Jets from black holes: a violation of physics?

Here’s a commonly asked question when it comes to learning about active galaxies:

How can jets launch from black holes when not even light itself can escape?

blackholebinary3Credit: NASA

The short answer is that the issue arises not with physics but with semantics. It’s very easy for astronomers to say that the jets launch from the central black hole, but what they really mean is that the jet is launched from a region very near the black hole. Since the jet is powered by accretion onto the black hole and since, on the scale of the whole galaxy, the black hole and surrounding accretion disk are essentially pointlike, then it’s easy to simply say the jets start at the black hole. But don’t worry, no material is flowing out from within the black hole’s event horizon so physics is not violated!

But the bigger question touches on some very active research areas at the moment:

What causes these jets to form?

This is very much still being debated within the astronomical community, made difficult because these jet-launching regions are so small and far away that we simply don’t have telescopes powerful enough to resolve an image. So we can’t directly see what’s going on. Instead we try to work things through from physical models and computer simulations. But that’s complicated because there’s a lot of turbulence, magnetic fields, and energetic fluids swirling around on a very small scale, and all of that gets messy very quickly.

So, more work still needs to be done, both observationally and theoretically.

But there is a general picture that is emerging:

A whirlpool of material surrounds and slowly feeds the blackhole. This is called the accretion disk.


Credit: NASA/Dana Berry, SkyWorks Digital

Accretion disks can have strong magnetic fields which you can imagine being twisted into helix along the axis as the disk rotates.

m87.diskCredit: NRAO and the Space Telescope Science Institute

We think this helical magnetic field can sweep up some of the charged material from the disk before it falls into the black hole, and propel it away from the centre in the form of a jet at near the speed of light! If you want, picture a very strong firehose blasting through space. But this analogy doesn’t do justice to these extraordinary machines. The jets are simply spectacular in their sheer power and size.
The following image shows a very nearby radio galaxy, Centaurus A, superimposed on a foreground of Australian radio telescopes and scaled to the correct angular size. So if we could suddenly see in radio light, this galaxy and its jets would dominate the sky!

cena_csiro_1063Credit: NASA/APOD and Ilana Feain, Tim Cornwell & Ron Ekers (CSIRO/ATNF);

Massive black holes aren’t the only places were we find powerful jets, and I think this fact is key in gaining understanding of how the jets form.

We see jets from newly-forming stars (called protostars), jets from binary star systems, jets from pulsars, jets from gamma-ray burst events…

A new Chandra movie of the Vela pulsar shows it may be "precessing," or wobbling as it spins.The Vela Pulsar Jet. Credit: Chandra X-ray Observatory

All on different scales, but all exhibiting the same sort of thin, powerful jet. Thus chances are, the physics is the same in all cases. Some universal engine is triggered when heavy accretion processes occur. The key is to compare jets on all these scales and look for the common physics that works in all cases. It’s very much a work in progress, including in my own research!

As a final note, this question and further details about accretion disks were discussed by myself and other astrophysicists in a recent Naked Scientist podcast: