Quantum Mechanics has the ability to surprise to owns and others. Recently, researchers have gone one step beyond performing a experiment that started 600 years ago, much before Plank, Einstein, or even Newton were born. An international team has performed a Bell test, an experiment to shows the non-locality of quantum mechanics, closing all loopholes present in most of the previous experiments. They have put a new limit to the locality, placed in 600 light-years. They have used light coming from stars that are, at least, 600 years far from Earth. This means that, if quantum mechanics is local, then the conditions of the experiment should have been stablished no less than 600 years ago. Albert Einstein and others didn't like the fact that Nature is not defined until measured. He didn't like the fact that the outcome of a quantum experiment is determined randomly from all possible outcomes, but the liked less the fact that the state of a system is not defined is no one is looking, measuring the result. John S. Bell found a mathematical expression to test whether a physical theory is local. By locality we understand that there is not any physical signal that can travel faster than light, as relativity implies. But there are experiments that allow to entangle two particles, forming a same quantum state. If those particles are sent away from each other, if the outcome is not defined, if there is not physical reality, you need to perform a measurement to create the outcome. Due to quantum relationships, the outcome of one particle will define the outcome of the other one, instantaneously! Since Bell established his inequalities several experiments have been performed, agreeing with quantum mechanics. But until 2015, all of those experiments made one or more assumptions, in what have been known as loopholes. Closing all the loopholes simultaneously is not easy, but two independent groups did it (here) . What this international group (Austria, USA, Germany, China) has done is use the light from two distant stars as a source of randomness, essential for the experiment (the papers has been published at Physical Review Letters). In the figure you can see the experimental scheme, where photons were sent from a central building, S, to two different buildings (A and B). The measurements settings were made using real-time observations of two distant stars (604 and 1930 light-years from Earth). The experiment assumes that the characteristics of the photons coming from the distant stars are independent and were established at the time of departure from the star, that nobody tampered them in their flight to Earth to affect the experiment.
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A recent paper published in Physical Review Letters puts new limits to the isotropy observed in the Universe, taking Copernicus' principle to be valid with more resolution than ever before. In my last year as an undergrad in college I took a general relativity course, I remember we spent two months studying cosmology. One of the first principles we were told of, was that the Universe is considered to be homogeneous and isotropic. This principle was assumed by Friedman, by Lemaitre, by Robertson and by Walker in what became known as the FLRW cosmological model. General relativity implies complex equations, 10 indeed, that are coupled to each other, and they cannot be solved in an analytical way unless we do some assumptions of symmetry to simplify them. Homogeneity and isotropy are two very reasonable ones. Although we see stars in particular positions and directions, at a great scale, all points in the universe and all directions seem equal, there is no preferred direction or placed in the Universe. This is not easy so verify observationally, or at least not in the 1930s, when cosmology was appearing to stage. But, nevertheless, assuming a homogeneous and isotropic universe has given a great success in describing it. Predictions based in FLRW cosmological models, on the composition and evolution of the universe, have been a great test to the assumptions. In the last two decades, thanks to the new observational data from the COBE, WMAP, and Planck space telescopes, some previous results have been verified with higher accuracy than ever before, such as the Cosmic Microwave Background radiation, or the composition of the universe. But these observations have also given birth to new questions, not easy to respond. Cosmology has incorporated new ingredients such as dark matter and dark energy. These are accepted by an important portion of the cosmology community, but there are other scientists that believe that there are no dark matter and/or dark energy, and the answer is in a different focus. There are important scientists trying to modify Einstein's equation of relativity to explain why the universe seems to be expending an an increasing rate, for example. Other researchers were assuming that the universe does not contain dark energy, instead, the observations have been ill-interpreted due to assuming that the universe is homogeneous and isotropic. Having some preferred directions could help explain the observations without needing to include the dark sector. In this recent publication, a research group in London, has analyzed data from the space telescopes concluding that the universe is highly isotropic. They have assumed that the universe, while still homogeneous, be non-isotropic. These assumptions lead to solutions of Einstein's equations known as Bianchi cosmologies. There are several of these posible solutions depending on the types of non-isotropy considered. Using data from the cosmic microwave background temperature and the polarization from the Planck satellite, the researchers conclude that the universe is very unlikely anisotropic. In the figure they represent the inhomogeneity pattern in the cosmic microwave background that would arise due to small anisotropic expansion of the universe. The authors constrain all model of the anisotropic expansion. Each map of the universe (each elipse) shows the temperature, the electric polarization (E), and magnetic polarization (B), these last magnified 30 times to see the inhomogeneities. Their results, after