#### Quantum Contemplations

When I think about the impact that Einstein’s greatest equation, *E = mc ^{2}*, has on my research, which involves constructing thought experiments about the nature of our universe, it’s hard not to reflect on how I got started in physics. My physicist tendencies go back to my earliest years in my native Trinidad. When I was a little boy in Basse Terre, I often swam in its bay, but I would never venture very far into the ocean. There was something frightening about the Caribbean’s periodic movement, the roaring sound of its waves, and the vastness of the horizon. I considered the sea a living thing, its motion like the heaving back of a giant creature. At night, I would theorize that the starry sky was an extension of the ocean. Even at age five, I was in awe of my natural surroundings, and I constantly wondered about the world around me, just like I do today.

When my family moved to the Bronx in New York City when I was eight, the ocean and the wide, star-filled skies of Trinidad were replaced with the din of traffic and a horizon of skyscrapers. But I quickly found other things to ponder. I recall, for example, being transfixed by the magic of a remote-controlled car I received one Christmas. I didn’t yet know about what Einstein called “spooky action at a distance,” a natural consequence of the bizarre world of quantum mechanics, though I did consider that invisible forces were at work. Later, in high school physics, I learned that remote-controlled cars work because electricity and magnetism can be generated and propagated to far distances at the speed of light—Einstein’s theory of special relativity borne out. It was magical to finally understand this; I felt like a wizard and my pencil was my wand.

My high school physics teacher, Mr. Daniel Kaplan, was my greatest inspiration. He didn’t care that I and my fellow students were a bunch of immigrant kids with low SAT scores and even lower expectations. A lot of my peers were very aware of the lowered expectations of us and were very sensitive to them. Which is why we were drawn to rare souls like Mr. Kaplan: he took us seriously. I came to school solely to be in his classroom and get my daily dose of his kindness and, of course, his knowledge about the laws of nature.

I had a conversation with him that has had a lifelong impact on me. It started with me asking, “Mr Kaplan, where do space and time come from?” He answered, “To understand that, Stephon, you’ll have to learn Einstein’s theory of general relativity. If you can master Einstein’s relativity, you’ll be a master of space and time.” This was all I needed to hear. I began to read everything I could get my hands on. Ten years later, I completed a Ph.D. in theoretical physics at Brown with a specialty in cosmology and string theory. But did I master space and time like Mr. Kaplan promised? Well, the answer is not a straight yes or no.

## MASTERING SPACE-TIME

Everyone has heard of *E = mc ^{2}, *but how many of us realize that our very existence here on Earth depends on the equation?

*E = mc*which outlines the equivalence of matter and energy, explains for starters how our sun provides warmth and life on Earth because of the continuous conversion of elements such as hydrogen into radiation energy. What the equation does not describe is

^{2},*how*energy is converted into matter and vice versa. To explain that process, one must consider how special relativity works in the quantum or microphysical domain, where my work is involved.

**We are well on the way to unlocking profound surprises about our universe.**

Special relativity famously tells us that if the speed of light is the same to different observers moving at different relative speeds, then at very high speeds space and time reveal their true faces. The most striking observation is that time is no longer absolute. Different observers can experience clocks ticking faster or slower depending on their individual states of motion. How can this be? Space and time are unified into a four-dimensional reality that is no longer separated into three spatial dimensions and one time dimension, as we commonly think about them. Einstein merged space and time into an entity he called space-time. This means that if you change your relationship with respect to space (by moving very fast) you will automatically change your relationship to time. If you think *this* is weird, read the next paragraph.

Not only are space and time unified, but space-time itself is relative. This physical reality was revealed when Einstein formulated his general theory of relativity. Under this description of the physical universe, all of space-time itself is dynamical. It is no longer a static stage as in a Broadway play, with actors dancing and singing across it. In Einstein’s view, space-time itself is an actor. It has a script of its own and responds to the actors of energy and matter. In general, it is incorrect to think that things live in a place called space-time. Our experience of objects living in space-time is a relational coincidence. This space-time script of general relativity is Einstein’s field equation. The field equation relates the contents of energy and matter to the dynamics of space-time. Matter and energy curve space-time, and the space-time curvature makes matter move.

This basic definition of general relativity begs a question. We know from*E = mc ^{2}* that energy can create mass “actors,” which can transform into energy “actors,” but what is responsible for the origin of the space-time “actor?” Where does the fabric of space-time come from? To properly address these questions we need a quantum theory of gravity, an explanation of the microscopic world, which is governed by the rules of quantum mechanics. That’s what I’m here for. I and other physicists are trying to combine into one unified picture the two most important achievements of 20th-century physics, quantum theory and Einstein’s relativity. If we can successfully combine these two theories and generate a quantum theory of gravity, we will solve one of the greatest puzzles of physics and achieve one of the deepest insights physicists have ever had into how our universe works and how the space-time that governs it came into being. My work takes on quantum mechanics, string theory, and relativity in hopes of solving this conundrum.

This basic definition of general relativity begs a question. We know from*E = mc ^{2}* that energy can create mass “actors,” which can transform into energy “actors,” but what is responsible for the origin of the space-time “actor?” Where does the fabric of space-time come from? To properly address these questions we need a quantum theory of gravity, an explanation of the microscopic world, which is governed by the rules of quantum mechanics. That’s what I’m here for. I and other physicists are trying to combine into one unified picture the two most important achievements of 20th-century physics, quantum theory and Einstein’s relativity. If we can successfully combine these two theories and generate a quantum theory of gravity, we will solve one of the greatest puzzles of physics and achieve one of the deepest insights physicists have ever had into how our universe works and how the space-time that governs it came into being. My work takes on quantum mechanics, string theory, and relativity in hopes of solving this conundrum.

**TOWARDS A THEORY OF STRINGS AND/OR LOOPS**

So how close are we? Currently, there are two main themes in the attempt to formulate a theory of quantum gravity: string theory and loop quantum gravity. At the moment, I am thinking a lot about how these two theories can relate to each other in complementary ways. String theory tells us that matter is like vibrating spaghetti when you get down to very small length scales and that everything, including space and time, emerges from the vibration of these strings. Loop quantum gravity, on the other hand, tells us that at even smaller scales, space and time become atomistic, and there is no space and time outside these atomistic networks of loops. Which is correct? I don’t know. This is why research in these areas is so exciting and challenging.

It is amazing that we have come so far since Einstein’s breakthrough 100 years ago. While we continue to understand how recent observations of the universe’s deepest secrets such as dark energy and dark matter jibe with relativity and our ongoing quest for quantum gravity, we are well on the way to unlocking profound surprises about our universe. I hope to be there every step of the way.