Posts Tagged ‘physics’

by Mike McCrae

Everything in our Universe is held together or pushed apart by four fundamental forces: gravity, electromagnetism, and two nuclear interactions. Physicists now think they’ve spotted the actions of a fifth physical force emerging from a helium atom.

It’s not the first time researchers claim to have caught a glimpse of it, either. A few years ago, they saw it in the decay of an isotope of beryllium. Now the same team has seen a second example of the mysterious force at play – and the particle they think is carrying it, which they’re calling X17.

If the discovery is confirmed, not only could learning more about X17 let us better understand the forces that govern our Universe, it could also help scientists solve the dark matter problem once and for all.

Attila Krasznahorkay and his colleagues from the Institute for Nuclear Research in Hungary suspected something weird was going on back in 2016, after analysing the way an excited beryllium-8 emits light as it decays.

If that light is energetic enough, it transforms into an electron and a positron, which push away from one another at a predictable angle before zooming off.

Based on the law of conservation of energy, as the energy of the light producing the two particles increases, the angle between them should decrease. Statistically speaking, at least.

Oddly, this isn’t quite what Krasznahorkay and his team saw. Among their tally of angles there was an unexpected rise in the number of electrons and positrons separating at an angle of 140 degrees.

The study seemed robust enough, and soon attracted the attention of other researchers around the globe who suggested that a whole new particle could be responsible for the anomaly.

Not just any old particle; its characteristics suggested it had to be a completely new kind of fundamental boson.

That’s no small claim. We currently know of four fundamental forces, and we know that three of them have bosons carrying their messages of attraction and repulsion.

The force of gravity is carried by a hypothetical particle known as a ‘graviton’, but sadly scientists have not yet detected it.

This new boson couldn’t possibly be one of the particles carrying the four known forces, thanks to its distinctive mass of (17 megaelectronvolts, or about 33 times that of an electron), and tiny life span (of about 10 to the minus 14 seconds … but hey, it’s long enough to smile for the camera).

So all signs point to the boson being the carrier of some new, fifth force. But physics isn’t keen on celebrating prematurely. Finding a new particle is always big news in physics, and warrants a lot of scrutiny. Not to mention repeated experiment.

Fortunately, Krasznahorkay’s team haven’t exactly been sitting on their laurels over the past few years. They’ve since changed focus from looking at the decay of beryllium-8 to a change in the state of an excited helium nucleus.

Similar to their previous discovery, the researchers found pairs of electrons and positrons separating at an angle that didn’t match currently accepted models. This time, the number was closer to 115 degrees.

Working backwards, the team calculated the helium’s nucleus could also have produced a short-lived boson with a mass just under 17 megaelectronvolts.

To keep it simple, they’re calling it X17. It’s a long way from being an official particle we can add to any models of matter.

While 2016’s experiment was accepted into the respectable journal, Physical Review Letters, this latest study is yet to be peer reviewed. You can read the findings yourself on arXiv, where they’ve been uploaded to be scrutinised by others in the field.

But if this strange boson isn’t just an illusion caused by some experimental blip, the fact it interacts with neutrons hints at a force that acts nothing like the traditional four.

With the ghostly pull of dark matter posing one of the biggest mysteries in physics today, a completely new fundamental particle could point to a solution we’re all craving, providing a way to connect the matter we can see with the matter we can’t.

In fact, a number of dark matter experiments have been keeping an eye out for a 17 megavolt oddball particle. So far they’ve found nothing, but with plenty of room left to explore, it’s too early to rule anything out.

Rearranging the Standard Model of known forces and their particles to make room for a new member of the family would be a massive shift, and not a change to make lightly.

Still, something like X17 could be just what we’re looking for.

This research is available on arXiv ahead of peer review: https://arxiv.org/abs/1910.10459

https://www.sciencealert.com/physicists-claim-a-they-ve-found-even-more-evidence-of-a-new-force-of-nature

By Corey S. Powell

Ever wonder what would have happened if you’d taken up the “Hey, let’s get coffee” offer from that cool classmate you once had? If you believe some of today’s top physicists, such questions are more than idle what-ifs. Maybe a version of you in another world did go on that date, and is now celebrating your 10th wedding anniversary.

