Science Blogs Physcial Sciences

Syndicate content
ScienceBlogs posts about Physical Science
Updated: 1 year 48 weeks ago

Calling London: Teaching Relativity to Your Dog on the Radio [Uncertain Principles]

6 March, 2012 - 17:50

sidebar_relativity_cover.jpgThe new book is out, which means it's time for lots of promotional efforts and links to radio shows and news articles and that sort of thing. Such as this one: I'll be talking about relativity and dog physics tomorrow night, Wednesday the 7th, on the Big Science radio program(me) at 9pm London time (in the frame of reference in which London is at rest, anyway). This'll be the first radio show for the new book, though I've done a few phone interviews for print publications (links as they become available...).

If you're in London, and have nothing better to do, tune in. (We are, after all, more popular than call girls in London...) Or listen via the Internet. Or don't. It's all good.

Read the comments on this post...

Categories: Education

A Brief History of Quantum Timekeeping [Uncertain Principles]

6 March, 2012 - 16:41

My course on the history and science of timekeeping has reached the home stretch, with students giving presentations in class for the remainder of the term. My portion of the course was wrapped up with two lectures on "quantum timkeeping," as it were: a lecture on the development of quantum mechanics:

History of Quantum Mechanics View more PowerPoint from Chad Orzel

And one on the development of atomic clocks:

A History of Atomic Clocks View more PowerPoint from Chad Orzel

These are pretty fast-moving, but by this point in the course, students were already working on their final projects, so these are mostly cultural sorts of presentations. The idea is to give them a bit of the flavor of quantum physics and how it plays into timekeeping, not for them to be able to solve problems relating to any of these topics.

Read the rest of this post... | Read the comments on this post...

Categories: Education

A Cosmic Smashup and a Dark Matter Mystery? [Starts With A Bang]

6 March, 2012 - 03:31

"I soon became convinced... that all the theorizing would be empty brain exercise and therefore a waste of time unless one first ascertained what the population of the Universe really consists of." -Fritz Zwicky You very likely know that there are four fundamental forces in the Universe: gravity, electromagnetism, and the weak and strong nuclear forces. While only some particles experience the nuclear and electromagnetic forces, anything with mass or energy -- which is everything we know of -- is subject to gravity.


(Image credit: CountInfinity by Ananth.)

The strong nuclear force binds all the nuclei heavier than hydrogen together, the weak force is responsible for radioactive decays and the (incredibly rare) neutrino interactions, but the two you're most familiar with are gravity and electromagnetism. Operating practically everywhere in the Universe, the two of them are the reason you can sit where you are right now as you read this.


(Image credit: SPH4C Physics.)

The reason that you are safely anchored down to the ground is the force of gravity, accelerating everything at Earth's surface downwards at 9.8 m/s2. But you are (most likely) not accelerating downwards, and that's because your body is composed of these electromagnetically-interacting particles -- atoms -- that the floor/ground/chair pushes back up on you at with an equal and opposite force to gravity.

The net result is that the electromagnetic force and the gravitational force cancel out, and so you remain where you are. When you toss a ball, it makes a parabola, as it gets accelerated down due to gravity. But once the ball hits the ground, it interacts electromagnetically, preventing it from falling through the floor. But imagine, hypothetically, that you instead tossed a neutrino, an object which doesn't interact electromagnetically? What happens then?


(Image credit: Donald E. Simanek.)

Because the neutrino doesn't interact electromagnetically or via the strong force, but does interact gravitationally, it would follow the same parabolic path that the ball did, until it ran into the surface of the Earth. At that moment, while the atoms in the Earth would collide with the atoms in the ball (and that's an electromagnetic interaction), because neutrinos don't interact electromagnetically, the neutrino would simply pass through the Earth's atoms, as though they weren't even there. It would plummet in an orbit -- a nearly perfect ellipse (nearly, because the Earth isn't a point mass concentrated at its very core) -- and would eventually return to its starting point. This is the same way a comet has an elongated, elliptical orbit with the Sun at one focus of the ellipse: due to the force of gravity alone.

The lack of strong and electromagnetic interactions, a vital characteristic of neutrinos, is also a characteristic of dark matter! What would happen if we took two giant blobs of matter -- a mix of dark and normal matter -- and let them go in space? What would happen?


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

Shown above is the results from such a simulation. In pink is the normal matter, made up of atoms, while in blue is dark matter. Initially, both the dark and normal matter move together, accelerating towards the other blob. When they first collide, the atoms smash into one another, slowing down and heating up! But collisions are electromagnetic interactions, so dark matter doesn't do it. Instead, the dark matter passes right through everything: through the normal matter in its own blob, through the other blob's dark matter, through the other blob's normal matter. So what we should see, shortly after a collision like this between, say two galaxy clusters in space, should be a separation in space between the dark matter (observable through the effect of weak gravitational lensing) and the normal matter (observable through the hot X-ray emissions that should come from the colliding gas).


(Image credit: X-ray, Optical, Lensing composite, courtesy NASA / CXC / CfA / M.Markevitch et al. / STScI / Magellan / U. Arizona / ESO WFI / D.Clowe et al.)

