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Jon Hiller
60,738 followers -
Electron Microscopist, Materials Scientist, Father, Son and Brother.
Electron Microscopist, Materials Scientist, Father, Son and Brother.

60,738 followers
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The Dark Matter Series

Thanks to everyone who plussed, shared, commented etc.  Here's the complete series in one place:

http://goo.gl/xpvDZX - Introduction
http://goo.gl/jGvBjw - The old model doesn't work
http://goo.gl/lAKQMX - Alternative gravity models don't work
http://goo.gl/0ee7dc - The dark matter model does work
http://goo.gl/f7Tzdk - Known dark matter isn't enough
http://goo.gl/nF1AcZ - We already know quite a bit

On a related note, several people have pointed out that G+ doesn't make it easy to catch every post, which is a particular problem when I do a connected series of posts.  I have several ideas on how to make following posts easier:

1. Tweet when I have a new post.  I do this already, but not everyone on G+ has or wants a Twitter account.  If you want to follow me, I'm @briankoberlein.

2. Edit posts so that there is a previously/next on every post.  That way if you find one, you can click through to others.

3. Make an RSS feed of my G+ posts.  There is apparently a way to do this, and people could have added it to their google reader accounts.

4. I could make a blog and post things on G+ and the blog.  The downside is you guys would have to help me find a name for it.

5. I could group posts into ebooks or something similar. 

Just to be clear, don't plan on shifting away from Google+.  There's a strong community here, and I plan on posting on G+ just as I have been for the foreseeable future.  But I also realize my posts have become very blog-like, and I'd like to make posts easier to follow if I can.

I'd love to hear your ideas/preferences/opposition.  

Image by +Keith Lohse  (http://goo.gl/Ogh79h).
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The Science on Google+ Community is getting ready to cross the 100K followers mark!!! Thanks for your support. To celebrate, we’re getting ready to make some structural changes to the community to increase engagement and to make it easier to find high quality posts. We will also be launching a new Science Hangout On Air series. Stay tuned!!

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Sorry Science Peeps, I've been off the radar for a bit.  Let's revisit this lesson :)
#scienceeveryday  when it's not #sciencesunday  
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Study reveals potential vaccination against HIV!

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Some great science stuff to do with your leftover Easter candy!

#scienceeveryday when it's not #sciencesunday

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Thanks to everyone who gave a +1 and/or a share to the series on six equations to live by.  I'm glad you liked them.  If you missed any of them here's the entire list:

Proven World: http://goo.gl/8P8bd (Introduction)
A Muse of Fire: http://goo.gl/YTwaK (How mass and energy are connected)
Mutual Attraction: http://goo.gl/YTwaK (Newton's gravity)
Time After Time: http://goo.gl/liQk0 (Special relativity and relative time)
Unity: http://goo.gl/kU1qh (Electricity, magnetism and light)
Memory Hole: http://goo.gl/fBZNc (Black hole information paradox)
Dying of the Light: http://goo.gl/yq0WD (Entropy and the end of time)

I'll likely do another series at some point, but starting tomorrow I'll go back to regular posts on astrophysics, physics and astronomy.  I have a few ideas for posts, but I'd also like to hear what you'd be interested in.  

If there's a topic you'd like me to write about, just add it in the comments.  
If you see a suggestion you like, give the comment a +1.  

And if you're willing, give this post a share and encourage your friends to add me to their circles.  There seems to be a lot of interest in my recent posts, and the more attention they get, the more motivation I have to keep writing them.

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It's outreach but also training our replacements.

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Cavitation Bubble Collapse:

To get a molecular-level understanding of nanobubble collapse near a solid surface, Priya Vashishta and his colleagues at the University of Southern California used supercomputers to simulate and unravel the complex mechanochemistry problem. The goal of this nanobubble collapse simulation, which was run on 163,840 cores, was to improve both the safety and longevity of nuclear reactors.

Science contributors:
Priya Vashishta, University of Southern California
Ken-ichi Nomura, University of Southern California
Adarsh Shekhar, University of Southern California

Visualization contributor:
Joseph A. Insley, Argonne

#scienceeveryday  when it's not #sciencesunday  
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Part 3:  Time After Time

Imagine you are travelling in a train.  If you were to walk down the aisle of the train, you would be moving at a walking pace relative to the other passengers, but someone watching the train go by would see you and all the other passengers race by at great speed.  In other words, your speed is relative.  It depends on what you are measuring it against.  Relative to another passenger your speed is slow, but relative to the ground your speed is fast.  That, in a nutshell, is relativity.

This concept of relativity dates back at least as far as Galileo (which is why it is sometimes called Galilean relativity).  Before Galileo’s time it may have been known, but it wasn’t a big deal because motion could always be measured relative to the fixed Earth.  But as we learned the Earth moves around the Sun, this raised an interesting philosophical puzzle.  Is there some great cosmic vantage point against which all speeds can be measured, or is it really the case that speed is always relative?  Is there such a thing as absolute speed?  

