with each other. The light is also known as free-free emissions, because it is produced by electrons colliding with hydrogen ions and not getting captured to form neutral hydrogen. Thus the electrons and ions are “free” both before and after the collision. Like thermal blackbody emissions, thermal bremsstrahlung can be identified by its overall spectrum. Free-free emissions typically occur where there is hot, ionized gas, such as near massive stars.

The last image in blue shows a type of light known as synchrotron radiation. When charged particles move through a magnetic field, they begin to move in a helix along the magnetic field lines. As they are accelerated in a circle they emit synchrotron radiation. This radiation is brightest when high-energy electrons are trapped in strong galactic magnetic fields. So the image show how the Milky Way’s magnetic field traps high energy electrons produced by things such as supernovae.

With all these images combined we can see where material is, where it’s collapsing under gravity, where it’s heating up, and where it’s energized. It’s a view of our galaxy that extends far beyond simply looking at images from different wavelengths. From the cosmic light we can learn not only what’s there, but also what it’s doing.
From the light of our galaxy we can see not only what's there, but the dynamics of how it's changing.
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Yellow=upper right
Red=upper left 
Green=Lower Left
Blue=margnetic field beyond
The all Comban One Galaxy Dynamic
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Have him in circles
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Dance of the Hag

On the face of it both electricity and magnetism are remarkably similar to gravity. Just as two masses are attracted to each other by an inverse square force, the force between two charged objects, or two poles of a magnet are also inverse square. The difference is that gravity is always attractive, whereas electricity and magnetism can be either attractive or repulsive. For example, two positive charges will push away from each other, while a positive and negative charge will pull toward each other. As with gravity, electricity and magnetism raised the question of action-at-a-distance. How does one charge “know” to be pushed or pulled by the other charge? How do they interact across the empty space between them? The answer to that question came from James Clerk Maxwell.

Maxwell’s breakthrough was to change the way we thought electromagnetic forces. His idea was that each charge must reach out to each other with some kind of energy. That is, a charge is surrounded by a field of electricity, a field that other charges can detect. Charges possess electric fields, and charges interact with the electric fields of other charges. The same must be true of magnets. Magnets possess magnetic fields, and interact with magnetic fields. Maxwell’s model was not just a description of the force between charges and magnets, but a also description of the electric and magnetic fields themselves. With that change of view, Maxwell found the connection between electricity and magnetism. They were connected by their fields. A moving electric field creates a magnetic field, and a moving magnetic field creates an electric field. Not only are the two connected, but one type of field can create the other. Maxwell had created a single, unified description of electricity and magnetism. He had united two different forces into a single unified force, which we now call electromagnetism.

Maxwell’s theory not only revolutionized physics, it gave astrophysics the tools to finally understand some of the complex behavior of interstellar space. By the mid-1900s Maxwell’s equations were combined with the Navier-Stokes equations describing fluids to create magnetohydrodynamics (MHD). Using MHD we could finally begin to model the behavior of plasma within magnetic fields, which is central to our understanding of everything from the Sun to the formation of stars and planets. As our computational powers grew, we were able to create simulations of protostars and young planets. Although there are still many unanswered questions, we now know that the dance of plasma and electromagnetism plays a crucial role in the formation of stars and planets.

While Maxwell’s electromagnetism is an incredibly powerful theory, it is a classical model just like Newton’s gravity and general relativity. But unlike gravity, electromagnetism could be combined with quantum theory to create a fully quantum model known as quantum electrodynamics (QED). A central idea of quantum theory is a duality between particle-like and wave-like (or field-like) behavior. Just has electrons and protons can interact as fields, the electromagnetic field can interact as particle-like quanta we call photons. In QED, charges and a electromagnetic fields are described as interactions of quanta. This is most famously done through Richard Feynman’s figure-based approach now known as Feynman diagrams.

Feynman diagrams are often mis-understood to represent what is actually happening when charges interact. For example, two electrons approach each other, exchange a photon, and then move away from each other. Or the idea that virtual particles can pop in and out of existence in real time. While the diagrams are easy to understand as particle interactions, they are still quanta, and still subject to quantum theory. How they are actually used in QED is to calculate all the possible ways that charges could interact through the electromagnetic field in order to determine the probability of a certain outcome. Treating all these possibilities as happening in real time is like arguing that five apples on a table become real one at a time as you count them.

QED has become the most accurate physical model we’ve devised so far, but this theoretical power comes at the cost of losing the intuitive concept of a force. Feynman’s interactions can be used to calculate the force between charges, just as Einstein’s spacetime curvature can be used to calculate the force between masses. But QED also allows for interactions that aren’t forces. An electron can emit a photon in order to change its direction, and an electron and positron can interact to produce a pair of photons. In QED matter can become energy and energy can be come matter.

