Category Archives: Quantum Physics
Reimagining the laser: new ideas from quantum theory could herald a revolution – The Conversation AU
Lasers were created 60 years ago this year, when three different laser devices were unveiled by independent laboratories in the United States. A few years later, one of these inventors called the unusual light sources a solution seeking a problem. Today, the laser has been applied to countless problems in science, medicine and everyday technologies, with a market of more than US$11 billion per year.
A crucial difference between lasers and traditional sources of light is the temporal coherence of the light beam, or just coherence. The coherence of a beam can be measured by a number C, which takes into account the fact light is both a wave and a particle.
Read more: Explainer: what is wave-particle duality
From even before lasers were created, physicists thought they knew exactly how coherent a laser could be. Now, two new studies (one by myself and colleagues in Australia, the other by a team of American physicists) have shown C can be much greater than was previously thought possible.
The coherence C is roughly the number of photons (particles of light) emitted consecutively into the beam with the same phase (all waving together). For typical lasers, C is very large. Billions of photons are emitted into the beam, all waving together.
This high degree of coherence is what makes lasers suitable for high-precision applications. For example, in many quantum computers, we will need a highly coherent beam of light at a specific frequency to control a large number of qubits over a long period of time. Future quantum computers may need light sources with even greater coherence.
Read more: Explainer: quantum computation and communication technology
Physicists have long thought the maximum possible coherence of a laser was governed by an iron rule known as the Schawlow-Townes limit. It is named after the two American physicists who derived it theoretically in 1958 and went on to win Nobel prizes for their laser research. They stated that the coherence C of the beam cannot be greater than the square of N, the number of energy-excitations inside the laser itself. (These excitations could be photons, or they could be atoms in an excited state, for example.)
Now, however, two theory papers have appeared that overturn the Schawlow-Townes limit by reimagining the laser. Basically, Schawlow and Townes made assumptions about how energy is added to the laser (gain) and how it is released to form the beam (loss).
The assumptions made sense at the time, and still apply to lasers built today, but they are not required by quantum mechanics. With the amazing advances that have occurred in quantum technology in the past decade or so, our imagination need not be limited by standard assumptions.
The first paper, published this week in Nature Physics, is by my group at Griffith University and a collaborator at Macquarie University. We introduced a new model, which differs from a standard laser in both gain and loss processes, for which the coherence C is as big as N to the fourth power.
In a laser containing as many photons as a regular laser, this would allow C to be much bigger than before. Moreover, we show a laser of this kind could in principle be built using the technology of superconducting qubits and circuits which is used in the currently most successful quantum computers.
Read more: Why are scientists so excited about a recently claimed quantum computing milestone?
The second paper, by a team at the University of Pittsburgh, has not yet been published in a peer-reviewed journal but recently appeared on the physics preprint archive. These authors use a somewhat different approach, and end up with a model in which C increases like N to the third power. This group also propose building their laser using superconducting devices.
It is important to note that, in both cases, the laser would not produce a beam of visible light, but rather microwaves. But, as the authors of this second paper note explicitly, this is exactly the type of source required for superconducting quantum computing.
The standard limit is that C is proportional to N , the Pittsburgh group achieved C proportional to N , and our model has C proportional to N . Could some other model achieve an even higher coherence?
No, at least not if the laser beam has the ideal coherence properties we expect from a laser beam. This is another of the results proven in our Nature Physics paper. Coherence proportional to the fourth power of the number of photons is the best that quantum mechanics allows, and we believe it is physically achievable.
An ultimate achievable limit that surpasses what is achievable with standard methods, is known as a Heisenberg limit. This is because it is related to Heisenbergs uncertainty principle.
Read more: Explainer: Heisenbergs Uncertainty Principle
A Heisenberg-limited laser, as we call it, would not be just a revolution in the design and performance of lasers. It also requires a fundamental rethinking of what a laser is: not restricted to the current kinds of devices, but any device which turns inputs with little coherence into an output of very high coherence.
It is the nature of revolutions that it is impossible to tell whether they will succeed when they begin. But if this one does, and standard lasers are supplanted by Heisenberg-limited lasers, at least in some applications, then these two papers will be remembered as the first shots.
Originally posted here:
Paint, creativity, the cosmos and how they all fit together are the focus of an upcoming event with artist and physicist Paul Biagi.
A virtual art event called Deep Reality: Art, Physics, the Unseeable and Space-Time takesplace on Oct. 30 and is co-sponsored by SciArt Santa Fe and the UNM College of Fine Arts.
During this free webinar, Biagi will discuss how he uses a variety of methods for applying paint in the desire to develop surfaces that act as visual metaphors for the processes of contemporary physics, including quantum physics. By building up transparent layers of acrylic glazes he hints at a world of waves and vibrations that lie beneath this reality. Besides the brush, he uses many different ways of applying the paint: pouring, troweling and spraying.
Click here to register for this event and learn more.
At 12 p.m., the painter, dancer and author will share his perspectives on consciousness, and the experience of a cosmos where all occurs simultaneously and is intimately entangled.
As Biagi said, The waves of the possible crash on the shores of the now.
Biagi began studying drawing, painting, and dancing when he was a graduate student in physics in the late 1960s. After completing his doctorate at the University of Colorado, he taught college-level Math and Physics.
He pursued his interest in art while teaching for over forty-five years and took early retirement to paint and draw while continuing his studies at The Santa Fe Community College. He then went on to complete a Post Baccalaureate program at The Maryland Institute College of Art.
Biagi has been represented by both Vivo Contemporary and Reflection galleries.
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Stanford professors Monika Schleier-Smith, a quantum physicist, and Forrest Stuart, a sociologist, both received genius grants from the John D. and Catherine T. MacArthur Foundation in honor of their exceptional creativity. The fellowship includes a $625,000 unconditional award to support extraordinarily talented and creative individuals as an investment in their potential.
Schleier-Smith works in experimental physics, attempting to better understand quantum mechanics by working with laser-cooled atoms. Stuarts work focuses on modern, urban poverty, specifically the rise in policing and the use of social media.
The Stanford Daily sat down with each professor individually to discuss the fellowship and their work.
TSD: The MacArthur genius grants recognize individuals who have shown originality and creativity. How has your creative thinking impacted your work?
MS: Im always open to exploring new ideas. And I will say, if theres a problem that everybody else is already working on, then I dont necessarily feel the need to go and work on that same thing. Im excited if theres something that seems like its not being explored and maybe it should be, even if its a little bit speculative.
I think there is an aspect where you have some ideas that come from within, but theres also an aspect of being able to bounce them off other people. The award is for me, but its also recognizing the creativity of the amazing team of students and postdocs who worked on all of this with me.
FS: I think that the style of work that I do, ethnography, which really privileges getting out of your office and spending long amounts of time in neighborhoods making intimate connections, really rethinks the way we think about why society works the way it does.
We can imagine the world as dictated by these large social structures: class, race, gender, politics, the economy. Its easy to imagine these things as superstructures that hang over our heads, that direct the way the world works. And I think ethnography kind of flips the lens. I think it demands some amount of creative thinking to try and just get your head around the ways that society is made up of these small, very mundane, seemingly innocent and innocuous interactions.
TSD: Often, we hear about academia being publish or perish in a way that isnt necessarily beneficial to true scholarship and creativity. Have you seen this issue in your studies? How have you combatted it?
MS: I will say that for myself, I certainly tend to focus on quality over quantity. I would rather invest time into really deeply understanding something and writing a better paper. And I think that might sound a little bit risky, but overall I think that quality, in the end, is appreciated.
