Jets that Appear Faster than Light
During the aftermath of the recent neutron star merger, ejecta from an initial explosion formed a shell around the black hole formed from the merger. A jet of material propelled from a disk surrounding the black hole first interacted with the ejecta material to form a broad “cocoon.” Later, the jet broke through to emerge into interstellar space, where its extremely fast motion became apparent. The jet appeared to be traveling four times the speed of light. This phenomenon is known as superluminal motion. It arises due to the jet traveling at nearly the speed of light and at a small angle relative to our line of sight. Since the jet is moving at nearly the speed of light, when a particle within the jet emits a bit of light the particle doesn’t fall far behind the bit of light it emitted. When a long time passes and the particle emits a second bit of light, it will do so close to the first one and when the light reaches us the particle will appear to have been moving faster than light.