# Microwave oven at relativistic speed

## Main Question or Discussion Point

Imagine a microwave oven on some kind of track such that it can reach speeds approaching c. If the microwave was switched on and shot past us at a speed so that as it traveled away from us, the relativistic doppler effect shifted the emitted microwaves into the visible spectrum - What would we see?
Could we 'see' the microwaves heating some food?

## Answers and Replies

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ghwellsjr
Gold Member
Microwave ovens are designed to keep the radiation inside so as not to cook the observer peeking through the window on the door of the oven. So there won't be any emitted microwaves for any observer traveling at any speed with respect to the oven to see.

However, if you modified a microwave oven so that some of the radiation could leak out or if you just think about a microwave antenna radiating a turkey sitting out in front of the antenna, then in order for the microwaves to get into the visible range, you would have to be traveling toward the turkey, not away from it. And if you did, the turkey would be blueshifted so that you would no longer be able to see it. You also wouldn't be able to see the radiation that is being absorbed by the turkey, because it is being absorbed. The heat radiation that would be re-emitted would be at the wrong frequency for you to see at that speed.

Why don't you think about a different problem? You have a turkey sitting in your car in the bright sunlight with the windows up. You can now see the radiation that is heating up the turkey but can you see what you want to see? I don't think so. I think what you want to see is the re-emitted heat radiation, correct? So why don't you get some goggles that allow you to see that? Then you could watch the turkey in your oven or in your car being cooked.

Nugatory
Mentor
Imagine a microwave oven on some kind of track such that it can reach speeds approaching c. If the microwave was switched on and shot past us at a speed so that as it traveled away from us, the relativistic doppler effect shifted the emitted microwaves into the visible spectrum - What would we see?
Could we 'see' the microwaves heating some food?
Microwaves have a longer wavelength than visible light, so we'd want to be looking as it was traveling towards us, blue-shifting the radiation into the visible spectrum. But with that correction, and if we use the scare-quotes around the word 'see' to cover all the practical problems of actually making such an observation... Yes, we could in principle 'see' the contents of the microwave illuminated by microwaves (or at least we could if the door to the oven were not opaque to microwaves).

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Microwave ovens are designed to keep the radiation inside so as not to cook the observer peeking through the window on the door of the oven. So there won't be any emitted microwaves for any observer traveling at any speed with respect to the oven to see.

However, if you modified a microwave oven so that some of the radiation could leak out or if you just think about a microwave antenna radiating a turkey sitting out in front of the antenna, then in order for the microwaves to get into the visible range, you would have to be traveling toward the turkey, not away from it. And if you did, the turkey would be blueshifted so that you would no longer be able to see it. You also wouldn't be able to see the radiation that is being absorbed by the turkey, because it is being absorbed. The heat radiation that would be re-emitted would be at the wrong frequency for you to see at that speed.

Why don't you think about a different problem? You have a turkey sitting in your car in the bright sunlight with the windows up. You can now see the radiation that is heating up the turkey but can you see what you want to see? I don't think so. I think what you want to see is the re-emitted heat radiation, correct? So why don't you get some goggles that allow you to see that? Then you could watch the turkey in your oven or in your car being cooked.
Thank you for such a great answer. I hope one day, that through my education in Physics I too will reach a point when I can give such an answer.
Rather than thinking about seeing the re-emitted heat radiation, I was more interested in whether or not I would see the food surrounded by some intense colour cloud.
I believe if we got past the impracticalities of seeing the oven, we would indeed see the microwaves but we would no longer see the food. But from what you've said on absorption, we could only see microwaves that aren't absorbed by the food, which I assume is a substantial amount.

ghwellsjr