These colliding clusters -- known as the Bullet Cluster -- strongly support a picture where galaxy clusters are made up predominantly of a large, diffuse halo of dark matter with a much smaller amount of normal matter in the form of collapsed, star-containing structures and gas.

We can then go back to the beginning of the Universe and simulate how galaxies should form in a Universe filled with dark matter, dark energy, and normal matter in the proportions we think are present.


(Image credit: V. Springel et al., for the Millenium Simulation, retrieved here.)

And we can compare it to the galaxies that are actually there in the observed Universe, and see how well the simulations agree with the data!


(Image credit: 2dF Galaxy Redshift Survey.)

The answer is extremely well, of course, but we don't merely rely on a visual inspection. Rather, we do our analysis quantitatively, and see what the best fit cosmological model is to this data. What do we find?


(Image credit: Cole et al. (2005), for the 2dFGRS.)

A Universe, dominated by dark energy, where 17% of the matter is normal matter and 83% is dark matter. The "wiggles" you see in the power spectrum, above, come from normal (baryonic / atomic) matter, which would go all the way down to the bottom of the graph were there no dark matter. This is one of the strongest arguments against a Universe without dark matter, and has been made even stronger in recent years by better observations by the Sloan Digital Sky Survey.

But the Bullet Cluster is not the only cosmic smash-up between galaxy clusters in the Universe, although it might be the simplest, earliest-stage one we've observed. Things are much less easy to decipher in, say, cluster Abell 520.


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

With the X-ray and lensing data clearly more complicated than in the Bullet Cluster, it is difficult to piece together exactly what's going on here. Perhaps this is an intermediate stage in a cluster merger? Perhaps something unusually violent is happening here? Or, spectacularly, perhaps dark matter is not behaving the way all other indicators are pointing?

The above image was released in 2007, and what you want to make sure to do is check that your observational data is solid. So they went back, got to use the Hubble Space Telescope to improve their observations, and got even better data, which was just released a few days ago. What -- in a new set of false-colors -- did they find?


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

What are we looking at here? From the NASA site itself: Starlight from galaxies, derived from observations by the Canada-France-Hawaii Telescope, is colored orange. The green-tinted regions show hot gas, as detected by NASA's Chandra X-ray Observatory. The gas is evidence that a collision took place. The blue-colored areas pinpoint the location of most of the mass in the cluster, which is dominated by dark matter. So in this case, the optical presence of the galaxies (orange), so well-aligned with the dark matter in the Bullet cluster, are independent of both the gas, in green, and the dark / overall matter, dominated by the blue color. Perhaps this is easier to see if we look at each of these components separately.


(Image credit: Same as above, retrieved from CFHT.)

There's some overlap of the dark matter with the luminous galaxies, which ought to move together with the dark matter, but there's also substantial overlap of the dark matter with the hot gas, which is an unexpected surprise!

Does this mean that dark matter's doing something weird? My first instinct -- mostly because the evidence for the standard theory dark matter is so overwhelming from so many different sources -- is to wonder whether this isn't either a later-stage collision, and/or if there isn't enough cool gas mixed in with the central hot gas to obscure a group of galaxies at the center that are actually there, but behind the gas? (This cosmic catastrophe, after all, is about 2.4 billion light-years away, and in three dimensions.) The jury is still out: "This result is a puzzle," said astronomer James Jee of the University of California, Davis, leader of the Hubble study. "Dark matter is not behaving as predicted, and it's not obviously clear what is going on. Theories of galaxy formation and dark matter must explain what we are seeing." If I make a composite of the three separate image components -- luminosity, mass, and X-ray emissions -- without those distracting background galaxies, what do we find?


Something very puzzling is going on here. If this is as young a collision as the Bullet Cluster, it's conceivable that dark matter is doing something very weird. We have very few examples of high-speed galaxy cluster collisions in the Universe, with Abell 520 and the Bullet Cluster being the two best measured ones, and yet they appear -- at first glance -- to be inconsistent with one another!

I'll definitely be following this story to see if there's a resolution, but if it turns out that this is as young a collision as the Bullet Cluster, there are no hiding galaxies, and the current picture of dark matter cannot explain what these galaxy clusters are doing, we may be learning an awful lot more than we bargained for awfully soon. Some are betting that's what will happen; I am by far more cautious, and would still happily bet on the standard picture of dark matter, with perhaps some complex kinematics -- maybe involving multiple mergers -- for the collision. For the time being, I'm content to agree with Ray Sanders, and say that the dark matter core of Abell 520 is mysterious. What's the solution to the mystery? I've made my wager; what's yours?

Read the comments on this post...

Categories: Education

Another Week of GW News, March 4, 2012 [A Few Things Ill Considered]

5 March, 2012 - 17:18

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

Read the rest of this post... | Read the comments on this post...

Categories: Education

Science and Engineering Education: What Happens in the Home Is Just as Important as the Classroom [USA Science and Engineering Festival: The Blog]

5 March, 2012 - 16:00

By Larry Bock
Founder and organizer, USA Science & Engineering Festival

Encouraging and motivating kids early in science and engineering via exciting, hands-on interactions in discovery may be one of the most important steps to boosting their interest and performance in these fields.

