I hope you’re all ready for today’s transit of Venus across the face of the Sun. You certainly can’t have missed that it’s happening, given the media hype. (Look at the website http://transitofvenus.nl/wp/, their post from yesterday in particular, for all sorts of useful resources, including timing for the event in various locations, and on-line resources for you to watch in case clouds interfere at your location. UPDATE: that website is overwhelmed. In the continental U.S. transit begins just after 6 pm Eastern, 3 pm Pacific, differing by a few minutes from place to place; and sunset will occur before the transit is over. One online location to watch the transit is http://events.slooh.com/) But — let me be the first to warn you — this is going to be very cool, but it isn’t going to be a spectacular event like a big meteor shower or a total solar eclipse or even a total lunar eclipse. It’s going to be subtle, slow, and potentially very boring, unless you have the right mindset (or a truly excellent telescope, properly filtered for sun viewing). So here are some suggestions: (more…)
Category: Astronomy
A reminder that the transit of Venus across the Sun will be visible a little over 24 hours from now (starting before sunset June 5th in the United States.) It’s the last of your lifetime, so make your plans now. Weather forecast across much of the eastern U.S. is pretty bad, I’m afraid. Of course there will be many places to watch on-line. For more information, I highly recommend http://transitofvenus.nl/wp/ . Also, though you and I can’t see Venus right now (too close to the Sun), the SOHO satellite can: click here for the latest photo, updated frequently, which shows the sun (at center, blocked out) and the sun’s corona around it, and a white dot approaching the sun, which is Venus (ignore the white horizontal lines, which are an imagining artifact).
The transit of Venus was used, historically, to figure out how big is the solar system (the sun and the planets). As I’ll explain tomorrow, it wasn’t that hard to figure out how large are the distances between the sun and the planets relative to one another, but getting the overall scale — the absolute distances — was a much harder problem. The first measurement accurate to a couple of percent came from the transits of Venus in the 1760s. I’ll explain (roughly) how this works tomorrow; the technique relies on the principle of parallax, a principle which lies behind our own ability to perceive depth, and is used by astronomers to measure distances to (relatively nearby) stars.
[Reminder: two events coming up in which I’ll be speaking: (more…)
POSTED BY Matt Strassler
ON June 4, 2012
First, a couple of things you might like to read:
- There was a long-overdue article from the New York Times as to how, after eight years of cuts from the Bush administration during good economic times, followed by additional inevitable cuts during the Great Recession, formerly world-leading scientific research efforts in the United States are on the verge of collapse, risking far more than the scientific research itself. The situation is far more dire, in my opinion, than the tone of the article implies; the brain drain of talent leaving the US is frightening and well underway, and the problems are by no means limited to particle physics and astrophysics.
- The article that I described last month by Moni Bidin et al. that claimed (loudly, in the press) that there was little evidence for dark matter in the Sun’s interstellar neighborhood (but far from the center of the Milky Way galaxy) has been discredited by one of the world’s leading astrophysicists, working with a younger collaborator. The claim (made without a big press release) of Jo Bovy and Scott Tremaine, from the Institute for Advanced Study in Princeton, is that one of the assumptions on which the Moni Bidin et al. argument was based is inconsistent with data and therefore wrong, ruining the argument. Bovy and Tremaine have replaced this assumption with a different one that is consistent with data, and they conclude, in contradiction to Moni Bidin et al., that the corrected argument leads to the conclusion that there is indeed dark matter in the vicinity — roughly the amount one would expect from other considerations. In short, the Moni Bidin argument, once corrected, actually leads to more evidence in favor of the existence of dark matter! A short description of the situation is given on the Resonaances blog.
Second: I’m starting to think about a new section for this website.
Particle physics, quantum mechanics, and all the strange-sounding stuff that many physicists study are often thought of as esoteric, abstract, and irrelevant to ordinary life. But in fact our world is directly impacted, in many different ways, by quarks and photons and quantum uncertainty and the like; these things are not abstract at all. To make this point more accessible, a website like this one needs a good introductory section to help beginners make their way into the world of particle physics. So I’m going to be constructing this section over the coming months. Feedback from readers as to whether the material is readable and sufficiently introductory will be very valuable, so please feel free to offer your comments.
