One of the things that Einstein started thinking about was this notion of simultaneity. How that applies to his special theory of relativity. And it's sort of an interesting idea, and let's do the following thought experiment. Let's say that we are sitting here on the Earth. Okay, and we have a telescope and we're looking up in the sky. And what we see is a star over here. That all of a sudden goes supernova. Right, so it blasts off all this outer layer of stuff all this light starts streaming out and heading towards us with our telescope. But let's say at the same time there is a star over here that also goes supernova. Okay, this is rather unlikely to happen, but let's pretend it does. Sends out stuff, but dominantly light is gonna come to us and we can observe it in our telescope. So here you are with your telescope light's going to come down from these two and you say, "aha!" Simultaneous supernovae. Plural is supernovae, right? So you would say, "oh, those two things went supernova at the exact same time." Okay? But what happens to the observer that is sitting over here on Planet X. Okay, they're sitting there with their telescope and they're looking at this thing and they get light from this one and then some time later, they get light from that one. So if this is planet A, and this is planet B they wouldn't say simultaneous. They would say, "aha! Supernova B came first and then Supernova A." And so in the physical world, there's no such thing as simultaneity. Two events that you observe as simultaneous are not necessarily observed by someone else as simultaneous. It depends not only on where you are located but it also depends on your relative speeds. Okay, so pretend somebody here on Earth measured simultaneous but somebody flying by would not necessarily measure the same simultaneous event. Okay, and it's this sort of simple thinking that led Einstein to realize that simultaneity? That's not a physical property. What are physical properties? Our forces, accelerations, energies, momentum things like that. Okay, there's no such thing as simultaneity. All right. So if we're not worried about simultaneity, what are we worried about?