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大家好，这是捉妖第一次发帖，还望表哥表妹们多多关照
今天的speed是一篇浪漫的物理文，浪漫之中略带物理，希望大家喜欢：）
Part I：Speaker
Wi-Fi Wobbling Hand Gestures Could Control Home
[Rephrase 1]
Transcript hided [Dialog, 1:15]
Thanks to wi-fi we can watch movies, play games, and check e-mail, from the comfort of the couch. And soon we may be able to repurpose our wi-fi signals so we can turn on the coffeemaker or turn off the TV with a simple flick of a finger, from anywhere in the house. Researchers described their prospective system, called WiSee, at the International Conference on Mobile Computing and Networking. [Qifan Pu et al., Whole-Home Gesture Recognition Using Wireless Signals]
The idea is similar in concept to the Xbox Kinect, which uses cameras to recognize a game player’s movements, and translates them into an action onscreen. But WiSee works without cameras. Instead, it uses the ambient wireless signals put out by our smart phones, laptops, routers and tablets.
When we move, we alter the patterns of these wi-fi signals. The WiSee receiver detects these disturbances and interprets the motions that caused them, like waving or swinging your fists. Of 900 gestures tested, WiSee could identify 94 percent.
The user assigns a particular motion to a specific gadget—the motion doesn’t have to be the one that works in the real world. So if all goes well, you may actually be able to punch your lights out.
Source:http://www.scientificamerican.com/podcast/episode.cfm?id=wifi-wobbling-hand-gestures-could-c-13-10-17
Part II：Speed
It’s too soon to declare supersymmetry a tragedy
By Tom Siegfried
[Time 2]
If Shakespeare were alive today, he’d write a tragedy about physics. I think he’d call it Romeo and SUSY.
It would start out with Romeo, a kind of avatar for all theoretical physicists, falling deeply in love with SUSY, a very beautiful but also very shy embodiment of the deepest insights into reality. Romeo had never actually met SUSY, though. She was kind of a magical, mythical creature, maybe a little like Hermione Granger. Romeo worshiped her nevertheless, entranced by the reports of her great beauty and intellectual sophistication.
For three decades Romeo searched far and wide for SUSY, eventually reaching her last conceivable hiding place, in a tunnel near Geneva. He got ever so close, but then a magnet blew up and he had to wait another few years. Finally he made it through the tunnel and didn’t find SUSY there. And so Romeo, in despair, declared his life a meaningless failure and had to decide between killing himself or writing a blog.
To hear many physicists talk these days, Romeo ought to go ahead and opt for worms as chambermaids and die with a kiss. On the other hand, there are a few who say there’s still some crimson in SUSY’s cheeks and lips. Or at least maybe she has an even hotter sister who is still alive.
OK, today’s real-life scenario isn’t so dire that SUSY fans should be poisoning themselves. But physicists do seem to be in a tizzy these days over the experimental findings, or lack thereof, at the Large Hadron Collider, the world’s most powerful atom smasher. (It occupies the tunnel outside Geneva.) Everyone exulted last year when the LHC found the Higgs boson. But upon reflection, many were saddened that the Higgs was the only big thing the LHC found. No SUSY!
[299 words]
[Time 3]
SUSY is shorthand for supersymmetry, the odds-on favorite to solve many of the mysteries about the physical world that have stumped theorists for decades. Supposedly the LHC should produce actual evidence for SUSY, but it hasn’t. And so some physicists have begun to declare SUSY dead, or at least on life-support. Consequently, they say, physics now faces one of the greatest crises in its history. A typical lament, from one recent paper:
“After three years of very successful experimental work at Large Hadron Collider (LHC) at CERN, theoretical physics is apparently in the greatest crisis in its history.” The experimental findings “have nearly eliminated supersymmetry as a possible physical theory. It seems inevitable that we have to face the Nightmare Scenario (i.e. no signs of new physics at LHC) and the unprecedented collapse of decades of speculative work.”
In September, Neil Turok, director of the Perimeter Institute in Canada, squirted lighter fluid into the flames during a lecture to students. “Theoretical physics is at a crossroads right now. In a sense we’ve entered a very deep crisis,” he said. SUSY (and other theories) predicted that the LHC would find new particles. “And they’re not there,” Turok declared. “And so to a large extent, the theories have failed.”
Well, maybe. But it might not really be so bad. Perhaps a little context is in order, starting with why physicists loved SUSY so much in the first place and then investigating the reasons for concern a little more deeply. There may turn out to be various ways the play could end without everybody having to die.
[264 words]
[Time 4]
First of all, supersymmetry seems like such a good idea to physicists because symmetry even without the super was so successful. (After all, isn’t a supernova better than a nova? Supermodel better than a model? Superman better than Clark Kent?) Symmetry showed its power in Einstein’s theory of relativity, for example, and later on in the development of the modern theory of particles and forces, the standard model (or as Nobel laureate Frank Wilczek likes to call it, the Theory of Matter).
