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The ideal clock is merely a convenient fiction, as theorists from the University of Warsaw (UW) and University of Nottingham (UN) have shown. In a study published in the journal Classical and Quantum Gravity they demonstrate that in systems moving with enormous accelerations, building a clock that would precisely measure the passage of time is impossible for fundamental reasons.
"In both theories of relativity, special and general, it is tacitly assumed that it is always possible to construct an ideal clock -- one that will accurately measure the time elapsed in the system, regardless of whether the system is at rest, moving at a uniform speed, or accelerating. It turns out, however, that when we talk about really fast accelerations, this postulate simply cannot apply," says Dr. Andrzej Dragan from the Faculty of Physics, University of Warsaw.
The simplest clocks are unstable elementary particles, for example muons (particles with similar properties to electrons but 200 times more massive). Usually, muons decay into an electron, muon neutrino, and an electron antineutrino. By measuring the decay times and averaging the results for muons moving slowly and those moving at nearly the speed of light, we can observe the famous slowing down of the passage of time: the faster the muons are moving, the less likely the experimenter is to see them decay. Velocity therefore affects the clocks' observed tempo.
What about acceleration? Experiments were performed at CERN in the late 1970s, measuring the decay time of muons undergoing circular motion accelerations even as great as billions of billions of times the acceleration of Earth's gravity (10^18 g). Such acceleration was found to have no impact on the disintegration times.
The Polish-British group of theorists from the universities of Warsaw and Nottingham, on the other hand, were looking at the description of unstable particles moving in accelerating motion in a straight line. The key point for their analysis turned out to be a fascinating effect predicted in 1976 by the Canadian physicist William Unruh.
"Contrary to intuition, the concept of a particle is not completely independent of the observer. We all know the Doppler Effect, for example, which causes a photon emitted by a moving source to appear bluer to an observer toward which the source is approaching, but redder to one it is receding from. The Unruh effect is somewhat similar, except that the results are more spectacular: in an certain area of space, a non-accelerating observer sees a quantum field vacuum, whereas an accelerating observer sees many particles," explains Dr. Dragan.
The equation describing the Unruh effect says that the number of particles visible within a quantum field varies depending on the acceleration experienced by an observer: the greater the acceleration, the more of them there are. These non-inertial effects may be due to the movement of the observer, but their source can also be a gravitational field. Interestingly, the Unruh effect is very akin to the famous Hawking radiation emitted by black holes.
The unstable particles which the physicists from the universities of Warsaw and Nottingham treated as a fundamental clocks in their analysis decay as a result of interactions with other quantum fields. The theory says that if such a particle remains in a space filled with a vacuum it decays at a different pace than when in the vicinity of many other particles interacting with it. Thus if in a system of extreme acceleration more particles can be seen as a result of the Unruh effect, the average decay times of particles such as muons should change.
"Our calculations showed that above certain very large accelerations there simply must be time disorders in the decay of elementary particles. And if the disturbances affect fundamental clocks such as muons, then any other device built on the principles of quantum field theory will also be disrupted. Therefore, perfectly precise measurements of proper time are no longer possible. This fact has further consequences, because losing the ability to accurately measure the passage of time also means problems with the measurements of distance," explains Dr. Dragan.
Until now it has been assumed that the concepts of time and space may lose their traditional senses only when certain phenomena predicted by hypothetical theories of quantum gravity begin to play a vital role. It is believed that the necessary conditions prevailed in the vicinity of the Big Bang.
"In our paper, we show that for problems with the measurements of space-time to arise, such extreme conditions are not needed at all. Time, and therefore space, most likely cease to be accurately measurable even in today's Universe, provided that we try to carry out the measurements in systems moving with great acceleration," notes Dr. Dragan.
The results from the physicists from Warsaw and Nottingham mean that at sufficiently high accelerations, the operational capabilities of any theory built on the notion of time, and thus also space, will be disrupted. This raises interesting questions. If in extremely accelerating systems we cannot build a clock that measures time accurately, is this exclusively a fundamental flaw in our measurement methods? Or maybe something is happening directly to time itself? And do properties which cannot be measured accurately even make physical sense?
Modern accelerators can accelerate particles with accelerations several orders of magnitude higher than in the experiments of the 70s. Thus today we can carry out experiments in which the Unruh effect should be visible -- and so changes in the decay time of particles triggered by acceleration should be observable, too. The conclusions of the Polish-British group of physicists on ideal clocks will thus soon be verified.
"If our predictions are confirmed experimentally, many things related to our understanding of space-time, the passage of time, and its measurement methods will have to be rethought from scratch. It could be... interesting," concludes Dr. Dragan with a smile.