But if you think this job falls to teachers alone, you're wrong. Parents can, and should, play an active and frequent role outside the classroom, especially in creative ways that keep children's innate sense of curiosity and exploration alive.

"Kids love the chance to try experiments, visit zoos, or watch science fiction movies," says Pendred Noyce, a physician, education advocate, and children's author. "Parents can keep excitement alive with activities that help kids develop a sense of science mastery, autonomy, and purpose."

Noyce knows well of what she speaks. Besides her aforementioned credentials, she is also a parent, and was herself inspired by her father, Robert Noyce, co-inventor of the integrated circuit (computer chip) and one of the founders of Intel. Today, as president of Tumblehome Learning, Inc., she creates science-related adventure books, biographies and hands-on learning kits for children.

We are proud to have Noyce as a featured author this April at the USA Science & Engineering Festival and Book Fair hosted by Lockheed Martin (the nation's largest celebration of science and engineering). Below, she shares five ways that parents can keep kids interested and motivated in science and engineering discovery:

1. Read great science books together -- fiction and non-fiction. Better yet, start a parent-child science book and activities club. Read about a topic and explore it further through experiments. This is the idea behind Tumblehome Learning -- great science stories coupled with hands-on exploration.

2. Visit as many museums, aquaria, and zoos as possible. Public libraries often provide patrons with free admissions tickets. Look for museum classes and opportunities for kids to become peer "explainers."

3. Create projects together, from birdhouses to burglar alarms. Making real things builds physical intuition, 3D perception, and confidence. A five-year-old girl who knows a flat-head from a Phillips-head screwdriver will dare to imagine a career in engineering.

4. Research summer programs or college outreach days. Better yet, let the kids do the research. The more their autonomy, the deeper their commitment.

5. Become a family of citizen scientists. Explore "Citizen Science" on the Web and join a project on anything from bird migration to protein folding to tracking invasive species. Your kids can experience the authentic excitement of contributing to the march of scientific knowledge.

The USA Science & Engineering Festival, with its exciting hands-on approach to inspiring kids in technology and innovation, will be encapsulating key elements of Noyce's timely advice this spring.

A truly world-class event, the Festival -- known widely for its unforgettable array of technology, innovators, science celebrities, Nobel Laureates, its nationwide satellite events, as well as amazing interactive exhibits and stage shows -- has rapidly evolved into a must-attend gathering for families and others.

The month-long Festival culminates the weekend of April 28-29 with a massive Expo celebration in Washington, DC, replete with over 2,500 interactive exhibits and more than 150 live performances by science celebrities, space explorers, best-selling authors, innovative entrepreneurs and world-renowned experts.

A virtual playground of learning and discovery for kids, the Expo adroitly integrates science and engineering in so many interesting ways that they are sure to inspire parents with "citizen scientists" ideas to explore with their kids at home. What's more, the event is free!

Here are just a few examples of the excitement that awaits families at the Festival Expo:

Meet Science Celebrities: Well-known personalities such as these will be on hand to help science come alive: Bill Nye the Science Guy; real-life neuroscientist Mayim Bialik who stars in the hit TV comedy The Big Bang Theory; Jamie Hyneman and Adam Savage of the MythBusters; Jeff Lieberman, host of Discovery Channel's Time Warp, and science-of-illusion maestro Apollo Robbins.

Award-Winning Authors: In addition to Pendred Noyce, meet and hear prominent and inspiring science authors such as: Homer Hickam, autobiographer of Rocket Boys (the book that formed the basis of the Hollywood movie October Sky); nationally-acclaimed children's science book writers Seymour Simon and Joy Hakim, and science fiction writer Robin Cook.

Tinkerers and Inventors: From demonstrations by MakerBot Industries, Fab Lab DC and others at the Expo (and at the partnering Robot Fest and DIY Pavilion), you'll see and experience the amazing technology that is spurring the DIY, or Do It Yourself, movement. This movement is making inventing easier and more cost effective for the average person, and inspiring future engineers!

Space Heroes and Heroines: Be inspired by such role models as: John Mace Grunsfeld, Ph.D., a five-time Space Shuttle astronaut; electrical engineer Anousheh Ansari, who in 2006 became the first female private space explorer, and legendary computer video game innovator Richard Garriott who became the sixth private citizen to journey into Earth's orbit.

Cutting-Edge Scientists and Engineers: Learn from leading scientists and engineers at the Expo's Career Pavilion about the joys and challenges of their work and the preparation needed. The Pavilion will also give students the chance to investigate these careers on their own as well as colleges, scholarships, internships, mentorships and after-school programs available.

Innovative Entrepreneurs: These are just some of the exciting entrepreneurs you'll meet who are changing the course of technology: Elon Musk, creator of rocket manufacturer SpaceX and co-founder of Pay Pal (the world's largest internet payment system), and George Whitesides, CEO and president of Virgin Galactic, the pioneering U.S.-based space tourism company.

Myriad Stage Performances and Exhibits: And from exciting presentations in music, magic and illusion, comedy, Hollywood movies, comic book superheroes, robotics and other frontiers, you'll discover that science is truly all around us.

It's true: Inspiring the next generation of innovators, as well as future "science citizens," depends as much on what happens outside the classroom with parents as it does inside our schools with teachers.