comparing the numerical simulations with the observations, point that the probability of having a non-isotropic universe is of 1 in more than one hundred thousand. Black holes, the most exotic bodies in the Universe, an absolutely abstract concept in our daily basis, might be objects of study in our laboratories. Jeff Steinhauer, from Israel Institute of Technology, has simulated in his lab the Hawking radiation that takes place in black holes. This allows to study and better understand the quantum nature around a black hole, which might help in the unification of quantum mechanics and general relativity. His work is published in Nature Physics. Black holes wars Black holes are exotic cosmic objects that arise after the cataclysmic death of vey massive stars. The extreme gravitational field in them doesn't allow neither light to scape from them. This makes it difficult to observe and study them. But General Relativity is a very powerful tool that allows theoreticians to explore them. Stephen Hawking, in 1974, studying some quantum properties around black holes, predicted that they are not indeed really black, but gray. Black holes should emit radiation due to quantum fluctuations that can make appear and disappear virtual elementary particles around the black hole (very near to its event horizon). It happens that these virtual particles come in pairs of particle and antiparticle that appear and annihilate in a very small fraction of time. But it can happen that one of them falls into the hole while the other can scape. Hawking also predicted that this radiation should be random, thermal radiation, although recently this is not that clear, and trying to explain it has become the information paradox, an open, hot and passionate debate between theoreticians (Hawking, Maldacena, Suskind). Quantum gravity Quantum mechanics applies to very small scales, to elementary particles, while general gravity applies to high gravitational fields, stars, black holes, galaxies, the Universe. The fact that we don't find accesible in Nature situations where we have elementary particles and high gravitational fields at the same time, makes it almost impossible to have observational data to test and build the theory. That is why in the last decade some efforts have been driven to find alternative ways to explore black holes in the lab. Simulating General Relativity in the Lab. There are some works in the field of photonics that simulate the behavior of light around a black hole. Also, using a Bose-Einstein condensate (BEC), a ultra-cold gas of atoms (one billionth of a degree above absolute zero) that can show macroscopic quantum behavior, Jeff Steinhauer, is doing experiments to create a sound-wave analog to a black hole. In his work, Steinhauer creates phonons, instead of photons, which are mechanical excitations and can be considered the equivalent of what photons are to light waves, but in sound waves. He stimulates the BEC with lasers to make them move at about 1 millimeter per second. This velocity is twice the speed of sound in the BEC. By doing this, he can trap phonons in the BEC just as photons are trapped in black holes. As the phonons are created spontaneously, just as virtual photons around the black hole, some of them can be trapped while their pair can scape, just mimicking Hawking radiation. Steinhauer has found that phonons falling inside the acoustic black hole where entangled to phonons escaping, indicating that they were cerated from the same perturbation, just as pair of virtual particles. Also, Steinhauer found that the sound radiation observed was compatible with a noise radiation, as would be the case of thermal radiation from a black hole. There are still some open questions to this work, such as the frequency analysis. The entanglement was clear for high energy phonons (high frequency), but was not that clear for low energy ones. Also, even though it could be though that these experiments could clarify the information paradox (unravel the characteristics of Hawking radiation to see if it can be extracted information from inside the black hole by using this radiation), it is not completely clear from this work. In any case, a very interesting door has been open that can seed light into the darkness of black holes. Physics! That great Science. The sublime description of the Cosmos, from the smallest subnuclear particles to the Universe as a whole. Physics is, and always has been a thrilling enterprise. It has not been a single decade without a new challenge or surprising discovery. Today, are we living the n-th golden age of Physics? At the end of the 19th century, some great physicists, among them the great Lord Kelvin (in the image), believed that all the laws of Nature were almost discovered. There were only a couple of small problems, and a few experimental improvements to calculate some parameters, but after that, nothing would remain to be discovered. Solving those small problems led to the birth of Quantum Mechanics and Relativity, which are at the basis of most of the Physics and technology of the 20th century. Right after the talk where Lord Kelvin made that claim Science witnessed one of the most exciting periods of Physics. Where are we now? Well, we have confirmed all of Einstein's predictions (being the latest the direct observation of gravitational waves), we have confirmed the Standard Model of elementary particle and found the Higgs boson, that explains some of the open questions of this model, we are using the weird properties of Quantum Mechanics, such as entanglement and superposition, in our commercial technology. But we have huge questions to answer, such as: what are dark matter and dark energy? After the discovery of the Higgs boson, most physicists pointed that the LHC particle accelerator was ready to discover new Physics, to detect new particles or interactions to guide us to answer open questions and find unexpected Physics. Indeed, a few weeks ago some researchers, analyzing data from LHC, claimed they had found a clear sign of a new Force of Nature. For more than sixty years we have been counting with four fundamental forces: gravity, electromagnetism, strong nuclear and weak nuclear. A fifth force has