The idea that there are multiple versions of you, existing across worlds too numerous to count, is a long way from our intuitive experience. It sure looks and feels like each of us is just one person living just one life, waking up every day in the same, one-and-only world.

But according to an increasingly popular analysis of quantum mechanics known as the “many worlds interpretation,” every fundamental event that has multiple possible outcomes — whether it’s a particle of light hitting Mars or a molecule in the flame bouncing off your teapot — splits the world into alternate realities.

Multiple splits, multiple worlds

Even to seasoned scientists, it’s odd to think that the universe splits apart depending on whether a molecule bounces this way or that way. It’s odder still to realize that a similar splitting could occur for every interaction taking place in the quantum world.

Things get downright bizarre when you realize that all those subatomic splits would also apply to bigger things, including ourselves. Maybe there’s a world in which a version of you split off and bought a winning lottery ticket. Or maybe in another, you tripped at the top of a cliff and fell to your death — oops.

“It’s absolutely possible that there are multiple worlds where you made different decisions. We’re just obeying the laws of physics,” says Sean Carroll, a theoretical physicist at the California Institute of Technology and the author of a new book on many worlds titled “Something Deeply Hidden.” Just how many versions of you might there be? “We don’t know whether the number of worlds is finite or infinite, but it’s certainly a very large number,” Carroll says. “There’s no way it’s, like, five.”

Carroll is aware that the many worlds interpretation sounds like something plucked from a science fiction movie. (It doesn’t help that he was an adviser on “Avengers: Endgame.”) And like a Hollywood blockbuster, the many worlds interpretation attracts both passionate fans and scathing critics.

Renowned theorist Roger Penrose of Oxford University dismisses the idea as “reductio ad absurdum”: physics reduced to absurdity. On the other hand, Penrose’s former collaborator, the late Stephen Hawking, described the many worlds interpretation as “self-evidently true.”

Carroll himself is comfortable with the idea that he’s but one of many Sean Carrolls running around in alternate versions of reality. “The concept of a single person extending from birth to death was always just a useful approximation,” he writes in his new book, and to him the many worlds interpretation merely extends that idea: “The world duplicates, and everything within the world goes along with it.”

How did we get here?

The mind-bending saga of the many worlds interpretation began in 1926, when Austrian physicist Erwin Schrödinger mathematically demonstrated that the subatomic world is fundamentally blurry.

In the familiar, human-scale reality, an object exists in one well-defined place: Place your phone on your bedside table, and that’s the only spot it can be, whether or not you’re looking for it. But in the quantum realm, objects exist in a smudge of probability, snapping into focus only when observed.

“Before you look at an object, whether it’s an electron, or an atom or whatever, it’s not in any definite location,” Carroll says. “It might be more likely that you observe it in one place or another, but it’s not actually located at any particular place.”

Nearly a century of experimentation has confirmed that, strange as it seems, this phenomenon is a core aspect of the physical world. Even Einstein struggled with the notion: What happened to all of the other possible locations where the object could have been, and all the other different outcomes that could have ensued? Why should an object’s behavior depend on whether or not somebody was looking at it?

In 1957, a Princeton student named Hugh Everett III came up with a radical explanation. He proposed that all possible outcomes really do occur — but that only a single version plays out in the world we inhabit. All the other possibilities split off from us, each giving rise to its own separate world. Nothing ever goes to waste, in this view, since everything that can happen does happen in some world.

For decades, Everett’s colleagues mostly brushed aside his explanation, treating it more like a ghost story than serious science. But nobody has found any flaws in Schrödinger’s equation; nor can they explain away its implications. As a result, many contemporary physicists — including David Deutsch at Oxford University and Max Tegmark at the Massachusetts Institute of Technology — have come to agree with Carroll that the many worlds interpretation is the only coherent way to understand quantum mechanics.

