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Two relativity tests are better than one

Einstein’s 1905 special theory of relativity is based on the idea that the speed of light is constant in all directions regardless of the relative motion of the observer. A consequence of Lorentz invariance, this property was first demonstrated by Albert Michelson and Edward Morley in their famous experiment of 1887.

Michelson and Morley split a beam of light in two and sent the beams off at right angles to two different mirrors. The beams were reflected back and recombined to form an interferometer. If the beams travelled at different speeds in the two directions — as they would if they were passing through a stationary aether through which the Earth was moving — then the two beams would be out of phase when they were recombined, leading to an interference pattern. No such pattern was revealed — ruling out the existence of an aether — and over the past 120 years the Michelson-Morley experiment has been refined and repeated to confirm that the speed of light is constant to one part in 10¹⁶.

However, there is one lingering doubt surrounding the Michelson-Morley experiment, according to Holger Mueller, a physicist at California’s Stanford University. Any Michelson-Morley experiment is also sensitive to possible changes to the length travelled by the light, and to changes in the refractive index of the medium that the light travels through.

Such physical changes could be caused by violations to Einstein’s theory and could prevent the Michelson-Morley experiment from detecting changes in the speed of light. For example, if both the speed of light and the length travelled by a light beam changed by the same factor, the changes would cancel each other out. A general theoretical framework for describing the violation of Lorentz invariance is defined in the standard model extension (SME).

In the past, most physicists simply assumed that the physical properties of Michelson-Morley experiments do not change, and have interpreted the results as proof of the Lorentz invariance of light. Now, however, Mueller and colleagues in Australia, Germany and France have worked out a way to separate possible changes in the speed of light from the variations in the physical properties of the apparatus.

The team performed two different Michelson-Morley experiments – one in Berlin involving infrared light in optical cavities and the other in Perth, employing microwave radiation in a pair of resonating cavities. The research will be described in an upcoming issue of Physical Review Letters.

The researchers used the SME to calculate possible changes to physical properties of both experiments as well as to the speed of light. While the SME predicts that the speed of light in both experiments should change by the same factor, the theory says that changes in the physical properties of the two experiments will change by different factors. By doing two experiments, Mueller obtained sets of equations that can be solved to separate the possible changes to the speed of light from physical changes.

Mueller told Physics Web that the experiments were both run for over a year, which meant his team could measure Lorentz violations that become evident only by a modulation of the experiment’s rotation relative to an inertial frame, such as the modulation provided by the Earth’s orbit. Making the measurements in different geographical locations meant that the experiments were sensitive to different combinations of Lorentz violations, which would boost their ability to separate the changes.

In all, the team was able to say that Lorentz invariance is not violated in 14 parameters associated with SME to an accuracy of around one part in 10¹⁶. While the team were able to boost the accuracy of some parameters by a factor of 50 over previous experiments, Mueller believes the real significance of the team’s work is the ability to simultaneously confirm the Lorentz invariance of light and matter without assuming the Lorentz invariance of the physical properties of the other.

Nanowires to probe biological samples

Until recently, many physicists thought it would be impossible to image objects with unrestricted resolution because of the so-called diffraction limit, which prevents objects that are much smaller than the probing light’s wavelength from being seen. However, techniques such as near-field imaging have been able to beat this limit by using a tiny light source that can get to within a wavelength of the surface of a sample. In this way, sub-wavelength surface features reveal themselves by reducing the transmitted intensity of the oncoming light. But so far there have been no light sources that can acquire sub-wavelength images from biological samples, which often need to be supported in warm, wet conditions.

Now, however, a group led by Peidong Yang at the Lawrence Berkeley National Laboratory in the US has discovered that nanowires made from potassium niobate can be used as optical light sources for sub-wavelength imaging. They began by dispersing many nanowires into water, and then used an infrared optical tweezer – a laser that can apply a force to nano-sized objects – to grab just one.

