If a microwave oven disk rotates to warm up food, why doesn't it go up/down/sideways? This has been in my mind for a while... Well, actually everytime I heat any food with a greater amount of liquid in it:


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*a microwave oven warms up food by inducing polar molecules in the food to rotate and produce thermal energy;

*it heat foods quickly and efficiently because excitation is fairly uniform in the outer 25–38 mm (1–1.5 inches) of a homogenous (high water content) food item. So, if there's an "heterogeneous, dense object" with liquid right in the center of it (beyond the 25-38mm margin they mentioned), that part won't be heated properly;

*however, the not so new ovens were added a disk at the bottom, so the food container can rotate while heating, I suppose for warming up the food uniformly, since the wave is guided in some pattern to the target;

*therefore, wouldn't the heating be more efficient if the container could be moved up, down or even sideways?

 A: I think this question is more about the engineering (and economics) of manufacturing microwave ovens.
The standing wave pattern can create hot and cool spots in larger food items that are not moving inside the oven.  The rotating plate is sufficient to move the food around so that most of the food is not stuck in a node that creates large temperature differences.  Food directly along the rotation axis moves very little, but it's only a small fraction.  In addition, this region is more likely to be on the interior where the direct heating effects are reduced and temperatures may have more of a chance to equalize after heating.
Moving the entire assembly in one direction would accomplish almost the same (admittedly it would eliminate the static axis) , but would require a more complex mechanism.  Also any translation would reduce the usable volume inside the oven.  A plate that shoved a reheating chicken onto one side of the oven might not be appreciated.
So it becomes a trade-off between more expensive ovens and the chance that small portions of the food on the rotation axis might not be as uniform as other areas.  It seems that most users perceive the microwave oven as a non-precision device and are not willing to pay significantly more for small improvements in cooking ability.
