Radio signals from pulsars are extremely weak, so weak that to catch them astronomers have to use big antennas, as the 64-m dish in Parkes (NSW, Australia), or arrays of antennae working together, as the VLA, the Very Large Array, in Socorro (New Mexico) formed by 27 elements each with a 25m dish.

The rotation of this special kind of stars, and in particular of some of the fastest spinning ones (the so-called millisecond pulsars, rotating up to 800 times per second!)  is so stable that their pulses can be used as ticks from a precise cosmic clock.  Thanks to this characteristic pulsars signals are used to perform a variety of astrophysical experiment in the deep space: measuring the delay in the time of arrival of a pulse at two different wavelengths allows us to estimate the density of free electrons in the interstellar medium (free electrons slow down more efficiently the radio waves emitted at smaller wavelengths); if a pulsar rotates around another star its distance to the Earth change as the Neutron Star moves into its orbit: when the pulsar is closer its pulsations will reach the Earth earlier, when it is further they will take longer. Measuring these differences in the time of arrival of several pulses allow us to determine the size and geometry of the double stellar system and to put constraints on the masses of the two stars; according to Albert Einstein’s theory of General Relativity masses deform the space-time. If the theory is true, the path that a pulse of radio waves emitted by a pulsar will follow, will be longer than expected in presence of a companion star massive enough to deform the space-time. Compact and massive stars, such as Neutron Stars or, even better, Black Holes, will produce big enough distortions in the space-time for this effect to be measurable when observing the ticking of a pulsar rotating around one such object: pulsars in this kind of binary systems are hence unique testbeds to prove the validity of Einstein’s theory. The best such laboratory discovered is the so-called Double Pulsar system J0737-3039A/B, in which the two Neutron Stars orbiting around each other are both emitting as radio pulsars. Thanks to the study of this system astronomers have been able to confirm that Einstein was right at least at 99.95%!! Another prediction of his General Relativity theory is that two bodies orbiting around each other will lose energy coming closer and closer together in time, eventually crushing in one another. The emission loss (and the final merger) generates waves in the space-time, the so-called gravitational waves. Thanks to a detailed study of a big number of pulsars over a large number of years it will also be possible to catch the ripples in the space time created by the gravitational waves. Few international projects, called pulsar timing array experiments, aiming at detecting such faint and intriguing signals are now underway. Should the astronomers succeed these findings will open an entirely new window in astrophysics, not based on the observation of the light (or, more in general on electromagnetic waves) emitted by stars, but based on the distribution of the masses in a distorted the space-time.