A unique feature of the potassium-niobate nanowires is that they can absorb two photons at a time from the optical tweezers and then reemit one at twice the frequency of the originals. This “frequency doubling” is useful because it produces green light that would be able to effectively highlight parts of biological samples coated with fluorescent dye – a common “tagging” method used in biological imaging.

Yang’s group used the optical tweezer to scan the nanowire over the surface of a thin sheet of glass imprinted with 50-nm thick lines of gold, and recorded the intensity of the reemitted light transmitted onto a detector. As the nanowire passed over each gold line, the intensity dropped, allowing them build up an image of the surface.

Because the nanowire light source needs no wires, it can be operated in liquids safely, making it ideal for biological samples. “We get many neat ideas from the physics community that fall flat when they are applied to biology,” Warren Zipfel, a biomedical engineer at Cornell University in the US, told Physics Web. “If this can really operate [in biological conditions], that’s a big plus.”

The US group says the next step is to refine the signal-processing techniques to make the imaging technique as practical as others such as atomic force microscopy.

Big stars have weather too

Magnetic fields emanating from stars and planets can usually be attributed to convection currents forcing molten metal to shift about in so called “dynamo action”, which converts mechanical energy into magnetic energy. In small stars like our Sun, the magnetic field can become so intense in regions on the surface that it traps heat, causing dark sunspots to form. But physicists believe that most larger stars do not have convection currents stretching to their surface and therefore would have a zero – or at least small – magnetic field. This should mean they are devoid of surface structure.

Recently, however, observations of irregular surface distributions of heavy elements on large blue “B-type” stars have swept aside this assumption. In particular, alpha Andromedae, which is 97 light-years from Earth, has a predicted magnetic field less than 10 Gauss, yet appears to have spots of mercury all over its surface.

Now, a group led by Oleg Kochukhov of Uppsala University has completed a study that traced these spots on alpha Andromedae over a seven year period. They used the Dominion Astrophysical Observatory in Canada and the Spectral Astrophysical Observatory in Russia to record how the strength and shape of the brightest line in the star’s characteristic mercury emission spectrum changed as the star rotated.

To interpret the results, Kochukhov’s group used a clever technique based on the Doppler effect, which makes the wavelength of light appear to compress or stretch depending on how the light is moving relative to an observer. By analysing the mercury line, they could tell whether it had wavelength components coming from parts of the star that were rotating towards or away from the telescopes. In this way, they could build up a detailed picture of how the mercury distribution changed on alpha Andromedae’s surface over time.

“The Doppler imaging technique applied in our investigation is probably the highest resolution remote sensing method astronomers have ever developed,” Kochukhov told Physics Web. “It is equivalent to reading a 1 cm newspaper title from a distance of 350,000 km, which is similar to the distance from the Earth to the Moon.”

The group discovered that the spots were distorted “clouds” that move over the surface of alpha Andromedae in a similar way to clouds on Earth. According to Kochukhov, these could exist because of a delicate balance between the star’s gravity pulling the mercury downwards and the pressure from radiation pushing it upwards. With minor perturbations, the balance would tip, causing regions of higher density mercury to accumulate. Just a 1.3% dip in gravity on the surface, for example, would create 300 times more mercury in that region, says Kochukhov.

Kochukhov thinks that by observing “weather” on stars, we might be able to get a better handle on our own. “[It] provides a new, interesting constraint that contributes to our understanding of how complex systems form,” he added.

LHC will switch on in May 2008, says CERN

“We’ll be starting up for physics in May 2008, as always foreseen, and will commission the machine to full energy in one go,” said LHC project leader Lyn Evans.

The €6.3bn Large Hadron Collider (LHC) will be the world’s largest particle accelerator, colliding protons at energies of 14 TeV to generate what physicists hope will be a slew of new particles, such as the Higgs boson and so-called supersymmetric particles.

