In this short video, we're going to discuss space time intervals. I guess I'm afraid of doing a space crime, so I don't want to do the space time. I put that case, we're going to have a space time here. So, what's the deal with these? Well, we've got these things called quantities called invariant quantities. So this came about because Einstein, he noticed that there's certain quantities like absolute time, absolute space does actually change when you go from one reference frame to another. In other words, they're not constant. That was bad when you're trying to do equations going from one to the other. So a nice easy thing to do is to try to analyze, hey, what are the things that do remain the same no matter what? So, that's why we have this idea here that some quantities, they are the same from one reference frame to another. So, these are here called invariant and that's why because they remain constant, they don't vary. That's what they're called invariant quantities. So what's an invariant quantity then? It's a quantity that remains constant. It remains the same from one reference frame to another. Now, there are a few invariant quantities that we know of. This one here, proper time interval, we've already seen those. That's delta t0. We've got proper length, which is just L0. And now we've got this new one, this idea of space time interval, which we're going to call delta s. Because this right here is going to be the new one. Okay, so this is this invariant quantity. Now that's called space time interval. So let's investigate a little bit further. So let's work on this delta s here. So this invariant quantity, remember, we're going to call this thing delta s. That's going to be the invariant quantity, this space time interval. That's what this thing here is called. We have an equation for it. And it goes like this. Delta s squared equals c delta t squared minus delta x squared. So this is an equation that you can use from your data booklet here. So notice that we often like to use ct in relativity. So units of ct, for example, when we're doing minkowski diagrams or space time diagrams, these kind of things, we often put ct, for example, on the y axis. This is still a unit of time, but it's interesting that we do like a speed of light times of time. It's actually quite interesting. But in any case, we have this space time interval. Like I said, it is an invariant quantity, this delta s here. And we have this equation that says delta s squared equals c delta t squared minus delta x squared. And we can use that if needed.