&quot;The Fabric of the Cosmos,&quot; a four-hour series based on the book by renowned physicist and author Brian Greene, takes us to the frontiers of physics to see how scientists are piecing together the most complete picture yet of space, time, and the universe. With each step, audiences will discover that just beneath the surface of our everyday experience lies a world we&#x27;d hardly recognize—a startling world far stranger and more wondrous than anyone expected.Brian Greene is going to let you in on a secret: We&#x27;ve all been deceived. Our perceptions of time and space have led us astray. Much of what we thought we knew about our universe—that the past has already happened and the future is yet to be, that space is just an empty void, that our universe is the only universe that exists—just might be wrong.Interweaving provocative theories, experiments, and stories with crystal-clear explanations and imaginative metaphors like those that defined the groundbreaking and highly acclaimed series &quot;The Elegant Universe,&quot; &quot;The Fabric of the Cosmos&quot; aims to be the most compelling, visual, and comprehensive picture of modern physics ever seen on television.

Acclaimed physicist Brian Greene reveals a mind-boggling reality beneath the surface of our everyday world.

The Fabric of the Cosmos

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Black holes are the most enigmatic and exotic objects in the universe. They’re also the most powerful, with gravity so strong it can trap light. And they’re destructive, swallowing entire planets, even giant stars. Anything that falls into them vanishes…gone forever. Now, astrophysicists are realizing that black holes may be essential to how our universe evolved—their influence possibly leading to life on Earth and, ultimately, us. In this two-hour special, astrophysicist and author Janna Levin takes viewers on a journey to the frontiers of black hole science. Along the way, we meet leading astronomers and physicists on the verge of finding new answers to provocative questions about these shadowy monsters: Where do they come from? What’s inside? What happens if you fall into one? And what can they tell us about the nature of space, time, and gravity? (Premiered January 10, 2018)

Take a mind-blowing voyage to the most powerful and mysterious objects in the universe.

Black Hole Apocalypse

One tablespoon of olive oil is dropped into a lake, and after ten minutes, a half-acre of waves are gone. How could the oil spread so far, and why does it calm the waves? Find out in this episode of <a href="https://www.youtube.com/channel/UCj1gfrsi8H8zTrmR0ft1Kjw">What The Physics?!</a>

One tablespoon of olive oil destroys half an acre of waves on this lake. What The Physics?!

Lake vs. Olive Oil

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Discover how contemporary researchers used computers to analyze a massive historical data set—127 million words collected from 239 years of criminal trials in London—to confirm a shift in public attitudes towards violence, in this video from NOVA The Violence Paradox.

Cultural Shifts in Attitudes Towards Violence

If a meteorite streaking through Earth’s atmosphere is too thin, it’ll tumble about. Too wide and it will flutter wildly. But if it’s taken on just the right cone shape, it’ll plummet straight to the ground.Roughly a quarter to a third of meteorites that make it to Earth converge on this “Goldilocks” cone shape and, until recently, scientists haven’t been able to explain why. New findings <a href="https://doi.org/10.1073/pnas.1815133116">published last month</a> in the journal PNAS show that the answer has to do with fluid mechanics, the way forces interact with liquids, gasses, and plasmas.
 

 
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 The story starts back in 2012, with a team of researchers from the Courant Institute at New York University holding balls of clay in a water tunnel to study erosion. The team observed that “if you hold an object fixed and let it erode, it makes a cone,” says study author Leif Ristroph, a mathematician at the Courant Institute who was also part of the team that made the <a href="https://www.pnas.org/content/109/48/19606">initial observations</a>.More specifically, he says, erosion always sculpted these cones so their tips, which pointed toward the oncoming water, formed a V with an angle of about 90 to 100 degrees. This tip angle happens to be very similar to those of cone-shaped meteorites found on Earth.A ball of clay stuck in water may not seem to have much in common with a meteorite crashing to Earth, but the forces at work on their surfaces aren’t all that different, Ristroph says. Meteorites entering the Earth’s atmosphere encounter intense heat that melts and reshapes them in a process similar to water eroding clay.With that in mind, the researches decided to use water to take a closer look to determine if this cone shape might be favorable for meteorite flight: They dropped solid aluminum cones into water and used a video camera to track their motion.
 

