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I cant understand that. If there isn't a material that makes it hard to pass, why there is a resistivity in vacuum?

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There is no resistivity in the vacuum (resistance would imply dissipation and nothing dissipates in the vacuum). There is only impedance. The impedance gives you the ratio of electric to magnetic field in an electromagnetic wave. This ration is dependent on the unit system chosen. In SI units it has the dimension of a resistance. You can choose other unit systems where it is just 1.

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More specifically it the imaginary part of a complex impedance, which implies the ability to store energy in a field and thus propagate energy eg. poynting vector.

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There is no resistivity in vacuum. You are familiar with RLC circuits right? R dissipates the energy in the circuit. Where as L and C stores the energy in the circuit. The same happens here. The vacuum stores and releases energy as it passes through the vacuum. In a perfect vacuum, number of photons emitted = number of photons received. But, in other mediums, number of photons emitted > number of photons received.

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