During construction, however, the accelerator has suffered repeated delays. CERN was caught out in 2001 when it emerged that the LHC was to cost 30% more than originally envisaged and was also running behind schedule. The committee that reviewed CERN’s operations in the light of these overruns recommended that the collider’s start date be put back from 2005 to 2007. Then, in March this year, a test of one of the magnets built by Fermilab in the US failed during a high-pressure test, which was designed to simulate the “quenching” that can occur when proton beams career off course into the magnets.

Although repair of the faulty magnet is currently in progress, CERN spokesperson James Gillies told Physics Web earlier this month that the delay would force CERN to cancel the low-energy “engineering run”, which was scheduled to take place in November. The machine’s operators were planning to use the low-energy run as an opportunity to gain experience steering the protons and detecting collisions before high-energy collisions take place.

CERN have now officially announced the cancellation of the engineering run, and now plan to slowly bring the LHC up to full-energy proton collisions after it starts up in May. “There’s no big red button when you’re starting up a new accelerator, but we aim to be seeing high energy collisions by the summer,” said Evans.

Currently, one of the eight sectors of the LHC has been cooled to its operating temperature of 1.9 K, and CERN say that cooling of a second sector is underway.

Telescope limitation could reveal exoplanets

Looking for exoplanets is tricky – normally, they cannot be observed directly with telescopes because any light they emit is drowned out by the glare from their parent star. In the last decade or so, however, astronomers have realized that they can observe them in special circumstances – for example, when an exoplanet’s orbit takes it behind its parent star so we no longer see the contribution of its light, causing the star to “wink”.

Astronomers wishing to observe a planet nearby to a star must also contend with the “diffraction limit”, which is a well-known limitation on the spatial resolution of an optical telescope. Even in the best telescopes, as light passes through the aperture it will diffract, causing alternating light and dark concentric rings – called “Airy rings” – to surround high-resolution images of stars and other objects. The radii of these rings depend on the wavelength of the light being observed, and no object can be seen that is smaller than the spot that appears at the centre of them.

Now, an international group led by Laird Close of the University of Arizona and including Niranjan Thatte of the University of Oxford have used a data-processing technique that exploits the Airy rings to retrieve the spectrum of stellar “companion” objects such as exoplanets from the glare of the parent star with a hundred times more spatial resolution than existing techniques.

The idea, which was first proposed in 2002 for space-based telescopes by the US-based astronomers William Sparks and Holland Ford, involves looking at how the Airy rings grow as images are taken at increasing wavelength. If a bright area remained in the same place as the wavelength changed, it would be an indication of a companion object. Therefore, by subtracting the part of the image containing the spreading rings – a procedure known as “spectral deconvolution” – an image of the companion object would be left.

Close and Thatte’s group have now performed spectral deconvolution using a special instrument on ESO’s ground-based Very Large Telescope called SINFONI – an “integral field spectrograph” – which can record a 2D map of a star’s light at 1000 wavelengths. They took data from AB Doradus, a system comprising two pairs of stars 48 light-years away, and analysed it computationally to separate the 2D spectra of one of the smallest stars, AB Doradus C, from the largest, AD Doradus A.

Although AB Doradus C has been imaged before using a less sophisticated technique, this is the first time a detailed image and spectrum has been recorded simultaneously. Astronomers had thought AB Doradus C would be a brown dwarf, a dim star that is too small to produce energy by nuclear fusion. Now, however, Close and Thatte have confirmed that it is just large enough – almost a tenth the mass of our Sun – to be a red dwarf.

Close and Thatte are now planning to use their technique in the hunt for exoplanets. But they will have to wait until the next generation of “extreme adaptive optics” instruments are completed for existing ground-based telescopes, which will be able to effectively cancel-out noise in the Earth’s atmosphere and thus increase image contrast in order to make out faint, distant objects. “This was more of a proof of technique,” Thatte told Physics Web. “Hopefully, though, it will one day enable us to find the Holy Grail – the spectrum of an Earth-like exoplanet.”

