Under the guidance of some of the top astronomy experts in the country, explore our amazing Universe - including up close views of the Earth's moon, Jupiter and other mysterious planetary objects - at the Stargazing Party, an exciting educational collaboration between the Festival, the Smithsonian's National Air & Space Museum (NASM), telescope manufacturer Celestron and other partners, on Saturday, April 28 at NASM in Washington, DC.

A hit with visitors at the inaugural Festival in 2010, the Stargazing Party is returning to the Festival Expo with an equally impressive lineup of evening celestial activities which include Bill Nye the Science Guy in a live recorded broadcast by Planetary Radio with host Mat Kaplan who will inspire other young astronomers to make their own unbelievable discoveries!

In addition, don't miss presentations by such prominent astronomy educators as Drs. Jeffrey Bennett and Jeff Goldstein who will give walking tours of the celebrated Voyage Scale Solar Model System located just outside the NASM and later discuss other fascinating facets of our quest to explore the universe.

Celestron will add hand-on excitement to this night with telescopes set up around NASM's Public Observatory to accommodate visitors' celestial viewing enjoyment.

Event Details:
April 28, 2012- 06:30 PM to 10:30 PM
Smithsonian Institution's National Air and Space Museum on the National Mall

6:30 pm - 7:45 pm: Walking Tours of the Voyage Scale Model Solar System (space is limited, sign up for one of the 3 thirty-minute tours at the museums info desk when you arrive)
7:30 pm: Telescopes ready for viewing around NASM's Public Observatory
7:45 pm: Doors open for Stargazing Party!
8:00 pm: Program in Moving Beyond Earth gallery, including welcome by Bill Nye the Science Guy and remarks by Celestron, and the Planetary Radio Live program
8:10 - 10:20 pm: Check out "Scale of the Universe" and "Human Exploration: the Journey Continues" in Milestones of Flight, plus "Forces of Flight" in How Things Fly, and look for hands-on Discovery Stations throughout the first floor of the Museum!

For more information on this free event, visit this link.

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Who says that if you scream in space no one will hear you?

A rare daytime meteor was seen and heard streaking over northern Nevada and parts of California on Sunday, just after the peak of an annual meteor shower.

Observers in the Reno-Sparks area of Nevada reported seeing a fireball at about 8 a.m. local time, accompanied or followed by a thunderous clap that experts said could have been a sonic boom from the meteor or the sound of it breaking up high over the Earth.

source

Here's an animation:

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"Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity - in all this vastness - there is no hint that help will come from elsewhere to save us from ourselves. It is up to us." -Carl Sagan Here on our planet, this is the one day that we take out of the year to stop and appreciate just how amazing the natural world really is, and how fortunate we are to have the Earth that we have. A wonderful but sad reminder of how fragile it is, and how quickly and easily we can affect it, comes through John Prine's great song,

Paradise.

Here on our planet, there are countless ways to celebrate what we have. But what if you weren't here on Earth; what if you were a distant space traveler, headed towards our Solar System for the first time?

(Image credit: SKY-MAP / WikiSKY.)

You'd see a rather unspectacular, whitish star. It would appear bright only because you were close to it. Even from our nearest star, Alpha Centauri, the Sun would only be the 5th brightest star in the night sky. If you knew the proper techniques, you could tell that there were gas giant planets around it, and -- if your tools were excellent -- some smaller, inner rocky worlds, too.

But unless you journeyed into the Oort Cloud, and then in past the Kuiper Belt, only at this point would it be easy for you to see Earth.

(Image credit: NASA / JPL.)

And even then, from 6 billion kilometers (4 billion miles) away, we'd appear as no more than a speck of dirt flying through the interplanetary dust.

But as you came in closer, you'd be able to see more and better details. That is, of course, if you knew to look for Earth. There are plenty of tempting distractions.

(Image credit: NASA / JPL / Space Science Institute / Cassini. Click for hi-res.)

From distant Saturn, our world is visible, seen here poking out from in between the rings. A close-up view shows that you can not only see that our planet is round in shape, but has a large moon, visible off of our upper-left limb.

If you came in all the way to the closest planet to the Sun, Mercury, your view would be different, but even more spectacular.

(Image credit: NASA / John's Hopkins University / Carnegie Institute of Washington.)

Our Earth, the largest disk in the photo, appears to be in either a full or nearly-full phase at all times from Mercury's vantage point, along with our Moon. Slightly more distant would be our view of Earth from Mars, which we were lucky enough to capture back in 2003 for the very first time, and in color to boot!

(Image credit: NASA / JPL-Caltech / Mars Global Surveyor.)

Seen from an outer planet, the Earth will run through all the phases, from new to crescent to gibbous to full. Our blue color, caused by our atmosphere and our oceans, surrounds patches of green and brown, where our continents poke out above the watery surface. Overlaid over all of it is the white clouds, which paint a transient covering above our world.

This is much more apparent, of course, the closer you get. When the Voyager 1 spacecraft was first leaving Earth on its journey, it snapped this far superior view than the one we can get from Mars.

(Image credit: NASA.

In fact, Voyager 1 and 2, in 1977, became the first spacecrafts to ever photograph the full Earth and Moon in the same picture.

But it's nothing compared to the first human view of the entire Earth, seen in December of 1968 by the Apollo 8 astronauts. As they emerged from behind the night side of the Moon -- the first humans ever to do so -- this was the sight that greeted them.

(Image credit: NASA / F. Borman, J. Lovell and B. Anders.)

The above photo, simply known as Earthrise, carries the following statement with it, courtesy of Anders: We came all this way to explore the moon, and the most important thing is that we discovered the Earth. Today, of course, there are thousands of satellites orbiting the Earth, taking photos of the entire planet in unprecedented detail.

(Image credit: NASA / Goddard Space Flight Center / GOES-13 / NOAA.)

This very hi-res image (click for it), taken just a few months after the BP Oil Spill, is perhaps a great example of how invisible the damage we can do to the Earth is. From even our best views from space, the Earth appears nothing if not pristine and magnificent, as this cropped section of the Gulf of Mexico shows.

So enjoy your Earth Day today in whatever way you choose. Remember that it's the only home we have, and that it's our job to take care of it, to clean up after ourselves, and to leave it in better shape than we found it. No matter where in the Universe you are, make the most of it!

