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Physics
Pulsars produce short bursts of electromagnetic
radiation
These are spread over a wide frequency range,
and observed by radio-telescopes
If space was an empty vacuum, all the signals would
travel at the same speed; however, due to free
electrons, different frequencies travel at different
speeds (dispersion)
The difference in travel time is proportional to distance,
so the distance can be deduced from the relative time
lag between different frequencies
De-dispersion – p. 3