gravity
esiegel | August 13, 2014“To be loyal to myself is to allow myself to grow and change, and challenge who I am and what I think. The only thing I am for sure is unsure, and this means I’m growing, and not stagnant or shrinking.” ‐Jarod Kintz
But for the Universe, although it's constantly changing, growing in a gravitational sense is a thing of its past. Sure, we continue to form new stars, galaxies continue to merge, and the structure we see on the largest scales continues to evolve.
Image credit: Bob Franke, via http://bf-astro.com/.
But there once was a time when there were no stars, no galaxies, and no groups or…
esiegel | July 24, 2014“Since the beginning of physics, symmetry considerations have provided us with an extremely powerful and useful tool in our effort to understand nature. Gradually they have become the backbone of our theoretical formulation of physical laws.” -Tsung-Dao Lee
We normally think of the Universe as being symmetric, in the sense that no one direction, location or configuration is inherently preferred over another. But at some level, the laws of physics aren't completely symmetric, as we see from the decays of certain elementary particles.
Image credit: author durbarsquare of http://readingpenrose.…
esiegel | July 22, 2014“Art has a double face, of expression and illusion, just like science has a double face: the reality of error and the phantom of truth.” -Publilius Syrus
Out in the depths of space, objects range from the incredibly small and low mass to the huge and super-heavy, shrinking down again for the most ultra-massive objects in the Universe.
Image credit: The Antiope Doublet asteroid / ESO, via http://www.eso.org/public/images/eso0718b/.
But what would happen, on all scales, if you took two such identical objects and merged them together?
Image credit: NASA.
Find out what happens when the…
esiegel | May 7, 2014“The discrepancy between what was expected and what has been observed has grown over the years, and we’re straining harder and harder to fill the gap.” -Jeremiah P. Ostriker
When it comes to the Universe, and the structure in it, from galaxies to groups to clusters to the grand cosmic web on the largest scales, you simply can't account for it all without dark matter.
Image credit: 2dF GRS, via http://www2.aao.gov.au/2dfgrs/Public/Survey/description.html.
But in principle, you might not need dark matter to do it; you might be able to modify the laws of gravity instead to take care of it.…
esiegel | February 21, 2014“If you put yourself in a position where you have to stretch outside your comfort zone, then you are forced to expand your consciousness.” -Les Brown
On the one hand, you've got gravitation, the most powerful force in the Universe, trying to attract all the matter in it together. If it had its way, not only would everything eventually merge together, but the Universe itself would recollapse!
Image credit: Eugenio Bianchi, Carlo Rovelli & Rocky Kolb, modifications by me.
But on the other hand, the Universe is not only expanding, but that expansion is accelerating, thanks to the presence…
esiegel | December 30, 2013"Tomorrow is the most important thing in life. Comes into us at midnight very clean. It's perfect when it arrives and it puts itself in our hands. It hopes we've learned something from yesterday." -John Wayne
There's a lot to look forward to on the precipice of the new year, and many of us will be up until midnight to ring it in. Well, if you're up that late, why not step outside and take a look at one of the deep-sky wonders of the Universe that won't be visible in the early part of the night for months! This Messier Monday, a near-moonless sky will greet you until the pre-dawn twilight, and…
esiegel | December 27, 2013"Never waste your time trying to explain who you are to people who are committed to misunderstanding you." -Dream Hampton
Perhaps no word in the English language generates as much misunderstanding as the word theory. In scientific circles, this word has a very specific meaning that's different from everyday use, and -- as a theoretical astrophysicist myself -- I feel it's my duty to help explain exactly what we mean when we use it. In this week’s Ask Ethan column, I'm pleased to pull out of our question/suggestion box the question of Ripley, who asks:
I often see that because there is no "100…
esiegel | December 20, 2013"To be is to be the value of a variable." -Willard Van Orman Quine
It's the end of the week once again, and the very end of our Year In Space 2014 Wall Calendar giveaway! So for this week's Ask Ethan column, after dipping into our question/suggestion box, I'm very pleased to let Vera from Italy know that her question was the winning one, as she asked:
I saw the video you posted about the variable star RS Puppis, by the Hubble Space Telescope, and I really would like a post about what exactly are variable stars.
Thank you very much Ethan!! :-)
So -- no burying the lede here -- there was a…
esiegel | December 11, 2013"Where you used to be, there is a hole in the world, which I find myself constantly walking around in the daytime, and falling in at night. I miss you like hell." -Edna St. Vincent Millay
It was just a little while ago that we were all speculating wildly -- and optimistically -- about Comet ISON, as it plunged towards the Sun from its origins in the very, very distant Solar System. As its perihelion date (the moment of closest approach to the Sun) drew near, you may have noticed something interesting about photos of the comet: it's tail appeared to get longer and longer!
Image credit &…
esiegel | December 5, 2013"Some say the world will end in fire,
Some say in ice.
From what I've tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice." -Robert Frost
Depending on where you are in the world right now, you might really be feeling the effects of the emerging winter, as cold snaps, freezes and snowstorms take hold across the northern hemisphere.
Image credit: NASA's Scientific Visualization Studio, Goddard Space Flight Center.
