Friday, December 16, 2016

The Light-Matter Interaction: Calling Theory into Question

Despite its reputation for social awkwardness, physics is fundamentally about interactions. Physics textbooks are filed with forces, fields, orbits, motion, and other concepts that describe things by the way that they interact with other things. Of all of the interactions, one of the most fundamental is how light interacts with matter. In research published in Physical Review A last week, a team of researchers called into question some generally accepted assumptions about this interaction.

An illustration of an electron beam traveling through a niobium cavity, a key component of the LCLS-II X-ray laser under construction at SLAC National Accelerator Laboratory. These cavities will power a highly energetic electron beam that will create up to 1 million X-ray flashes per second – more than any other current or planned X-ray laser.
Image Credit: SLAC National Accelerator Laboratory.

Let’s say that you want to describe how a basketball interacts with a court. You might start with an experiment, equations based on conservation of energy, or a force diagram. Once you have the basic understanding, you might look at how the interaction varies with the type of flooring, the inflation of the basketball, the angle and speed at which the ball impacts the floor, and so on, perhaps building a computer simulation to help. When addressing an interaction problem, it makes sense to start at a basic level and then venture into the nuances and special cases.

When you’re exploring things on a fundamental level, like at the level of how a photon interacts with individual electrons, the same approach can be useful. As you work your way up to more specialized cases, such as how extremely intense laser pulses interact with an electron, the rules of the interactions can change. Over time scientists have developed mathematical techniques and models to describe these fundamental interactions.

The last 30 years have seen incredible advances in lasers, enabling us to develop powerful medical and industrial tools and opening up new areas of science. Facilities that can deliver more powerful laser pulses than we’ve ever seen before are currently under construction, with new science and new applications on the horizon. Exciting opportunities are ahead.

In order to take full advantage of these opportunities though, we need to have a solid theoretical understanding of the way light and matter interact on the quantum level. In this new work, Erez Raicher (Hebrew University and Soreq Nuclear Research Center), Shalom Eliezer (Soreq NRC and Polytechnic University of Madrid), and Arie Zigler (Hebrew University) show that scientists may have to revisit some things they thought they already knew before we get there.

For the first time, the team calculated what happens when an intense laser pulse interacts with an electron in a rotating electric field. This interaction, a type of scattering, leads to the emission of an energetic photon.

First, the researchers developed a mathematical description of the photon emission rate. Their result is complicated, but they showed that you can make some simplifications and get a valid approximation that works well in computer simulations. Next, they used a computer program to calculate the distribution of photons that would be emitted if you used an existing powerful x-ray laser or optical laser in this interaction.

When the researchers compared their results to the expected distribution based on a widely accepted but more general model, things didn’t match. This calls into question a standard model of radiation that has been accepted for 50 years, and the researchers say that it could significantly influence theoretical studies of how light and matter interact going forward.

What happens now? It should be possible to test these predictions with existing laser facilities and see how well experimental results match both predictions. Like a ball bouncing on a court, exploring the interaction from both experimental and theoretical angles gives us a more complete understanding. With a better understanding we can probe deeper, creating more powerful tools, techniques, and models of our world.

Kendra Redmond

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