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"April is the cruellest month, breeding
Lilacs out of the dead land, mixing
Memory and desire, stirring
Dull roots with spring rain." -T. S. Eliot Of course you all know the refrain, "April showers bring May flowers," but there's one April shower that brings fireballs instead: the Lyrids!

(Image credit: John Chumack, retrieved from Bob King.)

Like all meteor showers, the Lyrids come from a comet's dust trail that forms a great ellipse with respect to our Solar System. Once per year, the Earth, in its orbit around the Sun, passes through this dusty debris. When this happens, the Earth, moving at over 10,000 miles-per-hour with respect to these dust grains, cause the dust to vaporize in a fiery plunge as they collide with our atmosphere.

(Image credit: Gehrz et al. 2006, retrieved from Michael Kelley.)

Every year, right around April 22nd, this meteor shower peaks, delivering anywhere from 10 to 100 meteors per hour visible beneath dark skies.

"Why is there so much variability," you ask? Well...

(Image credit: ESA. Courtesy of MPAe, Lindau, retrieved from space.com.)

These dusty debris trails that fill our Solar System all come from comets. When a comet passes close enough to the Sun, it spews off gas, dust and ions, leaving an elliptical trail in roughly the same orbit as the comet itself. But comets spend most of their time far away from the Sun, so that these big bursts of debris only get emitted at rare intervals.

Are we going to pass through one of these big bursts, or are we going to spend this year passing through a lull in the debris trail? Like every year, we never know until we get there. The Lyrid meteor shower is a particularly tough one to predict, because the comet responsible for it, Comet Thatcher (after the 19th Century astronomer, not the 20th Century Iron Lady), is not only perpendicular to the plane of our Solar System, it also has a disturbingly long period of 415 years, meaning it won't be back until 2276!

(Image credit: Astroclock 2010 blog.)

But the comet isn't the interesting thing: the dusty debris which brings us the meteor shower is! The peak of the meteor shower, which is the best time to observe it, should occur close to midnight in North America during the night of April 21st / morning of April 22nd, making it the perfect way to usher in Earth Day!

No matter where on Earth you are, here's where you want to look.

(Image credit: me, using stellarium.)

The bright star Vega, the fifth brightest star in the night sky (and #2 in the Northern Hemisphere), should be high enough in the sky by 10 PM to easily identify it. (Vega features prominently in my summer sky tutorial, here.)

There's a small (but prominent; visible even in most cities) parallelogram nearby, just slightly closer to the horizon. Combined with Vega, that's how you can easily identify the constellation Lyra. The meteor shower should originate just a few degrees away, to the upper right of the parallelogram. But don't look directly at that spot, and don't use a telescope! The meteors originate from there, but you'll see them streaking away from that point, in random directions!

(Image credit: Wally Pacholka from the 2001 Leonids, retrieved here.)

Over the course of an hour, you should see anywhere from 10 to 100 meteors, depending on how good this year's Lyrids are. You'll have the added bonus of a moonless sky; the waxing crescent will be so minuscule that it will have completely set by time the constellation Lyra is visible. The meteors themselves are worth the price of admission, but every once in a while, the Lyrids give the gift of a true fireball: a meteor so bright it outshines the entire sky combined, lasts for a few seconds, and can even cast prominent shadows.

(Image credit: Baltimore Sun, retrieved from here.)

No promises, of course, as it's impossible to predict these things, but if I've got clear skies, you can bet I won't miss the opportunity to spend some time enjoying the wonders of the night!

For those of you who want real-time updates on the Lyrid meteor shower, including reports from around the world as they come in, well, what's the point of running the best Science news service on the web if you can't make that happen? So follow the Meteor Showers & Comets trap and stay on top of it. However you do it, make sure you enjoy the show Saturday night and into Sunday morning, and know that my eyes and millions of others will be gazing upwards with you!

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I'm in the mood for some math today, so here's an amusing little proof I recently showed to my History of Mathematics class. We shall derive the formula

\[ \frac{\pi^2}{6}=1+\frac{1}{4}+\frac{1}{9}+\frac{1}{16}+\frac{1}{25}+\dots \]

Note that the denominators of the fractions on the right are all perfect squares.

The problem of evaluating the sum on the right has a pedigree going back to the 1600s, when various mathematicians, including the famed Bernoullis, tried unsuccessfully to solve it. It was Leonhard Euler who polished it off at the age of 28 in 1735, thereby announcing himself as a force to be reckoned with in mathematics.

Euler's solution is one of those exceedingly clever arguments which, if you have any taste for mathematics at all, just has to bring a smile to your face. We need two main pieces of machinery. The first is the Taylor series for the sine function. If you can think back to whenever you took calculus, you might recall that it looks like this:

\[ \sin x=x-\frac{x^3}{3!}+\frac{x^5}{5!}-\frac{x^7}{7!}+\frac{x^9}{9!}-\dots \]

If we divide through by x we obtain:

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"Science progresses best when observations force us to alter our preconceptions." -Vera Rubin I want you to think about the Universe. The whole thing; about everything that physically exists, both visible and invisible, about the laws of nature that they obey, and about your place in it.

It's a daunting, terrifying, and simultaneously beautiful and wondrous thing, isn't it?

(Image credit: NASA, ESA, S. Beckwith (STScI) and the HUDF Team.)

After all, we spend our entire lives on one rocky world, that's just one of many planets orbiting our Sun, which is just one star among hundreds of billions in our Milky Way galaxy, which is just one galaxy among hundreds of billions that make up our observable Universe.

Yes, we've learned an awful lot about what's out there and our place in it. As best as we can tell, we've learned what the fundamental laws are that govern everything in it, too!

(Image credit: Mark Garlick / SPL, retrieved from the BBC.)

As far as gravitation goes, Einstein's theory of general relativity explains everything from how matter and energy bend starlight to why clocks run slow in strong gravitational fields to how the Universe expands as it ages. It is arguably the most well-tested and vetted scientific theory of all time, and every single one of its predictions that has ever been precision-tested has been verified to be spot-on.

(Image credit: Contemporary Physics Education Project.)

On the other hand, we've got the standard model of elementary particles and interactions, which explains everything known to exist in the Universe, and all the other (nuclear and electromagnetic) forces that they experience. This, also, is arguably the most well-tested and vetted scientific theory of all time.

