Home / Opinion / Columns /  One photograph, one supernova, and three different images of it

Here’s news for a routine Friday in November: a star exploded about 11 billion years ago. You’re hardly impressed, ok. After all, there are stars that explode at the end of their lives, we know that. They turn into massive fireballs, known as supernovas. But while this is a familiar phenomenon, it’s not that we see supernovas all the time.

But the Hubble Space Telescope saw this one, even though it’s very far away and light from it took a long time indeed to reach us. 11 billion years long. Thus, in looking at it, we are looking far back in time, at a star in a relatively young universe—then less than three billion years old. Yet, as awe-inspiring as the idea of looking so far back in time is, it is familiar, too.

So, why did this supernova sighting make the news? Well, the Hubble Telescope did see it; more correctly, it took a photograph of a part of the sky in which the supernova is visible. But here’s what’s astonishing. That one photograph has three different images of the supernova.

You read that right. Three different images, and in fact, they differ in time by a week. This is as if you got your Shyamoli-maasi to pose for a photograph—only to find, when you look at the image later, that she appears three times in the same frame. She’s wearing the sari she wore to your birthday party last week; the salwar kameez she wore at her teaching job the next morning; and the swimsuit she used when she joined you at Juhu beach when you took the photograph two days ago. Would such a photograph spook you? Certainly it would—and yet it’s a fair analogy to Hubble’s image of the supernova.

How did this happen? Here’s a clue. The three supernova images surround a much larger and brighter spot of light. That’s an enormous cluster of galaxies called Abell 370, somewhere between the supernova and us. This is important, for it’s because of the cluster that we see multiple images of the supernova. Put it another way: if Abell 370 wasn’t where it is, we’d most likely see only one image of the supernova.

Your Shyamoli-maasi can offer another analogy here. Let’s say she lives a mile directly east of you. There’s an arrow-straight path across a large field that she takes regularly, to walk over to your home. You’re fond of her, so you often stand in your doorway and watch her approach. Since you can see her home, you could watch her from the moment she left there, if you wanted.

At some point, the municipality acquires the field and erects a cricket stadium on it. No longer can you see your maasi’s home — the stadium is in the way. No longer can she walk to your place in a straight westerly line. Instead, she approaches the stadium from her home, walks around it to its northern tip, and from there, she walks in a straight line to your front door.

Of course you know she’s coming from her home, east of you. But at the moment on her walk that she appears, at the northern edge of the stadium, she is no longer directly east of you. Instead, she’s northeast. If you didn’t know any better, on sighting her you might say she lived to the northeast, because that’s the direction she’s approaching from.

Similarly, if she turned southwards on the other side of the stadium and came around from there, you might conclude she lived southeast of you.

Something like that is happening with this supernova and us observers here on Earth. Light from there sets out in a straight line towards us. If there was nothing between it and us, the light would reach us, no problem, in 11 billion years. When it does, we’d see the star where it really is in the sky—or really was, since it actually exploded 11 billion years ago.

But as it happens, there is something between it and us, Abell 370.

What that cluster does to the light from the supernova is, for all of us who learned that light travels in straight lines, simply astonishing. But it also verifies one of the great Albert Einstein’s remarkable hypotheses. In a word, the light bends.

More correctly, it appears to bend. Think of a large rubber sheet that you hold taut. Drop a cricket ball on it, and the sheet bends with the weight of the ball. Much like that cricket ball, at least conceptually, Abell 370 bends and distorts the “fabric" of space-time. Light from the supernova behind follows the contours of that distortion. Some of it gets blocked by the galaxy cluster, no doubt. But some of it does follow the contours —bends —all around the cluster. Once on the other side, it resumes its straight-line path towards us on Earth.

And when that light reaches us, we see it as originating to one side of the intervening cluster. That is, to us, the supernova is actually visible on that side, rather than out of sight behind Abell 370.

Which is just what Einstein predicted: the phenomenon of “gravitational lensing". The gravity of the galaxy cluster attracts the light from the supernova, diverting it from its path, acting like a lens of a kind. What’s more, the light takes different paths around the cluster—like Shyamoli might around the stadium—so we see three images of the supernova. What’s still more, those paths vary in length, so light takes different lengths of time to traverse them.

So, the three images are of the supernova at—please take a moment to comprehend the wonder of this—different instants in time.

All-in-one photograph from Hubble. Don’t know about you, but this just takes my breath away.

I’m also suddenly reminded of the street corner near where I live. Leading away from it is St Sebastian Road. Till not long ago, the corner sported three signs identifying that one road. Each spelled “Sebastian" differently. Not one got it right.

Once a computer scientist, Dilip D’Souza now lives in Mumbai and writes for his dinners. His Twitter handle is @DeathEndsFun.

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