 
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 This is where they witnessed the Goldilocks principle at work. Cones with tip angles that were too slim or too wide moved in unstable paths through the water. But cones that hit a sweet spot between 60 and 100 degrees oriented themselves so that their tips pointed down and fell straight—just as a similarly shaped meteorite would fall through air.The reason for this stability is the way forces from the surrounding fluids—­water in the 2012 experiment, and air in the case of meteorites—act on the cones as they fall, Ristroph says. As fluid is displaced by the cones, it flows around them, only to come back together in the cones’ wakes, like the turbulent surf behind a fast-moving boat. While narrow or wide cones get pushed around by these fluid forces, Goldilocks cones are just the right shape to stay stable.When to team looked back at their data, they realized the same angles kept popping up and theorized that these erosion and stability effects might be related, says study author Jun Zhang, a physicist and mathematician at the Courant Institute and NYU Shanghai.Erosion leads to a cone shape, which leads to stability, which enables further erosion. “These two mechanisms reinforce each other,” says Zhang.But there may yet be more to the story.Meteorites aren’t just made of clay, Maitrayee Bose, an isotope cosmochemist at Arizona State University who was not involved in the study, said in an email. Meteorites can be made of harder materials, like iron and stone, which could respond very differently to erosion stresses when falling through the atmosphere, she says.
 <figure class="richtext-image full-width"><img alt="Wiluna.jpg" class="richtext-image full-width" height="621" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/Wiluna.width-800.jpg"/><figcaption>A cone-shaped meteorite found east of the Wiluna township in Western Australia. Image Credit: Jon Taylor, Flickr, Wikimedia Commons<div class="credit"></div></figcaption>
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 And it’s not just what meteorites are made of that needs to be considered.At supersonic speeds—like those of a falling meteorite—flight dynamics get very complicated, and surface pressure and temperatures can skyrocket, Ed White, an aerospace engineer at Texas A&amp;M University who was not involved in the study, said in an email. A water tunnel experiment may not be enough to properly capture everything that’s going on, he says.Ristroph acknowledges these limitations as well. “We’ve sort of solved two parts of the problem individually,” he says, “but how that all works out in the full problem is not yet solved.”Given the complexities of meteorite flight, it may be difficult to replicate every aspect of it in a lab. “We have our work cut out for us,” Ristroph says.
 

 
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A ‘mock meteorite’ made up of a ball of clay. Researchers used this model to show how objects moving through fluids like water will naturally erode into cone shapes. Image Credit: NYU's Applied Mathematics Laboratory

Researchers eroding clay in water may have uncovered secrets of meteorites’ aerodynamic stability.

Cone-shaped meteorites are ‘just right’ for plummeting to Earth

David Camm lost 13 years of his life to eight drops of blood.In 2000, the Indiana state trooper was <a href="https://www.law.umich.edu/special/exoneration/Pages/casedetail.aspx?caseid=4291">charged with murder</a> after finding his wife and two children shot to death in their home. During the three trials that followed, the prosecution brought in bloodstain experts who argued that the flecks of blood on the t-shirt Camm had worn the night of the murder were “high-velocity impact spatter”—proof, they said, that he was the shooter. Analysts called by the defense, on the other hand, testified that the blood was actually a transfer stain that had smeared onto Camm’s clothing after he’d tried to help his children.Testifying for the defense, forensic scientist Robert Shaler disputed the claims of the bloodstain experts on both sides, insisting that the blood’s scant patterning was “too little information from which to draw any meaningful conclusion,” Pamela Coloff at ProPublica <a href="https://www.propublica.org/article/bloodstain-pattern-analysis-jury-wrongful-conviction-acquitted-exonerated">reported last year</a>.Nevertheless, spatter evidence was used against Camm, who was sentenced to 195 years in prison. He remained behind bars until his acquittal in 2013, after the conviction of another man whose DNA had been found at the crime scene several years before.
 

