Is the Speed of Light Really Constant in a Vacuum?

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Pierre007080
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Speed of light in a "vaccuum"

My question is: What is this "vaccuum" vacant of? Unless the vacuum is at -273 degrees C it cannot be vacant of electromagnetic waves due to black body radiation from the material surrounding it. If this is true, then the statement that light can propagate through a vacuum has never been proven.
 
on Phys.org

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