Binary Pulsars and Gravitational Waves

Nobel Prizes in Astrophysics & Cosmology – Part 6

(A Twelve Part Series)

Russel Hulse and Joseph Taylor

Einstein's General Theory of Relativity predicted that gravitational waves could carry energy from a system, but no one had seen evidence for gravitational waves prior to Hulse and Taylor's results. This was the first physical evidence of the existence of gravitational waves.

-       NASA

Diagram from New Scientist magazine

 

The Nobel Prize is equated with the pinnacle of human achievement in both popular perception and professional esteem.  Since it was first awarded in 1901, the annual Nobel Prize for Physics has gone to major contributions in Astrophysics and Cosmology related fields only on eleven occasions. The first four awards (1967, 1974, 1978, 1983) were the subjects of earlier articles (see here 1,2,3,4). The next was in 1993, jointly to Russel Hulse and Joseph Taylor Jr for their discovery of binary pulsars in 1974.

 

Russel Hulse (right) and Joseph Taylor Jr

Pulsars – Nature’s Cosmic Timekeepers

Discovery

Pulsars were discovered in 1967 by Jocelyn Bell Burnell (see picture below), then a graduate student at Cambridge (see also here), while analyzing radio signals from a newly built radio telescope. The signals were strikingly regular—sharp pulses repeating every 1.337 seconds. Soon, the astrophysical explanation became clear: pulsars are rapidly rotating neutron stars. 

This discovery provided the first direct observational evidence for neutron stars, objects that had been theoretically predicted in the 1930s following the discovery of the neutron. What began as a mysterious radio signal has become one of the most powerful tools in astrophysics.

What Is a Pulsar?

A pulsar is a highly magnetized, rapidly rotating neutron star that emits beams of electromagnetic radiation from its magnetic poles. If one of these beams sweeps across the Earth as the star rotates, we observe a pulse—much like the flash from a lighthouse (see representation below).

Key characteristics:

·       Mass: ~1.4 times the mass of the Sun

·       Radius: ~10–12 km

·       Density: Comparable to that of an atomic nucleus

·       Rotation period: From milliseconds to several seconds

·       Magnetic field: 10⁸–10¹⁴ gauss (trillions of times Earth’s field) 

Birth of a Pulsar

Pulsars are born in core-collapse supernovae. When a massive star exhausts its nuclear fuel:

1.     The core collapses under gravity.

2.     Protons and electrons combine to form neutrons.

3.     The collapse halts due to neutron degeneracy pressure.

4.     Conservation of angular momentum spins the core up dramatically.

5.     Magnetic field lines are compressed and intensified.

The result is a spinning neutron star—sometimes visible as a pulsar embedded within a supernova remnant (see picture below), such as the famous Crab Pulsar. 

Crab Nebula

The Pulsar Emission Mechanism

The pulsar’s magnetic axis is usually tilted relative to its rotation axis. Charged particles are accelerated along curved magnetic field lines near the magnetic poles, producing radiation via:

·       Curvature radiation, produced by ultra-relativistic charged particles accelerating along the extremely curved magnetic field lines near the neutron star’s surface,

·       Synchrotron radiation, the intense electromagnetic radiation (from infrared to X-rays) produced when high-energy charged particles, like electrons, are forced to travel in curved paths by magnetic fields as in particles accelerators

·       Inverse Compton scattering, a process where low-energy photons gain significant energy by scattering off high-energy, relativistic electrons

This emission spans a wide range of the electromagnetic spectrum:

·       Radio (most common)

·       Optical

·       X-ray

·       Gamma-ray

The pulse regularity reflects the star’s rotation, making pulsars extraordinarily stable clocks. 

Types of Pulsars

(a) Normal (Rotation-Powered) Pulsars

·       Periods: ~0.1–10 s

·       Gradually slow down due to energy loss

·       Example: Vela Pulsar

(b) Millisecond Pulsars

·       Periods: 1–10 ms

·       “Recycled” by accretion from a binary companion

·       Extremely stable—rival atomic clocks in precision

(c) Binary Pulsars

·       Found in orbit with another star or neutron star

·       Provide laboratories for testing general relativity

(d) Magnetars (a rare type of neutron star with incredibly powerful magnetic fields)

·       Ultra-strong magnetic fields (~10¹⁴–10¹⁵ gauss)

·       Emit X-ray and gamma-ray bursts

(Not all magnetars show classic pulsar behavior)

Binary Pulsars

A binary pulsar is a pulsar that is gravitationally bound to a companion star, forming a binary system. The companion may be a white dwarf, a neutron star, or—more rarely—a massive main-sequence star. What makes binary pulsars extraordinary is not merely their existence, but the fact that their clock-like regularity allows astronomers to test fundamental physical laws with unprecedented precision.

Binary pulsars have transformed gravity from a largely theoretical subject into an observational science.

Discovery of the First Binary Pulsar

The first binary pulsar, PSR B1913+16, was discovered in 1974 by Russell Hulse and Joseph Taylor using the giant Arecibo radio telescope (see diagram and picture below). Its pulse arrival times showed systematic Doppler shifts, revealing that the pulsar was orbiting another compact object.

