“None of this is intuitive”
I am reminded of an oft-repeated crackpot claim that physics should be simple and intuitive, usually in terms of relativity or QM. But they are typically novices at actual physics, and probably never studied rotational systems.
(Reference: The Chewbacca defense)
26 Sure Signs You’re A Physics Graduate
Yeah, pretty much.
At first glance it’s pretty good, although there are one or two things I think aren’t expressed well. One is the divergence of E equation. It’s written as being equal to zero, with the explanation that this is true when there are no charges arund. Well, the other form that’s discussed,
\(\nabla\cdot{\bf E} = \rho/\epsilon_{\tiny 0}\)
is always true. If there are no charges around, rho is zero. I’m not a big fan of equations that are written for specific cases. The first thing that happens is you forget the assumptions and caveats, and then when you try to apply it in general, it fails to work. Use the general equation and then apply the boundary conditions. You’ll be better off in the long run.
Then there is this canard:
Longer wavelengths include heat (infra-red waves)
No! Heat is NOT a part of the EM spectrum. It’s true that for room-temperature items and thereabouts, the bulk of the energy radiated is in the infrared part of the spectrum, but not all IR is from thermal sources, so equating the two is wrong. Furthermore, when you get hotter, you start getting visible light. Like from a stove burner or the sun — all of that light we can see? It’s still radiant heat!
Waxbows: The Incredible Beauty of a Blown Out Candle
Up close the mist looks almost like how water vaporizes at the bottom of a waterfall. Indeed, the wax droplets are enough like water that they reflect and refract light to create a very special sight; I call it a waxbow.
Wide Left: Study Shows that Holders Play Key Role in Field Goal Accuracy
Using the model, the researchers found that if the ball is leaning to the left or right, it will affect the trajectory of the football. And the more it leans, the more pronounced the effect, which is the result of complex interactions between the rotational motion and aerodynamic forces acting on the football.
“For example,” Mazzoleni says, “if the ball is tilted 20 degrees to the left for a 45 yard field goal attempt, it will sail up to 3.5 feet to the left before hooking back to the right.” And any football fan can tell you that 3.5 feet can be the difference between winning and losing. (Just ask the University of Nebraska.)
The paper’s abstract indicates they checked with real results
A case study was performed for which experimental data were available, showing the trends of the flight of the ball captured in our simulations in actual game situations.
The title is a quote attributed to Garo Yepremian, a pretty good kicker who played for several teams from ’66 to’81, including the ’72 Dolphins. Sorry, Garo, the holder is blameless for that horrible decision to try and pass the ball in Super Bowl VII.
This is really cool. Well, technically it’s hot.
I especially like how the drops move uphill in seeming defiance of gravity, since you can’t see the invisible transition to vapor that’s doing the work.
Veritasium has their response to the bullet/block I experiment I linked to and explained, and then explained some more. I absolutely love that they were overwhelmed with responses as people tried to figure it out.
One nit: momentum is not always, always, always, always conserved. It’s conserved when you define the system such that there is no net external force on it. The bullet-block system can be said to conserve momentum during the collision, because it happens quickly and the perturbing impulse is small. But over a longer time, the momentum clearly changes, as the block rises and then falls. One would have to incorporate the earth to have momentum be conserved, but that wouldn’t help solve the original problem. Defining the system so that one can come to a solution is a large step in the setup to any problem.
Denis Joye captured these lightning spectra during a thunderstorm over Paris. They reveal the extreme conditions inside a lightning stroke.
He used a 540 line/mm diffraction grating in front of the camera lens. A slit is usually needed to get good line spectra. Here the very narrow lightning strokes make a slit quite unnecessary.
We see sharp spectrum emission lines superimposed on weaker continuum light.
University of St Andrews scientists create ‘fastest man-made spinning object’
The team then used the miniscule forces of laser light to hold the sphere with the radiation pressure of light – rather like levitating a beach ball with a jet of water.
They exploited the property of polarisation of the laser light that changed as the light passed through the levitating sphere, exerting a small twist or torque.
Light, of course, can have angular momentum, so if the ball was changing the light from linear to circular polarization (or vice versa), the angular momentum change would have to come from the ball, so it would spin.
If my calculator skills are not failing me, the ball had around 10^17 h-bar of angular momentum, which isn’t a lot for a macroscopic object. 10^17 photons is less than a tenth of a Watt-second’s worth of visible light, or 10 mW for less than 2 minutes.
Addendum:
Nick over at Fine Structure points out that the Nature Communications paper is currently free.
The technique they are using is actually using the material as a half-wave plate, which is twice as effective at imparting angular momentum to the sphere as I had described. When circularly polarized light is incident upon it it switches the direction of the polarization, imparting 2*h-bar of angular momentum.
It’s time for the seemingly semi-annual announcement (which you may have already seen) about the new work coming out of some lab (often it’s NIST), where a new experimental technique, or new atom or ion, or some other ingenuity or heroic effort allows them to come up with a better frequency standard measurement. In this month’s game of Clock Clue it’s NIST (plus collaborators), in an optical lattice, with neutral Ytterbium.