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The above post is reprinted from materials provided by Faculty of Physics University of Warsaw. Note: Materials may be edited for content and length.

In an article published in the Oct. 5 issue of Physical Review Letters, a team of University of Chicago physicists explains how to tune a laser to make atoms attract or repel each other in an exotic state of matter called a Bose-Einstein condensate.
"This realizes a goal that has been pursued for the past 20 years," said Cheng Chin, professor in physics at the University of Chicago, who led the team. "This exquisite control over interactions in a many-body system has great potential for the exploration of exotic quantum phenomena and engineering of novel quantum devices."
Many research groups in the United States and Europe have tried various ideas over the last decade. It was Logan Clark, a graduate student in Chin's group, who came up with the first practical solution. He has now demonstrated the idea in the lab with cesium atoms chilled to temperatures just billionths of a degree above absolute zero, and the technique can be widely applied to other atomic species.
Clark compared the process to a billiards game, when one ball encounters another. "Normally, as soon as the surfaces touch, the balls repel each other and bounce away," Clark said. In Chin's lab, cesium atoms replace the billiard balls, and ordinarily they repel each other when they collide. But by turning up the laser while operating at a "magic" wavelength, Clark showed that the repulsion between atoms can be converted into attraction.
"The atoms exhibit fascinating behavior in this system," he said. By exposing different parts of the sample to different laser intensities, "We can choose to make the atoms attract or repel each other, or pass right through each other without colliding."
Alternatively, by oscillating their interactions, analogous to making the billiard balls rapidly grow and shrink while they roll, the atoms stick to each other in pairs.
The researchers explained two fundamental ways that lasers influence the atomic motion. One is to create potentials, like a bump or valley on the billiard table, proportional to laser intensity. The new way is to alter how billiard balls collide.
"We want our laser to control collisions, but we don't want it to create any hills or valleys," Clark said. When the laser is tuned to a "magic wavelength," the beam creates no hills or valleys, but only affects collisions.
"This is because the magic wavelength happens to be in between two excited states of the atom, so they 'magically' cancel each other out," he said.
Magic is a concept that has no place in science, though the word does enjoy fairly common use among atomic physicists. "Generally it is used to refer to a wavelength at which two effects cancel or are equal, in particular when this cancellation or equality is useful for some technological goal," Clark said.

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The above post is reprinted from materials provided by University of Chicago. Note: Materials may be edited for content and length.