But we can all be pretty sure that this is temporary,…
esiegel | November 15, 2013"I wouldn't know a spacetime continuum or a warp core breach if they got into bed with me." -Patrick Stewart
It's the end of the week once again, and so it's time for another Ask Ethan segment! There have been scores of good questions to choose from that were submitted this month alone (and you can submit yours here), but this week's comes from our reader garbulky, who asks:
Why does gravity decrease the further away you are from the object? I've read that it does decrease with distance squared but not why it does this.
This question seems so simple, and yet the answer -- to the limits of our…
esiegel | November 9, 2013"One of the most tragic things I know about human nature is that all of us tend to put off living. We are all dreaming of some magical rose garden over the horizon instead of enjoying the roses that are blooming outside our windows today." -Dale Carnegie
Our new Ask Ethan segment has been really popular, and the questions and suggestions keep pouring in. It's your Universe too, and if there's something you want to know about it, you should ask! (So keep it up!) This week's question is one of the biggest of them all, and it comes courtesy of John L. Ferri, who asks,
I have a difficult time…
esiegel | November 1, 2013"Galileo got it wrong. The Earth does not revolve around the Sun. It revolves around you and has been doing so for decades. At least, this is the model you are using." -Srikumar Rao
It's the end of the week, so that means its time to take on another one of your questions from the question/suggestion box, and continue our ongoing Ask Ethan series! Even though there's a backlog of hundreds of questions, you should keep sending the new ones in, as all questions are fair game for any segment. This week's question comes from reader Brian Mucha, who asks us:
Where did the sun and planets get their…
esiegel | October 30, 2013“I followed him at the time and thought he was hysterical. He was the first serial killer, a new kettle of fish, because we didn't have the detection techniques in those days.” -Matthew Bright
You can't get mad at a team for trying the improbable, hoping that nature cooperates. Some of the most famous discoveries of all time have come about thanks to nothing more than mere serendipity, and so if we can test something at low-cost with an insanely high reward, we tend to go for it. Believe it or not, that's the mindset that's driving the direct searches for dark matter.
Image credit:…
esiegel | October 17, 2013"It is not when truth is dirty, but when it is shallow, that the lover of knowledge is reluctant to step into its waters." -Friedrich Nietzsche
Although the innermost planets, from Mercury through Saturn, were known since ancient times, it's only since the advent of the telescope that we've discovered what really lives in our Solar System. Over the past four centuries, the wonders of not only the distant Universe, but also our nearby neighborhood, have been uncovered in spectacular detail.
Image credit: NASA and – I believe – G. Bacon (STScI).
The third and fourth largest planets were…
esiegel | October 11, 2013"If you look for truth, you may find comfort in the end; if you look for comfort you will not get either comfort or truth only soft soap and wishful thinking to begin, and in the end, despair." -C. S. Lewis
And yet, when you search for the truth, you often find answers that butt up against your sensibilities, your preconceptions, and even your very notions of common sense. Such is the case in this week's Ask Ethan, where longtime reader and commenter MandoZink asks:
I have a question that has perplexed me for most of my life. Recently I sought out and re-read more expert explanations of the…
esiegel | October 9, 2013"Even in hindsight, I would not change one whit of the Voyager experience. Dreams and sweat carried it off. But most of all, its legacy makes us all Earth travelers among the stars." -Charley Kohlhase
In the early days of space exploration, it was quite a feat just to get up and out of Earth's atmosphere. There's are two good, simple reason for that, of course: first, it takes a lot of energy to go up that high...
Image credit: Nathan Bergey of http://psas.pdx.edu/orbit_intro/.
and second, if you don't get your spacecraft moving really fast, you're just going to fall back to Earth once you…
esiegel | September 20, 2013"Observations indicate that the universe is expanding at an ever increasing rate. It will expand forever, getting emptier and darker." -Stephen Hawking
Today's Ask Ethan comes to us via our question/suggestion box from reader MIUFish, who asks:
I read something recently about some people who were doing work looking at the possibility that the mass of things has been decreasing, as an alternate explanation - or, perhaps, interpretation - to some of the questions regarding the expansion of the universe and so on.
Is there any seriousness to this? Do you know anything about it?
Here's what…
esiegel | September 18, 2013"A cosmic mystery of immense proportions, once seemingly on the verge of solution, has deepened and left astronomers and astrophysicists more baffled than ever. The crux ... is that the vast majority of the mass of the universe seems to be missing." -William J. Broad
It was in the 1930s, looking at dense clusters of galaxies (like Coma, below), that Fritz Zwicky first noticed that the mass in the Universe didn't add up.
Image credit: Adam Block / Mount Lemmon SkyCenter / University of Arizona.
We knew how gravity worked, so it was pretty straightforward -- based on how the galaxies within…
esiegel | September 12, 2013"The great oak of Astronomy has been felled, and we are lost without its shadow." -Subrahmanyan Chandrasekhar, on the passing of Jan Oort
In our new semi-regular series, readers from all over the world (and in low-Earth orbit, too, because why not!) are invited to send in their questions and suggestions for a chance to have them answered here on this blog! Today, our question comes from Robert Meegan, who asks,
Have any Oort Cloud objects been detected in situ, as opposed to when they whipped through the inner solar system as comets? I would presume that any detection would need to be…