And you would think that if our understanding of things were perfect, if we knew all about the structure of the Universe, the matter in it, and the laws of physics that it obeyed, we'd be able to explain everything. Why? Because all you'd have to do is start out with some set of initial conditions -- immediately following the Big Bang -- for all the particles in the Universe, apply those laws of nature that we know, and see what it turns into over time! It's a hard problem, but in theory, it should be not only possible to simulate, it should give us a sample Universe that looks just like the one we have today.

(Image credit: NASA / WMAP Science Team.)

But this doesn't happen. In fact, this doesn't happen at all. This picture I painted for you above is all true, on the one hand, but we also know that it isn't the whole story. There are other things going on that we don't fully understand.

Here, as best as I can present the full history in a single blog post, is the whole story.

(Visualizations & Simulations: Ralf Kähler, Tom Abel, and Oliver Hahn (KIPAC).)

As we come forward from the event of the Big Bang, our Universe expands, cools, while the entire time experiencing the irresistible force of gravity. Over time, a number of extremely important events happen, including, in chronological order:

1. the formation of the first atomic nuclei,
2. the formation of the first neutral atoms,
3. the formation of stars, galaxies, clusters, and large-scale structure,
4. and how the Universe expands over its entire history.

If we know what's fundamentally in the Universe and the physical laws that everything obeys, we'll arrive at quantitative predictions for all of these things, including:

1. what nuclei form and when in the early Universe,
2. what the radiation from the last-scattering-surface, when the first neutral atoms are formed, looks like in great detail,
3. what the structure of the Universe, from large scales down to small scales, looks like both today and at any moment in the Universe's past,
4. and how the scale, size, and number of objects in the observable Universe have evolved over its history.

We have made observations measuring all of these things, quantitatively, extremely well. Here's what we've learned.

(Image credit: NASA / Goddard Space Flight Center / WMAP101087.)

What we consider to be normal matter, that is, stuff made up of atoms, is highly constrained by a variety of measurements. Before any stars formed, the nuclear furnace of the very early Universe fused the first protons and neutrons together in very specific ratios, depending on how much matter and how many photons there were at the time.

What our measurements tell us, and they've been verified directly, is exactly how much normal matter there is in the Universe. This number is incredibly tightly constrained to be -- in terms that might be familiar to you -- about 0.262 protons + neutrons per cubic meter. There could be 0.28, or 0.24, or some other number in that range, but there really couldn't be more or less than that; our observations are too solid.

(Image credit: Ned Wright.)

After that, the Universe continues to expand and cool, until eventually the photons in the Universe -- which outnumber the nuclei by more than a billion-to-one -- lose enough energy that neutral atoms can form without immediately being blasted apart.

When these neutral atoms finally form, the photons are free to travel, uninhibited, in whatever direction they happened to be moving last. Billions of years later, that leftover glow from the Big Bang -- those photons -- are still around, but they've continued to cool, and are now in the microwave portion of the electromagnetic spectrum. First observed in the 1960s, we've now not only measured this Cosmic Microwave Background, we've measured the tiny temperature fluctuations -- microKelvin-scale fluctuations -- that exist in it.

(Image credit: WMAP Science Team / NASA. For those of you who like your maps shown on Mercator projections, click here for that view.)

These temperature fluctuations, and the magnitudes, correlations and scales on which they appear, can give us an incredible amount of information about the Universe. In particular, one of the things they can tell us is what the ratio of total matter in the Universe is to the ratio of normal matter. We would see a very particular pattern if that number were 100%, and the pattern we do see looks nothing like that.

Here's what we find.

(Image credit: Pavel Kroupa.)

The necessary ratio is about 5:1, meaning that only about 20% of the matter in the Universe can be normal matter. This doesn't tell us anything what this other 80% is. From the Cosmic Microwave Background alone, we only know that it exerts a gravitational influence like normal matter, but it doesn't interact with electromagnetic radiation (photons) like normal matter does.

You can also imagine that we've got something wrong about the laws of gravity; that there's some modification we can make to it to mimic this effect that we can re-create by putting in dark matter. We don't know what sort of modification could do that (we haven't successfully found one, yet), but it is conceivable that we've just got the laws of gravity wrong. If a modified theory of gravity could explain the fluctuations of in the Microwave Background without any dark matter at all, that would be incredibly interesting.

But if there really is dark matter, it could be something light, like a neutrino, or something very heavy, like a theorized WIMP. It could be something fast-moving, with a lot of kinetic energy, or it could be something slow-moving, with practically none. We just know that all of the matter can't be the normal stuff we're used to, and that we've come to expect. But we can learn more about it by simulating how structure -- stars, galaxies, clusters, and large-scale structure -- forms in the Universe.

(Video credit: DEUS Consortium.)

Because the types of structures you get out -- including what types of galaxies, clusters, gas clouds, etc. -- exist at all times in the Universe's history. These differences don't show up in the Cosmic Microwave Background, but they do show up in the structures that form in the Universe.

What we do is take a look at the galaxies that form in the Universe and see how they cluster together: how far away from a galaxy do I have to look before I see a second galaxy? How early in the Universe do large galaxies and clusters form? How quickly do the first stars and galaxies form? And what can we learn about the matter in the Universe from this?

(Image credit: E.M. Huff; SDSS-III; South Pole Telescope / Zosia Rostomian.)

Because if the dark matter -- which doesn't interact with light or normal matter -- has lots of kinetic energy, it will delay the formation of stars, galaxies, and clusters. If the dark matter has some but not too much, it makes it easier to form clusters, but still hard to form stars and galaxies early on. If the dark matter has virtually none, we should form stars and galaxies early. Also, the more dark matter there is (relative to normal matter), the more smooth the correlations will be between galaxies on different scale, while the less dark matter there is means that the differences in correlations between different scales will be very stark.

The reason for this is that early on, when clouds of normal matter starts to contract beneath the force of gravity, the radiation pressure increases, causing the atoms to "bounce back" on certain scales. But dark matter, being invisible to photons, wouldn't do this. So if we see how big these "bouncing features" are, known as baryon acoustic oscillations, we can learn whether there's dark matter or not, and -- if it's there -- what its properties are. The thing we construct, if we want to see this, is just as powerful as the graph of the fluctuations in the microwave background, a couple of images above. It's the much lesser-known but equally important Matter Power Spectrum, shown below.

(Image credit: W. Percival et al. / Sloan Digital Sky Survey.)

