Q&A: a gravitational wave astronomer
Voyager 1’s discovery of a ‘plasma hum’ outside our Solar System may be critical for finding future gravitational waves
What is Voyager 1 doing now? It’s over 40 years since the Voyager 1 and 2 spacecraft were launched in the late 1970s. They have now left the sphere of the Sun’s influence – the heliosphere – and they are both sampling the conditions in the interstellar medium. This connects up with what I do, using ground-based radio telescopes to also study the interstellar medium.
How did you use data from
Voyager 1 in your recent study in Nature Astronomy?
We analysed the signal from Voyager 1’s Plasma Wave System (PWS), which uses antennae that hang off the spacecraft and measures low frequency radio waves. We look for oscillations that are at a frequency which characterises the density of the ionised gas (plasma) in the insterstellar medium
What did you find?
We learned that the plasma density changes as Voyager 1 moves along, and some of that change is due to turbulence in the interstellar gas. It’s thin gas, less than one particle per cubic centimetre – but it’s important on an astronomical scale. Other people had used this instrument to see specific events caused by big flares called coronal mass ejections. We were asking: is there a more continuous signal? We came up with a way of pulling that signal out of the noise in the data and that’s how we identified the ‘hum’.
What do you think is happening in the interstellar medium to cause this ‘hum’?
It is electromagnetic waves coming from the oscillation of the plasma, in particular ionised hydrogen – protons and electrons unbound from each other. The electrons are oscillating back and forth past the heavier protons. The question is, what excites this? With the big events we’ve previously seen, the trigger is a coronal mass ejection. For this signal, though, the cause is not really understood. It could be some residual effects from coronal mass ejections, or it might just be a property of having hot gas that’s moving, or it might be extremely energetic particles from cosmic rays. Whatever the mechanism, it’s not a single event – it’s something fairly persistent.
Could we listen to the ‘plasma hum’?
No, it’s not really sound, it’s a radio signal that’s been measured. However, it’s at the same frequency that we can hear, so you can translate the electromagnetic signal into sound the way a radio does.
What are the implications of your findings?
There are several implications. One is on the role of turbulence in the interstellar gas. New stars are forming out of gas clouds and there’s turbulence there as well. So there are these deep physical connections between the very thin gas and dense gas that ultimately forms planets and stars.
The other is cosmic rays: they are energetic particles, so how do they get that energy? The understanding is that shock fronts accelerate those particles; but they don’t move in a straight line, they make a random ‘drunkard’s walk’ because of turbulence in the interstellar medium. Voyager 1 may tell us about what that looks like on a smaller scale.
The last connection is more astronomical. We know about twinkling stars, their light varies due to turbulence in Earth’s atmosphere. We see a similar effect on radio waves viewed through interstellar turbulence; it can limit our precision in measurements.
Why does interstellar turbulence matter?
The NANOGrav project we are involved with at Cornell University uses pulsars as gravitational wave detectors. LIGO (the Laser Interferometer Gravitational-Wave Observatory) measures the gravitational waves to look at neutron stars using a ground-based detector that’s a set of lasers. Instead we’re using radio telescopes to look for gravitational waves from bigger black holes. These produce waves with long wavelengths, like a lightyear, so when they propagate through the Solar System it causes space-time to contract, or stretch. That’s what we are trying to measure. It’s a tiny, tiny effect so we have to worry about this interstellar medium stuff to make precise measurements. Gravitational wave detection is the biggest driver for us understanding all this.