FS: The expectations, I think, have been increasing by the year. People are expected to publish at this breakneck speed. And I do think that if we were to pump the brakes a little bit, and really read peoples work rather than read their CV I think that would be an environment which would really foster some creativity. When I was on the tenure track, I would have loved to spend a few months longer, say, on paper, or a few months longer collecting some extra interviews, or a few months longer in a field site, doing some observations, because I knew there was some extra problem that was even deeper than the one that I had set my sights on that I could crack open.
TSD: What does winning this honor mean for you and your work?
MS: Its amazing how much publicity it got, and its already given me a couple of opportunities to explain quantum mechanics to a broader audience. Im always happy when theres a chance to explain why quantum physics is cool. The grant is something that Im still turning over in my mind, but right now I am really appreciative of the recognition for my group and for the awesome work that theyre doing.
FS: This is a very nice sign of recognition that the kind of work that Im doing, and the kind of work that my students are doing, is not just good, but really important for people besides sociologists. In some circles in social science, the ethnographic approach, where we spend lots of time in communities and often marginalized communities, can get some flack. But clearly, the world wants the kind of work that were doing, so that recognition is really important. In terms of moving forward, I have plans for a future project thats kind of outside of the box. And now I can execute it without having to go beg funders to give my project some extra consideration.
This interview has been condensed and lightly edited for clarity.
Contact Kirsten Mettler at kmettler at stanford.edu.
Following is a transcript of the video.
Batman: Why did you say that name?
Lois: It's his mother's name!
Jim Kakalios: What are the odds of two men at random both having mothers named Martha? Hi, I'm Jim Kakalios. I'm a physics professor at the University of Minnesota and the author of "The Physics of Superheroes." Today, we're going to look at the physics behind some DC Universe films.
"Suicide Squad" (2016)
Jim: Must not have been too deep. Wow, that would be, like, about 11 meters, you know, 30 to 40 feet up. I did a little bit of research. It turns out that there's a whole sport that is high diving into shallow pools. [laughing] And the world record is, like, 11.5 meters that someone jumped and fell into a pool that had only 30 centimeters, about a foot of water. If you just drop an object from, say, 11, 12 meters, and ask how fast is it going when it reaches the bottom, if you would neglect air resistance, it's going about 44 meters per second, maybe 80, 90 miles per hour. But if you come in horizontal, you're maximizing air resistance. And so you're converting your gravitational potential energy into kinetic energy, but some of that energy goes into work, pushing the air out of the way, because you have a large surface area. And so the amount of energy available to you as kinetic energy is reduced. So your velocity is reduced. A parachute travels much faster when it's tied up in the backpack than when it's spread out. When the parachute is spread out, it falls much slower than in the backpack. So, similarly, the Joker comes down the way these extreme jumpers do, horizontally, to maximize his air drag and decrease his speed. So instead of 44 meters per second, let's say he's coming in at 20 meters per second. Let's say it's about 45 miles per hour. The force is still quite high. It's about over 3,000 pounds to stop him in the space of about 1 meter, once he hits the water. But there, again, if he could stretch out the time by spreading out that force over as much of a large area as possible, the pressure is reduced. In fact, the world record for highest jump into shallowest water, 11.52 meters height into 30 centimeters of water, was by Darren Taylor. Joker may have actually beaten that, but I give the Joker an eight out of 10, 'cause I wanna stay on Mr. J's good side.
"Justice League" (2017)
[crashing] Let's ignore issues like air drag [laughing] and his source of energy, that he would need to eat constantly in order to have enough caloric energy to run so fast. In the human brain, processes happen faster than we tend to experience them here. But no signal gets sent from neuron to neuron at a speed that's roughly faster than a millisecond, a thousandth of a second. The blink of an eye is 100 milliseconds. So that's still very fast, compared to the processes on the timescales that we live. Let's assume that the Flash is running at a stride that he covers a meter, 3.3 feet, in a millisecond. So that's 1,000 meters per second. 1,000 meters per second is 2,250 miles per hour, which turns out to be almost exactly three times the speed of sound. So he could travel at Mach 3. He can't actually run on a vertical surface, because in order to run, we require friction. You push backwards with your foot, and the ground, because forces come in pairs, the ground pushes back on you and propels you forward. But that frictional force requires there to be some weight on the surface that you're pressing down. If the wall is completely vertical, then no part of his weight is pressing against the wall. Maybe his first step presses against the wall, but then it no longer does. If he's traveling at 2,250 miles per hour, you can ask how fast does he fall due to gravity in that timescale, and in about a millisecond he's only fallen about 10 to the -6 meters, which is about the width of a red blood cell. So he could appear to be running across the wall and not fall down because the effect of gravity, like, the amount that he falls is imperceptible. But, again, if he's trying to go faster than the human mind can respond, then this is why, if he doesn't look where he's going, he's gonna trip. We speedsters have to stick together, and he gets an eight out of 10.
"Batman v Superman: Dawn of Justice" (2016)
Batman: Why did you say that name?
Lois: It's his mother's name!
Jim: What are the odds of two men at random both having mothers named Martha? This movie that came out just a few years ago, and each of them is in their 30s. Then their parents were born in the '50s or '60s. How common was the name Martha back then? In the United States, at least. Not that common. [laughs] Martha in the 1950s was only the 49th most popular female name. In the 1960s it was the 94th most popular female name. Now, of course, these characters' names were set in the comic books that go back to the '40s and the '50s. In the 1930s, Martha was the 24th most popular name in America for women. So the question is, how do we calculate this probability? If the case is given Bruce has a mother named Martha, what is the probability that Clark has a mother named Martha? Well, then the probability Bruce has a mother named Martha is 1, because we're specifying it. So it's just multiplied by the probability that Clark has a mother named Martha. So that way, if it's in the 1960s, the odds of this happening are only 0.2%. If you just say Bruce and Clark are two random individuals, what is the probability that they each have a mother named Martha? The probability goes way down. Think about a dice. You have six sides to a dice. What's the probability that you roll a one? One out of six, 16%. What's the probability that you roll two ones? Snake eyes, with two die. Well, you need one-sixth on one times one-sixth on the other. It's one outcome out of 36 possible outcomes, or 2.7%. So it's much less likely if you're saying two random individuals, what's the probability that their mothers are both named Martha? If they were born in the 1930s, it's 0.005%. If it's in the 1960s, it's 0.0004%. Let's assume that they're using the names in this movie that were set in the comics that were coming back in the 1930s. Then they'd get a little bit of a boost in probability, but not much. Still very unlikely. I would have to give this no better than a two out of 10.
"Wonder Woman" (2017)
Wonder Woman, I'm not gonna complain about issues of her strength or speed and reflexes. We're gonna grant the character a one-time miracle exemption from the laws of nature. In the comics, it was said that Wonder Woman's bracelet was made of Amazonian metal, which is why she was able to deflect the bullets. We can actually figure out how strong the bracelet would have to be. We look at the momentum of a bullet coming in. Bullet has a weight of maybe 20 grams, but it's going 1,000 feet per second, approximately, let's say. Deflects back, so it has a momentum in and a momentum out. So there's a change in momentum of twice the initial momentum. To change the momentum requires a force applied for a given time. If we say the time of the ricochet is a millisecond, a thousandth of a second, then the force the bracelet has to supply is about 2,700 pounds. Quite high. The surface area is very low. So the pressure that the bullet exerts on the bracelet: 70,000 pounds per square inch. What kind of metal can handle that? Pretty much all of them. Cold rolled steel, stainless steel. They're all strong enough to support a compressive pressure of 70,000, 75,000 pounds per square inch. I'm not gonna argue with her strength, but the bracelet itself, I give this a 10 out of 10.
