Sorry (not sorry) to rant about this, but how does a paper get to be a PRL (and editor's suggestion) with this falsehood in the first sentence of the abstract? Gravity *does* couple to spin! More below if you want to know.

[From journals.aps.org/prl/abstract/1… in @PhysRevLett] @PhysRevLett It's clear that the authors are not gravity experts; and neither were any referees or editors in the process; gravity experts know that spin couples to curvature. Even classically, we have the MPD equations en.wikipedia.org/wiki/Mathisson…. But there is a quantum-mechanical version, too.

The expansion of the universe can't be compared to speed--it doesn't make sense to say it's expanding "faster than the bullet can move". In one second, every piece of the universe (say 1 meter) grows proportionally to its length, with the same factor. Let's talk about it!

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https://twitter.com/UberFacts/status/1477240441615622145

That coefficient is called the Hubble parameter (usually the Hubble constant, except it changes over cosmic history!). Today, the Hubble parameter is approximately 70 km/s/Mpc. What does this mean? Every second, a megaparsec gets longer by 70 kilometers.

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I don't understand the topology of the orthonormal frame bundle of the 2-sphere. Is it topologically RP^3? Or is it S^3? I read a paper once saying that it's S^3, but that seems wrong now. Shouldn't the ONF bundle be a homogeneous SO(3) space, so a quotient of SO(3) by a subgroup? That can't make S^3, can it?

🚨 New paper day! 🚨
Numerical renormalization group-based approach to secular perturbation theory
-> arxiv.org/abs/2106.08410 <- read here

This paper took waay longer than expected, because, you know, pandemic and all that.

A thread 🧵, 1/? The term "secular effects" in math/physics refers to something small that builds up slowly over a long time, eventually creating a big effect. The example closest to my heart is that gravitational waves are tiny, but GW emission makes a binary eventually inspiral and merge. 2/?

This is a Penrose illustration appreciation thread. Post your favorite figures below. Every physicist's first nontrivial fiber bundle:

I have a foggy memory of being at a hotel bar after some physics conference reception, and drinking next to Roger Penrose. He let us youngsters explain our research to him, which I think included explaining how pulsar timing can detect gravitational waves. 1/ I feel like he already knew how pulsar timing worked, but it's also possible he didn't. Either way, it was very kind of him to humor us young physicists and let us explain something to him! 2/

Ok, since people were asking… Here's an easy way to see that Hawking radiation is not due to particle–anti-particle pair at the horizon. First thing to remember is a that a black hole is not a point, it has a size: its Schwarzschild radius Rs~2GM/c^2 (figure below)
1/ We'll measure other lengths in terms of this distance. It turns out that there is a whole region of space close to a black hole that's responsible for emitting the Hawking radiation. It doesn't have a sharp cutoff—there is a density that goes down farther away from the BH. 2/

@TheAngkar @Harkaway @_Astro_Nerd_ @SusannaLHarris DOOM @TheAngkar @Harkaway @_Astro_Nerd_ @SusannaLHarris Lol sorry I can't be much more specific, but here's what we do know:
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Exactly 100 years ago today, the Moon's shadow passed directly over the Earth: a total solar eclipse. On that fateful day, several astronomers were working to photograph the stars by the edge of the sun at the moment of totality. This observation would rocket Einstein to fame. 1/ In Newton's theory, there is no reason for light to be bent by gravity. You can, however, derive some amount of bending by appealing to conservation of energy. But that amount of bending is also wrong. The right answer comes from Einstein's theory of general relativity. 2/

One cool bit of science you can learn from the #GravitationalWaves from neutron stars is about the "equation of state" of nuclear matter 1/? this is something that simply can't be measured directly in labs or colliders on Earth. Astronomy is the only direct way to measure it. 2/?