One of my first tasks is to talk about the architecture of the universe — how its parts are formed from smaller parts, on down to the smallest objects we know about so far. Of course there are many other introductions to this subject already on the web (and a few of them are even correct!) What I hope will make my presentation a little different is the inclusion of some insights into not only what we know about the structure of the universe but also how we know it, and additionally some comments on how our lives are impacted by each level of detail in the architectural hierarchy.
The first phase of this process will be to look at how ordinary matter that we are made from and surrounded by is constructed from the basic ingredients of electrons, quarks (and anti-quarks and gluons), and the strong nuclear, electromagnetic and gravitational forces. The figure below gives a preview of what’s coming (and you can click the figure for a larger version.)
POSTED BY Matt Strassler
ON May 25, 2012
What would the Grand Canyon look like if it had sunset light without the sunset shadows? Sunday’s annular solar eclipse provided a hint of an answer:
Quite a first visit to the North Rim of the canyon. Maximum eclipse occurred an hour before sunset, and the sun set with a small piece of the moon’s silhouette still covering its disk. As a result, the amount of sunlight remained low for the entire hour, bathing the canyon in the low light that allows its layers of color and geological time to be more easily seen.
Meanwhile, in the other direction the sun was still far too bright to look at with the naked eye, or photograph without a filter. Lacking both a proper filter and a tripod, this is all I could manage with my camera, I’m afraid:
I think that when you look at photos of an eclipse (certainly I find this for myself) it is easy to miss the visceral nature of the experience. When you are watching it happen, you can see (through a proper filter, or with a projection), second by second, the slow but steady glide of the moon across the sun. You can detect the ring of sunlight changing shape, from a perfect circle to one that is thicker on one side than the other, and finally turning back into a crescent. The process is a dynamic one, as well as a visual feast. And this is part of what makes it so beautiful — not just what one sees with the eyes, but what one feels as a witness to the steady motion of the heavens.
[p.s. don’t miss the other two crescents to see right now: crescent Venus and crescent Moon near each other in the western sky tonight!]
POSTED BY Matt Strassler
ON May 22, 2012
Hi all! I said that posts would be sparse for a couple of weeks, but I didn’t realize quite how sparse! But I’m gradually getting back on-line here.
Right now, what’s occupying my attention for the next 48 hours is all in the sky: three crescents, and a ring. I’ll be brief, but if you want more explanation about the geometry involved, you might want to read my very relevant article about geometry and the beauty of the heavens. (more…)
POSTED BY Matt Strassler
ON May 20, 2012
I’m preparing an article on a very important type of energy that I’ve avoided writing about so far — the energy that comes from the interaction among fields. I’ve avoided it because it’s tricky to figure out how to explain it. But it’s important, for this form of energy is responsible for all the structure in the universe, from atoms to galaxies. The article’s not quite ready yet, so today I’ve just got some good reading material for you, including the heavy, the weird, the amusing, and the optimistic. (more…)
POSTED BY Matt Strassler
ON April 26, 2012
Two interesting claims about dark matter this week, and on the face of it, completely contradictory, but in fact, not obviously so. Before saying one word more, let me repeat my mantra — something that all physicists know but relatively few non-scientists appreciate — most claims of a radical new result turn out to be largely or completely wrong. This is not because physicists are stupid but because doing science at the forefront of knowledge involves using novel techniques that might have unknown pitfalls, and also because a single small mistake can create a fake effect (as we saw most recently with the OPERA neutrino speed measurement.) And because nasty statistical accidents can play tricks on you.
Both claims that I’m about to describe use novel techniques, and their analyses have not been repeated by anyone else. At this point you should understand that both are tentative, and (based on the history of radical claims) the odds are against them. Both might be wrong. That said, both analyses look to me as though they’ve been reasonably well done, and if a mistake has been made, it will require someone far more expert in dark matter studies than I am to point it out.