A field guide to many worlds

The many worlds interpretation raises all kinds of puzzling questions about the multiple versions of reality, and about the multiple versions of you that exist in them. Carroll has some answers.

If new universes are constantly popping into existence, isn’t something being created from nothing, violating one of the most basic principles of physics? Not so, according to Carroll: “It only looks like you are creating extra copies of the universe. It’s better to think of it as taking a big thick universe and slicing it.”

Why do we experience one particular reality but none of the others? “What other one would you find yourself in?” Carroll says, amused. “It’s like asking why you live now instead of some other time. Everyone in every world thinks that they’re in that world.”

Carroll also has a disappointing response for one of the most compelling questions of all: Could you cross over and visit one of the other realities and compare notes with an alternate-world version of yourself? “Once the other worlds come into existence, they go their own way,” Carroll says. “They don’t interact, they don’t influence each other in any form. Crossing over is like traveling faster than the speed of light. It’s not something that you can do.”

War of the many worlds

One criticism of the many worlds interpretation is that while it offers a colorful way to think about the world, it doesn’t deliver any new insights into how nature works. “It is completely content-less,” says physicist Christopher Fuchs of the University of Massachusetts, Boston.

Fuchs favors an alternative called Quantum Bayesianism, which offers a path back to an old-fashioned single reality. He argues that the universe changes when you look at it not because you are creating new worlds but simply because observation requires interacting with your surroundings. No coffee dates, no other lives for you. “In this way, measurement is demoted from being something mystical to being about things as mundane as walking across a busy street: It’s an action I can take that clearly has consequences for me,” he says.

Coming at the critique from a different angle, Oxford’s Roger Penrose argues that the whole idea of many worlds is flawed, because it’s based on an overly simplistic version of quantum mechanics that doesn’t account for gravity. “The rules must change when gravity is involved,” he says.

In a more complete quantum theory, Penrose argues, gravity helps anchor reality and blurry events will have only one allowable outcome. He points to a potentially decisive experiment now being carried out at the University of California, Santa Barbara, and Leiden University in the Netherlands that’s designed to directly observe how an object transforms from many possible locations to a single, fixed reality.

Carroll is unmoved by these alternative explanations, which he considers overly complicated and unsupported by data. The notion of multiple yous can be unnerving, he concedes. But to him the underlying concept of many worlds is “crisp, clear, beautiful, simple and pure.”

If he’s right, he’s not the only Sean Carroll who feels that way.

https://www.nbcnews.com/mach/science/weirdest-idea-quantum-physics-catching-there-may-be-endless-worlds-ncna1068706

by Matt Williams

The idea of traveling to another star system has been the dream of people long before the first rockets and astronauts were sent to space. But despite all the progress we have made since the beginning of the Space Age, interstellar travel remains just that – a dream. While theoretical concepts have been proposed, the issues of cost, travel time and fuel remain highly problematic.

A lot of hopes currently hinge on the use of directed energy and lightsails to push tiny spacecraft to relativistic speeds. But what if there was a way to make larger spacecraft fast enough to conduct interstellar voyages? According to Prof. David Kipping, the leader of Columbia University’s Cool Worlds lab, future spacecraft could rely on a halo drive, which uses the gravitational force of a black hole to reach incredible speeds.

Prof. Kipping described this concept in a recent study that appeared online (the preprint is also available on the Cool Worlds website). In it, Kipping addressed one of the greatest challenges posed by space exploration, which is the sheer amount of time and energy it would take to send a spacecraft on a mission to explore beyond our solar system.

Kipping told Universe Today via email: “Interstellar travel is one of the most challenging technical feats we can conceive of. Whilst we can envisage drifting between the stars over millions of years – which is legitimately interstellar travel – to achieve journeys on timescales of centuries or less requires relativistic propulsion.”

As Kipping put it, relativistic propulsion (or accelerating to a fraction of the speed of light) is very expensive in terms of energy. Existing spacecraft simply don’t have the fuel capacity to get up to those kinds of speeds, and short of detonating nukes to generate thrust à la Project Orion, or building a fusion ramjet à la Project Daedalus, there are not a lot of options available.

