We're now in this situation where people just assume that science can compute everything, that if we have all the right input data and we have the right models, science will figure it out. If we learn that our universe is fundamentally computational, that throws us right into the idea that computation is a paradigm you have to care about. The big transition was from using equations to describe how everything works to using programs and computation to describe how things work. And that's a transition that has happened after 300 years of equations. The transition time to using programs has been remarkably quick, a decade or two. One area that was a holdout, despite the transition of many fields of science into the computational models direction, was fundamental physics.

If we can firmly establish this fundamental theory of physics, we know it's computation all the way down. Once we know it's computation all the way down, we're forced to think about it computationally. One of the consequences of thinking about things computationally is this phenomenon of computational irreducibility. You can't get around it. That means we have always had the point of view that science will eventually figure out everything, but computational irreducibility says that can't work. It says that even if we know the rules for the system, it may be the case that we can't work out what that system will do any more efficiently than basically just running the system and seeing what happens, just doing the experiment so to speak. We can't have a predictive theoretical science of what's going to happen.

STEPHEN WOLFRAM is a scientist, inventor, and the founder and CEO of Wolfram Research. He is the creator of the symbolic computation program Mathematica and its programming language, Wolfram Language, as well as the knowledge engine Wolfram|Alpha. His most recent endeavor is The Wolfram Physics Project. He is also the author, most recently, of A Project to Find the Fundamental Theory of Physics. Stephen Wolfram's Edge Bio Page

An event has a view of the world. First, let me tell you what I mean by a view. A view is the information that that event has about how it fits into the rest of the world. That includes who its parents were (by which I mean the events in its past that gave rise to it) and how much energy and momentum was propagated to it from them. If I am an event, my view of the world is what I see when I look around. I see light comes to me from the past, which I perceive as a pattern of colors, which come from photons of different energies striking my eye. That's my view; it's a property of a moment. That contains all that I, as an event, know about how I fit into the rest of the world.

Now, if you know the things that I just said were real—the events, the causal relations, the distribution of energy and momentum flowing—I can tell you what the view of each event is, but I can also flip it around. There's a dual description in which I just say what the views are and that's the whole description. So, I just say there's a view, and that view is that makes a kind of picture. You see the sky, a two-dimensional sphere around you, and there are some colors, which are photons coming in of different energies—that's the view. I can hypothesize that all that exists in the world is views and a process that continually makes new views out of old views. That's what I call the causal theory of views.

LEE SMOLIN, a theoretical physicist, is a founding and senior faculty member at the Perimeter Institute for Theoretical Physics in Canada. His main contributions have been so far to the quantum theory of gravity, to which he has been a co-inventor and major contributor to two major directions, loop quantum gravity and deformed special relativity. He is the author, most recently, of Einstein's Unfinished Revolution. Lee Smolin's Edge Bio Page

One of the great books in science was published in 1824 by a young Frenchman called Sadi Carnot. It is one of the most wonderful books, the title of which is Reflections on the Motive Power of Fire. In about six pages, Carnot explains how you would make a steam engine that would work with the absolute maximum efficiency possible. It was almost entirely ignored, and he died before anything much could come out of it. It was rediscovered in 1849 when William Thomson, who later became Lord Kelvin, wrote a paper that publicized this work. Within a couple of years, thermodynamics had been created as a science.

It caused a tremendous lot of excitement from the 1850s onwards. The key thing about this work of Carnot's is that if you have a steam engine, the steam has to remain in a cylinder in a box. You want the steam engine to work continuously, so you keep on having to bring the steam and the cylinder back to the condition it was before. It's remarkable that the development of what's called statistical mechanics—to understand how steam behaves—led to the discovery of entropy, one of the great discoveries in the history of science, and with it the mystery of the arrow of time. And it all followed out of this work of Carnot on how steam engines work. And moreover, it was very anthropocentric thinking about how human beings could exploit coal to drive steam engines and do work for them. At that stage, nobody was thinking about the universe as a whole; they were just thinking about how they could make steam engines work better.

This way of thinking, I believe, has survived more or less unchanged to this day. You still find that people who work on this problem of the arrow of time are still assuming conditions that are appropriate for a steam engine. But in the 1920s and early 1930s, Hubble showed that the universe was expanding, that we live in an expanding universe. Is that going to be well modeled by steam in a box? My belief is that people haven't realized that we have to think out of the box. We have to think in different ways. My collaborators and I keep on finding ways in which the mathematics that was developed before to understand systems confined in a box have to be modified with quite surprising consequences and, above all, possibly to explain why we have an incredibly powerful sense of the passage of time, why the past is so different from the future.

JULIAN BARBOUR is a theoretical physicist specializing in the study of time and motion; emeritus visiting professor in physics at the University of Oxford; and author of The Janus Point (forthcoming, 2020) and The End of Time. Julian Barbour's Edge Bio Page

What is fascinating to me is that we are now hoping, with modern measurements, to probe the early Universe. In doing so, we’re encountering deep questions about the scientific method and questions about what is fundamental to physics. When we look out on the Universe, we’re looking through this dirty window, literally a dusty window. We look out through dust in our galaxy. And what is that dust? I like to call it nanoplanets, tiny grains of iron and carbon and silicon—all these things that are the matter of our solar system. They’re the very matter that Galileo was looking through when he first glimpsed the Pleiades and the stars beyond the solar system for the first time.

When we look out our telescopes, we never see just what we're looking for. We have to contend with everything in the foreground. And thank goodness for that dust in the foreground, for without it, we would not be here.

Professor BRIAN KEATING is an astrophysicist with the University of California San Diego’s Department of Physics. He and his team develop instrumentation to study the early universe at radio, microwave, and infrared wavelengths. He is the author of over 100 scientific publications and holds two U.S. patents.  Brian Keating's Edge Bio page