"Batman Begins" (2005)
Lucius: It's called memory cloth. Notice anything? Jim: If you're gonna dress up like Dracula to beat up criminals, you gotta have a cape. Lucius: Regularly flexible. But put a current through it... molecules realign; becomes rigid. Jim: Shape-memory fabrics. Then becomes rigid; you can use it for hang gliding. Those things actually exist. We're used to phase transitions, where, say, ice melts; becomes water. Or water boils; goes from the liquid to the vapor phase, become steam. But there are other types of phase transitions that we've discovered where materials go from one crystal structure to a different crystal structure. And that's pretty much a one-way transformation. Some materials can have a reversible transformation. You can undo it by warming or some other process. It used to be that the shape-memory materials were metals for the most part, like nitinol, which is used in eyeglass frames. Presumably, Wayne Technology has developed a fabric that, via the application of an electric field of maybe some electric current passing through it, undergoes this phase transition and undergoes this change. There is the Wearable Technology Lab at the University of Minnesota, where they're developing materials that compress under the application, say, of a temperature, that are being used to develop new suits for astronauts. Since it's Batman, I give it eight out of 10.
[screaming] I was gonna say, that's gonna leave a mark. Jon Osterman is accidentally locked into a chamber that removes his intrinsic field. In the comics, it is explained that the intrinsic fields are all the fundamental forces except gravity. Well, the fundamental forces of nature, there are four, gravity, and then there's electromagnetism, the strong nuclear force, which holds the nucleus together, and the weak nuclear force is responsible for some radioactive decays. Without electromagnetism, there's nothing to hold the atoms together. Without the strong force, there's nothing to hold the nuclei together. And so he would be actually taken apart at the very subatomic level. Once he is reborn as Doctor Manhattan, he seems to have independent control of his quantum mechanical wave function. So, quantum mechanics says that every object can be described by a mathematical function called the wave function that contains all the information about the object. If he is able to access his total wave function, it has the entire history of the object that's contained in it, which is why he experiences time as backwards and forwards as well. If you can control over your wave function, there's a phenomenon called quantum mechanical tunneling that would enable you to go from one position to another instantaneously. So presumably that's what he's doing when he's teleporting. And from the "wave" in the term "wave function," there is a wave aspect to nature, which enables certain phenomena like diffraction patterns. You send light through a grating, and you see a series of spots due to the fact that the light has interference through the multiple slits. You send an electron through a grating, and it shows the same type of interference pattern, even though it's a particle. And that's because the wave function interacts with the slits the same way light does. Doctor Manhattan presumably is diffracting himself. And that's why we see multiple versions of him in the clip. So I'll give him an eight out of 10. [rousing orchestral music]
Superman changing the flow of time. He does this in the comics all the time, like, traveling faster, so fast that he breaks the "time barrier." Well, how fast is he going here? We can figure that out by looking at the distance he travels and how long it takes him. And the ratio of those two is his speed. We know the radius of the Earth. It's a bit over 6,300 kilometers, so we can conclude that the radius of his orbit is a bit over 10,000 kilometers. All right, so the distance he goes in one orbit is 2 pi R. So it's 60,000 kilometers in one orbit. How long does that take? Well, we can count how long it takes him to make, say, 10 orbits. And we can conclude that his speed is about 400 to 500 million meters per second. The speed of light is 300 million meters per second. So he's going much faster than the speed of light. So he gets points for indeed violating physics [laughing] and doing that. Of course, he loses points, because if you're going that fast, I don't know how you're constantly changing direction. Because in order to change direction, you need some other force to be able to pull you in towards the center of your circular orbit. So he loses points for that. So I would say, overall, five out of 10.
Aquaman talks to fishes. If you give him a miracle exemption from the laws of nature to account for the power, this part is just perfectly fine. When you think, there are electrical currents in your brain. And they're constantly changing direction, stopping, starting, moving from one neuron to another. Any change in electrical current creates an electromagnetic wave. It's the basis of radio. So you create electromagnetic waves just by you thinking like that. These are extremely weak electromagnetic waves, roughly a billion times weaker than the radio waves that are in the room with you right now. We never think about the fact that we're surrounded by radio waves, unless we can't get a cellphone signal. Aquaman has presumably a special ability to be sensitive to these very weak electromagnetic waves when he thinks and when he's communicating with other species. Seawater is a million times more conductive to electric fields than regular deionized water. So, he's in seawater, which is a great conductive medium for the electromagnetic waves. The fishes have a special organ to detect these electromagnetic waves. Sharks in particular, their organs are so sensitive that you take a AA battery, 1 1/2 volts, and you attach one wire up in, say, Boston, off in the Boston Harbor, and the other wire off the coast of Florida, and the shark would be able to detect that electric field. Extremely sensitive. Once you make the miracle exemption that he has this ability, I give Aquaman 10 out of 10.
"Man of Steel" (2013)
[suspenseful music] Superman's X-ray vision. We can see when light shines into our eyes, either directly from a light or reflected off of an object. Kryptonians apparently can also see by emitting light, being both the source of illumination and the detector. He can apparently emit light in both the high-energy part of the spectrum, X-rays, and the low-energy part of the spectrum, infrared light, heat vision. X-rays come, the wavelength of an X-ray is about the size of an atom, and it gets scattered by the electrons in an atom. The more electrons, the more the scattering. Water has 10 electrons, H2O molecule. The calcium in your bones has 20 electrons. So the bones, your skeleton, are much more effective at scattering X-rays than the best of your tissues, which is why your skeleton shows up so starkly in an X-ray image. They say X-ray vision can't penetrate lead, and it's true. Lead has 82 electrons, which makes it a very effective scatterer of X-rays. But gold has 79 electrons. It should be just as good a scatterer. So, the images that we see here for Clark's X-ray vision, that, I'm giving a good grade to, but it gets downgraded by being able to actually create photons of light from the eye that get reflected back and detected again. So there I'm gonna have to downgrade him. I'll give him maybe five out of 10.
"Harley Quinn: Birds of Prey" (2020)
[screaming] You don't have to be crazy to do that, but it helps. So, she has some superpower that enables her to create these very high-intensity sound waves. The sound waves are the same type of propulsive force in an explosion. Say, dynamite or something exploding. People get knocked down, windows break, buildings can be shattered because of the air molecules traveling so fast, slamming into an object, and it's the air molecules, and they're doing it. OK. That...[laughs] That's gonna cost you some points. Here she's hanging onto the motorcycle, maintaining her balance. I don't doubt that there are skilled athletes that can actually do this. And then as she catches up with the other car and is holding onto it and then moves relative to it, well, if she's holding onto the car, she's traveling at the same speed as the car. So her relative velocity to the car is zero. Then even when she moves a little bit forward at all, that only requires her a small additional relative velocity forward to get to the front of the car. Harley Quinn maintaining perfect roller-skating form during this explosion, not so good. So I give this maybe a six out of 10.
Shazam: Your phone's charged. Your phone's charged.
Shazam: And your phone's charged. Well, you think you can do better?
