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Einstein's ultimate laboratory: a recently discovered binary pulsar gives scientists a golden opportunity to test general relativity.

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In 1609 Galileo Galilei aimed his telescope at the heavens, establishing modern astronomy. Ever since, modern science and astronomy have connected us with the universe in a way that allows us to ask whether the laws of physics derived here on Earth are also valid in outer space. Of particular interest is gravitation, exemplified by Newton's realization that the fall of an apple and the movement of the planets are governed by the same force.

Although gravity determines cosmic evolution, it's an extremely weak force. That concept may be difficult to accept if you bang your head on the floor, but the electromagnetic force between an electron and a proton is 1040 stronger than the corresponding gravitational force. The most stringent experimental tests of gravity must therefore involve very massive bodies such as the Sun or even more exotic objects such as neutron stars and black holes.

The study of Mercury's orbit around the Sun provided clear indications that Newton's theory was not the full story. Nevertheless, Newtonian gravitation reigned supreme for more than 200 years, until Albert Einstein's general theory of relativity replaced it by explaining the observed discrepancies.

General relativity (GR) works daily in our GPS navigation systems, but is GR our final word in the understanding of gravity? GR has passed every experimental test with flying colors (S&T: July 2005, page 33). But despite decades of effort, theorists have been unable to unify GR with quantum mechanics, the well-established physics of the microworld.

Do we accept GR or quantum physics? Do we have to consider alternatives or modifications to GR in order to derive the Theory of Everything? Only experiments will decide this question, and astronomers are pushing to increasingly precise tests of GR under more extreme conditions. One of the most exciting such efforts involves a pair of incredibly dense objects known as the Double Pulsar.

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Decaying Orbit

The Double Pulsar, also known by its catalog number J0737-3039 for its coordinates in Canis Major, is a physicist's dream come true. Discovered at Australia's Parkes Observatory in 2003 by an international team led by Marta Burgay (University of Bologna, Italy), it's the only known system that consists of two active radio pulsars. Pulsar A rotates every 22 milliseconds, while its companion, B, rotates with a period of 2.7 seconds.

Binaries consisting of two neutron stars were known before, and the 1974 discovery of the first known system (B1913+16) led to the 1993 Nobel Prize in Physics for Joseph Taylor and Russell Hulse. But these other systems contain only one visible radio pulsar in orbit around an unseen second neutron star. We obtain a lot more information about J0737 because we have received regular pulsed signals from two objects instead of one.

Better yet, the two neutron stars in J0737 have the smallest separation of any known binary-neutron-star system. J0737's two members orbit each other in less than 145 minutes with orbital speeds of a million kilometers per hour (S&T: March 2004, page 22). With the stars being separated by a mere 900,000 km, the orbit is only about twice the size of the Earth-Moon system. The tightness of the orbit and the fast orbital speed enhance the effects of GR.

But that separation is not constant. I belong to an international group of radio astronomers that routinely monitors this system using the 64-meter Parkes Telescope, the 100-meter Robert C. Byrd Green Bank Telescope in West Virginia, and the 76-meter Lovell Telescope at Jodrell Bank Observatory in England. We have followed the pulsars' movements by monitoring their pulsed signals, allowing us to measure how the gravitational field of each companion affects the orbital motion. In GR, unlike in Newtonian physics, the orbital motion is determined only by the curvature of spacetime, whose curvature itself is determined by the presence of mass. The Double Pulsar thus provides the perfect experimental set-up for tests of GR: we have two precise clocks, attached to heavy test-masses that move under the influence of strong gravitational fields in the curved spacetime around each other.

Our measurements show that the separation is shrinking every day by 7.42 [+ or -] 0.09 millimeters (0.29 [+ or -] 0.004 inch), leading to a collision in 85 million years. This shrinkage is perfectly in line with the predictions of GR. The fact that we can measure this minuscule change precisely, in particular with the system being about 2,000 light-years from Earth, is only possible because fast-rotating pulsars such as A are incredibly accurate cosmic timekeepers (see "Why Pulsars Are Great Clocks").

