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Category Archives: Quantum Physics
The Physics of Christmas: Wormholes and Other Tricks Santa Might Use to Get His Job Done – Discover Magazine
Posted: December 26, 2019 at 8:07 pm
The jolly old elf known as Santa Claus seems to have been cursed with an impossible task: deliver millions of presents to specific locations scattered across the surface of an entire planet.
As all good kids know, the man in the red suit always seems to pull his task off with aplomb. Its an admirable feat, but it does strain our credulity, not to mention the laws of physics.
But perhaps Santa is more than an elf perhaps hes also a brilliant physicist. With some back-of-the-back-of-the-envelope calculations (and some generous assumptions about currently theoretical concepts), there might yet be a way for Santa to complete his task without resorting to fiction. We turned to physicists to find out.
In what might be the easiest solution, Santa could use wormholes. Wormholes are like tunnels through space-time, shortcuts that would allow Santa to travel enormous distances so quickly that his efforts would be indistinguishable from magic.
Wormholes are theoretically possible (Einsteins special relativity allows for them), and they would be the optimal strategy for Santa, says Dan Holz, a professor of physics at the University of Chicago.
Without using wormholes, Holz says, Santa would have to travel near the speed of light to deliver his gifts in one night. The constant acceleration and deceleration Santa and his toys would have to withstand going to and from each house would be problematic it would kill a normal human. That is, unless you have a very impressive sleigh and Santa is a very impressive specimen.
However, by using wormholes, Holz says, Santa could skip all of the starting and stopping. Santa can keep going through a wormhole, doing what needs to be done at whatever leisurely pace he prefers and given his belly, its easy to imagine thats the option Santa would take. On the downside, Holz says, wormholes require a form of matter that probably doesnt exist.
Another strategy for Santa would be to travel back and from the North Pole to his destinations carrying just enough presents to complete one mission at a time, although even carrying just enough toys for three households would require Santa to travel faster than the speed of light, says John Freeouf, a professor of physics at Oregons Portland State University.
An early illustration of Santa Claus, by editorial cartoonist Thomas Nast, published in Harper's Weekly in 1881. (Credit: Wikimedia Commons)
We dont have the ability to create wormholes, and acceleration would create a major problem both for Santa and his packages, says Rhett Allain, associate professor of physics at Southeastern Louisiana University. So, Santas mission might require magic. But, on the other hand, maybe magic is just physics we dont understand yet.
But what about a preemptive strike? Allain proposes. During the year, Santa makes nanobots and they move around all over the world. And when hes ready, they 3D-print packages in the kids houses. This solves the problem by getting the mass there slowly, rather than all at once.
However, Holz says, using nano-elves means Santa is less involved. I really like the idea of Santa eating the cookies, but [instead] the nanobots would take the cookies to use as the raw material to build the toys.
Could Santa use other universes? The many-worlds interpretation of quantum mechanics asserts there is a multitude of universes parallel to the one we live in.
You can imagine there are many universes, Holz says. There is always some universe where Santa has shown up at your house and given you a present. But what we want to do is have Santa show up at every house in this universe.
Maybe it makes you feel better to know that in some universe you got a present, Holz says. But for most kids, what they care about is the universe theyre currently experiencing and whether they got a present in that universe.
You could have a philosophical discussion in the morning with the kids, Holz says. You could say youre sorry they didnt get any presents, but there is definitely a universe out there where they got a present and they got exactly what they wanted. The multiverse would say thats true, but I have a feeling that for most kids, thats cold comfort and for most physicists as well.
Allain says he uses examples like Santa to teach physics. They create an emotional investment in students.
As educators, we lie all the time, Allain says. For example, many teachers tell students that gravity is equal to an objects mass multiplied by the gravitational constant (its actually more complicated than that). Thats a good model, but its not the full story. We can make the models more complicated, but we try to keep it as simple as possible, he says.
Santa, Holz notes, is a relevant approximation of whats actually happening: parents providing the gifts and eating the cookies. As parents, he says, youre using what physicists call a black box, where there are some processes under the hood that are not relevant to, in this case, whats under the tree.
But its nice to have some magic in ones life and to call that black box Santa, he says.
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Posted: at 8:07 pm
1. New Horizons nails the most distant flyby in history
Just 33 minutes after ringing in the New Year, scientists at the Johns Hopkins University Applied Physics Laboratory cheered and threw confetti a second time. The New Horizons spacecraft had just conducted a flyby of a Kuiper Belt object 4 billion miles from Earth. And as the sun rose on January 1, New Horizons beamed back its first close-up images of the 19-mile-long peanut-shaped space rock, officially named 2014 MU69.
Images eventually revealed 2014 MU69 (initially nicknamed Ultima Thule), to be a surprisingly flat contact binary, a body composed of two once-separated rocks that slowly gravitated toward each other until they lightly touched and fused. Scientists believe the flyby data could offer insight into how planets formed in our solar system billions of years ago.
In November, NASA changed the rocks nickname from Ultima Thule, a term with links to the Nazi party, to Arrokoth, which means sky in the Powhatan/Algonquian language.
2014 MU69 is revealed to be a two-tiered snowman. According to the New Horizons team, this image supports the idea that planets in our system formed as bits of raw planetary matter coalesced over time. Image Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute
Twelve years after a first patient was declared to be rid of HIV, another person achieved a similar milestone this year. In March, with the help of a stem cell transplant from a virus-resistant donor, the anonymous individual entered long-term remission from HIV.
In both cases, remission followed a transplant of bone marrow from a person with a mutation in the gene that encodes the protein CCR5, which many HIV strains use to infiltrate cells. Neither treatment was originally intended to eliminate the infection itself, but to treat blood cancers that had spread in both individuals.