 
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 Camm’s case is only one of many that have come under fire for their heavy reliance on bloodstain pattern analysis, a subfield of forensic science criticized in <a href="https://www.ncjrs.gov/pdffiles1/nij/grants/228091.pdf">a report released by the National Academy of Sciences</a> in 2009 for being “more subjective than scientific” and rife with “enormous” uncertainties.But that might be in the process of changing—thanks in part, perhaps, to the work of a team of physicists that’s marrying the study of blood spatter to fluid dynamics. Their <a href="https://aip.scitation.org/doi/10.1063/1.5111835">latest model</a>, published today in the journal Physics of Fluids, can now accurately match a bullet of a certain size and shape to the size, number, and speed of blood droplets produced by a gunshot wound.The approach is all about bringing bloodstain pattern analysis up to speed with modern science, says <a href="https://www.exponent.com/professionals/c/comiskey-patrick">Patrick Comiskey</a>, who conducted the research as part of his graduate work at the University of Illinois, Chicago. Unlike an individual investigator’s interpretation, he says, physics is something “you can’t change.”No one scientific finding or model will solve all the problems associated with bloodstain pattern analysis, says <a href="https://forensics.wvu.edu/faculty-and-staff/faculty/suzanne-bell">Suzanne Bell</a>, a forensics expert at West Virginia University who was not involved in the research. Still, she says, “any time you bring in fundamental science to forensic applications…[it] will help build up the base knowledge we didn’t have necessarily before. People have been working with bloodstains for hundreds of years, but we haven’t seen the depth of investigation that studies like this bring.”Shaky foundationsBridging the existing gaps between forensics and science isn’t trivial. There’s an inherent challenge to analyzing crime scenes, which are, in a sense, the world’s worst time capsules: At best, they partially preserve the remains of a complex, messy problem that can only be solved in reverse.In many cases, the pattern left by a bloodstain, including the number, size, location, and overall distribution of droplets, can reveal a lot about the shooter, victim, and weapon involved. But some of the methods that blood spatter analysts have used in the past to reverse engineer the trajectory and point of origin of a stain <a href="https://www.sciencedirect.com/science/article/pii/S0379073813002417">may have been too simple</a>, leaving out some of the intricacies of how blood moves and deforms after leaving the body, says <a href="https://www.me.iastate.edu/directory/faculty__trashed/daniel-attinger/">Daniel Attinger</a>, a mechanical engineer at Iowa State University.
 <figure class="richtext-image full-width"><img alt="shutterstock_1039696972.jpg" class="richtext-image full-width" height="533" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/iStock-961710198_lSqcTWD.width-800.jpg"/><figcaption>Many factors, including the speed of a bullet, the distance blood spatter travels, and the type of surface it lands on, affect the number, size, location, and distribution of the droplets that make up a bloodstain. Image Credit: undefined undefined, iStock<div class="credit"></div></figcaption>
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 Part of the problem is that blood isn’t an easy fluid to work with. Liquids like water flow at the same rate no matter how hard they’re shaken or stirred, but the same isn’t true of certain mixtures like ketchup or blood, which actually get runnier if more force is applied to them. As a result, blood drips slow, but sprays fast.Blood is also incredibly dynamic: It teems with living cells and active enzymes, and its properties shift under the influence of minute fluctuations in temperature, or the presence of drugs. It’s the “most complex fluid” he’s ever studied, Attinger says.As soon as it leaves a human body, blood begins to cool and coagulate, effectively thickening, while also warping under the combined influence of gravity and drag from the air it’s moving through.That’s all before blood meets <a href="https://www.sciencedirect.com/science/article/pii/S0379073818310600?via%3Dihub">the surface it ultimately lands on</a>—the properties of which can seriously affect the impressions left behind. If blood’s landing pad has some bounce, it will affect the way the fluid splashes; if it’s rough or full of holes, it’ll change how the blood distributes itself. And while some stains burst on impact, producing a spray of disconnected droplets, others sprout spines that snake away from the bloody epicenter like skeletal fingers.Like several other disciplines of forensics based in pattern analysis, the study of blood spatter arose and <a href="https://www.pnas.org/content/115/18/4541">matured largely outside of the scientific community</a>, Bell says. Unlike DNA analysis, which was honed in research laboratories for the purposes of basic biology before being applied to forensics, blood spatter has a far more subjective heritage, based on experiences, anecdotes, and empirical observations. “To get their data, analysts would walk into a room, fill a sponge with blood, and hit it,” she says.There was nothing intrinsically wrong with this practice, and this kind of experimentation remains necessary, she says. But the practice lacked strong scientific foundations, and for years, little was done to explore the fundamental dynamics behind the paths droplets of blood took after impact.Thicker than water, and far more complexThis rift in knowledge is where fluid dynamics comes in: Many of the subtleties of blood in motion can be captured with equations, Comiskey says. He, Attinger, and their colleagues have spent years crunching the numbers, coming up with increasingly complex theoretical models that account for more and more of the blood spattering process. The team did their fair share of hands-on experiments, too, firing bullets at everything from <a href="https://pdfs.semanticscholar.org/9fea/843e16fd242d07d5db8b13606f4a3649dd0d.pdf">sponges soaked in pig’s blood</a> to suspended sacks filled with the <a href="https://aip.scitation.org/doi/10.1063/1.5111835">real stuff taken from human bodies</a>. After collecting the spatter on clean, smooth surfaces like butcher paper, they’d see how the results of their computer simulations stacked up, and adjust accordingly.
 <figure class="richtext-image full-width"><img alt="spatter setup.jpg" class="richtext-image full-width" height="494" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/shooting_setup_3.1.width-800.jpg"/><figcaption>The Attinger lab&#x27;s experimental setup for collecting blood spatter. Image Credit: Courtesy of Daniel Attinger, Iowa State University<div class="credit"></div></figcaption>
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 The researchers’ models can now demonstrate a mathematical relationship between the <a href="http://aip.scitation.org/doi/full/10.1063/1.5111835.">shape</a> and speed of a bullet and the distribution of the resulting blood spatter. Some things are fairly intuitive—like the fact that faster bullets will generate smaller droplets, or that the farther a fleck of blood travels, the more it’s going to be influenced by the pull of gravity. Other factors, though, are more subtle, such as the ways in which droplets are affected by the gas that escapes the muzzle of a gun after it’s fired.“We now have a <a href="http://aip.scitation.org/doi/full/10.1063/1.5111835.">pretty generalized model</a>,” Comiskey says. “I can say, more or less, that if I have this number of droplets, and they’re this size, and so on, then it was this bullet, this velocity, and [the shooter or victim was at] this location.”
 

