X-ray astronomy puts Einstein to the test

From tracing the way light bends around a neutron star to detecting the effects of a black hole tugging on space itself, X-ray astronomy is uniquely suited to testing and observing predictions of general relativity, Albert Einstein's landmark 1915 theory of gravitation.

Thus far, Einstein's predictions are passing with flying colors. But will Einstein have the last word on gravity? X-ray astronomers hope to subject general relativity to even greater scrutiny by traveling (albeit virtually) to where gravity is at its extreme and the effects of general relativity most pronounced: the innermost regions of black holes and neutron stars. The payoff will be a greater understanding of the dominant force in the large-scale universe and, perhaps, a unified theory of the fundamental forces of nature – the central goal of physics today.

In a mere 40 years, the field of X-ray astronomy has witnessed nearly a billion-fold improvement in telescope sensitivity since June 18, 1962, when a sounding rocket laden with a payload of research equipment discovered the first celestial X-ray source.

"We have come a long way, from the first inkling that a black hole – Cygnus X-1 – was in hand," said Jean Swank of NASA Goddard Space Flight Center, Project Scientist for NASA's Rossi X-ray Timing Explorer. "Now we can reasonably hope to map spacetime close to the invisible abyss of gravity."

General relativity defines gravity as a result of mass distorting both space and time – a four-dimensional concept called spacetime. Nowhere is this more evident than in the regions around a black hole and neutron star – two examples of mass at extreme density, and thus sources of extreme gravity in a small area.

X-ray telescopes are well suited to study these regions for two fundamental reasons. First, the power unleashed by matter crashing onto a neutron star or falling into a black hole shines predominantly in X-rays, particularly in the regions closest to these objects, where gravity is at its strongest. Also, X-rays can penetrate through the obscuring veil of dust and gas surrounding black holes and neutron stars, which blocks the passage of other forms of radiation.

The field of X-ray astronomy continues to verify key predictions of general relativity with greater certainty as X-ray telescope technology continues to improve. Some of these methods include: gravitational redshifting, which is gravity tugging at a photon, or light particle, as it tries to escape; frame-dragging, which is a spinning object twisting the actual fabric of space along with it; gravitational lensing, which is the path of light bent by gravity; and Einsteinian orbits (an innermost stable orbit).

For example, in 1995, astronomers using the Japanese-built ASCA X-ray satellite observed the first clear indication of gravitationally redshifted light around a black hole. ASCA detected a broad iron line, which is a spectral feature in the emission of hot iron atoms around a black hole revealing that strong gravity was stealing energy from the emitted light. This verified Einstein's prediction that black holes create a gravitational well, which a photon of light must climb out of on its journey towards Earth. Today, the Chandra X-ray Observatory and the XMM-Newton satellite observe this phenomenon with greater precision.

Also, the Rossi Explorer has observed some of the more exotic predictions of general relativity. First in 1998 and again with better accuracy in 2001, scientists observed the frame-dragging phenomenon. Einstein's equations predicted that a spinning object with strong gravity would take spacetime, as well as any matter within its gravitational influence, for a spin along with it. This effect would make it hard for any object to fall onto the black hole directly, because the object would be accelerated in orbit along the same spin as the black hole, and would resist falling more than an object that was not affected by frame-dragging.

General relativity has many consequences regarding what happens to stars. It determines that neutron stars, made in supernovae from stars that have consumed all their nuclear fuel, cannot be more massive than about three solar masses. More massive collapsed remnants cannot be neutron stars (they become black holes instead). So far, we have no contradiction.

General relativity also has consequences for the environments of neutron stars and black holes. Material cannot orbit too closely. As opposed to Newtonian law, which permits matter to orbit at any radii, general relativity predicts an innermost stable orbit. Again, so far there is no contradiction. In fact, the Rossi Explorer has determined that one certain black hole is spinning by virtue of an extremely tight, stable orbit – an orbit predicted for a spinning black hole of a given mass. Also, scientists observe quasi-periodic oscillations, or X-ray flickering, that, so far, are best explained by the effects of general relativity and the concept of an innermost stable orbit.

Yet as far as X-ray astronomy has come, scientists have been unable to test general relativity completely. Such an undertaking could point the way to a better theory of gravity, the same way that Einstein's improved upon Newton's theory. The endeavor could also bridge general relativity with quantum theory, an independent pillar of modern physics.

General relativity describes the domain of the large-scale, the force of gravity. Quantum theory, part of the Standard Model, describes the small-scale, the subatomic. The quantum forces are electromagnetic radiation (light, photons), strong forces (the kind that holds together protons and neutrons to form a nucleus), and weak forces (seen in radioactive decay). Gravity, as of yet, does not fit into the Standard Model; the so-called graviton, a particle of gravity, has not been found.

Yet gravity does not act alone. General relativity defines this force as the result of mass and energy (united in E=mc²). Mass (matter, atoms) and radiation (photons) are controlled by quantum theory. Thus, the connection between the two theories is indeed logical and tantalizingly close at hand. Particle physicists refine quantum theory with the help of particle accelerators, smashing protons and electrons to reveal the matter and energy within. Astronomers must develop tests to better scrutinize gravity. Future X-ray observatories – some in development, others proposed for launch at the close of the next decade – will move us closer to a black hole, step by step.

One possible future mission, the Micro-Arcsecond X-ray Imaging Mission (MAXIM), is a follow-up to the Rossi Explorer and would provide more precise timing of the X-ray flickering characteristics of neutron stars and black holes. This would reveal fainter features in the quasi-periodic oscillations, which may be a signature of general relativity. MAXIM's mission would include imaging a black hole event horizon. With an image of the event horizon and accretion disk as a point of reference, scientists would be able to determine exactly from where X-rays of varying energies arise. Knowing precise distances from the black hole to X-rays of specific energies would allow for accurate tests of Einstein's math.

Updated: June 2011