What is a gravitational lens?: windows to deep space



The concept of “gravitational lensing” is important to astronomy and physics in general, but what does it really mean?

During the last few days the concept of “gravitational lensing” has become fashionable. The first image taken with the James Webb Space Telescope, a milestone that amateurs and professionals have been waiting for more than a decade. The protagonist of the image turned out to be SMACS 0723, a cluster of galaxies that gravitationally act on each other. It may sound fascinating, and it is, but there’s something else that makes SMACS 0723 a great candidate for the James Webb debut: it acts as a gravitational lens.

It is unthinkable to say what we have said and not explain what that combination of words means. “Gravitational lensing” is a concept that only becomes intuitive if we know a little about general relativity, and if we know that little, we have probably already read about gravitational lensing, so intuition alone will do little good here.

The key to understanding what all this consists of (although we will delve into them in the next paragraphs) is that the cluster of these galaxies adds such an enormous mass in that region of space that its force of gravity is capable of bending the light that passes near him. That effect can bend light in a similar way to a giant glass lens, thus allowing us to see objects far away. Clarified this, it is time to talk about them a little more in detail.

Blame it on Einstein
At the beginning of the last century, Albert Einstein published a series of works that changed the way we understood gravity. It was no longer as “simple” as Newton had believed, at that time, thanks to general relativity, we were beginning to understand gravity as a warping of space-time. Normally (and although it involves a series of important problems) it is often said that gravity warps space-time as if we put a bowling ball on a trampoline. The mass of the object will cause the tarp to curve more or less, leaving it in a hole in the fabric. If I throw a couple of marbles onto that sinking trampoline, they’ll fall down the ledge and, if they’re on the right track, start to spiral as they fall around the bowling ball.

This is more or less how gravity would work, except that, of course, space-time is not a two-dimensional canvas that only sinks downwards… it has three dimensions and deforms in a different way, more like a mesh made of cubes where those closest to the object shrink, bringing their vertices closer to whatever is causing the warp. Now, if this is true, an object with enough mass could exert a gravitational attraction capable of bending even the same light, despite its speed and the almost negligible mass of the photons that compose it. Now, what seems like just a curiosity, has surprising implications, because depending on how they deform that light, we can see fascinating things.

A lens without glass
The lens of a magnifying glass or telescope is shaped like a lentil, thicker in the center than around the edges. This causes the light that passes through the glass to change its direction and the rays that were previously parallel, when exiting through the other side of the lens, bend towards each other, concentrating on a specific point. That’s what makes images focus precisely on our retina or children can burn things simply using a magnifying glass and sunlight. Now, if the key to this magnifying effect of the image is due only to a specific bending of light, and we have already said that gravity can bend the path of light rays… then it is to be expected that gravity can work as a giant magnifying glass.

What happens, specifically, is that light rays from very, very distant objects that would have traveled parallel and we would not have been able to see, reach the cluster of galaxies (for example), there, gravity pulls on some of them, of which pass at the right distance so that their trajectory is corrected until they surround the cluster and bend in such a way that they focus on us, as if we were the retina of a cosmic eye where gravity has worked as a lens, focusing the image. That is the idea of gravitational lensing and the first one was found in 1919 thanks to a solar eclipse observed by Arthur Eddington. So that’s what the SMACS 0723 cluster that James Webb has been observing does and, thanks to that magnification, it allows us to see much further than those 4.6 million light-years away that the cluster was when light that comes to us came out of him. In a way, gravitational lenses are windows into deep space.

Carroll, Bradley W, and Dale A Ostlie. An Introduction To Modern Astrophysics. Cambridge University Press, 2018.