Jim: Yep, and the phone blows up, because phones are not supposed to be charged up by lightning! The batteries work by a chemical reaction. Chemical A and chemical B go to chemical C and D, and they do it in such a way that they build up electrical charges on the terminals, and then you can use those electrical charges to power your phone. And once you've used up all of the chemical A and B, you can force the reaction to run backwards, and go backwards from C to D back to A and B. And that's recharging your battery. Zapping it in this way is not only not going to recharge the battery, but it's gonna fry all of the microchips that you have in the phone. He's a nice boy, but I give this only a two out of 10.
If you're gonna dress up like Dracula to beat up criminals, you gotta have a cape.
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When Susan Scott first started looking for gravitational waves more than 25 years ago, many scientists were sceptical of finding anything.
Nearly 100 years after Einstein first proposed these tiny ripples in the fabric of spacetime existed, Professor Scott was part of a 1000-strong international team that finally detected them in 2015.
"That detection involved two black holes colliding and the two amazing projections from Einstein's theory are black holes and gravitational waves and they came together in that one event," she said.
"It's like the most magical story in science."
On Wednesday night, Professor Scott of the Australian National University was one of four scientists and the first female physicist to be awarded Australia's top science prize for their pioneering work discovering gravitational waves opening a new window to the universe.
She shares the $250,000 Prime Minister's Prize for Science with David Blair, of the University of Western Australia (UWA), Peter Veitch of the University of Adelaide, and David McClelland of the Australian National University.
"Australia has a presence in this field now because of the work we have done over more than 30 years," said Professor Scott, who is a head investigator of the ARC Centre of Excellence for Gravitational Wave Discovery (OzGrav).
Australia is one of four nations that signed up to the Advanced LIGO Project, which made the historic discovery.
The field of gravitational wave research in Australia was kickstarted in the 1980s by Professor Blair, who built an early detector in Western Australia.
"This is something we spent a long, long time doing," he said.
"But also we were doing this because we wanted Australia to be part of this discovery and it's wonderful that this has been recognised."
"This is a prize for physics in Australia."
Professor Scott heads a team of scientists who are instrumental in analysing the data and deciphering signals picked up by gravitational wave detectors.
She also led Australia's efforts to follow up the detection of gravitational waves by optical telescopes to detect kilonova explosions created by merging neutron stars.
Professor Scott said she hoped the award will inspire young women to pursue a career in physics.
"It could have easily been the case that the four recipients were all males because that's how the field was when I started out," she said.
"I think it's great for our young early career scientists to see ... that they can go as far as they possibly can with their careers."
The experimental physicist started her career working with Professor Sir Roger Penrose, who has just won a Nobel Prize for his ground-breaking work that proved black holes were possible according to Einstein's general relativity theory.
But it wasn't until she returned to Australia from the United Kingdom in 1990 that she thought she'd concentrate on gravitational waves.
"When I got to Canberra I got to thinking 'Yeah, there's something in these gravitational waves'."
"I don't think I would have embarked on it if I hadn't convinced myself that the waves really were an implication of the theory," she said.
"I'm just glad it didn't take another 20 years."
Professor Blair has been developing technologies to detect gravitational waves for 40 years and set up a research centre at Gingin in Western Australia.
Research and technologies developed by Professor Blair and his team were instrumental in the detection of gravitational waves by the LIGO Observatory in 2015.
When Professor Blair started working on gravitational wave projects in the US in the 1970s he thought it would just take a couple of years.
"Never did I guess that 40 years later I'd still be trying to detect gravitational waves.
"Even now when you know how to do the calculations and can calculate their sensitivity it is still pretty hard to really believe we can measure these things," he said.
He said the success of the amazing quest to find gravitational waves was a tribute to the hard work of an immense team of people and the support of his university.
"I particularly thank the UWA for nurturing us and nurturing this vision of physicists can aim for the impossible and do the impossible," he said.
It was at UWA where he worked with Peter Veitch and David McClelland.
Professor Veitch was one of Professor Blair's PhD students.
Now at the University of Adelaide, his focus is on developing advanced lasers and optics now used in the Advanced LIGO detector.
One of the problems the LIGO Project faced, was that the high-powered laser beams used inside the detectors get slightly distorted as they travel through the instrument.
"We came up with this new technology that was able to measure these effects with the sensitivity about a factor of 30 better than anything else in the world," Professor Veitch said.
"We're developing technology that did not exist until we came up with it," Professor Veitch said.
"Fundamentally, it's just great to see that Australia values its scientific contribution to this momentous event."
He's also pleased that the prize is recognising all four of them, although is quick to point out they represent a much larger team of collaborators.
"The four of us have been working together for quite a long time, but there have been many people that have worked with us as well," Professor Veitch said.
"Within physics, if you want to work on these really big projects that answer quite fundamental questions, it is rare that it's a single person or even a couple of people."
Professor McClelland and his team pioneered optical and quantum technologies that enable components inside the detectors to work together in harmony, and made the LIGO detectors so sensitive they can detect a gravitational wave signal every week when they're operating.
He said the prize is "a culmination of 30 years' worth of effort towards one of the most exciting outcomes in the history of physics".
"What we understand about the universe to date, has only been by looking at light and electromagnetic waves and some neutrinos.
"This is a new window to explore the universe," Professor McClelland said.
Professor McClelland said the quartet was humbled to accept the prize on behalf of the Australian effort, and they hoped it would lead to bigger things in gravitational wave detection in Australia.
"In the long run, we would like to have one of these magnificent detectors onshore in Australia," he said.
In order to pinpoint where the gravitational waves sources are on the sky what's needed is a real network of detectors: a big detector in Europe, a big detector in the United States and a big detector in the southern hemisphere.
"Australia is one of the few places in the world where we can find sites suitable," Professor McClelland said.
"It'll inspire the next generations the way that the astronomy in the 1990s in Australia, in the 1980s, inspired researchers to build the SKA, the Square Kilometre Array."
The Prime Minister's Prize for Science was one of seven prizes awarded on Wednesday night. There are four other science and innovation prizes and two science teaching prizes.
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Is math really the language of nature? This physicist is on a quest to find out. – News@Northeastern
One of the most vivid memories Martin Rodriguez-Vega has of his hometown of Comalalocated in the western coastal state of Colima, Mexico, and famous for its all-white buildingsis seeing flocks of birds flying into beautiful sunsets.
Rodriguez-Vega recalls watching the birds swooping in the sky as if dancing harmoniouslyeach bird flying adroitly to form large and intricate patterns in the air.
At the time, the interaction between those birds was simply something upon which Rodriguez-Vega liked to gaze. Later, as a doctoral student in theoretical physics, he came to understand that the birds behavior was an example of a complex phenomenon scientists call emergence: patterns or behaviors that form or emerge thanks to the dexterity of the individual parts of a dynamic system, such as birds in a flock.
Working collectively, these parts give rise to a type of group behavior that would be impossible to generate without the coordinated dynamics of its individual components.
The birds want to stay with the group together, but they dont want to be too close together because otherwise they collide, he says. So theres this competition of wanting to be close, but not so close, which leads to these emergent patterns. Its quite interesting.
More than a decade later, Rodriguez-Vega, now a postdoctoral research fellow at Northeastern, is studying a similar type of behaviorexcept that instead of analyzing the dynamics of bird flight, he and a group of physicists are studying, modeling, and testing the collective behavior of subatomic particles. Their goal is to explore the hidden properties of quantum materials, which are known to display exotic qualities that emerge from the arrangement of electrons.
At the subatomic scale, electrons are responsible for the conduction of electricity in all materials. But by stimulating quantum materials with specialized laser pumps or ultra-cold temperatures, scientists can make electrons rearrange collectively, giving those materials new properties.