The system consists of:

·       Two neutron stars

·       An orbital period of about 7.75 hours

·       A highly elliptical orbit

This discovery earned Hulse and Taylor the 1993 Nobel Prize in Physics. 

The giant Arecibo radio telescope in Puerto Rico (now destroyed and decommissioned)

Why Pulsars Make Ideal Clocks

Pulsars, especially millisecond pulsars, are extraordinarily stable rotators. In a binary system:

·       Orbital motion modulates pulse arrival times

·       Relativistic effects introduce tiny but measurable deviations

·       These deviations can be tracked over years or decades

Timing precision can reach microseconds or better, turning binary pulsars into natural laboratories for precision gravity measurements.

Orbital Dynamics and Relativistic Effects

Binary pulsars exhibit several relativistic phenomena predicted by Einstein’s General Theory of Relativity (GR):

(a) Periastron Advance

The point of closest approach in the orbit slowly rotates, analogous to Mercury’s perihelion advance, but vastly stronger. In PSR B1913+16, this advance is about 4.2° per year.

(b) Gravitational Redshift and Time Dilation

The pulsar’s clock runs slower when it is deeper in the gravitational field of its companion, producing periodic variations in pulse timing.

(c) Shapiro Delay

When pulses pass close to the companion star, they are delayed due to spacetime curvature. This effect provides precise measurements of the companion’s mass and orbital inclination. 

The gradual decay in the orbit of Taylor & Hulse's binary pulsar due to gravitational wave radiation. The observed data (dots) agree with the predictions of general relativity (solid line)

Indirect Detection of Gravitational Waves

Perhaps the most profound contribution of binary pulsars is the first indirect evidence for gravitational waves.

According to GR:

·       A binary system should lose energy via gravitational radiation

·       The orbit should slowly shrink

·       The orbital period should decrease at a precisely calculable rate

For PSR B1913+16, the observed orbital decay matches GR’s prediction to better than 0.2%, a triumph for Einstein’s theory decades before direct detection by LIGO (to be discussed in a future article in this series).

The Double Pulsar System

In 2003, astronomers discovered PSR J0737–3039, the only known system where both neutron stars are observable as pulsars.

Key features:

·       Orbital period: ~2.4 hours

·       Extremely relativistic orbit

·       Multiple strong-field GR effects measured simultaneously

This system provides the most stringent tests of GR in the strong-field regime available outside black holes. 

Evolution and Formation

Binary pulsars form through complex evolutionary pathways:

1. Two massive stars form a binary system.

2. One star undergoes a supernova, forming a neutron star.

3. Mass transfer from the companion can “recycle” the pulsar, spinning it up to millisecond periods.

4. The second star may later undergo its own supernova.

Survival through one or two supernova explosions requires finely tuned conditions, making such systems rare and valuable.

Broader Significance

Binary pulsars bridge theory and observation in a way few astrophysical systems can. They allow us to:

·       Measure neutron star masses with high precision

·       Confirm energy loss through gravitational radiation

·       Observe relativistic effects in real time

They are, in essence, cosmic experiments set up by nature itself.

Conclusion

Binary pulsars stand as one of the strongest empirical pillars supporting general relativity. Long before gravitational waves were directly detected, these systems quietly demonstrated their existence through meticulous timing. Even today, they remain indispensable tools for exploring gravity, matter, and spacetime under extreme conditions.

Russell Alan Hulse (1950 -) – A Biographical Sketch 

Russell Alan Hulse was born on November 28, 1950, in New York City, USA. From an early age, he displayed a strong aptitude for mathematics and physics, nurtured by the vibrant scientific culture of post-war America.

Hulse pursued his undergraduate education at Cooper Union for the Advancement of Science and Art, New York, graduating in 1970 with a degree in physics. He then entered the University of Massachusetts Amherst for graduate studies, where his interests gravitated toward radio astronomy and astrophysics—fields that were undergoing rapid growth following the discovery of pulsars in 1967. 

Doctoral Work and Discovery of the Binary Pulsar

Hulse’s doctoral research, supervised by Joseph H Taylor Jr (with whom he shared the Nobel prize), proved to be historic. Using the giant 305-m Arecibo radio telescope in Puerto Rico—then the most sensitive radio telescope in the world—Hulse undertook a systematic pulsar survey as part of his PhD thesis. 

In July 1974, Hulse identified a pulsar with an unusual, periodically varying pulse arrival time. Careful analysis revealed that the pulsar was in a tight binary orbit with another compact object, most likely another neutron star. This object was designated PSR B1913+16, now universally known as the Hulse–Taylor binary pulsar. 

This was the first known binary pulsar ever discovered. 

