Quirks and Quarks

Freshly detected 'ghost particle' offers a new way to observe the universe

Detection of a neutrino from 'the most violent astrophysical processes' gives scientists a new way to understand the cosmos.
A neutrino, having interacted with a molecule of ice, produces a secondary particle called a muon, that moves at relativistic speed in the ice, leaving a trace of blue light behind it. (Nicolle R. Fuller/NSF/IceCube)

Neutrinos are one of the fundamental particles of the universe, but very little is known about these "ghost particles" because their properties make them elusive to detect.

When the IceCube Neutrino Observatory in the South Pole detected a highly-energetic neutrino last September, that triggered an internationally coordinated effort to hunt down its source. It turns out to be from a blazar galaxy some 4 billion light years away and the results have opened the door to a new way of studying the universe — with neutrinos.

Darren Grant is an Associate Professor of Physics at the University of Alberta and a spokesperson at the IceCube Neutrino Observatory. Here's Quirks & Quarks summer host Britt Wray's conversation with Prof. Grant:

This interview has been edited for length and clarity.

Britt Wray: We've seen extraterrestrial neutrinos before from the sun and a supernova. Why is this new source a groundbreaking discovery for astronomers?

Darren Grant: The neutrinos that we study are at really the extreme energies in the universe. So what we're trying to do are detect the neutrinos from the most violent astrophysical processes. And what we've done is is constructed the world's largest neutrino observatory to do this. Coming back directly to your question, what makes these neutrinos truly special are that they're at significantly higher energies than the neutrinos that we have detected from astrophysical sources previously. To give you an example, the neutrino that triggered everything last September in IceCube was at approximately 300 tera electron Volts —  so nearly a factor of 300 million times more energetic than the neutrinos that come prolifically from the fusion production in our sun.

BW: And what were the main findings after seeing that this very high energy neutrino was out there?

In this artistic composition, based on a real image of the IceCube Lab at the South Pole, a distant source emits neutrinos that are detected below the ice by IceCube sensors, called DOMs. (IceCube/National Science Foundation)

DG: Yeah it was a fantastic ride. This neutrino showed up in our detector and it was a beautiful event. It was a muon-type neutrino and then it produces a muon and that left this beautiful track and that track allows us to point back into the cosmos and look at where the neutrino was coming from. What was discovered was that there was a blazar basically less than a degree from the direction that this neutrino arrived.

BW: What's a blazar?

DG: So blazars are really phenomenal. They are one of the most energetic objects that we know about in the universe. What they are is a supermassive black hole at the centre of these huge elliptical galaxies. That supermassive black hole is rotating and it's accreting matter from the material that surrounds it. Now when it does this, it actually produces these really energetic jets that come out along the axis of rotation for them for the black hole, and when those jets are pointed towards the earth, that's what we call a blazar, because you're basically looking down the barrel of this super energetic jet and it's pointing at the Earth.

BW: How does this discovery of the high energy neutrino open up a new way of observing the universe?

DG: It gives you a new window to look at the universe. It's much the way when people started using high energy gamma rays to look at the universe or X-rays or radio. It gives you a new handle of how to observe the universe through a brand new particle. And that's really what is incredibly exciting with this. I always think of this looking back based on what I read about you know in the late 60's, early 70's when people were really making the first measurements of the cosmic microwave background. That was a new way to really look at the universe and study it. And I think we're taking those same first steps now with neutrinos.

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