In this context, symmetry is a mathematical concept, but it has its nonmathematical illustrations. At its most basic, symmetry means that changing one thing doesn’t change everything, or to sloganize it, symmetry is change without change. (Physicists prefer to say invariance under transformation, but it’s the same idea). If you look at a circle in a mirror, you’ve flipped left and right, but the circle looks exactly the same. It possesses mirror image symmetry. If you rotate a snowflake by 60 degrees, it looks just like it did before you rotated it. Rotational symmetry.
All these and other symmetries can be described with equations that seem to be very helpful in making sense of the universe. Einstein was particularly skillful when it came to symmetry. His theory of relativity embodies the principle that the laws of physics stay the same no matter how you move. In the 1920s, Hungarian physicist Eugene Wigner pioneered the application of symmetry math to particle physics, describing particle properties using “quantum numbers” that emerge from equations describing symmetries. Symmetry equations led Murray Gell-Mann to propose the existence of quarks in 1964.
[268 words]
[Time 5]
Further pursuit guided by symmetry math led to the creation, in the 1970s, of the standard model, which provides a precise description of all of nature’s basic particles and forces. Some of those particles had not been discovered at the time, but they all have since, including the Higgs boson just last year. Symmetry has a better track record than even Bill Belichick, and is much more polite.
For a long time, though, it has been clear that the standard model symmetries do not solve all the problems that nature poses. During the 1970s, just as the standard model was taking shape, various physicists began to explore additional symmetries. In particular, several investigators discerned a symmetry (eventually called supersymmetry) relating the matter particles (technically, fermions) with the force particles (bosons). In 1981 Savas Dimopoulos and Howard Georgi produced the math describing the complete supersymmetric version of the standard model. For each fermion in the standard model, there should exist a supersymmetric partner boson. Each standard boson should have a superpartner fermion. So nature ought to possess twice the number of known particles.
Dimopoulos once told me that doubling the number of particles in nature wasn’t as radical as it might have seemed. He pointed out that similar considerations had multiplied the number of particles in nature before. In the late 1920s, Wolfgang Pauli figured out that electrons must have a two-valued property later known as spin. That was really the same thing as doubling the number of electrons, Dimopoulos said. And a few years later Paul Dirac proposed the existence of antimatter — a new “antiparticle” for every known particle. Dimopoulos and Georgi were simply doing the same thing again.
[279 words]
[Time 6]
There was one tiny (actually, big) problem: no evidence for any such particles. Superparticles therefore must be much more massive than their standard partners — otherwise they would already have been noticed. Hence the need for powerful atom smashers, capable of reaching energies high enough to make such particles. By most calculations, the LHC should have had enough energy to produce the lightest of the superparticles. But even though the LHC succeeded in finding the Higgs, it failed to find any signs of SUSY. To hear some physicists tell it, there never was a story of more woe.
But let’s remember that Shakespeare isn’t writing this story. It may be fair to say that the simplest version of SUSY (known as the minimal supersymmetric standard model) seems a little shaky. But that’s not the end of the play.
Many variations on SUSY have been proposed — in other words, SUSY has siblings and cousins, perhaps not as beautiful as SUSY, but still possibly able to help solve some of the outstanding problems facing physics today. SUSY’s no-show at the LHC may not imply supersymmetry’s absence in nature, but merely that superpartner particles in the correct version of SUSY are too heavy for the LHC to see.
Heavier SUSY particles do pose a problem, though. They seem to complicate the theory more than necessary, and may defy a criterion that physicists call “naturalness,” as Nathaniel Craig of the Institute for Advanced Study in Princeton pointed out in recent lectures, published at arXiv.org.
“The march of null results suggests that we were mostly wrong about precisely how supersymmetry would appear at the LHC,” Craig writes.
It turns out that whether SUSY lives or dies may hinge on how physicists decide to define what is natural, and whether they should insist that a theory be as simple as possible, or parsimonious. And that turns out to be a rather complicated task. To be continued.
[319 words]
Source:
https://www.sciencenews.org/blog/context/it%E2%80%99s-too-soon-declare-supersymmetry-tragedy
Part III： Obstacle
Unbreakable cryptography,The devil and the details
From the print edition: Science and technology, Published: Sep 21st 2013
[Paraphrase 7]
Quantum cryptography has yet to deliver a truly unbreakable way of sending messages. Quantum entanglement may change that.
The current way quantum theory is employed in cryptography, known as “prepare and measure”, works by distributing a secret key, encoded in the way light is polarised, to two people (known conventionally as Alice and Bob) who wish to talk privily with each other. This key is used to encrypt a message so that it cannot be understood, even if it is intercepted. Prepare-and-measure looks good in theory because an eavesdropper (Eve) listening in will perforce give herself away by measuring the light’s polarisation, and thus disrupting the system. If that happens, Alice and Bob can ditch the compromised key and ask for another.
However, if Eve can somehow tinker with the sending and receiving equipment (for example by blinding it with a special kind of laser, as happened in one famous quantum hack in 2010, or getting the manufacturer to do something similar), she can hide her disruption, leaving Bob and Alice none the wiser. The technique therefore ceases to be secure. Given recent revelations about Western-government activities in this area, and strong suspicions about pressure the Chinese government puts on the country’s computer and telecoms firms, users’ fears that their equipment might not be all it says it is are hardly paranoid. The Bell test promises to assuage those fears.
