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Ethan Siegel
Works at NASA's The Space Place
Attended University of Florida College of Liberal Arts and Sciences
Lived in Bronx, New York
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Ethan Siegel

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"But if the eyes of our telescopes on Earth were especially keen, the exact moments of opposition — where the Sun is directly opposite to Saturn in the skies — would have brought on a spectacular phenomenon: an intense brightening at one particular point, known as the opposition effect."

Saturn was just at opposition this past Saturday, appearing bigger, brighter and less shadowed than at any other time of the year. But have you ever heard of the 'opposition effect'? One of the coolest optical phenomena ever, coming to us courtesy of Cassini!
VideoOnce per orbit, as the Earth completes its annual revolution around the Sun, it also passes and laps the outer worlds, who move so slowly they take decades -- or, in the case of Neptune and Pluto, centuries -- to complete even one orbit around our central star. It's for [...]
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These wonderful probes that carry our senses within range of the planets and moons that give us answers to questions we didn't know to ask - and even more questions...
A front row seat to the exploration of the solar system.
Thank you Ethan.
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“There are 12 men on an island. 11 weigh exactly the same amount, but one of them is slightly lighter or heavier. You must figure out which. The island has no escapes, but there is a see-saw. The exciting catch? You can only use it three times.”

It was the toughest brain teaser of the season on television, resulting in a hilarious #nerdfail for detective Santiago. Here's not just the solution, but all possible solutions, worked out in illustrated form.
The most famous logic puzzle from the best police comedy on television, and how to (finally) solve it!
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A bit more about my reasoning.  The closer the heavier or lighter person is to the outside of the slide, the faster the slide will hit the ground because of the greater bending moment.

If person C, D ,I or J is the odd weight the see-saw will move faster second time around.  If A, G, F or L is the odd weight it will move more slowly.

If D, E, F, G, H or I are the odd weight the see-saw will dip in the opposite direction in the second test. 
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"But the Universe’s dark ages aren’t totally, 100% dark. Sure, there’s no visible light around, but there is a little bit of light that does get created before you ever form a star, and it’s due to one of the simplest structures in all of the Universe: a humble, simple, neutral atom."

In the beginning, there was light. Then it went away. It took 50-to-100 million years for the Universe to make that mistake again; here's the science of what happened in the meantime.
After the CMB, before the first stars, there was nothing to see. Or was there?
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Very interesting, thank you.
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"4.) The Earth’s crust “floats” atop the mantle, and mountains work like floating icebergs: there’s far more crust underneath the mountains than there is in the oceans! Need to account for the atmosphere, or the atmosphere that gets displaced by oceans or mountains? That’s the Free-air correction. Need to account for the fact that there’s “extra mountain” (or any land mass) above sea level? That’s the Bouguer correction.

But what about the fact that the crust is low in density? If you want a mountain sticking up, high above sea level, you’ve got to remember that the crust is atop the mantle, which means the thickest crust of the Earth occurs where the highest mountains are, and the thinnest crust is where the deepest ocean trenches are!"

Counterintuitively, the least amount of mass would be beneath your feet if you climbed up to the top of the highest mountain. Come find out why!
How gravity teaches us that the mountains we see extend far underground.
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+Ethan Siegel, thank you. I look forward to it!
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"Mathematics is in the first line a discipline of thought. Cleaned of the vagueness of human language, mathematics is a tool to deduce consequences from assumptions. It is incorruptible by human fragilities and knows no pity, it’s the gatekeeper of objectivity."

If you’re not a theoretical physicist yourself, you might think that physics is physics — we ask questions about the Universe, do experiments/make observations, and get the answers — and math is just a tool that we use to help us get there. But that really sells the power of mathematics short. For a physical theory to be valid, there are a whole host of mathematical properties that theory needs to possess, including being free of logical inconsistencies, making predictions about observables, and that those predictions agree with observations. Yet when we look at our theory of gravitation at the smallest scales and with the strongest gravitational fields, our theory itself fails, which is precisely why we need a quantum theory of gravity.
Why physicists are compelled to find a quantum theory of gravity.
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+phil priestman  Because uniqueness: yes :)
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Ethan Siegel

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“But this eruption is special, having been going on for hundreds of millions of years, due to the size of the eruption that’s many millions of light years across. If it were caused by accreting matter, it would have had to accrete nearly a billion solar masses worth of material. The combined radio (VLA), visible (Hubble) and X-ray (Chandra) data suggest another interpretation: an ultramassive black hole in excess of 10^10 solar masses powers the outburst.”

What’s causing this monstrosity at the heart of MS 0735.6+7421? Come find out on Mostly Mute Monday.
A galaxy cluster that’s been actively devouring matter for hundreds of millions of years blows all the records away.
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The sheer scale of stellar events in general is difficult to grasp, this... wow!

You can get a rough idea of distance in an image by the population of objects. The further out we look there are fewer stars and more galaxies.
Here, we see only two stars that are within our galaxy, the two with diffraction spikes on the right and left - the rest are external galaxies.
For this AGN to present at this scale at that distance is further than I can reach mentally... the fantastic energy from this galaxy paints space time out to a million light years with its influence - almost half the distance between us and Andromeda.
Thanks for another thought provoking lesson +Ethan Siegel​!
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"I really did get a lot of comments like this (I mean, you can go read them), mostly from people justifying asteroid defense by the “risk assessment” viewpoint argument as follows:

* If we get hit by a civilization-killer asteroid, everything is gone. It’s the biggest catastrophe ever, it’s game over for all of humanity, it’s a hundred million year setback for evolution, and it’s literally the worst-case scenario.
* Therefore, if there’s even a finite probability of it happening, finite probability multiplied by infinite value consequence equals infinite importance, and therefore we should stop it.