Eventually, scientists hope to use their knowledge of quantum materials to transform technology, helping to produce faster electronics, better supercomputers, and smarter communication devicesall of which rely on the behavior of electrons.
One of the most striking examples of emergent quantum properties is superconductivity: the ability of materials such as aluminum to conduct electricity perfectly, and at ultra-fast speeds, under extremely frigid temperatures.
A single electron would never be able to present this type of behavior, Rodriguez-Vega says. But with every electron together, [the material] can transition into a new state in which all the electrons can move together through it without resistance.
The impacts of superconductors and other quantum materials are likely to drive a technological revolution. And Rodriguez-Vega wants to be part of it.
In addition to developing theories that predict and explain the behavior of potential quantum materials, Rodriguez-Vega works with researchers to conduct experiments and test his theories in a lab. By shooting the materials with special lasers to probe their properties, the researchers hope to find easier and cheaper ways to harness the power of quantum materials.
Recently, Rodriguez-Vega was part of a team that used that technique to discover new ways in which magnetite, a mineral that scientists have long used to study magnetism, can turn from a metal that transports charge into a material that stops charge.
That ability to combine theory with experiments is what drives Rodriguez Vega to devote his life to research. The most thrilling part of being a physicist, he says, is seeing equations he has written accurately predict the outcome of an experiment.
The thrill is about the deeper meaning, because what youre writing on the paper and solving in your computer is just your interpretation of what could be going on with nature in general, he says. That gives you this sense that things really work, that perhaps math is really the language of nature. Its really amazing.
Rodriguez-Vega grew up the son of two accountants in a small, agricultural town of Mexico, and says his parents played an integral role in opening the doors for him to pursue a life in physics. When he first applied to graduate school in the United States, the fees for language and admission tests amounted to nearly two months of his parents salary, he says.
At the time, they did everything in their possibility for me to do the tests, and then, when I was accepted, to gather the money to get the plane tickets, and get the security deposit for a place to live, he says. That was pretty incredible on their side.
Rodriguez-Vega also credits his career as a theoretical physicist to the support and mentorship of scientists such as Paolo Amore, his professor at the University of Colima, and Enrico Rossi, who took Rodriguez-Vega under his wing when he migrated to the U.S. to pursue a doctoral degree at the College of William & Mary in Williamsburg, Virginia.
When you live in a small town, a little disconnected from the notion of science, its quite common for people to think that if youre going into one of these careers, the only possible outcome for you is to be a schoolteacher, Rodriguez-Vega says. The idea of being able to get an education, to devote your life and try to create knowledge to answer some questions out there is not something that was on my mind.
Rodriguez-Vega thinks mentors in science are important not only because of the technical knowledge they pass on to their students, but because they can help budding scientists who face a huge wall of challenges and knowledge to break into the scientific community.
A career in science is much more manageable, he says, when you have somebody sitting on top of that wall, giving you a hand to climb over it.
The knowledge that [scientists] accumulated at this point is really large, he says. At least for me, it was quite easy to feel overwhelmed by the complexity of things.
As part of the Quantum Matter and Correlated Electron Theory Lab, Rodriguez-Vega tries to dedicate as much time as possible to young scientists who need it, even if that entails discussions that have nothing to do with physics. Sometimes, he says, students just need to chat about their lives in general, as well as the challenges of graduate school.
Gregory Fiete, a professor of physics who leads the lab, says that thanks in large part to Rodriguez-Vega, his students are developing new skills in a cutting-edge area of physics. Rodriguez-Vega has been training them to use machine learning techniques to scan large sets of data in order to spot potential quantum materials.
Fiete also recognizes the importance of helping students advance, and says Rodriguez-Vega is the epitome of a model scientist. Hes a capable scientist who can do the technical aspects of the work, Fiete saysthe numerical and analytical partsbut hes also a reliable mentor for junior researchers in the group.
Ultimately, everything challenging that any person undertakes requires help and assistance in life, whether you are a dancer, a musician, a scientist, whatever you might be, Fiete says. You need someone else to help you learn and someone else to bounce ideas off of, or maybe be inspired by.
Those relationships between mentors and students are essential for the advancement of science, Fiete says, and for the democratization of the research enterprise worldwide.
One of the things that I find inspiring about Martin is just how far he has come, Fiete says. Not everyone has the same starting point in life, and he had to overcome a lot of obstacles in early stages to get where he is today.
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Life and Work: Teaching in the Time of COVID: A Tale of Three Universities – All Together – Society of Women Engineers
In adverse circumstances, actions that might have been rejected at other times may become the best option. The pandemic and accompanying measures to socially distance confronted every higher education institution with the unprecedented challenge of teaching and learning from afar.
Pivoting from classroom to online instruction was a complete disruption of normal operations and caught the university community off guard. Many of us had only taught in our classrooms supplemented by online learning management systems (LMS), such as Blackboard, Canvas, Moodle, Google Classroom, and other services used to manage and support instruction. Changing teaching methods midstream posed an immense challenge.
From an instructional design perspective, lessons taught in the normal face-to-face classroom need restructuring to be effective in an online setting. But in this crisis, we were expected to change our teaching methods quite literally overnight.
Personally, the thought of teaching my scheduled undergraduate engineering design seminar online was daunting. I was faced with the question of how to reframe a collaborative workshop, where students work in teams to develop a concept from design to prototype, into an online distance learning experience.
Starting on day one of the lockdown in Austria, my department began holding daily meetings on Microsoft Teams, where we shared our progress in adapting to the new situation. Being a member of the IT and systems management faculty at the FH Salzburg University of Applied Sciences ensured our technical dexterity, as well as the need to show our prowess and ability to adapt to these new circumstances. As a faculty team, we were determined to offer our students opportunities to complete the current semester workload online and discussed how best to accomplish that.
Fortunately, I had previously implemented many features of the LMS into my normal classes, and at the beginning of the lockdown used them to teach asynchronously, posting recorded voice-over PowerPoint presentations for students to follow at their own pace.
During the first weeks of the lockdown, I participated in several online workshops and was introduced to new virtual tools and collaborative spaces on various platforms. My own online learning experiences not only taught me how to use the platforms and tools, but they also enabled me to develop empathy for my students by experiencing firsthand what it was like to be thrown into an unfamiliar learning environment.
Even at the best of times, not every student is equally motivated, and the uncertainty caused by the pandemic is an additional factor that could negatively influence their outlook on learning. As an educator, it was imperative for me to be aware of the students well-being at this stressful time, as well as allowing them enough time to process the content and tasks associated with the course.
Providing an engaging learning opportunity for my students required investing a significant amount of time to implement new methods and prepare the interactive online workshops I envisioned. Providing this learning environment in a limited time frame was a source of stress.
When the engineering design workshops were up and running, my students responded well to the new format and actively participated. After taking part in discussions to find topics of interest, they worked in project teams to design digital solutions to problems facing society.
Central command was my home office, which consisted of my private computer, my work laptop, tablet, and smartphone. Sometimes I felt like an air traffic controller as I directed students into breakout channels to do group work, opened interactive whiteboards with prepared activities, set timers, posted assignments, and gave direct feedback on their progress. To encourage communication, I asked students to turn on their cameras and frequently visited the project teams in their breakout rooms; or, they called me, and we met for a video chat on their project status.
The importance of providing the students opportunities to interact was something that I had underestimated. Many of them were somewhat isolated and really missed student life and the informal exchanges that take place at university. In addition, adjusting to virtual communication took some getting used to; therefore, it was important for the group dynamic to make time for small talk. The effects of social distancing and isolation on our ability to learn, focus, and be motivated could not be overlooked.
