X-rays and science: from molecules to galaxies
By Richard B. Gunderman
We often think of x-rays strictly in terms of medical diagnosis, but in fact they have played a huge role in scientific discovery beyond medicine. Though they are part of the same electromagnetic spectrum that includes visible light, their different properties enable them to reveal phenomena that the naked eye cannot perceive. For example, x-rays have opened up our understanding of the worlds of the very small and the very large, enabling us to grasp the structure of some of the most essential molecules in living organisms. They have also spawned remarkable insights into the origin, structure, and ongoing evolution of the universe of which we are a part.
Some would say that the single most important biological discovery of the 20th century concerned the structure of DNA, which is sometimes referred to as the “master molecule” of life. In 1962, James Watson, Francis Crick, and Maurice Wilkins shared the Nobel Prize for this discovery, but much of the groundwork had been laid over the preceding decades. Perhaps the single most important technique for making inferences about DNA’s structure was x-ray crystallography, which is performed by projecting x-rays onto a crystalline solid. The resulting images make it possible to determine how its atoms are positioned relative to one another.
The father-son team of William Bragg and William Bragg developed much of the theory behind x-ray crystallography. Ironically, the elder Bragg had been the first person on the continent of Australia to use x-rays for medical purposes when he diagnosed the fracture of a bone in the younger Bragg’s arm when the boy was only 5 years old. Many years later, it would be the younger Bragg who nominated Watson, Crick, and Wilkins for their Nobel Prize. Later researchers, including Maurice Wilkins and Rosalind Franklin, used the Braggs’ technique to show that DNA had a long, threadlike structure, though neither one deduced its now well-known double-helical structure.
Rosalind Franklin was a particularly interesting figure. Franklin battled sexism throughout her life, and eventually earned her PhD at Cambridge University. During her postdoctoral work in Paris, she was able to improve on Wilkins’ techniques and produce much more precise images of DNA’s molecular structure. According to Watson, it was Franklin’s x-ray crystallographic images that made it possible for him and Crick to arrive at the double-helix model. Unfortunately, Franklin did not share in the Nobel Prize because she was diagnosed with ovarian cancer at the age of only 36 years. Nobel Prizes are not awarded posthumously, and Franklin had succumbed 5 years before the award was made.
James Watson was an American wunderkind who earned his PhD at Indiana University at only 22, while the British Francis Crick was in his mid-30s and had not yet received his PhD at the time he and Watson made their breakthrough. They were competing against other remarkable scientists, including the American Linus Pauling, whom many consider the greatest American scientist of the 20th century. Pauling is also one of only a handful of people to receive two Nobel prizes. Pauling mistakenly published a triple-helix model of DNA, leaving the field open for Watson and Crick, relying on better x-ray crystallographic data, to produce the correct double-helix model.
X-rays have also opened up our understanding of some of the largest and most extraordinary objects in the universe, up to and including the universe itself. X-rays are a particularly high-energy form of electromagnetic radiation, meaning that they are generally released by processes producing a temperature thousands of times higher than the heat of the sun. For many years, astronomers could rely only on visible light for their observations, but around 1960 the world of x-ray astronomy began to open up. This is due to the fact that cosmic x-rays are filtered out by the earth’s atmosphere, but about this time it began to be possible to put x-ray detection devices into space on rockets.
For example, in 1962, a rocket launched to assess x-ray emissions by the moon identified what became known as the first extra-solar x-ray source. Known as Scorpius X-1, it is a neutron star approximately 9,000 light years away. Its visible light emission is only 1/400 that of the dimmest star detectable in the night sky, but, at least from earth’s point of view, it is the strongest source of x-rays in the sky. Its x-ray output is approximately 60,000 times greater than the luminosity of the sun. We now believe that these tremendously energetic x-ray emissions originate as the immense gravitational pull of the neutron star draws off material from a companion star, converting much of its matter to energy.
What exactly is a neutron star? A neutron star is an incredibly dense object. It has been estimated that one containing half a million times the mass of the earth would fit into a sphere with a diameter equal to that of Brooklyn, New York. It is created by gravitational collapse following the explosion of a massive star, a so-called supernova. Most of the atomic components are released in the explosion, but the neutrons remain and collapse in on themselves, creating an extraordinarily dense, hot, and rapidly rotating object. The gravitational pull of neutron stars is so great that they bend light, in a phenomenon known as gravitational lensing.
Black holes are even more bizarre x-ray sources. The represent the final stage of the evolution of very massive stars, and it is thought that the supermassive black holes at the centers of some galaxies may have masses equivalent to billions of suns. As a result, they collapse and compress matter to an even greater extent than a neutron star, to the point that the entire mass of the earth would fit into the palm of a hand. These objects bend space-time to such an extent that even light cannot escape, and the passage of time itself ceases, in the sense that there is no longer any before or after. Such an object cannot be directly observed, and its presence is inferred based on the way it gobbles up matter.
Specifically, as matter approaches a black hole, it is accelerated to relativistic speeds near the speed of light. Once such matter reaches a certain point, known as the event horizon, it is impossible for anything to escape, and all light that reaches the horizon is absorbed, making a black hole impossible to visualize. But as matter approaches the event horizon, it is heated to an incredible degree, emitting huge quantities of x-rays. Moreover, it emits lesser quantities of energy as visible light, helping to form some of the brightest light sources in the universe. Though most of the light is blocked by intervening debris, the x-rays can travel vast distances and reach our detectors.
Wilhelm Roentgen, the German physicist who discovered X-rays in 1985, could not have imagined the immense impact this new invisible light would have on the course of science. In addition to their use in crystallography and astronomy, x-rays can also be used in microscopy, fluorescence, and spectroscopy. They have many additional applications, such as inspecting welds, producing precise three-dimensional images of objects such as violins, and scanning passengers and baggage at the airport. It is remarkable to think that, though x-rays themselves are invisible to the eye, they continue to play a huge role in revealing the world around and within us.
Richard Gunderman, MD PhD, is a Professor of Radiology, Pediatrics, Medical Education, Philosophy, Liberal Arts, and Philanthrophy at Indiana University, Indianapolis, Indiana, and winner of the 2012 Alpha Omega Alpha Robert Glaser Distinguished Teacher Award. He is the author of X-Ray Vision: The Evolution of Medical Imaging and Its Human Implications. He will be hosting a webinar on this subject on Wednesday, 29 May 2013, 3:00-4:00 p.m. Eastern.