While the intervention is likely to be effective in only a small fraction of HIV-positive individuals, the 2019 case shows that its efficacy was more than a one-time event.
In April, we were able to feast our eyes on the first-ever image of a black hole. The black hole, whose image was generated from data captured by a network of eight radio observatories that make up the Event Horizon Telescope, dwells in the center of a galaxy some 55 million light-years away from Earth.
As popular as the black hole image was, another aspect of the story that quickly unfolded online: the contribution of 29-year-old MIT scientist Katie Bouman, who crafted an algorithm to help translate the telescope data into the black hole image. Bouman, captured in a photo with a laptop, beaming behind her folded hands, quickly became a symbol for womens accomplishment in astronomy and computer science. But, Marina Koren writes for The Atlantic, This one image tapped into a multitude of questions about the role of women in science, the myth of the lone genius, and the pressure scientists have to promote themselves and their work on social media.
No one algorithm or person made this image, Bouman later wrote, referring to the black hole picture, in a Facebook post. It required the amazing talent of a team of scientists from around the globe and years of hard work to develop the instrument, data processing, imaging methods, and analysis techniques that were necessary to pull off this seemingly impossible feat.
CRISPR-Cas9 is a tool that lets scientists cut and insert small pieces of DNA at precise areas along a DNA strand. | Photo credit: Public Domain
In April, researchers began the first CRISPR-Cas9 gene-editing clinical trials in people in the United States. In the trials, scientists used CRISPR, a powerful gene-editing technique derived from an ancient bacterial immune system, to combat cancer and blood disorders. That month, two cancer patientsone with myeloma and one with sarcomawere treated using CRISPR.
In the cancer and blood disorder trials, researchers remove some cells from a patients body, edit the cells DNA using CRISPR, and inject the cells back in, now hopefully armed to fight disease, Tina Hesman Saey writes for Science News. But in another trial, conducted by Editas Medicine in Cambridge, Mass., researchers are using CRISPR to edit DNA directly in the human body by snipping a small piece of DNA out of cells in the eyes of people with an inherited form of blindness, Saey writes.
The trials come on the heels of Chinese scientist He Jiankuis gene-editing on twin girls born in November 2018, which was widely criticized as premature and highly unethical. .
Mammal fossils like this one, discovered by a team of paleontologists and paleobotanists led by Tyler Lyson in Corral Bluffs, Colorado, fill in a missing piece of the timeline of life. Image Credit: HHMI Tangled Bank Studios
This year, scientists gleaned new insight into the day the dinosaur-killing asteroid crashed into Earth 66 million years ago, and the first million years after the impact.
In April, paleontologist and graduate student Robert DePalma claimed to unveil an unprecedented time capsule of the asteroid-induced catastrophe. He reported finding scorched tree trunks and hundreds of well-preserved fossil fish beneath sediment at a site in North Dakota, forming a snapshot of the first minutes and hours after impact. (Some experts remain cautious about the finding, due in part to the fact that DePalmas discovery was first announced in a New Yorker article before publication of the peer-reviewed paper.)
Then, in October, new fossils that capture the million-year timeline of life after the dinosaurs died were revealed. Discovered in Colorados Corral Bluffs by paleontologist Tyler Lyson and his team, the fossils showcase the extraordinary resilience of life in the wake of disaster and help reveal the evolutionary journey of the mammals that survived the asteroid.
With the use of artificial intelligence on the rise, one serious flaw continued to make headlines in 2019: racial bias. In October, researchers announced that a particular algorithm, which predicts who might benefit from follow-up care and affects 100 million Americans, underestimates black patients need for additional treatment. The algorithm underestimates the health needs of black patients even when theyre sicker than their white counterparts.
Additionally, the U.S. remains one of the most dangerous developed nations in which to be pregnant and give birth, particularly for minorities. Pregnancy-related deaths are rising in the United States and the main risk factor is being black, Mike Stobbe and Marilynn Marchione write for AP News. A CDC report concludes black women, along with Native Americans and Alaska natives, are three times more likely to die before, during or after having a baby, and more than half of these deaths are preventable, Stobbe and Marchione write.
Also this year, researchers further investigated why black scientists are less likely to receive funding from the National Institutes of Health (NIH) than their white counterparts. A study published in October illustrated that topic choice contributes to the lower rates of NIH awards going to black scientists. Specifically, Jeffrey Mervis writes for Science Magazine, black applicants are more likely to propose approaches, such as community interventions, and topics, such as health disparities, adolescent health, and fertility, that receive less competitive scores from reviewers.
A replica of a fragment of a Denisovan finger found in Denisova Cave, Siberia, in 2008. Image Credit: Thilo Parg, Wikimedia Commons
New findings in 2019 added to anthropologists understanding of Denisovans, a species of early human that likely shared the planet with Homo sapiens as recently as 50,000 years ago.
This fall, scientists learned that although Denisovans DNA ties them more closely to Neanderthals, their fingers may have looked more like ours, suggesting Neanderthals broader digits evolved after their lineage split off from the Denisovans around 410,000 years ago. A few more fossils, Bruce Bower writes for Science News, plus genetic analyses indicated Denisovans were close relatives and occasional mating partners of Neanderthals and Homo sapiens tens of thousands of years ago. But there was too little evidence to say what Denisovans looked like or how they behaved.
Physicists reached a milestone in quantum computing this year, a method of computing that uses quantum physics to solve complex problems quickly.
In October, Google said it had achieved quantum supremacy. Its AI Quantum Team presented evidence that it had built a quantum computer that needs only 200 seconds to solve a problem that would have taken IBMs Summit, the worlds most powerful supercomputer, 10,000 years to crack. Though IBM disputed the claim, others in the computing community are tentatively optimistic about the breakthroughs promise. If validated, it may bring us closer to a future of ultra-efficient computing.
