If the exterior portion of a thermos is magically floating inside the rest of the thermos so that it’s not touching anything (it also has no opening at the top where one can access its contents) and the vacuum around it is a complete vacuum (I know it’s not possible), and it’s filled with hot water, will the water remain hot forever?
No. The vacuum portion will prevent heat loss by conduction, but you will still have heat loss by radiation.
nm
OK, so make the outer envelope perfectly reflective, and we’re good to go. Except that perfect reflectors and absolute vacuums aren’t possible. Levitating the inner vessel is not impossible, I think.
No matter how perfectly magical your surroundings, quantum effects would prevent the “forever” from coming true.
This is pretty close to what a Thermos bottle does (also known as a Dewar, after its inventor, James Dewar). A Dewar is a glass bottle inside another glass bottle, with a vacuum in the space between them. Both bottles are coated with high reflectivity metal to lower the thermal radiation from the hot side and reflect back whatever radiation is emitted. The OP alluded to the Achille’s heal of this system, the opening for access to the contents.
A modern addition is to surround the inner vessel with many layers of aluminized mylar (so-called “super insulation”) to further reduce the radiative loss.
A proper Klein bottle thermos would keep things hot forever, but a real klein bottle can’t exist in space as we know it.
How about gravity waves from the movements of water molecules? In a trillion years you’d be lukewarm…
Good point, gravitational radiation will zip through superinsulation like there is no tomorrow. It’ll take way more than a trillion years for that to matter, but forever is a long time.
You’d probably cool off via neutrino processes quicker than via gravitational radiation, and you’d probably get electromagnetic radiation tunneling out faster than either, and for practical purposes, the losses due to the various ways in which the system is not quite ideal (not perfectly reflective, or imperfect vacuum, etc.) would probably in turn dwarf that.