The Rogues' Gallery

Aliens Cloak Distant Pulsar (Either that or General Relativity Did It)

Bob Novella January 15, 2015 4 Comments

Astronomers have precisely determined the mass of a pulsar for the first time (right before the Pulsar completely disappeared from view).

When astronomers found the binary system J1906 in 2006, they quickly realized they had something special. The objects rotating around each other were both compact, massive, and incredibly close. One partner was hard to identify exactly; most likely being either a neutron star or white dwarf. The other was a no brainer, it’s lighthouse-like radio signals clearly made it a pulsar.

Clearly these two degenerate partners have an intense relationship but just how intense? Well, they both weigh in at around 1.3 solar masses. Not a huge weight, all things considered, but the pulsar packs all that mass into just a few miles which is pretty extraordinary when you think about it. The real kicker though is that they are both separated by only about 900,000 miles, about a 100th of the distance between the earth and sun. Those kinds of masses in such close proximity also translate into an amazing orbital period of only 4 hours (the 2nd fastest ever found).

Such incredible masses, so close (and moving at about 200 miles per second around each other) means that Einstein would love the chunk of space these stars inhabit. By that I mean these conditions are ideal to showcase the weird behaviour described by his General Relativity. For that reason, these stars are also referred to as relativistic binaries.

This manifests itself as a warping of space-time so extreme that it actually slowly alters the orientation of the spin axis of the pulsar. This is called Geodetic Precession which is easy to imagine if you think of a spinning top that is no longer pointing straight up but wobbling back and forth as it continues to rotate.

The earth precesses as well. Our axis of rotation points the earth’s north pole almost directly to what we call the Pole star or North star. If you were at the north pole and looked up, this star (Polaris) would be almost directly overhead. Due to precession however, we will not always look to Polaris as the North Star. In 14,000 AD, precession will point us most directly to the star Vega instead. Earth’s precession is caused by the gravitational pull of the moon and sun on our equatorial bulge (the spinning earth bulges at the equator and flattens at the poles). Yet, our mass warps the space we travel in like this pulsar does, only to a much lesser extent. For this reason a very tiny part of earth’s precession is due to the warping of spacetime.

The relativistic binaries were closely studied for years and it eventually became clear that geodetic precession would cause the beams of collimated radio waves that were aimed right at earth to slowly drift off target as the pulsar’s axis of rotation precessed in the warped spacetime.

And guess what? That’s exactly what happened.

Abstract

Animation of Binary System: Exquisite visual depiction of exactly what’s happening.

Image Credit: