Archive for the ‘At the seaside’ Category

The painful search for gravitational waves

Wednesday, July 30th, 2008

G-wave data

Gravitational wave detectors have a sorry history of disappointing results.

Joseph Weber at the University of Maryland first claimed to have spotted these waves in 1969. He did it by listening to the way a giant cylindrical bars vibrate, thinking that passing gravitational waves would cause them to ring like a bell. Nobody has been able to reproduce these results and they remain strongly disputed today.

Various groups still listen out for gravitational waves using Weber-like detectors. But the Ferraris in this field are a new generation of laser interferometers that are much more sensitive to the bending and squeezing of space that these waves cause as they pass by.

The trouble is that none of these detectors has ever spotted a gravitational wave either, despite the investment of hundreds of millions of dollars. One way of increasing the sensitivity is to use two or more interferometers in different parts of the world to look for wave simultaneously.

Now the results of the first  combined search using four detectors (three LIGO detectors in the US and the GEO600 in Germany) have been published and the results are again disappointing.  They took data over a period of month between 22nd February and 23rd March 2005, giving them a decent amount of data to play with. But…

“No candidate gravitational wave signals have been identified”

says the team, ominously.

That’s embarrassing because these combined searches should be sensitive enough to pick up gravitational waves from sources such as supernovae and from black holes as they collide.

So why aren’t they seeing anything? One possibility is bad luck, that there weren’t any events during the the time  the data was being taken. That seems unlikely. Another possibility is that the problem is closer to home, perhaps in the equipment, analysis or even the theory itself.

Whatever the problem, they don’t seem to be able to put their finger on it. This data is three years old which means it’s been given one almighty going over before publication.

So I wonder how these guys are feeling given that hundreds of millions of dollars and several years of work has so far produced zilch.

Ref: First Joint Search for Gravitational-Wave Bursts in LIGO and GEO600 Data

Why small black holes cannot grow

Monday, July 28th, 2008

Quantum mechanics places a fundamental limit on the minimum quanta of energy that can be associated with a bit of energy.  It’s about 10^-50 Joules, which ain’t much.

That has important implications for black holes, says Scott Funkhouser, a physicist at The Citadel, the military college of South Carolina, in Charleston. As black holes accretes mass, its total energy increases but the energy per bit decreases,  he says.

And this decrease must always be greater than the minimum quanta of energy allowed by quantum mechanics. That only happens when black holes are bigger than 10^11 kg, which is only about a thousandth of the mass of Halley’s Comet.

That’s small on astrophysical  terms. But it should also come as a relief to anybody worried about the possibility of the production of tiny black hole at particles accelerators such as the LHC. According to Funkhouser, these black holes must be safe because they cannot grow.


Ref: The Minimum Mass of a Black Hole that is Capable of Accretion in a Universe with a Cosmological Constant

Musical relativity

Monday, July 21st, 2008

Musical relativity

Here’s a neat idea for a concert that’s going to blow a few minds if it ever takes to the stage.

A combination of three or more notes played together is called a chord. We know that certain musical chords sound happy while others sound sad (although nobody knows why). The mood of a piece of music then depends on the combination of chords being played. More than a few weighty tomes have been written about the way one chord can be transformed into another and the effect this has on the mood of the music.

But Kaca Bradonjic, a physicist at Boston University, says that musicians appear to have ignored one of the fundamental ways of changing the pitch of a note: the Doppler shift. He points out that it ought to be possible for an observer moving at a specific velocity to hear a sad sounding note as a happy one and vice versa.

Which means that the mood of a piece of music depends on the relative  velocities of the audience and performers.

He calculates for example that to hear a C major chord as a C minor, the listener would need to be travelling at about 43 miles per hour, directly away from the source. That’s a fair speed. And the accelerations necessary to vary this effect from one note to another during a concert would make this one helluva roller coaster ride.

Talking of which, a (very quiet) roller coaster might be the perfect venue for  the first concert of this type.

Ref: Relativity of musical mood

The science of the Grateful Dead

Friday, July 18th, 2008

Grateful Dead

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 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: A Grateful Dead Analysis: The Relationship Between Concert and Listening Behavior


How to build a warp drive

Wednesday, July 16th, 2008

 Alcubierre drive

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: Putting the “Warp” into Warp Drive

If invisibility cloaks don’t work, try the invisibility sheet

Friday, July 11th, 2008

Invisibility sheet

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: Hiding Under the Carpet: a New Strategy for Cloaking

Surfing solves puzzle of water snail locomotion

Wednesday, June 25th, 2008

Snail surfing

Snails move using a mechanism called adhesive locomotion. Through muscular contraction and expansion of their foot, they transmit a force to the ground through a thin layer of mucus which is adhesive at low strains but otherwise flows like a liquid.

