The LHC as a photon collider

Quartic gauge coupling: A Feynman diagram showing how protons radiate photons that then interact and produce W bosons.

Quartic gauge coupling: A Feynman diagram showing how protons radiate photons that then interact and produce W bosons.

Yes, that’s correct: photon collider.

The Large Hadron Collider is known for smashing together protons. The energy from these collisions gets converted into matter, producing new particles that allow us to explore the nature of our Universe. The protons are not fired at one another individually; instead, they are circulated in bunches inside the LHC, each bunch containing some 100 billion (100,000,000,000) particles. When two bunches cross each other in the centre of CMS, a few of the protons — around 25 or so — will collide with one another. The rest of the protons continue flying through the LHC unimpeded until the next time two bunches cross.

Sometimes, something very different happens. As they fly through the LHC, the accelerating protons radiate photons, the quanta of light. If two protons going in opposite directions fly very close to one another within CMS, photons radiated from each can collide together and produce new particles, just as in proton collisions. The two parent protons remain completely intact but recoil as a result of this photon-photon interaction: they get slightly deflected from their original paths but continue circulating in the LHC. We can determine whether the photon interactions took place by identifying these deflected protons, thus effectively treating the LHC as a photon collider and adding a new probe to our toolkit for exploring fundamental physics. Continue reading The LHC as a photon collider

Hawking Evaporation is Inconsistent with a Classical Event Horizon

Borun D. Chowdhury, Lawrence M. Krauss
A simple classical consideration of black hole formation and evaporation times as measured by an observer at infinity demonstrates that an infall cutoff outside the event horizon of a black hole must be imposed in order for the formation time of a black hole event horizon to not exceed its evaporation time.
We explore this paradox quantitatively and examine possible cutoff scales and their relation to the Planck scale.
Our analysis suggests several different possibilities, none of which can be resolved classically and all of which require new physics associated with even large black holes and macroscopic event horizons:
(1) an event horizon never forms, for example due to radiation during collapse (resolving the information loss problem),
(2) and/or quantum effects may affect space-time near an event horizon in ways which alter infall as well as black hole evaporation itself.
Read more at http://arxiv.org/pdf/1409.0187v1.pdf

Self-Replicating Cracks

cracks

Unusual cracks in thin film moderately adherent to a substrate (scale bar 100µm)

Dry mud and glaze on pottery will crack when placed under stress, often producing a crisscrossing fracture network on the surface. Most of these seemingly random crack patterns are well understood, but certain coatings, such as metal and silica nanofilms, can exhibit spiral-shaped cracks and other regular patterns that cannot be produced by known fracture mechanisms. A new study in Physical Review Letters reveals that these peculiar patterns can arise when new cracks are formed by the spontaneous replication of an initial crack template.

The standard picture of crack formation in a thin film is based on the competition between elastic forces and fracture energy. Under tensile stress, the elastic energy builds up until it is greater than the energy that it takes to break the film, at which point the stress is released and the film splits along a straight channel. This theory predicts that a new crack will tend to be deflected in the close vicinity of a previous one and cross it perpendicularly. As a result, disordered crack patterns are expected.

Joël Marthelot of ESPCI Paris-Tech in France and his colleagues investigated anomalous, ordered cracks (e.g., Archimedean spirals and alleys of crescents) in spin-on-glass coatings—a type of coating used in modern electronics and laser technologies. The team observed that new cracks formed at a set distance from previous cracks, suggesting that cracks can replicate, in a periodic fashion, an initial crack pattern. To explain their findings, they propose a new fracture mechanism in which stress on the film is released by simultaneous fracturing and delaminating (i.e., peeling off of the surface). The model explains how the crack-separation distance depends exclusively on the thickness of the film and how various ordered patterns can result from different triggering mechanisms. – Michael Schirber – physics.aps.org – arxiv.org/pdf

Quantum imaging with undetected photons

Phase image of an object opaque for 810 nm light

Phase image of an object opaque for 810 nm light (arxiv)

Physicists have devised a way to take pictures using light that has not interacted with the object being photographed.

