Magnetic fields can crack like a whip. If you shake the handle of a whip, a wave travels along the whip, increasing in speed until it snaps at the end. The energy of that initial shake can only move along the whip, hence the motion of the wave. In a similar way, the jets of a black hole can shake like a whip due to magnetic fields interacting with the plasma of the jet.

Model of an s-wave. Credit: NASA/JPL-Caltech

When jets of ionized plasma interact with magnetic fields, transverse waves can travel along the ionized jet similar to the way waves travel along a whip. These transverse waves are known as Alfvén s-waves. As the black hole wobbles, the energy of the oscillations most easily travel along the jet, producing these waves. In a recent paper in the Astrophysical Journal, these Alfvén waves have been observed.

In the paper, they are referred to as superluminal Alfvén waves, because they appear to travel faster than light. That doesn’t mean the waves actually are breaking the light barrier. Their superluminal appearance is actually a well known effect due to the fact that the waves are traveling near the speed of light with the jet pointed mostly in our direction. The light from the base of the jet takes much longer to reach us than light from the end, so the waves appear to travel faster than light.

What’s important about this work is that it allows us to better understand the complex interactions of astrophysical plasmas, which (despite what some claim) astrophysicists have been studying for quite some time.

Paper: M. H. Cohen et al. Studies of the Jet in BL Lacertae. II. Superluminal Alfvén Waves. ApJ 803 3 (2015)

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The figure below is a set of radio images of a mass of plasma (a jet) leaving the quasar 3C 279.  If you look at the scale at the bottom, this mass of plasma appears to travel about 25 light years in about 7 years.  In other words, it appears to be traveling more than 3 times faster than the speed of light.  How is this possible if (supposedly) nothing can travel faster than light?

The answer is that such a “superluminal” jet is actually an optical illusion.  What’s really happening is that the jet is moving at a large fraction of the speed of light in our general direction.  As an example, suppose the jet were to move at an angle from our line of site such that for every 4 light years it moves toward us, it also moves 3 light years across the sky (perpendicular to our line of sight).  This means it will have actually traveled 5 light years (think of a 3-4-5 right triangle).

Since the jet travels at close to the speed of light, it takes a bit more than 5 years to travel that distance.  During this time, the light that left the jet at the beginning has travelled about 5 light years.  At this point, the jet has moved 4 light years toward us, so new light from the jet is only about 1 light year behind the light that left at the beginning.  This means as the light reaches us, we will observe the beginning to end in only about a year.  But the jet has also traveled 3 light years across our field of view.  As a result, we will observe an apparent motion of 3 light years in about a year, giving the appearance of superluminal motion.

Sometimes appearances can be deceiving.

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