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Robert “The Bobby Yaga” McNees

@mcnees.bsky.social

15K followers 700 following 7.9K posts

Professor and physicist. Black holes, quantum gravity, cosmology. Rocky Top. Tar Heel. Reposts are spooky action at a distance. These are my views, not my employer's. Just terrible at this.

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · Nov 9

Probably the best thing I ever posted on that other site, wish I could permanently transplant it to my timeline here.

A very popular tweet I posted in 2020 that says “Somehow I've lucked out and have an 8yo who thinks secretly reading under the covers past her bedtime is an act of rebellion, and it hasn't yet occurred to her that her flashlights never seem to run out of batteries.”

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Reposted by Robert “The Bobby Yaga” McNees

James Petruzzi @hawaiianimages.bsky.social · 48m

Sure thing, here's the Pelican Nebula, taken thru a dual-band Ha/O[III} filter and my 10" RC, with a bonus Herbig-haro object at nearly exact center of frame. #Astronomy #Astrophotography

The Pelican Nebula, with blue assigned to ionized oxygen and red assigned to hydrogen. Numerous tendrils of dark gas wend their way across and there are stars shining around and through the neubla

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 59m

Anyway, our night sky is mostly dark, at least for now. But it's blazing in the poetic sense, if not the catastrophic one, and we should enjoy it while we can, so share some of your favorite photos on the night sky in the replies. (21/21)

Chanda Prescod-Weinstein 🌌 @chanda.blacksky.app · 8h

Regarding capitalism, the Milky Way's neighbor Andromeda offered the following comment: BITCH, PLEASE

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Andromeda. Saturation way turned down. Easy to see two of the satellites -- the bigger bright spots.

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

This is a fair point!

Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Unfortunately, they haven't updated since 2018. You'll have to adjust for that.

Altair, brightest star in the constellation Aquila, twelfth-brightest star in our night sky, is 16.7 light years from Earth and is currently getting its first listen to "I Gotta Feeling" by the Black Eyed Peas. (20/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

The finite speed of light applies just as well to terrestrial radio broadcasts reaching the stars. If you favorite song came out 20 years ago, that broadcast has now traveled outwards 20 lightyears from Earth. You can visualize this with lightyear.fm. (19/n)

Lightyear.fm

Radio broadcasts leave Earth at the speed of light. Scroll away from Earth and hear how far the biggest hits of the past have travelled. The farther away you get, the longer the waves take to travel t...

lightyear.fm

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

For that, we need to imagine lots of stars at truly great distances.

The time it takes the light of such stars to reach us plumbs the depths of cosmological history. Over those timescales the Universe has grown much, much larger, and the ratio a(then) / a(now) is truly small. (18/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

There are plenty of stars that are nearby in astronomical terms, and their light reaches us on time scales over which the Universe hasn’t expanded very much at all. But there aren’t nearly enough of them to lead to Olber’s paradox. (17/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Starlight that is just now reaching Earth from Ross 614 in Monoceros, faint and visible only with a telescope, was emitted around the time my daughter was born. Light that left Alhena (third-brightest star in Gemini) when my grandfather was a toddler, will arrive here this evening. (16/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Distances to stars are vast. Even the nearest ones (besides our Sun) are around 4-5 light years away. So it takes quite a while for light to reach us from the closest stars, and much longer for distant stars. (15/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

A few details for folks just encountering these concepts.

Why does starlight take so long to reach us? Light travels at a finite speed of about 300 million meters (186,000 miles) per second. Give it a whole year and it travels around 9.5 quadrillion meters (5.9 trillion miles). (14/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Unless weird tech bros have their way, that is. (13/n)

A US startup plans to deliver ‘sunlight on demand’ after dark. Can it work – and would we want it to?

Satellites beaming sunlight down to Earth sound like science fiction – and they have astronomers very worried.

theconversation.com

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Thus, modern cosmological models that explain our observations of the universe will naturally avoid Olber's paradox and allow us to enjoy a dark night sky. (12/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

In the past our universe was much smaller, so this ratio (raised to the fourth power!) becomes very tiny for light emitted further back in time by more distant sources. Even if the proper luminosity at early times was very large, it won’t overpower this infinitesimal factor. (11/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

So when we add up the radiant energy received here on earth, the contribution emitted at an earlier time t is suppressed by a factor of ( a(t) / a(now) )^4 , where a(t) is called the "scale factor" — it tells us how cosmological distances have changed over time. (10/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

On top of that, we know that the universe has expanded over its history. This expansion affects the density of sources throughout the universe and also red-shifts the light they emit. (9/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

One way out is to drop the assumption of an eternal universe. Light from distant stars hasn't yet reached us or heated up the interstellar medium. This is part of the resolution offered by modern cosmology, which extends back to a "hot big bang" at some *finite* time in the past. (8/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Olbers (and Cheseaux, 80 years before) tried to explain the dark night sky by invoking some interstellar medium that absorbs most of the energy.

But that doesn't work in an eternal universe; thermodynamics says the medium would heat up and radiate as much energy as it absorbs! (7/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

That’s the paradox. The decreasing apparent luminosity of distant stars should be more than made up for by the increasing number of distant stars. If this is the case, the total radiant energy received here on Earth should be infinite.

The night sky should be catastrophically ablaze. (6/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

But here's the thing: if there's a uniform distribution of stars throughout the universe, then the number of stars further than a distance r but closer than r + Δr will be proportional to r² Δr. (5/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

So more distant stars don’t appear as bright as they would if they were closer, for the same reason that a flashlight doesn't seem as bright from 100 yards away as it would right up against your eye. Your eye covers a smaller patch of a larger sphere and collects less energy. (4/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

A star at distance r with luminosity L will have an apparent brightness proportional to L/r². It’s easy to see why: the star radiates in all directions, and the energy it emits is spread over a sphere of area 4π r² as it moves outwards. (3/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

What exactly is the paradox?

For a long time, astronomers thought the universe was infinite and eternal, with a roughly uniform distribution of stars. (2/n)

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

“Why is the night sky dark, if we live in an infinite universe?”

Kepler, Halley, and Cheseaux all pondered this apparent paradox, but the question is commonly attributed to Heinrich Olbers, who was born #OTD in 1758. 🧪 🔭

Images: hyperphysics.phy-astr.gsu.edu/hbase/Astro/..., Wellcome Collection

A diagram shows arrows radiating from a central point, indicating directions, with lots of white disks (stars) surrounding the central point.

An etching of Hans Olbers. He wears a heavy coat, vest, and high collared white shirt. His wispy hair is receding.

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Let me know if you are interested in selling one of the signed prints of the cover art!

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Robert “The Bobby Yaga” McNees @mcnees.bsky.social · 1h

Oh, I think my wife has a piece in that one! (Buying Bread on Devon)

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