Join us at the Festival for an unforgettable family experience in science and technology! For more information, visit our website here.

Read the comments on this post...

Categories: Education

Another Week of GW News, February 26, 2012 [A Few Things Ill Considered]

5 March, 2012 - 06:34

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

Read the rest of this post... | Read the comments on this post...

Categories: Education

Another Week of GW News, February 19, 2012 [A Few Things Ill Considered]

5 March, 2012 - 06:24

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

Read the rest of this post... | Read the comments on this post...

Categories: Education

Beyond oil and water not mixing [Greg Laden's Blog]

3 March, 2012 - 14:46

Send these young women into space!

Read the comments on this post...

Categories: Education

Periodic Table Table [Greg Laden's Blog]

1 March, 2012 - 14:41

With elements inside.

This is important if you need to get your dog's ears to lie flat.

Read the comments on this post...

Categories: Education

Remember How Tiny We All Are [Starts With A Bang]

1 March, 2012 - 08:05

"The Earth reminded us of a Christmas tree ornament hanging in the blackness of space. As we got farther and farther away it diminished in size. Finally it shrank to the size of a marble, the most beautiful marble you can imagine." -James Irwin With everything that goes on in this world, from our daily lives to concerns around the globe, it's easy to forget just how vast the Universe is, and how small we all really are. You had so much fun playing with the Interactive Scale of the Universe tool a couple of weeks ago that I had to just give you a few things to ponder. We think of the Earth, our entire world, as a pretty large place. But let's put this in perspective.


(Image credit: Windows to the Universe, Randy Russell, SOHO/ESA/NASA/JPL/U. of Arizona.)

The entirety of our planet, immense in both size and mass, is tiny compared to the Sun. The Sun is 109 times larger in diameter, over 300,000 times more massive, and has sunspots bigger than our planet.

But in the context of the Milky Way, our Sun -- and even the entire Solar System -- pales in comparison.


(Image credit: Brown, Trujillo, and Rabinowitz.)

Our Sun, 1.4 million kilometers in diameter, has a Solar System that extends out beyond Pluto, to a distance of about 140 billion kilometers. That's the distance to Sedna at its farthest point from the Sun, the most distant Solar System object ever discovered. But even that great distance is nothing when placed into the context of our galaxy.


(Image credit: NASA / CXC / M. Weiss, Harvard-Smithsonian CfA.)

Because the distance to Sedna is just 1.5% of one light year, meaning it takes light around five days to go from the Sun to Sedna. But our galaxy is 100,000 light years across, or about ten million times the diameter of our Solar System. And it ought to be. Our galaxy alone contains hundreds of billions of stars, most of which have solar systems not unlike our own.

But compared to the rest of what's out there, our galaxy is terribly insignificant.


(Image credit: Mark Subbarao, Dinoj Surendran, and Randy Landsberg for the SDSS team.)

Although "only" about 250,000 galaxies are shown in the above image, the entire Universe is estimated to have at least hundreds of billions of galaxies, spread out over a spherical region about a million times larger in diameter than our galaxy is. In other words, you and everything you know resides on a tiny, wet rock nearly a million times less massive than the star that powers it, in a solar system one ten-millionth the diameter of our galaxy, which contains at least hundreds of billions of stars not so different from ours, in a Universe filled with hundreds of billions of galaxies, and maybe perhaps more.

You. Are. Tiny.

It also inspired me to dig up this old (2008) video, that helps put into perspective just how big the Universe is. Sometimes, pictures can't do the same justice that a well-put-together video visualization can. And the Universe? It's really, really, really big!

[Some of the numbers you just heard are known to be larger now; we now believe that practically all stars have planets (and there are even planets that have no stars), the number of stars in the Milky Way may be closer to 400 billion, and there may be -- when dwarf galaxies are included -- upwards of one trillion galaxies in the Universe.]

Remember how big this Universe is, and how tiny we all are. But despite all of this, we all get to be a part of it, here, on the most beautiful marble you can imagine.

Read the comments on this post...

Categories: Education

How to Teach Relativity to Your Dog in Nature Physics [Uncertain Principles]

29 February, 2012 - 19:53

The Subject: header pretty much says it all: How to Teach Relativity to Your Dog is reviewed in Nature Physics. I am inordinately pleased with the existence of this-- not because I expect it to sell a significant number of books, but because a serious technical publication recognized it as worth writing up, despite the silly title.

Of course, Nature being Nature, it's paywalled, so you can only read the full thing via the above link if you have institutional access, or know a nice person who will email you a copy when you ask for it on Twitter. The review itself is about what I would expect-- the reviewer, Roger Jones, is a little uncertain about the talking-dog conceit, and dings me for incomplete labeling of some diagrams-- but generally pretty good.

Amusingly, Jones also says that I'm the victim of poor timing, since the publishing schedule didn't allow a more substantial discussion of the OPERA result, which broke while we were in the proof stages of the process. This is funny, because his review is also a victim of timing-- when he wrote it a few weeks ago (presumably), that would've looked like a better criticism of the book than it does after last week's news about problems with the OPERA measurement. (See Matt Strassler's latest update for more detail on the current state of affairs...)