This form of imaging uses pairs of photons, twins that are ‘entangled’ in such a way that the quantum state of one is inextricably linked to the other. While one photon has the potential to travel through the subject of a photo and then be lost, the other goes to a detector but nonetheless ‘knows’ about its twin’s life and can be used to build up an image.

Normally, you have to collect particles that come from the object to image it, says Anton Zeilinger, a physicist at the Austrian Academy of Sciences in Vienna who led the work. “Now, for the first time, you don’t have to do that.”

One advantage of the technique is that the two photons need not be of the same energy, Zeilinger says, meaning that the light that touches the object can be of a different colour than the light that is detected. For example, a quantum imager could probe delicate biological samples by sending low-energy photons through them while building up the image using visible-range photons and a conventional camera. The work is published in the August 28 issue of Nature.

Zeilinger and his colleagues based the technique on an idea first outlined in 1991, in which there are two paths down which a photon can travel. Each contains a crystal that turns the particle into a pair of entangled photons. But only one path contains the object to be imaged.

According to the laws of quantum physics, if no one detects which path a photon took, the particle effectively has taken both routes, and a photon pair is created in each path at once, says Gabriela Barreto Lemos, a physicist at Austrian Academy of Sciences and a co-author on the latest paper.

In the first path, one photon in the pair passes through the object to be imaged, and the other does not. The photon that passed through the object is then recombined with its other ‘possible self’ — which travelled down the second path and not through the object — and is thrown away. The remaining photon from the second path is also reunited with itself from the first path and directed towards a camera, where it is used to build the image, despite having never interacted with the object.

The researchers imaged a cut-out of a cat, a few millimeters wide, as well as other shapes etched into silicon. The team probed the cat cut-out using a wavelength of light which they knew could not be detected by their camera. “That’s important, it’s the proof that it’s working,” says Zeilinger.

The cat was picked in honor of a thought experiment, proposed in 1935 by the Austrian physicist Erwin Schrödinger, in which a hypothetical cat in a box is both alive and dead, as long as no one knows whether or not a poison in the box has been released. In a similar way, in the latest experiment, as long as there is nothing to say which path the photon took, one of the photons in the pair that is subsequently created has both gone and not gone through the object, she adds.

Previous experiments have tried to do something similar in a process known as ghost imaging. But the latest method is simpler, says Mary Jacquiline Romero, a physicist at the University of Glasgow, UK. In ghost imaging, even though only one photon interacts with the object, both photons need to be collected to reconstruct the image, whereas in the Vienna team’s work only one photon needs to be detected. As ghost imaging needs both photons to produce the image, some physicists have questioned whether the effect is truly quantum or could be explained by classical physics – an argument Zeilinger says would be difficult to make with this experiment.

Robert Boyd, a physicist at the University of Rochester in New York, says that the experiment is so intriguing he wishes he had thought of it first. “That’s the greatest compliment that a scientist can give,” he says.
Read more atwww.scientificamerican.com

Van der Pol and the history of relaxation oscillations

Jean-Marc Ginoux, Christophe Letellier

Relaxation oscillations are commonly associated with the name of Balthazar van der Pol via his eponymous paper (Philosophical Magazine, 1926) in which he apparently introduced this terminology to describe the nonlinear oscillations produced by self-sustained oscillating systems such as a triode circuit.
Our aim is to investigate how relaxation oscillations were actually discovered. Browsing the literature from the late 19th century, we identified four self-oscillating systems in which relaxation oscillations have been observed:
i) the series dynamo machine conducted by Gerard-Lescuyer (1880),
ii) the musical arc discovered by Duddell (1901) and investigated by Blondel (1905),
iii) the triode invented by de Forest (1907)
and, iv) the multivibrator elaborated by Abraham and Bloch (1917).
The differential equation describing such a self-oscillating system was proposed by Poincare for the musical arc (1908), by Janet for the series dynamo machine (1919), and by Blondel for the triode (1919). Once Janet (1919) established that these three self-oscillating systems can be described by the same equation, van der Pol proposed (1926) a generic dimensionless equation which captures the relevant dynamical properties shared by these systems. Van der Pol’s contributions during the period of 1926-1930 were investigated to show how, with Le Corbeiller’s help, he popularized the “relaxation oscillations” using the previous experiments as examples and, turned them into a concept….
… Read more at http://arxiv.org/pdf/1408.4890v1.pdf

