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Comment by Arnavion | Hacker News Reader

Blogspam (but with worse editing) of https://phys.org/news/2020-08-fastest-star.html

Thanks - this is much better.

> it visibly warps spacetime

The explanation for this part of the title was completely missing from the original article. They _might_ have been talking about the red shift?

> For years, S2 was thought to be the closest star to SgrA, but then S62 was discovered. As a team recently discovered, it's a star about twice as massive as the sun that orbits the black hole every 10 years. By their calculations, at the closest approach, its speed approaches 8% of the speed of light. That's so fast that time dilation comes into play. An hour at S62 would last about 100 Earth minutes.

But time dilation at 8% of light speed is less than half a percent. The so-called "blogspam" has that part much more right: "At eight percent lightspeed, the time dilation effect for S62 just barely becomes noticeable"

So where did they get '100 minutes'?

I'm not sure what affect the black hole itself would have, maybe that makes a difference?

A back of the napkin calculation says 60m should be about 62.5m at that velocity. I'm not sure where that came from either.

edit: 60.2, forgot to square the ratio.

If you got a 2.5 minute difference, did you forget to square the .08? My napkin says that 60 / sqrt(1 - .08^2) is 60.2

I sure did forget, thanks for the catch.

> where did they get '100 minutes'?

It's phys.org. Don't expect them to actually check the math. They're way off.

> I'm not sure what affect the black hole itself would have, maybe that makes a difference?

No, gravitational time dilation due to altitude above the hole is negligible for this case; even the distance of closest approach is still 215 times the horizon radius of the hole.

They got 100 minutes by using 80% of the speed of light instead of 8%. Oops.

WA for the Lorentz factor math because I'm lazy: https://www.wolframalpha.com/input/?i=sqrt%281+-+x%5E2%29+%3...

At 8% of c time dilation & length contraction are about 0.3%. That’s definitely measurable, but whoever wrote this article clearly calculated values for 80% of c rather than 8%.

I chuckled at the retro colloquialism "thats booking it". Which was current in probably the late 70's early 80s of the last century.

That said, given the time dilation, one wonders if LIGO could pull out the orbit from enough samples.

I hear that phrase still today , often enough that I’d consider it current.

Wow, that's wicked cool :-)

Is there a particular group associated with it these days? Back in the day it was hot rodders who were talking about fast cars. I wonder what the linguistic definition of a colloquialism's lifetime is, or if there even is one.

I say both wicked cool and booking it, FWIW. Mid-30s and not sure what group I might be associated with. I guess some people would call me a hipster maybe. Haven't owned a car yet.

> Back in the day it was hot rodders who were talking about fast cars.

I’m not sure where you’re from, but this is just simply not correct. I have never been in that group and I know lots of people who regularly use that phrase. It’s a fundamental colloquialism.

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