Archivi tag: special relativity

Testing different aspects of the twin paradox using a physical system

Immaginiamo due gemelli, identici, tranne per il fatto che uno dei due possieda una navicella spaziale. Quest’ultimo decide di fare un viaggio verso una stella distante, diciamo qualche decina di anni-luce, mentre l’altro gemello rimane a terra. Viaggiando con una velocità pari a circa il 75% -85% rispetto a quella della luce, il gemello raggiunge la stella e fa ritorno a terra, incontrando il proprio gemello decisamente più vecchio di lui. Questo fenomeno, noto come paradosso dei due gemelli, è dovuto alla dilatazione dei tempi come viene descritto nella teoria della relatività speciale. Di fatto, Albert Einstein predisse che orologi soggetti ad accelerazioni diverse misurano il tempo in maniera differente. Per quanto strano possa sembrare, l’effetto della dilatazione dei tempi è stato verificato più volte in laboratorio e viene continuamente utilizzato nei sistemi GPS.

The GPS is able to provide you with your position by timing very precisely the signals emitted by satellites, and to this end it needs to take into account the time dilation due to the different accelerations of the satellites. While GPS is one of the most precise systems we have, it can locate your smartphone with an error margin of a few metres. The precision could be improved by using the most precise clocks that we know on Earth, known as quantum clocks because they are ruled by the laws of quantum mechanics. There are plans funded by space agencies to launch these clocks into orbit. It is natural to think that a GPS consisting of quantum clocks would also need to take into account relativistic effects.

However, we do not fully understand how to combine quantum mechanics and relativity. The inability of unifying both theories remains as one of the biggest challenges of modern science.

Predictions in the 1970s said that there is a physical phenomenon that is both quantum and relativistic called the Dynamical Casimir Effect. But it wasn’t until 2011 that an experimental setup could be developed to test the prediction. Here is what theory predicted: if light is trapped between mirrors that move at velocities close to the speed of light, then they will generate more light than there is in the system. Even if initially there is no light between the mirrors, just vacuum, light shows up because the mirror turns the quantum vacuum into particles. This is supposed to happen because vacuum at the quantum level is like a sea of pairs of particles that are constantly emitting and absorbing light. They do this at incredible speeds, but if the mirror moves that fast too some of these particles are reflected by the mirror before disappearing and can be observed. But setting up such a system has proved difficult. In 2011, this difficulty was circumvented in the experiment conducted by Per Delsing at Chalmers University of Technology in Sweden. In this case the mirrors were different. They were magnetic fields inside a Superconducting Quantum Interferometric Device (SQUID), but they behaved exactly like mirrors, making light bounce back and forth. Unlike physical mirrors, these magnetic fields could be moved at incredible speeds. Einstein used to think of clocks as light going back and forth between mirrors. Time can be inferred from the distance between the mirrors divided by the speed of light, which remains constant no matter what. But he never thought about particles being created by motion, a prediction that was made many years after his death.

In a recent work, with colleagues at the University of Nottingham, Chalmers University and University of Warsaw, we have taken inspiration from the 2011 experiment.

We propose using a similar setup to test different aspects of the twin paradox using a physical system, which haven’t been tested so far.

Although it won’t involve human twins, the possibility of achieving enormous speeds and acceleration allows the observation of time dilation in a very short distance. Also, all previous experiments that have tested the theory have involved atomic clocks, which are “point-clocks”, that is, what measures time in these atomic clocks is confined to a tiny point in space. Our experiment would instead use something that has finite length. This is important because, along with time, Einstein’s theory predicts that length of the object changes too. We believe our experiment would test that aspect of the theory for the first time. We have found that particle creation by motion, which was observed in 2011, has an effect on the difference in time between the clock that is moving and the one that is static. Using this setup, while we can reconfirm that time dilation occurs, the more interesting application would be to help build better quantum clocks, by means of a better understanding of the interplay between quantum and relativistic effects.