As you can clearly see, we do see these "bouncing" features, as those are the wiggles in the curve, above. But they're small bounces, consistent with 20% of the matter being "normal" matter and the vast majority of it being smooth, "dark" matter. Again, you might wonder if there isn't some way we could modify gravity to account for this type of measurement, rather than introducing dark matter. We haven't found one yet, but if such a modification were found, it would be awfully compelling. But we'd have to find a modification that works for both the matter power spectrum and the cosmic microwave background, the way that a Universe where 80% of the matter is dark matter works for both.

This is from the structure data on large scales; we can also look on small scales, and see whether small clouds of gas, in-between us and very distant, bright objects from the early Universe, are thoroughly gravitationally collapsed or not; we look at the Lyman-alpha forest for this.

(Image credit: Bob Carswell.)

These intervening, ultra-distant clouds of hydrogen gas teach us that, if there is dark matter, it must have very little kinetic energy. So this tells us that either the dark matter was born somewhat cold, without very much kinetic energy, or it's very massive, so that the heat from the early Universe wouldn't have much of an effect on the speed it was moving millions of years later on. In other words, as much as we can define a temperature for dark matter, assuming it exists, it's on the cold side.

But we also need to explain the smaller-scale structures that we have today, and examine in gory detail. This means when we look at galaxy clusters, they, too, should be made of 80% dark matter and 20% normal matter. The dark matter should exist in a big, diffuse halo around the galaxies and the clusters. The normal matter should be in a couple of different forms: the stars, which are extremely dense, collapsed objects, and the gas, diffuse (but denser than the dark matter) and in clouds, populating the interstellar and intergalactic medium. Under normal circumstances, the matter -- normal and dark -- is all held together, gravitationally. But every once in a while, these clusters merge together, resulting in a collision and a cosmic smash-up.

(Image credit: NASA/CXC/CfA/M.Markevitch et al.; NASA/STScI; Magellan/U.Arizona/D.Clowe et al.; NASA/STScI; ESO WFI; Magellan/U.Arizona/D.Clowe et al..)

The dark matter from the two clusters should pass right through one another, because dark matter doesn't collide with normal matter or photons, as should the stars within the galaxies. (The stars not colliding is because the cluster collision is like firing two guns loaded with bird-shot at one another from 30 yards away: every single pellet should miss.) But the diffuse gas should heat up when they collide, radiating energy away in the X-ray (shown in pink) and losing momentum. In the Bullet Cluster, above, that's exactly what we see.

(Image credit: NASA/CXC/STScI/UC Davis/W.Dawson et al., retrieved from Wired.)

Ditto for the Musket Ball Cluster, a slightly older collision than the Bullet Cluster, that's just recently analyzed. But others are more complicated; cluster Abell 520, for example, below, appears to have too much gravity associated with a location that ought to have only normal matter and not dark matter.

(Image credit: NASA / CXC / CFHT / UVic. / A. Mahdavi et al.)

If we look at the individual components, you can see where the galaxies are (which is also where the dark matter ought to be), as well as the X-rays, which tell us where the gas is, you'd expect the lensing data -- which is sensitive to the mass (and hence, dark matter) -- to reflect that.

(Image credit: NASA, ESA, CFHT, CXO, M.J. Jee and A. Mahdavi.)

Instead, we see evidence for the gas creating a large amount of lensing, which shouldn't be. So, perhaps something funny is going on here. Maybe this is evidence in favor of modified gravity and against dark matter, as some contend. Or, perhaps, there's an explanation consistent with dark matter, and we simply have an unusual mass distribution in this type of smash-up.

But we can go to even smaller scales, and look at individual galaxies on their own. Because around every single galaxy, there should be a huge dark matter halo, comprising approximately 80% of the mass of the galaxy, but much larger and more diffuse than the galaxy itself.

(Image credit: ESO/L. Calçada.)

Whereas a spiral galaxy like the Milky Way might have a disc 100,000 light-years in diameter, its dark matter halo is expected to extend for a few million light-years! It's incredibly diffuse because it doesn't interact with photons or normal matter, and so has no way to lose momentum and form very dense structures like normal matter can.

What we don't yet have any information about, however, is whether dark matter interacts with itself in some way. Different simulations give very different results, for example, as to what the density of one of these halos ought to look like.

(Image credit: R. Lehoucq et al.)

If the dark matter is cold and doesn't interact with itself, it should have either an NFW or a Moore-type profile, above. But if it is allowed to thermalize with itself, it would make an isothermal profile. In other words, the density doesn't continue to increase as you get close to the core of a dark matter halo that's isothermal.

Why a dark matter halo would be isothermal isn't certain. Dark matter could be self-interacting, it could exhibit some sort of exclusion rule, it could be subject to a new, dark-matter-specific force, or something else that we haven't thought of yet. Or, of course, it could simply not exist, and the laws of gravity that we know could simply need modification. On galactic scales, this is where MOND, the theory of Modified Newtonian Dynamics, really shines.

(Image credit: University of Sheffield.)

While the NFW and Moore profiles -- the ones that come from the simplest models of Cold Dark Matter -- don't really match up with the observed rotation curves very well, MOND fits individual galaxies perfectly. The isothermal halos do a better job, but lack a compelling theoretical explanation. If we only based our understanding of the "missing mass" problem -- whether there was extra, "dark" matter, or whether there was a flaw in our theory of gravity -- on individual galaxies, I would likely side with the MOND-ian explanation.

So when you see a recent headline like Serious blow to dark matter theories?, you already have a hint that they're looking at individual galaxies. Let's see what this is about.

(Image credit: ESO/L. Calçada.)

A paper released just two days ago took a look at stars relatively close to our solar neighborhood, and looked for evidence of this inner distribution of mass from the theoretical dark matter halo. You'll notice, looking a couple of images up, that only the simplest, completely collision-less models of Cold Dark Matter give that large effect in the cores of dark matter halos.

So let's take a look at what the survey shows.

(Image credit: C. Moni Bidin et al., 2012.)

Indeed, the simple (NFW and Moore) halo profiles are highly disfavored, as many studies before have shown. Although this is interesting, because it demonstrates their insufficiency on these small scales in a new way.

