Category Archives: Physics

Spooky Speeding

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A pretty cool experiment that puts a lower bound on a speed of entanglement has been performed. The experimenters entangled photons, separated them, and then made their measurements.

Physicist Nicolas Gisin and colleagues at the University of Geneva in Switzerland split off pairs of quantum-entangled photons and sent them from the university’s campus through two fiber-optic cables to two Swiss villages located 18 kilometers apart. Thinking of the photons like traffic lights, each passed through specially designed detectors that determined what “color” they were when entering the cable and what color they appeared to be when they reached the terminus. The experiments revealed two things: First, the physical properties of the photons changed identically during their journey, just as predicted by quantum theory–when one turned “red,” so did the other. Second, there was no detectable time difference between when those changes occurred in the photons, as though an imaginary traffic controller had signaled them both.

The results show that any information connection between them would have to occur at at least 10,000 times the speed of light, which is interpreted as a pretty good indication that it’s an inherent behavior of quantum mechanics, and this “communication” isn’t actually taking place. (see also Bohm’s Bummed and the summary at Physics and Physicists)

Or not.

nature news has an article entitled Physicists spooked by faster-than-light information transfer. LiveScience’s article is Spooky Physics: Signals Seem to Travel Faster Than Light. Which is really strange, because at least in the nature summary, they discuss how it isn’t evidence of superluminal communication

A second test ensured that the scientists in the two villages weren’t missing some form of communication thanks to Earth’s motion through space. According to Einstein’s theory of relativity, observers moving at high speeds can have different ‘reference frames’, so that they can potentially get different measurements of the same event. The Geneva results could possibly be explained if the two photons were communicating through a frame of reference that wasn’t readily apparent to the scientists.”

But theoretical calculations have shown that performing tests over a full spin of the globe would test all possible reference frames. The team did just that, and they got the same result in all cases.

The bottom line, says Gisin is that “there is just no time for these two photons to communicate”.

So why use a headline that says or implies that there is FTL information transfer, when the conclusion is that there isn’t?

Deadman's Curve

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Matt discusses the disaster of grades in The Final Countdown

I just finished grading three problems worth of the final exam (the other two TAs are taking care of the rest), and I think the exam can be safely described as a debacle. It was a disaster. The scores haven’t been tallied up yet, but I think there’s a good chance the mean score will be within one standard deviation of zero. And, I dunno, about 4 or 5 standard deviations away from 100.

One mitigation technique I’ve mentioned before is to have a database of questions, so that you know the expected results, but this doesn’t necessarily work well in a university environment, and doesn’t apply here as it was already noted that the exam was made from scratch.

But there’s another option. I’ve taken and TA’d several classes where grades were neither curved, strictly speaking, nor were graded on a standard “90 and above is an A” basis. The professor who taught the class for which I TA’d pointed out that the students had a hard time adjusting to the concept that the average score was going to be about 50, since they had never encountered that system before. When it first happened to me as an undergraduate, the professor put it rather succinctly — why bother asking a lot of questions that everybody can answer? If 65 is the lowest passing score, then you’re asking a whole bunch of points worth of questions that don’t demonstrate adequate knowledge of the material. The idea was to cut out 50 points of that and add in questions that do require “passing” knowledge to answer and adjust the grading accordingly. The exams were much more complete in testing comprehension, since you could ask more questions about a particular topic. It’s not unlike the strategy taken during oral exams — asking questions until the target can’t answer them anymore. That’s when you’ve tested the depth of knowledge and comprehension.

Threading the Needle

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Another cool find by Zapperz: Threading Light Through the Opaque

Freshly fallen snow is blinding white because the jumble of flakes scatter light in all directions. Such scattering also implies that little light passes through snow, so that if you’re ever buried deep in it, you’ll find yourself in the dark. But according to theoretical physicists, it should always be possible to fiddle with light waves to make them wend their way through such a disordered material, no matter how thick. And now a duo of experimenters has demonstrated that feat.

Do Not Fear the Banana

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Zapperz has a short post on an article that appeared in the NY Times, chumming the waters of fear about radiation from granite countertops. I see that Chad has promised and delivered a bit of a rant, pointing out that popular media could and should do science. The problem is that they don’t — not in the living section, and sometimes not in the science section, and almost certainly not on the op-ed page.

But that’s not actually what piqued my interest here. It’s mediocre reporting, to be sure; the author makes sure to give “both sides” of the story, even though science boils down there being experimentally verified claims or not, so reporting knee-jerk reactions to the ticking of a geiger counter isn’t particularly responsible. But there was a snippet that reminded me of a conversation I was having last week.

Indeed, health physicists and radiation experts agree that most granite countertops emit radiation and radon at extremely low levels. They say these emissions are insignificant compared with so-called background radiation that is constantly raining down from outer space or seeping up from the earth’s crust, not to mention emanating from manmade sources like X-rays, luminous watches and smoke detectors.

And not to mention — because they don’t — people. That’s right: YOU are radioactive. An adult contains something like 140 g of potassium, of which about 16.5 mg will be K-40, which is radioactive with a 1.26 billion year half-life, yielding about 4400 decays per second. You also have C-14 in you, adding in another 3000 decays per second. The C-14 decay, and 89% of the K decays give betas, which will be deposited in your body. The other 11% of the K-40 decays have a 1.46 MeV gamma, and about half of them will be deposited in your body as well. This ends up being tens of millirem of dose per year.

