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Light can be slowed down to a walking pace and even stopped in a medium of hot rubidium vapour. If an electromagnet was placed in this medium and turned on would the popagation of the magnetic field be slowed down or stopped?

Do virtual photons change speed in a medium?

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Careful - light can be slowed down and stopped but photons are not.

If I'm reading you right: Slow light refers to the speed of a pulse of light not the speed of the waves, or the photons, that the pulse is composed of.

So the short answer to your question about the electromagnet in this media is "no".

Some notes:

The slower speed of light waves in a media can be thought of as an emergent effect of the underlying QM involving real photons - that always travel at the same speed between interactions.

Virtual photons don't travel in the way commonly thought of for real photons - it's more that there is a probability of an electromagnetic interaction represented by a particle term in the perturbation theory. In Field theory they appear as disturbances in the field with a different symmetry to the real ones.

At a beginner level it is probably best to think of virtual particles as intermediate steps in a calculation.

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