So you ask yourself, do these small-scale studies, the ones that favor modified gravity, allow us to get away with a Universe without dark matter in explaining large-scale structure, the Lyman-alpha forest, the fluctuations in the cosmic microwave background, or the matter power spectrum of the Universe? The answers, at this point, are no, no, no, and no. Definitively. Which doesn't mean that dark matter is a definite yes, and that modifying gravity is a definite no. It just means that I know exactly what the relative successes and remaining challenges are for each of these options. It's why I unequivocally state that modern cosmology overwhelmingly favors dark matter over modified gravity. But I also know -- and freely admit -- exactly what it will take to change my scientific opinion of which one is the leading theory. And you're free to believe whatever it is you like, of course, but there are very good reasons why the modifications to gravity that one can make to have gravity succeed so well without dark matter on galactic scales fail to address the other observations without also including dark matter.

And we know what it isn't: it isn't baryonic (normal matter), it isn't black holes, it isn't photons, it isn't fast-moving, hot stuff, and it probably isn't simple, standard, cold and non-interacting stuff either, like most WIMP-type theories hope for.

(Image credit: Dark Matter Candidates, retrieved from IsraCast.)

I think it's likely to be something more complicated than the leading theories of today. Which isn't to say that I think I know exactly what dark matter is or how to find it. I'm even sympathetic to certain degrees of skepticism expressed on that account; I don't think I would claim to be 100% certain that dark matter is right and our theories of gravity are also right until we can verify dark matter's existence more directly. But, if you want to reject dark matter, there's a whole host of things you'll need to explain some other way. Don't completely ignore large-scale structure and the need to address it; that's a surefire way to fail to earn my respect, and the respect of every cosmologist who studies it.

And that's, as best as I can express it in a single blog post, the whole story on dark matter. I'm sure there are plenty of comments; let the fireworks begin!

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This post was written by Brookhaven Lab science writer Justin Eure.

Let's start with a number, by chance a palindrome: 1441. Imagine taking that many photographs of a single object, a soccer ball, say - obsessively capturing it from every angle to expose all the details. Those 1441 images provide all the evidence needed to illustrate and understand the three dimensional structure of that soccer ball. Each shot reveals another curve of the sphere, another line in the checkered pattern, another scuff or scratch along the surface.

Building a 3D image gets tricky with objects billions of times smaller. (courtesy of Electric-Eye on flickr)

An adept programmer or the right piece of software can then combine those photographs to generate an accurate 3D model of that ball. More than a thousand 2D images may take time to align, but at least the structure is simple. But what if that soccer ball was itself the size of a skin cell, its patterns were smaller than airborne viruses, and you still needed over a thousand photos to know its structure?

That's the challenge that a team at Brookhaven National Laboratory overcame with a new transmission x-ray microscope (TXM), which successfully combined 1441 images of a lithium-ion battery electrode into a detailed 3D structure. The TXM, hooked up to an x-ray beamline at Brookhaven's National Synchrotron Light Source, generates unparalleled image resolutions, as demonstrated in a new paper published in Applied Physics Letters. Read the rest of this post... | Read the comments on this post...

I have not watched these yet myself, but will do as soon as I download and convert them for my iPhone, but I have no doubt they are up to Peter Sinclair's usual high standards.

Embedded below are part's 1 and 2 of Weird Winter - Mad March. Enjoy, discuss!

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Yesterday's post on applying intro physics concepts to the question of how fast and how long football players might accelerate generated a bunch of comments, several of them claiming that the model I used didn't match real data in the form of race clips and the like. One comment in particular linked to a PDF file including 10m "splits" for two Usain Bolt races, including a complicated model showing that he was still accelerating at 70m into the race. How does this affect my argument from yesterday?

Well, that document is really a guide to fancy fitting routines on some sort of graphing calculator or something. Which is fine as far as it goes, but I think it attributes too high a degree of reality to those unofficial split times, which are obtained from some unidentified web site. They proceed to fit a bunch of complicated functions to the data, but I think they're overthinking it.

Let's look at the actual data, graphed in more or less the way you would expect to see it in an intro physics class: as a plot of position vs. time:

The black circles represent the times from a race in 2008, the white circles times from a race in 2009. They're practically right on top of each other, because in absolute terms, the difference in times is pretty tiny.

Their first step is to fit a straight line to the data, which works remarkably well, even though it can't possibly be right. Looking at the graph, though, it does look awfully linear, particularly if you threw out the first point or two. That seems pretty consistent with the "accelerate to a maximum speed and stay there" model I assumed in the previous post, especially given that we don't know anything, really, about how these numbers were obtained.

Of course, the real test is to look at the speed as a function of time:

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The European Space Agency has made its selection for the next Large Mission to be flown by ESA, with a launch window in about 2022

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Over at Grantland, Bill Barnwell offers some unorthodox suggestions for replacing the kickoff in NFL games, which has apparently been floated as a way to improve player safety. Appropriately enough, the suggestion apparently came from Giants owner John Mara, which makes perfect sense giving that the Giants haven't had a decent kick returner since Dave Meggett twenty years ago, and their kick coverage team has lost them multiple games by giving up touchdowns to the other team.

Anyway, one of Barnwell's suggestions invoked physics, in a way that struck me as puzzling:

Idea 3: The receiving team's returner is handed the ball on his own goal line. His blockers must be positioned on the 20-yard line. The "kicking" team's players are positioned on the opposition's 40-yard line. Once the whistle blows, it's a traditional kick return.

Advantages: Everyone on the field will still be colliding, but because the kick-coverage team will have been running for 20 yards as opposed to 45, there won't be anywhere near as much momentum in those collisions. That should produce fewer injuries.

Now, if you know anything about introductory physics, you know that momentum is mass times velocity (for speeds much less than the speed of light). This doesn't have any direct relationship to the distance somebody has run to get to that point, unless they're accelerating the whole time. But it seems awfully unlikely to me that any of the whack jobs covering kicks in the NFL are actually speeding up appreciably between 20 and 45 yards-- they probably hit their top speed well before that.

Ah, but is there a way to use our knowledge of introductory physics to test this idea? That is, can we estimate the distance over which an NFL player is likely to be accelerating? Well, I would hardly be posting this if I didn't have a model for this sort of thing, so let's have a run at it (heh).

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"Truth is mighty and will prevail. There is nothing wrong with this, except that it ain't so." -Mark Twain

"It doesn't matter how beautiful your theory is, it doesn't matter how smart you are. If it doesn't agree with experiment, it's wrong." -Richard Feynman Every day that you set forth in the world is a new opportunity to learn something about it. Every new observation that you make, every new test you perform, every novel encounter or piece of information you pick up is a new chance to be a scientist.

How so?

(Image credit: Alan Chen.)

