Oh, this is just awesome, from a geeky reference frame: The case of the 500-mile email
“What’s the problem?” I asked.
“We can’t send mail more than 500 miles,” the chairman explained.
I choked on my latte. “Come again?”
“We can’t send mail farther than 500 miles from here,” he repeated. “A
little bit more, actually. Call it 520 miles. But no farther.”
(There’s a factor of two that’s not explained in the story, for round-trip vs. one-way, but still …)
[P]olymers in a solution that isn’t at a uniform temperature behaved in an unexpected way, with the longer molecules drifting toward the coldest region and the very shortest ones drifting toward the warmest region.
Reminiscent of the Brazil Nut sorting problem
Magnetic self-assembly in slow motion
90 1/8″ small magnets arranged in a 9×10 matrix with a larger magnet brought close. Beautiful self-assembly occurs
Slower and slower motion, so you can see what’s happening.
Hi, I’m Rob Gallagher and I’m a MagnetNerd.
[…]
Neodymium Magnets are just plain cool. It’s totally amazing how something so small can have such strong magnetic forces. I enjoy showing others the amazing things that can be done with Neodymium Magnets. So far I have created 53 Videos of my experiments and designs on YouTube and have incorporated most of them into this website.
More magnet-relates stuff than you can shake a magnetic stick at.
Super clocks: More accurate than time itself
An article discussing the progression of atomic clock technology, and also relating to something I posted earlier, a discussion of what happens when the next generation of atomic clocks is deployed: the clocks won’t be the limiting factor in determining the time.
To tell the time consistently, all clocks need to be at a known height relative to Earth’s “geoid”, an imaginary surface that links points at which the gravitational field has the same strength. But the height of this geoid varies over time at any given place by up to 20 centimetres, because of effects such as tectonic movements, glacial melting and changes in ocean levels, and varying atmospheric pressure. Changes of that magnitude could wreak havoc with any attempt to establish a global time standard at an accuracy of 1 part in 1018 or better
One of the things that always glossed over in these discussions is that almost everything that is called a clock is actually a frequency standard, which is part of a clock. Clocks run continuously, because you are measuring a phase, and frequency standards don’t. Now, there’s a caveat here in “running continuously,” because even commercially-available clocks will skip measurement cycles to do self-diagnostics. During those skips in measurement, as with the time between measurements, the frequency is maintained with some oscillator. Usually this is a quartz crystal, which typically has excellent short-term stability. The important point becomes how long the clock is running on the “flywheel” oscillator and what kind of degradation that introduces. The latest generation of frequency standards run for several hours, but then are shut down for extended periods of time, which is not surprising for a cutting-edge kind of experiment. But while the frequency standard is not running, the clock’s performance approaches whatever the flywheel performance is, whether that’s cesium beam clocks or hydrogen masers, etc., or some ensemble made up of several clocks.
So when these stories appear touting the great performance of cutting-edge clocks, there’s an unwritten implication that we will be getting an improvement in the flywheel operation as well, to be able to leverage this improved frequency-standard performance.
Quantum twist: Electrons mimic presence of magnetic field
[O]n the surface of certain materials collective arrangements of electrons move in ways that mimic the presence of a magnetic field where none is present. The finding represents one of the most exotic macroscopic quantum phenomena in condensed-matter physics: a topological Quantum Spin Hall effect.
Bouncing atoms take a measure of gravity
Pretty cool. The experiment uses a technique used in interferometry, where a laser standing wave induces an absorption-emission cycle of an atom, so it receives a momentum “kick” of twice the photon momentum (one kick for absorption, and once for emission), effectively making a diffraction grating out of light if they were moving through the standing wave, and uses this to bounce the falling atoms.
If the colour of the laser light and the frequency of pulses are set correctly, the atoms will be set bouncing and the acceleration due to gravity can be deduced from the experimental parameters and Planck’s constant. The team managed to sustain this bouncing for about 100 cycles, which they say is the equivalent of dropping the atoms about 2 cm in a standard experiment.
