“Maybe the aliens don’t want to mess up their radio astronomers with transmitters at the hydrogen line frequency, but pi times the hydrogen frequency is something that a lot of people might think of.”īut things are moving on. “Maybe that makes sense,” comments the SETI Institute’s Dr Seth Shostak. The 42 six-metre dishes of the Allen Telescope Array, run by the SETI Institute and the University of California, is conducting a survey of roughly 100 nearby stars at a frequency of pi times 1,420MHz. Some searches have attempted looking at 2,840MHz instead, which is twice the neutral hydrogen line, whilst others have searched at other ‘magic frequencies’ such as 4,462MHz (pi times neutral hydrogen). NASA’s official strategy was to search not just the water hole, but the entire microwave window (the part of the radio spectrum that can pass through the atmosphere and reach the surface) from 1,000MHz (1GHz) to 10,000MHz (10GHz). The majority of SETI radio searches have thus centred around the water hole, but it hasn’t had total exclusivity. Indeed, this possibility was identified as early as 1959, by Giuseppe Cocconi and Philip Morrison in their seminal 1959 Nature paper. Given the importance of the hydrogen line in particular to astronomy, radio telescopes are scanning this area of the spectrum all the time, and could easily see a signal if they happen to be looking in the right direction at the right time. Sandwiched between the hydrogen and hydroxyl lines, the water hole band of radio frequencies are relatively quiet (hence, a radio ‘hole’), and anyone broadcasting on one of the frequencies would come through loud and clear. The microwave window is found between 1 and 10 GHz, with the water hole between 1,420MHz (hydrogen, H) and 1,666MHz (hydroxyl, OH). Measuring the Doppler shifts of the great interstellar clouds of hydrogen that line the Milky Way’s spiral arms allows astronomers to measure the rotation curve of the Galaxy and infer its mass, or do the same for any of the myriad other galaxies our telescopes can see. It was first measured in 1950 by Edward Purcell and Harold Ewen at Harvard, six years after Dutch astronomer Hendrik van de Hulst predicted that it could be detected by radio instruments and could be used to trace the structure of the Galaxy. Neutral hydrogen forms a doughnut-shaped disc that encapsulates the Milky Way’s spiral arms. So what do astronomers do with this? The most common atom in the Universe is hydrogen, and the majority of visible matter in our Galaxy is interstellar hydrogen gas. When the switch takes place – which for a single hydrogen atom is a very rare occurrence, but given all the countless neutral hydrogen atoms in the Galaxy the laws of probability suggest that it’s a surefire bet that there are plenty undergoing the switch at any one time – the electron moves to a lower energy state and a radio photon is emitted, with a frequency of 1,420MHz. Particles have a property called spin, and every now and then the spin axes of the electron and the proton within a neutral hydrogen atom will switch from a parallel configuration to an anti-parallel one (to grasp what anti-parallel means, an analogy is to think of two bar magnets, lined up side by side, but with their north and south poles pointing in different directions to one another). The 21cm line, the most famous in astronomy, is produced by a quantum peculiarity in the nature of the electrons within the hydrogen atom. Combine hydroxyl and neutral hydrogen, and you’ve got a water molecule, H 20. It’s a band of radio frequencies sandwiched between emission from neutral hydrogen – 1,420 MHz (a wavelength of 21cm) – and emission from molecules of hydroxyl (1,666MHz, 18cm wavelength), which is composed of one atom of hydrogen, and one atom of oxygen. Image: J Dickey (UMn)/F Lockman (NRAO)/Sky View. Rather fittingly, this cosmic oasis is known as the water hole.Īn all sky survey of neutral hydrogen in the Milky Way. There is, however, an alternative, a place we can come together and communicate with one another. Unless they have developed the means of superluminal travel and can hop across the light years with free abandon, they too must quail at the immensity of space that acts as a possibly insurmountable barrier to us ever physically meeting. “Where shall we meet our neighbours? At the water hole, where species have always gathered” – Dr Bernard Oliver, former head of NASA’s SETI programme.įor oxygen breathing, water-drinking carbon-based life forms such as ourselves, space is a barren desert, with vast stretches of unbearable nothingness between the oases of life that we are searching for.Īs it is for us, so it must also be for any extraterrestrial intelligence out there.
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