Scientific Significance 

The binary pulsar had extraordinary properties:

·       Orbital period: ~7.75 hours

·       Orbital velocities: ~300 km/s

·       Highly relativistic regime 

Because pulsars act as extremely precise cosmic clocks, minute changes in their orbital motion could be measured with unprecedented accuracy. Continued monitoring revealed that the orbital period was gradually decreasing. This decay matched, to high precision, the energy loss predicted by Albert Einstein’s General Theory of Relativity due to the emission of gravitational waves. Thus, Hulse’s discovery provided: 

· The first indirect experimental evidence for gravitational waves

· The strongest test of general relativity in the strong-field regime available at the time

Later Career 

After completing his PhD, Hulse pursued a more applied scientific career. He worked at:

· National Radio Astronomy Observatory (NRAO)

· Bell Laboratories

· Princeton University

· Later, in plasma physics and fusion research, including work at Princeton Plasma Physics Laboratory

Despite moving away from astrophysics, Hulse’s early contribution remains foundational. 

Nobel Prize 

In 1993, Russell A Hulse was awarded the Nobel Prize in Physics, jointly with Joseph H. Taylor Jr., “for the discovery of a new type of pulsar, a discovery that has opened up new possibilities for the study of gravitation.” 

Joseph Taylor Jr (1941 - ) – A Biographical Sketch 

Joseph Hooton Taylor Jr was born on March 29, 1941, in Philadelphia, Pennsylvania, USA. His early interests included electronics, radio communication, and astronomy—skills that later merged seamlessly in radio astrophysics. 

Taylor earned his bachelor’s degree from Haverford College in 1963, followed by a PhD from Harvard University in 1968, specializing in astronomy and astrophysics. His doctoral work already reflected a strong emphasis on precision measurement and observational rigor. 

Academic Career and Mentorship 

After Harvard, Taylor joined the faculty at:

· University of Massachusetts Amherst

· Later Princeton University, where he became the James S McDonnell Distinguished University Professor of Physics

Taylor was known not only for his scientific insight but also for his meticulous approach to experimental design and data analysis. It was under his guidance that Russell Hulse conducted the pulsar survey that led to the Nobel-winning discovery. 

Precision Timing and Relativistic Tests 

Following the discovery of PSR B1913+16, Taylor led a long-term observational program spanning decades. His key contribution lay in:

· Refining pulsar timing techniques to microsecond accuracy

· Extracting relativistic parameters from orbital dynamics

Taylor and collaborators measured several relativistic effects:

· Orbital decay due to gravitational radiation

· Periastron advance (analogous to Mercury’s perihelion advance, but vastly stronger)

· Gravitational redshift and time dilation

The observed orbital decay agreed with Einstein’s predictions to better than 0.2%, an astonishing level of confirmation. 

Broader Contributions 

Beyond the binary pulsar:

· Taylor played a central role in establishing pulsar timing as a precision tool in astrophysics

· His methods laid the groundwork for modern pulsar timing arrays, now used to directly detect low-frequency gravitational waves

· He contributed to radio astronomy instrumentation and observational standards

Nobel Prize

In 1993, Joseph H. Taylor Jr. shared the Nobel Prize in Physics with Russell Hulse.

The Nobel Committee emphasized that Taylor’s work transformed pulsars from astronomical curiosities into fundamental laboratories for relativistic physics. 

The Nobel-Winning Discovery in Perspective 

The Hulse–Taylor binary pulsar represents a pivotal moment in modern physics:

· It bridged astrophysics and gravitational theory

· It provided the first compelling evidence for gravitational waves, decades before their direct detection by LIGO in 2015

· It demonstrated that the universe itself could serve as a precision experimental apparatus

In retrospect, the discovery stands as a textbook example of how careful observation, ingenious instrumentation, and theoretical insight can converge to validate one of the deepest ideas in physics. 

Epilogue: Fate of the giant Arecibo Telescope

The 305 m single-dish radio telescope built into a natural sinkhole in Puerto Rico was decommissioned and destroyed after decades of service.

· In August and November 2020, critical support cables broke, damaging the telescope’s suspended receiver platform.

· On December 1, 2020, before a planned controlled decommissioning could be completed, the central receiver platform and support structure collapsed onto the dish, destroying the telescope.

 Decision Not to Rebuild:

· After the collapse, the US National Science Foundation (NSF) evaluated options and decided not to rebuild the giant telescope. Instead, the NSF shifted focus to other uses for the site.

· This decision reflected the high cost and engineering challenges of reconstructing another world-class telescope of the same scale.

What’s Happening Now:

· The original telescope no longer exists, but the data it collected over nearly six decades is preserved and being re-used. Researchers continue to analyze archived observations using modern techniques to extract new scientific insights.

· The site is being repurposed for science education and outreach. The NSF Arecibo Center for Culturally Relevant and Inclusive Science Education, Computational Skills, and Community Engagement (Arecibo C3) has been established to foster STEM learning and community engagement.

Legacy and Future Ideas:

· Arecibo made major scientific contributions, from pulsar discoveries to asteroid radar studies and SETI searches. Though the telescope is gone, its legacy endures in scientific literature and ongoing data use.

· There have been proposals and concepts for next-generation instruments that could sit at the Arecibo site or elsewhere. These include array-based designs that aim to capture some of Arecibo’s unique capabilities, though none has yet been funded or begun construction.