The Conversation: How to test the twin paradox without using a spaceship

arXiv: The twin paradox with macroscopic clocks in superconducting circuits

27° Texas Symposium on Relativistic Astrophysics

The 27th Texas Symposium on Relativistic Astrophysics will be held in downtown Dallas December 8 – 13, 2013. It is organized by the Department of Physics at The University of Texas at Dallas (UTD) and is chaired by Wolfgang Rindler and Mustapha Ishak. The Symposium will include both invited and contributed talks and posters. This will be a special and historically meaningful Jubilee meeting, marking the 50th anniversary, almost to the day, of the very first of these Texas Symposia, held in Dallas in December 1963. We are excited to welcome hundreds of international astrophysicists back to Dallas fifty years later, both to celebrate the past 50 years of Texas Symposia and relativistic astrophysics and to kick off the next 50 years of remarkable discoveries.

The Symposium will cover the following topics:


  • Cosmic acceleration/dark energy
  • Cosmic microwave background
  • Early universe (Inflation, Cyclic Model, CCC cosmology …)
  • Galaxy formation and reionization
  • Inhomogeneous cosmologies, averaging, and backreaction
  • Large-scale surveys
  • Quantum gravity/cosmology and string cosmology
  • Weak gravitational lensing
  • Experimental/observational cosmology – other topics
  • Theoretical cosmology – other topics
Compact objects and galactic/cluster scales
  • Black holes, mergers, and accretion discs
  • Galaxy evolution and supermassive black holes
  • Imaging black holes
  • Microlensing and exoplanets
  • Neutron stars, pulsars, magnetars, and white dwarfs
  • Nuclear Equation of State for Compact Objects
  • Singularities
  • Strong gravitational lensing
  • Supermassive black hole binaries
  • Tidal disruption of stars by supermassive black holes
  • Compact object observations – other topics
  • Compact object theory – other topics
High-energy astrophysics and astroparticle physics
  • Active galactic nuclei and jets
  • Cosmological implications of the Higgs and the LHC
  • Dark matter astrophysics
  • Dark matter experiments and data
  • Gamma-ray bursts, SNe connection, and sources
  • High-energy cosmic rays (VHE, UHE, mechanisms, etc.)
  • High-energy gamma-rays
  • Nuclear Astrophysics
  • Supernovae and their remnants
  • High-energy astrophysics/astroparticle physics – other topics
Testing general relativity and modified gravity
  • Alternative theories of gravity
  • Strong-field tests of general relativity
  • Testing general relativity at cosmological scales
  • Testing general relativity – other topics
  • Modified gravity – other topics
Gravitational waves
  • Electromagnetic counterparts of gravitational wave sources
  • Ongoing and planned gravitational wave experiments
  • Gravitational wave theory and simulations
  • Results and progress from gravitational wave searches
  • Supernovae and Gravitational Wave Emission
  • Gravitational waves – other topics
Numerical relativity
  • Computer algebra and symbolic programming
  • Locating black hole horizons
  • Numerical simulations
  • Relativistic magnetohydrodynamics
  • Numerical relativity – other topics
Other ongoing and future experiments and surveys
  • ACT, AMS, BOSS, CFHT, Chandra, DES, Euclid, Fermi, HETDEX, HSC, JWST,
  • (to be completed after abstract submissions)
And also:
History of relativistic astrophysics
History of the Texas Symposium and interface with other anniversaries
The Kerr solution – 50 years later

Exploring the nature of dark matter through the A’ particle

Per definizione, la materia scura è invisibile anche se la sua presenza viene “osservata” grazie ai suoi effetti gravitazionali. Nonostante ciò, secondo alcune teorie moderne che tentano di descrivere questa materia elusiva le sue proprietà potrebbero essere in definitiva rivelate. Infatti, alcuni scienziati del MIT hanno sviluppato un metodo che potrebbe permettere di verificare la validità di una teoria al fine di dare credito, o meno, alle sue previsioni.

The work is described in a paper in the journal Physical Review Letters co-authored by MIT physics professors Richard Milner and Peter Fisher and 19 other researchers. “We’re looking for a massive photon“, Milner explains. That may seem like a contradiction in terms: photons, or particles of light, are known to be massless. That’s why they travel at the speed of light, something that, according to Einstein’s theory of relativity, is impossible for anything that possesses mass. However, an exotic particle that resembles a photon, but with mass, has been proposed by some theorists to explain dark matter, whose nature is unknown but whose existence can be inferred from the gravitational attraction it exerts on ordinary matter, such as in the way galaxies rotate and clump together. “Now, an experiment known as DarkLight, developed by Fisher and Milner in collaboration with researchers at the Jefferson National Accelerator Laboratory in Virginia and others, will look for a massive photon with a specific energy postulated in one particular theory about dark matter“, Milner says. The idea is more than just a theoretical prediction, he adds: “There are hints of such a particle from other experiments, making it worthwhile to pursue a definitive answer“. But the previous hints, consisting of what Milner calls “anomalous moments of the muon”, do not rise to statistical significance.