There was a point when I could've made a more substantive change to include additional stuff about the OPERA results, but I elected not to, because the whole thing was too provisional to say much about. I'm fairly happy with that decision at the moment, though if OPERA fixes their problems and still finds the same result, I might tip back the other way. Oh, well. Something for the Second Edition, should I ever be so lucky...

Read the comments on this post...

Categories: Education

How Good Is BookScan Anyway? [Uncertain Principles]

29 February, 2012 - 18:56

One of the big stories in genre Internet news was Seanan McGuire's post last week, about reactions to the early release of some copies of her book, and the hateful garbage thrown her way by people outraged that the ebook didn't slip out early as well. And let me state right up front that the people who wrote her those things are lower than the slime that pond scum scrapes off its shoes. That's absolutely unconscionable behavior, and has no place in civilized society.

That said, Andrew Wheeler picked up on something that also struck me as odd, namely the way McGuire was so upset about paper copies of the book being sold before the release date. Wheeler does a nice job, using numbers from Nielsen BookScan, of showing exactly why this might matter: McGuire's past sales suggest that, if everything broke just right, rapid sales in the first week could put her book on the extended New York Times bestseller list, which is a Big Deal. That would require, however, that her book sell a lot of copies in the first week, which is hurt by having some copies slip out a week early. So there's some reason why she and her publisher should be concerned about the early release.

Of course, this relies on BookScan, which is an imperfect measurement-- Wheeler includes the usual explanation: "BookScan captures, by general consensus, somewhere from 2/3 to 3/4 of the book outlets in the USA." But exactly how good a measure is it? which leads to this graph:


This shows the sales for the trade paperback of How to Teach Physics to Your Dog, normalized so as to obscure the proprietary values, for a period of several weeks. Black circles are numbers from BookScan (which I've used before in modeling sales), green triangles are point-of-sale values provided by Scribner, which capture all the books sold.

This shows more or less what you'd expect: the two track each other pretty well, with the BookScan numbers generally a bit lower (there's one point that's actually higher, which I think happened because I miscopied the number and included some hardcover sales). The big spike in the data is the week before Christmas, with sales almost five times the average of the other weeks.

This is, however, subject to some rather stringent limitations.

Read the rest of this post... | Read the comments on this post...

Categories: Education

The Physics of Leap Day [Starts With A Bang]

29 February, 2012 - 18:30

"When in doubt, make a fool of yourself. There is a microscopically thin line between being brilliantly creative and acting like the most gigantic idiot on earth. So what the hell, leap." -Cynthia Heimel Once every four years, the elusive entity that is today -- February 29th -- comes along. The historical origins and urban legends associated with it are incredibly interesting, but the reason there's any such thing as Leap Day at all is because of the physics of planet Earth.


(Image credit: Mrs. Snyder at the Seven Hills School.)

The Earth, of course, is rotating on its axis while simultaneously revolving around the Sun. Rotation, as we all learn, is responsible for sunrise, sunset, moonrise, moonset, the Coriolis effect, and the rotation of all the stars in the night sky about the poles. Revolution, on the other hand, is responsible for the seasons; when your hemisphere tilted away from the Sun, that's when you have your winter (and minimum daylight), and when your hemisphere is tilted towards the Sun, that's when you have your luminous summer.

And you probably learned that a day is 24 hours, due to the rotation, while a year is 365 days (with an occasional 366 for leap years), taking care of the revolution. It turns out it's a little more complicated than that, so let's dive in!


(Image credit: Larry McNish at RASC Calgary Centre.)

The Earth completes a full rotation in less than 24 hours: 23 hours, 56 minutes and 4.09 seconds, to be more precise. But even though we've spun around a full 360 degrees, we've progressed just a little bit in our orbit around the Sun. If we insisted on using the 23:56:04.09 figure as our day, the Sun would be out at midnight for half the year! To fix the motion of the Earth around the Sun, we need those extra 3 minutes and 56 seconds to orient ourselves correctly. That takes care of what a day is, but what about a year? A revolution -- for the Earth to return to the same position with respect to the Sun -- might be an interesting astronomical thing to mark, it isn't a useful definition for a year on Earth.

In order for the Earth to achieve the same seasonal position in its orbit around the Sun -- and trust me, if you live on Earth, you'll want to mark your calendars by the seasons -- you'll need for the Earth to be oriented the exact same way with respect to the Sun as it was exactly one revolution ago. We could do this from winter solstice to winter solstice, when the Earth's north pole (for me) points maximally away from the Sun, or any other arbitrary point in its orbit. This way of measuring the year, known as the tropical year, is actually a little shorter than the astronomical measurement of a year we might be tempted to make.


(Image credit: Greg Benson at Wikimedia Commons.)

Because the Earth only needs to revolve slightly less than 360 degrees around the Sun to make one tropical year. The difference is tiny -- 359.986 degrees instead of 360 -- but enough to make the tropical year about 20 minutes shorter than the sidereal (or astronomical) year. This difference is known as precession, and it explains why the pole star in the night sky appears to change very slowly over a period of about 26,000 years. (25,771 years, for the sticklers.)


(Image credit: retrieved from Tom's Astro Blog / Marian Ware.)

Combine all three of those effects together -- rotation, revolution, and precession -- and you can answer the question of how many days will it take the Earth to make a tropical year?