The rest of the gammas escape, which means that you are a 6.5 nanoCurie gamma source. (Sleep with someone else 8 hours a night, all cuddled up? That’s around a millirem of dose each year. Not a cuddler? Here’s your excuse — your exposure decreases as you move away.) Do you use potassium in your water softener or as an alternative to table salt? What about bananas? That’s a 300 picoCurie source there, and you’re eating it. If you leave it alone, it’s only about 20 picoCuries of gamma.

The point here isn’t to make anyone afraid of bananas. You need potassium, and K-40 is along for the ride. But reporting like this gives no context, and paints a very simplistic “all radiation is bad” picture, when some dose is simply inescapable. It accentuates and panders to our inability to properly assess risk for unusual circumstances, especially with the mention of radon testing kits at the close of the article.

An Individual Medley of Physics

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Michael Phelps has more going for him than immense talent. In Quest for Speed, Olympic Swimmers Use Physics over at Physics Buzz. There’s a brief discussion of the fluid mechanics topics involved in improving swim times, from analysis of swimmers’ motions to the pool design that reduces turbulent water to the new swimsuits.

But what goes unmentioned is the underwater photo, showing the phenomenon known as Snell’s Window. Snell’s law tells us that light moving from one index to another will refract; light entering the pool on its way to the camera must bend toward the normal, meaning that the light entering the lens is compressed from a hemisphere into a cone, and the index of water (1.33) dictating the cone’s apex of 97.5º. Outside of that angle we have total internal reflection from the pool’s bottom; this light cannot escape the pool as it can’t refract and enter the air.

Here’s another picture showing Snell’s Window.

Those Not-So-Good Old Days

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Via Physics Buzz, The things I didn’t believe in graduate school

Graduate school is a hazing ritual, designed to make sure you really truly want to be allowed through the gates into the Ivory Tower. Those who don’t want it hard enough, generally can’t make it through the gauntlet of tests, homework sets, research walls and periodic failures of weather, equipment, software, satellites, and sometimes all of the above. (I experienced all of the above.)
Nonetheless, there are certain things I deeply miss. Today for some reason, I have both journal clubs and observing on my mind. These are two things I never really realized would go away to the degree that they have, and I feel like an entire part of my life was amputated when I wasn’t watching.

[…]

What I hadn’t realized in my return to academia was how little time life as an academic would leave me to just learn. As a graduate student, I remember being aware that the journal clubs were only occupied by postdocs and graduate students. Our advisor (the faculty member who said to us – go off and form a journal club!) would ask us for summaries of what we learned. I hadn’t realized this was his way of identifying cool new papers rather then his way of checking up on us. I remember noticing that the seminars on various sub-fields (stars, galaxies, planets…) were often empty of faculty, with everyone only showing up for the weekly out of town colloquium speakers.

I concur about a lot of the assessment of grad school. It doesn’t just boil down to being smart — obviously there’s a threshold there, but being stubborn enough to slog your way through it all is another component. I almost packed it in at one point, at the point right after my best friend died, the IRS started hassling me (because the grad school “forgot” to tag my fellowship as such, so the IRS assumed it was self-employment income, and claimed I owed them $1500) and then my advisor reminding me that 40 hours a week doesn’t cut it in grad school. Yeah, I know, I’m a little distracted at the moment.

We tried starting up a journal club at work a few years ago, but it didn’t last. It’s not just academia that leaves little time for other pursuits. One thing I’ve noticed is that in the six months I’ve been blogging I’ve been more aware of other research and have done more journal reading than in the last several years.

Chuckles From Above

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Catching up with my blog reading. Via Physics and Physicists, an ArXiv paper by L. B. Okun, The Einstein Formula: E0=mc^2 “Isn’t the Lord Laughing?” detailing some history of “relativistic mass” and the confusion surrounding the term.

The article traces the way Einstein formulated the relation between energy and mass in his work from 1905 to 1955. Einstein emphasized quite often that the mass m of a body is equivalent to its rest energy E0. At the same time he frequently resorted to the less clear-cut statement of equivalence of energy and mass. As a result, Einstein’s formula E0 = mc2 still remains much less known than its popular form, E = mc2, in which E is the total energy equal to the sum of the rest energy and the kinetic energy of a freely moving body. One of the consequences of this is the widespread fallacy that the mass of a body increases when its velocity increases and even that this is an experimental fact.

[…]

Why is it that the weed of velocity-dependent mass is so resistant? First and foremost, because it does not lead to immediate mistakes as far as arithmetic or algebra are concerned. One can introduce additional ‘quasi-physical variables’ into any selfconsistent theory by multiplying true physical quantities by arbitrary powers of the speed of light. The most striking example of such a ‘quasi-quantity’ is the so-called ‘relativistic mass.’ If calculations are done carefully enough, their results should be the same as in the original theory. In a higher sense, however, after the introduction of such ‘quasi-quantities,’ the theory is mutilated because its symmetry properties are violated. (For example, the relativistic mass is only one component of a 4-vector, while the other three components are not even mentioned.)