According to Einstein, the moving pulsars produce ripples in spacetime that carry away orbital energy as gravitational waves. Taylor and Hulse won the Nobel Prize because B1913's orbit is decaying, the first indirect evidence of gravitational waves. Gravitational-wave detectors such as LIGO are designed to detect this signal (see "The Hunt for Gravitational Waves"). The fact that we could measure the orbital shrinkage from gravitational-wave emission only a few years after the system's discovery is remarkable. It reflects the fact that the Double Pulsar is the most relativistic binary system known, and so it's not surprising that we also measure a plethora of other predicted GR effects.

Orbital Precession

We observe the same relativistic effect that provided the first clues that Isaac Newton's theory was not the final word on gravity. During the 19th century, French mathematician Urbain J. J. Le Verrier noticed that Mercury's orbit slowly rotates in space, and that the rate of this orbital precession is small but incompatible with Newtonian physics. He offered a number of explanations, including the existence of an unseen planet (Vulcan) affecting Mercury's motion. But it was not before Einstein published GR in 1916 that the small perihelion advance of 0.00012[degrees] per year could be fully explained by Mercury'smotion in the curved spacetime around the Sun.

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We see the same effect in the Double Pulsar, but spacetime around the pulsars is much more deeply curved, so the effect is much larger. We measure that the orbit of the two pulsars is precessing at a rate of 16.8995 [+ or -] 0.0007[degrees] per year. While it takes 3 million years for Mercury to complete one full orbital precession, it takes only 21.3 years in the Double Pulsar! In these 21.3 years, the slightly eccentric orbit is reorienting itself to Earth, allowing us to view the system from all sides, mimicking a spacecraft that allows us to travel all around the orbit.

Shapiro Delay

The curvature of spacetime is also directly observable due to a fortuitous orientation of the orbit relative to Earth. We're incredibly lucky that both pulsars point their beams directly toward us, and that we also see the orbit nearly edge-on. This alignment has two fascinating consequences. First, we observe 30-second-long eclipses at superior conjunction, when the two pulsars and Earth form a perfectly straight line, causing one pulsar to block the other's signal.

Second, around conjunction the radio signals from one pulsar pass very close to its companion en route to Earth. At this moment, the signals propagate through heavily curved spacetime, which takes slightly longer than the signals emitted away from conjunction, when they travel through flatter spacetime. This extra path length causes the pulses to arrive at our telescopes about 100 microseconds later than usual, in agreement with GR's predictions.

This effect is known as the Shapiro delay, for astronomer Irwin Shapiro, who first detected its effects in the solar system in 1964. By timing the exact delay as it changes with orbital phase, we can measure the mass of the neutron star that causes the curvature in spacetime and the orbital inclination, that is, how close the signal passes the companion's surface. The orbit is indeed nearly precisely edge-on, only 1.5 [+ or -] 0.5[degrees] away from perfect alignment, and we know the radio signals pass the surface at a mere 20,000 km.

Gravitational Time Dilation

GR also predicts that gravitational fields alter the rates of time passing at different depths of a gravitational field. As a result, clocks run slower when they experience stronger gravity. Physicists have demonstrated gravitational time dilation on Earth using atomic clocks on airplanes by noting that clocks at higher altitudes (farther from Earth's center of mass) run slightly faster that clocks at lower altitudes. The effect in these terrestrial experiments amounts to mere nanoseconds.

But the gravitational time delay in the Double Pulsar is much larger. We can easily measure the consequences of this effect because the orbit is slightly eccentric, and the separation of the pulsars (and hence the experienced gravitational field) changes during the orbit. The result is a periodic variation of the pulsar clock rate measured (together with a special relativistic effect) with an amplitude of 386 [+ or -] 3 microseconds.

Calculating the Pulsar Masses

We clearly confirm all of these fascinating GR effects, but in order to test the theory's validity, we have to go one step further. The theory must also predict the precise magnitude of the measured effects.

For all "reasonable" theories of gravity, including GR, the magnitude of each effect depends on the unknown masses of the two pulsars. By measuring two relativistic effects, we can calculate the pulsar masses--assuming GR is correct. But we can test this assumption by measuring several other relativistic effects. For every combination of two GR effects, we should be able to calculate the same pulsar masses if the theory is a correct description of nature. If GR fails to predict the correct magnitude of even one of these relativistic effects, we have to reject the theory.

