ScienceWise - Autumn 2013

Moon beams

Why one scientist isn’t worried about alien invasions

The Square Kilometre Array project (SKA) is an international collaboration to build the largest and most powerful radio telescope the world has ever seen. Part of the instrument will be located in the deserts of Western Australia which are currently home to the Murchison Widefield Array (MWA) radio telescope.

Graduate student Ben McKinley from the ANU Research School of Astronomy and Astrophysics, and supported by the Centre of Excellence for All-Sky Astrophysics (CAASTRO), is part of a team of astronomers analysing scientific data from the MWA, which is the low-frequency precursor instrument to the SKA. This work will ultimately inform the design and construction of the giant Square Kilometer Array and help scientists to interpret the vast amounts of data that the SKA will produce. 

“What I’m especially interested in is detecting the radio waves emitted by hydrogen in the early universe, that have been stretched to long wavelengths by the expansion of the Universe” Ben says, “And that’s precisely what the MWA was designed to do.”

But just as visual astronomers struggle with light pollution, radio astronomers have to contend with the blanket of human generated radio waves that bathe the Earth.

“There are very few places on Earth which are so sparsely populated that man-made radio waves don’t impact observations,” Ben says, “Western Australia is one and that’s one reason why we’ll be locating part of the SKA there.” 

In addition to a low-noise location, scientific instruments also need to be calibrated before they can produce accurate measurements so the scientists were looking for a suitable reference source to calibrate the MWA. They came up with the idea of using the Moon.

Any object warmer than absolute zero emits electromagnetic radiation – something physicists call black body radiation. The hotter the body is shorter the wavelength. An intensely hot light bulb filament emits visible light where as a warm human body emits mostly in the mid infrared – which is what burglar alarm sensors can detect. Although the peak of the moon’s black body emission isn’t at radio wavelengths, it does provide enough radio signal to register on a radio telescope and could be used as reference that is smooth and predictable. 

However, when the astronomers turned the telescope to the moon they saw a huge spike in the frequency range between 87.5 and 108 MHz. This corresponds precisely with the band allocated to FM radio transmissions, which leak into space and are reflected back down to Earth by the Moon. “It was kind of annoying in a way because it interfered with our calibration of the telescope,” Ben explains, “But then it occurred to me that perhaps we could use this to calculate just how much radio frequency energy we are radiating into space in the FM band.”

Over the years, many people have questioned whether such Earth based radio and television transmissions might be detected by possible alien civilizations out in Space and if that might attract them to visit. This idea has formed the basis of numerous science fiction movies foretelling either benign aliens that help humanity with its problems or evil invaders wiping us out and taking over the planet.

“I was interested in the question of just how easy would it be for an extraterrestrial civilization to detect us by our radio and TV transmissions,” Ben explains. “And the reflections from the moon provided the perfect way to measure the total integrated radio output of the Earth, at least within the FM radio band.” 

So how easy would it be to detect us? 

“Not at all,” Ben says, “Although it may seem like a lot of transmission, it’s miniscule compared to the radio noise the sun produces. The only way you’d be able to identify our ‘intelligent’ emissions is if you had a radio telescope with enough resolution to see the Earth and Sun separately, and enough sensitivity to detect such a faint signal. This translates to having a very big telescope!”

“If you had something as powerful as the Square Kilometre Array located on the nearest star Proxima Centauri, then you might just be able to do it. Much further out into the galaxy and I think even with super advanced technology the laws of physics would make it almost impossible to detect us.” Ben says.

So if you’re waiting for aliens to come help us mop up all the CO2 we’ve pumped into the atmosphere or cure cancer perhaps you’ll be waiting a long time. And if you’ve been worried that fleets of menacing flying saucers will descend on us from above, well, perhaps you can relax a little!

“Personally I don’t believe that aliens would be that likely to be hostile,” Ben says, “If they’d managed to develop the technology for interstellar travel without using it to destroy themselves in the meantime, then they probably wouldn’t destroy us either. But in any case, I seriously doubt that’s they’d find us from our FM radio emissions! ”

You can find out more about this story on you tube:

http://youtu.be/aNXnqkGHBAA

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