 
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 One of the most important factors, Comiskey says, is being able to predict how blood breaks up into smaller drops, a process called atomization. Blood spatter progresses in two directions—<a href="https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.2.073906">backward, toward the shooter</a>, and <a href="https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.3.063901">forward, parallel to the path of the bullet</a>—and neither is very straightforward. The backward case is actually the simpler one, because it doesn’t really matter what the bullet is hitting: Either way, it’ll produce a kind of recoil. Forward spatter, however, produces more droplets that can <a href="https://www.sciencedirect.com/science/article/pii/S0379073817301536?via%3Dihub">travel up to twice as fast</a>. It also involves messier physics, because the blood is pushed so quickly ahead of the bullet that it shatters like glass instead of deforming like a typical fluid. The term for this phenomenon is an apt one: “chaotic disintegration.”Comiskey compares the two physical processes to running a faucet into a cup already filled with liquid. Backward spatter is the upward splash that occurs when the water hits what’s in the cup. Forward spatter is what’s produced when the jet of water is so fast and strong that it blasts through the bottom of the cup. In this case, the cup’s composition and contents can have huge effects on the resulting spray.Attinger acknowledges that these models and scenarios are still oversimplifications. Sponges and sacks of blood aren’t human bodies, and the walls and floors of crime scenes usually aren’t lined with paper. The different types of fabric used in clothing also present a complication that hasn’t yet been accounted for in crime scene reconstruction. Still, experiments like these, in which the researchers can control and keep track of all the main variables at play, are necessary starting points, he says.“None of this fundamentally changes what happens if you hit a sponge full of blood,” Bell says. “But it helps you understand why it produces the pattern it produces.”Blood will tell—but just how much?The researchers hope that their models will someday become sophisticated and realistic enough to be translated into a chart that spatter analysts can use in the field. An even loftier goal, Comiskey says, would be a cell phone app or handheld device that could scan photos of stains and spatter, crunch the numbers, and spit out a likely rundown of what transpired: the type of gun, the bullets used, the locations of the people involved, each with a score of the algorithm’s confidence in its results.That possibility is a long way off, though, says <a href="https://forensicscience.vcu.edu/people/miller-edd.html">Marilyn Miller</a>, a forensics expert at Virginia Commonwealth University who was not involved in the research. Perfect experimental conditions manufactured in a research lab are a far cry from the “highly textured, highly interrupted world of a crime scene,” she says.All the same, the team’s results offer clarity and coherence to the study of blood spatter, Miller says. “This is the kind of fundamental research that should be the basis for all forensic sciences.”
 <figure class="richtext-image full-width"><img alt="backspatter fabric strips.jpg" class="richtext-image full-width" height="434" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/backspatter_fabric_strips_zTtNv0S.width-800.jpg"/><figcaption>In their simulations, researchers at Iowa State University collect blood spatter (in this case, backward spatter) on two different types of fabric. Image Credit: Courtesy of Daniel Attinger, Iowa State University.<div class="credit"></div></figcaption>
 </figure>
 Attinger, Comiskey, and their colleagues also aren’t alone in their work: Several other research groups have previously investigated the fluid dynamics of blood spatter, including teams at <a href="https://www.ncjrs.gov/pdffiles1/nij/grants/251439.pdf">Georgia Tech</a>, <a href="https://www.sciencedirect.com/science/article/pii/S0379073818310600">Boston University</a>, and <a href="https://aapt.scitation.org/doi/abs/10.1119/1.3591319?journalCode=ajp">Washington State University</a>.Practically speaking, the mathematical models that come out of these labs could be incredibly powerful tools to test hypotheses generated in the field, Bell says. “The evidence might suggest that a blow was struck here, using this,” she says. “The model can tell you if that’s within the realm of possibility, or unlikely. It’s not going to be, ‘This is what happened,’ but it can assign a probability [to a scenario].”And that kind of uncertainty—or lack thereof—can make a big difference as evidence makes its way into a courtroom, Bell says. “This is all about making something that wasn’t quantitative, more quantitative.”But this knowledge won’t do any good if it’s not made available to blood spatter analysts in an understandable, approachable way, Miller says. The fact remains that most experts in this discipline haven’t been trained to interpret research papers grounded in fluid dynamics.Bell agrees, stressing that much of the scientific gap in these disciplines is about communication, rather than a lack of effort or any kind of willful ignorance. “The problem isn’t that labs don’t want to do better,” she says. “It’s a resource issue: So much is demanded of [the personnel in forensics laboratories], who are busy doing cases...every minute they give to training, learning, and digesting this research takes away from casework.”Tweaking training regimens for blood spatter analysts or encouraging academic researchers to join forensics labs might make a dent in these issues, Bell says. But more sweeping reforms will be needed to actually put research like this in a position where it can have a real impact.
 <figure class="richtext-image full-width"><img alt="timeseries spatter.jpg" class="richtext-image full-width" height="398" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/full_time_series.width-800.jpg"/><figcaption>Selected frames of video that recorded a simulation of a shooting. A 9 millimeter bullet was shot at a target about 35 inches away. Image Credit: Courtesy of James Michael and Reetam Das, Iowa State University<div class="credit"></div></figcaption>
 </figure>
 Part of this will require bringing other relevant voices, like those of judges and attorneys, into the bigger conversation, says <a href="https://www.innocenceproject.org/teams/#SarahChu">Sarah Chu</a>, the Innocence Project’s Senior Advisor on Forensic Science Policy, who was not involved in the research. Accuracy and scientific rigor are critical, but research also needs to be evaluated for its impact on the criminal justice system itself, as well as its ability to be ethically applied in the field, she says.But the forecast isn’t necessarily gloomy, Chu says. In the decade since the 2009 National Academy of Sciences report, a lot has changed. In 2013, the National Institute for Standards and Technology and the Department of Justice established the <a href="https://www.nist.gov/topics/organization-scientific-area-committees-forensic-science">Organization of Scientific Area Committees (OSAC) for Forensic Science</a>, which was explicitly tasked with shoring up the scientific foundations of forensics.OSAC’s subcommittees, which vet methodology for 25 forensic science disciplines, including <a href="https://www.nist.gov/topics/forensic-science/bloodstain-pattern-analysis-subcommittee">bloodstain pattern analysis</a>, are some of the few venues in which judges, attorneys, scientists, and other stakeholders are able to come together, Chu says.Maintaining the momentum gained in the past 10 years, she says, will mean staying engaged and highlighting the goals that traditionally disparate communities have in common, rather than the qualities that might set them apart.“There hasn’t been a lot of overlap between the world of these scientists and [bloodstain pattern analysts],” she says. “Hopefully publications like these signal the need for that to change.”
 