Just when you thought Saturn couldnt get any more awesome, it secured yet another claim to fame: the most known moons of any planet in our solar system (sorry, Jupiter).
On October 7, the International Astronomical Unions Minor Planet Center announced that researchers discovered an additional 20 moons orbiting Saturn, bringing its grand total to a whopping 82. Jupiter, the largest and oldest planet in our solar system, has 79.
The latest discoveries were made possible by Hawaiis Subaru telescope. A team led by Carnegie Sciences Scott S. Sheppard first eyed them in the spring of 2017, but because faraway moons are dim and tough to spot, researchers used Subaru to scan the skies periodically throughout the following years to confirm their finding. Then, they used a computer algorithm to link the data through time and confirm that the moons were indeed reliably orbiting Saturn.
Less than a week after the U.N. climate talks came to a close in Madrid this month, Australia recorded its hottest day ever, one day after its previous record. Just a few months ago, wildfires raged across not only the American West and Australian Outback but also Europe and the Amazon, an occurrence that many climatologists believe may have been exacerbated by climate change-induced drought and high temperatures. And in May, a United Nations report claimed that one million plant and animal species are on the verge of extinctionmore than in any other period in human historywith alarming implications for human survival. The warming climate, which heightens the effects of overfishing, pesticide use, pollution, and urban expansion is a major driver, the report concludes.
Three weeks ago, a bleak climate report, also from the U.N., predicted that global carbon emissions will climb despite promises from almost 200 nations to address climate change, propelling temperatures upward and threatening to shatter the threshold of 2C that scientists say would invite dramatic changes to ecology and the economy, Nathaniel Gronewald writes for Science Magazine. And many declared this months COP25 climate talks to be a massive failure.
But climate activists, particularly teens, have seized the spotlight this year. Greta Thunberg, a 16-year-old Swedish climate activist, was just named TIMEs Person of the Year. And at COP25, official youth constituency representatives expressed their disappointment to leaders and officials, Kartik Chandramouli writes for Mongabay. Do you want to be remembered as the ones who had the chance to act but decided not to as betrayers of our generation, of indigenous people and communities desperately fighting on the ground? Youth representatives said. We are rising, we are fighting and we will win.
Posted: at 8:07 pm
I remember discussions with Bohr which went through many hours till very late at night and ended almost in despair; and when at the end of the discussion I went alone for a walk in the neighboring park I repeated to myself again and again the question: Can nature possibly be so absurd as it seemed to us in these atomic experiments? Werner Heisenberg, Physics and Philosophy (1958)
In 1905, Albert Einstein argued that light behaves not only as a continuous wave but sometimes as an individual particle. This led to the development of Quantum Mechanics in the mid-1920s by Werner Heisenberg, Niels Bohr, Erwin Schrdinger, and others. Einstein questioned parts of the theory with his God does not play dice with the universe quote. Even 100 years later, Quantum Mechanics still troubles us with its strange phenomena.
Heisenberg (12/5/1901 2/1/1976) was a key pioneer in quantum mechanics. He published a breakthrough paper in 1925, and additionally with Max Born and Pascual Jordan described the matrix formulation of quantum mechanics. Heisenberg won the Nobel Prize in 1932 for the creation of quantum mechanics. In 1927, Heisenberg stated his famous Uncertainty Principle as:
The more precise the measurement of position, the more imprecise the measurement of momentum, and vice versa.
This is a fairly easy idea to understand. To measure the objects location, some energy (usually light) must impact the object. For big objects, this effect is trivial. But when an object is extremely small (like an electron) the light imparts energy to the particle. Thus the momentum (mass times velocity) changes. Einstein agreed with this theory, as it can be shown within the structure of classical physics.
But other parts of Quantum Theory are difficult, such as the dual (particle and wave) nature of light. In the famous two slit experiment, a light (or electron) source is effectively split into two separate waves that later combine, resulting in the well-known interference pattern of classical mechanics. Since Einstein showed that light can sometimes behave like a particle, how does a particle exhibit special wave properties going through either slit? You can send only one electron at a time and the effect is still valid. Even today, people still struggle with this phenomenon:
Could it be that each electrons somehow splits, passes through both slits at once, interferes with itself, and then recombines to meet the second screen as a single, localized particle?
To find out, you might place a detector by the slits, to see which slit an electron passes through If you do that, then the pattern on the detector screen turns into the particle pattern of two strips The interference pattern disappears. Somehow, the very act of looking makes sure that the electrons travel like well-behaved little tennis balls.
This is similar to whats involved in the Uncertainty Principle but very weird! No wonder Heisenberg remarked:
Quantum theory provides us with a striking illustration of the fact that we can fully understand a connection though we can only speak of it in images and parables.
Centre for Quantum Technologies (CQT) Hands Over Satellite Operations of SpooQy-1 CubeSat to SpeQtral – Business Wire
Posted: December 15, 2019 at 12:42 am
SINGAPORE--(BUSINESS WIRE)--SpeQtral announced today that it has taken over operations of the SpooQy-1 nanosatellite on behalf of the Centre for Quantum Technologies (CQT) at the National University of Singapore.
SpooQy-1 is a shoebox-sized, 3U CubeSat hosting a quantum payload developed at CQT. It was launched April 2019 and subsequently deployed from the International Space Station on 17 June 2019. The quantum payload is the worlds first entangled photon source compact enough to fit on a CubeSat and qualified for the harsh space environment.
The primary objective of the SpooQy-1 mission is to produce and characterize entangled photon pairs in space such that they violate the CHSH (Clauser-Horne-Shimony-Holt) Bells inequality. This is a core capability for future quantum communication networks. The CQT team is analysing scientific data from the mission and expects to publish results on the sources performance in 2020.