But what of water snails that move upsidedown along the underside of a liquid surface? Water snails seem to move their foot in the same undulating way as their terrestrial cousins but adhesive locomotion can’t answer for their albeit small, velocity because there’s nothing to stick to.

Today, Sungyon Lee, an engineer at  the Massachusetts Institute of Technology in Cambridge  and a few pals put forward their own suggestion. Their idea is that the undulating motion of the foot deforms the surface of the water. And this generates a pressure that causes the mucus, which is sandwiched between the foot and water surface, to flow.

In other words, water snails surf on waves of their own making.

Neat idea but Lee and company have more work to do to make their argument water tight.

First, it isn’t clear whether their model can account for the kinds of speeds water snails actually achieve (whatever these are).

And second, the model assumes that water snail mucus is newtonian. That’s probably wrong. Terrestrial snail mucus is non-newtonian and that is crucial for locomotion.

I’d be willing to bet a bowl of steaming escargot a la poulette that water snail mucus also turns out to be non-newtonian and that this is crucial for amplifying whatever forces snails use to surf.

Ref: Crawling Beneath the Free Surface: Water Snail Locomotion

First test of exotic space thruster ends in explosion

Friday, May 23rd, 2008


In 2006, Mason Peck at Cornell University in Ithaca dreamt up with an entirely new way to control satellites orbiting planets that have a magnetic field. The idea is based on the Lorentz force: that a charged particle moving through a magnetic field experiences a force perpendicular to both its velocity and the field.

So the plan is to somehow ensure that the spacecraft becomes electrically charged as it moves through the planetary magnetic field which should then generate a force that can alter the orbit or orientation of the vehicle. The big advantage of so-called Lorentz actuated orbit control is that it requires no propellant. That’s a big deal since the amount of fuel a spacecraft can carry is the main factor that determines its lifespan. Propellant-free propulsion could significantly increase their operaitng lives.

Today, Peck along with William Gorman and James Brownridge at the State University of New York at Binghamton present the results of the first experimental trials of the idea. The work was funded by NASA but it has to be said: it doesn’t look entirely promising.

The team tested the ability of various objects to hold a charge in a vacuum while being bombarded with plasma, as would be the case in orbit. To generate the charge on the test object, they attached it to a sample of radioactive Americium-24, an alpha-particle emitter, and applied a voltage. The electric field carries away the positively charged alpha particles leaving the object highly charged.

I’ll let the team take up the tale:

Microscopic arcing was observed at voltages as low as -300 V. This arcing caused solder to explode off of the object.

Obviously, a proplusion system that explodes while it is in operation needs some more work.

The early pioneers of experimental propulsion systems such as Robert Goddard and Werner von Braun all had to cope with catastrophic failures, so Peck, Gorman and Brownridge are in good company. And as long as nobody gets hurt, a decent explosion livens up any experiment.

So stick with it fellas. Something tells me that if NASA funds the future development of this system, we’re going to be in for some fun.

Ref: Experimental Study of a Lorentz Actuated Orbit

Why ET will phone using neutrinos not photons

Tuesday, May 20th, 2008

Neutrino communication

The search for extraterrestrial intelligence assumes that ET will be communicating using photons. But despite decades of listening out, we’ve heard nothing.

But today, John Learned from the University of Hawaii and pals say forget photons. We should be looking for evidence of ET using neutrinos.

The reason is that any civilisation advanced enough to colonise the galaxy would need a reliable way to communicate over intragalactic distances and photons simply don’t pass muster. There is a huge amount of noise in the electromagnetic spectrum, photons are easily scattered and would almost certainly be absorbed if they had to travel from one side of the galaxy to the other.

By contrast, the neutrino spectrum is relatively noise free and neutrinos intereact so weakly with matter that a signal could travel unhindered from one side of the galaxy to the other.

They propose testing the idea by generating a neutrino signal using a particle acclerator to genreate Z nought particles which decay into neutrinos of a relatively easily detectable energy. They would encode information in the time structure of the beam, like Morse code.

What’s more, Learned and co say that the kind of neutrino signals that ET might be expected to beam should be detectable by the generation of neutrino detectors now under construction.

Ref: Galactic Neutrino Communication

Phobos could form Saturn-like ring around Mars

Wednesday, May 14th, 2008


The martian moon Phobos is spiralling towards Mars at a rate of 20 cm a year

The question is when will it hit. On the arxiv today, calculates that we have about 11 million years before it smashes in to the Red Planet. That’s plenty of time to visit although he also points out that Phobos may break up long before that because of tidal forces. If that’s going to happen it’ll probably occur about 7 million years from now.

But it’s not all bad news. Sharma says if tidal forces get the better of Phobos, it should form a Saturn-like ring around Mars. Now that would be worth seeing.

Ref: Theoretical Formulation of the Phobos, moon of
Mars, rate of altitudinal loss.