Planck's constant is a thing we all hear about in high school physics classes, but no one really talks about why it's cool.
Textbooks will talk dryly about "blah blah theories of quantum gravity" and teachers will repeat that "something something a photon's energy to its frequency. And sure, these things are true. But when people say stuff like that, they're really burying the lede.
Planck's constant is about the limits of what's physically possible in the Universe!
A Planck length is a unit of length which can be derived from Planck's constant. It is the smallest length which is possible in our Universe.
And it's unimaginably tiny. A hundredth of a billionth of a billionth of the size of a proton. But this is significant. Anything smaller than a Planck length cannot exist. It would not make physical sense.
Planck lengths are basically Universe pixels.
A Planck time is the time taken for light to travel one Planck length. As Einstein proved, it's impossible for anything to travel faster than light. That's the upper speed limit for anything travelling through the spacetime fabric of our Universe.
So the Planck time is arguably the smallest unit of time which can possibly exist and still have any meaning.
At the other end of the scale you have the upper limits which are also absurd.
A Planck temperature is 140 thousand billion billion billion degrees. That's the highest possible temperature in our Universe. Nothing hotter can exist.
A Planck density is roughly 100 billion billion times the mass of the Sun squeezed into the size of an atomic nucleus. It is the highest density which can possibly exist. But that one's theoretical. A black hole would form long before anything could get that dense.
@InvaderXan Nothing hotter can exist in our universe under the same rules as everything else that is, it is possible that above the plank temperature, there are other, hotter temperatures, but at those temperatures the laws of physics literally change, we don't understand exactly how they change or what to, all we know is that they would.
@starwall Honestly, I'm skeptical of any hypothesis which requires us to literally move the goalposts, particularly where there's no evidence that those goalposts are even movable. It's questionable how many of those hypotheses would survive Occam's razor.
@InvaderXan Well, at a certain temperature in the past of the universe, the laws of physics were different. various independent forces were conjoined and worked differently, for the first brief moments of reality this is what the universe was like. the plank temperature is basically a definition of that limit which we can't look past, where the math breaks down. it's a line in the sand marking where our theories cannot tread past with any certainty. Absolute Hot is a hypothesis, whereas at least with Absolute Zero we've basically proven it to be impossible to get to in the lab
@InvaderXan this is one of those extremely cool regions of physics where the answer is still, basically "dunno"