Originally posted by Acolyte
Temperature doesn't really measure this - for example, a puddle of water at room temperature has much more internal kinetic energy than a lump of copper of the same mass and temperature. Temperature measures the propensity of something to transfer its internal kinetic energy to its surroundings. So once the space around the oven has photons in it (from the oven), its temperature is above absolute zero.
We should clarify. There are many definitions of temperature, one of them being "the average kinetic energy of the components of an object", another being "the propensity of an object to give up kinetic energy to another object". There are others, too. No definition is useful or applicable in all cases.
When we measure temperature with a standard thermometer, we actually are measuring the average kinetic energy of the part of the object that the thermometer is in conact with. Of course, this type of measurement is not particularly useful if we have less than about 10^6 particles to measure (an extremely small number of particles compared), about what is required for a good statistical average. And the lump of copper and the puddle do have the same average kinetic energy, provided they are both at the same temperature (room temperature).
The zeroth law definition describes two bodies being of equal temperature if they are in thermal contact and they are in thermal equilibrium (no energy transfer). This definition works too, but it's hard to test a small collection of atoms by getting another small collection of atoms and seeing if there is any energy transfer.
There's also a definition that refers to the change in entropy with the addition of energy to a system. This one allows us to define negative temperatures which are neat, but measuring the entropy of a real system is not always easy.
Having said all that, Acolyte is exactly right when he says that as the object radiates heat in the form of electromagnetic waves, the vacuum is no longer a vacuum. Outer space is generally reported as having a background temperature of about 3 K for this reason.
So I guess the oven would heat up space until it was in thermal equilibrium with it, making the oven pretty f'n cold eventually.
Now I guess the heat will continue to travel off into space until it hits something, or hits the edge of the universe. According to Acolyte, energy will never catch the edge of the universe because it's moving at the speed of light. My question is, what's on the other side of the edge? What does it mean to be on the other side of existence? How do we define the edge of existence?