For whom the Bell tallies
Bell-based cryptography also works by generating a key based on the polarisation of light. But it begins by using a special machine to produce the particles of light (called photons) in which the message will be encoded. This machine turns them out as entangled pairs. One member of each pair goes to Alice, and one to Bob. For each photon she receives, Alice chooses at random which of two predetermined polarisation angles to measure. For each measurement, she can get one of two results: either the photon will appear aligned with her polarisation axis (call that a one) or perpendicular to it (call it zero). This can be used to encode a digital bit. Bob, for his part, also measures his photon’s polarisation. Both of his axes, too, have been arbitrarily set.
Conventional odds in the world of classical physics predict Alice’s and Bob’s bits will match three times out of four. Add in quantum entanglement, though, and the odds increase to just over 85%. This was the essence of Bell’s insight.
If Alice and Bob’s measurements agree more often three-quarters of the time, it suggests their photons are entangled. That means they cannot have been intercepted, since any attempt by Eve to do so would inevitably cause them to untangle. If Alice and Bob then each add a third, identical polarisation angle, they can use this extra bit, which they know they must share, to encode the cryptographic key.
The trick is to turn this insight into a practical device, given that 85% is a theoretical maximum that real-world equipment never achieves, because it does not always succeed in turning out entangled photon pairs and may also mislay some of those it does. Christian Kurtsiefer and Valerio Scarani, two of Dr Ekert’s colleagues at CQT, have been trying to deal with this. Their photodetectors reach efficiencies of 97-98% and their sources of entangled photons are now “pretty much perfect”, Dr Ekert says—producing unentangled pairs less than 1% of the time. The main remaining problem is losses in the fibres, filters, lenses and polarisers that link the source of entangled photons to the detectors. The more such optics there are, the lower the efficiency tends to be. For a system that goes beyond a laboratory bench efficiency quickly drops to 70-80%, which is below the 82% that theory suggests is the minimum needed if the Bell test is to be valid.
A team led by Paul Kwiat of the University of Illinois at Urbana-Champaign is trying to deal with this not by reducing the losses but rather by reducing the sensitivity of the system to such losses. Dr Kwiat attempts this by entangling photons in a slightly different, weaker way. Thus entangled, they are more vulnerable to measurement errors but more stable against losses. Last month the team presented their findings to the third International Conference on Quantum Cryptography, in Waterloo, Ontario. They think they have managed to design and implement a scheme in which 75% efficiency is enough to ensure the validity of the Bell test.
Allison Rubenok and her colleagues at the University of Calgary, in Alberta, and Liu Yang and Chen Tengyun, from the University of Science and Technology of China, use yet another approach. Each group introduces a third party who sits between Alice and Bob.
In this scheme, rather than creating and sending entangled photons, Alice and Bob begin by polarising their light pulses at random. Each then sends his or her pulse, whose polarisation is known only to the sender, to the third party, who performs a variant of the Bell test on the two incoming signals. The outcome is successful for those combinations of photons from Alice and Bob that are in the same quantum state. These outcomes do not need to be secret: once publicly announced, they allow Bob and Alice to pick the sequence of bits associated with them as their private encryption key. Crucially, for complicated reasons that have to do with the nature of the protocol, this scheme does not require near-perfect detectors.
Moreover, as the two groups report in the latest issue of Physical Review Letters, it works even if the connecting optical fibres become long enough to be of practical use. Dr Rubenok and her team have tested their version of the scheme on a spool of optical fibre more than 80km (50 miles) long, and also on 18km of working cable installed in Calgary. Dr Liu and Dr Chen have run it successfully over a link 50km long.
It all therefore looks quite promising. Unlike prepare-and-measure cryptography, no tinkering with the photon-generator could go undetected. But inevitably, there is a wrinkle. As Jonathan Barrett, another Oxford academic, points out, the interception need not take place in this bit of the system at all. Instead, the detector Alice bought from a manufacturer subverted by Eve could surreptitiously record the quantum-key data Alice receives, store them in a conventional memory, and then broadcast them to Eve later. That would enable her to decode Alice’s earlier messages to Bob.
Dr Ekert reckons this threat is more theoretical than real. The non-quantum parts of the system, whose weakness Dr Barrett is pointing out, are easier to test for interference than the quantum parts that prepare-and-measure bugs hide in—and anyone concerned enough about security to bother with quantum cryptography in the first place is certainly going to scrutinise his equipment pretty thoroughly before he uses it. Soon, then, those who wish to communicate completely privily may be able to do so, whatever the world’s Eves might try throwing at them.
[1116 words]
Source:
http://www.economist.com/news/science-and-technology/21586529-quantum-cryptography-has-yet-deliver-truly-unbreakable-way-sending
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发表于 2013-10-21 22:11:45
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uantum cryptography has yet to deliver a truly unbreakable way of sending messages. Quantum entanglement may change that.