The answer, as precisely as we can figure for 2012, is 365.242188931 days. If we just had 365 days in the year every year, we'd be off by nearly a month every century, which is pretty lousy. Putting in a leap year (with an extra day) every 4th year gets us closer, giving us 365.25 days in a year. (This was how we kept time with the Julian Calendar, which we followed for 1,600 years!) Still, this difference was significant enough that, by 1582, we had put in 10 too many days. For this reason, October 5th through October 14th of 1582 never existed in Italy, Poland, Spain and Portugal, with other countries skipping 10 days at a later date. The Gregorian calendar, which we now follow, is exactly the same as the Julian calendar, except instead of having a leap year if your year is divisible by 4 (as 2012 is), you don't get a leap year on the turn-of-the-century unless your year is also divisible by 400! So even though 2,000 was a leap year, 1,900 wasn't and 2,100 won't be, but 2,400 will be again. When did your country make the switch?


The adoption of the Gregorian calendar gives us a calendar of with -- over time -- 365.2425 days in the year. In comparison with the present, actual figure of 365.242188931 days, it will take over 3,200 years for us to be off by a single day, which is certainly good enough for a little while.

But if we want to be planning for the long term, we shouldn't simply be thinking about this difference. We should be thinking about the fact that the Earth's rotation rate is changing, and over long enough amounts of time, so should our definition of what a "day" is!

What am I talking about? Two things happen that change the Earth's rotation rate, and they push the day in opposite directions.


(Image credit: USGS.)

Every time we have an earthquake, that's mass inside the Earth rearranging itself so that -- by the conservation of angular momentum -- its rotation speeds up a little bit. For instance, last year's Japanese earthquake shortened the day by 1.8 microseconds, and the 9.1 Sumatra earthquake in 2004 shortened the day by 6.8 microseconds. On the other hand, there are two bodies out there with large gravitational effects on the Earth!


(Image credit: Rick Taylor.)

The Sun and the Moon both exert gravitational pulls on the Earth, all while the Earth itself rotates. If the Earth were just a point in space, this wouldn't matter; the Earth would make its elliptical orbit around the Sun, the Earth-Moon system would orbit their center of mass, and nothing would change. But because the Earth is a sphere, both the Sun and the Moon exert greater gravitational pulls on the side of Earth that's closer to them than on the side that's farther away.


(Image credit: the COMET program. Registration required.)

Throw in the Earth's rotation, and you not only get tides, you also get tidal braking, which causes the Earth's rotation to slow down!


(Image credit: Purdue University.)

The slow-down is small but pretty consistent, at an average of 14 microseconds per year, a much larger effect than the speedup due to earthquakes. And over geological times, this really adds up! If we go back to the daily patterns left in the soil from the tides -- known as tidal rhythmites -- we can calculate what the period of Earth's rotation was from it.


(Image credit: Touchet formation by Williamborg.)

If we look at the most ancient one we know of on Earth, from 620 million years ago, we find that a day back then was a little under 22 hours long!

If you extrapolate this tidal braking back to when the Earth was first formed, 4.5 billion years ago, you'll find that a day was originally only around 23,000 seconds, or six-and-a-half hours!


(Image credit: Primeval Earth by Don Dixon.)

And the best part about this is that the Earth continues to slow down! Every 18 months or so, because of the difference between 86,400 seconds and an actual day, we add an extra leap second to our clocks (for now). Wait around for around four million years or so, and the day will lengthen by about 56 seconds, enough that we won't even want leap year anymore; a year will have exactly 365 Earth days!

So appreciate this leap day and our attention to detail to getting the Earth's seasons to remain constant from year-to-year, but also be aware that our Earth, however imperceptibly, means that these leap days, too, shall pass.

Read the comments on this post...

Categories: Education

The Most Distant, Dark Galaxy Ever Found! [Starts With A Bang]

29 February, 2012 - 01:36

"One mustn't look at the abyss, because there is at the bottom an inexpressible charm which attracts us." -Gustave Flaubert The deepest depths of space, out beyond our atmosphere, our Solar System, and even our galaxy, hold the richness of the great Universe beyond. Stretching for billions of light years in every direction, there are structures large and small, dense and sparse, everywhere we've ever dared to look.


(Image credit: R. Jay GaBany,

In addition to the visible, luminous matter we see in the image above, there's both non-luminous normal matter and dark matter. The non-luminous matter is made out of protons, neutrons, and electrons, but doesn't emit light. This includes things such as gas, dust, planets, and astrophysicists: in other words, most normal matter in the Universe. But when we take everything we know about normal matter, including how much there is of it, how its pulls together under the force of gravity throughout the Universe, we find that there needs to be about five times as much dark matter as all the normal matter combined.

One of the easiest ways to figure this out and measure it is by looking at some chance locations in the Universe where there are two massive structures directly lined up, one-behind-the-other, relative to our line-of-sight.


(Image credit: ESA, NASA, K. Sharon and E. Ofek.)

Above is what happens when you have a galaxy cluster with both a quasar and a background galaxy directly behind it. This result -- of multiple images and/or distorted, arcing appearances of the background source(s) -- comes about because of gravitational lensing. This intervening mass bends and magnifies the light from the background source, allowing us to see incredibly distant objects that would be otherwise invisible.

