Mysterious deep-space flashes repeat every 157 days

https://ui.adsabs.harvard.edu/abs/2017ApJ...837L..23L/abstra...

Man, don't be too modest. With these few lines I'd be tempted to award you a Nobel prize if I were the judge :)

(exaggerating a bit, but more seriously: you seem to have your chops in this department).

Given your arithmetic, I assume by "maximum speed" you mean the speed of light, in which case travel wouldn't take 25k years. It would in fact be instantaneous. At relativistic speeds, distances contract and time dilates.

Now, this is all moot because this:

> The idea that aliens would use these to propel ships is rather ludicrous, and highly unlikely.

... is absolutely correct. No amount of energy will propel a massive object to the speed of light.

edit: I realized I may have misinterpreted. From the perspective of someone watching their own kind travel that expanse, it would indeed appear to the outside observer to take 25k years to travel.

That said, I don't think distances/timescales is a great argument against aliens, since it makes a lot of assumptions. They could use robotic ships, or generation ships (no telling how long their generations might last.) If they're moving at a significant fraction of the speed of light, relativistic effects could be dramatically reducing their perceived mission duration. Maybe the biology of their species is such that cloning or suspended animation is straight forward.

We're talking about aliens, right? Who says they must have human life spans? Perhaps a 25k year trek is a walk in the park for them.

I find this assumption (that life on other planets will have the same duration as life on Earth) very often in discussions of possible discovery of or interaction with alien civilisations. It is a huge assumption, on par with assuming that aliens must have a head, two hands and two feet and communicate by making sounds, like we do.

Accordingly, the fact that we are limited in our ability to travel from star to star because of short duration of our life spans relative to the speed of light, is not to be taken for granted as a universal characteristic of all life in the universe.

"At its equator it is spinning at approximately 24% of the speed of light, or over 70,000 km per second."

https://en.wikipedia.org/wiki/PSR_J1748%E2%88%922446ad

Magnetars, even more so - and can have a measurable effect on the Earth's atmosphere:

https://slate.com/technology/2012/12/cosmic-blast-magnetar-e...

https://en.wikipedia.org/wiki/SGR_1806%E2%88%9220

It's an old idea: Pioneer and later spacecraft have a plaque showing our location relative to distances to pulsars we can see from Earth.

https://en.wikipedia.org/wiki/Pulsar

https://en.wikipedia.org/wiki/Pulsar#/media/File:Pioneer_pla...

edit: I guess if you're a Type 3 civilization with nothing better to do, you wouldn't have to set up lighthouses but you could better direct your attention to moving galaxies around or whatever.

Edit: clarified

As an example, our day on Earth is roughly 1440 minutes, while on Mars it is 1477 minutes. So on Mars it would be 153 & 65 day intervals.

I deliberately chose the closest comparison to Earth. Had I chosen any other planet in our solar system, the numbers would have been much, much more different (on Jupiter it would be 406 & 173 days, respectively).

EDIT: Also an Earth day is not a constant value. It would have been much shorter 3 billion years ago and this cycle wouldn't be repeating in prime numbers. This theoretical alien intelligence would therefore need to not only be able to recognize life on Earth in the first few hundred million years it existed. It would also need to be able to accurately model the Earths orbit over the next 3 billion years in order to customize the message for when it would be received. All that work to send the message and they still needed to luck out that the 3 billion year journey happened to reach us at nearly the perfect time for our technology to advance enough so we could hear it.

https://arxiv.org/pdf/2003.03596.pdf