Of Toyotas, Cosmic Rays and Dark Energy
A recent blog post on Alan Boyle’s Cosmic Log talks about how cosmic rays could be the source of Toyota’s problems. Cosmic rays are high energy particles from outer space that constantly bombard the Earth. If they were to hit sensitive electronics, they have more than enough energy to cause problems, including flipping bits that could lead to serious errors.
How far-fetched is this?
Without really commenting on Toyota’s woes, the issue of cosmic rays affecting electronics is not new, and is certainly a concern. I first learned of it while on the summit of Mauna Kea. I was there to research the Keck Telescopes for The Edge of Physics. I was particularly interested in a spectrograph called DEIMOS (the DEep Imaging Multi-Object Spectrograph), a 12-foot-high, 20-foot-long, 8.6-ton monster.
DEIMOS is capable of taking spectra of about 140 galaxies simultaneously. I was in the Keck II control in Waimea on the Big Island, watching astronomer Sandy Faber and her graduate student putting DEIMOS through its paces.
What intrigued me was that they were taking three “science” exposures for every patch of sky they were observing. I was told that it was to deal with any accidental flipping of bits by cosmic rays. DEIMOS uses a gigantic CCD array to capture the data, and if any pixel were to be hit by a cosmic ray it could momentarily brighten it. Now, when you are gathering faint light from galaxies billions of light years away (sometimes a single photon at a time), a cosmic ray hit could more than mess up the data. Here’s a paragraph from The Edge of Physics that talks about this issue:
Meanwhile, DEIMOS’s flexure compensation system was acting up. Faber talked about the spectrograph as if it were alive:“It’s gotten completely deranged,” she said. A few frantic moments later, everything was fixed, and DEIMOS was ready for use. The telescope, too, was pointing correctly. Faber set DEIMOS in motion with a few clicks of a computer mouse. Somewhere up on the summit, inside the Keck II dome, a slitmask moved into position. Once the telescope was locked and tracking, DEIMOS started moving with it. An image of the sky as seen by DEIMOS appeared on the computer screen in Waimea. Four known stars could be seen in square holes at the edges of the slitmask. These were the alignment stars, and they were slightly off-center. Yan sent commands to the telescope, fine-tuning its position, and checked again. The alignment was better, but not perfect. One more command. A less experienced team might have checked again to see if the stars were centered in their boxes, but Faber and Yan moved on to observing the galaxies. And sure enough, not just the four alignment stars but each one of the nearly 140 galaxies had lined up with their respective slits. Inside DEIMOS, a grating moved into place. The light from each galaxy was now being split into a spectrum. Three exposures followed. Taking three spectra meant that for each pixel they would have three readings. If any pixel were hit by a cosmic ray, making it brighter than usual, it would be discarded (a cosmic ray is unlikely to hit the same pixel twice in three exposures), and the average of the two dimmer readings would be used.
Well, what has all this go to do with Dark Energy? The DEIMOS spectrograph is being used to survey 50,000 distant galaxies. Their spectra will be used to figure out their distance and velocity, which then can be used to study how the expansion of the universe has changed with time. This, in turn, will lead to a better understanding of Dark Energy – the energy of spacetime that is thought to be causing the expansion of the universe to accelerate.