The cream from the physics arXiv blog this week:
Archive for July, 2008
The best of the rest from the physics arXiv this week:
Good to see that Deadheads are alive and well at Los Alamos National Laboratory in New Mexico. In the heart of one of the world’s most secret weapons labs, these guys are hard at work developing a science of the Grateful Dead, the 60s psychedelic band that played together until 1995. Today, they take the world of theoretical physics by storm with their results.
Marko Rodriguez from the Center for Non-Linear Studies at Los Alamos and a couple of pals have studied the listening behaviour of Grateful Dead fans using statistics from the online music service last.fm. They then compare the number of times songs were downloaded to the number of times they were played in concert. (The vast majority of Grateful Dead releases were recorded live at concerts so in many cases these really are the actual songs played at concerts.)
Rodriguez and co report a strong correlation but not a perfect one. This prompts them to ask why the correlation isn’t perfect and to answer the question with a detailed analysis of changes in the band and the nature of the songs themselves (although why they expect the correlation to be perfect, they don’t say).
The team says the work gives an unprecedented insight into American concert tour culture and the bands that bring this culture to fruition. If you’re a Deadhead you might agree.
We can only hoping that Los Alamos has an equally dedicated team down the corridor working on the traumatic break up of the Swedish supergroup Abba.
Ref: arxiv.org/abs/0807.2466: A Grateful Dead Analysis: The Relationship Between Concert and Listening Behavior
Liquid mirror telescopes are amazing contraptions. They start life as a puddle of mercury in a bowl. Set the whole thing spinning and the mercury spreads out in a thin film up the sides of the bowl.
The result is a fabulously cheap mirror that can be used for a variety of astronomical surveys. If we ever put a telescope on the moon, many astronomers have suggested that it should be one of this type.
It won’t have escaped your attention that liquid mirrors have important limitations. First, they can only point straight up. One or two people have played with fluids that have a higher viscosity than mercury and so can be tilted a few degrees this way or that but with limited success. And second, they cannot be made adaptive to correct for blurring introduced by the Earth’s atmosphere.
But that may change thanks to some interesting work being done by Denis Brousseau at Université Laval in Quebec et amis. Their machine controls the shape of the surface of a liquid mirror using a magnetic field. Mercury cannot be used, however, because it is too dense and changing its shape requires impractically powerful fields.
Instead the team have used a suspension of ferromagnetic nanoparticles in oil. A thin highly reflectivity layer of silver particles can then be spread across the surface of the ferrofluid to create a mirror.
Brousseau and co use an array of tiny coils behind the liquid to create a field that deforms the fluid surface as required. Their tests show this can be done fast and furiously enough to cope with the usual array of optical aberrations that the atmosphere throws up.
However, it may also be possible to use this technique to tilt liquid mirrors further than ever before. Ferrofluids can easily be made much more viscous than mercury and so combat the deforming pull of gravity. But they can also be deformed in a way that opposes gravity during each rotation of the supporting bowl. That could make them much more tiltable than mercury mirrors.
Of course, such a mirror would be mechanically more complex than the spinning bowls we have today and correspondingly more expensive. And sending one to the moon seems an unnecessary extravagance given the absence of an atmosphere there.
But here on Earth they could be made much more useful. It’s a combination of new-found utility and value for money that many astronomy projects on a budget will find irresistible.
Ref: arxiv.org/abs/0807.2397: Wavefront Correction with a Ferrofluid Deformable Mirror: Experimental Results and Recent Developments
Is faster than light travel allowed by the laws of physics? There’s no harm in speculating, right?
In 1994, Michael Alcubierre, a physicist at the National Autonomous University of Mexico in Mexico City, put warp drive on a firm (-ish) theoretical footing for the first time. His thinking was that what relativity actually prevents is faster-than-light-travel relative to the fabric of spacetime. But it places no restrictions on the way in which spacetime itself can move and stretch.
The Alcubierre drive consists of a device that somehow contracts space in front of your spacecraft, bringing your destination effectively closer, while expanding space behind it. The spacecraft sits in a bubble of flat space in the middle. So while the bubble can travel at any speed across the universe, the spacecraft can be almost stationary relative to the space in which it sits.
Clever idea. And today Gerald Cleaver and Richard Obousy from Baylor University in Texas, take it further by explaining how it might actually be possible to stretch spacetime into the Alcubierre bubble.
Their idea is based on the possible existence of extra dimensions that are curled up with a radius so small that we never experience them. They say:
“The basic idea is that by altering the radius of an extra dimension, it would be possible, in principle, to adjust the energy density of spacetime.”
And that would allow the kind of space-time stretching that could create an Alcubierre drive.
There’s one drawback. Cleaver and Obousy calculate that the energy needed to distort the space around a spacecraft-sized object is about 10^45 Joules or the total energy of an object the size of Jupiter if all its mass were converted into energy.
Still, if you’re glass half-full kind of physicist, you’ll take that as encouragement.