So let me describe them in turn, to the best of my ability. (more…)
POSTED BY Matt Strassler
ON April 20, 2012
There is going to be some amount of debate regarding dark matter in the next few weeks, so I’ve written an article on one of the best ways to go looking for new signs of dark matter out in space.
The reason we are almost entirely convinced that the universe has lots of matter that doesn’t shine is that we can see many signs of its gravitational effects — for instance, its effect on the motions of stars within galaxies, its ability to bend light a la Einstein, etc. It’s almost certain that most of a galaxy is dark matter. And over the years we’ve convinced ourselves this dark matter almost certainly can’t be made from any type of particle that we already know about.
But to learn more about what it is, we need to find signs of some of its non-gravitational effects, if it has any. One possibility is that dark matter particles, if and when they collide, might annihilate into ordinary known particles. If those known particles are photons, we might be able to detect them. A good way to look for them would be to point a suitable telescope toward the center of the Milky Way, our galaxy, which is one place where we expect dark matter particles to be especially numerous, and collisions among them to be especially common.
In the article I just finished, I explain how this can be done. One goes looking for photons from the galactic center, makes a plot of the number of photons observed at a particular energy, and looks for a bump in the plot — an exceptional number of photons with the same energy.
And the reason I’m doing this now is that there is a new paper claiming that a signal of this type may have been seen (with a claimed significance of 3.3 standard deviations, after including the look-elsewhere effect.) This is a paper by a theorist, analyzing publicly available data taken by the experimental group that operates the Fermi Large Area Telescope satellite. One should note that the record of theorists making discoveries using experimentalists’ data is very poor. Typically there are either detector-related or statistics-related issues that theorists screw up. And there are risks of bias — I am not yet sure whether the rather sophisticated analysis method used by this theorist was chosen in a blinded fashion. [For instance, did he choose his method first and then look at the data, or did he already know there was a hint of a peak in the data before he started designing his method?] So I would be skeptical of this claim for now. (And the theorist, knowing he’s out on a limb, was careful [and wise] to put the word “Tentative” in his title.) However, stranger things have happened, so I wouldn’t dismiss this claim out of hand either, at least not until the Fermi experimentalists tell us that in their opinion the theorist over-estimated the statistical significance of this particular bump. We’ll be looking forward to what they have to say.
I’ll have a few more details about this for you soon.
POSTED BY Matt Strassler
ON April 19, 2012
Why does the sight of the Moon draw our gaze and silence our voices? What is it about the planets, those exceptionally bright points of light that wander among the stars, that we instinctively find so beautiful? Is it perhaps that they make us dream of faraway, unreachable places? Is it that they are beacons in the night, nature’s candle flames, helping keep fear at bay, and offering us hope amid darkness? Or is it perhaps that they seem to float — we do love things that float, whether they be autumn leaves, balloons, clouds or birds — suspended in the sky, in apparent defiance of the force of gravity which keeps us pinned to the Earth?
This last thought offers a certain delicious irony… for in truth the planets and the Moon, in their procession above our heads, obey gravity’s dictates.
The next few days, weather permitting, will give us a chance to contemplate these questions. Our planet’s natural satellite, on its monthly trip around the Earth, will pass three of the brighter planets in the sky, creating one lovely spectacle after another. Of course, the Moon really passes nowhere near the planets, just as your outstretched hand, when it blocks your view of the Moon, is nowhere near the Moon itself. It is all a matter of perspective — of geometry, of cavernous spaces, of the play of light, and of the elegant choreography of the solar system. But this perspective is not something we sense easily. Our eyes can perceive no depth for objects so far away, and so our brains form a two-dimensional picture from the three dimensions of the universe, projecting the Moon, the planets and the stars, at extraordinary distances from one another, onto a psychologically flat black screen of the night sky. It takes great mental effort to see things as they are, and not as they appear. This, too, is worthy of contemplation.
First, on the night of February 22nd, just after sunset (don’t be late!), one of the most delicate of nocturnal sights awaits: Click here to read more…
POSTED BY Matt Strassler
ON February 21, 2012
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