Scientists just figured out why black holes make multiple copies of faraway galaxies by warping light. Basically, the light gets bent so much it loops around the black hole, creating these weird, stretched-out repeats of the same galaxy. How far apart those copies show up depends on how fast the black hole is spinning – the faster it spins, the closer together the images appear. This could help us test out gravity theories, and someday we might even see stuff like supernovas happening over and over because the light keeps circling the black hole.

Someone on Reddit named Harry had a pretty popular comment about this, getting over 60 upvotes. He was wondering about the time gap between each repeat image, especially for something like a supernova. Like, how long would it take to see the 500th image after the first one? Obviously it depends on the black hole’s size and mass, but it’s a cool thing to think about.

You’ve probably heard about black holes, those crazy spots in space where gravity is so intense that not even light can get away. And yeah, things get pretty weird around them—space and time don’t act normal at all.

When you get close to a black hole, space bends in such a wild way that light starts curving around it. If the light gets too close, it can actually loop around the black hole a bunch of times. That means if we’re looking at a galaxy or something far away, we might get lucky and see multiple copies of it, though each one gets more warped than the last.

Multiple versions of galaxies

Here’s how it works. Imagine a galaxy way out in space, sending light in every direction. Some of that light skims past a black hole and gets bent a little. Other light gets even closer, doing a full loop around the black hole before heading our way. The closer you look to the black hole, the more copies of that same galaxy you’ll see, each one appearing nearer to the edge of the hole.

Now, here’s the weird part. If you want to see the next image, how much closer do you have to look? Turns out, it’s about 500 times closer. Math folks might recognize that number as e to the power of two pi, but honestly, that’s just fancy talk for “a lot.”

For decades, we knew this was the number, but nobody really understood why. That is, until Albert Sneppen, a grad student at the Cosmic Dawn Center, cracked the code using some seriously clever math. It’s one of those things that seems obvious once someone explains it, but getting there? That’s the real trick.

“There’s something incredibly beautiful about now understanding why the images repeat so elegantly. Moreover, it opens up new possibilities for testing our knowledge of gravity and black holes,” says Albert Sneppen.

Working through a math proof is pretty satisfying on its own, but it also helps us get a better handle on how wild this whole thing really is. That “500” number isn’t just random – it’s tied right into how black holes and gravity do their thing. Now we can actually use those repeating images to test out and double-check our gravity theories, which is pretty cool when you think about it.

Rotating black holes

Sneppen’s method introduces an exciting new aspect: it can be applied not only to “simple” black holes but also to rotating black holes — and, in reality, all black holes spin.

“It turns out that when the black hole spins really fast, you don’t have to look 500 times closer for the next image. It’s much less — the distance could be 50, or 5, or even just 2 times closer to the edge of the black hole,” explains Sneppen.

So here’s the thing—if every new image had to zoom in like 500 times closer to the black hole, all those pictures would just get squished into one big ring shape, kinda like what you see in that diagram over there. That would make it super hard to spot multiple images. But with spinning black holes, there’s actually more room for those extra images to show up, which means we might actually get to prove this whole theory pretty soon. And that’d be huge, because we’d learn way more about black holes and even the galaxies hiding behind them.

Oh, and another cool part—the longer light takes to loop around the black hole, the more delayed those images get. Like, imagine a star in some far-off galaxy going supernova. We could actually see that explosion over and over again, which is just wild to think about.

Albert Sneppen’s article has just been accepted for publication in the journal Scientific Reports, and can be read here: Divergent reflections around the photon sphere of a black hole.

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