The most interesting parts of the Universe are under pressure – a lot of pressure. Deep inside planets, especially exoplanets like massive super-Earths, gravity crushes matter into exotic shapes not found on planetary surfaces. At the centers of stars, pressures are so high that hydrogen atoms are tightly compressed until they fuse into helium, releasing energy in a process called fusion that keeps the Sun bright.
The key role that very high pressures play in shaping the most important natural processes requires scientists to continually push the boundaries of technology. It is therefore ironic that today my colleagues and I study the high pressures of the Universe not by squeezing matter with steel plates or diamond anvils, but with the most intangible of substances: light .
It may seem strange that light could be used to crush samples of matter to the densities found at the center of an exoplanet or giant star. After all, no one has ever been pushed into the street by a ray of sunlight. So how do scientists convert light into pressure? The answer is that they don’t use just any type of light. They use lasers – very large lasers.
About a mile from my office in the Department of Physics and Astronomy at the University of Rochester is a huge building housing the Laser Energy Laboratory (LLE). There you will find the 60-beam Omega laser system which is a great example of Big Science. Firing once an hour, Omega can deliver up to 60 terrawatts of power to targets placed in the center of a huge, football-shaped chamber dotted with instruments. LLE is also home to the Center for Matter at Atomic Pressures, a National Science Foundation program of which I am a part that uses LLE lasers to, among other things, push the frontiers of interior exoplanet science.
What is happening inside this target chamber that allows Omega to explore high densities? Laser light is consistent, which means that it is made up of electromagnetic waves of a single wavelength. This is different from what comes out of, say, a light bulb, which emits light of multiple wavelengths (i.e. “white” light). The coherence of laser light means that a focused beam can deliver extremely high energies to a small target in a very short time. It’s the delivery of so many things power (energy per time, like Joules per second) which gives Omega and laser systems like this the punch they need to drive matter at high pressures.
From laser to pressure
But the link between intense lasers and high pressure is not direct. Instead, it requires a critical intermediate step.
When lasers reach their target, they couple with matter by pouring energy into the random movements of the constituent particles of matter (such as electrons). These random movements are essentially heat. The lasers quickly overheat the outer layers of the target, which respond by projecting the material outward as in an explosion. However, just like a rocket, these explosions can be harnessed to generate directed movement. In this case, a strong shock wave can be projected in the direction opposite to the explosion, i.e. towards the target. As the shock wave, traveling at tens of kilometers per second, passes through the target, it compresses the material to densities that could not be achieved otherwise. Even though the shock only takes a few nanoseconds to pass through the target, that’s long enough for all the sensors and diagnostics arrayed around the target chamber to capture details of the material in these exotic high-pressure states.
The use of this type of “rocket effect” is also the basis of how scientists performed their first-ever equilibrium fusion experiments within the laser system at the National Ignition Facility at Lawrence Livermore National Lab (NIF is the big brother of LLE). For fusion studies, lasers are fired at a tiny spherical capsule (about the size of a BB) containing an isotope of hydrogen. The outer layers of the capsule “ablate,” meaning the heat generated by the lasers is removed from the outer shell. The ablation acts as a kind of spherical rocket engine pushing the capsule to collapse – an implosion – and squeeze it until the hydrogen fuses into helium.
Thus, light can be transformed into heat, which can then be transformed into movement, and the effect of this movement can be transformed into strong pressure. It’s an elegant method that requires large machines that can focus on very small targets, giving scientists deep insight into the pressures that shape the Universe.