What started as a simple force has become a fairy dance of charge and light. Through this dance we left the classical world and moved forward in search of the strong and the weak.

Tomorrow: The strong force answered the question of how positive charges could be bound in the nuclei of atoms, and allowed us to understand the origin of matter itself.
Electromagnetism can produce a force between charges or magnets, but it is much more than a simple force.
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+Matt McIrvin - thank you, your comments in this thread are truly illuminating and serve as a timely and profound contribution to Mr Koberlein's article. Consequently, I had a tremendously nifty experience as I read your descriptions of dynamic field interactions. I took up the study of electronics just shy of two years ago now, and have gained a modest understanding of some of the principles and maybe a handful of fundamentals. No more than a hobbyist, you're right in your assertion that EE's don't put much thought into field tensor strength/shape or any of that, as knowledge of it isn't really required to have a well designed circuit. Interactions of these fields are accounted for and I'll wager you know this already, like you understand that such an acute scrutiny of electronic's true fundamentals might only, over-task some budding engineers! But not me- my nature is inquisitive to a fault, and even before I successfully breadboarded my first blinkie I first wanted to know how any why these components functioned together as they do. I was discouraged from seeking those answers, verbally at first and then later on by virtue, as every rock I turned over didn't provide me with information I felt was satisfactory. Until I read your comments. It was an interesting sensation as I visualized these curious fields and thoughts of understanding finally began to coalesce, at the exact same time my mind was blown!..
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The Four Horsemen

In physics it’s often said that there are four fundamental forces: gravity, electromagnetism, strong, and weak.  The reason we list them as four forces has a bit to do with the history of our understanding of them. In the 1700s, the forces of electricity and magnetism were considered to be separate, but by the mid 1800s James Clerk Maxwell unified them into a general theory of electromagnetism. Soon general relativity was first validated, a unified theory of electromagnetism and gravity known as the Kaluza-Klein model was developed. This classical model ran into difficulties integrating with quantum theory, so the model fell out of favor. Later, electromagnetism was unified with the weak to form the electroweak model, but since their unified behavior only appears at high energies we continue to treat them as distinct forces. 

Most people are familiar with gravity and electromagnetism in their daily lives, while the strong force holds atomic nuclei together, and the weak … something something … radioactivity. Even many physics majors aren’t given more than a cursory overview of the strong and weak forces, so while we all know the list, we’re less clear in describing them. While some fields of study can get away with focusing on one or two of these forces, all four of them are central to astrophysics.

So this week I’ll look at these four forces, specifically within the context of astrophysics:

Gravity: forge of the universe

Electromagnetism: forge of planets and stars

Strong: forge of atoms

Weak: forge of life

We’ll start with gravity first. It’s the force everyone knows, but no one fully understands. It all starts tomorrow.
There are four fundamental forces in the universe, and each of them plays a crucial role in astrophysics.
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+CynicalCell Sounds a far better way to spend that day than what I did;-)
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Wrong Way Triton

Triton is the largest moon of Neptune. It is distinctive because it is the largest moon in the solar system with a retrograde orbit, meaning its orbit is opposite to Neptune’s rotational direction. This means Triton didn’t form along with Neptune, but rather was captured once Neptune formed. Just how Triton was captured isn’t clear, but one possibility is that Triton was once a binary object, and a close encounter with Neptune led to the capture of Triton and the expulsion of its partner.

Triton’s orbit is almost perfectly circular, which is a bit surprising. It likely had a much more elliptical orbit when it was captured, and over time its orbit was circularized by tidal forces as well as gas and dust particles moving in a prograde orbit. The tidal forces also worked to sync its rotation with its orbit, so that now one side of Triton always faces Neptune, just as one side of our Moon faces Earth. When it was initially captured, tidal forces would have been much stronger, which heated the moon’s interior. To this day Triton is still geologically active, though it is now driven by radioactive heating in its core. It’s possible that Triton has liquid water in its interior similar to Europa.

Triton has a chemical composition similar to Pluto, so there has been some speculation that the two bodies share a common origin. What we do know is that Triton is a Kuiper belt object like Pluto. When New Horizons makes its flyby of Pluto later this year, one of the questions we’ll try to address is just how closely related Pluto and Triton actually are.
Triton is Neptune's largest moon, and the largest moon it the solar system to orbit backwards.
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Nice catch, Neptune! Imagine Triton as a short period comet whizzing closer and closer to us every couple of hundred years or so. Retrograde, prograde, there is no right or wrong. Just be glad it's not headed this way.
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Karma Chameleon

While the search for dark matter particles often hits the news, there are also efforts underway to detect dark energy particles. As with dark matter, the experiments thus far have largely determined what dark matter isn’t rather than what it is. 