At The University of Maine, Karen Horton, P.E., professor and coordinator, mechanical engineering technology, encouraged her students to stay connected by providing evening Zoom office hours. This made her easily available to students, and more than a third of them sought her out, which is far more than would have normally shown up for office hours.
The downside of teaching online was that it required more time. Developing, posting, and providing students with online teaching materials and the additional resources needed to teach online meant that her workload increased by 50%. I went from typically working about 40 hours to working 60 hours. But it was absolutely necessary to invest the additional time in preparing the online lessons, she said.
Transforming from an offline to an online classroom requires a higher level of clarity in instruction and documentation for the sake of students understanding. Additionally, there is the time-consuming learning curve associated with effectively using the learning platform and online tools for teaching.
To prepare for distance learning classes, Horton explained, I reconfigured my teaching material by breaking down the long-form lecture into shorter recordings of about 15 minutes and posted them as asynchronous videos with PowerPoint presentations. This proved effective so much so that, when we do return to normal operations, I will continue to provide lectures asynchronously with required assignments and have smaller groups doing in-class exercises.
For Horton, there was an unexpected benefit from the experience. By dividing my lectures into shorter parts and giving more required assignments, I was able to improve outcomes and could encourage student engagement by promoting self-review of provided solutions, she said.
The coronavirus crisis hit just as Caterina Cocchi, Ph.D., began a new position as professor of theoretical solid-state physics, at the University of Oldenburg, Germany. I was scheduled to teach quantum mechanics, when it was announced that throughout Germany, universities could only deliver the current semester courses in digital form.
Much like her counterparts elsewhere, the pandemic forced me to reconsider my teaching methods, she said. I was already familiar with Zoom and had used it frequently for scientific exchanges with colleagues and collaborators.
Faced with this new challenge, Dr. Cocchi, who is also chair of theoretical solid-state physics at Oldenburg, said, My concerns focused on how distance teaching could work without student proximity and a blackboard to write on. At the beginning, I didnt have an iPad to use as a whiteboard, so I switched to slides for my livestreaming lectures. To my surprise, it worked like a charm. With this one example, along with many others, she notes that the biggest lesson I learned during this emergency is that I should give myself credit for being creative and flexible.
Just as she was settling into the routine, another problem emerged when a student population of more than 15,000 tried to connect simultaneously to the online courses. Faculty were warned that they were exceeding infrastructure limitations.
Livestreaming (my preference) was strongly discouraged, as it was expected to overload the bandwidth. Many colleagues decided to record their lectures and post them on the internal teaching platform for students to access. I didnt like this idea and decided to livestream my lectures. Now, I am glad I did.
With the benefit of several months behind her, Dr. Cocchi reflected on her decision to continue livestreaming her lectures: Even though I couldnt see my students, at least I knew they were there. I could watch them connecting online, and they had the chance to ask questions over chat and even engage in simple polls. If I had uploaded my recorded lectures, the distance between us would have been much greater. With a touch of humor, she added, I already felt more like a radio DJ than a teacher, but at least I could broadcast live. Recording lectures would have turned teaching into delivering a series of podcasts. I love podcasts, but quantum mechanics cannot be learned while doing the laundry or working out.
Summing up her experience, Dr. Cocchi said, What I most appreciate about teaching remotely is that I am at home. I dont have to worry about public transportation, and everything I need is just a click away. Still, it is difficult not to see my students. Even if they dont ask questions, just looking at their faces is enough for me to know whether they are following the lesson or not. These interactions are missing on the digital platform. My only wish is to be able to meet my students once or twice.
Expressing a sentiment shared by her colleagues, Rishelle Wimmer, in Austria, and Karen Horton, in Maine, United States, Dr. Cocchi said, There is nothing more precious for me as a teacher than to see the spark in the students eyes when he or she comes to understand a new concept.
Rishelle Wimmer is a senior lecturer in the information technology and systems management department of the FH Salzburg University of Applied Sciences. She studied operation research and system analysis at Cornell University and holds a masters degree in educational sciences from the University of Salzburg. She currently serves on the SWE editorial board and the research advisory committee and has been the faculty advisor for the Salzburg SWE affiliate since FY17.
Thirty years from today, a new world powered by robotics, artificial intelligence and quantum physics will be upon us. In that future, if a person incidentally ventures into a virtual library and looks for a digital history book on the 2020s, the reader will surely come upon a narrative on The Great Pandemic.
The story may describe how people around the world gave a heart-warming welcome to the year 2020. People celebrated the incoming year at squares, parties and cathedrals in New York, London, Lagos, Cape Town, Sydney and Tokyo. 2020 arrived peaceably and joyfully on Jan. 1 in most countries.
However, an uninvited and invisible guest tightly stuck to the New Year. This was a highly contagious and dangerous virus later named Covid-19. The virus brought on respiratory infections and physical anomalies in humans. It was transferred to humans from animals and killed millions and infected many more millions.
Covid-19 negatively impacted the world economy, driving it down to the tune of many trillions of dollars.
Inequality between rich and poor, powerful and vulnerable, and the people stuck in between, has existed for millennia. That wealth inequality is overt in a society such as these Virgin Islands. The spectrum here ranges from the likes of the super wealthy like Sir Richard Branson, Henry Jarecki and Larry Page who live largely invisibly in these islands to the poor who live hand to mouth on handouts from charity, family and friends.
Much of the world went into lockdown in March 2020 when it was clear that shutting down society was the one sure way of controlling the spread of the contagion. That shutdown was complete in some places, eerily turning great cities of the world into veritable ghost towns.
The lockdown period changed the lives of millions. The better-off, however, were able to spend time with family, as they did not feel the pinch of lost income.
Residents especially workers in travel and tourism, a mainstay for employment in the VI and elsewhere lost jobs, livelihoods and even homes. They were unable to earn the accustomed income.
Lockdown forced residents to stay at home. Businesses went under and shut for good.
However, the people at the top of the wealth pyramid were unscathed. In fact, by some cruel trick, billionaires and stockholders in technology became wealthier as the world became a virtual marketplace as a result of Covid-19.
Towards the bottom of the pyramid, the adversity and suffering increased greatly. Migrant workers, daily wage earners, waitresses, bartenders, small-business owners, cleaners, taxi drivers, and so on were severely impacted. These workers were driven into poverty and great suffering.
The people at the middle of the pyramid such as government workers and middle managers in private firms were affected, but not as drastically as those at the bottom of the pyramid.
The world of pandemic recession will get worse, sadly. Employment is a lagging indicator of economic recovery. So when the world begins to recover, probably in early 2022 after a vaccine for Covid-19 is widely available, only then will investor and business confidence return.
Consumer confidence appears after investor and business confidence increases. When people start to spend, aggregate demand increases. Then shut businesses will reopen, and managers and business owners will begin to invest in equipment and stock and hire workers again.
The world economy will resume normality and growth.
When that will happen? Only God knows.
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Albert Einsteins twin paradox is one of the most famous thought experiments in physics. It postulates that if you send one of two twins on a return trip to a star at near light speed, they will be younger than their identical sibling when they return home. The age difference is a consequence of something called time dilation, which is described by Einsteins special theory of relativity: the faster you travel, the slower time appears to pass.