In the Double Pulsar we can perform four such tests of GR, which are all independent of one another. While doing this, we can use some additional information that is only available in this system--the relative mass of both pulsars. Since both pulsars orbit a common center of mass, the more massive pulsar is closer to the center of mass than the other. By measuring the size of their orbits, we can immediately infer the pulsars' relative masses.

This is unique and valuable information. Whatever theory is assumed to determine the two pulsar masses, it must produce the same measured relative masses. GR does exactly this. The comparison of the expected result compared to the experimentally measured values shows a perfect agreement. For the most precise test, we calculate a ratio of expected and observed value of 1.0000 [+ or -] 0.0005. This is by far the best test of GR in a strong gravitational field. At the same time, we have obtained a very precise measurement of the pulsar masses: The fast-rotating pulsar, A, has a mass of 1.3381 [+ or -] 0.0007 Suns, while pulsar B is slightly lighter, with a mass of 1.2489 [+ or -] 0.0007 Suns.

Eclipses

It's amazing that we can obtain these results only by precisely measuring the arrival times of the pulses of both sources. But there's further information encoded in the properties of the radio emission. Indeed, we can learn a lot from studying the eclipses of pulsar A that occur at superior conjunction. They are caused by the doughnut-shaped magnetosphere of pulsar B, which is filled with absorbing plasma that blocks pulsar A's light for nearly 30 seconds.

But the blockage is incomplete. Due to the geometrical configuration and limited size of B's magnetosphere, some of A's pulses can still be detected during the eclipse. Our careful eclipse monitoring reveals that the shape of the eclipse and the pattern of detected pulses are slowly changing. This is only possible if the geometry, namely the orientation of B's magnetosphere, which is a result of B's spin axis, varies systematically. We expect such a variation because of yet another predicted GR effect.

It turns out that the pulsars' rotation and orbital motions are affecting each other. This relativistic effect, known as spin-orbit coupling, leads to a precession (or wobble) of B's rotation axis. The rate of change is small, since GR predicts that it takes 71 years for one complete wobble cycle. Nevertheless, the impact of this precession is clearly visible in the data and we can model it to derive a measurement of the precession rate. In fact, last year the spin-precession caused B's signal to disappear because its beam no longer points in our direction. It will remain absent for an unspecified period of time, since we don't know the exact shape of the beam.

The agreement of the measured precession rate with GR's prediction is again remarkable, representing not only a fifth test in this system but also the first high-precision test of relativistic spin-precession in strong gravitational fields. It's deeply satisfactory that after 1919, when a total solar eclipse first allowed astronomers to test Einstein's theory of gravity, eclipses of two dead stars have offered a completely new and exciting confirmation of his theory.

A Remarkable Story

The precise agreement between our observations and GR's predictions not only strongly supports the validity of Einstein's theory--at least for the strengths of the gravitational fields probed by the Double Pulsar--but it also puts severe constraints on alternative theories of gravity. These constraints will improve with time, as the precision of our measurements continually improves. If GR has to be modified to explain the observations of dark energy or dark matter, the new theory will probably look very similar to GR in many respects. Finding discrepancies with GR--should they exist--would not diminish Einstein's unique contribution to our understanding of the physical world but it would instead be a major event that might be the starting point of new physics.

We're lucky to live in such exciting times. Soon, the Large Hadron Collider in Switzerland will or will not confirm the Standard Model of particle physics. We are simultaneously on the brink of directly detecting gravitational waves. Together with observations of the Double Pulsar, and potentially with systems where pulsars orbit black holes, we will be able to test GR to the breaking point. With the combination of all results, obtained in a multitude of complementary experiments, we're moving a step closer to the Theory of Everything. Pulsars will clearly be part of this journey, and I dare to think that Einstein would have liked this unexpected way of testing his theory.

RELATED ARTICLE:

COLLISION COURSE S&T illustrator Casey Reed depicts the double pulsar system 85 million years from now, shortly before the pulsars collide and merge. When the pulsars are close to merger, they emit powerful gravitational waves that alter the fabric of the surrounding spacetime.

RELATED ARTICLE: Anatomy of the Double Pulsar system.

DOUBLE PULSAR The pulsars in J0737-3039 whirl around a common center of mass in only 2 hours, 25 minutes. The binary is so compact that it's only about twice the size of the Earth-Moon system--making it by far the mostly tightly packed known neutron-star binary system.

