They calculated it, and could only say "less than 25% of C" which isn't even a rotational speed measurement?!? Get bent. If you don't want to say, fine, don't. But don't tell me you've calculated something and then A) not give me that number, and B) give me a number that isn't the number you calculated and can't be used to determine the number you calculated without additional data that you also haven't provided.
this post was submitted on 25 May 2024
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Perhaps the article is over simplifying, but even if it isn't, to be able to calculate an upper bound for something we didn't have before is valuable. With more data, they'll be able to understand the range of spin speeds in similar objects, and how those correlate to mass and age. Once they have a solid baseline, they can start to look at outliers and try to understand why those are different. Science is a learning journey, not necessarily a destination.
I'm fine with even just an "order or magnitude" ballpark number. But again, they did not give us a rotational speed.
Are you complaining about the actual article or this sciencealert.com coverage of the article
I've only seen the sciencealert.com link, so that one.
It might be that I expect technical proficiency from science journalists, which might be a high bar.
Dude, ofc they couldn't observe the black hole itself, that's the speed of the disc around it.
As for the rotation, even the article states 15 days. The impressive part is (with approximations) figuring out the speed of the disc (you can't just zoom in on a billon light years & measure distances).
15 days. Great. What's the circumference? Oh, they didn't give that either. "15 days" is still not a rotational velocity, it's linear.
I'm not giving these folks grief for doing near impossible measurements, I'm giving the article grief for claiming a measurement was made, and then not stating said measurement.
What's wrong with it rotating one quarter the speed of light?
Because linear velocity and angular velocity (the speed at which an object spins) are two different quantities, as is already apparent from the units (m/s and rad/s). Saying that something rotates with 1/4 c is simply not a useful statement. It would be useful if you were saying that, for example, an object at the event horizon travels at that speed. But that speed is dependent on your altitude above the massive body. The same angular velocity (rotational speed) at greater heights translates to lower speeds than further below.
The same thing that's wrong with saying I'm going 1250 rpm down the road. It may be correct, but doesn't actually mean anything without more data.
Think about how fast a point on the equator of the Earth moves relative to a point a few cm away from the North Pole.
In one full rotation of the Earth, the point on the equator will have traveled 40,000 km, and the point by the North Pole will have traveled a meter.
So... it's that it's a useless way to express rotation.
eli5 how