 So if we have one of these processes that we've looked at many times before, we have an A state, a barrier, and a B state. A, B, and barrier. There will be two free energy differences here, right? Going from A to B, and from, sorry, from A to B, we're gonna need to go from A to F sharp. And going from B to A, we're gonna need to go from FB to F sharp. One in that direction and one in that direction. Both of these we can describe with kinetics. So the rate of the reaction AB is gonna be proportional to some sort of fundamental constant K0, multiplied by E raised to minus F sharp minus FA divided by RT. And similarly, KBA is gonna be equal to K0 multiplied by E raised to minus F sharp minus FB divided by RT. You know this by now. The trick here though, is that now I would like to start from experiments. Maybe I could try to measure these rates to see how quickly the folding around folding happens. That's a great idea in theory, but it's way harder in practice than one thinks. Because folding, these equilibrium processes are not like normal chemical reactions. Assuming that I start with the protein that is unfolded, and then when the protein is folded, let's say that it starts to emit light, fluorescence or something, then I can just count the amount of fluorescence, right, and see how fast it folds. Well, not so fast, because if the protein starts here, when it's moved over here, I will certainly see it, but there will be a certain fraction of the proteins here that move in the other direction. That's not what I wanted to observe. So it's virtually impossible for me to observe only the proteins going in that direction or only the proteins doing that direction. Almost all experiments end up studying both directions. There are some tricks around this, and one of them is called Sotho-Stopped Flow. So assuming that there is a reaction where I can mix two small test tubes, and mixing might just be that I'm changing the protein or something. If I have a small syringe here that I push on, and then I have a second small syringe here that I push on, if these lead to some sort of mixing chamber, and then I have a long, thin vial here, maybe just a millimeter across or so, and then these syringes are fairly large. Now, I know the volume here, and I know what the volume of these are, so what if I push so much so that I know that with the force I'm pushing, I'm gonna end up with a velocity here that is exactly one meter per second. So the molecule that is here, one second later, it's gonna be one meter away. The beautiful thing if I now put an experiment here and measure say light, well here I'm gonna be exactly zero seconds since they measured. If I measure one meter away, I'm gonna be one second after they measure, and if I measure one centimeter away, I'm gonna be 10 milliseconds after they're mixing. But the point here, one centimeter away that will keep being one millisecond after they're mixed. And the products that have been, that have spent longer, they will keep moving away. This is complicated. We don't wanna do it this way, but for now let's just assume that I could measure these rates AB or BA separately and see what that will take us if we plot them.