Laser technique could help redefine the kelvin

The Boltzmann constant, k_B, is a fundamental constant of nature that relates the kinetic energy of an ensemble of microscopic particles — such as gas molecules — to its temperature. As a result, it provides the crucial link between the microscopic world of atoms and molecules and the macroscopic properties of matter such as pressure. So far, there is only one technique — measuring the speed of sound in argon gas – that can determine k_B to an accuracy of about 2 parts-per-million (ppm). Other techniques for determining k_B include measuring noise in a resistor; determining the dielectric constant of a gas; and measuring the radiation emitted from a black body. However, none of these techniques has yet to reach ppm accuracy.

Having a number of independent measurements of k_B – those based on techniques that are subject to different systematic errors – at the ppm level is particularly important to the Paris-based International Committee for Weights and Measures (CIPM), which is planning to redefine the kelvin in 2011 using k_B.

The kelvin is currently defined as 1/273.16 of the temperature difference between absolute zero and the triple point of very pure water held at a certain pressure. While the triple point technique can define the kelvin to better than 1 ppm, the problem with this definition is that it is based on a specific physical situation, which must be reproduced whenever a highly accurate definition of the kelvin is required. The CIPM, however, wants to define the kelvin and other SI units in terms of each other and the fundamental constants. In the case of the kelvin, this involves the second and k_B — and therefore an accurate value of the kelvin could be obtained from the second, which is known to an extremely high degree of accuracy of about one part in 10¹⁶.

Now, Christian Chardonnet and collegues at Université Paris 13 – Institut Galilée, claim to have developed a laser spectroscopy technique that could provide an alternative way of measuring k_B to ppm accuracy. The technique exploits the fact that the thermal motion of a molecule – ammonia in Chardonnet’s experiment — smears out peaks in its optical absorption spectrum in a process called thermal broadening. The width of the broadening is determined by k_B as well as the pressure and temperature of the gas and the frequency of the light being absorbed. By measuring the width of the broadening as a function of pressure at a fixed temperature and frequency, Chardonnet and colleagues were able to determine k_B to an accuracy of about of about two parts in ten thousand.

Although this is nowhere near the accuracy needed by the CIPM, the researchers believe that, in principle, the technique could be improved to 1 ppm. Specifically, the team intend to use more stringent temperature control in their experiment in order to stabilize the temperature to better than 1 ppm. They also intend to increase the amount of ammonia in the path of the laser and improve the stability of the laser, which should allow more data to be accumulated in a shorter time.

Single atom entangles two photons

When two photons are entangled, the quantum state of one photon is revealed automatically by measuring the state of the other — a property that is crucial to the operation of quantum computers. Whereas ordinary computers use bits of information that are either 1 or 0, a quantum computer would use quantum bits of information, or qubits, that can be in a superposition of both 1 and 0 at the same time. A 1 could represent, say, a horizontally polarized photon, while 0 represent a vertically polarized photon. By combining N such qubits, these could entangled to represent 2^N values at the same time, which would, in principle, allow a quantum computer to outperform a classical computer for certain tasks.

Some physicists believe that quantum computers could involve entangled photons moving from node to node in an optical system – with the nodes performing logical operations on the photons. In order for such a computer to work, it would need a way of transferring quantum information from the entangled photons to the nodes and vice versa. It would also require a way of generating entangled photon pairs and sending them off in the appropriate directions. Little of this technology is available today and hence quantum computation remains a distant dream.

Such nodes could consist of an atom trapped by a standing wave of light in an optical cavity. However, researchers have so far only been able to get such an atom to emit a single photon that is entangled with the atom itself. Now, however, Gerhard Rempe and colleagues at the Max Planck Institute for Quantum Optics in Garching, Germany and Axel Kuhn at the UK’s Oxford University, have extended this technique to use a single atom to create an entangled pair of photons.

