Professor of Physics and Astronomy, Barnard College of Columbia University; Author, Black Hole Blues and Other Songs from Outer Space
Physicist, Columbia University; Author, How The Universe Got Its Spots

I believe there is an external reality and you are not all figments of my imagination. My friend asks me through the steam he blows off the surface of his coffee, how I can trust the laws of physics back to the origins of the universe. I ask him how he can trust the laws of physics down to his cup of coffee. He shows every confidence that the scalding liquid will not spontaneously defy gravity and fly up in his eyes. He lives with this confidence born of his empirical experience of the world. His experiments with gravity, heat, and light began in childhood when he palpated the world to test its materials. Now he has a refined and well-developed theory of physics, whether expressed in equations or not.

I simultaneously believe more and less than he does. It is rational to believe what all of my empirical and logical tests of the world confirm—that there is a reality that exists independent of me. That the coffee will not fly upwards. But it is a belief nonetheless. Once I've gone that far, why stop at the perimeter of mundane experience? Just as we can test the temperature of a hot beverage with a tongue, or a thermometer, we can test the temperature of the primordial light left over from the big bang. One is no less real than the other simply because it is remarkable.

But how do I really know? If I measure the temperature of boiling water, all I really know is that mercury climbs a glass tube. Not even that, all I really know is that I see mercury climb a glass tube. But maybe the image in my mind's eye isn't real. Maybe nothing is real, not the mercury, not the glass, not the coffee, not my friend. They are all products of a florid imagination. There is no external reality, just me. Einstein? My creation. Picasso? My mind's forgery. But this solopsism is ugly and arrogant. How can I know that mathematics and the laws of physics can be reasoned down to the moment of creation of time, space, the entire universe? In the very same way that my friend believes in the reality of the second double cappuccino he orders. In formulating our beliefs, we are honest and critical and able to admit when we are wrong—and these are the cornerstones of truth.

When I leave the café, I believe the room of couches and tables is still on the block at 122nd Street, that it is still full of people, and that they haven't evaporated when my attention drifts away. But if I am wrong and there is no external reality, then not only is this essay my invention, but so is the web,edge.org, all of its participants and their ingenious ideas. And if you are reading this, I have created you too. But if I am wrong and there is no external reality, then maybe it is me who is a figment of your imagination and the cosmos outside your door is your magnificent creation.