helium (hê´lê-em), (He), gaseous element, first observed spectroscopically in the sun during a solar eclipse in 1868. Its noncombustibility and buoyancy make this extremely unreactive, INERT GAS the most suitable of gases for BALLOONS and AIRSHIPS. Deep-sea divers often breathe a helium-and-oxygen mixture; because helium is less soluble in human blood than nitrogen, its use reduces the risk of the bends (DECOMPRESSION SICKNESS; DIVING, DEEP-SEA). Liquid helium is essential for low-temperature work (LOW-TEMPERATURE PHYSICS; SUPERFLUIDITY). Helium is also used in arc welding and gas-discharge lasers. Abundant in outer space, helium is the product of hydrogen fusion in STARS.
noun Symbol He
A colorless, odorless inert gaseous element occurring in natural gas and with radioactive ores. It is used as a component of artificial atmospheres and laser media, as a refrigerant, as a lifting gas for balloons, and as a superfluid in cryogenic research. Atomic number 2; atomic weight 4.0026; boiling point -268.9°C; density at 0°C 0.1785 gram per liter.
[From Greek hêlios, sun (so called because its existence was deduced from the solar spectrum).]
lo fi noise pop indie rock entity Helium
member: Mary Timony
Why does inhaling helium make your voice sound funny?
The secret to this trick lies in the fact that helium is less dense than air. The less dense the medium, the faster your vocal chords will vibrate, in this case making your voice sound higher pitched.
A lower pitch can be achieved
by inhaling a gas that's heavier than air,
such as xenon, but a party decorated with xenon
filled balloons doesn't sound as fun.
Source: New Scientist
05 September 1998
After years working with the world's weirdest fluid,Richard Packard and Seamus Davis thought they'd seen it all -- until the fluid in their lab started whistling. Michael Brooks reports.
SUPERFLUID helium is to liquids what Salvador Dali is to artists--full of surreal surprises. Put some in a bowl, set the bowl spinning, and the liquid inside will remain stationary. Draw a cupful out of the bowl, suspend it a few centimetres above the remaining liquid, then stand back and rub your eyes--the fluid in the cup will cheat common sense by pouring itself, drop by drop, back into the bowl. A drop climbs up the inside of the cup, then runs down the outside. When it falls, another begins climbing, and themagic continues until the cup is dry.
When you have spent most of your life working with a liquid which brings the weirdness of the quantum world into our own, you would think nothing could surprise you. You'd think that once you had seen it defy gravity and climb the walls of its container, you would have seen it all. And that's just what physicists Richard Packard and Seamus Davis thought--until the liquid in their lab started whistling.
Last year, Packard, Davis and their colleagues at the University of California at Berkeley linked two pools of superfluid by a tiny hole, and out came an eerie sound like a falling bomb, a whistling that gradually decreased in pitch. They had discovered the quantum whistle.
In the days that followed, the two researchers couldn't stop playing with its strange sounds, but now things are becoming a bit more serious. For behind the light-entertainment value of this whistle lies a host of exotic applications. The group has already used the phenomenon to fashion a gyroscope that harnesses a superfluid's weirdness to measure how fast the Earth is spinning. They are hard at work on other devices too, some of which could be used to put Einstein's relativity theories through more demanding tests than ever before.
Conceptually, a superfluid is little more than a liquid that lives by quantum rather than classical rules. To make one, you need to cool a substance down to extremely cold temperatures so that its particles move very slowly. According to quantum theory, this causes the wavesassociated with the particles to stretch out and becomemore influential--the quantum nature of one can affect the behaviour of others. But most liquids turn to solids long before this can happen.
What makes helium special is that it remains a liquid at all temperatures. Near absolute zero, when the quantum waves associated with its atoms begin to overlap, a superfluid is born: the atoms suddenly lose their individuality and the liquid collapses into a Bose-Einstein condensate, a quantum substance in which the atoms move in lockstep. It's a strange and super-slippery stuff that canflow through pipes with no friction at all, and perform all sorts of other stunts.
In 1963, physicist Brian Josephson of Cambridge University predicted that merely linking together two tiny pools of superfluid would cause a natural quantum oscillation, in which the liquid would rush back and forth through the link. Packard and his colleagues were searching for theseoscillations in early 1997, but, like others before them, they weren't finding anything. The screens of their oscilloscopes--used to detect and display rapid oscillations--showed nothing interesting, and Packard's frustrated graduate students were ready to give up. Their sophisticated research equipment was getting them nowhere.
When Packard suggested that they take a pair of headphones and listen for the signal, the students were less thanenthusiastic. "They kept arguing that there was no point because there was nothing there," he says. He kept at them to try it, but they resisted. "They really didn't want to do it--in the end they simply argued that they couldn't do it because they didn't have any headphones in the lab."