In the mid-1800s, physicists came to understand that light was a wave.  At the time it was thought that all waves travel through a medium.  Sound waves travel through air, water waves travel through water, and so on.  That means there must be a medium through which light travels.  Physicists couldn’t observe this medium, but they called it the luminiferous (light-bearing) ether.  There soon began a hunt to observe the ether, because the ether was a way to measure absolute speed.

If you drop a pebble in a calm lake, you can see the ripples flow outward at a particular speed.  The ripples flow with the same speed in every direction.  But if you were moving in a boat and dropped a pebble into the water, the ripples would seem to move slower in the direction of the boat’s motion, and faster in the opposite direction.  Because of the boat’s motion the speed of the ripples would be different in different directions.  The same would be true with the ether.  Since the Earth must be moving through the ether, the speed of light must be different in different directions.  

In 1887, Albert Michelson and Edward Morley performed an experiment to measure this difference in the speed of light.  But what they found was the speed of light was always the same.  No matter what direction light travelled, no matter how they oriented their experiment, the speed of light never changed.  This was not only surprising, it violated the principle of relativity.  After all, if you stand on a moving train and toss a ball, the speed of the ball relative to the ground is the speed of the ball plus the speed of the train, not just the speed of the ball.  Basically what Michelson and Morley found was that if your “ball” was light, the speed of your ball relative to the  train and the speed of the ball relative to the ground is the same.  It seemed the speed of light (and only the speed of light) is absolute, and this made no sense at all.

Then in 1905 Albert Einstein published a solution to the problem, known as special relativity.  He demonstrated that if the speed of light is absolute, then time must be relative, as given in the equation below.  It relates the different times of two observers, say you and me.  In this case, T is your time as you measure it, T’ is your time as I measure it, V is your speed relative to me, and C is the speed of light.  What it says is that your time appears slower to me than it does to you.  The faster you move relative to me, the slower your time appears to me.  This sounds insane.  How can time be relative?  It is, however, very real.    

We can see how this works if we imagine a clock made with light.  Take two mirrors and place one above the other and facing each other, then bounce a pulse of light between them.  We can measure time by counting the number of times the light bounces off a mirror.  Each bounce is like the tick or tock of a mechanical clock.  If you could watch the pulse of light, you would see it move up and down between the mirrors at the speed of light.  Up and down at a constant rate.  Now suppose you took your clock on a fast moving train.  Standing in the aisle of the train, you would see the light pulse move up and down at the same rate as before.  Up and down at the speed of light.  

But as I watch you speed past, I see something slightly different.  I would also see the pulse move at the speed of light, but from my view the light can’t move straight up and down because it must also be moving along with you.  I would see the pulse move diagonally up then diagonally down, which is a slightly longer distance between each bounce.  That means it would take the light longer to travel from bounce to bounce.  So from my point of view the ticks and tocks of your clock are slower than the ticks and tocks as you see them.  Your clock appears to be running slow because of your motion relative to me.  The faster you move relative to me, the more your clock will slow down from my point of view.

You might think this effect only occurs because the clock relied on light to tell time, but this effect is real for everything.  If you have a GPS in your phone or car, you rely on relative time being true every time you use it.  A GPS determines your location by receiving signals from satellites orbiting the Earth.  Those satellites broadcast their time and location, which your GPS uses to determine your position, so it is vitally important that the satellites broadcast the proper time.  But the satellites are moving at high speed relative to you, which means their clocks run slightly slow.  To give you the accurate time the satellites have to account for that slowdown effect.  When your phone tells you where the nearest coffee shop is, it’s using special relativity to do it.

So how does all this relate to astrophysics?  It’s one of the ways we know the universe is expanding.  When we observe the light from distant galaxies, the light appears more red than we would expect.    The more distant the galaxies, the more their light is redshifted.  This effect is known as the Doppler effect, and it is due to the fact that the galaxy is moving away from us.  The galaxies are moving away from us because the universe is expanding.  But suppose over long periods of time light just naturally reddens?  How do we know astronomers are not being fooled?

Special relativity tells us we’re not.  We can observe supernovae in nearby and distant galaxies, and what we find is that when a supernova goes off in a distant galaxy it happens more slowly than a supernova in a closer galaxy.  The time of a distant supernova appears slower to us because the distant galaxy is moving away from us at a faster rate than the closer galaxy.

Strange as it is, special relativity works.  Time after time.


