A passion for space: April 2014

Listen carefully: sound could help us solve one of the enduring mysteries of the Universe

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The audible ‘footprint’ of a gravitational wave could be found in a pulsar’s radiation beam

The famous tagline used to advertise the 1979 film Alien was “In space no one can hear you scream”. Technically, that’s true. In the vacuum of space sound waves are unable to propagate as there’s no medium to carry them. So why are the sounds of the Universe the subject of March’s episode of The Sky at Night? Well, it’s because sound has helped us discover some amazing things about celestial bodies and may help us solve one of the mysteries that continues to puzzle us.

Good vibrations
My first encounter with sounds in space came with the Cassini-Huygens space probe. After its seven-year journey of 
more than 1.5 billion km the spacecraft arrived at Saturn and released the Huygens lander, which tumbled through the dense atmosphere of Saturn’s moon Titan. It 
sent back sounds and images as it fell, and I remember being amazed at hearing sounds from bodies in our Solar System.

Now we’re doing more with sound and we don’t have to send up a spacecraft to detect it, as vibrations emanating from a body can sometimes be seen. In the early 1960s oscillations were observed over the surface of the Sun. The oscillations were detected by measurements of red or blue shifts of small surface patches, which were able to tell us if the area being observed was moving towards or away from us.

The source of the oscillations was unknown at the time and it was suggested that they were due to sound waves generated inside the Sun. Observations later confirmed this and the science of helioseismology was born. Just as seismologists here on Earth can use tectonic events to probe our planet’s interior, so similar techniques can be used to probe the Sun and reveal structure that we otherwise could not see.

Now it’s hoped that oscillations generated by other objects in space could be used to detect one of the true enigmas of modern physics: gravitational waves.

The demise of a star can leave behind some interesting debris. Some leave a white dwarf, some collapse to form black holes while others turn supernova and can leave behind a neutron star – a body the size of 
a city (~10km) with the mass a little less than 
1.5 times that of our Sun.

A pulsar is a type of neutron star that emits a beam of radiation that appears to pulsate – a bit like the light from a lighthouse – due to a misalignment between its rotational and magnetic axes. If this beam is in the right orientation, its radio waves can be detected on Earth and heard as a clicking sound. The frequency of the clicking can be incredibly stable. So stable, in fact, that it may enable us to detect a gravitational wave, as a wave’s presence could cause a minuscule delay in the arrival the pulse’s clicks on Earth. If a delay is found in a number of pulsars then the passing of a gravitational wave could be detected.

So even if screaming is futile, listening is not, as the detection of sound waves and oscillations out there is giving us new insight into the Universe around us.

This column appeared in the April 2014 issue of BBC Sky at Night Magazine



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