been postulated and gone several times in the last sixty years. But this time all was pointing to no doubt experimental result. Well, ..., last week, other analysis pointed out that those measurements were not consistent enough with that conclusion. Not yet time for a fifth force. Another of the open questions that has been concerning Physicists for the last few years has to do with the elusive neutrino. Fortunately, since 2010 we have the Ice-Cube Neutrino Observatory, in Antarctica. It is a huge (one cubic kilometer) neutrino detector. It is the biggest on the planet and highly interesting research is been carried on in it for the last five years. The neutrino was postulated by Wolfgang Pauli in the 1930s to solve some observations that suggested that energy could not be conserved. Pauli himself thought that we would not be able to detect neutrinos (extremely light, neutral particle) as it interacts extremely weakly with other particles. Trillions of neutrinos go through your body every second and none of them will interact with your atoms. It took 25 years but it was finally detected. Now, we have even observed neutrinos coming from the center of the Sun. The image at the right is a picture of the core of the sun taken with neutrinos (not with photons). Neutrinos come in three families (electron, muon and tau) and they change their flavor as they travel through space. This is what the Standard Model allows and what has been found in observations. But some scientists have postulated that there could be four families of neutrinos. This fourth neutrino would be a 'sterile' neutrino. It would be even much harder to detect than the other three. This sterile neutrino would be detected by catching it when transforming into another type of neutrino. The existence of the sterile neutrino would help explain the mass of the neutrinos, and it would also help understand dark matter. But it would open big questions on the validity/limitations of the Standard Model. This week the journal Physical Review Letters publishes a paper by the IceCube collaboration pointing that there is no room for the sterile neutrino in the observations. The neutrinos observed are generated through collisions from cosmic rays in the higher atmosphere. They go though the Earth and reach IceCube at the other end (see picture below, from PRL). In their brief travel though the planet they can transform into another type of neutrino and be observed at the detector. With a confidence of 99%, the experimenters claim that there is no sterile neutrino. It seems that the New Physics is been coaxed, but for sure it will come sooner than later. If Nature is surprising, Quantum Physics is mindbogling. It can be non-intuitive, it can lead to results difficult to accept, but it does it job when it comes to describe Nature at the elementary particles level. Although that is not its only realm. In a recent publication (Nature Communications 7, Article number: 12172, July 2016), scientists from the University of Vienna (Austria) and the Rockefeller University (USA) go one step beyond and answer affirmatively to the question: Can a person see a single photon? We physicists are used to doing experiments shielding our setups from any external perturbation, just to be sure that what we are measuring is what we want to study. When we do experiments in quantum optics this goal is not easy. Thermal fluctuations, electronic noise, straight light or even quantum fluctuations can blur our measurements. This differentiation from the micro-world to the macro-world makes us think that quantum effects can not be detected at the macroscopic level, that we are not going to be able to create Schrödinger´s cat with an actual cat (besides having all youtube followers attacking us). But this is not true. Otherwise we would not have transistors, computers, mobile phones, magnetic resonance imaging, and many more. There is one question that has been in the air and in the conferences for many years: What is the limit of perception of the human eye? In the 1980s and 1990s some researchers tried to answer it, but the equipment of the moment could only conclude that the human eye could detect pulses of five photons. By that time, researchers could not create single photons in a controllable way. They used light attenuated from a laser. But this type of light is called coherent, and when it is pulsed and attenuated, each pulse has not a well defined number of photons. There is a statistics, called Poissonian, that tells us the probability of having zero, one two, three, ... photons in each pulse. We can talk about mean photon number but we cannot know exactly how many photons will have each one of the pulses. Now, by using spontaneous parametric downconversion (SPDC), these physicists have created correlated pairs of photons. From each pair, one was sent to a detector, to herald the creation of the pair, and the other was sent to the eye of a person. This way, the researchers knew when a photon was crated and could wait for the response of the person. Of course, the room was completely dark and had been dark for a while, so the patient could adapt her vision to darkness and have the eye more sensitive to light. The research conducted followed a statistical analysis of the detections made by three different people, trying to quantify the goodness of the detection and to distinguish all the detections from a purely random process. It seems that our human eye is sensitive enough to a single photon. Even though this does not guarantee that we can detect every single photon sent to our eye. Also, the research was carried out only on three individuals. A more robust statistics should have been more conclusive, though. In any case, this surprising result closes one question that remained open for a few decades and opens the door do quantum experiments were human beings can be part of the detection side. Maybe we could read soon on research on quantum entanglement resolved by a person herself. Cheers, QuantumMan. |
Andrés AragonesesPhysicist, working in quantum optics and nonlinear dynamics in optical systems. Loves to communicate science. Archives
January 2018
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