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ORBITAL DECAY The two pulsars emit gravitational waves as they orbit the center of mass, drawing orbital energy from the system and causing the separation to decrease 7.42 mm (0.29 inch) per day. This diagram shows the decay of pulsar A's orbit in steps of 5 million years (pulsar B is omitted for the sake of clarity). The rate of decay significantly increases near the end.

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ORBITAL PRECESSION Due to the very strong relativistic curvature of spacetime caused by two neutron stars in close proximity to each other, J0737-3039's orbit precesses (rotates) by a whopping 16.9[degrees] per year, 141,000 times the rate of the precession of Mercury's orbit around the Sun. The two pulsars complete a full precession cycle in only 21.3 years.

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SPIN-ORBIT COUPLING The pulsars are so massive and close to each other that relativistic effects create a linkage between the pulsars' rotation and orbital motions. These interactions cause pulsar B's tilted spin axis to precess (wobble). Though the measured wobble beautifully matches general relativity's prediction, it unfortunately caused B's signal to disappear in 2009. Pulsar A's spin axis is perpendicular to the orbital plane, and so far has not shown any signs of precession.

S&T: CASEY REED/ SOURCE: MICHAEL KRAMER

RELATED ARTICLE: The Double Pulsar's origins.

Both pulsars in J0737-3039 formed in supernova explosions. Usually, a supernova is violent enough to disrupt the binary system. In the standard scenario, the more massive pulsar (pulsar A) was born first, while pulsar B's somewhat less massive progenitor star needed a little more time to evolve. Once B's progenitor expanded into a red giant, matter flowed from B to A, spinning up A to its current rotation period of 22 milliseconds. Pulsar B formed in a later supernova, creating the 2.7-second pulsar.

Interestingly, astronomers measure a very small space velocity for the Double Pulsar of only 10 km per second, suggesting that the second explosion may have been rather "gentle," and may have involved an intermediate step in which a white dwarf at the center of B's progenitor gravitationally collapsed to a neutron star.

RELATED ARTICLE:

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LIGO LOUISIANA The U.S. National Science Foundation has funded LIGO, which consists of laboratories in Louisiana and Washington that can detect gravitational waves from systems such as the double pulsar. But unless the pulsars are very close to merger, their gravitational waves will be so weak that even LIGO won't be able to detect them. This image shows the LIGO facility near Livingston, Louisiana.

RELATED ARTICLE: The hunt for gravitational waves.

Most theories of gravity, including general relativity, predict that spacetime is curved by the presence of masses. If these masses are accelerated, the curvature changes and propagates outward as a wave. The existence of gravitational waves was proven indirectly with observations of the first binary pulsar (PSR 1913+16), discovered by Joseph Taylor and Russell Hulse in 1974.

To measure the impact of gravitational waves directly, one needs to detect the movement of large test masses that move when a gravitational wave passes through. Gravitational wave detectors such as LIGO attempt to measure the impact of cosmic gravitational waves in terrestrial labs. The discovery of J07373039 suggests that many more such sources can be expected in our Galaxy, increasing LIGO's expected success rate by a factor of 5 to 10.

RELATED ARTICLE: Why pulsars are great clocks.

Pulsars, like the one at the heart of the Crab Nebula (right), are highly magnetized, rotating neutron stars that emit radio emission along their magnetic axis. As on Earth, a pulsar's magnetic axis is inclined to its rotation axis, so that the rotating radio beam sweeps across the sky like a cosmic lighthouse. If Earth happens to lie in the covered area of sky, sensitive radio telescopes can detect the radio beam as a periodic radio pulse, with a period that corresponds to the neutron star's rotational period.

The regularity of this pulsed signal is incredible. As a neutron star crams about 1.4 solar masses into a city-sized sphere only 20 kilometers in diameter, this super-dense object acts like a massive flywheel whose rotation is very difficult to disturb, making it a superb cosmic clock that astronomers can exploit.

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Michael Kramer is director of the Max-Planck-Institut fur Radioastronomie in Bonn, Germany, and holds a professorship for astrophysics at the University of Manchester in the United Kingdom.
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Title Annotation:Extreme Double Star; Albert Einstein
Author:Kramer, Michael
Publication:Sky & Telescope
Geographic Code:1USA
Date:Aug 1, 2010
Words:3230
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