What Does A Theoretical Physicist Do?

How do you introduce yourself, scientifically?

My name is David Kolchmeyer and I am a theoretical physicist. I’m interested in quantum gravity, which is a theory of gravity that obeys the rules of quantum mechanics. Quantum mechanics is the fundamental framework upon which much of my field is built. I’m most interested in the properties of black holes, which are a good system for studying quantum gravity.

What are the implications or broader impacts of your work? What kinds of questions are you trying to answer?

There are no near-term technological applications of my work, so the impact is expanding our knowledge of the universe. 

One of the things I want to understand is how black holes store and process information. If you throw matter into a black hole, what happens to the information that is encoded in that matter? One way to think about this question is ‘What happens to a person who goes into a black hole? What would their experience be?’. 

That might seem like a simple question, but people have reached contradictions in trying to understand it. Stephen Hawking’s original proposal was that information is destroyed. If true, that would be a radical departure from everything else we know in quantum mechanics – there is a concept called unitarity, the idea that information cannot be destroyed.

Quantum mechanics predicts that black holes emit radiation. Eventually, they will emit all their energy away, and there will be no black hole left. If you had sufficiently powerful instruments and could collect and analyze all of the radiation that came from the black hole, would you be able to figure out what information entered the black hole during its formation? Or, is that information destroyed? Black holes do exist in nature, but it is extremely difficult to measure their radiation, so we rely on theoretical models in order to understand them.

What do your data look like?

Many scientists start with a natural phenomenon, and they assume that it is governed by a theory, and then they do experiments and collect data to probe that theory. In my work, I usually start with a particular theory and then explore its implications. The ‘data’ that we use are the conclusions about the behavior of a black hole, for example, that we can deduce from the definitions of the theory.

If the theory is complete, you can do your calculations and tell an interesting story. But sometimes the theory might be incomplete. This is where artistry might be required; you have to be creative and construct a new theory. We write down equations and create mathematical models to formalize our ideas about how black holes behave. 

Once you develop this model, this set of equations, what do you hope it reveals?  How do you use equations to come to conclusions? 

Equations are the language we have for communicating ideas or intuitions about how a system works. There are two sides to every story: there are the words, the intuition, and there are the equations, the math. I personally don’t think in equations. I think of an intuitive picture for how a system works. Then, in order to convince others that my idea is correct I have to capture it using an equation; I present the equation, showing what every term represents, and relate it to an intuitive understanding of a black hole, for example. 

How do you verify that your models/equations are valid?

A model has to be self-consistent, and it has to tell a plausible physical story that agrees with prior discoveries. 

Can you explain some of the steps that go into generating these models? What does a typical day look like for you?

Some days I’m engaged in meetings with collaborators and my advisor, and some days I’m working alone. A typical day involves reading papers and thinking about discussions I’ve had in meetings. These discussions are really important for establishing our goals and next steps. The work itself is writing down and solving equations that tell us the consequences of our models and intuitions. A lot of that is done with paper and pencil, with the aid of computer programs. You need to figure out what equations you should try to solve; they can be quite complicated, so you want to have confidence that the equation you are trying to solve is meaningful. 

What makes a theoretical physicist different from other scientists?

One thing that sets us apart is that, while we are happy to study the actual universe that we live in, we also enjoy studying other universes. These other universes have different laws of nature, and they may have different numbers of spatial dimensions. Most other scientists aren’t as concerned with other kinds of universes. But there is a mathematical basis for studying other universes, and the details of those different universes are essential to understanding more basic principles of physics. When modeling other kinds of universes, it is not true that anything goes, which is why we think that what we are doing is still relevant. 

Is there a common misconception about your field? 

There is a misconception that we can simply make things up. It’s significant that predictions made by theoretical physicists have eventually been confirmed. For example, it took 100 years for Einstein’s gravitational waves to be observed in nature. Within theoretical physics, there are some subfields that are more concerned with the outcome of experiments, and other more ‘formal’ subfields (including mine), where we set aside experiments and think at a more abstract level.

Is there anything you would tell people interested in becoming a theoretical physicist?

It’s extremely exciting! Theoretical physics is full of surprises. You might think that a problem is really hard to solve, but then one day you get a moment of clarity and you can write up your results. It is truly a field of exploration. And it’s exciting that somehow the exploration can all take place in your mind, but at the same time the work has implications for our understanding of the real world. It’s amazing how much you can explore and understand just by taking an established set of interesting and deep principles and pursuing them ruthlessly until their logical conclusions. It’s also cost-efficient research – all I need is a good box of pencils.

To learn more about David’s work:

• Check out this article in Quantamagazine on black holes

This interview with David Kolchmeyer was conducted and edited for space and clarity by Malinda J. McPherson in October 2020. 

Malinda J. McPherson is a PhD candidate in the Harvard University/MIT Program in Speech and Hearing Bioscience and Technology.

Cover image by WikiImages from Pixabay.

This piece is part of our special edition on the day-to-day lives of researchers working in many different fields of science. Are you interested in learning about a different type of scientist? Check out the rest of the special edition!