You have a conception of how things work in this world. You've pieced it together as a combination of your experiences, your knowledge, and the working hypotheses that you've accepted as the best mirror of reality. And every new shred of evidence you pick up about reality interrogates these hypotheses, daring your picture of reality to hold up to this level of scrutiny.

No matter who you are, no matter how smart you are, no matter how brilliantly you've drawn the conclusions you've drawn from the evidence you've gathered, there will come an instance where the evidence you encounter will be irreconciliable with the picture of reality you presently hold. And when that moment happens, your response will mean absolutely everything.

(Image credit: Glennbeck.com.)

Because there is the possibility that your view of reality -- the way you make sense of things -- is flawed in some way. You have to open your self up to at least the possibility that you are wrong. It is a humbling admission, that you may be wrong, but it's also the most freeing thing in the world. Because if you can be wrong about something, then you can learn.

(Image credit: Dave Koerner at Northern Arizona University.)

The discovery that planets move about the Sun in ellipses required exactly that; were it not for Kepler and his ability to accept that his earlier models were flawed, and then abandon them and create new and improved ones, physics and astronomy would likely have been set back an entire generation. And if you, yourself, can do this in your own life, you can find a better explanation for the phenomena you encounter in this world. You can bring your understanding of the world more closely in line with what reality actually is. In other words, you can do what all good scientists do, and in the end, learn something amazing.

(Image credit: NASA, retrieved from Universe Today.)

But if you can't admit that you might be wrong, if your picture of reality is unchangeable despite any evidence to the contrary, if you refuse to assimilate new information and new knowledge and re-evaluate your prior stance on an issue, then you will never learn.

Anything.

Perhaps as an adult you're entitled to that right; you are, after all, free to believe whatever you want. But if you're a student in school? Your job is to learn. If you don't do your job, particularly if you don't even try to do your job, it's your teachers duty -- and I would say responsibility -- to fail you.

At least, it should be. Recently, some incredibly appalling things have been happening in education that completely undermine this, including the banning of the words 'dinosaur' and 'evolution' from standardized tests and the passage of Tennessee's "academic freedom" bill that allows teachers to teach counterfactual scientific information to their students about biological evolution and climate science, among other topics.

(Image credit: Listverse / Mike Devlin.)

And this is unfathomable to me. See that creature above? That's a black wolf. Know what's interesting to me about it? The black wolf doesn't occur in nature! The mutation for black fur did not occur until after the domestic dog had been in existence for thousands of years. If ever you see a black wolf, that tells you that at some point in their lineal history, there was a wolf that engaged in breeding with a domestic dog that had that (dominant) black fur mutation.

Biology, of course, doesn't stop with evolution. What I just explained to you is an explanation that requires genetics to understand, which is encoded in an organism's DNA. But before you get to DNA, before you even get to genetics, at a more basic level you must have an understanding of evolution. If you want to understand disease: evolution. If you want to understand whales and dolphins: evolution. (I mean come on, they've got freakin' leg bones!)

(Image credit: retrieved from distraff at selectsmart.com.)

Same deal with global warming; there are plenty of people asserting that the Earth isn't warming anymore (yes, there really are), despite all studies showing that it totally is, if you look at the data without cheating. For example, last year (2011) was "only" the 11th-warmest year on record since records began in 1880. But last year was also a La Niña year, which is characterized by cooler temperatures. It was also the hottest La Niña year of all time, since 1880.

The question I always ask people who dig in even deeper when their view on an issue is challenged by new data is the following: What evidence would it take to change your mind on this issue? For the "Is the Earth continuing to warm" question, you may very well get your wish in 2012 or 2013; one of the next two years could easily become the new warmest-year-on-record.

(Image credit: NASA / GISS global average temperature data.)

Believe it or not, it's actually harder for many of us to admit that we could be wrong about something the less we know about it! Why's that? A neat little psychological effect known as the Dunning-Kruger effect. In a nutshell, it says that people who are incompetent at something (e.g., biology, climate science, etc.) lack the very skills necessary to evaluate the fact that they are incompetent!

This results in people who know almost nothing about a particular topic who are willing to opine at length, argue with experts, and declare -- incorrectly -- that they are right and you are an idiot. Here's the original graph from the original Dunning-Kruger paper, illustrating exactly that.

(Image credit: Justin Kruger and David Dunning, 1999.)

But if we recognize that our present understanding may not be the final answer, and we can absorb that ego-bruise from possibly not being in the right when we thought we were, we can step forward. There are plenty of people working to help make it easier for us all to do exactly that. I'm not exempt from this either, even in areas where my knowledge actually is far above average. Last week, I wrote about when ultramassive stars die, and a number of people challenged some of the contentions I made. Yes, some of them may have been jerks about it, but they also had information that I didn't. Despite being a theoretical astrophysicist, I don't know all there is to know about all aspects of astrophysics, and I never will.

(Image credit: NASA / CXC / M. Weiss.)

So I went out and learned what it was that I didn't know, and now my picture of how supermassive stars die is -- while possibly still imperfect -- improved over what it was. And the next time I go to explain it, there will be at least two things that I can do a better, more accurate job of explaining, and there will be at least one misstep I won't make again.

It doesn't make me any less of a person or any less of a scientist that I didn't get everything right the first time I put it all together; on the contrary, it makes me human. I've been refining what I know and how things make sense to me my entire life, and I'll continue to do that tomorrow. There is no part of that picture of reality that I hold so dear that overwhelming evidence to the contrary couldn't change my mind. I would be surprised at a great number of things, but I wouldn't be stuck.

I know exactly what types of evidence would change my mind about the theories, hypotheses and ideas that make up my world view. Remember the words of Carl Sagan: When you make the finding yourself -- even if you're the last person on Earth to see the light -- you'll never forget it. I hope that I never reach the point where I think I'm always right; I hope I can always gather new information and knowledge, have that crisis when my preconceptions conflict with new data, and admit when I was wrong. Because I don't want to ever stop learning; no matter how much I know, there's always going to be a whole Universe out there to explore.