In the meantime, CQT and SpeQtral have signed an agreement allowing SpeQtral to manage ongoing operations. Formed as a spin-out company to commercialize quantum communications technologies developed at CQT, SpeQtral will monitor the long-term performance of the quantum payload for radiation damage and other degradation effects in the space environment. This information will help guide the development of long-lived quantum systems in space, necessary for the commercial deployment of space-based QKD systems.
Establishing a partnership for the SpooQy mission plays to all our strengths: at the Centre for Quantum Technologies, we will concentrate on scientific objectives, while SpeQtral focuses on commercial applications, says Artur Ekert, Director of CQT.
SpooQy-1 is pioneering quantum technologies for space-based quantum key distribution (QKD) systems, said Chune Yang Lum, co-founder and CEO of SpeQtral, Being involved in this mission gives SpeQtral know-how that serves our goal of delivering next-generation secure communication networks.
SpeQtral is developing space-based, quantum communication built on technologies developed at the Centre for Quantum Technologies (CQT) at the National University of Singapore. The team has developed technologies that harness the unique properties of quantum physics to enable encryption methods that can secure communications with forward security. SpeQtral is the only team with heritage from a successful on-orbit demonstration of a quantum light source on a CubeSat, and is committed to bringing future-proof security to the commercial world. Learn more at http://www.speqtral.space.
About Centre for Quantum Technologies
The Centre for Quantum Technologies (CQT) is a national Research Centre of Excellence in Singapore. It brings together physicists, computer scientists and engineers to do basic research on quantum physics and to build devices based on quantum phenomena. Experts in this new discipline of quantum technologies are applying their discoveries in computing, communications and sensing. The Centre was established in December 2007 with support from Singapores National Research Foundation and Ministry of Education. CQT is hosted by the National University of Singapore and also has staff at Nanyang Technological University. Learn more at http://www.quantumlah.org.
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Posted: at 12:42 am
In 1986, two relatively unknown physicists, working in a laboratory on a Swiss hilltop, made a discovery that started a revolution.
It was the Woodstock of condensed matter physics, said Enrico Rossi, associate professor of physics at William & Mary. People were so excited. It changed everything.
Physicists J. Georg Bednorz and K. Alex Mueller discovered superconductivity in ceramic material, specifically lanthanum-based cuprate perovskite, and created the first high-temperature superconductor.
The discovery earned them the 1987 Nobel Prize in Physics and held the promise that one day it could be feasible to transmit electricity and information over vast distances with virtually no loss of current or data.
They were basically playing with this ceramic material and found that it became a superconductor at temperatures well above absolute zero, well above the limit that theory predicted was possible, Rossi said.
The typical flow of electric current, the kind that powers the average household, is a charge carried by electrons that move through a circuit made from copper wiring. The electrons move from one atom to another as they travel through the wiring, which creates a current that provides power to the home.
With copper, and almost any other material, there is a certain level of resistance against the moving electrons, sort of like air resistance pushes back on a thrown tennis ball. The less resistance, the better the electrons can move and the current will flow more freely. Superconductivity is a phenomenon in which the resistance against an electric current flowing through a material is zero.
The problem with superconductivity is that it happens at very low temperatures, close to 0 Kelvin or -459.67 degrees Fahrenheit. The idea of room-temperature superconductivity is something like the El Dorado of materials science, Rossi explained.
Such a discovery would pave the way for ultrafast computers, far more efficient power transmission and high-speed trains that could travel hundreds of miles per hour with little power. For now, the City of Gold remains elusive. Bednorz and Mueller earned their Nobel for reaching superconductivity at 35 Kelvin, or -396.67 degrees Fahrenheit.
There was the hope that we could go all the way up to room temperature, Rossi said. That would be a true revolution, because you could have no dissipation in everyday connections. But were stuck in lower temperatures and, from an academic perspective, we still dont understand why these ceramic materials are superconducting.
Rossi says part of the difficulty may be that ceramic materials have a complicated chemical structure that makes it challenging to identify the key ingredients that lead electrons to superconduct. He and Xiang Hu, a postdoc research assistant in the universitys Department of Physics, are co-authors on a paper recently published in Physical Review Letters, the American Physical Societys flagship publication.
The duo collaborated with researchers from Microsoft Quantum and the Polish Academy of Sciences to examine what leads electrons to superconduct in twisted bilayer graphene. Their work was supported by an NSF-CAREER grant, the Office of Naval Research, the Army Research Office and the United States-Israel Binational Science Foundation.
Twisted bilayer graphene is a material made from taking a one-atom-thick layer of carbon atoms and folding it over on itself at a slight angle, 1.05 degrees. By folding it at that precise angle (what physicists call the magic angle), the atoms line up in such a way that the material becomes a superconductor.
The fundamental mechanism that leads to superconductivity might be the same as in the ceramic materials, but the chemical structure of twisted bilayer graphene is much simpler, Rossi explained. The revelation started a new field called "twistronics" and opened the door for researchers like Rossi and Hu to study the underlying physics of superconductivity.
Its really about the way the system is engineered, Rossi said. Take, for example, something like chalk. You can break a brick of chalk really easily, but that same material makes shells, which can last for centuries. It has to do with the nature of how the atoms are arranged. That same principle applies for creating superconductivity in graphene.
Rossi and the team of researchers found that the specific arrangement of atoms, and the way such arrangement affects the quantum state of each electron, can explain why electrons in graphene superconduct.
It helps to think of the phenomenon as a kind of quantum square-dance, with the folded matrix on which the atoms are arranged as the dance floor. The dancers are electrons, who, as the evening goes on, couple up with other electrons in groupings called Cooper pairs, a key element of superconductivity.