Ref: arxiv.org/abs/0807.1957: Putting the “Warp” into Warp Drive
Our old friend HD 189733b is in the news again this week. As a Jupiter-sized gaseous planet orbiting a yellow dwarf in the constellation of Vulpecula, HD 189733b has become one of the best studied exoplanets.
The reason is that it’s relatively big and close to its sun, which shines through the atmosphere as the planet transits.
This phenomenon has allowed astronomers to measure many properties of HD 189733b’s atmosphere. For example, they have found that this ball of gas is rich in water and methane and that to our eyes the planet would appear a rich dark blue, a bit like Uranus.
Last year, the Spitzer Space Telescope produced a heat map of the planet showing global temperature differences. Unsurprisingly, HD 189733b is warmer at the equator than at the poles.
Astronomers also think the atmosphere is filled with particles of the condensates of iron, silicates and aluminium oxide. And today, Sujan Sengupta from the Indian Institute of Astrophysics in Bangalore, India, adds another nugget of information. It seems that these particles must be distributed in a thin layer of cloud in the upper atmosphere.
So if you’re wondering what the weather is like on HD 189733b, the answer is cloudy.
What’s emerging is the most amazing picture of a planet orbiting another sun, something that only a few years ago would have been deemed impossible.
To put this in perspective, our understanding and knowledge of HD 189733b is comparable to, and in some ways better than, our pre-Voyager knowledge of Neptune and Uranus in the 1970s.
That’s truly astounding. There can’t be many more persuasive examples of the fact that we’re currently living through a golden age of astronomy.
Ref: arxiv.org/abs/0807.1794: Cloudy Atmosphere of the Extra-solar Planet HD189733b : A Possible Explanation of the Detected B-band Polarization
It’s hard to get your head around dark energy, this universe-accelerating stuff that is supposed to fill the cosmos. Dark energy was invented to explain measurements that seem to show that the most distant supernovas all appear to be accelerating away from us. The thinking is that something must be pushing them away and that stuff is dark energy.
But for many astrophysicists, dark energy is a difficult pill to swallow. It requires the universe to be fine tuned in a previously unexpected, and frankly, unimaginable way.
So astronomers have begun a systematic investigation of all the assumptions on which the notion of dark energy depends. Nothing is sacrosanct in this hunt–these guys are tearing up the floorboards in the search for an alternative hypothesis. And that means revisiting some of our most fundamental assumptions.
One of these is the Copernican principle, that the universe is more or less the same wherever you happen to be. Principles don’t come much more fundamental than this but the evidence in its favour, at least on the scale that dark energy seems to behave, is pretty thin.
In fact, a number of theorists have calculated that the supernova data can be explained without the need for dark energy if our local environment were emptier than the universe as a whole. But to make this idea work, the earth must be sitting in the middle of a void that is roughly the size of the observable universe and that’s not compatible with the Copernican principle, not by a long shot.
Now Timothy Clifton and pals at the University of Oxford in the UK have worked our how to tell whether such a void exists or not. They say that the next round of highly accurate measurements of nearby supernova should be able to tell us whether we’re in a void or not. So we shouldn’t have long to wait.
Either way, astronomers will find it hard to settle that troubling sensation in the pit of their stomachs. The truth is that when it comes to swallowing uncomfortable ideas, dark energy may turn out to be a sugar-coated doughnut compared to a rejection of the Copernican principle.
Ref: arxiv.org/abs/0807.1443: Living in a Void: Testing the Copernican Principle with Distant Supernovae
The rough diamonds from the physics arxivblog this week:
The best of the rest from the physics arXiv this week:
When it comes to invisibility cloaks, nobody has done more to advance the field than John Pendry, a theoretical physicist at Imperial College, London. It was he who suggested the idea in the first place and mapped out how one could be built in theory. He even got his hands dirty by collaborating with the team of engineers who first built a working cloak.
So when he pronounces on the subject, we sit up and listen.
Pendry has clearly been worrying about the limitations of invisibility cloaks. For a start, they work only in the microwave part of the spectrum and at a single specific freqeuncy. (Optical invisibility cloaks seem as far away as ever because of problems with light absorption.)
The cloaks must be made of exotic materials with properties that vary throughout their structure and are in any case unobtainable in nature and so have to be designed and made by hand.
The resulting cloaks are not perfect and probably never will be. To hide an object completely, the permittivity and permeability of these metamaterials must take infinite values at some points.
So what to do? Pendry argues in a paper on the arxiv that instead of making objects invisible, you can hide them just as well by making them look like a flat conducting sheet. An eminently sensible suggestion.
The advantage of this approach, he calculates, is that it readily works for visible light and over a wide range of frequencies. What’s more, it can be done with ordinary materials that are available today.
All that’s needed is to hide your object under a material that he calls an isotropic dielectric. He’s even done a number of simulations to show how such a material would make anything it covers look like a flat conducting sheet.
Pendry doesn’t bother with the practical details of how to make an isotropic dielectric material. But maybe he doesn’t need to. He wouldn’t by any chance be referring to water, would he?
Ref: arxiv.org/abs/0806.4396: Hiding Under the Carpet: a New Strategy for Cloaking