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Logging the Onset of The Bottleneck Years
This weekly posting is brought to you courtesy of H. E. Taylor. Happy reading, I hope you enjoy this week's Global Warming news roundup

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My Google vanity search for my name and the book titles is really frustratingly spotty, often missing things in major news outlets that I later find by other means. For example, I didn't get a notification about this awesome review in the Guardian, from their children's book section:

I am a ten year old who likes Physics. What is Physics, you might ask! Well, Physics is the science of pretty much everything around you. It asks big questions like where did we come from? How long ago was the Big Bang? Quantum Physics is the part of physics which talks about atomic and sub atomic particles, basically very very small particles. It is a little tricky to understand the behaviour of these particles. So I decided to buy How to Teach Quantum Physics to your Dog. It is an extremely funny book in which Chad Orzel, the writer, teaches his pet dog, Emma, Quantum Physics. Emma loves to chase bunnies and squirrels in the garden, but the problem is that she cannot predict where they will be so they are able to dodge her every time. Now this is also a problem we have in Quantum Physics. So learning Quantum Physics should help Emma catch bunnies...

(The replacement of "Emmy" with "Emma" is a really persistent problem, even among adults, so I just shrug that off. I suspect it's the work of over-zealous copy editors who haven't read the book.)

He goes on to call it "one of the most amazing books I have read." I wonder if I can get that on the jacket? "'One of the most amazing books I have read' -- 'Tintin,' age 10."

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Rockville High School in Rockville, MD was visited by Dr. April Croxton, Marine Biologist from the National Oceanic and Atmospheric Administration (NOAA). Dr. Croxton is one of the AT&T Sponsored Nifty Fifty X 2 Speakers. WTOP's "Man About Town" Bob Madigan was on hand to broadcast Dr. Croxton's presentation and even catch some student comments about this inspiring event.

Listen to WTOP's coverage with Bob Madigan here.

The USA Science & Engineering Festival's AT&T Sponsored Nifty Fifty Program sends more than 100 top scientists and engineers to DC-area schools leading up to the the FREE Expo which will take place at the DC Convention Center on April 28-29. The program has been a huge success this year and we are so grateful to all of our Nifty Fifty Speakers! We believe it is so important for students to hear from real live scientists and engineers to see the potential they can reach with STEM.

We hope to see you at the Festival in 2 weeks!

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"If we knew what it was we were doing, it would not be called research, would it?" -Albert Einstein Our galaxy is but one among hundreds of billions in the cosmos, nearly all of which contain supermassive black holes at the center. Ours happens to be "only" a few million times as massive as our Sun, as well as quiet.

(Image credit: ESO, R. Genzel et al. at MPI fur Extraterrestrische Physik.)

In other words, our galaxy's supermassive black hole is behaving right now, by not viciously shooting off high-energy jets of particles and light at some poor, innocent passers-by.

But other galaxies are not so well-behaved.

(Image credit: NASA and the Hubble Heritage Team / STScI / AURA.)

For example, some 53 million light-years away in the Virgo Cluster, the galaxy Messier 87 shoots a jet some 5,000 light-years long out of its central supermassive black hole. This behemoth has the advantage of having a black hole a few billion times as massive as our Sun (rather than a few million), and happens to be in the process of devouring a large amount of matter at the moment.

(Image credit: NASA, NRAO/NSF, John Biretta (STScI/JHU), and Associated Universities, Inc.)

These supermassive black holes possess some of the strongest and largest magnetic fields in the Universe, and as matter falls in -- whether its from stars, planets, asteroids, gas or dust -- it gets broken apart into individual atoms, which become ionized. Like all charged particles, they get accelerated by these magnetic fields, resulting in, among other things, the intense emission of radiation along the magnetic-axis of the black hole!

(Image credit: CosmoVision, W. Steffen et al. / UNAM, retrieved here.)

Galaxies with these powerful, collimated jets emitted from their central black holes are known as Active Galaxies, and this radiation covers the entire spectrum, from gamma-rays through visible light all the way down to radio frequencies. Nearly always, this radiation comes off in an extremely narrow beam, so that any one particular Active Galactic Nucleus (AGN) is very unlikely to fry us, here on Earth.

(Image credit: NASA / JPL-Caltech.)

But, keep in mind, there are hundreds of billions of galaxies out there in the Universe, and that even with a tiny percentage of them containing active nuclei and an even tinier percentage pointing right at us, it is a very big Universe. And for those of you who've come by here recently, you'll recall that we've just completed surveying the entire Universe in the infrared, imaging -- literally -- hundreds of millions of these galaxies.

(Image credit: NASA / JPL-Caltech / WISE Team / UCLA.)

With the entire sky imaged by the infrared WISE satellite in four wavelengths -- higher energies in blue and lower energies progressing through green and into red -- hotter objects like stars and galaxies should pretty much always show up with a blue or greenish-blue color. Very dusty regions will show up as red or green, as that gas is heated up by stars to be much warmer than the interstellar medium, but still far too cold to give off visible light.

But remember, nearly half of the "points of light" imaged by WISE are galaxies, some of which are active. And every once in a very rare while, these active galaxies have a jet pointed right at us.

(Image credit: NASA / GSFC Conceptual Image Lab.)

These rare but fascinating objects are known as Blazars, because they're "blazing quasars." Normally found with Gamma-Ray telescopes, you wouldn't necessarily think that an infrared telescope would be a great Blazar-finding tool. But Francesco Massaro and his team have done exactly that, with great success. Here's how:

(Image credit: NASA / JPL-Caltech / Kavli.)

With a uniquely flat spectrum across WISE's four color filters, Blazars appear white compared to everything else. Why white? Unlike practically every other point of light in the image, which shows up colored based on the thermal temperature of the emitting objects (hotter appear blue, cooler appear red), the light from Blazars comes from that characteristic radiation of charged particles being accelerated by their black hole's magnetic field: synchrotron radiation! The synchrotron radiation shows up with a roughly equal brightness in all four of WISE's filters, giving it a whitish appearance compared to all other compact object.

These objects are very rare and normally hard-to-find, but because of their unique and easily identifiable appearance to WISE's eyes, more than 200 new blazars have been seen and thousands of old ones confirmed, with estimates that thousands more are in the all-sky image, just waiting to be discovered. This is particularly remarkable, considering that there only are around three thousand known Blazars in the entire Universe! (3,081 as of October, to be more precise.)

(Image credit: Gene Smith's Astronomy Tutorial at UCSD.)

In other words, this infrared satellite could, if we're lucky, wind up doubling the number of known Blazars! As the study's lead author notes: Blazars are extremely rare because it's not too often that a supermassive black hole's jet happens to point towards Earth. We came up with a crazy idea to use WISE's infrared observations, which are typically associated with lower-energy phenomena, to spot high-energy blazars, and it worked better than we hoped. What an unexpected, serendipitous use for an infrared satellite: as a gold-mine for Blazars, which just happen to be some of the most interesting objects in the Universe.