The team first fired a laser pulse at a trapped atom, causing it to emit a single photon. As a result of this process, the atom and the photon are entangled. A microsecond or so later, a second laser pulse was fired at the atom, causing it to emit a second photon. Crucially, the second pulse cause the entanglement to be transferred from the atom to the second photon, and the two photons become an entangled pair.

Rempe told Physics Web that the photons could then be sent to interact with two different atoms. As a result of this interaction, the two atoms would become entangled with each other. This, he said, could form the basis of a “quantum repeater”, which is an essential component of a quantum computer.

According to Rempe, an important benefit of the scheme is that an entangled photon pair can be produced “at the push of a button”, unlike other methods, which create entangled pairs in random manner.

The researchers have managed to operate their scheme with a 1.3% probability that a photon pair is entangled. While this is on par with other entanglement schemes for quantum computing, the team is currently working to better localize the atom inside the cavity, which Rempe said will improve the efficiency of the system.

Fundamental constant is pretty much constant

Fundamental constants are very finely tuned for our existence – if the strong force were just 1% stronger than it is today, for example, carbon could not be produced in stars, and we would not be here at all. This is one reason why many physicists are eager to check whether certain fundamental constants have changed over the history of the universe.

One such fundamental constant is the ratio of the electron and proton masses, µ. Traditionally this has been measured by analysing data from a ground-based telescope that is pointed at a quasar – a compact but very bright nucleus of a young galaxy which serves as a “beacon” in distant space. The spectrum of light from quasars covers a broad range of wavelengths, but some of these wavelengths can be absorbed by molecules in older galaxies as the light travels through the cosmos. These wavelengths, called absorption lines, correspond to the molecules “jumping” to higher rotational or vibrational energy levels, and are governed by µ.

Because light from quasars can take billions of years to arrive at Earth, the value of µ measured from these distant sources can be compared with the value of µ measured in a lab experiment to see if the constant has changed over time. But now Victor Flambaum and Michael Kozlov of the University of New South Wales in Australia have made the technique more accurate by incorporating an analysis of an “inversion spectrum”, which is produced when atoms in molecules absorb light and quantum-mechanically tunnel to a higher energy level. Because the probability of tunnelling is more dependent on µ than the absorption lines in the rotation spectrum, it enables any time variations in µ to be calculated more accurately.

Flambaum and Kozlov took existing data from the Effelsberg radio telescope in Germany of light coming from a quasar through the galaxy B0218+357, 6.5 billion light-years from Earth, and examined both its inversion spectrum of ammonia molecules and its rotational absorption spectra of other molecules such as carbon monoxide. They then compared the spectra to those from present-day lab experiments. They found that µ cannot have decreased by more than 4 × 10^-16 per year, and cannot have increased more than 2 × 10^-16 per year – an estimate ten times more accurate than the previous best.

Last year, a group led by Wim Ubachs of VU University in Amsterdam, the Netherlands, found using the older technique that µ could be decreasing over time. If true, this would mean that the most fundamental theories in physics, such as Einstein’s theory of relativity, would have to be reconsidered. Flambaum told Physics Web, however, that his more accurate results show that µ is unlikely to have changed, so our current understanding of physics is safe. Moreover, he added that if more data is collated his analysis technique should enable theorists to determine variations in µ even more accurately.

Single crystals go supersolid

Supersolidity was first predicted in 1969 by Russian theorists Alexander Andreev and Ilya Liftshitz. They said that lattice vacancies, which usually only occur at finite temperature, could still exist at temperatures close to absolute zero in weakly-bound elements such as helium due to quantum “zero point” energy. By cooling solid helium to low temperatures, these vacancies could all collapse into the same ground state, becoming what is known as a Bose-Einstein condensate (BEC). In this supersolid state, vacancies would behave as a coherent entity, moving throughout the rest of the solid effortlessly like a superfluid.