The DarkLight experiment is designed to provide solid confirmation of the massive photon’s existence. If it does exist, that would represent a major discovery.

It’s totally beyond anything we understand about the physical world“, Milner says. “A massive photon would be totally different from anything allowed by the Standard Model, the bedrock of modern , he says. To prove the existence of the theorized particle, dubbed A’ (“A prime”), the new experiment will use a particle accelerator at the Jefferson Lab that has been tuned to produce a very narrow beam of electrons with a megawatt of power. That’s a lot of power, Milner says: “You could not put any material in that path without having it obliterated by the beam”. For comparison, he explains that a hot oven represents a kilowatt of power. “This is a thousand times that“, he says, “concentrated into mere millionths of a meter”.

The new paper confirms that the new facility’s beam meets the characteristics needed to definitively detect the hypothetical particle, or rather, to detect the two particles that it decays into, in precise proportions that would reveal its existence.

Doing so, however, will require up to two years of further preparations and testing of the equipment, followed by another two years to collect data on millions of electron collisions in the search for a tiny statistical anomaly. “It’s a tiny effect“, Milner says, but “it can have enormous consequences for our theories and our understanding. It would be absolutely groundbreaking in physics“. While DarkLight’s main purpose is to search for the A’ particle, it also happens to be well suited to addressing other major puzzles in physics. It can probe the nature of a reaction, inside stars, in which carbon and helium fuse to form oxygen, a process that accounts for all of the oxygen that now exists in the Universe. “This is the stuff we’re all made of“, Milner says, “and the rate of this reaction determines how much oxygen exists”. While that reaction rate is very hard to measure, the DarkLight experiment could illuminate the process in a novel way: “The idea is to do the inverse. Instead of fusing atoms to form oxygen, the experiment would direct the powerful beam at an oxygen target, causing it to split into carbon and helium. That would provide an indirect way of determining the stellar production rateRoy Holt, a distinguished fellow in the physics division at Argonne National Laboratory in Illinois, says “This work is a novel and significant technical development that not only opens a new window to search for a new [particle], but also for new studies in nuclear physics“. “If the planned experiment detects the A’ particle”, he says, “it would signal that  could actually be studied in a laboratory setting“.

MIT: Seeing the dark: New MIT-led experiment could finally shed light on the mysteries of dark matter
arXiv: Transmission of Megawatt Relativistic Electron Beams Through Millimeter Apertures

Understanding complexity in the early Universe with simpler models

Il comportamento statistico di alcuni sistemi complessi, come ad esempio l’Universo primordiale, può essere analizzato se viene ridotto ad un insieme di sistemi più semplici. Oggi due fisici, Petr Jizba della Czech Technical University a Praga e Fabio Scardigli ora alla Kyoto University in Giappone, hanno pubblicato i risultati di un lavoro di ricerca che riguarda le previsioni teoriche del comportamento dinamico di tali sistemi cosmologici.

Their work focuses on complex dynamical systems whose statistical behaviour can be explained in terms of a superposition of simpler underlying dynamics.

They found that the combination of two cornerstones of contemporary physics, namely Einstein’s special relativity and quantum-mechanical dynamics, is mathematically identical to a complex dynamical system described by two interlocked processes operating at different energy scales.

The combined dynamic obeys Einstein’s special relativity even though neither of the two underlying dynamics does. This implies that Einstein’s special relativity might well be an emergent concept and suggests that it would be worthwhile to further develop Einstein’s insights to take into account the quantum structure of space and time (post). To model the double process in question, the authors consider quantum mechanical dynamics in a background space consisting of a number of small crystal-like domains varying in size and composition, known as polycrystalline space. There, particles exhibit an analogous motion to pollen grains in water, referred to as Brownian motion. The observed relativistic dynamics then comes solely from a particular grain distribution in the polycrystalline space. In the cosmological context such distribution might form during the early Universe’s formation. Finally, the authors’ new interpretation focuses on the interaction of a quantum particle with gravity, that, according to Einstein’s general relativity, can be understood as propagation in curved space-time. The non-existence of the relativistic dynamics on the basic level of the description leads to a natural mechanism for the formation of asymmetry between particles and anti-particles. When coupled with an inflationary cosmology, the authors’ approach predicts that a charge asymmetry should have been produced at ultra-minute fractions of seconds after the Big Bang. This prediction is in agreement with constraints born out of recent cosmological observations.