In twisted bilayer graphene, the electrons are forced to move slowly. If you slow them down, make the velocity very small, and you allow the electrons to spend more time in the same place, they start interacting and pairing up, Rossi said. Naively, you would also expect that to lead to pure superconductivity, because the electrons have enough time to form pairs and form a lot of them. However, if these Cooper pairs are all by themselves, doing their own thing, then the system is not going to superconduct and conventional results suggest that this would be the case in twisted bilayer graphene.
In simpler terms, to achieve superconductivity, the quantum square-dance must become a giant conga line with all the Cooper pairs joining together. If couples keep to themselves, the material doesnt superconduct. Rossi, Hu and their collaborators discovered how this happens in twisted bilayer graphene, despite the extremely small velocity of the electrons.
The way I like to explain it is that somehow they all need to link arms, Rossi said. Imagine there is a chain of people and theyre all going forward, but then they hit an obstacle. If the pairs arent linked together, then one pair will stop when they hit the obstacle and the other may keep going.
If only half the pairs are getting around the obstacle, Rossi explained, then the amount of current the system can carry is cut in half, causing electrical resistance. Half of the pairs are getting stuck. If all the electrons are able to link together, then they can pull each other past obstacles and the electrical resistance shrinks to zero.
The strength of this linkage is really important, Hu said. Using previous results, one would conclude that such strength would be vanishingly small in twisted bilayer graphene. The fact that it's possible for the linkage to be present in twisted bilayer graphene has not been examined before now.
Once the researchers realized that the conga line formation in twisted bilayer graphene was crucial for superconductivity, they set out to figure out why it happens. Instead of looking at the dancers, the team looked at how they dance.
They found it was actually the individual nature of each couple (the electron pairs individual attributes, analogous to spin) that had the greatest impact on linkage. It had a greater impact than their size or speed or how much time they spent on the quantum dance floor.
At first, the focus was on the velocity. When it goes to zero, you can form couples and thats great, but its not enough, Rossi said. You need to be able to make all those couples somehow link up. Thats what you need to get superconductivity. The assumption was that this linking was also due to the velocity, but that was neglecting the fact that there is another way. It has to do with the individual features of the quantum states. Its a contribution people hadnt considered before.
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Posted: December 13, 2019 at 2:13 pm
Quantum physics is the study of the behavior of matter and energy at the molecular, atomic, nuclear, and even smaller microscopic levels. In the early 20th century, scientists discovered that the laws governing macroscopic objects do not function the same in such small realms.
"Quantum" comes from the Latin meaning "how much." It refers to the discrete units of matter and energy that are predicted by and observed in quantum physics. Even space and time, which appear to be extremely continuous, have the smallest possible values.
As scientists gained the technology to measure with greater precision, strange phenomena was observed. The birth of quantum physics is attributed to Max Planck's 1900 paper on blackbody radiation. Development of the field was done by Max Planck, Albert Einstein, Niels Bohr, Richard Feynman, Werner Heisenberg, Erwin Schroedinger, and other luminary figures in the field. Ironically, Albert Einstein had serious theoretical issues with quantum mechanics and tried for many years to disprove or modify it.
In the realm of quantum physics, observing something actually influences the physical processes taking place. Light waves act like particles and particles act like waves (called wave particle duality). Matter can go from one spot to another without moving through the intervening space (called quantum tunnelling). Information moves instantly across vast distances. In fact, in quantum mechanics we discover that the entire universe is actually a series of probabilities. Fortunately, it breaks down when dealing with large objects, as demonstrated by the Schrodinger's Cat thought experiment.
One of the key concepts is quantum entanglement, which describes a situation where multiple particles are associated in such a way that measuring the quantum state of one particle also places constraints on the measurements of the other particles. This is best exemplified by the EPR Paradox. Though originally a thought experiment, this has now been confirmed experimentally through tests of something known as Bell's Theorem.
Quantum optics is a branch of quantum physics that focuses primarily on the behavior of light, or photons. At the level of quantum optics, the behavior of individual photons has a bearing on the outcoming light, as opposed to classical optics, which was developed by Sir Isaac Newton. Lasers are one application that has come out of the study of quantum optics.
Quantum electrodynamics (QED) is the study of how electrons and photons interact. It was developed in the late 1940s by Richard Feynman, Julian Schwinger, Sinitro Tomonage, and others. The predictions of QED regarding the scattering of photons and electrons are accurate to eleven decimal places.
Quantum physics is sometimes called quantum mechanics or quantum field theory. It also has various subfields, as discussed above, which are sometimes used interchangeably with quantum physics, though quantum physics is actually the broader term for all of these disciplines.
Causality in Quantum Physics - Thought Experiments and Interpretations
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Posted: at 2:13 pm
Image by agsandrew / Shutterstock.com
In this quantum physics introduction for beginners we will explain quantum physics, also called quantum mechanics, in simple terms. Quantum physics is possibly the most fascinating part of physics there is. It is the amazing physics that becomes relevant for small particles, where the so-called classical physics is no longer valid. Where classical mechanics describes the movement of sufficiently big particles, and everything is deterministic, we can only determine probabilities for the movement of very small particles, and we call the corresponding theory quantum mechanics.
You may have heard Einsteins saying Der Alte wrfelt nicht which translated to English roughly means God does not roll dice. Well, even geniuses can be wrong. Again, quantum mechanics is not deterministic, but we can in general only determine probabilities. Since we are used to fairly big objects in our everyday life, quantum mechanics and its laws may at first seem strange and quantum theory is often considered to be complex. But for example electrons and photons are sufficiently small that quantum physics is needed, and on this website we will show you that understanding the basics of quantum physics is easy and fun.
In the following paragraph we will describe a thought experiment that we perform at two different length scales: With bullets as known from pistols (the large scale) and with electrons (the very small scale). While the experiment is essentially the same but for the size, we will show you how the result is very different. This will be your first lecture in quantum mechanics.