 
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The science behind blood spatter analysis (also known as bloodstain pattern analysis) is even messier than you think. Image Credit: Yeti studio, Shutterstock

Researchers are using fluid dynamics to try to improve the study of crime scene blood spatter.

There will be blood, and physics, too: The messy science of bloodstain pattern analysis

Imagine you have a pair of quantum dice. You put the dice in your cup, throw them, and they are both a “six.” You do it again, and they’re both a “three.” You do it again, and they’re both a “one.” No matter how many times you throw the dice, they are always the same number. This is a seemingly random process, but quantum entanglement ensures that these dice will always act in unison.The thought of tackling quantum entanglement in your classroom can be intimidating. Even Albert Einstein was skeptical of the concept, calling it “spooky action at a distance,” and didn’t think it was possible. If this was a tough sell for Einstein, how can you teach this idea to middle and high school students?
 

 
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 Quantum physics are the laws that govern subatomic particles. It can help explain the movements, interactions, and behaviors of very small things like photons and electrons. How small are particles in the subatomic world? Imagine something a million times smaller than the width of a human hair, or making a soccer ball into the size of the Earth and moving inside. This is how we can understand the scale of the world of atoms and subatomic particles.Quantum physics is rooted in probability. An electron doesn’t have a specific location in space until it is looked for or detected. When we picture an atom, we often imagine its electrons orbiting the atom’s nucleus in clear defined paths. However, electrons are not the ever-present tiny spheres we imagine them to be. They are really a “fuzzy wave of probability,” ready to be in an infinite amount of locations around the nucleus of an atom until we look at it. The simple act of observing an electron changes the electron’s location.
 

 
 Quantum entanglement takes this idea one step further, and is a phenomenon where two particles act in unison, no matter the distances between them. Unlike a row of dominoes that rely on the actions of each previous domino to be knocked down over time, entangled particles change instantly once one is detected, no matter how far apart they may be. For example, if a particle on Earth is entangled with a particle in the Andromeda galaxy, they will change at the exact same time once a change is made to one of the particles.While quantum physics pertains to the smallest units in the universe, the ways in which it impacts our daily lives are immense. Quantum theory played a role in the Manhattan Project and the atom bomb and in the development of lasers and transistors. Our increasingly digital world relies on quantum entanglement to send encrypted messages and quantum computers with massive processing power to solve problems a normal computer could never accomplish.Quantum physics impacts the technology students use every day. Use these resources from NOVA broadcasts, NOVA Digital, and What the Physics!? to introduce quantum concepts to your students.
 