EPJ: Removing complexity layers from the universe’s creation
EPJ C: Special relativity induced by granular space

New ways to hunt for elusive neutrinos

particle_collisionsOgni secondo, trilioni di particelle chiamate neutrini attraversano il nostro corpo. Queste particelle elusive hanno una massa così piccola che non è stata ancora misurata. Inoltre, esse interagiscono debolmente con la materia che è quasi impossibile rivelarle e ciò rende molto complicato studiare le loro proprietà.

Since arriving at MIT in 2005, Joseph Formaggio, an associate professor of physics, has sought new ways to measure the mass of neutrinos. Nailing down that value, and answering questions such as whether neutrinos are identical to antineutrinos, could help scientists refine the Standard Model of particle physics, which outlines the 16 types of subatomic particles (including the three neutrinos) that physicists have identified. Those discoveries could also shed light on why there is more matter than antimatter in the Universe, even though they were formed in equal amounts during the Big Bang. “There are big questions that we still haven’t answered, all centered around this little particle. It’s not just measuring some numbers; it’s really about understanding the nature of the equation that explains particle physics. That’s really exciting”, Formaggio says. Formaggio, the only child of Italian immigrants, was the first in his family to attend college. Born in New York City, he spent part of his childhood in Sicily, his parents’ homeland, before returning to New York. From an early age, he was interested in science, especially physics and math. At Yale University, he studied physics but was also interested in creative writing. The summer after his freshman year, in search of a summer job, he “called every publishing house in New York City, all of which resoundingly rejected me”, he says. However, his call to the Yale physics department yielded an immediate offer to work with a group that was doing research at the Collider Detector at Fermilab. That led to a senior thesis characterizing the excited states of the upsilon particle, which had recently been discovered. As a student, Formaggio was drawn to both particle physics and astrophysics. At Columbia University, where he earned his PhD, he started working in an astrophysics group that was studying dark matter. Neutrinos were then thought to be a prime candidate for dark matter, and the mysterious particles intrigued Formaggio. He eventually joined a neutrino research group at Columbia, which included Janet Conrad, a professor who is now at MIT. While a postdoc at the University of Washington, Formaggio participated in experiments at the Sudbury Neutrino Observatory (SNO), located in a Canadian nickel mine some 6,800 feet underground. Those were the first experiments to show definitively that neutrinos have mass, albeit a very tiny mass. Until then, “there were definitely hints that neutrinos undergo this process called oscillation where they transmute from one type to another, which is a signature for mass, but all the evidence was sort of murky and not quite definitive”, Formaggio says. The SNO experiments revealed that there are three “flavors” of neutrino that can morph from one to the other. Those experiments “basically put the nail in the coffin and said that neutrinos change flavors, so they must have mass”, Formaggio says. “It was a big paradigm shift in thinking about neutrinos, because the Standard Model of particle physics wants neutrinos to be massless, and the fact that they’re not means we don’t understand it at some very deep level”. Another possible discovery that could throw a wrench into the Standard Model is the existence of a fourth type of neutrino (post). There have been hints of such a particle but no definitive observation yet. “If you put in four neutrinos, the Standard Model is done”, Formaggio says, “but we’re not there yet”.

In his current work, Formaggio is focused on trying to measure the mass of neutrinos.

In one approach, he is working with an international team on a detector called KATRIN, located in a small town in southwest Germany. This detector, about the size of a large hangar, is filled with tritium, an unstable radioactive isotope. When tritium decays, it produces neutrinos and electrons. By measuring the energy of the electron released during the decay, physicists hope to be able to calculate the mass of the neutrino, an approach based on Einstein’s E=mc2 equation. “Because energy is conserved, if you know how much you started out with and how much the electron took away, you can figure out how much the neutrino weighs”, Formaggio says. “It’s a very hard measurement but I like it because the experiment is a giant electromagnetic problem”. The KATRIN detector is under construction and scheduled to begin taking data within the next two years. Formaggio is also developing another tritium detector, known as Project 8, which uses the radio frequency of electrons to measure their energies. Formaggio hopes that one day, tritium-based detectors could be used to find neutrinos still lingering from the Big Bang, which would require even larger quantities of tritium. “There are many holy grails in physics, and finding those neutrinos is definitely one of them. People look at the light from the Big Bang, but that’s actually closer to 300,000 years old, or thereabouts. Neutrinos from the Big Bang have been around since the first second of the Universe”, Formaggio says.