Consider first a machine gun that fires bullets to a wall. Between the wall and the machine gun there is another wall that has two parallel slits that are big enough to easily allow a bullet to pass through them. To make the experiment interesting, we take a bad machine gun that has a lot of spread. This means it sometimes shoots through the first slit and sometimes through the second, and sometimes it hits the intermediate wall.
If we block the second slit, all bullets that reach the outer wall will have come through the first slit. If we count the number of bullets as a function of the distance from the center of the outer wall, we will find a curve distribution that could be similar to a Gaussian curve. We can call this probability curve P1.
If we block the first slit, all bullets that reach the outer wall will have come through the second slit. The probability curve will be mirrored around the center, and we call it P2.
If we open both slits, all bullets at the outer wall will have come through either slit 1 or 2. What is typical for classical mechanics in this situation is that then the total probability distribution P can be determined as the sum of the previously-mentioned probability distributions, P = P1 + P2.
Now consider the same experiment on a much smaller scale. Instead of bullets from a machine gun we consider electrons that for example can stem from a heated wire that is parallel to the two slits in an intermediate wall. The electron direction will have a natural spread. The slits are also much smaller than before but quite a bit broader than a single electron.
Consider again the case that the second slit is blocked. For proper sizes of the slits and distance between the wire and the walls, the probability distribution P1 will be similar to before. Similarly, if we block the slit 1, we will for proper distances find a probability distribution P2 similar to before.
What do you expect will happen if we do not block any slit? Will we find a probability distribution P = P1 + P2 as before? Well, after all we said you may guess that this is not the case. Indeed, we will instead find a probability distribution that has various minima and maxima. That is, for x = 0 there would be the strongest peak of electrons, for a certain +-Delta x there wouldnt be any electrons at all, but for +-2 Delta x there would be another peak of electrons, and so on.
How can we explain these results? Well, the explanation is rather straight forward if we assume that electrons in this specific case do not behave as particles, but as waves. Waves? you may ask. Well, consider a plain of water, and the same wall as before and the same intermediate wall with a double slit as before. At the place where the machine gun or the wire where, consider a pencil punching periodically downwards into the water. If you do this, you will get concentric waves around the point where you punch the water, until the intermediate plain with the two slits.
Behind each slit, there will be a half circle of concentric waves, up to the point where the new waves from the two slits cross each other. There, the waves from the two slits can add up or eliminate each other. As a function of the periodic punching you will find points where the height of the wave is always the same. There will be other places where the wave is sometimes very high and sometimes very low. At the outer wall, these two phases will be repeatedly following one another. The places where there is a lot of variation correspond to the places where there are the most electrons. The places with no variation correspond to the places where there are no electrons on the wall at all.
So, why do electrons in this case behave like waves and not like particles? Well, this is the thing where you will not find a satisfying answer. You just need to accept it.
What if you do not believe this? Well, the thought experiment with the electrons is rather difficult to perform with the proper scale of all elements of the experiment. But there is another very similar experiment that you can do at home. Instead of the electrons you use the photons (light particles) from a laser which you can buy for a few bucks. You let the laser shine through a double slit, darken the room, and look at the outer wall. And boom! What you see is not just two light lines on the outer wall, but a pattern of light line, dark line, light line, dark line, and so on. The intensity of the lighter region becomes less far away from the center. It corresponds exactly to the result of our thought experiments with electrons.
Why does the laser experiment give the same result as the thought experiment with electrons? It is quite easy: Light particles, called photons, are also very small and therefore behave quantum mechanically. And like electrons, they behave like waves in this specific situation. As a side remark, research has shown that light behaves like particles in another respect: If one reduces the intensity a lot, one will find single light spots from single photons on the wall. This means the light behaves like particles as well. One therefore talks about the particle-wave duality of photons or electrons.
What do you wait for? Do the experiment, and you will become a believer of quantum mechanics, or more generally phrased, of quantum physics.
The pattern with maxima and minima is called an interference pattern, since it comes about by the interference of the waves through slit 1 and slit 2. It has been found that you only get this interference pattern if you do not by other means (some additional measurement instrument) watch through which of the two slits the electrons or photons pass. If you do measure which of the two ways the particles pass by any other means, the interference pattern goes away. You will then find the sum distribution P = P1 + P2 as in the classical experiment.
A measurement device for electrons would typically disturb the electrons. More precisely, their momentum p would typically change due to a measurement device, while the place x of its path would become known more precisely. In general, there will be some uncertainty left in the momentum and in the place of the electron. It was postulated by Heisenberg that the product of these uncertainties can never be lower than a specific constant h: Delta x times Delta p >= h. Noone ever managed to disproof this relation, which is at the heart of quantum mechanics. Essentially it says, we cannot measure both momentum and place with arbitrary precision at the same time.
We said that for proper distributions you will find a similar result P1 and P2 as in the classical case. However, for other sizes one can achieve an interference pattern even for the single slits. This is the case when the slit is so broad that one can achieve an interference of the wave stemming from one side of the slit with the wave stemming from the other side of the slit.
We said above that quantum physics becomes relevant for small particles whereby we mean that naturally, quantum effects are only seen for small particles. However,the theory itself is thought to provide correct results for large particles as well. Why is it then, that quantum effects (which cannot be explained with classical theory) become increasingly difficult to observe for larger particles? Larger compound particles in general experience more interaction both within themselves and with their surroundings. These interactions typically lead to an effect physicists call decoherence which simply put means that quantum effects get lost. In this case (for sufficiently large matter), quantum physics and classical physics yield the same result.