Ytterbium Clock Sets New Stability Mark
An international team of researchers has built a clock whose quantum-mechanical ticking is stable to within 1.6 x 10^-18 (a little better than two parts in a quintillion).
This is pretty awesome work (do they get bored with being awesome on a regular basis?) But now comes my standard disclaimer: this isn’t really a clock, it’s a frequency standard. Side note: I had a communication from someone doing a little background on a similar situation and they made the comment that it seems that people in the timing community are kind of sensitive about the distinction between frequency standards and clocks. I don’t know this to be true — I’m the only one who seems to spend any effort making the distinction. I’m not terribly upset by it (I understand why clock is used) and I can’t speak for anyone else. Everyone in the community already knows, so they aren’t confused by it, and they probably don’t care all that much about what goes on in the popular press. But I blog, and this is the sort of thing that matters more in the science communication field, and it affects me when someone says they read about a new clock that NIST build and am I working on that too? And if it happens to me, I’m sure it’s part of certain discussions that happen above my pay grade.
In other words, it matters with respect to people who fund these efforts. I’m reasonably sure there are higher-level inquiries, asking if we’re working on this sort of thing, and why the hell not, and/or not understanding the difference in measuring frequency and time. If you don’t see the difference, you might think that there’s a duplication of effort going on. Even if you get the distinction, you might think this is a technology we should be investigating*.
So let me explain with an analogy that might be easier to understand than timing.
Imagine you are navigating a vessel in eternal fog — there is no way to do any kind of observing for a navigational fix. You want to follow a path — let’s say you want to go exactly north, so you can think of a line drawn on a map, going north, from where you are. That’s the course you wish to follow. (we’re assuming a flat earth here, so all lines north are parallel)
You have a compass that’s pretty good but not perfect. There is going to be some steering “noise” because of this. If the compass exhibits 1 degree of error, that means your velocity vector is going to randomly point anywhere from 1 degree port to 1 degree starboard, randomly. On average your direction will be correct, but that’s for your velocity vector. For your displacement, which is what’s important to you, there will be a random walk, because that’s what the integral of white noise becomes — a random walk. Put another way, even though the direction averages to zero, the errors do not cancel — being off by some angle to port is not immediately followed by being off to starboard by the same amount — the steering error is never undone. It accumulates with each random jiggle of the compass, and there’s nothing you can do about it.
The result is that your good compass means you will random walk some distance to the side of your ideal path that you’d have for a perfect compass. You’re traveling north, and when you reach your destination you might have a random walk to the east by a mile, and that’s bad. You want a better compass.
Let’s say you have a much, much better compass. Good to an arc second instead of a degree — that’s 3600 times better. If you could use it all the time, your 1 mile lateral random walk becomes a few feet. For all intents and purposes, it’s perfect.
However, for some reason, you can’t use it all the time. (insert any plot twist you like for a reason why). Let’s say you can only use it half a day. While you’re using it you accumulate essentially no error in your path, but when you are stuck using the old compass, you still accumulate your error. Since you can use the perfect compass half the time, your random walk error is cut in half, even though the new compass is 3600 times better. The actual improvement in performance is a combination of two things: the precision and the duty cycle.
It’s the same with clocks. Since you are counting “ticks” to keep time, it means that time is an integral of frequency — any clock with white frequency noise will random walk away from perfect time. And you can only count ticks when a clock is running. What do you do when it’s not running? It’s the worst clock in the world when it’s not running! So you have to a have a flywheel — some other clock (in practice a group of them, sometimes called a timing ensemble) to keep time when your über-cool device isn’t running. Even if you add a device that’s 100 times better, its improvement to your timekeeping is limited by its duty cycle, just as with the compasses.
In this case, they ran for 7 hours to make one stability measurement. How often can they do that? Every 3 days? That’s a 10% duty cycle, and even though its stability is 100 times better than currently used clock systems, it would only represent a 10% improvement in your timing ensemble’s performance. Depending on the size of your ensemble, you might see the same (or better) improvement just by adding another continuously-running clock to it, and averaging them all together — ideally, the stability of an ensemble of identical clocks depends on the square root of the number of clocks.
The Ytterbium device is really neat, with stability of a part in 10^18 being a big achievement. There is a lot of neat physics you can do with one, or better yet, two of them. But for the application of timekeeping, the ability to run essentially continuously is very important, and timekeeping is primarily what a clock is for. The better analogy in this case is a stopwatch rather than a clock, just in case you care about the distinction. That doesn’t make for a good headline, though: NIST builds a better stopwatch sounds a bit dismissive and I don’t want to diminish the accomplishment in any way, which is why clock is going to be used even though it’s technically wrong. Until the technology becomes robust enough to run all the time, though, it’s not something that’s going to become part of a true clock.
*it happened when Bose-Einstein Condensates were in the news. Lots of questions about whether we were going to make a clock out of a BEC.