MIT: On the hunt for neutrinos

Why nature is quantum?

Sappiamo che gli scienziati sono come dei bambini perchè si pongono sempre la domanda sul ‘perché’ delle cose. Tra queste, una domanda a cui si deve ancora rispondere riguarda come mai la natura si comporta secondo le leggi della fisica quantistica. Oggi, due ricercatori, Corsin Pfister e Stephanie Wehner del Centre for Quantum Technologies presso la National University di Singapore stanno cercando di affrontare la questione in un articolo pubblicato su Nature Communications.

We know that things that follow quantum rules, such as atoms, electrons or the photons that make up light, are full of surprises. They can exist in more than one place at once, for instance, or exist in a shared state where the properties of two particles show what Einstein called “spooky action at a distance“, no matter what their physical separation. Because such things have been confirmed in experiments, researchers are confident the theory is right. But it would still be easier to swallow if it could be shown that quantum physics itself sprang from intuitive underlying principles. One way to approach this problem is to imagine all the theories one could possibly come up with to describe nature, and then work out what principles help to single out quantum physics. A good start is to assume that information follows Einstein’s special relativity and cannot travel faster than light. However, this alone isn’t enough to define quantum physics as the only way nature might behave. Corsin and Stephanie think they have come across a new useful principle. “We have found a principle that is very good at ruling out other theories“, says Corsin. In short, the principle to be assumed is that if a measurement yields no information, then the system being measured has not been disturbed. Quantum physicists accept that gaining information from quantum systems causes disturbance. Corsin and Stephanie suggest that in a sensible world the reverse should be true, too. If you learn nothing from measuring a system, then you can’t have disturbed it. Consider the famous Schrodinger’s cat paradox, a thought experiment in which a cat in a box simultaneously exists in two states (this is known as a ‘quantum superposition’). According to quantum theory it is possible that the cat is both dead and alive until the cat’s state of health is ‘measured’ by opening the box. When the box is opened, allowing the health of the cat to be measured, the superposition collapses and the cat ends up definitively dead or alive. The measurement has disturbed the cat. This is a property of quantum systems in general. Perform a measurement for which you can’t know the outcome in advance, and the system changes to match the outcome you get. What happens if you look a second time? The researchers assume the system is not evolving in time or affected by any outside influence, which means the quantum state stays collapsed. You would then expect the second measurement to yield the same result as the first. After all, “If you look into the box and find a dead cat, you don’t expect to look again later and find the cat has been resurrected“, says Stephanie. “You could say we’ve formalised the principle of accepting the facts“. Corsin and Stephanie show that this principle rules out various theories of nature. They note particularly that a class of theories they call ‘discrete’ are incompatible with the principle. These theories hold that quantum particles can take up only a finite number of states, rather than choose from an infinite, continuous range of possibilities. The possibility of such a discrete ‘state space’ has been linked to quantum gravitational theories proposing similar discreteness in spacetime, where the fabric of the Universe is made up of tiny brick-like elements rather than being a smooth, continuous sheet. As is often the case in research, Corsin and Stephanie reached this point having set out to solve an entirely different problem altogether. Corsin was trying to find a general way to describe the effects of measurements on states, a problem that he found impossible to solve. In an attempt to make progress, he wrote down features that a ‘sensible’ answer should have. This property of information gain versus disturbance was on the list. He then noticed that if he imposed the property as a principle, some theories would fail. Corsin and Stephanie are keen to point out it’s still not the whole answer to the big ‘why’ question: theories other than quantum physics, including classical physics, are compatible with the principle. But as researchers compile lists of principles that each rule out some theories to reach a set that singles out quantum physics, the principle of information gain versus disturbance seems like a good one to include.

See also: Is nature – deep down – actually discrete?

CQT: New principle may help explain why nature is quantum
arXiv: If no information gain implies no disturbance, then any discrete physical theory is classical