In recent years, attention has shifted toward the idea of using lightsails and nanocraft to conduct interstellar missions. A well-known example is Breakthrough Starshot, an initiative that aims to send a smartphone-sized spacecraft to Alpha Centauri within our lifetime. Using a powerful laser array, the lightsail would be accelerated to speeds of up to 20 percent of the speed of light – thus making the trip in 20 years.

“But even here, you are talking about several terra-joules of energy for the most minimalist (a gram-mass) spacecraft conceivable,” said Kipping. “That’s the cumulative energy output of nuclear power stations running for weeks on end… so this is why it’s hard.”

To this, Kipping suggests a modified version of the “Dyson Slingshot,” an idea proposed by venerated theoretical physicist Freeman Dyson, the theorist behind the Dyson Sphere. In the 1963 book Interstellar Communications (Chapter 12: “Gravitational Machines”), Dyson described how spacecraft could slingshot around compact binary stars in order to receive a significant boost in velocity.

As Dyson described it, a ship would be dispatched to a compact binary system where it would perform a gravity-assist maneuver. This would consist of the spaceship picking up speed from the binary’s intense gravity, adding the equivalent of twice their rotational velocity to its own, and is then flung out of the system.

While the prospect of harnessing this kind of energy for the sake of propulsion was highly theoretical in Dyson’s time (and still is), Dyson offered two reasons why “gravitational machines” were worth exploring:

“First, if our species continues to expand its population and its technology at an exponential rate, there may come a time in the remote future where engineering on an astronomical scale may be both feasible and necessary. Second, if we are searching for signs of technologically advanced life already existing elsewhere in the universe, it is useful to consider what kind of observable phenomena a really advanced technology might be capable of producing.”

In short, gravitational machines are worth studying in case they become possible someday, and because this study could allow us to spot possible extraterrestrial intelligences (ETIs) by detecting the technosignatures such machines would create. Expanding upon this, Kipping considers how black holes, especially those found in binary pairs, could constitute even more powerful gravitational slingshots.

This proposal is based in part on the recent success of the Laser Interferometer Gravitational-Wave Observatory (LIGO), which has detected multiple gravitational wave signals since 2016. According to recent estimates based on these detections, there could be as many as 100 million black holes in the Milky Way galaxy alone.

Where binaries occur, they possess an incredible amount of rotational energy, which is the result of their spin and the way they rapidly orbit one another. In addition, as Kipping notes, black holes can also act as a gravitational mirror – where photons directed at the edge of the event horizon will bend around and come straight back at the source. As Kipping put it:

“So the binary black hole is really a couple of giant mirrors circling around one another at potentially high velocity. The halo drive exploits this by bouncing photons off the “mirror” as the mirror approaches you, the photons bounce back, pushing you along, but also steal some of the energy from the black hole binary itself (think about how a ping pong ball thrown against a moving wall would come back faster). Using this setup, one can harvest the binary black hole energy for propulsion.”

This method of propulsion offers several obvious advantages. For starters, it offers users the potential to travel at relativistic speeds without the need for fuel, which currently accounts for the majority of a launch vehicle’s mass. And there are many, many black holes that exist throughout the Milky Way, which could act as a network for relativistic space travel.

What’s more, scientists have already witnessed the power of gravitational slingshots thanks to the discovery of hyper-velocity stars. According to research from the Harvard-Smithsonian Center for Astrophysics (CfA), these stars are a result of galactic mergers and interaction with massive black holes, which kick them out of their galaxies at one-tenth to one-third the speed of light – around 30,000 to 100,000 km/s (18,600 to 62,000 mps).

But of course, the concept comes with innumerable challenges and more than a few disadvantages. In addition to building spacecraft that can endure being flung around the event horizon of a black hole, a tremendous amount of precision is required – otherwise, the ship and crew (if it has one) could be pulled apart in the maw of the black hole. Additionally, there’s simply the matter of reaching one:

“[T]he thing has a huge disadvantage for us in that we have to first get to one of these black holes. I tend to think of it like a interstellar highway system – you have to pay a one-time toll to get on the highway, but once you’re on, you can ride across the galaxy as much as you like without expending any more fuel.”