But what if we introduce quantum theory into the problem? Physicists Alexander Smith of Saint Anselm College and Dartmouth College and Mehdi Ahmadi of Santa Clara University tackle this idea in a study published today in the journal Nature Communications. The scientists imagine measuring a quantum atomic clock experiencing two different times while it is placed in superpositiona quirk of quantum mechanics in which something appears to exist in two places at once. We know from Einsteins special theory of relativity that when a clock moves relative to another clock, the time shown on it slows down, Smith says. But quantum mechanics allows you to start thinking about what happens if this clock were to move in a superposition of two different speeds.
Superposition is a strange aspect of quantum physics where an object can initially be in multiple locations simultaneously, yet when it is observed, only one of those states becomes true. Particles can be placed in superposition in certain experiments, such as those using a beam splitter to divide photons of light, to show the phenomenon in action. Both of the particles in superposition appear to share information until they are observed, making the phenomenon useful for applications such as encryption and quantum communications.
Some atoms, meanwhile, can act as atomic clocks, with their rate of decay noting the passage of time. In their paper, Smith and Ahmadi describe how an atomic clock placed in superposition could experience time dilation, just like Einsteins twins experiment, if one of the superposition states is moved at several meters per second while the other remains stationary. Instead of the atom simply being in two states at onceas described in the Schrdingers cat experimentthe states would actually age differently. Its kind of like Schrdingers clock, Smith says.
Vlatko Vedral, a physicist at the University of Oxford, who was not involved in the study, says the idea allows for a rare opportunity to merge quantum mechanics with relativitytwo areas of physics that infamously do not mix well. You can actually combine the superposition principle in quantum mechanics with this notion of time dilation in relativity, he says. Its exactly Einsteins twins but now applied to the same system. Thats the twist. The final state is really amazing, because the atom is back in the same position where you started, but internally, it feels two different times. Its in a superposition of being older and younger at the same time.
Though the effect is far too small to be noticeable to humans, this idea of quantum time dilation could have repercussions for high-precision quantum clocks. And crucially, the new study suggests it might be possible to measure the effect experimentally. Im hoping this paper really prompts people to try to do this in the lab, Vedral says. And Smith suggests an experimental proposal could be drafted in the near future, perhaps using spectroscopy to split light, to look for this signature of quantum time dilation. We might be able to see this in the next five to 10 years, he says. I dont think its science fiction by any means.
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No sooner had the radical equations of quantum mechanics been discovered than physicists identified one of the strangest phenomena the theory allows.
Quantum tunneling shows how profoundly particles such as electrons differ from bigger things. Throw a ball at the wall and it bounces backward; let it roll to the bottom of a valley and it stays there. But a particle will occasionally hop through the wall. It has a chance of slipping through the mountain and escaping from the valley, as two physicists wrote in Nature in 1928, in one of the earliest descriptions of tunneling.
Physicists quickly saw that particles ability to tunnel through barriers solved many mysteries. It explained various chemical bonds and radioactive decays and how hydrogen nuclei in the sun are able to overcome their mutual repulsion and fuse, producing sunlight.
But physicists became curious mildly at first, then morbidly so. How long, they wondered, does it take for a particle to tunnel through a barrier?
The trouble was that the answer didnt make sense.
The first tentative calculation of tunneling time appeared in print in 1932. Even earlier stabs might have been made in private, but when you get an answer you cant make sense of, you dont publish it, noted Aephraim Steinberg, a physicist at the University of Toronto.
It wasnt until 1962 that a semiconductor engineer at Texas Instruments named Thomas Hartman wrote a paper that explicitly embraced the shocking implications of the math.
Hartman found that a barrier seemed to act as a shortcut. When a particle tunnels, the trip takes less time than if the barrier werent there. Even more astonishing, he calculated that thickening a barrier hardly increases the time it takes for a particle to tunnel across it. This means that with a sufficiently thick barrier, particles could hop from one side to the other faster than light traveling the same distance through empty space.
In short, quantum tunneling seemed to allow faster-than-light travel, a supposed physical impossibility.
After the Hartman effect, thats when people started to worry, said Steinberg.
The discussion spiraled for decades, in part because the tunneling-time question seemed to scratch at some of the most enigmatic aspects of quantum mechanics. Its part of the general problem of what is time, and how do we measure time in quantum mechanics, and what is its meaning, said Eli Pollak, a theoretical physicist at the Weizmann Institute of Science in Israel. Physicists eventually derived at least 10 alternative mathematical expressions for tunneling time, each reflecting a different perspective on the tunneling process. None settled the issue.
But the tunneling-time question is making a comeback, fueled by a series of virtuoso experiments that have precisely measured tunneling time in the lab.
In the most highly praised measurement yet, reported in Nature in July, Steinbergs group in Toronto used whats called the Larmor clock method to gauge how long rubidium atoms took to tunnel through a repulsive laser field.
The Larmor clock is the best and most intuitive way to measure tunneling time, and the experiment was the first to very nicely measure it, said Igor Litvinyuk, a physicist at Griffith University in Australia who reported a different measurement of tunneling time in Nature last year.
Luiz Manzoni, a theoretical physicist at Concordia College in Minnesota, also finds the Larmor clock measurement convincing. What they measure is really the tunneling time, he said.
The recent experiments are bringing new attention to an unresolved issue. In the six decades since Hartmans paper, no matter how carefully physicists have redefined tunneling time or how precisely theyve measured it in the lab, theyve found that quantum tunneling invariably exhibits the Hartman effect. Tunneling seems to be incurably, robustly superluminal.
How is it possible for [a tunneling particle] to travel faster than light? Litvinyuk said. It was purely theoretical until the measurements were made.
Tunneling time is hard to pin down because reality itself is.
At the macroscopic scale, how long an object takes to go from A to B is simply the distance divided by the objects speed. But quantum theory teaches us that precise knowledge of both distance and speed is forbidden.
In quantum theory, a particle has a range of possible locations and speeds. From among these options, definite properties somehow crystallize at the moment of measurement. How this happens is one of the deepest questions.
The upshot is that until a particle strikes a detector, its everywhere and nowhere in particular. This makes it really hard to say how long the particle previously spent somewhere, such as inside a barrier. You cannot say what time it spends there, Litvinyuk said, because it can be simultaneously two places at the same time.
To understand the problem in the context of tunneling, picture a bell curve representing the possible locations of a particle. This bell curve, called a wave packet, is centered at position A. Now picture the wave packet traveling, tsunami-like, toward a barrier. The equations of quantum mechanics describe how the wave packet splits in two upon hitting the obstacle. Most of it reflects, heading back toward A. But a smaller peak of probability slips through the barrier and keeps going toward B. Thus the particle has a chance of registering in a detector there.
But when a particle arrives at B, what can be said about its journey, or its time in the barrier? Before it suddenly showed up, the particle was a two-part probability wave both reflected and transmitted. It both entered the barrier and didnt. The meaning of tunneling time becomes unclear.
And yet any particle that starts at A and ends at B undeniably interacts with the barrier in between, and this interaction is something in time, as Pollak put it. The question is, what time is that?
Steinberg, who has had a seeming obsession with the tunneling-time question since he was a graduate student in the 1990s, explained that the trouble stems from the peculiar nature of time. Objects have certain characteristics, like mass or location. But they dont have an intrinsic time that we can measure directly. I can ask you, What is the position of thebaseball? but it makes no sense to ask, What is the time of thebaseball? Steinberg said. The time is not a property any particle possesses. Instead, we track other changes in the world, such as ticks of clocks (which are ultimately changes in position), and call these increments of time.