There are two basic ways to account for dark energy. One is known as the cosmological constant. In this model, dark energy is an inherent aspect of the structure of space and time. Thus, throughout the universe there is a constant, uniform expansion of spacetime that gives the effect of dark energy. This model is the simplest way to account for dark energy, as it’s just a matter of adding a term to the usual general relativity equations. It also agrees with observations so far. But simply adding a term to your equations seems like a bit of a tweak model. General relativity doesn’t require a cosmological constant, it just allows for one. There’s no reason why there should be such a constant other than the fact that it fits observation. So lots of alternatives have been proposed.

The most popular type of alternative is to propose some type of scalar field. The idea is that the universe would be filled with a scalar field that results in dark energy. That may seem even more crazy than a cosmological constant, but the Higgs boson is a result of a scalar Higgs field introduced to account for particle mass, and we’ve actually detected it. There are several variations of the scalar field idea, but most of them can’t be tested using current data. But one version known as chameleon fields has just been tested, and failed the test.

The chameleon field is a “fifth force” field that interacts with itself  to produces the effects of dark energy in deep space, but also gets inhibited by the presence of mass. In this way you get cosmic expansion between galaxies, but you don’t see its effect in galaxies (or in our solar system). Since the presence of mass makes it “hidden,” it acts as a kind of cosmic chameleon, hence the name. Normally I wouldn’t put much credence in a “just so” model like this, but a few months ago it was demonstrated that the model could actually be tested. Because of its chameleon effect, the field could be “trapped” within a vacuum cavity. By making the matter in the chamber as little as possible, the chameleon field would strengthen in the chamber. As a result, the effective gravitational force within the vacuum is altered. Using an atom interferometer (basically a double-slit experiment using atoms instead of electrons) the change in gravity could be measured. What the team found was that there was no measured effect to the limits of their experiment.

This basically rules out the chameleon field and similar models. There’s still a few ways the model could be tweaked to still exist within the limits of this experiment, but it doesn’t look good for chameleon fields. That’s not particularly surprising, since most proposed models will be wrong. What makes this interesting is that we’re now actually testing dark energy models in the lab.

Paper: Paul Hamilton, et al. Atom-interferometry constraints on dark energy. arXiv:1502.03888 [physics.atom-ph] (2015)
One proposed model for dark energy known as the chameleon field has been put to the test, and failed.
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Brian Koberlein

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E.T. Phone Home

In the movie Contact, astronomers receive a radio signal from the star Vega. Buried within the signal is a broadcast of Hitler’s speech for the opening of the 1936 Olympic games. The television signal had made the 25 light year journey to Vega, which let the aliens know we’re here. The idea that our television and radio signals are gradually reaching ever more distant stars is a popular one, but in reality things aren’t so simple.

The opening ceremony of the 1936 Olympics was the first major television signal at a frequency high enough to penetrate Earth’s ionosphere. From there you could calculate that any star within about 80 light years of Earth could detect our presence. There’s even a website that shows which TV shows might be reaching potentially habitable worlds. But the problem with this idea is that it isn’t good enough for the signal to reach a distant star, it also needs to be powerful and clear enough to be detectable.

For example, the most distant human-made object is Voyager I, which has a transmission power of about 23 Watts, and is still detectable by radio telescopes 125 AU away. Proxima Centauri, the closest star to the Sun, is about 2,200 times more distant. Since the strength of a light signal decreases with distance following the inverse square relation, one would need a transmission power of more than 110 million Watts to transmit a signal to Proxima Centauri with the strength of Voyager to Earth. Current TV broadcasts (at least in the States) is limited to around 5 million Watts for UHF stations, and many stations aren’t nearly that powerful.

One might argue that an advanced alien civilization would surely have more advanced detectors than we currently have, so a weaker signal isn’t a huge problem. However the television signals we transmit aren’t targeted at space. Some of the signal does leak out into space, but they aren’t specifically aimed at a stellar target the way Voyager I’s signal is aimed at Earth. They also lack a clear mechanism for how transform the signal to an image. On Earth this works by implementing a specific standard, which any alien civilization would need to reverse engineer to really watch TV. On top of that, there is the problem of scattering and absorption of the signal by interstellar gas and dust. This can diminish the power and distort the signal. Even if aliens could detect our signals, they might still confuse it with background noise.