The challenge of how humanity might go about reaching the nearest suitable black hole will be the subject of Kipping’s next paper, he indicated. And while an idea like this is about as remote to us as building a Dyson Sphere or using black holes to power starships, it does offer some pretty exciting possibilities for the future.

In short, the concept of a black hole gravity machine presents humanity with a plausible path to becoming an interstellar species. In the meantime, the study of the concept will provide SETI researchers with another possible technosignature to look for. So until the day comes when we might attempt this ourselves, we will be able to see if any other species have already made it work.

Read more at: https://phys.org/news/2019-03-black-holes-conquer-space-halo.html#jCp

by Jonathan O’Callaghan

You might be forgiven for thinking our understanding of classical physics had reached its peak in the four centuries since Isaac Newton devised his eponymous laws of motion. But surprising new research shows there are still secrets waiting to be found, hidden in plain sight—or, at least in this case, within earshot.

In a paper published in Physical Review Letters, a group of scientists has theorized that sound waves possess mass, meaning sounds would be directly affected by gravity. They suggest phonons, particlelike collective excitations responsible for transporting sound waves across a medium, might exhibit a tiny amount of mass in a gravitational field. “You would expect classical physics results like this one to have been known for a long time by now,” says Angelo Esposito from Columbia University, the lead author on the paper. “It’s something we stumbled upon almost by chance.”

Esposito and his colleagues built on a previous paper published last year, in which Alberto Nicolis of Columbia and Riccardo Penco from Carnegie Mellon University first suggested phonons could have mass in a superfluid. The latest study, however, shows this effect should hold true for other materials, too, including regular liquids and solids, and even air itself.

And although the amount of mass carried by the phonons is expected to be tiny—comparable with a hydrogen atom, about 10–24 grams—it may actually be measurable. Except, if you were to measure it, you would find something deeply counterintuitive: The mass of the phonons would be negative, meaning they would fall “up.” Over time their trajectory would gradually move away from a gravitational source such as Earth. “If their gravitational mass was positive, they would fall downward,” Penco says. “Because their gravitational mass is negative, phonons fall upwards.” And the amount they would “fall” is equally small, varying depending on the medium the phonon is traveling through. In water, where sound moves at 1.5 kilometers per second, the negative mass of the phonon would cause it to drift at about 1 degree per second. But this corresponds to a change of 1 degree over 15 kilometers, which would be exceedingly difficult to measure.

Difficult it might be, but such a measurement should still be possible. Esposito notes that to distinguish the phonons’ mass, one could look for them in a medium where the speed of sound was very slow. That might be possible in superfluid helium, where the speed of sound can drop to hundreds of meters per second or less, and the passage of a single phonon might shift an atom’s equivalent of material.

Alternatively, instead of seeking minuscule effects magnified by exotic substances, researchers might look for more obvious signs of mass-carrying phonons by closely studying extremely intense sound waves. Earthquakes offer one possibility, Esposito says. According to his calculations, a magnitude 9 temblor would release enough energy so that the resulting change in the gravitational acceleration of the earthquake’s sound wave might be measurable using atomic clocks. (Although current techniques are not sensitive enough to detect the gravitational field of a seismic wave, future advancements in technology might make this possible.)

Sound waves having mass are unlikely to have a major impact on day-to-day life, but the possibility something so fundamental has gone unnoticed for so long is intriguing. “Until this paper, it was thought that sound waves do not transport mass,” says Ira Rothstein from Carnegie Mellon University, who was not involved in this research. “So in that sense it’s a really remarkable result. Because anytime you find any new result in classical physics, given that it’s been around since Newton, you would have thought it would be completely understood. If you look carefully enough, you can find fresh [ideas] even in fields which have been covered for centuries.”