But in the tunneling scenario, theres no clock inside the particle itself. So what changes should be tracked? Physicists have found no end of possible proxies for tunneling time.
Hartman (and LeRoy Archibald MacColl before him in 1932) took the simplest approach to gauging how long tunneling takes. Hartman calculated the difference in the most likely arrival time of a particle traveling from A to B in free space versus a particle that has to cross a barrier. He did this by considering how the barrier shifts the position of the peak of the transmitted wave packet.
But this approach has a problem, aside from its weird suggestion that barriers speed particles up. You cant simply compare the initial and final peaks of a particles wave packet. Clocking the difference between a particles most likely departure time (when the peak of the bell curve is located at A) and its most likely arrival time (when the peak reaches B) doesnt tell you any individual particles time of flight, because a particle detected at B didnt necessarily start at A. It was anywhere and everywhere in the initial probability distribution, including its front tail, which was much closer to the barrier. This gave it a chance to reach B quickly.
Since particles exact trajectories are unknowable, researchers sought a more probabilistic approach. They considered the fact that after a wave packet hits a barrier, at each instant theres some probability that the particle is inside the barrier (and some probability that its not). Physicists then sum up the probabilities at every instant to derive the average tunneling time.
As for how to measure the probabilities, various thought experiments were conceived starting in the late 1960s in which clocks could be attached to the particles themselves. If each particles clock only ticks while its in the barrier, and you read the clocks of many transmitted particles, theyll show a range of different times. But the average gives the tunneling time.
All of this was easier said than done, of course. They were just coming up with crazy ideas of how to measure this time and thought it would never happen, said Ramn Ramos, the lead author of the recent Nature paper. Now the science has advanced, and we were happy to make this experiment real.
Although physicists have gauged tunneling times since the 1980s, the recent rise of ultraprecise measurements began in 2014 in Ursula Kellers lab at the Swiss Federal Institute of Technology Zurich. Her team measured tunneling time using whats called an attoclock. In Kellers attoclock, electrons from helium atoms encounter a barrier, which rotates in place like the hands of a clock. Electrons tunnel most often when the barrier is in a certain orientation call it noon on the attoclock. Then, when electrons emerge from the barrier, they get kicked in a direction that depends on the barriers alignment at that moment. To gauge the tunneling time, Kellers team measured the angular difference between noon, when most tunneling events began, and the angle of most outgoing electrons. They measured a difference of 50 attoseconds, or billionths of a billionth of a second.
Then in work reported in 2019, Litvinyuks group improved on Kellers attoclock experiment by switching from helium to simpler hydrogen atoms. They measured an even shorter time of at most two attoseconds, suggesting that tunneling happens almost instantaneously.
But some experts have since concluded that the duration the attoclock measures is not a good proxy for tunneling time. Manzoni, who published an analysis of the measurement last year, said the approach is flawed in a similar way to Hartmans tunneling-time definition: Electrons that tunnel out of the barrier almost instantly can be said, in hindsight, to have had a head start.
Meanwhile, Steinberg, Ramos and their Toronto colleagues David Spierings and Isabelle Racicot pursued an experiment that has been more convincing.
This alternative approach utilizes the fact that many particles possess an intrinsic magnetic property called spin. Spin is like an arrow that is only ever measured pointing up or down. But before a measurement, it can point in any direction. As the Irish physicist Joseph Larmor discovered in 1897, the angle of the spin rotates, or precesses, when the particle is in a magnetic field. The Toronto team used this precession to act as the hands of a clock, called a Larmor clock.
The researchers used a laser beam as their barrier and turned on a magnetic field inside it. They then prepared rubidium atoms with spins aligned in a particular direction, and sent the atoms drifting toward the barrier. Next, they measured the spin of the atoms that came out the other side. Measuring any individual atoms spin always returns an unilluminating answer of up or down. But do the measurement over and over again, and the collected measurements will reveal how much the angle of the spins precessed, on average, while the atoms were inside the barrier and thus how long they typically spent there.
The researchers reported that the rubidium atoms spent, on average, 0.61 milliseconds inside the barrier, in line with Larmor clock times theoretically predicted in the 1980s. Thats less time than the atoms would have taken to travel through free space. Therefore, the calculations indicate that if you made the barrier really thick, Steinberg said, the speedup would let atoms tunnel from one side to the other faster than light.
In 1907, Albert Einstein realized that his brand-new theory of relativity must render faster-than-light communication impossible. Imagine two people, Alice and Bob, moving apart at high speed. Because of relativity, their clocks tell different times. One consequence is that if Alice sends a faster-than-light signal to Bob, who immediately sends a superluminal reply to Alice, Bobs reply could reach Alice before she sent her initial message. The achieved effect would precede the cause, Einstein wrote.
Experts generally feel confident that tunneling doesnt really break causality, but theres no consensus on the precise reasons why not. I dont feel like we have a completely unified way of thinking about it, Steinberg said. Theres a mystery there, not a paradox.
Some good guesses are wrong. Manzoni, on hearing about the superluminal tunneling issue in the early 2000s, worked with a colleague to redo the calculations. They thought they would see tunneling drop to subluminal speeds if they accounted for relativistic effects (where time slows down for fast-moving particles). To our surprise, it was possible to have superluminal tunneling there too, Manzoni said. In fact, the problem was even more drastic in relativistic quantum mechanics.
Researchers stress that superluminal tunneling is not a problem as long as it doesnt allow superluminal signaling. Its similar in this way to the spooky action at a distance that so bothered Einstein. Spooky action refers to the ability of far-apart particles to be entangled, so that a measurement of one instantly determines the properties of both. This instant connection between distant particles doesnt cause paradoxes because it cant be used to signal from one to the other.
Considering the amount of hand-wringing over spooky action at a distance, though, surprisingly little fuss has been made about superluminal tunneling. With tunneling, youre not dealing with two systems that are separate, whose states are linked in this spooky way, said Grace Field, who studies the tunneling-time issue at the University of Cambridge. Youre dealing with a single system thats traveling through space. In that way it almost seems weirder than entanglement.
In a paper published in the New Journal of Physics in September, Pollak and two colleagues argued that superluminal tunneling doesnt allow superluminal signaling for a statistical reason: Even though tunneling through an extremely thick barrier happens very fast, the chance of a tunneling event happening through such a barrier is extraordinarily low. A signaler would always prefer to send the signal through free space.
Why, though, couldnt you blast tons of particles at the ultra-thick barrier in the hopes that one will make it through superluminally? Wouldnt just one particle be enough to convey your message and break physics? Steinberg, who agrees with the statistical view of the situation, argues that a single tunneled particle cant convey information. A signal requires detail and structure, and any attempt to send a detailed signal will always be faster sent through the air than through an unreliable barrier.
Pollak said these questions are the subject of future study. I believe the experiments of Steinberg are going to be an impetus for more theory. Where that leads, I dont know.
The pondering will occur alongside more experiments, including the next on Steinbergs list. By localizing the magnetic field within different regions in the barrier, he and his team plan to probe not only how long the particle spends in the barrier, but where within the barrier it spends that time, he said. Theoretical calculations predict that the rubidium atoms spend most of their time near the barriers entrance and exit, but very little time in the middle. Its kind of surprising and not intuitive at all, Ramos said.
By probing the average experience of many tunneling particles, the researchers are painting a more vivid picture of what goes on inside the mountain than the pioneers of quantum mechanics ever expected a century ago. In Steinbergs view, the developments drive home the point that despite quantum mechanics strange reputation, when you see where a particle ends up, that does give you more information about what it was doing before.