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Stephen Granade
Science lecturer, robotics researcher, award-winning interactive fiction author, Disasterpiece Theatre and WhatTheCast podcaster, and occasional programmer. Also the LOLTrek guy.
Science lecturer, robotics researcher, award-winning interactive fiction author, Disasterpiece Theatre and WhatTheCast podcaster, and occasional programmer. Also the LOLTrek guy.


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Prepping a talk on how hard it is to get to space and, once you're there, to maneuver around. I believe this will illustrate things nicely.

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When we talk about the event horizon of a black hole, what do we really mean?
Point of No Return

If you toss a ball up into the air, it will fall back to the ground.  Toss one faster, and the ball will travel higher before returning to the ground.  Of course this raises the question of just how fast you would have to throw a ball for it to never fall back down.  Put another way, could you throw a ball so fast that it would escape the Earth’s gravitational pull forever?

In practice the answer to this question is fairly complicated, because things like air resistance would slow the ball down, and calculating the air resistance of a ball depends on its speed, the density of air, etc.  But we can calculate the speed for the hypothetical case where we only need to overcome gravity.  In that case the answer is really simple.  It is just the square root of the planet’s surface gravity times its diameter.

In the case of the Earth, the surface gravity is about 9.8 m/s, and its diameter is about 12.7 million meters, so if we multiply these numbers together and take the square root, we get a speed of about 11 km/s (or about 25,000 mph).  This is known as the escape velocity.  At that speed, the ball would have enough energy to overcome the Earth’s gravitational pull.  

Since the surface gravity depends on mass and size, you can actually calculate the escape velocity of a (spherical, non-rotating) planet or star simply by knowing its mass and radius.  The escape velocity increases for a larger mass or a smaller radius.  For example, the escape velocity of the Sun is about 620 km/s, but a white dwarf with the same mass has an escape velocity of about 6,000 km/s, since it is about a hundredth the size of the Sun.  

If a mass like the Sun were squeezed smaller and smaller, the escape velocity would get faster and faster.  Of course there is a limit to how fast you can go, which is the speed of light.  If an object had an escape velocity faster than light, then nothing could escape its gravitational pull.  If you think such an object would be a black hole, you’re almost right.

In our hypothetical example, if the escape velocity were the speed of light, we could still toss an object in the air.  If we tossed a ball at a speed approaching the speed of light, it would travel very far from the surface before falling back to the ground.  Relativistic gravity doesn’t work that way, but our simple model actually gives us clues about relativistic gravity.

For any given mass, we can calculate the size we would have to squeeze it down to for the escape velocity to be the speed of light.  For the Sun, we’d need to squeeze it into a sphere with a radius of about 3 kilometers.  This radius is known as the Schwarzschild radius of the Sun.  

For every mass you can calculate a Schwarzschild radius.  This means that the escape velocity can be used to relate the mass of an object to a distance.  In other words, for every mass there is a corresponding length.  General relativity describes gravity as a warping of space, and this connection between mass and Schwarzschild radius shows up again.

In general relativity, the radius of a (non-rotating) black hole is the Schwarzschild radius of its mass.  It’s tempting to say that the escape velocity of a black hole is the speed of light, but that’s not quite how it works.  For a black hole, the Schwarzschild radius defines the surface known as the event horizon.  If you were to cross the event horizon of a black hole, you would be forever trapped.  Space is warped in such a way you can only move inward, never outward.  Once in a black hole you are trapped in an ever shrinking sphere.

Of course this also means that anything crossing the event horizon of a black hole is unable to provide any information to us.  We could never see past a black hole’s event horizon, so we can’t be sure what lies inside.  This is why the “size” of a black hole is often given by its event horizon.  It is not a physical surface, but a surface beyond which we have no information.

It is the point of no return. 

Image:  Artist’s conception of a black hole (

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The first of the new videos for Dragon Con 2013.
It's time to show some new videos for Dragon Con 2013. On a related note, these kaiju battles are making me crave seafood. Anybody else hungry?

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While searching Flickr I found the original source of the Invisible Pommel Horse lolcat.

I'm not sure what's more awesome, stumbling across the 2006 source of the captioned cat picture or recognizing it on sight.

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Scientists were able to tell what letter a person was looking at by scanning their brain.

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As +Carl Zimmer explains, humans can drive evolution -- look at drug-resistant bacteria for one example. But here's something new: it's possible that we're helping some animals evolve larger brains.

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This is what it's like to have your helmet filling with water while you're spacewalking.

This is, bar none, one of the most frightening things I've read in a while. It combines claustrophobia and drowning with the need to move slowly and methodically.

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Meet Kepler-78B
A planet made of lava, with an 8-hour "year."

It's called Kepler-78B, 700 light years from Earth - and about the same size as our planet. One revolution around its star takes 8.5 hours - about 1,000th the time it takes Earth to go around the sun. And because its surface temperature is over 5,000 degrees Fahrenheit, it's one big boiling ocean of lava. Read more:

(Image: Artist's rendition by ESO/L. Calçada)

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CANDELS helped show us how galaxies have evolved throughout time.
CANDELS in the dark

If you look at different galaxies in the universe, you begin to notice that they seem to fall into some broad types.  Edwin Hubble was the first to categorize galaxies into different types, and he divided them into ellipticals, spirals and irregulars.  He then further broke these categories into subgroups, and arranged them into what is now known as Hubble’s Tuning Fork diagram.  I’ve written about Hubble’s initial classification in an earlier post (

When Hubble first proposed the tuning fork diagram, there were some who suggested this might be indicative of the evolution of galaxies, with irregulars collapsing into ellipticals, and ellipticals gradually flattening into spirals.  This type of galactic evolution doesn’t work.  Observations of more distant (and therefore older) galaxies show they aren’t primarily irregular or elliptical.  But this raises a broader question about galactic evolution, specifically about how the tuning fork morphology for galaxies evolved over time.  For this we have the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS).  

The CANDELS survey used data from the Hubble Space Telescope to survey a range of galaxies from the closest to more than 11 billion light years away.  Observing galaxies at increasing distances also means observing galaxies as they were in the past.  So CANDELS looked at galaxies over an 11 billion year period.  Grouping galaxies by age, they could then be assembled into the Hubble morphology.  This allows us to look at how the tuning fork diagram changed over time.

The result can be seen in the image below (  What we find is that the diagram has remained fairly unchanged for the last 11 billion years.  The most distant galaxies are still in the process of forming, but they already have a structure similar to later galaxies of their type.  In astronomer-speak, it seems galaxies matured early on.

But this image is a bit misleading.  Since it is a publicity image meant to emphasize the point of the findings.  The distant galaxies in the image look like modern nearby galaxies, and they are remarkably clear for being 11 billion light years away.  That’s not quite how they actually look.

If you want to see how they really look, and learn more about the details about CANDELS, you can find that here: 

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(This assumes your home has a working particle accelerator producing a proton beam, natch.)
Bad things. Bad things happen when you stick your head in a particle accelerator.
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