Astronomers detect regular rhythm of radio waves, with origins unknown

Re:Astronomers detect regular rhythm of radio waves, with or... by cheerlessbog on Sunday June 28, 2020 @08:51AM (#23668390)

A broader question that puzzles me, not specific to this phenomenon. Pulse length is often used to bound the size of a source in space (it cannot be smaller than light takes to traverse the object). But it’s possible that two phenomena can be correlated without being close, if there is a delayed action (akin to setting two alarm clocks and separating them in space). How can we confidently use this as evidence of size?

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Re:Astronomers detect regular rhythm of radio waves, with or... by throwaway4220 on Monday June 29, 2020 @05:26PM (#23683308)

> Pulse length is often used to bound the size of a source in space (it cannot be smaller than light takes to traverse the object).

Sorry for the possibly noob question, but why is this? I can't think of an intuitive explanation

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Re:Astronomers detect regular rhythm of radio waves, with or... by cheerlessbog on Tuesday June 30, 2020 @01:04AM (#23686732)

I guess the assumption is that the phenomenon starts at a point, spreads across the object sized l at c, and lasts for only a period of time t at any location. Therefore the pulse ends after l/c + t = T. By measuring T you can put an upper bound on l.

Example of this inference, from the Wikipedia on Fast Radio Bursts (but I have seen in many contexts):

> The sources are thought to be a few hundred kilometers or less in size, as the bursts last for only a few milliseconds

Does that make sense?

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Re:Astronomers detect regular rhythm of radio waves, with or... by cheerlessbog on Tuesday June 30, 2020 @01:12AM (#23686778)

I just realized what I described is not giving a lower bound.

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