But I would dispute both of those charges."

Get your probability / expectation value lesson and a whole lot more on this edition of the comments of the week!
“An asteroid or a supervolcano could certainly destroy us, but we also face risks the dinosaurs never saw: An engineered virus, nuclear war, inadvertent creation of a micro black hole, or some as-yet-unknown technology could spell the end of us.” -Elon Musk We’ve covered a ton this week on Starts With A Bang, ranging from the earliest times…
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Let's remember what science is and what subjective existential vantage is in comparison. Let's try to remember the difference between an excuse and a theory. Let's try for a little adult honesty.
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"Now, the idea that you could be killed by a space rock — or that all of humanity could be wiped out due to one — is no doubt terrifying. But humans are notoriously bad at estimating the risk due to infrequent but catastrophic events, and the only cure for that is quantitative science."

When it comes to risk assessment, there’s one type that humans are notoriously bad at: the very low-frequency but high-consequence risks and rewards. It’s why so many of us are so eager to play the lottery, and simultaneously why we’re catastrophically afraid of ebola and plane crashes, when we’re far more likely to die from something mundane, like getting hit by a truck. One of the examples where science and this type of fear-based fallacy intersect is the science of asteroid strikes. With all we know about asteroids today, here's the actual risk to humanity, and it's much lower than anyone cares to admit.
Sure, they wiped out the dinosaurs, but do they really pose a risk to humans?
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Listen, +Chris Reeve, you speak of checking facts but my profile isn't very long and finding in it this aphorism of my own:

Ambiguities are like microbes: the pathogenic ones steal attention.

should have been sufficient to prevent your impression that your unconscious bias assertion machine matched my pattern of ignorance. 
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"Currently orbiting at a distance of 13,600 kilometers off of the asteroid’s surface, or some forty times more distant than the International Space Station is from Earth’s surface, Dawn will descend over the coming months, mapping out Ceres with a variety of instruments, and learning the elemental composition of this material.

Right now, we can already be sure of some things that it isn’t, but the coming months will shed so much light — at such improved resolution — on what’s really causing this most mysterious of features. There’s something reflecting the light here, something that’s unlike anything else on Ceres’ surface, and we’re about to find out what."

What's causing the white spots at the bottom of one of Ceres' largest craters? There are three viable possibilities, but none of them involve aliens.
A series of mysterious white features lurk at the bottom of one of its most massive craters. Here’s what they could be, …
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Notice that many of Ceres' craters have central peaks, and that the largest bright spot is right in the centre of that crater, as if its central peak (if it had one) was shaved off.

What is less obvious in these images is a broad, slightly-darker, fan-shaped area originating inside the crater and extending to its right for several crater diameters (that crater is 57 miles in diameter. You could put Dallas/Fort Worth in there). The fan-shaped area extends from about 11:30 to 4 o'clock, with its apex more or less between the two bright spots.

My guess (for what it's worth) is that something struck the crater's central peak at a very low angle and the bulk of the debris - water ice - splashed on the floor of the crater. The rest flew past the crater and settled beyond. Water ice that would have sublimated by now, leaving behind the darker dust. A low-velocity impact, and fairly recently too.
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"As the comet nears the Sun in its orbit, the increased solar radiation vaporizes the ices on its surface, causing ionization, surface disintegration and the development of cometary tails. As it approaches perihelion, its speed increases from just a few km/s all the way up to many hundreds, with the longest tails extending for distances of 500 million kilometers. The various colors come from atomic transitions of various atoms and molecules, such as the green coma coming from the excited diatomic carbon molecule and blue coming from cyanogen."

What makes cometary tails so brilliant? Find out in pictures, videos and no more than 200 words on this week’s Mostly Mute Monday!
The unique sight of a comet only scratches the surface of an amazing story.
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Theoretical Astrophysicist / Writer / Educator
  • NASA's The Space Place
    Columnist, 2013 - present
  • Trap!t
    Head Editor: Science/Health, 2011 - present
  • Starts With A Bang!
    Science Writer, 2008 - present
  • Lewis & Clark College
    Visiting Assistant Professor of Physics, 2009 - 2011
  • University of Portland
    Professor/Lab Coordinator, 2008 - 2009
  • Steward Observatory/University of Arizona
    Postdoctoral Research Associate, 2007 - 2008
  • University of Wisconsin
    Faculty Assistant, 2006 - 2007
  • University of Florida
    Teaching/Research Assistant, Fellow, 2001 - 2006
  • King/Drew Medical Magnet High School
    Teacher, 2000 - 2001
Map of the places this user has livedMap of the places this user has livedMap of the places this user has lived
Bronx, New York - Yonkers, New York - Evanston, Illinois - Torrance, California - Gainesville, Florida - Madison, Wisconsin - Tucson, Arizona - Portland, Oregon - Houston, Texas - Rome, Italy
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Science writer, professor and theoretical astrophysicist
Theoretical Astrophysicist, Science Writer and Communicator, expert in (some aspects of) dark matter and dark energy, physical cosmology, and sometimes professor, teacher and educator.

Creator and writer of Starts With A Bang!, the 2010 Physics Blog of the Year! Author of over 1,000 articles, featured in Esquire, the St. Petersburg Times,'s Page 2, and many others.

Competitive beardsman and amateur acrobat / halloween-costumer extraordinaire.
  • University of Florida College of Liberal Arts and Sciences
    Physics, 2001 - 2006
  • Northwestern University
    Physics, Classics, Integrated Science Program, 1996 - 2000
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