One critique of loop quantum gravity is that it contradicts certain predictions of Einstein, namely that the speed of light is constant for all wavelengths. What do you make of this critique? The theory has evolved a lot over the last 20 years, and the current version is not incompatible with Einstein’s predictions — the speed of light is indeed constant at all physical wavelengths. That said, there are some things about loop quantum gravity that still need resolving. We’re not sure how the different versions of the theory are equivalent to one another. We have a problem in which particle scattering seems to generate infinite amounts of low-energy radiation. And solving the equations is still a very complicated task that we’re working to simplify. Our community has wasted a lot of time searching after speculative ideas. What we need instead is to digest the knowledge we already have. The main shortcoming is the lack of experiments supporting it. However, there’s hope on the horizon. There are some proposals to use loop quantum gravity to make sense of signatures in the cosmic microwave background radiation that’s left over from the Big Bang. And there’s another new idea I’m very excited about: If loop quantum gravity is right, there should exist tiny black holes weighing around 10 micrograms that are long-living and that interact only gravitationally. We’re thinking about ways to detect a background “wind” of these particles. And perhaps these tiny black holes are actually what we call dark matter, a mysterious widespread astronomical phenomenon that we have not yet understood. Detection will be difficult, but it’s not out of the game. I’m hopeful there will be some experiment that will make the larger community see loop quantum gravity as the natural explanation. It’s far from clear that we cannot account for all of these phenomena using the existing theories that have worked so well for 100 years. If we are to hold on to our existing theories, what picture do they paint about the nature of reality when taken together? Rethinking space and time pushed me to view reality in a completely different way — not as a universe made of objects with defined properties, but as a network of interactions. This is the “relational” interpretation of quantum mechanics. In some sense, it’s a continuation of the trend in modern physics that we have seen with general relativity and quantum mechanics — a strong push toward perspectivalism. We’re used to velocity being relative: The velocity of this table is different with respect to me, with respect to [that pigeon flying] outside, or with respect to the sun. Einstein showed us that time and length are also relative to different observers. Relational quantum mechanics takes this idea a step further. It argues that all properties of an object — its color, location, size, etc. — are in principle only definable in relation to another system. We need to give up the idea that there are material things which we’re describing from the outside. The best way of conceptualizing reality in light of modern science is in terms of the relative information that pieces of nature have about one another. We can only say how the world looks from our limited, biased perspective. This is very radical, because you can no longer say, “This is a list of things in the world, and this is how they are.” We have to live with this lack of total description over reality.