As for why this has never been spotted before, Esposito is uncertain. “Maybe because we are high-energy physicists, gravity is more our language,” he says. “It’s not some theoretical mumbo jumbo kind of thing. In principle people could have discovered it years ago.”

https://www.scientificamerican.com/article/sound-by-the-pound-surprising-discovery-hints-sonic-waves-carry-mass/

Back in 1961, the Nobel Prize–winning physicist Eugene Wigner outlined a thought experiment that demonstrated one of the lesser-known paradoxes of quantum mechanics. The experiment shows how the strange nature of the universe allows two observers—say, Wigner and Wigner’s friend—to experience different realities.

Since then, physicists have used the “Wigner’s Friend” thought experiment to explore the nature of measurement and to argue over whether objective facts can exist. That’s important because scientists carry out experiments to establish objective facts. But if they experience different realities, the argument goes, how can they agree on what these facts might be?

That’s provided some entertaining fodder for after-dinner conversation, but Wigner’s thought experiment has never been more than that—just a thought experiment.

Last year, however, physicists noticed that recent advances in quantum technologies have made it possible to reproduce the Wigner’s Friend test in a real experiment. In other words, it ought to be possible to create different realities and compare them in the lab to find out whether they can be reconciled.

And today, Massimiliano Proietti at Heriot-Watt University in Edinburgh and a few colleagues say they have performed this experiment for the first time: they have created different realities and compared them. Their conclusion is that Wigner was correct—these realities can be made irreconcilable so that it is impossible to agree on objective facts about an experiment.

Wigner’s original thought experiment is straightforward in principle. It begins with a single polarized photon that, when measured, can have either a horizontal polarization or a vertical polarization. But before the measurement, according to the laws of quantum mechanics, the photon exists in both polarization states at the same time—a so-called superposition.

Wigner imagined a friend in a different lab measuring the state of this photon and storing the result, while Wigner observed from afar. Wigner has no information about his friend’s measurement and so is forced to assume that the photon and the measurement of it are in a superposition of all possible outcomes of the experiment.

Wigner can even perform an experiment to determine whether this superposition exists or not. This is a kind of interference experiment showing that the photon and the measurement are indeed in a superposition.

From Wigner’s point of view, this is a “fact”—the superposition exists. And this fact suggests that a measurement cannot have taken place.

But this is in stark contrast to the point of view of the friend, who has indeed measured the photon’s polarization and recorded it. The friend can even call Wigner and say the measurement has been done (provided the outcome is not revealed).

So the two realities are at odds with each other. “This calls into question the objective status of the facts established by the two observers,” say Proietti and co.

That’s the theory, but last year Caslav Brukner, at the University of Vienna in Austria, came up with a way to re-create the Wigner’s Friend experiment in the lab by means of techniques involving the entanglement of many particles at the same time.

The breakthrough that Proietti and co have made is to carry this out. “In a state-of-the-art 6-photon experiment, we realize this extended Wigner’s friend scenario,” they say.

They use these six entangled photons to create two alternate realities—one representing Wigner and one representing Wigner’s friend. Wigner’s friend measures the polarization of a photon and stores the result. Wigner then performs an interference measurement to determine if the measurement and the photon are in a superposition.

The experiment produces an unambiguous result. It turns out that both realities can coexist even though they produce irreconcilable outcomes, just as Wigner predicted.

That raises some fascinating questions that are forcing physicists to reconsider the nature of reality.

The idea that observers can ultimately reconcile their measurements of some kind of fundamental reality is based on several assumptions. The first is that universal facts actually exist and that observers can agree on them.

But there are other assumptions too. One is that observers have the freedom to make whatever observations they want. And another is that the choices one observer makes do not influence the choices other observers make—an assumption that physicists call locality.

If there is an objective reality that everyone can agree on, then these assumptions all hold.

But Proietti and co’s result suggests that objective reality does not exist. In other words, the experiment suggests that one or more of the assumptions—the idea that there is a reality we can agree on, the idea that we have freedom of choice, or the idea of locality—must be wrong.

