The preamplifiers of the National Ignition Facility are the first step in increasing the energy of laser beams as they make their way toward the target chamber. NIF recently achieved a 500 terawatt shot — 1,000 times more power than the United States uses at any instant in time. Despite our uses and applications for energy, it remains notoriously difficult to define. (DAMIEN JEMISON/LLNL)

Ask Ethan: What Is Energy?

We speak about it, argue over it, and even fight wars for it. We know it when we see it. But just what is energy, anyway?

When it comes to being a human on planet Earth, energy affects practically every aspect of our lives. The energy content of a room determines its temperature; the ability to use it in a directed fashion is how we transport ourselves; we harness it to cook our food; the energy we burn in our bodies is necessary to keep us alive. From the energy of motion to stored energy to distributing or conserving it, energy affects all aspects of our lives. But even defining what energy is can be an awfully big challenge. That’s why Raza Usman asked, for this edition of our Ask Ethan column:

We talk about energy and we know that there are various forms of energy (PE, KE …) and you can do work with it, and it has to be conserved, and energy and matter are interchangeable, etc. But what is energy?

Physics can say a lot about energy, but even the top theoretical physicists have trouble concocting a definition that everyone can be happy with.

During an inspiral and merger of two neutron stars, a tremendous amount of energy should be released, along with heavy elements, gravitational waves, and an electromagnetic signal, as illustrated here. There are a variety of energy types that come into play in an event like this, and yet we still lack an unambiguous, universally applicable definition of energy itself. (NASA/JPL)