Now you may wonder: At which size does this happen?.While one doesnt naturally observe quantum effects in large particles, ingenious people have managed to specifically prepare test environments which showed quantum effects for an ever growing size of particles. Already 1999 an experiment showed a quantum superposition in particles as large as C60 molecules (original article). A2013 articlealready claims to observe quantum superpositions in molecules that weighmore than 10000 atomic mass units. The question of where the achievable limit lies, and whether one can be sure that experiments really demonstrate quantum behavior, is still of interest. That these questions are not finally concluded is also reflected in a more recent article on the American Physical Society site. In principle, if one would be able to somehow get rid of decoherence effects in specifically prepared systems, the theory itself imposes no upper size limits on where quantum effects could be shown.
The aspect of the length scale for quantum physics that we just discussed was the particle size which typically is on the microscopic scale. A completely different matter is the length scale of how far you can move or separate such particles afteran initial interaction, without loosing quantum effects. You can view the two-slit experiment as showingan interaction between particles at the slit. If you tried out the experiment yourself, you probably realized, that the distance between the slit and the wall were you observe interference patterns can easily be some meters not microscopic at all!
Other experiments prepare two particles in a special quantum superposition called entanglement which, by the way, lies at the heart of quantum computation and then separate these particles. In someexperiments, it was possible to show interactions between these particles despite a separation over many miles. Essentially, if one measures the state of one such particle, one can thereafter predict the state of the other particle (within errors), despite the large separation between the particles. A recent experimentdemonstrated this entanglement effect over extreme distances. Particles were sent to a satellite and back to earth a fairly large scale distance compared to the size of a human.
In this quantum physics introduction we told you that both photons and electrons behave as both particles and waves. This particle-wave duality is not understandable with classical mechanics. It results in us only being able to predict probabilities, while one classically can make deterministic predictions. You can easily test these results at home by performing the two-slits experiment with a laser pointer. Have fun! We hope you enjoyed this quantum physics introduction for beginners. If you havent read it yet, you should continue with our article What Everyone should Know about Quantum Physics. And if you want to learn even more, why not have a look at our article Best Quantum Physics Books for Beginners?
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Posted: at 2:13 pm
Quantum mechanics is the branch of physics relating to the very small.
It results in what may appear to be some very strange conclusions about the physical world. At the scale of atoms and electrons, many of the equations of classical mechanics, which describe how things move at everyday sizes and speeds, cease to be useful. In classical mechanics, objects exist in a specific place at a specific time. However, in quantum mechanics, objects instead exist in a haze of probability; they have a certain chance of being at point A, another chance of being at point B and so on.Three revolutionary principlesQuantum mechanics (QM) developed over many decades, beginning as a set of controversial mathematical explanations of experiments that the math of classical mechanics could not explain. It began at the turn of the 20th century, around the same time that Albert Einstein published his theory of relativity, a separate mathematical revolution in physics that describes the motion of things at high speeds. Unlike relativity, however, the origins of QM cannot be attributed to any one scientist. Rather, multiple scientists contributed to a foundation of three revolutionary principles that gradually gained acceptance and experimental verification between 1900 and 1930. They are:
Quantized properties: Certain properties, such as position, speed and color, can sometimes only occur in specific, set amounts, much like a dial that "clicks" from number to number. This challenged a fundamental assumption of classical mechanics, which said that such properties should exist on a smooth, continuous spectrum. To describe the idea that some properties "clicked" like a dial with specific settings, scientists coined the word "quantized."
Particles of light: Light can sometimes behave as a particle. This was initially met with harsh criticism, as it ran contrary to 200 years of experiments showing that light behaved as a wave; much like ripples on the surface of a calm lake. Light behaves similarly in that it bounces off walls and bends around corners, and that the crests and troughs of the wave can add up or cancel out. Added wave crests result in brighter light, while waves that cancel out produce darkness. A light source can be thought of as a ball on a stick being rhythmically dipped in the center of a lake. The color emitted corresponds to the distance between the crests, which is determined by the speed of the ball's rhythm.
Waves of matter: Matter can also behave as a wave. This ran counter to the roughly 30 years of experiments showing that matter (such as electrons) exists as particles.
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Has Physics Ever Been Deterministic? New Insights on the Relationship Between Classical and Quantum Physics – SciTechDaily
Posted: at 2:13 pm
Bagatelle or pin-board game. Credit: Lorenzo Nocchi
Researchers from the Austrian Academy of Sciences, the University of Vienna and the University of Geneva, have proposed a new interpretation of classical physics without real numbers. This new study challenges the traditional view of classical physics as deterministic.
In classical physics it is usually assumed that if we know where an object is and its velocity, we can exactly predict where it will go. An alleged superior intelligence having the knowledge of all existing objects at present, would be able to know with certainty the future as well as the past of the universe with infinite precision. Pierre-Simon Laplace illustrated this argument, later called Laplaces demon, in the early 1800s to illustrate the concept of determinism in classical physics. It is generally believed that it was only with the advent of quantum physics that determinism was challenged. Scientists found out that not everything can be said with certainty and we can only calculate the probability that something could behave in a certain way.
But is really classical physics completely deterministic? Flavio Del Santo, researcher at Vienna Institute for Quantum Optics and Quantum Information of the Austrian Academy of Sciences and the University of Vienna, and Nicolas Gisin from the University of Geneva, address this question in their new article Physics without Determinism: Alternative Interpretations of Classical Physics, published in the journal Physical Review A.
Building on previous works of the latter author, they show that the usual interpretation of classical physics is based on tacit additional assumptions. When we measure something, say the length of a table with a ruler, we find a value with a finite precision, meaning with a finite number of digits. Even if we use a more accurate measurement instrument, we will just find more digits, but still a finite number of them. However, classical physics assumes that even if we may not be able to measure them, there exist an infinite number of predetermined digits. This means that the length of the table is always perfectly determined.