 
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 <h2>Einstein’s Quantum Riddle Resources</h2><a href="https://mass.pbslearningmedia.org/resource/nveqr-sci-entanglement/quantum-entanglement-einsteins-quantum-riddle/">Quantum Entanglement</a>: Conceptualize quantum entanglement, the idea that particles can instantaneously influence each other even when they are spatially separated, in this video from NOVA: Einstein’s Quantum Riddle.<a href="https://mass.pbslearningmedia.org/resource/nveqr-sci-electronduality/waveparticle-duality-of-electrons-einsteins-quantum-riddle/">Wave Particle Duality of Electrons:</a> Conceptualize the nonintuitive idea that electrons can behave both as a wave and a particle (wave–particle duality) in this video from NOVA: Einstein’s Quantum Riddle.<a href="https://mass.pbslearningmedia.org/resource/nveqr-sci-evidence/collecting-evidence-for-quantum-entanglement-einsteins-quantum-riddle/">Collecting Evidence for Quantum Entanglement</a>: Learn why different experimental designs were needed to collect evidence for quantum entanglement in this media gallery of videos from NOVA: Einstein’s Quantum Riddle.<h2>What the Physics!? Resources</h2><a href="https://www.pbs.org/wgbh/nova/video/seeing-quantum-with-the-naked-eye/">Seeing Quantum with the Naked Eye</a>: How can you train yourself to be a quantum detector? You can detect quantum mechanics all over—if you know how to look for it. In this episode of What the Physics!? find out how simply squinting at a lamp reveals the quantum nature of light.<a href="https://www.pbs.org/wgbh/nova/video/the-biggest-puzzle-in-physics/">The Biggest Puzzle in Physics:</a> Sometimes the biggest puzzle in physics seems like the worst relationship in the universe. Quantum mechanics and general relativity are the two best theories in physics, but they have never been able to get along. Find out why in this episode of What the Physics!?<h2>NOVA Digital Resources</h2><a href="https://mass.pbslearningmedia.org/resource/arct14.sci.nvqconf/quantum-confidential/">Quantum Confidential</a>: Watch how a technique called quantum cryptography could save a state secret from falling into enemy hands in this video from NOVA Digital.<a href="https://mass.pbslearningmedia.org/resource/nvnd-sci-voterfraud/wgbh-nova-digital-using-quantum-physics-to-prevent-voter-fraud/">Quantum Physics to Protect Votes:</a> Voters want to be assured that every election is fraud-proof and hack-free. With the help of quantum mechanics, it’s possible to do just that — encrypt and transmit a vote in ways that can’t be tampered with.
 

 
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Quantum physics impacts the technology students use every day. Use these resources from NOVA broadcasts, NOVA Digital, and What the Physics!? to introduce quantum concepts to your classroom.

Bring "Spooky Action at a Distance" into the Classroom with NOVA Resources

Why this card trick might fool your eyes.
Watch a ten of clubs transform into an ace of hearts in this magic trick, which demonstrates “change blindness,” a phenomenon that causes gaps in visual.

The Science Behind This Slight of Hand Hero

Why this card trick might fool your eyes.

The Science Behind This Sleight of Hand

If there’s anything more mesmerizing than iridescent swirls dancing across a soap bubble, it’s the sight of icy stars whirling around that bubble as it freezes.So when a <a href="https://www.youtube.com/watch?v=H7pqoCJQp2I">YouTube video</a> of a bubble freezing made its way to mechanical engineer Jonathan Boreyko and his team at Virginia Tech, they weren’t just awed. They wanted to understand the physics behind the stunning display of swirling ice crystals.“A lot of things in research are more motivated by a desire for truth or beauty,” Boreyko says, “like just trying to understand a new and mysterious thing.”
 

 
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 The team’s <a href="https://www.nature.com/articles/s41467-019-10021-6">study</a>, published last month in the journal Nature Communications, reveals the physics of the “snow globe effect,” a phenomenon that causes hundreds of tiny ice crystals to float across a bubble’s surface and freeze simultaneously in a whimsical display.<div>
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What Boreyko and his team set out to determine was where the hundreds of small ice crystals in the snow globe effect come from. Typically, when a relatively small quantity of a liquid freezes—like when water turns into an ice cube—the freezing process begins from a single point of origin. That point creates an expanding boundary between frozen and unfrozen liquid called a freeze front. As the freeze front expands outward, it cools the remaining liquid, causing it to change phase and become ice.But in bubbles, Boreyko and his team noticed, this process can be different. When the researchers froze some bubbles in a walk-in freezer and took photos and videos of them, they saw what looked like hundreds of freeze fronts floating simultaneously across the bubbles’ surfaces.
 

