Lenses and rings

"Spacetime tells matter how to move; matter tells spacetime how to curve."

This rather mind-blowing statement by groundbreaking American physicist John Archibald Wheeler summarizes, in one sentence, Einstein's General Theory of Relativity.  The presence of matter warps the fabric of spacetime, and that curvature affects how objects are able to move through it.  In a sense, gravity isn't pulling on you right now; you're simply occupying a position in space where the mass of the Earth curves space so much that you're constrained to moving with it as it rotates on its axis.  The Earth itself traces an elliptical path around the Sun because the Sun's huge mass contorts the space around it; the Earth is following the shortest possible path through a spacetime that is itself curved.

If this is hard for you to picture, you're not alone.  It's easier if you reduce the dimensions by one, and picture a two-dimensional sheet deformed into a third spatial dimension by a heavy weight, like a bowling ball resting on a trampoline.  If you roll a marble toward it, it will follow the curvature of the surface -- not because the bowling ball is somehow attracting the marble, but because the sheet itself curves.

[Image licensed under the Creative Commons OpenStax University Physics, CNX UPhysics 13 07 spacecurve, CC BY 4.0]

So what this means is that gravity can affect even something that doesn't have mass -- like light.  Light takes the shortest possible path through the space it crosses, so the common-sense assumption is that this would be a straight line, consistent with the Euclidean geometry we all learned in high school.

The thing is, space isn't Euclidean.  Oh, it's close enough, on small scales and when you're not close to ginormously massive objects; the famed Greek mathematician did pretty well, given what information he had access to.  It's just that there are objects in the universe that are so massive that spacetime curves dramatically -- and light near them no longer travels in a straight line, but follows the curvature of the space it's passing through.  The effect is called gravitational lensing, because the light bends as if it were passing through a curved glass lens.

As you might expect, this distorts your view of whatever the light is coming from.  And the results can be nothing short of bizarre -- such as the image we just got to see this week from the Euclid Space Telescope of an "Einstein ring," where two massive astronomical objects are in perfect alignment with the Earth, so that the light from the farther one is bent as it passes around the nearer, creating a ghostly halo.

The ring is light coming from a single object which is directly behind the central bright galaxy; the mass of the galaxy has warped the space the light is passing through, stretching the background image into a circle [Image is in the Public Domain courtesy of NASA]

"An Einstein ring is an example of strong gravitational lensing," said Conor O'Riordan, of the Max Planck Institute for Astrophysics, who was lead author of the paper analyzing the ring, which was published in the journal Astronomy & Astrophysics.  "All strong lenses are special, because they're so rare, and they're incredibly useful scientifically.  This one is particularly special, because it's so close to Earth and the alignment makes it very beautiful."

"Close," of course, is a relative term.  The foreground galaxy, NGC 6505, is 590 million light years away; the background galaxy -- the one whose light has been distorted into a ring -- is 4.42 billion light years away.  But still, the fact that they've lined up so precisely that the lensing effect creates the image of a ring is pretty spectacular.

The coolest thing about this, though, is that it is a visible and tangible demonstration of a principle in physics that is kind of out there by anyone's estimation.  The results of the General Theory of Relativity -- phenomena like time dilation and Lorenz contraction -- are so bizarre that it's easy to say, "Oh, come on, that can't possibly be true."  (Never mind that even a relatively lightweight object like the Earth is massive enough that our GPS satellites have to adjust for relativistic effects -- or within a couple of days, our global positioning data would become so inaccurate as to be useless.)

But in this case, the effect is also strangely beautiful, isn't it?  It's hard to look at the photographs from Euclid and JWST and Hubble and not be overawed by how magnificent the universe is.  And the more we understand it -- like finding a glittering ring that falls right in line with Einstein's predictions -- the more astonishing it becomes.

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