So Packard went to a local electronics store, bought a $50 pair of headphones out of his own pocket, and presented them to his students. "The connector's wrong," they said. He went back to the shop and bought an adaptor. Graduate student Sergey Pereverzev reluctantly plugged in the headphones and flicked a switch to start up the experiment. His jaw dropped. What he heard was just what the theory had predicted: a high-pitched whistle that gradually became lower in tone, like the sound of a falling bomb.
People got so excited, Packard recalls, that the headphones only lasted four days. "Pulling them on and off, they tore them apart." But not before they were able to do the tests and measurements that pinned down the discovery, which they reported last year in Nature (vol 388, p 449).
So what makes the whistle? According to Josephson'scalculations, oscillations should occur in any two pools of superfluid connected by a tiny hole. All you need is a small pressure difference between them. In an ordinary liquid, the fluid would simply flow from one side to the other. But a superfluid has other ideas.
Since each fluid is a quantum substance, it has a "wave function"--an undulating wave-like form that describes its properties. This wave function depends on, among other things, a superfluid's pressure, which means that the wave functions of the two superfluid pools differ. This leads to a kind of confusion involving the atoms at the boundary between the pools. Roughly speaking, these atoms try to occupy both regions at once and end up doing a rapid quantum dance back and forth between the two.
The idea is simple enough. And yet it took ten years to detect the oscillations. To do it, Packard and his colleagues had to create what is known as a "weak link". This is a hole just large enough to allow the superfluids' wave functions to overlap, yet small enough to prevent the liquids from merging into one.
Making the perfect weak link relies on creating a connection with a diameter roughly equal to the "healing length" of the superfluid--the length over which the wave function remains more or less constant. For the most common form of helium--helium-4, which has two neutrons and two protons in its nucleus--that would mean punching a hole between the reservoirs that was only 0·1 nanometres in diameter, which for now is technically impossible. But helium-3, a less common isotope having only one neutron in its nucleus, has a far larger healing length, so a weak link can be 500 times larger.
Even with helium-3, however, there is a problem. A good, weak link would produce a whistling so tiny as to be undetectable. So Packard's team linked their superfluid baths with a grid of 4225 identical perforations in a tiny silicon wafer. They were hoping that the apertures would together produce oscillationslarge enough to be detected. As it turns out, they were lucky.
They started by embedding the silicon wafer in a stiff membrane, which they glued to the bottom of an aluminium washer. A chamber filled with superfluid was made by gluing a flexible, metal-coated membrane to the top of the washer. The object was then immersed in superfluid to create two pools--one inside and one outside--connected by the tiny holes in the silicon wafer.
To apply a pressure difference, the team momentarilydeformed the flexible membrane with a voltage, socompressing the helium-3 trapped within the structure. This caused an oscillation across the weak links, just as Josephson had predicted in 1963. An ultra-sensitive motion sensor placed next to the washer detected the movement and sent the signal up to an oscilloscope, or the headphones.
As the membrane slowly returned to its original shape, the pressure difference slowly decreased. Consequently, theoscillation frequency--proportional to the pressure difference--also fell off slowly. This explains why it was so hard to see the signal on the oscilloscope. In sweeping over a range offrequencies, it left no single "spike" on the screen. But the human ear is adept at hearing sounds with changing pitches--so the falling bomb sound was clear through the headphones.
The group is still working out a complete theoretical model for its whistling superfluid, but that hasn't stopped them seeking applications. Eventually, they hope to incorporate it into the world's most sensitive gyroscope.
Gyroscopes use rotating bodies to sense shifts in the direction of movement. They are essential for navigation on board ships and aeroplanes, providing an absolute reference for their orientation and movement. Using helium-4, the Berkeley team has already produced a superfluid gyroscope that can detect changes in the Earth's rotation speed with an accuracy of 0·5 per cent. Using helium-3 and "whistling links", they believe they can do much better.
The design of a superfluid gyroscope is based on the fact that these quantum liquids like to remain perfectly motionless. More specifically, they prefer to remain in a state of zero angular momentum. This can happen if the fluid remains completely motionless, or, if the "amount" of rotation clockwise and anticlockwise compensate one another. If you push one part of a superfluid one way, another part will move in the opposite direction to compensate. But if the flow velocity at any point gets too high, the superfluid can save energy by allowing a "phase slip", the sudden creation of a vortex-like tornado in the superfluid. This removes excess energy from the fluid by pinching all the rotation down into a tiny tube.
