A white dwarf star with mass below the Chandrasekhar limit has settled down into a stable configuration. I guess that's okay if you're into quiet endpoints for stellar evolution. But it's no neutron star or black hole, imo.

Subrahmanyan Chandrasekhar was born #OTD in 1910. He established an upper limit on the mass of stable white dwarf stars, and made numerous contributions to astrophysics and relativity.

Above the Chandrasekhar Limit – about 1.44 solar masses – stars eventually explode and then collapse into a neutron star or black hole.

Photo: Stephen Lewellyn / AIP

@NationMeta No, it blocks the page *serving the question*. The page displays for a few seconds, showing a question that includes the word “pot,” then the filters redirect her to another page explaining that the previous page has triggered the filters.

@sollat lol

My daughter couldn’t finish her math homework tonight, because the assignment includes a question about how much soil you can put in a certain “pot,” and the school Chromebook has a filter installed that keeps getting triggered by an unsafe word.

@_thegeoff @davidho The true vacuum bubble asymptotes to the speed of light, so there may be just enough lead time for the photons to reach your eyes and send nerve impulses to your brain before The Ending.

“I just always assumed, despite the fact that the US hadn’t sent any women up there, or people of color, that I was going to go.”

Dr. Mae Jemison (@maejemison) was born #OTD in 1956. Doctor, peace corps volunteer, first Black woman in space, and first astronaut on Star Trek.

@simonbp Maybe tied for first.

Crossing Brougham Bridge in Dublin #OTD in 1843, William Hamilton had a flash of insight. He finally understood how to multiply quaternions after puzzling over the problem for years. He scratched the result into the stone:

i² = j² = k² = ijk = -1

Image: Wikimedia

Alright y’all, it’s mid-October. Only Halloween music from here on out.

Here’s a playlist to get you started, please leave your faves and recommendations in the replies.

https://music.apple.com/us/playlist/hell-toup%C3%A9e/pl.u-7yzmSe7q1N

@nantucketebooks Note, however, that energy loss from the GKZ effect would slow it down over a much shorter distance (though it’d still be moving very fast)

@nantucketebooks That sounds about right. The time dilation factor is something like 3x10^{-12}, so 10^5 ly in about 3x10^{-7} years. That’s around 10s in its frame of reference.

The name "Oh-My-God Particle" was coined by John Walker, who also founded the company Autodesk!

https://www.fourmilab.ch/documents/OhMyGodParticle/

(Also, microscopic black holes would likely flare out in a burst of Hawking radiation almost as soon as they formed. Their own radiation pressure would prevent anything from getting close enough to be swallowed. They are perfectly safe. Cute, even.)

Indeed, such ultra-high energy particles must be common throughout the universe. They probably participated in many very nearly head-on collisions over the past 13 billion years and the Universe is still here.

No accelerator here on Earth will ever collide particles at energies comparable to a head-on collision between two OMG Particles, so you can safely ignore most* doomsday scenarios the next time a new collider is switched on.

* For more details go read @AstroKatie’s book!

The OMG particle was about 50 million times more energetic than protons accelerated by the LHC. Thanks to special relativity, its collision with a stationary nucleon in our atmosphere would only have been ~100x more energetic than the LHC's head-on collisions.

This tells us the LHC can’t set off exotic catastrophes like vacuum decay, microscopic black holes, or strange matter formation. Otherwise, ambient ultra-energetic collisions in our atmosphere would have triggered such scenarios long ago.

On the other hand, if it was a heavier nuclei, like Iron,then there'd be a little more wiggle room. Its energy would have to exceed 10²¹ eV before the energy-per-nucleon was high enough to trigger the effect pointed out by GZK.

A heavy nucleus with all its protons would also have more charge and therefore be more susceptible to deflection by our galaxy's magnetic field. So it could have been produced by some relatively nearby event and ended up on a trajectory that sent it here.

It is not clear how such a particle, accelerated to tremendous energies by some astrophysical source, could even reach the Earth.

At those energies a proton would be subject to the "Greisen-Zatsepin-Kuzmin cutoff," causing it to lose energy and slow down dvia interactions with the ambient Cosmic Microwave Background.

So if the OMG particle was a single proton, it must have been produced nearby, cosmically speaking, to arrive with that much energy.

Let's think about that.

The speed of light is about 3 x 10⁸ m/s. So 4 parts in 10²⁴ is a difference of around one femtometer per second. A femtometer is 10⁻¹⁵ m, which is about the size of a proton.

Light moves fast enough to circle the Earth at the equator about 7.5 times per second, and the OMG Particle – if it was a proton – would have lagged behind it by a distance comparable to the size of a proton! That is very, very fast for a massive particle.

It isn't clear whether the Oh-My-God Particle was a single proton, or a heavier nucleus consisting of several protons and neutrons.

If the Oh-My-God Particle was a proton then it had about 50 million times as much energy as the most energetic proton accelerated in a human-built collider (the 6.5 TeV protons at the LHC).

It would've been moving slower than c, the speed of light, by only about 4 parts in 10²⁴.