Interferometers

I was interested to read that Miller was counting fringes with white light. Maybe that was the best he could do, but a single bright line from a sodium arc would have produced a clearer result.

I consulted for a customer bitten on the ass by polychromatic white light, sloppy experimental design. They were looking at charge leakage in the gate transistors of liquid crystal displays. They had two setups that they used interchangably to measure this, one with narrow-band filtered green light, one with white light. They did not differentiate between them, but used the results to turn the knobs on the manufacturing process. Sometimes they would get leakage, sometimes not. Well, it turns out that the blue portion of the white light carried enough photon energy to activate conduction paths that green light did not. Their actual problems were many. They weren't cleaning off all the gunk left behind from a photoresist removal step. The grain boundaries in the amorphous silicon were sometimes collinear to the transistors, which would leak when photons activated them. They needed to change the process, and the alignment of the transistors, and how they measured the results. People make mistakes, and one advantage of bringing in consultants is that consultant mistakes don't always overlap those of the core team.

The most accurate interferometer I've seen up close is the LIGO Laser Interferometer Gravitational Wave Observatory up at Hanford. "Up close" meaning carefully the central mirrors, the control center, and some examples of the equipment being upgraded, as well as conversatoins with the physicist/computer-geeks that run the place (on the same Linux distro I use). LIGO is to the experiments of a century ago as the Saturn V is to a bottle rocket, though that analogy does not capture enough orders of magnitude.

All these experiments sit under an atmosphere weighing many tons per square meter. If overhead mass density entrains aether, a building doesn't matter much. LIGO runs its multi-kilometer monochromatic laser beams through above-ground steel tubes adding another percent or so of mass, far less than the atmosphere or a building roof, and is measured to accuracies that make gnat's eyelashes look like continents. If there was aether drag of more than micrometers per second, this monster (and its twin in Louisiana) would see it. There isn't.

What they are looking for is gravitational waves, and the dirty little secret is that they would need an instrument with a thousand times more resolution to see them, ant farts compared to the tsunami that an aether wind would produce. To see those ant farts at all, much less measure their magnitude, would require an orbiting interferometer many times the scale of the Earth, and far from the gravitational perturbations of Earth and moon. Of course, you would still need to cancel out the tidal effects of all the big masses scattered about, all the planets out to Neptune, and smaller objects like kilometer- scale asteroids passing nearby. So you would probably end up with a new kind of asteroid detector, rather than something that detects gravity waves from co-orbiting pulsars or supernovae explosions thousands of light-years distant.

We learn more from watching those pulsars slowly wind down (again verifying general relativity), and watching the time difference between the arrival of the neutrino pulse from supernova 1987a and the arrival of light 8 minutes later, as expected from our stellar models.

One of the big events of the summer, beginning about now, is the infrared light reaching Keck Observatory in Hawaii, from the collision of a gas cloud with the very-very-big-mass at the center of the galaxy (which the evil astronomers call a "black hole"). We get to watch the huge burst of energy as some of that gas falls in. I got a chance to talk to Dr. Andrea Ghez, the head of the UCLA Galactic Center Research Team, and learn about her observations and expectations. There are almost 100 stars orbiting the big invisible mass with enough velocity to see high doppler shifts. We won't see any gravitational waves from the infall, but we see plenty of gravitational light-bending, and expect to see LOTS of redshift in the infalling gas as it approaches what evil astronomers call the event horizon. Don't try this at home, kids!

BTW, my late friend Robert Forward did his graduate work at University of Maryland, building the first "Weber bar" gravitational wave detector. They did not expect to see anything, but like most first experiments, they would learn about the errors that such equipment generates. In the extremely unlikely event that they actually found something unexpected, they would have turned physics upside down and got a trip to Stockholm out of it. This is actually the same motivating impulse for LIGO - yet another rehersal for the giant experiments of the future, and a chance to discover the unexpected anomalies that justify experimental physics.

Bob was one of the most "out of the box" thinkers that I know, the first to design efficient "antimatter factories" for particle accelerators, the first to design survivable orbiting tethers, and one of the first to design useful space missions for them. One of the great regrets of my life is that I did not shut down a startup I was building, and instead spend the last six months at his side, changing bedpans and writing down his last ideas. That has made me more diligent about writing down my own ideas.

Interferometers (last edited 2014-06-10 19:49:01 by KeithLofstrom)