Of course, there is another way out for those hanging on to the conventional view of reality. This is that there is some other loophole that the experimenters have overlooked. Indeed, physicists have tried to close loopholes in similar experiments for years, although they concede that it may never be possible to close them all.

Nevertheless, the work has important implications for the work of scientists. “The scientific method relies on facts, established through repeated measurements and agreed upon universally, independently of who observed them,” say Proietti and co. And yet in the same paper, they undermine this idea, perhaps fatally.

The next step is to go further: to construct experiments creating increasingly bizarre alternate realities that cannot be reconciled. Where this will take us is anybody’s guess. But Wigner, and his friend, would surely not be surprised.

Ref: arxiv.org/abs/1902.05080 : Experimental Rejection of Observer-Independence in the Quantum World

https://www.technologyreview.com/s/613092/a-quantum-experiment-suggests-theres-no-such-thing-as-objective-reality/

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A new uncertainty principle holds that quantum objects can be at two temperatures at once, which is similar to the famous Schrödinger’s cat thought experiment, in which a cat in a box with a radioactive element can be both alive and dead.

By Meredith Fore

The famous thought experiment known as Schrödinger’s cat implies that a cat in a box can be both dead and alive at the same time — a bizarre phenomenon that is a consequence of quantum mechanics.

Now, physicists at the University of Exeter in England have found that a similar state of limbo may exist for temperatures: Objects can be two temperatures at the same time at the quantum level. This weird quantum paradox is the first completely new quantum uncertainty relation to be formulated in decades.

Heisenberg’s other principle
In 1927, German physicist Werner Heisenberg postulated that the more precisely you measure a quantum particle’s position, the less precisely you can know its momentum, and vice versa — a rule that would become the now-famous Heisenberg uncertainty principle.

The new quantum uncertainty, which states that the more precisely you know temperature, the less you can say about energy, and vice versa, has big implications for nanoscience, which studies incredibly tiny objects smaller than a nanometer. This principle will change how scientists measure the temperature of extremely small things such as quantum dots, small semiconductors or single cells, the researchers said in the new study, which was published in June in the journal Nature Communications.

In the 1930s, Heisenberg and Danish physicist Niels Bohr established an uncertainty relation between energy and temperature on the nonquantum scale. The idea was that, if you wanted to know the exact temperature of an object, the best and most precise scientific way to do that would be to immerse it in a “reservoir” — say, a tub of water, or a fridge full of cold air — with a known temperature, and allow the object to slowly become that temperature. This is called thermal equilibrium.

However, that thermal equilibrium is maintained by the object and the reservoir constantly exchanging energy. The energy in your object therefore goes up and down by infinitesimal amounts, making it impossible to define precisely. On the flip side, if you wanted to know the precise energy in your object, you would have to isolate it so that it could not come into contact with, and exchange energy with, anything. But if you isolated it, you would not be able to precisely measure its temperature using a reservoir. This limitation makes the temperature uncertain.

Things get weirder when you go to the quantum scale.

A new uncertainty relation
Even if a typical thermometer has an energy that goes up and down slightly, that energy can still be known to within a small range. This is not true at all on the quantum level, the new research showed, and it’s all due to Schrödinger’s cat. That thought experiment proposed a theoretical cat in a box with a poison that could be activated by the decay of a radioactive particle. According to the laws of quantum mechanics, the particle could have decayed and not decayed at the same time, meaning that until the box was opened, the cat would be both dead and alive at the same time — a phenomenon known as superposition.

The researchers used math and theory to predict exactly how such superposition affects the measurement of the temperature of quantum objects.

“In the quantum case, a quantum thermometer … will be in a superposition of energy states simultaneously,”Harry Miller, one of the physicists at the University of Exeter who developed the new principle, told Live Science. “What we find is that because the thermometer no longer has a well-defined energy and is actually in a combination of different states at once, that this actually contributes to the uncertainty in the temperature that we can measure.”

