The Biggest Telescope in The Universe

Vinnie rocks back in his chair.  “These gravitational lenses, Cathleen.  How do you figure their apertures and f-numbers, space being infinite and all?”

She takes a breath to answer, but I cut in.  “Whoa, I never got past a snapshot camera.  How about you explain Vinnie’s question before you answer it?”Bird and lenses

“You’re right, Sy, most people these days just use their cellphone camera and have no clue about what it does inside.  Apertures and f-numbers are all just simple geometry.  Everything scales with the lens’ focal length.”

“That’s how far away something is that you’re taking a picture of?”

“No, it’s a characteristic of the lens itself.  It’s the distance between the midpoint of the lens and its focal plane, which is where you’d want to put the sensor chip or film in a camera.  The aperture is the diameter of the light beam entering the lens.  The optimal aperture, the image size, even the weight of the lens, all scale to the lens focal length.”

“I can see image size thing — the further back the focal plane, the bigger the image by the the time it gets there.  It’s like a lever.”

“Sort of, Vinnie, but you’ve got the idea.”

“The aperture scales to focal length?  I’d think you could make a lens with any diameter you like.”

“Sure you could, Sy, but remember you’d be using a recording medium of some sort and it’s got an optimum input level.  Too much light and you over-expose, too little and you under-expose.  To get the right amount of light when you take the shot the aperture has to be right compared to the focal length.”

“Hey, so that’s the reason for the old ‘Sunny 16‘ rule.  Didn’t matter if I had a 35mm Olympus or a big ol’ Rollei, if it was a sunny day I got good pictures with an f/16 aperture.  ‘Course I had to balance the exposure time with the film’s speed rating but that was easy.”

“Exactly, Vinnie.  If I remember right, the Rollei’s images were about triple the size of the little guy’s.  Tripled focal length meant tripled lens size.  You could use the same speed-rated film in both cameras and use the same range of f-stops.  The rule still works with digital cameras but you need to know your sensor’s ISO rating.”

“Ya got this, Sy?  Can we move on to Cathleen’s gravity lenses?”

“Sure, go ahead.”

“Well, they’re completely different from … I’ll call them classical lenses. That kind has a focal plane and a focal length and an aperture and only operates along one axis.  Gravitational lenses have none of that, but they have an infinite number of focal lines and rings.”

Gravitational lens and galaxy“Infinite?”

“At least in principle.  Any observation point in the Universe has a focal line running to a massive object’s center of gravity.  At any point along the line, you could look toward an object and potentially see all or part of a ring composed of light from some bright object behind it.  Einstein showed that a completed ring’s  visual angle depends on the deflector’s mass and the three distances between the observer, the deflector and the bright object.”

“The way you said that, there could be a bunch of rings.”

“Sure, one for each bright object shining onto the lens.  For that matter, the deflector itself could be complex — the gravity of a whole cluster of galaxies rather than the single black hole we’ve been assuming as an example.”

“That diagram reminds me of Galileo’s telescope, just a three-foot tube with an objective lens at the far end and an eyepiece lens to look through.  But it was enough to show him the rings of Saturn and the moons of Jupiter.”

“Right, Sy.  His objective lens was maybe a couple of inches across.  If its focal point was halfway down the tube, his scope’s light-gathering power would match an f/9 camera lens.  Gravitational lenses don’t have apertures so not an issue.”

“So here we are like Galileo, with a brand new kind of telescope.”

“Poetic, Vinnie, and so right.  It’s already shown us maybe the youngest galaxy, born 13 billion years ago.  We’re just getting started.”

~~ Rich Olcott

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The Fellowship of A Ring

Einstein ring 2018

Hubble photo from NASA’s Web site

Cathleen and I are at a table in Al’s coffee shop, discussing not much, when Vinnie comes barreling in.  “Hey, guys.  Glad I found you together.  I just saw this ‘Einstein ring’ photo.  They say it’s some kind of lensing phenomenon and I’m thinking that a lens floating out in space to do that has to be yuuuge.  What’s it made of, and d’ya think aliens put it there to send us a message?”

Astronomer Cathleen rises to the bait.  I sit back to watch the fun.  “No, Vinnie, I don’t.  We’re not that special, the rings aren’t signals, and the lenses aren’t things, at least not in the way you’re thinking.”

“There’s more than one?”

“Hundreds we know of so far and it’s early days because the technology’s still improving.”

“How come so many?”

“It’s because of what makes the phenomenon happen.  What do you know about gravity and light rays?”

Me and Sy talked about that a while ago.  Light rays think they travel in straight lines past a heavy object, but if you’re watching the beam from somewhere else you think it bends there.”

I chip in.  “Nice summary, good to know you’re storing this stuff away.”Gravitational lens 1

“Hey, Sy, it’s why I ask questions is to catch up.  So go on, Cathleen.”

She swings her laptop around to show us a graphic.  “So think about a star far, far away.  It’s sending out light rays in every direction.  We’re here in Earth and catch only the rays emitted in our direction.  But suppose there’s a black hole exactly in the way of the direct beam.”

“We couldn’t see the star, I get that.”

“Well, actually we could see some of its light, thanks to the massive black hole’s ray-bending trick. Rays that would have missed us are bent inward towards our telescope.  The net effect is similar to having a big magnifying lens out there, focusing the star’s light on us.”

“You said, ‘similar.’  How’s it different?”Refraction lens

“In the pattern of light deflection.  Your standard Sherlock magnifying lens bends light most strongly at the edges so all the light is directed towards a point.  Gravitational lenses bend light most strongly near the center.  Their light pattern is hollow.  If we’re exactly in a straight line with the star and the black hole, we see the image ‘focused’ to a ring.”

“That’d be the Einstein ring, right?”

“Yes, he gets credit because he was the one who first set out the equation for how the rays would converge.  We don’t see the star, but we do see the ring.  His equation says that the angular size of the ring grows as the square root of the deflecting object’s mass.  That’s the basis of a widely-used technique for measuring the masses not only of black holes but of galaxies and even larger structures.”

“The magnification makes the star look brighter?”

“Brighter only in the sense that we’re gathering photons from a wider field then if we had only the direct beam.  The lens doesn’t make additional photons, probably.”

Suddenly I’m interested.  “Probably?”

“Yes, Sy, theoreticians have suggested a couple of possible effects, but to my knowledge there’s no good evidence yet for either of them.  You both know about Hawking radiation?”

“Sure.”

“Yup.”

“Well, there’s the possibility that starlight falling on a black hole’s event horizon could enhance virtual particle production.  That would generate more photons than one would expect from first principles.  On the other hand, we don’t really have a good handle on first principles for black holes.”

“And the other effect?”

“There’s a stack of IFs under this one.  IF dark matter exists and if the lens is a concentration of dark matter, then maybe photons passing through dark matter might have some subtle interaction with it that could generate more photons.  Like I said, no evidence.”

“Hundreds, you say.”

“Pardon?”

“We’ve found hundreds of these lenses.”

“All it takes is for one object to be more-or-less behind some other object that’s heavy enough to bend light towards us.”

“Seein’ the forest by using the trees, I guess.”

“That’s a good way to put, it, Vinnie.”

~~ Rich Olcott