This is part of the beauty of doing an All-Sky Survey; if you can just learn one new technique for finding/identifying one class of object, you're suddenly going to catch every single one in the Universe that your instrument is sensitive to.

And that's how a simple infrared satellite can capture thousands of the most energetic cosmic cannons in the Universe!

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As threatened a little while ago, this is the first of ten hopefully weekly posts looking back at the ten years this blog has been in operation. This one covers the period from the very first post on June 22, 2002 to June 21, 2003.

When I started doing this look back, I was more than a little afraid that it would prove cringe-inducing. It's been ten years, after all, and in that time I've gone from a wet-behind-the-ears, recently married assistant professor to a tenured father of two and a published author. That's enough external change that I was expecting my early posts to seem, well, pretty juvenile.

That wasn't the case, though. I mean, there are some definite changes in the general style of the blog, but all in all I was pleasantly surprised at how well a lot of it held up. Some of the pieces I wrote in the early days are a surprisingly good match to stuff I've written recently on the same topics. Which either means that I've always been brilliant, or that I've plateaued as a writer, I'm not sure which.

The tagline of the blog from the very beginning has been "Physics, Politics, Pop Culture," so I'll use those as headings to organize the recap of noteworthy posts.

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Under the guidance of some of the top astronomy experts in the country, explore our amazing Universe - including up close views of the Earth's moon, Jupiter and other mysterious planetary objects - at the Stargazing Party, an exciting educational collaboration between the Festival, the Smithsonian's National Air & Space Museum (NASM), telescope manufacturer Celestron and other partners, on Saturday, April 28 at NASM in Washington, DC.

A hit with visitors at the inaugural Festival in 2010, the Stargazing Party is returning to the Festival Expo with an equally impressive lineup of evening celestial activities which include Bill Nye the Science Guy in a live recorded broadcast by Planetary Radio with host Mat Kaplan who will inspire other young astronomers to make their own unbelievable discoveries!

In addition, don't miss presentations by such prominent astronomy educators as Drs. Jeffrey Bennett and Jeff Goldstein who will give walking tours of the celebrated Voyage Scale Solar Model System located just outside the NASM and later discuss other fascinating facets of our quest to explore the universe.

Celestron will add hand-on excitement to this night with telescopes set up around NASM's Public Observatory to accommodate visitors' celestial viewing enjoyment.

Event Details:
April 28, 2012- 06:30 PM to 10:30 PM
Smithsonian Institution's National Air and Space Museum on the National Mall

6:30 pm - 7:45 pm: Walking Tours of the Voyage Scale Model Solar System (space is limited, sign up for one of the 3 thirty-minute tours at the museums info desk when you arrive)
7:30 pm: Telescopes ready for viewing around NASM's Public Observatory
7:45 pm: Doors open for Stargazing Party!
8:00 pm: Program in Moving Beyond Earth gallery, including welcome by Bill Nye the Science Guy and remarks by Celestron, and the Planetary Radio Live program
8:10 - 10:20 pm: Check out "Scale of the Universe" and "Human Exploration: the Journey Continues" in Milestones of Flight, plus "Forces of Flight" in How Things Fly, and look for hands-on Discovery Stations throughout the first floor of the Museum!

For more information on this free event, visit this link. And keep updated on other exciting Festival activities taking place in the month of April (including Expo Weekend) here.

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In it's increasingly bizarre need to inflict it's animal rights morality on everybody, PETA's Ingrid Newkirk has criticized Jennifer Lawrence for scenes in Winter's Bone and the Hunger Games, which show her hunting and eating animals.

The actress was dubbed "the coolest chick in Hollywood" by Rolling Stone, and in the magazine's latest issue she recounts her on-screen squirrel-skinning scene in the 2010 movie "Winter's Bone."

"I should say it wasn't real, for PETA. But screw PETA," she told the magazine.

In response to the actress's comment, PETA president Ingrid Newkirk told Gothamist, "[Lawrence] is young and the plight of animals somehow hasn't yet touched her heart. As Henry David Thoreau said, 'The squirrel you kill in jest, dies in earnest.' We are told that this squirrel was hit by a car, but when people kill animals, it is the animals who are 'screwed,' not PETA, and one day I hope she will try to make up for any pain she might have caused any animal who did nothing but try to eke out a humble existence in nature."

Gag me with a spoon. Lawrence's initial instincts were correct. Screw PETA. In these scenes and movies characters are grappling with survival in the face of starvation and poverty. PETA seems to think the appropriate ending for Katniss would have been a moral vegan death from starvation in district 12 rather than being a life-affirming, kickass hunter. And I guess Ree should have morally died from exposure in the Ozarks. The producers bought her a squirrel from a local hunter, and she realistically portrayed the skinning of an animal by hunters for food. I think what really upsets PETA about these portrayals is that they realistically show what humans will do to survive, that hunting and eating animals comes naturally to us, and there's nothing wrong with hunting for food.

Let's hope Lawrence doesn't back down, for some reason I think she won't:

The actress, who spent a month in Missouri with a rural family learning to shoot rifles and chop wood in preparation for "Winter's Bone," and was trained by four-time Olympic archer Khatuna Lorig for her role as Katniss in "The Hunger Games," also told Rolling Stone, that when she is done with her next movie she is "thinking about buying a house. And a big dog. And a shotgun."

I'm liking her more and more.

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I'm re-instituting the quota system for the moment-- no blogging until I make some substantive progress on the current work-in-progress-- but I'll throw out a quick post here to note a media appearance: Physics World has a podcast about books on quantum physics up today:

Since its inception in the early part of the 20th century, the theory of quantum mechanics has consistently baffled many of the great physicists of our time. But while the ideas of quantum physics are challenging and notoriously weird, they seem to capture the public imagination and hold an enduring appeal. Evidence of this comes in part from the numerous popular-science books that have been written on the topic over the years. This episode in the Physics World books podcast series looks at the popularity of quantum mechanics in science writing.

This features some comments from a telephone interview I did with them, about why I thought it was worth writing yet another book about quantum physics. They also talked with Marcus Chown and Robert Crease, and go a little bit into the great Brian Cox argument.

So, if you've got 20 minutes to kill, check it out.

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