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…)
ON May 20, 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…)
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.
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…
ON February 21, 2012
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