system
Given the following post, respond as if you're commenting on Reddit:
human
What happens if someone seals a completely empty box in space and brings it to earth?

My thermos has a vacuum, but my coffee still gets cold. If I bring home a vacuum from space, what will happen? Will I be able to use it to keep my coffee hot forever? Will the box close in on itself? 

Thanks in advance for all the answers!
gpt
If the box is not strong enough, the pressure difference between the inside (0) and the outside (1 bar) will cause it to collapse.

If the box is strong enough and it is sealed well, then you'll just have a box with a near-vacuum inside it. This isn't anything extraordinary, since we can produce vacuums that are more pure (that is, fewer particles per unit of volume) in labs than what you can find in space in the general neighbourhood of our planet. Since no seal is perfect, over time the vacuum will become less and less pure as atmospheric particles leak in.

You won't be able to put your coffee into it after bringing it back to Earth without opening the box and breaking the vacuum, so that might be an issue. But suppose that you are able to do so in some way, then a generic box will likely be less effective than a thermos.

To see why, it is important to know how heat is transferred. Heat transfer happens through 3 primary modes: Convection, conduction and radiation. The first is the movement of heat by the movement of a gas or liquid. A breeze blowing away hot air of a campfire or the hot air near a radiator rising to the ceiling, for example. Conduction is the transfer of heat through a material or between different materials. Some materials conduct heat very well (metals for example), others conduct quite poorly (air), but stuff is the least conductive when there's no material to conduct the heat (vacuum). 

Last, but not least, is radiation. Every object or substance emits radiation, with intensity (and spectrum, but that's not relevant right now) depending on its temperature. If all objects are at the same temperature, they'll receive roughly the same amount of radiation as they emit and things stay pretty much the same. If one object is much hotter, it will emit more radiation than it receives from the cooler surroundings and the object will cool down (and the surroundings will heat up, slightly).

Now, in case of a thermos, the contents are confined and there is no flow out of the thermos, so convection doesn't play a role. In order to minimize conduction, the contents are, as much as possible, isolated from the outside world by a vacuum. This makes the heat loss through conduction rather small, but still non-zero. In order to shield against radiation heat loss, the walls of a thermos are made reflective, which ensures that most of the radiated heat is reflected back.

Unless your hypothetical box filled with space-vacuum that was somehow filled with coffee has the same reflective walls that a thermos has, it's going to be less effective than your thermos since more heat is lost through radiation. If it does have reflective walls, it'll have more or less similar effectiveness, depending on the construction of the box and the quality of the thermos you're comparing to.