Packard and his team have exploited this effect to build a highly sensitive device for measuring rotation. Their apparatus lives on a 1-centimetre-square silicon chip. An etched channel spirals around the edge of the chip. At one outer end of the spiral is a relatively large (1 millimetre in diameter) hole, and at the other end is a tiny 1 micrometre hole. When immersed in a superfluid, there is a circular path around which the fluid can flow. Essentially, the device forms a ring of superfluid with a weak link fixed inside it.
As the Earth spins, so does the chip. And this is enough to cause a compensating flow in the superfluid--a small backflow through the tiny aperture. This is too small to measure, so Packard's team has had to develop an ingenious means of watching the superfluid's motion. They covered one side of the chip with a plastic membrane. When set vibrating, this membrane pumps superfluid back and forth in the channel, so inducing a corresponding flow through the tiny aperture.
At a certain point in the cycle of this alternating flow, the fluid reaches a critical velocity, which forces a vortex through the aperture. This causes a jump in the position of the membrane, and a glitch on a nearby position sensor. The glitches would happen at a fixed rate even if the device wasn't rotating. But rotation changes how the glitches occur.
By slowly turning the cryostat containing the silicon chip from an east-west orientation through to north-south, Packard and his team could watch as the effect of the Earth's rotation was gradually added to the oscillating flow velocity of the superfluid, changing the glitch's position in the cycle. This change gave them a way to measure the effect of the Earth's rotation.
By adding more turns and increasing the loop area, Packard believes it may be possible to improve the gyroscope's sensitivity by up to 10 000 times. But they are currently working at the limits of their laboratory, and conducting experiments in the dead of night, when no one is around to ruin the results by, for example, flushing a distanttoilet. Further improvements will mean quitting theBerkeley campus to escape such vibrations. Eventually, the gyroscope may even have to be calibrated in space.
One of the possible applications of the instrument is in geodesy, which is concerned with surveying and mapping the Earth. Studying the vibrations and rotation of the planet can reveal what is happening in its interior. The signals involved are exceedingly tiny, and the only way the team will be able to tell if the gyroscope is up to the task is to detach it from terrestrial vibrations--by putting it in a satellite and letting it float.
If it isn't up to the task, that would probably be due to the noise introduced by the vortices as they pass through the aperture. A gyroscope made to a different design, using the whistling helium-3 weak links, doesn't rely on creating vortices. Instead, it uses quantum interference effects to detect rotation. "Our belief is that the noise is going to be a good deal smaller in this system," Packard says.
Helium-3 might make the ultimate gyroscope, but it has its own problems, Packard admits. "It would need to be a thousand times colder than the helium-4, so it's technologically more difficult. Whether one would want to do it depends on whether there's a scientific problem that justifies the effort."
Modern aircraft and submarines employ ring lasergyroscopes, in which revolving beams of light detectchanges in orientation and position. Packard concedes that they are already as good as they need to be. Who wants to make a gyroscope that needs cooling to near absolute zero? "It's clear," he says, "that nobody's going to put this in an aeroplane when laser gyroscopes are already good enough to get you from New York to London."
But superfluid gyroscopes
could be put to work in other fields. Their
quantum sensitivity may, for example, be sufficient
to finally settle a century-old argument about Einstein's
general theory of relativity. That's because superfluids
on Earth pick out what physicists call an "absolute
inertial frame"--they have an unnerving ability to
keep still while their containers revolve around them.
But what constitutes true "stillness", and to what does the helium anchor itself so as not to rotate?
In an absolute inertial frame, the laws of physics are just what Einstein's special theory of relativity says they are. In particular, a body at rest should remain that way. We rotate as the Earth spins, so we clearly don't live in such a frame. The superfluid shows us how much we're rotating with respect to the ultimate state of no rotation. "The question," says Packard, "is whether this is the same frame in which the distant stars are at rest. General relativity would say that it is not."
Einstein's general theory of relativity deals with gravity,and says that the proximity of the spinning Earth should change the inertial frame of anything near it. To test this, you could make a highly sensitive superfluid gyroscope, move it around, and work out what it considers to be the absolute inertial frame near the Earth. Then you can couple the superfluid gyroscope to a telescope that points at a distant star. The aim is to find out if that distant star is moving with respect to the inertial frame as detected by the gyroscope on the Earth.
The measurements involved would have to be more accurate than anything currently possible, and Packard is not sure they will ever get the required accuracy. "We'll continue the development and see what happens," he says.
Whatever the future holds, he is confident that his team will discover more about superfluids. This laid-back, optimistic approach exemplifies Packard's philosophy of science. He sees his research as more of a leisure pursuit than a career. Some make model aeroplanes, some people make superfluids whistle. In a world where liquids climb walls, who's to say what's strange?
Michael Brooks is a freelance journalist based
East Sussex From New Scientist, 05 September 1998