Comment by DoctorOetker

> Space is a vacuum. i.e. The lack-of-a-thing that makes a thermos great at keeping your drink hot.

1) The heat can be transported by a heat carrier conducting heat standing still.

2) The heat can be transported by a heat carrier in motion.

3) The heat can be transported by thermal radiation.

The first 2 require massive particles, the latter are spontaneous photons.

A thermos bottle does not simply work by eliminating the motile mass particles.

Lets consider room temperature as the outer thermos temperature and boiling hot water as the inner temperature, that is roughly 300 K and 400 K.

Thermal radiation is proportional to the fourth power of temperature and proportional to emissivity (which is between 0 and 1).

Lets pretend you are correct and thus thermally blackened glass (emissivity 1) inside the vacuum flask would be fine according to you. That would mean that the radiation from your tea to the room temperature side would be proportional to 400^4 while the thermal radiation from room temperature to the tea would be proportional to 300^4. Since (400/300) ^ 4 = 3.16 that means the heat transport from hot tea to room temperature is about 3 times higher.

If on the other hand the glass was aluminized before being pulled vacuum the heat transports are proportional to 0 * 400 K ^ 4 and 0 * 300 K ^ 4 . So the heat transport in either direction would be 0 and no net heat transport remains.

If you believe the shiny inside of your thermos flask is an aesthetic gimmick, think again.

You are making a non-comparison.

Imagine comparing a diesel engine car to an electric car, but first removing the electric motor. Does that make a fair comparison???