[Kryptid]

this is the best i/ve heard it explained.... quoted from eric hawthorn

'This is a trickier question than it may seem.

In quantum mechanics, there is no such thing as a persistently isolated system. Even a black hole radiates, due to quantum fluctuations across its event horizon (Hawking).

So in a precise analysis, we could say that the second law, which applies only to isolated systems, is inapplicable. It would be true of isolated systems if they existed, but they don’t.

However, physics these days talks about “effective” theories. These theories work within limited regimes, such as within an energy range. At the boundaries of the regime/range, that theory falls apart and no longer explains things. It’s fair to say I think that a lot of the most up to date physics consists only of these limited-applicability effective theories. I think we have to say that the second law of thermodynamics is also only such an effective theory.

This is a trickier question than it may seem.

In quantum mechanics, there is no such thing as a persistently isolated system. Even a black hole radiates, due to quantum fluctuations across its event horizon (Hawking).

So in a precise analysis, we could say that the second law, which applies only to isolated systems, is inapplicable. It would be true of isolated systems if they existed, but they don’t.

However, physics these days talks about “effective” theories. These theories work within limited regimes, such as within an energy range. At the boundaries of the regime/range, that theory falls apart and no longer explains things. It’s fair to say I think that a lot of the most up to date physics consists only of these limited-applicability effective theories. I think we have to say that the second law of thermodynamics is also only such an effective theory."

But what does that have to do with the Karman line and, more specifically, "Karman line technologies" (which I still don't know what you mean by that)?

[evan_au]

In quantum mechanics, there is no such thing as a persistently isolated system.

That's why the laws of thermodynamics are a useful thought experiment, which define a fundamental limit to which real systems can aspire, but never quite reach.

This is different from the "sound barrier", a practical limit that plane designers wished to overcome - apparently, aircraft stresses appeared to increase asymptotically as you approach the sound barrier.
- This barrier was eventually overcame by a piloted vehicle in 1947.
- But firearms manufacturers had defeated this "barrier" long before
https://en.wikipedia.org/wiki/Sound_barrier

This is different from the Karmen Line, which is a rather arbitrary line, where properties slightly above it and not significantly different than just below it.
- It was first conquered by a German rocket in 1944
- On its return, it also went supersonic
https://en.wikipedia.org/wiki/MW_18014

With our present level of knowledge, it is believed that the speed of light in a vacuum ("c") is a fundamental limit, in that the LHC (and prior particle accelerators) have tested that c is an asymptotic limit for subatomic particles.

Even a black hole radiates, due to quantum fluctuations across its event horizon (Hawking).

Similarly, it is believed that it is impossible for an intact object to return beyond the event horizon of a black hole.
- It's energy (and probably its information, too) is released in tiny increments over a time longer than the current age of the universe
- This spreads an initially compact, energetic object (eg an astronaut) into an extremely diffuse sphere of low-energy photons, demonstrating the operation of the laws of thermodynamics even with Black Holes
https://en.wikipedia.org/wiki/Black_hole_thermodynamics