Category Archives: Experiments

I Come to a Different Conclusion

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There’s a video out there in the ether that purports to measure time dilation in a car. I’ve already shown that this can, and does, happen, but you need to have some pretty expensive toys at your disposal to make the measurement.

For a good experiment (I’m perhaps charitably assuming this wasn’t just an out-and-out fraud), one would also want to measure the stopwatches against each other to make sure they were running at the same rate, and calibrate them if they weren’t. Ideally you’d want several clocks, but that’s a little advanced for this level of execution. Then, you’d want to make sure that you weren’t perturbing the clocks with different environments, like temperature differences, so make sure you aren’t blasting the AC on the stopwatch. Finally, you’d want to predict the difference to compare it to the measured difference. Ignoring effects from any elevation changes, a half-hour trip at 60 mph is going to give you a dilation of around 7 picoseconds.

My conclusion is that your stopwatches suck.

Release the Press Hounds!

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I was poking around the toobz (looking for a citation or link to something about “slow light”) and ran across this press release from last year that made me clench and then start grinding my teeth. I have no idea who vets these things, but OMFG, it’s bad. The press latches onto these ideas that are just wrong, and use cutesy buzzwords and phrases to try and connect the story to the urban-legend version of physics that the popular-press readers know, partly because that’s what gets fed to them by the popular press. It becomes that much harder to undo the damage once the bad information gets ingrained, much like when superluminal physics gets reported, only to invariably find it’s anomalous dispersion, and nothing has “broken the lightspeed barrier” or in any way violated relativity.

Here’s the press release: Light and Matter United

Let me say, at the outset, that Lena Hau, et. al, do some amazing, quantum jaw-dropping atomic physics, and I’m not making any arguments or objections about that work. What I’m critiquing is how that work is being reported.

Lene Hau has already shaken scientists’ beliefs about the nature of things. Albert Einstein and just about every other physicist insisted that light travels 186,000 miles a second in free space, and that it can’t be speeded-up or slowed down. But in 1998, Hau, for the first time in history, slowed light to 38 miles an hour, about the speed of rush-hour traffic.

It’s well-known that light traveling through a medium does so at a speed slower than c, and the light that was slowed down wasn’t in a vacuum, so WTF? It was in a specially-prepared sample of a Bose-Einstein Condensate (BEC), called Electromagnetically-Induced Transparency (EIT), which creates a narrow window (in frequency-space) where light won’t be absorbed, and near a resonance you get a change in index of refraction. A rapidly-varying index of refraction, as you get here with a sharp resonance, will slow down the group velocity of light by a large factor. As one can read in the paper (pdf),

[W]e obtain a nonlinear refractive index of 0.18 cm2W-1. This nonlinear index is ~106 times greater than that measured in cold Cs atoms

So the experiment was way cool, but not something that shakes one’s belief about relativity, and the whole bit about the speed of light in a vacuum is a head-fake.
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Vive La Difference

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Electrons and photons in a Mach-Zehnder interferometer show differences in their behavior

The output signal hit a minimum every time the two electron waves cancelled and a maximum when the waves maximally reinforced one another. But as they increased the current, the interference pattern waxed and waned in amplitude in an unexpected way, disappearing altogether at certain points. Researchers in France found similar results the following year.

Please Pass the Lamb Dip

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Zapperz, back from vacation, notes that Willis Lamb passed away recently. The Lamb shift, the energy splitting of the 2s and 2p states of the hydrogen atom, was a huge confirmation of quantum electrodynamics and garnered him the Nobel prize, and you can read more about that here. But that’s not the only effect named after him. Another artifact is the Lamb dip.

The Lamb dip is not a sauce, nor is it related to sheep dip. It arises in a certain geometry of spectroscopy: if you pass a near-resonant laser through an atomic vapor, some of the light will be absorbed. If the laser’s frequency is scanned, you will map out an absorption profile of the atoms, but because they are moving, the absorption depends not only on the transition frequency, but also on the motion of the atoms, which causes a Doppler shift. So your absorption profile is really a representation of the thermal motion of the atoms. At any one frequency the light will be absorbed by those atoms whose motion places them in resonance with the light.

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One More Thing . . .

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The other thing that struck me about bait-and-switch was this

I gravitated toward a scientific life with fantasies of sci-fi movies running through my head, with large machines emitting lightning at the flip of a huge Frankenstein-type switch, or several people poring over softly-glowing computer screens as an experiment produces fantastic data in real-time, and great discoveries are made. I thought this kind of thing actually happened even as I started grad school (even if I had never seen it in my various research summers…)

It doesn’t happen often, but it does happen (depending on your definition of “great”). Back in my first postdoc, at TRIUMF, we trapped radioactive potassium atoms for nuclear-decay tests of the standard model. Or, more precisely, we planned to do this, since the research had progressed only to the point where stable potassium had been trapped when I started working there. Not too long after my arrival we were scheduled for a few stretches of beam time, with an appropriate target to produce the radioactive isotopes we were trying to trap.

Since these were radioactive isotopes, the exact frequency for trapping them was unknown, though the presence of stable isotopes meant (in principle) that the isotope shift could be calculated to some degree of accuracy and narrow down the range of frequencies for the trapping and repump interactions. Since the linewidth of the transition is somewhere around 5 MHz, and you should be able to see a trap with a laser detuning of somewhere between a half a linewidth and several linewidths to the red of resonance, we set up to scan in discrete steps of several MHz, pausing at each step to look for fluorescence at the center of the trapping region — literally looking: we integrated the output from a CCD camera and displayed it on a computer screen, along with a graph of the total fluorescence.
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Oh Dear, Have You Put On Some Mass?

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The topic comes up, as it sometimes does, of the mass-energy equivalence from relativity. There are different tangents to this — what does the equivalence really mean, can you really turn energy into mass, does a photon have rest mass, what is the difference between relativistic mass and rest mass, and is the use of relativistic mass grounds for justifiable homicide, or is one compelled to stop at maiming?

E = mc2 is the equation everyone knows, but what many don’t know is that the equation already assumes one is at rest. The actual equation is E2 = p2c2 + m2c4, which reduces to the more familiar form when the object is at rest. The implications of this are that photons have no mass, the mass term for massive particles doesn’t change when you move — that energy is in the kinetic term, (which renders relativistic mass moot) and also that the mass will increase if you add energy that does not appear in the kinetic term, i.e. extra energy in the center-of-momentum frame appears as mass.

The last concept showed up at Cosmic Variance recently, in the context of the mass of a spinning top

The spinning gyroscope has more energy than the non-spinning one. As a test, we can imagine extracting work from the spinning gyroscope — for example, by hooking it up to a generator — in ways that we couldn’t extract work from the stationary gyroscope. And since it has more energy, it has more mass. And the weight is just the acceleration due to gravity times the mass — so, as long as we weigh our spinning and non-spinning gyroscopes in the same gravitational field, the spinning one will indeed weigh more.

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Classic Physics: Light is an EM Wave

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Classic Science Paper: Otto Wiener’s experiment (1890) at Skulls in the Stars.

By 1890, then, scientists were interested in seeing whether similar results held for light waves: it seems that a number of scientists remained unconvinced that light truly was just another manifestation of electromagnetic waves! One big obstacle stood in the path of such studies: the smallness of the wavelength of light. Hertz’s radio waves had a wavelength of meters, but visible light has a wavelength on the order of 500 nanometers, or 500 billionths of a meter! Such distances cannot be directly observed with the naked eye, so experimental ingenuity was required – and Otto Wiener provided it.

(My own summary of some “classic” physics is progressing)