## Transcript

**Question:** Is e=mc2 changing?**Stephon Alexander:** Yes. E=mc2, meaning that there is an exact equivalence between energy- like, take energy of light- okay? And actually matter, meaning take, you know, my ring- you know, it’s a piece of metal. So what makes- why could we say that my gold ring- my graduation ring- is the same as any? So this is the bold statement Einstein made, and it’s actually true. That’s how the sun does its job- converts matter into radiation energy. And that still works there- but here’s the issue- when we go to the realm of quantum gravity or even gravity, there are two definitions of matter, actually. You see, the definition of matter that we’re talking about right here is gravitational matter, meaning that this thing has gravity, right? It has weight, right? It’s localized and it’s at some well-defined- so the energy is highly concentrated here, okay? But there’s actually energy associated with motion, so if you’re- this is so-called inertial mass- so there’s gravitational mass and there’s inertial mass, meaning that I can move- if I’m driving in my car and the car starts turning quickly, I feel some force, right? That pushes me to the edge of the car- that’s because the car changed its state of motion. So there’s an inertial mass, mass that resists the change of motion. Einstein’s big breakthrough was to realize that they’re one and the same thing. When we use E=mc2, we are talking about the mass of something, okay? Without reference to any change in its motion- meaning any acceleration, okay?

But when we talk about gravity as a whole, general relativity, and we talk about how the sun affects the earth, we have to actually take into account the equivalence of these two things. And what’s one of the things that E=mc2 is missing is how does the theory of gravity actually- equivalence between inertial mass and rest mass affecting actually the mass, okay? The gravitational mass- so it really boils down to the question of, how do we describe the very origin of mass from a quantum gravitational point of view? Really, that’s true to the Einsteinian philosophy, okay? Of background independence. Okay? So, yeah, so-

**Question:** What are your current research interests?**Stephon Alexander:** So, one of the things that I study is- I’m interested in the question of unification of the four forces, so I’m interested in how, you know, so far, we have a unified description of three of the four forces. You know, the subnuclear forces and electromagnetic forces have a- there’s a paradigm that unifies these three forces, and there’s gravity, which is a force that, you know, we’re most familiar with. I’m interested in that unification. String theory provides some insights into that unification, but I’m also interested in how, in alternative ways, I wish that unification could happen. So, one of the things I’m- and using cosmology, using actually observations, actually- there’s a lot of data now available to us in the field of cosmology. Using that data, and also there’s data that’s anomalous, that we can’t make any sense of- and using those things as input or, if you’re gonna say, motivation, or as a guide to helping us find really this unification of all the four forces. So that’s sort of the general- I use cosmology to test ideas in unification and quantum gravity and string theory and particle physics.