Faster-than-light detection following cosmic collision not what it appears

The first cosmic collision to be seen in both gravitational waves and light has also produced jets that appear to travel four times faster than light itself.

As if the encounter between the two neutron stars, known as GW170817, wasn’t an extreme enough event, it might also help explain the nature of the highest energy explosions in the universe, known as short gamma-ray bursts (SGRBs). They may be due to jets of relativistic material from such collisions, claims a recent paper in the journal Nature.

Even at relativistic speeds, over the course of 150 days a jet’s motion will appear tiny when 130 million light years distant in its host galaxy. Its journey would be approximately equivalent to the distance between one end of a grain of rice and the other, if the rice was on board the International Space Station and the observer was on Earth. 

To detect this tiny change emanating from GW170817 required combining three radio facilities – the Very Long Baseline Array (VLBA) in Hawaii, US, the Karl G Jansky Very Large Array (VLA) in New Mexico, US, and the Robert C Byrd Green Bank Telescope (GBT) in West Virginia, US – effectively creating an interferometer that spanned the globe.{%recommended 6412%}

Between day 75 and day 230 after the gravitational wave event, radio images of the jet appear to have traversed 2.7 milliarcseconds – equivalent to 1.3 light years – in just 155 days. 

Including all other relevant factors, the team “measured an apparent motion that is four times faster than light. That illusion, called superluminal motion, results when the jet is pointed nearly toward Earth and the material in the jet is moving close to the speed of light,” explains lead author, from Caltech, US.

However, this faster-than-light, or “superluminal”, motion doesn’t represent a breakdown of Einstein’s famous Theory of Special Relativity, but is in fact an optical illusion. If a jet is nearly pointed at us, then the two measured locations at day 75 and day 230 aren’t just moving away from the collision but are also moving towards us. That means that the day 230 location is closer than we might have thought, so when we project it back to the same distance as the source to infer the “sideways motion” we infer a much greater speed. 

Astrophysicist Adam Deller from Australia’s Swinburne University of Technology states that to infer such a high apparent speed means “this jet most likely is very narrow, at most five degrees wide, and was pointed only 20 degrees away from the Earth’s direction. To give this apparently ‘superluminal’ sideways motion, the material in the jet also has to be blasting outwards at over 97% of the speed of light.” 

The collision of the two neutron stars created an incredibly energetic event seen initially in gravitational waves, then an expanding “cocoon” of material that was visible across the electromagnetic spectrum. Approximately 60 days later, twin jets successfully broke clear at relativistic velocities and ultimately produced this apparent superluminal motion. 

These jets result in a highly focussed beam of emission, a relativistic effect predicted by Einstein’s Theory of Special Relativity, that can make these events the brightest known explosions in the universe. 

Astronomers have long witnessed such extreme events, termed short gamma-ray bursts, and GW170817 has strengthened the connection between them and colliding neutron stars.

The chances that a narrowly focussed jet happens to point close to Earth is rare, with this work suggesting that the for every GW170817-like event pointing towards us, a thousand don’t.