It works the other way, too. From the light that we observe from these background objects, we can infer all sorts of things -- like the mass and how it's distributed -- of the intervening, foreground object, as well as how perfectly/imperfectly it's aligned with the background ones.

The results are often breathtaking and, at least to me, always spectacular.


(Image credit: ESA/Hubble & NASA, retrieved from APOD.)

The "horseshoe" above represents a nearly perfect alignment of two sources. Almost perfect, but not quite. If the background and foreground sources were perfectly aligned, the background source would be bend into a uniform-brightness circle -- a great cosmic rarity -- known as an Einstein ring.

If you could zoom into a nearly-but-not-quite-perfect Einstein ring that was lensed by a black hole, the sight would surely, for as long as you remained an intact being, blow your mind. For what you'd find would be an infinite sequence of these rings, progressively decreasing in brightness, as you approached the event horizon.


(Image credit: Public Domain image, retrieved from Wikipedia.)

But I digress. These Einstein rings are lensed by galaxies, not black holes. The circles they make are never exactly perfect, but some come close. In particular, here's a (falsely-colored) image of one that was recently discovered. This one is particularly interesting for the sheer distances involved: the foreground galaxy -- the one doing the lensing -- is a luminous red galaxy located 9.8 billion light years away. But the background galaxy, the one bent into the ring, is an even more spectacular 17.3 billion light years away. And as you'll notice, it forms a nearly, but not quite, perfect ring.


(Image credit: S. Vegetti et al., retrieved from

This system, known as JVAS B1938+666, is much more than just a pretty ring of nearly-perfectly aligned galaxies. You remember, when you form a ring like this, one of the things you'll learn is how the foreground mass is distributed. In addition to the central, luminous red galaxy, there's also a dark concentration of mass, a bit off from the center, of about two hundred million Suns.

Incorrectly reported by many as a purely dark matter galaxy, this is simply a standard dwarf galaxy, but it's so far away that the light from its stars are insufficiently bright to be seen, even with a ten-meter telescope!


(Image credit: David Lagattuta / W. M. Keck Observatory.)

So what's really going on here? You can take a look at the full paper (S. Vegetti et al., 2012) for yourself, but let's break it down in simple terms, and (hopefully) clear up the confusion surrounding this. Almost everywhere in the Universe, the structure you form is about 80-85% dark matter and 15-20% normal matter.

Everywhere. In our galaxy, in galaxy clusters, even in superclusters on the largest visible scales. In these large objects, the gravitational forces are huge, and the gas, dust, and all the forms of normal matter stay bound to their parent object, no matter what you do to it.


(Image credit: NASA, ESA, and A. Aloisi (ESA & STScI).)

But in dwarf galaxies, the little guys, large bursts of star formation can be so powerful that they can eject normal matter out of the galaxy itself! This allows them to, over time, become galaxies that are even more dark-matter-dominated than other, more massive objects in the Universe. In the image above, I Zwicky 18 is full of young stars, indicating an intense burst of star formation that's no more than 500 million years old. There are older stars in there, too, which are more like 10 billion years old, but this latest burst, as perhaps the image below shows even more clearly, will turn this galaxy into an even more strongly dark-matter-dominated object in the future.


(Image credit: Centro de Astrofísica da Universidade do Porto, retrieved from here.)

As this intense burst of star formation happens, bright, hot, young stars form, burn brightly, and die in spectacular supernova explosions. The radiation and outward flux from these objects can heat up and energize the normal, non-luminous matter so thoroughly that it can achieve escape velocity, kicking it out of the dwarf galaxy! What gets left behind are the relic, long-lived stars, the old stellar corpses, and the dark matter.

But what's really important here is that this is exactly what we know should happen! Despite what you may have read elsewhere, there's no reason to believe these objects are 100% dark matter, and there's definitely no reason to believe these observations pose a problem for structure formation. Some people argue that dark matter simulations predict a different density of dwarf galaxies than our Local Group has, and therefore dark matter is wrong. But we have to look at the entire Universe! This one fact does not mean that our Local Group is a good representation of the rest of the Universe; in fact we already know many ways in which it is definitely not! So we go to the paper itself, wherein they've found the most distant dwarf galaxy ever, which says: Our results are consistent with the predictions from cold dark matter simulations at the 95 per cent confidence level, and therefore agree with the view that galaxies formed hierarchically in a Universe composed of cold dark matter. And this is as close to a definite (12-σ significance!) detection as one could get: see for yourself!


(Image credit: S. Vegetti et al., Figure 3 in the pdf.)

You're always going to make waves by claiming that you've done something sensational, like disproven dark matter or found an object made out of 100% dark matter and 0% normal matter, but that's not what we have here.

We've just discovered, at nearly 10 billion light years away, the most distant low-mass, dwarf galaxy in the Universe. And that should be impressive enough; do you know how hard these things are to find?


(Image credit: NASA, ESA, and The Hubble Heritage Team (STScI/AURA).)