In 2004, Moses Chan of Pennsylvania State University in the US was the first to find evidence for supersolidity when he and his graduate student Eun-Seong Kim noted a small change in the rotational inertia of a sample of helium-4 supported inside a torsion oscillator below a temperature of 230 mK. This, they concluded, meant 1% of the sample had remained at rest in the lab frame by condensing into a supersolid. But according to a more recent theory, that strength of supersolid signal would not be possible from zero-point vacancies alone, and so is not explainable by Andreev and Liftshitz’s original theory.

Earlier this year, Nikolay Prokofev and Boris Svistunov of the University of Massachusetts and several others suggested that Kim and Chan’s results – and other supersolid signals found since then by other groups – could be coming from superfluid grain boundaries. These are defects just three atoms wide that separate regions of different crystal orientation. But now Chan – this time with graduate students Anthony Clark and Joshua West – has ruled out this possibility by repeating his experiment with single crystals of helium-4, which contain no grain boundaries

To ensure that he was working with single crystals, Chan managed to “seed” the growth of solid helium at a cold spot within the torsion oscillator. “We can be confident that our samples are single crystals or at worst comprise just a few large crystals in the sample cell,” he said. He used two oscillators: one for samples of commercially pure helium-4 (containing 300 parts per billion helium-3) and the other for “isotopically” pure helium-4 (containing one part per billion helium-3).

Chan’s group found that single helium crystals still gave a supersolid signal, albeit reduced. They also found that samples of the purer, isotopic helium-4 had a stronger signal than samples containing the commercially pure grade. To see if this was actually an effect of purity, they added helium-3 to the purer helium-4 samples, but the signal remained just as strong.

Chan told Physics Web that he suspects dislocations are the cause of supersolidity, and that the stronger signal from the purer sample could be due to the differences between the two torsion oscillators affecting the crystallization of the helium. “All we can say is that in a single, solid crystal we still see [a supersolid signal], and it is not due to grain boundaries,” he said. “But why there is still a variation in [the signal], we don’t really know.”

The hunt for unparticles is on

All particles exist in a state with a certain energy, momentum and mass. In most of the Standard Model, particles of the same type cannot exist in another state with all these properties scaled up or down by a common factor – electrons, for example, always have the same mass regardless of their energy or momentum. But this is not always the case: massless particles, such as photons, can exist with their properties scaled equally. This immunity to scaling is called “scale invariance”.

Although theories exploiting scale invariance have been devised before, physicists accustomed to the Standard Model have been reluctant to think how they could be applied. This is because they would involve things unlike particles – the only known “stuff” in the universe – that do not necessarily have zero mass.

Georgi, on the other hand, has now considered how we could deduce experimentally whether any of this weird scale-invariant stuff, which he dubs “unparticles”, could exist. He suggests that the reason we haven’t seen any unparticles so far is that they couple with normal matter more weakly as energies get lower. If he is right, this means evidence for unparticles might show up in future experiments such as the Large Hadron Collider (LHC). “The very confusing question of ‘What does unparticle stuff look like?’ gets replaced by a simpler question: ‘How does unparticle stuff begin to show up as the energy of our experiments is increased?'” he said.

Unparticles would have properties in common with neutrinos, which have almost zero mass and are therefore nearly scale invariant. Neutrinos barely interact with matter – most of the time physicists can only infer their presence by calculating the “missing” energy and momentum after an interaction. By looking at the same interaction many times, a probability distribution is built up that tells more specifically how many and what sort of neutrinos are involved.

Georgi thinks that a similar technique could be used to search for evidence of unparticles. According to scale invariance, a distribution containing unparticles would become apparent because, oddly, it would look like a distribution for a fractional number of massless particles.

“Even if we don’t [see unparticles at the LHC], I believe that analyses like this are useful because they can shake us out of preconceptions that could cause us to miss important physics as the energy of our machines grows,” he said.

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