OK, so, there are three really important fundamental constants in physics: G, c, and hbar. G describes the strength of the gravitational force, and so if you’re working with gravity, it shows up all over the place. It shows up so much that, if you’re working with gravity, it’s easiest to pick units where G = 1. hbar (written as a letter h with a line through the upper part) is the fundamental quantum of angular momentum, and shows up all over the place in quantum mechanics. So, like the gravity people, quantum folks usually pick units such that hbar = 1. And c is the fundamental speed of the Universe (among other things, it’s the speed of massless particles like photons). If you’re doing relativity, then c shows up all over the place, and so, again, it’s customary to choose units where c = 1.

Now, it’s quite common to be working with relativity and gravity at the same time, in which case you use units where both G and c are equal to 1. Likewise, it’s common to be working with both relativity and quantum mechanics, in which case you use units where c and hbar are both 1. In either of these cases, setting those constants equal to 1 means that everything can be measured in the same units. Typically, the gravity people end up using length units for everything and the quantum people end up using energy units for everything, but that’s a minor detail: You could use any units.

But what if you’re doing something where you’re working with gravity, relativity, and quantum mechanics all at once? In practice, that’s quite rare, because we know almost nothing about the interaction of gravity with quantum mechanics. But if you were doing that, you’d want to use units where all three of those fundamental constants are equal to 1. And that means that you effectively don’t have any units left at all: All quantities are just measured in dimensionless numbers. Instead of a time of 1 day, or 1 second, or 1 meter/c, you can have a time that’s just 1. That time is called the Planck time. Similarly, a length of just 1 is the Planck length, and a mass of just 1 is the Planck mass, and so on. And from mass, length, and time, you can construct the other quantities of interest in physics, and so get a Planck energy and so on.

What do these quantities mean physically? Well, in some cases, the answer is obvious: The “Planck speed”, for instance, just brings us back to c, and the “Planck angular momentum” is just hbar. But in most cases, we don’t know of any particular physical significance: It’s just a notational convenience. It might be the case that, once we develop a theory of quantum gravity, it’ll turn out that we find significance for the others: For instance, *if* spacetime is quantized, and *if* the quantization is such that there’s a smallest length possible, then our best guess for what that smallest length would be would be somewhere in the vicinity of the Planck length. But nobody would be particularly surprised if it was something like half the Planck length or pi times the Planck length, and it might be that our best guess is just *completely* off. Or, of course, it might be that there is no such thing as a “smallest length”: That’s just a guess, too.

Nor are all of the Planck quantities even all that extreme. The Planck mass, for instance, is far greater than the mass of any known subatomic particle, but far less than the mass of any object you’re familiar with. Some bacteria have a mass about equal to the Planck mass. And the Planck momentum is a perfectly reasonable amount of momentum to encounter in a high school physics lab.