In our world, a thermometer may tell us an object is between 31 and 32 degrees Fahrenheit (minus 0.5 and zero degrees Celsius). In the quantum world, a thermometer may tell us an object is both those temperatures at the same time. The new uncertainty principle accounts for that quantum weirdness.

Interactions between objects at the quantum scale can create superpositions, and also create energy. The old uncertainty relation ignored these effects, because it doesn’t matter for nonquantum objects. But it matters a lot when you’re trying to measure the temperature of a quantum dot, and this new uncertainty relation makes up a theoretical framework to take these interactions into account.

The new paper could help anyone who’s designing an experiment to measure temperature changes in objects below the nanometer scale, Miller said. “Our result is going to tell them exactly how to accurately design their probes and tell them how to account for the additional quantum uncertainty that you get.”

https://www.livescience.com/63595-schrodinger-uncertainty-relation-temperature.html

time

by MIKE MCRAE

For around a century it’s been thought that particles don’t have defined properties until we nail them down with a measurement.

That kind of quantum madness opens up a whole world of counter-intuitive paradoxes. Take this one, for example – it’s possible for a single particle to experience two sequences of events at the same time, making it impossible to know which came first.

Physicists from the University of Queensland designed a race course for light that forced a single particle to traverse two pathways at once, making it impossible to say in which order it completed a pair of operations.

In boring old everyday life you could roll a single ball down a ramp and have it ring bell A and then ring bell B. Or, if you’d prefer, you could roll it down another ramp and have it ring B before A.

If you want to get fancy you could even set up a rig so one bell causes the other bell to ring.

None of this is mind blowing, since we’re used to events in the Universe having a set order, where one thing precedes another in such a way that we presume an order of causation.

But nothing is so simple when we accept that reality is a blur of possibility prior to it being measured.

To demonstrate this, the physicists created a physical equivalent of something called a quantum switch, where multiple operations occur while a particle is in a superposition of all its possible locations.

Keeping it simple, the team set up a pathway that split apart and converged again in an interferometer, with access to each fork dependent on the polarisation of the light entering it.

Light waves travelling down each fork in the pathway would then merge and interfere to create a distinctive pattern depending on its properties.

In this particular case, the two light waves were actually the same photon taking both paths at the same time.

Before being measured, a photon can be either vertically or horizontally polarised. Or, more precisely, it’s polarised both vertical and horizontal at the same time until a measurement confirms one over the other.

Since this undefined photon’s polarisation is both vertical and horizontal, it enters both pathways, with the vertically polarised version of the photon barrelling down one channel and the horizontally polarised version heading down the second.

Following the two paths, the team had the quantum equivalent of those bells we mentioned earlier – in the form of lenses that subtly changed the shape of the photon.

The horizontal polarisation would hit ‘bell’ A before striking B, while the vertical polarisation would strike ‘bell’ B, and then A.

An analysis of the interference pattern of the reunited photon revealed signs of this mess of possible sequences.

On one hand, it’s easy to imagine two separate light particles – one horizontally polarised, the other vertically polarised – passing each lens in separate orders.

That’s not what happened, though. This was a single photon with two possible histories, neither of which set in reality until they’re measured.

While the events A and B were independent in this quantum switch, they could be linked to affect one another. A could cause B, or B could cause A … all depending on which history you wanted after the event.

Putting aside daydreams of travelling back in time to undo that big mistake (what were you thinking?!), this does have one possible practical application in the emerging field of quantum communications.

Transmitting photons down a noisy channel could be disastrous for their quantum information, quickly making a mess of their precious superposition. Sending them down channels fitted with a quantum switch, however, could in principle give the quantum information an opportunity to get through.

A paper the team published on the pre-peer review website arxiv.org back in July shows how a quantum switch applied to two noisy channels can allow a superposition to survive.

Whatever weird clockwork is going on in reality’s basement, we won’t pretend to understand it. But the very fact physicists are able to craft it into new technology is truly mindblowing in itself.

This research was published in Physical Review Letters.

https://www.sciencealert.com/quantum-switch-causation-superposition-applied-technology