We’re coming up on the golden anniversary of some very important experiments that were milestones in confirming relativity and were enabled by a breakthrough in nuclear physics, the Mossbauer effect. Mossbauer’s discovery (published in 1958) of the Mossbauer effect (what were the odds of that happening?) was that nuclei in a lattice had essentially no recoil when emitting gammas, since effectively they shared the mass of the entire sample. Normally, the conservation of momentum from the recoil of a nucleus shifted the gamma’s energy out of resonance, meaning that the gamma would not be reabsorbed by an identical nucleus; even though the recoil from the emission of a 100 keV gamma would only cause a shift of a few thousandths of a eV in the gamma’s energy, this is significantly larger than the width of the transition. However, effectively increasing the mass of the emitter by even a small fraction of Avogadro’s number — which you can do with just a speck of material — all but eliminates that energy shift, and the ground-state nucleus will absorb the photons emitted by the excited state.
This incredible new tool set the stage for several experiments in General and Special Relativity. One is the famous Pound-Rebka experiment that took place at the Jefferson Lab tower at Harvard. The premise of the experiment was that a photon climbing or falling in a potential well would be red- or blueshifted, and this could be compensated for by moving the source; when the Doppler shift canceled the gravitational effect, the photons would be on resonance and be absorbed by the target, but at other speeds would not be absorbed. This would cause a variation in the number of photons striking a detector. The gravitational redshift is small, \(gh/c^2 = 2.45 x10^-15 \) and the Doppler shift necessary to compensate is just \(7 x 10^-7 m/s\) . This sensitivity is doubled by reversing the experiment and looking for the blueshift of a falling photon. The source, Fe-57 (from a decay in Co-57) has a transition at 14.4 keV, and is narrow (about 10^-8 eV) owing to a ~100 ns lifetime.
The simplicity of the basic experiment masks some subtleties of device. The source and absorber needed to be specially prepared; the Co was diffused into a thin Fe sheet so that the source was in a very thin layer near the surface, and for the target a thin layer of Fe was electroplated onto a Be disc. These were vertically separated by 22.5 meters, and to reduce absorption by air, this space was taken by a mylar bag filled with Helium. The source was put on a transducer, i.e. a speaker cone, and oscillated at low frequency. To eliminate thermal effects, since the difference in thermal motion between the source and target materials could shift the nuclei out of resonance, they were stabilized to the same temperature.
The second order Doppler shift resulting from
lattice vibrations required that the temperature
difference between the source and absorber be
controlled or monitored. A difference of 1ºC
would produce a shift as large as that sought, so
the potential difference of a thermocouple with
one junction at the source and the other at the
main absorber was recorded. An identical system
was provided for the monitor channel.
The results agreed to about 10%, and a later experiment by Pound and Snider agreed to 1%
But it doesn’t end there. A lesser-know cousin to this experiment was carried out to observe the frequency shift in a rotating system. Once again using the Mossbauer spectroscopy of Fe-57, the source and target were mounted on the axle and rim of a cylinder, which was then rotated at some speed. In this case, one can look at the effect either by viewing this as a pseudo-gravitational potential or as a kinematic time dilation effect (both approaches, not surprisingly, yield the same answer), with the fractional frequency shift of \(v^2/2c^2 \) . The cylinder was rotated at different speeds and the increase in the counting rate was observed, as the target moved out of resonance with the source due to the frequency shift of the target.
Gravitational Red-Shift in Nuclear Resonance
Phys. Rev. Lett. 3, 439 – 441 (1959)
R. V. Pound and G. A. Rebka, Jr.
(Theory)
Apparent Weight of Photons
Phys. Rev. Lett. 4, 337 – 341 (1960)
R. V. Pound and G. A. Rebka, Jr.
(Experiment)
Measurement of the red shift in an accelerated system using the Mossbauer effect in Fe-57
Phys. Rev. Letters. 4, 165 (1960)
H. J. Hay, J. P. Schiffer, T. E. Cranshaw, and P. A. Egelstaff
Measurement of Relativistic Time Dilatation using the Mössbauer Effect
Nature 198, 1186 – 1187 (22 June 1963)
D. C. Champeney, G. R. Isaak and A. M. Khan
Doppler effect reversed by metamaterial
This latest research takes the principles of negative electromagnetic refraction and applies them to acoustic vibrations. Here the parameters to be made negative are material density and modulus, the latter relating to a material’s elasticity. Until now engineers have only created metamaterials with either of these properties, but Kim and colleagues have successfully combined them to create the world’s first “double-negative” acoustic metamaterial (arXiv:0901.2772v2 ). Their acoustic tube is constructed from thin membranes under tension fed by a carefully controlled air flow, and this manages to create a negative phase velocity for sound travelling through.