Tomorrow:  Flying kites in a thunderstorm leads us to a single elegant theory describing lightning, magnets and light.  Don’t try this at home, just stay tuned for Part 4.
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Part 2:  Mutual Attraction

Medieval astronomy was dominated by the writings of Aristotle.  Aristotle divided motion into earthly lines and heavenly circles, so the planets must surely move about us in perfect circles.  Astronomers soon learned this wasn’t true, but the physics of Aristotle was so deeply rooted in the minds of scholars that astronomers imposed circular motion upon the heavens for a thousand years.  When a single circle could not describe the motion of a planet, they placed circles upon circles (known as epicycles), each rotating just so, to match a planet’s motion.  As more precise measurements of the planets were made, more epicycles were needed.  

Then in the early seventeenth century Johannes Kepler published three simple rules that described the motion of the planets.  They are now known as Kepler’s laws of planetary motion.  Kepler did not use circles to move the planets.  He allowed them to move in a more general shape, known as an ellipse.  What made Kepler’s approach so radical is that an ellipse is neither a circle nor a line.  It is a geometric form that connects the two, unifying earthly and heavenly motion.  Kepler’s theory was the first step toward modern astrophysics, giving us an accurate description of planetary motion.  But Kepler’s laws were still merely a description of motion.  Kepler gave us form, but not function.

It was Isaac Newton who gave us the mechanism.  In the late seventeenth century Newton published his Principia, which described a world governed by a simple set of rules for forces and motion.  The equation below is one of these rules, and is known as Newton’s law of gravity.  In it F represents the force between two bodies (the subscript G just denotes it is a gravitational force), the M’s are the masses of the two bodies, R is the distance between them, and G is a number known as the gravitational constant.  What the equation says is that bodies are drawn to each other through gravitational attraction.  The strength of their attraction is greater if they are close together, and lesser if they are more distant.  This force of attraction exists between any two bodies.  Between Sun and planet, between Earth and moon, and between me and you.

Newton’s triumph was that he could use his rules to explain why the planets moved in ellipses.  They didn’t move in ellipses just because, they were driven to move by forces that followed simple rules.  Rules you could test here on Earth.  It is hard to overstate the effect Newton’s work had on our view of the universe.  At the beginning of the 1600s the universe was one of epicycles and celestial spheres.  By the end that century the universe was driven by fundamental physical laws we could prove and understand.

One thing Newton couldn’t do was determine the value of his gravitational constant.  The only gravitational forces he could observe were between the planets Moon and Sun, and no one had any idea what their masses were.  Without them, the value of G couldn’t be determined.  A solution wasn’t found until 1797 when Henry Cavendish devised a clever experiment.  He placed lead balls in wooden frame suspended by a thin wire that was free to twist.  He then placed larger lead balls near the frame.  By measuring just how much the frame twisted, Cavendish could measure the gravitational attraction between masses, and thereby determine the value of G.  This experiment is now known as the Cavendish experiment, but it could also be called "weighing the heavens."  With the gravitational constant known, astronomers could observe the motions of the Sun and planets to determine their mass.  It is a technique we still use today to measure the mass of stars, planets, and even galaxies.

There is, however, a mysterious consequence of Newton’s equation.  The force of gravity is always attractive, and the closer two bodies are the stronger their attraction.  It would seem then that if large enough masses got close enough together the gravitational attraction would be so strong that the objects would be crushed under their own weight.  Gravity would pull ever stronger, squeezing the objects more and more, making them smaller and smaller until they finally collapsed into a single, infinitely dense point.  A gravitational singularity.

This was such a bizarre idea that astronomers long thought it was impossible.  Surely there must be some unknown physical mechanism that would prevent singularities.  But in the early 1900s, Einstein’s theory of general relativity was confirmed, and the singularity problem became more severe.  In essence Einstein combined Newton’s gravity with relativity.  If you remember from yesterday (http://goo.gl/YTwaK) mass and energy are connected.  This means the energy of gravitational attraction is itself gravitationally attractive.  Put simply, not just mass, but gravity itself is heavy.  Put enough mass in a small enough volume, and it will collapse under its own gravitational weight.  Einstein’s theory made gravitational singularities inevitable.  Near such a singularity the gravitational attraction is so strong that nothing can escape its pull, not even light, which is why they are now known as black holes.

In 1974, radio astronomers discovered an intense energy source at the center of our galaxy.  Named Sagittarius A*, it appeared to be a large black hole.  By the dawn of the twenty-first century, astronomers were able to observe stars orbiting this galactic black hole.  The motions of these stars follow the ellipses of Kepler, driven by Newton’s gravity.  By observing their motions, and with the equation below, we can determine the black hole’s mass (http://goo.gl/MPJNU).  In the center of our galaxy, just 27,000 light years away, is a black hole with a mass of more than four million Suns.

Newton’s equation gave us the mechanism behind the motion of the planets.  It tells how we are connected to everything in the universe through mutual attraction.  

It has also revealed the gravitational dragon that rests at the heart of our galaxy.
   
Next time:   How a beam of light overturned 300 years of physics, and changed our view of the universe.  Part 3, coming tomorrow.
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