Is General Relativity Vindicated Again? Maybe!

Top: Line-of-sight components of the orbital velocities of the radio pulsar J0348 and its white-dwarf companion, measured, respectively, by radio-pulse timing and spectral Doppler shift.  Bottom:  All binary radio pulsars with measured masses and no significant tidal or mass losses are plotted by mass (with corresponding gravitational binding energy) and orbital-velocity parameter β.


Amazingly, in many scientific circles general relativity or GR remains as controversial as the Darwinian theory of evolution in biology. This despite the fact that for all intents, at least within the confines of our own solar system, GR has been vindicated. For example, it very well predicts the annual advance in perihelion of the planet Mercury, and fairly well predicts the deviation of starlight as it passes near to the Sun in our line of sight. Recall here that the closest thing to a strong field object in the solar system is the Sun whose radius is more than 100,000 times its Schwarzschild radius of 3 km. (The  Schwarzschild radius R(s) = 2GM/c²,  where G is the Newtonian gravitational constant, c is the speed of light in vacuo, and M is the gravitating mass. Once a stellar remnant collapses within this radius, light cannot escape and the object is no longer visible .)

The situation is dramatically different for neutron stars, which comprise the components of what we call "extreme binary pulsars."  The neutron star is an ultradense stellar remnant of a  core-collapsed supernovae. A one solar-mass (1 M_⊙) neutron star has a radius of order 10 km, only a few times its R_s. And whereas the gravitational binding energy of an ordinary star is a negligible fraction of its mass, the binding energy of a neutron star can reduce the total mass of its unassembled constituents by as much as 20%.

Thus, the binary pair, labeled J0348+0432, in which the most massive component is a neutron star closely orbited every 2.46 hours by a much lighter white-dwarf star, has attracted astrophysical attention. Though 7000 light-years away, J0348 is quite observer friendly. The white dwarf’s unusually bright hydrogen spectrum yields high-resolution Doppler-shift data and much information about its intrinsic properties. And the neutron star is a radio pulsar whose lighthouse-like radio beam, sweeping Earth every 39 milliseconds, provides an excellent long-term timing reference.

This is the very type of object that would be useful in vindicating GR in an extra-solar context. But why does GR remain so suspect? First, it has problems with quantization, so hasn't yet been reconciled to the other powerful theory of modern physics: the quantum theory. It also has problems with spacetime infinities (i.e. the singularities at the center of black holes), and has difficulty incorporating cosmic inflation (because of superluminal rates of expansion associated with it) and finally GR can't quite cope with the unification of fundamental forces (gravitation, electromagnetism, the strong and weak nuclear forces).

Enter the extreme binary pulsar J0348+0432. The findings so far conform with it being included in a class called  “clean, relativistic” binaries—those with relativistic velocities and negligible losses due to tidal dissipation or mass transfer, hence they lose energy primarily by gravitational radiation. In GR, the lowest-order gravity-wave production by an extended dynamical source is called quadrupole radiation. But many variations on GR predict that dipole radiation will, under the right circumstances, sap a binary’s orbital energy much faster than the quadruple radiation

The report of a team at Max Planck Institute in Bonn, Germany now appears to have supported that this is the case for J0348+0432. (The new binary pulsar was first spotted by team member Ryan Lynch (McGill University) in accumulated data from a 2007 radio-telescope survey).

Figure 1 shows the 2.46-hour oscillation of the line-of-sight velocity components of the white dwarf and the pulsar as measured, respectively, by spectral Doppler shifts and pulsar timing. The ratio of their oscillatory amplitudes measures the ratio q ≡ M_p/M_wd of their masses to be 11.7 ± 0.1. The pulsar mass M_p = qM_wd was determined to be a record (2.01 ± 0.04) M_⊙. The two masses plus the orbit’s period and its line-of-sight velocity components yield a detailed description of the binary orbit: Its plane is inclined 40° from the plane of the sky, and the white dwarf’s orbital velocity is about 0.2% of the speed of light. Its separation from the pulsar is about half the diameter of the Sun.

Figure 2 compares J0348 with other binary pulsar systems, with regard to pulsar mass, orbital velocity, and gravitational binding energy. The figure shows another 2-M_⊙ pulsar orbited by a white dwarf. But that binary’s orbital velocity is much slower (see Physics Today, January 2011, page 12). On the other hand, the plot shows a unique double pulsar—two pulsars orbiting their center of mass with a relative velocity slightly faster than that of J0348.

Given the new binary’s measured parameters, GR predicts that its present 2.46-hour orbital period P_b should be decreasing by about 8 µs per year as the orbit shrinks due to energy loss by gravitational radiation. To test that prediction, Lynch and Paulo Freire began continual pulsar timing with Puerto Rico’s Arecibo radio telescope in April 2011. Now, based on two years of timing data, the orbital period’s measured time derivative is 1.05 ± 0.18 times the GR prediction. So thus far there’s no evidence of new physics.  Fig. 3 (below)



shows the constraints imposed on the masses of the binary pulsar J0348 by measurements of the white-dwarf mass M_wd, the mass ratio q, and the time derivative of the orbital period. In each case, the triplet of lines indicates one standard deviation.

The yellow segment is the 1-standard-deviation confinement imposed on the binary’s mass plane by the measurement, assuming that GR is the correct theory. The fact that the intersection of the measured q and M_wd lines, which involve no assumptions about GR, falls nicely in the middle of the calculated swath indicates that GR has thus far passed the team’s radiative test. With increased observing time t over the next few years, the uncertainty on should shrink rather rapidly—like t^−5/2.

Will it satisfy all critics? Hardly!  For example, one group at Boston University has objected based on Augur electron simulations - that challenge the claim that the high energy spectral peak is produced by Augur electrons. Thus, they claim there is "no correlation between electron energies and any information about the intrinsic properties of the pulsar." (The GR proponents have tried to tie the electronic behavior of the instrumental detection LEDs to electrons tunneling out of a quantum well to form peaks in the spectrum observed.)

In any case, as with all good physics we shall have to await confirmation, or at least wait another few years to see if indeed the uncertainty on   shrinks.  For another GR-related blog post, see:

http://brane-space.blogspot.com/2012/11/solar-oblateness-may-be-constant-over.html