The closest galaxy to us, the Sagittarius Dwarf Galaxy, is about the same size: 1-200 million solar masses, and it took the freakin' Hubble Space Telescope to take this picture of it! Considering it's only seventy thousand light years away, maybe we can cut ourselves some slack for not being able to see the starlight from its twin ten billion light years away!

And despite the great cosmic distance, we are still able to find it, all thanks to gravitational lensing. Are you not impressed?

Read the comments on this post...

Categories: Education

The MAA Reviews the BSB [EvolutionBlog]

28 February, 2012 - 23:01

The Mathematical Association of America has now posted a thorough review of the Big Sudoku Book. The review is by Mark Hunacek, of Iowa State University. His verdict:

This is a delightful book which I thoroughly enjoyed reading. In some sense it is hardly surprising that I would enjoy it, since I enjoy Sudoku, mathematics and good writing, and this book combines all three of these. However, I doubt anyone needs to enjoy all three to enjoy this book; a person with very limited background in mathematics, or a person without much experience solving Sudoku puzzles, could still find something of interest here. While it is not intended as a text for any kind of traditional course in mathematics, I did keep thinking, as I read it, that it would be fun to offer an honor's seminar based on it. Finally, I should also mention that it is quite reasonably priced: it is (as of this writing, at least) available for sale for just over twenty dollars, with a Kindle edition available for ten. I would, however, enthusiastically recommend this book even if it cost more.


Read the rest of this post... | Read the comments on this post...

Categories: Education

How to Teach Relativity to Your Dog: The Big Idea [Uncertain Principles]

28 February, 2012 - 17:23

I'm home with The Pip today, so no extended typing for me, but I pre[ared for this by typing something up ahead of time, and getting John Scalzi to post it for me, as part of his Big Idea series:

In a way, a book about Einstein's theory of relativity is uniquely suited to a series about Big Ideas. Relativity, at its heart, is a theory built on a single Big Idea:

The laws of physics do not depend on how you're moving.

For all its fearsome reputation, everything stems from that single,simple idea. Whether you're moving or standing still, floating in space or on the surface of a planet, you will see the laws of physics work in exactly the same way.

There's a bunch more, but you can read it over at John's blog. And if you're sold on that, well, How to Teach Relativity to Your Dog is released today, so you can get it immediately.

Read the comments on this post...

Categories: Education

How to Teach Relativity to Your Dog is Officially Out Today [Uncertain Principles]

28 February, 2012 - 16:11

The Pip says, "Hi, folks. My daddy's book is released today, and he's shameless enough to use me to promote it:"


"I can't read it yet, because I'm just a baby, but I can report that it was very satisfying to drool on. So you should definitely buy a copy, maybe two."

"Also, dig the awesome stuffed alligator toy I got from my Aunt Erin and Aunt 'Stasia. It crackles, and it has a mirror! It's so cool!"

Read the comments on this post...

Categories: Education

My First Podcast! [EvolutionBlog]

28 February, 2012 - 05:38

If you have a few minutes to kill, go check out this podcast over at Sol Lederman's website “Wild About Math.” Laura Taalman and I discuss the BSB (by which I mean the Big Sudoku Book). We talk Sudoku, math, education and plenty of other stuff. We originally planned on talking for thirty minutes. but everything went so well that we ended up hanging around for fifty. So go enjoy and let me know what you think!

Read the comments on this post...

Categories: Education

How to Teach Relativity to Your Dog in the Washington Post [Uncertain Principles]

28 February, 2012 - 01:43

If you're allergic to hype, you might want to tune this blog out for the next couple of days, because How to Teach Relativity to Your Dog is officially released tomorrow, so it's all I'm going to talk about for a little while. Because, well, I'm pretty excited.

And tonight's exciting finding is that it's mentioned in the Washington Post:

If "Physics for Dummies" left you baffled, maybe it's time to go a step further: Why not physics for pets? In "How to Teach Relativity to Your Dog," physics professor Chad Orzel attempts to explain Einstein's theory of relativity via a dialogue with his dog, Emmy. Orzel breaks down complex concepts -- time dilation, relative motion, black holes, the big bang -- by applying their physics to canine-friendly situations, like chasing rabbits and determining whether a dog can eat enough kibble to run at the speed of light. Rather than barking or growling, Emmy leavens the mood with requests for walks; and when the academics get heavy, she interjects to beg for clarification. Obviously, real-life dogs will not walk away from the book with a grasp of the universe's mechanics, but the human sort of non-scientist can get some benefit.

OK, that one paragraph is the sum total of the mention, and I'm not sure if that's in the print edition or just the web site (then again, who reads print newspapers any more?), but, dude! The Washington Post acknowledged my book! Woo-hoo!

"All right!," says Emmy. "The Post represents the conventional wisdom of political elites, so if they say nice things about our book, it's only a matter of time before Congress dishes me some of that sweet, sweet pork I keep hearing about. Mmmmm.... pork...."

Read the comments on this post...

Categories: Education

Ground Resonance Shakes Apart Chopper? [Greg Laden's Blog]

28 February, 2012 - 00:30

I know many of you enjoy discussing resonance. Check this out:

Read the comments on this post...

Categories: Education

WebElements: the periodic table on the WWW []

Copyright 1993-2011 Mark Winter [The University of Sheffield and WebElements Ltd, UK]. All rights reserved.