That doesn’t mean it’s impossible to communicate between stars. It just means that communication would require an intentional effort on both sides. If you really want to communicate with aliens, you need to make sure your signal is both clear and readable. To make it stand out among all the electromagnetic noise in the universe, you’d want to choose a wavelength were things are relatively quiet. One good region is known as the water hole, which spans a range from 18 to 21 cm. Hydrogen (H) emits at about 21 cm, and hydroxyl (HO) has a strong emission at about 18 cm. Together they can form water, hence the name for the quite gap in between. You also need to make your signal easy to recognize as an artificial signal. In Contact the aliens did this by transmitting a series of prime numbers.

In 1974 humanity made its most famous effort to send a signal to the stars. It was a radio transmission sent from the Arecibo observatory, and consisted of 1,679 binary digits, lasting three minutes. Since 1,679 is the product of the primes 23 and 73, the bits can be arranged into an image of those dimensions. There have been other efforts to send messages to the stars, but they haven’t been as powerful or as simple.

Beyond a few light years, our leaky TV broadcasts are likely undetectable. As we’ve switched to digital television and lower transmission powers they’ve become even harder to detect.  Any aliens looking for us will have to rely on other bits of evidence, such as the indication of water in our atmosphere or chlorophyl on Earth’s surface, just as we will strive to detect such things on distant worlds. Either way, the first message received won’t be a complex text of information. It will simply be a recognition of life on another world.
It's a popular idea that our television shows could be seen by aliens light years away, but our signals aren't quite that clear.
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The term alien is superficial. The cosmology of our persons has been verified to be stellar. How then can any mass be foreign? Only by justification of metrics, defined by finite perception, could such a term be rational.
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Brian Koberlein

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Galaxy X

The distribution of hydrogen in the Milky Way is something we’ve measured to a good degree of precision. Combined with computer models we can start to look at the dynamics of our galaxy. For example, back in 2009 a comparison was made between the observed distribution of atomic hydrogen in the Milky Way and possible effects of dark matter. Basically there are variations or ripples in the galactic hydrogen that don’t match up with the known distribution of visible matter, however computer simulations showed that these ripples could be caused by a localized clump of dark matter. That is, it seemed small satellite galaxy comprised mostly of dark matter is perturbing the gas and dust in our galaxy. While it was an interesting idea, proving that the ripples could be caused by dark matter isn’t the same as demonstrating that they are.  But a new paper published in Astrophysical Journal Letters has strengthened the idea.

If such a dark matter “galaxy x” exists, then we should be able to find it. The problem is that a mostly dark matter dwarf galaxy wouldn’t be particularly bright, and what light it does emit could be dimmed by gas and dust in the way. But the simulations predicted a region where the cluster of dark matter should be, so the team began a search in that region. They used public data from the ESO Public survey VISTA
Variables of the Via Lactea (VVV), gathered at infrared wavelengths. Near infrared wavelengths are useful because they are less affected by interstellar gas and dust. When they analyzed the data, the team found four Cepheid variable stars clustered in the same region of the sky near the galactic plane. Cepheid variables are useful because they vary in brightness in a specific way, and we can use that fact to determine their distance. When the team did this they found the stars were all about 294,000 light years away, give or take a bit. It would be very unusual to find four Cepheid variables so close together just by chance, so it is most likely the case that they are part of a previously unknown dwarf galaxy.

In the popular press this new work is generally being presented as the “discovery” of a dark matter galaxy, but that isn’t quite the case. This new work doesn’t conclusively prove a dark matter galaxy. What the work has done is taken an earlier prediction on the existence of a dark matter dwarf galaxy, and found a clustering of stars in the general location predicted by their model. This clustering of variable stars is consistent with their model. Once again it demonstrates the predictive power of dark matter models. It also demonstrates how useful public data can be, since data gathered for one project can be used in several others.

Paper: Sukanya Chakrabarti et al. Tidal imprints of a dark subhalo on the outskirts of the Milky Way. MNRAS 399 (1): L118-L122. (2009)

Paper: Sukanya Chakrabarti et al. Clustered Cepheid Variables 90 kiloparsec from the Galactic Center. Astrophysical Journal Letters 1502: 1358 (2015)
New research finds evidence of a dark matter dwarf galaxy orbiting the Milky Way.
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+Brian Koberlein​ excellent link, and excellent advice! Thanks!
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The Universe is amazing, let me tell you.
An astrophysicist and physics professor at Rochester Institute of Technology.  Author of "Astrophysics Through Computation" with David Meisel.  Creator of the science outreach project Prove Your World, developing a science television show for children.

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