This existential question has been raised by a series of experiments conducted recently at the Large Hadron Collider and the Relativistic Heavy Ion Collider that smash various atomic particles together at nearly the speed of light in order to create tiny drops of primordial soup: the quark-gluon plasma (QGP) that cosmologists are convinced dominated the universe microseconds after the Big Bang before the universe cooled down enough for atoms to form. In fact, the flow characteristic of these droplets is a major topic at a scientific conference, Quark Matter 2015, taking place this week in Kobe, Japan.
As part of the Large Hadron Collider's CMS detector team, Professor of Physics Julia Velkovska, post-doctoral fellow Shenguan Tuo and assistant research professor Shengli Huang at Vanderbilt University have been at the middle of these discoveries.
In 2010, the LHC successfully created sub-atomic blobs of QGP by colliding lead ions together. Smashing these two massive ions -- each containing hundreds of protons and neutrons -- had generated the tremendous temperatures, more than 250,000 times hotter than the core of the sun, that are required for the primordial state of matter to form.
(Unfortunately, the physicists don't have a direct way to measure the number of particles in the quark-gluon plasma, so they use the number of subatomic particles that are created when the plasma evaporates as a measure of their size.)
"Lead ions are very large, each containing hundreds of protons and neutrons. When you smash them together at very high speed, they generate blobs of plasma that produce thousands of particles when they cool down," said Velkovska. "But when the LHC switched to proton-lead ion collisions, we didn't think the collisions would contain enough energy to produce the plasma."
However, Tuo, as part of his doctoral thesis, made detailed measurements of the behavior of the particles produced by these smaller proton-lead collisions and discovered that they were in fact producing liquid droplets that were about one tenth the size of those produced in the lead-lead collisions.
"Everyone was surprised when we began finding evidence for liquid behavior," said Tuo. "It caused some very intense debates."
One of the key properties of a liquid is the ability to flow. Looked at from the point of view of the individual particles in a liquid, the ability to flow means that each particle is exerting an attractive force on its neighbors that is strong enough to effect their movement but not strong enough to lock them together like they are in a solid. So their movements are coordinated and, when released from a container, they retain information about the container's shape. Tuo's measurements showed that small numbers of particles produced in the proton-lead collisions originated on the ellipsoidal surfaces of small QGP droplets.
Because of the computational difficulty involved, physicists normally look for these correlations between pairs of particles, but Velkovska, Tuo and their CMS collaborators took it several steps further. They searched for correlations between groups of four, six and eight particles. In some cases, they went to the extraordinary length of computing the correlations between all the particles in a given collision.
"These measurements confirmed that we were seeing this coherent behavior even in droplets producing as few as 100 to 200 particles," Tuo said. The results were published in Physical Review Letters in June. But that wasn't the end of the story.
The recreation of the quark gluon plasma (QGP) dates back to 2005. Velkovska and her Vanderbilt colleagues -- physics professors Victoria Greene and Charlie Maguire -- were members of the PHENIX science team at RHIC, located at Brookhaven National Laboratory, when they announced that they had created this new state of matter by colliding gold ions together at relativistic velocities. The big surprise was that this primordial material behaved like a liquid, rather than a gas.
To see what happened at even higher energies, the Vanderbilt group joined the CMS science team at the LHC located at the European Laboratory for Nuclear and Particle Physics in Geneva. The more powerful particle collider succeeded in duplicating the RHIC results, first as expected, by smashing lead ions together and then, unexpectedly, in the proton-lead collisions.
The proton-lead results prompted the scientists in the PHENIX team to re-analyze data that had been collected at RHIC in 2000, when the collider had smashed deuterium ions (proton-neutron pairs) and gold ions together at much lower energies than those in the LHC. The re-analysis, led by Shengli Huang, found that the proton-neutron pairs formed two hot spots in the gold ion when they collided which then merged into an elongated drop of QGP.
The RHIC researchers decided to test this further by adding a new run that collided helium ions (two protons and a neutron) with gold ions, and found that the same thing happened, except that three hot spots formed and merged into the QGP droplet. The results were just published in Physical Review Letters.
"Although the LHC collisions release 25 times more energy than the RHIC collisions, we don't see much difference in the droplet-formation process: Once you have reached the threshold, adding more energy doesn't seem to have much effect," said Velkovska. "I guess you can't get more perfect than perfect!"
Not only have the physicists found that the quark-gluon plasma is a liquid, the physicists have also established that it is nearly a perfect liquid: That is a liquid with zero viscosity that flows without any resistance. If you swish a perfect liquid in a glass and set the glass down, then the liquid will continue to swirl around as long as it is not disrupted.
Curiously, the phenomenon that most closely resembles the properties of the hottest known liquid is one of the coldest known liquids: lithium atoms that have been cooled to temperatures one-billionth of a degree above absolute zero using a device called a laser trap. When released from the trap these ultra-cold atoms also behave as a perfect liquid with near-zero viscosity.
"These are both strongly coupled systems. This appears to be an emergent property of such systems," Velkovska has concluded.

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The above post is reprinted from materials provided by Vanderbilt University. Note: Materials may be edited for content and length.