 
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 To look more closely at the physics behind the multitude of freeze fronts, the team developed a mathematical model to simulate bubbles freezing. The modeling, Boreyko says, was the hardest part. “Once all the experiments were done, it took over a year to fully model and understand the physics behind the stuff we were seeing,” he says.What the team discovered follows the fundamental process of freezing itself: The moment a liquid freezes, it gets slightly warmer. This is because as a liquid’s molecules rearrange and fuse into a solid, excess energy is released in the form of heat, called latent heat.When a bubble is in a freezing environment and lands on a cold surface, the first thing to freeze is the bottom of the bubble. As this freeze front expands, it releases latent heat into the adjacent liquid surface of the bubble. This warms the water at the edge of the freeze front ever so slightly and causes it to flow toward the top of the bubble.As more latent heat is released, the water flow grows stronger. This puts stress on the freeze front and causes extremely tiny ice crystals to break off and slide across the bubble’s surface. These crystals then begin to grow simultaneously, giving the impression that there are multiple freeze fronts forming at once.“It actually is only one freeze front,” Boreyko says, adding that the cold surface provides a point of origin to start the growth of ice crystals.“This paper was not about ice, really. It was more about liquid flow,” says Kenneth Libbrecht, a physicist at Caltech specializing in ice crystals and snowflake formation who was not involved in the study. The study’s unique finding, he says, is that freezing creates liquid flows on the surface of bubbles that create a feedback loop, which enhances the freezing process itself.This new understanding of the snow globe effect could someday help experts engineer better ways to freeze things, including biological material like human tissue.
 <figure class="richtext-image full-width"><img alt="Image-1-1 (1).png" class="richtext-image full-width" height="228" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/Image-1-1_1.width-800.png"/><figcaption>The progression of the &quot;snow globe effect&quot; in three images. Image Credit: Farzad Ahmadi and Christian Kingett<div class="credit"></div></figcaption>
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 Currently, the low-temperature freezing of human tissue, called cryonics, is mostly limited to freezing small tissue samples or single cells, says Steven Van Sciver, a professor studying cryogenics and cryonics at Florida State University who was not involved in the study. Larger samples, such as full organs, he says, become damaged during the freezing and thawing process.“Most of what you see in Star Wars and Star Trek and Hollywood is things like freezing people and bringing them back to life,” Van Sciver says. “And most of it is a hoax.”But, to researchers looking for better ways to freeze things, bubbles present a unique opportunity, he says. “Understanding how these crystals are formed in very thin bubbles and how they move around could be helpful in understanding biological freezing done with cryogenics.”But Boreyko says his team didn’t have an end use in mind when they started this project, just a desire to understand what they were seeing.“People are always thinking about applications,” Libbrecht says. “I think that’s the wrong way to think about it. In this case, the research is very inexpensive....It’s OK to spend a little bit of money trying to understand what’s going on around you.”
 

 
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A bubble freezing on a block of ice inside of a freezer. Image Credit: Farzad Ahmadi and Christian Kingett

Freezing soap bubbles look like snow globes. This whimsical effect could help us improve biological freezing techniques—and is incredibly fun to watch.

The physics of freezing soap bubbles is cooler than you’d think

At some point or another, most of us have played travel agent to a dandelion. Blowing on their tufted tops is a childhood rite of passage for many, and a simple puff of air is all it takes to send the seeds of these wayfaring weeds wafting into the breeze.But even when our lungs are the source of the initial gust, we humans can’t take all the credit for dandelion fight. Thanks to evolution’s incessant tinkering, these seeds have become some of nature’s deftest dispersers, tumbling inches, feet, sometimes miles from their original source. Now, two <a href="https://www.nature.com/articles/s41586-018-0604-2">recent</a> <a href="https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.4.071901">studies</a> reveal the physics behind their fanciful flight.
 

 
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 In the wake of their golden days of flowering, the tops of dandelion plants mature into dozens of flight-ready seeds, each tethered to its own personal parachute. But what crowns these seeds aren’t the billowing cloth contraptions we’re used to. Rather, they’re tufts of haphazard fluff, each made up of about 100 individual bristles that fan out into a sparse, delicate plume called a pappus.To suss out how these strange little structures keep seeds aloft, a team of University of Edinburgh engineers led by <a href="https://www.eng.ed.ac.uk/about/people/dr-cathal-cummins">Cathal Cummins</a> went into the weeds. Using a tiny, vertical wind tunnel that suspended seeds at a fixed height, simulating flight, the researchers were able to observe how air interacted with the pappus’ filaments.The trick behind the seeds’ airborne acrobatics, it turned out, was less about the bristles themselves than the gaps between them. High-speed cameras showed that, after air flowed up and around the pappus, it was quickly rerouted into the center of a swirling, elongated whirlpool called a <a href="https://www.youtube.com/watch?v=jzq5gfeHfuc">vortex ring</a> that pushed the air back down toward the top of the pappus, where it ascended once more. This fast-moving funnel was so good at recycling air that it generated an ever-present pocket of low pressure just above the seed, sucking it upward into an easy, breezy cruise—and delaying its inevitable descent.
 <figure class="richtext-image full-width"><img alt="Image One Madeleine Seale, Alice Macente.png" class="richtext-image full-width" height="517" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/41185693185_a07e06b42d_o.width-800.jpg"/><figcaption>Fun fact: Each unit of a dandelion is actually home to hundreds of individual flowers clustered together. Because dandelions can reproduce both sexually (through pollination) and asexually, every seed emerging from a dandelion’s head can have a different genetic makeup. Image Credit: Theo Crazzolara, flickr<div class="credit"></div></figcaption>
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 Dandelions are by no means the first organisms (<a href="https://science.sciencemag.org/content/329/5990/406.long">or even the first plants</a>) that have been shown to DIY their own vortex rings to achieve liftoff. But as Cummins and his colleagues <a href="https://www.nature.com/articles/s41586-018-0604-2">reported</a> last October in the journal Nature, their pappuses have an especially aerodynamic structure, with bristles so thin that they take up just 10 percent of the total space in the seed’s frizzy updo. This gives the pappus a lot of porosity—a measure of its relative “emptiness,” explains study author <a href="https://www.ed.ac.uk/profile/madeleine-seale">Madeleine Seale</a>, who specializes in plant biomechanics.And dandelion seeds, it would seem, have a knack for turning nothing into a whole lotta something. Compared to a solid, parachute-shaped structure of about the same size, the pappus is actually four times as efficient at creating drag, the force that counteracts gravity to keep the seed airborne, the researchers found.
 