Do we live in a 2-D hologram?

New Fermilab experiment will test the nature of the universe

A Fermilab scientist works on the laser beams at the heart of the Holometer experiment. The Holometer will use twin laser interferometers to test whether the universe is a 2-D hologram. Credit: Fermilab

A Fermilab scientist works on the laser beams at the heart of the Holometer experiment. The Holometer will use twin laser interferometers to test whether the universe is a 2-D hologram. Credit: Fermilab

A unique experiment at the U.S. Department of Energy’s Fermi National Accelerator Laboratory called the Holometer has started collecting data that will answer some mind-bending questions about our universe – including whether we live in a hologram.

Much like characters on a television show would not know that their seemingly 3-D world exists only on a 2-D screen, we could be clueless that our 3-D space is just an illusion. The information about everything in our universe could actually be encoded in tiny packets in two dimensions.

Get close enough to your TV screen and you’ll see pixels, small points of data that make a seamless image if you stand back. Scientists think that the universe’s information may be contained in the same way and that the natural “pixel size” of space is roughly 10 trillion trillion times smaller than an atom, a distance that physicists refer to as the Planck scale.

“We want to find out whether space-time is a quantum system just like matter is,” said Craig Hogan, director of Fermilab’s Center for Particle Astrophysics and the developer of the holographic noise theory. “If we see something, it will completely change ideas about space we’ve used for thousands of years.”

Quantum theory suggests that it is impossible to know both the exact location and the exact speed of subatomic particles. If space comes in 2-D bits with limited information about the precise location of objects, then space itself would fall under the same theory of uncertainty. The same way that matter continues to jiggle (as quantum waves) even when cooled to absolute zero, this digitized space should have built-in vibrations even in its lowest energy state.

Essentially, the experiment probes the limits of the universe’s ability to store information. If there is a set number of bits that tell you where something is, it eventually becomes impossible to find more specific information about the location – even in principle. The instrument testing these limits is Fermilab’s Holometer, or holographic interferometer, the most sensitive device ever created to measure the quantum jitter of space itself.

Now operating at full power, the Holometer uses a pair of interferometers placed close to one another. Each one sends a one-kilowatt laser beam (the equivalent of 200,000 laser pointers) at a beam splitter and down two perpendicular 40-meter arms. The light is then reflected back to the beam splitter where the two beams recombine, creating fluctuations in brightness if there is motion. Researchers analyze these fluctuations in the returning light to see if the beam splitter is moving in a certain way – being carried along on a jitter of space itself.

“Holographic noise” is expected to be present at all frequencies, but the scientists’ challenge is not to be fooled by other sources of vibrations. The Holometer is testing a frequency so high – millions of cycles per second – that motions of normal matter are not likely to cause problems. Rather, the dominant background noise is more often due to radio waves emitted by nearby electronics. The Holometer experiment is designed to identify and eliminate noise from such conventional sources.

“If we find a noise we can’t get rid of, we might be detecting something fundamental about nature – a noise that is intrinsic to space-time,” said Fermilab physicist Aaron Chou, lead scientist and project manager for the Holometer. “It’s an exciting moment for physics. A positive result will open a whole new avenue of questioning about how space works.”

The Holometer experiment, funded by the U.S. Department of Energy Office of Science and other sources, is expected to gather data over the coming year.
Read more at www.fnal.gov