Imagine now to play a variant of the Bagatelle or pin-board game (as in figure), where a board is symmetrically filled with pins. When a little ball rolls down the board, it will hit the pins and move either to the right or to the left of each of them. In a deterministic world, the perfect knowledge of the initial conditions under which the ball enters the board (its velocity and position) determines unambiguously the path that the ball will follow between the pins. Classical physics assumes that if we cannot obtain the same path in different runs, it is only because in practice we were not able to set precisely the same initial conditions. For instance, because we do not have an infinitely precise measurement instrument to set the initial position of the ball when entering the board.
The authors of this new study propose an alternative view: after a certain number of pins, the future of the ball is genuinely random, even in principle, and not due to the limitations of our measurement instruments. At each hit, the ball has a certain propensity or tendency to bounce on the right or on the left, and this choice is not determined a priori. For the first few hits, the path can be determined with certainty, that is the propensity is 100% for the one side and 0% for the other. After a certain number of pins, however, the choice is not pre-determined and the propensity gradually reaches 50% for the right and 50% for the left for the distant pins. In this way, one can think of each digit of the length of our table as becoming determined by a process similar to the choice of going left or right at each hit of the little ball. Therefore, after a certain number of digits, the length is not determined anymore.
The new model introduced by the researchers hence refuses the usual attribution of a physical meaning to mathematical real numbers (numbers with infinite predetermined digits). It states instead that after a certain number of digits their values become truly random, and only the propensity of taking a specific value is well defined. This leads to new insights on the relationship between classical and quantum physics. In fact, when, how and under what circumstances an indeterminate quantity takes a definite value is a notorious question in the foundations of quantum physics, known as the quantum measurement problem. This is related to the fact that in the quantum world it is impossible to observe reality without changing it. In fact, the value of a measurement on a quantum object is not yet established until an observer actually measures it. This new study, on the other hand, points out that the same issue could have always been hidden also behind the reassuring rules of classical physics.
Reference: Physics without determinism: Alternative interpretations of classical physics by Flavio Del Santo and Nicolas Gisin, 5 December 2019, Physical Review A.DOI: 10.1103/PhysRevA.100.062107
Posted: at 2:13 pm
Quantum computers will make easy work of our current encryption systems, putting some of the worlds most sensitive data at risk. And John Prisco, CEO of the security company Quantum Xchange, tells Inverse that the time for new encryption is already here.
Traditional cryptography relies on a system of public and private encrypted keys that protect data by creating a decryption process that relies on solving incredibly complex math. Namely, the factoring of prime numbers. For todays computers, trying to solve the answer through brute force (e.g. guessing as many different answers as possible) would be nearly impossible. But for quantum computers, such computational hurdles would be trivial.
Before computers were as powerful as they are today, that [kind of cryptography] was going to be good for a million years, says Prisco. [But] a million years got truncated into just a handful of years.
But such computational might, for the time being, is still fairly theoretical. Google was only able to achieve quantum supremacy (a benchmark that compares its computational abilities to a classical computer) this year and quantum systems are far from office staples. Yet, Prisco tells Inverse that waiting until these machines become more widespread to begin improving our encryption methods would be too late.
People are stealing data today and then harvesting [and] storing it, says Prisco. And when they crack the key, then theyve got the information. So if you have data that has a long shelf life, like personal information, personnel records, you really cant afford to not future proof that.
And government agencies says Prisco, are worried about this too. In 2017 NIST (National Institute of Science and Technology) put out a call for new, quantum-resistant algorithms. Out of the 82 submissions it received, only 26 are still being considered for implementation. But Prisco tells Inverse that simply creating algorithms to combat these advanced computers wont be enough. Instead, we need to fight quantum with quantum.
Thats where Priscos company, Quantum Xchange, comes in. Instead of focusing on quantum-resistant algorithms, Quantum Xchange creates new encryption keys that themselves rely on the physics of quantum mechanics.
Just as todays keys are made up of numbers, says Prisco, their quantum key (called QKD) would be made up of photons.
[The QKDs] photons are encoded with ones and zeros, but rather than relying on solving a difficult math problem, it relies on a property of physics, says Prisco. And that property is associated with not being able to observe a photon in any way, shape, or form without changing its quantum state.
This quantum property that Prisco refers to is a law of physics called the Heisenberg Uncertainty Principle. According to this principle, the quantum state of the QKD is only stable as long as its not observed. So, even if a nefarious actor were to steal the QKD, Prisco tells Inverse, the very act of stealing it would count as observation and would thus change the QKD altogether and render it moot.
You could steal the quantum key, says Prisco, but it would no longer be the key that was used to encrypt and therefore it would no longer be able to decrypt.
Prisco tells Inverse that he believes this new generation of quantum keys would remain resilient as long as the laws of quantum physics did. So in theory, a very, very long time.
While other experts have estimated that it will be ten years until such quantum attacks really start taking place, Prisco tells Inverse he believes it will be less than five. And waiting to develop these technologies will not only put our data at risk, but could put us behind the curve when it comes to competing with other countries in this arena as well. Particularly China, who Prisco says is outspending the U.S. 10-to-1 in quantum technology.
Going forward, Prisco says that the U.S.s best bet will be to incorporate both the quantum-resistant algorithms being developed by NIST and other government agencies as well as a quantum key like their QKD.
Im a proponent for combining what NSA and NIST are doing with quantum-resistant algorithms with quantum keys, says Prisco. You know, it may seem like a revolutionary concept in the United States but I can tell you that Chinas doing this, all of Europes doing this Russias doing this. Everybody kind of realizes that the quantum computer is an offensive weapon when it comes to cryptography. And that the first defensive weapon one can deploy are the quantum keys, and then quantum-resistant algorithms when theyre available.
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