 
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 There was a second effect, too: By generating super-efficient drag, the pappus’ porosity also has the perk of keeping the seed’s flight stable.When upward-bound air travels up and around a falling object, it can create a sort of wobble—a bit like a flag flapping in a gust of wind, just on a much smaller scale. But because the pappus is porous, some air flows through it instead, streaming between the bristles of the seed’s thin toupee. The more porous the parachute, the smoother its flight.What wasn’t quite clear, however, was if the seeds’ stability was affected by the number of filaments in each pappus. That’s where the <a href="https://journals.aps.org/prfluids/abstract/10.1103/PhysRevFluids.4.071901">second paper</a>, published today in the journal Physical Review Fluids, comes in. Taking a theoretical approach, a team led by fluid dynamicist <a href="https://people.epfl.ch/francois.gallaire?lang=en">François Gallaire</a> of the Swiss Federal Institute of Technology Lausanne devised a mathematical model of the super-steady nature of dandelion vortex rings.The results revealed a fragile balancing act between two opposing forces. The fewer filaments atop a seed, the higher its porosity, and thus stability—good news for a skyfaring seed. But stability also comes at the cost of drag, which increases as the number of bristles goes up. A fair compromise, the researchers found, is for the seeds to hit a sweet spot around 100 bristles per pappus. This figure matched up with what’s found on real-life dandelions, Gallaire says. There’s some wiggle room, but at lower numbers, the seed would have trouble staying afloat; too much higher, and things might start to get shaky.“It is super cool that dandelions and this mathematical model came up with the same number,” says <a href="https://biology.williams.edu/profile/jedwards/">Joan Edwards</a>, a plant biologist at Williams College who was not involved in either study. “Adaptation is one of the hardest questions to answer in evolutionary biology...but now, we can theoretically explain why a pappus has this number of filaments, and that’s really exciting.”<div>
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But the theoretical model does more than add credence the first paper’s experimentally-driven results, Seale says. “With a model, they can explore things we can’t explore experimentally very easily...including how these structures can be used in new and different ways,” she says.For instance, plugging different numbers into the Swiss team’s equations could aid the design of better microdelivery systems, says <a href="https://www.mcdb.ucla.edu/faculty/siobhanb">Siobhan Braybrook</a>, who studies plant form and function at the University of California, Los Angeles, but was not involved in either study. Someday, pappus-like structures could play a role in tasks like airdropping fertilizer or transporting teeny robots.One thing that’s probably not in our future, though, is a dandelion-inspired parachute for people or their parcels, Braybrook adds. We humans (and most of the things we buy) are far too big and bulky for the same physical principles to apply. “For this design to be relevant...you need to talk about objects that are incredibly light or incredibly small,” she says.
 <figure class="richtext-image full-width"><img alt="HaloVortexRingLiftsDandelionSeeds (1).png" class="richtext-image full-width" height="600" width="800"
 src="https://www.pbs.org/wgbh/nova/media/images/HaloVortexRingLiftsDandelionSeeds_1.width-800.png"/><figcaption>The dandelion&#x27;s sparse, tufted pappus creates a swirling pocket of air called a vortex ring in the space above it. The pappus is mostly empty space, but the 100 or so filaments that comprise it hit a &quot;sweet spot&quot; between creating just enough drag to keep it aloft, while maintaing the seed&#x27;s stability. Image Credit: Courtesy of Cathal Cummins, Madeleine Seale, Enrico Mastropaolo, Ignazio Maria Viola, and Naomi Nakayama<div class="credit"></div></figcaption>
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 Unless, that is, we fiddle with one pesky constraint: gravity itself. “If we were on a different planet...we might need to come up with different ways of dealing with drag,” Braybrook says. “In that kind of situation, [this model] could be incredibly relevant.”We’re not quite ready to deploy those tactics just yet. In the meantime, there are still exciting opportunities to study some marvels of innovation right here on Earth: Many other plants spread their seed by wind; even some animals have evolved specialized structures that help them navigate through air or water.As research on these organisms and more continues to take flight, a one-size-fits-all equation probably won’t apply. But maybe that’s part of the beauty of this dazzlingly simple system.“Nature has optimized something so beautiful,” Braybrook says. “For us, it’s only as we start to develop [new technologies] that we start thinking about these problems...but nature’s already done the work. We have the best simulations ever in the products of evolution.”
 

 
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With two recent studies, the physics behind dandelion seed dispersal is now taking flight. Image Credit: Bess Hamitii, Shutterstock

Two research teams went into the weeds to quantify the magic behind the flight of the dandelion seed.

Topic: Physics & Math

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