Stuck at home? Bored? How about a thread about falling into a #blackhole?

You may have heard about spaghettification, where tidal forces tear you apart (the force on your feet is stronger than the force on your head). https://twitter.com/joey_neilsen/status/1172581394717061120?s=20

You may also have heard that relative to a distant observer, time slows down in a gravitational field. What does this mean for a trip into a black hole? It means that if (from a safe distance) you watch me approach a black hole, you will see me move more and more slowly.

In fact, you will NEVER see me reach the event horizon. There are two reasons for this, and they are both the same reason. First, because my clock ticks more and more slowly relative to yours, my pace will become positively glacial. I'll just literally never get there!

The other reason is that because my clock ticks more and more slowly, the frequency of any light that you see from me will decrease. If I'm wearing a blue spacesuit, you'll see its color change. But as the color shifts out of the visible, it'll get darker too!

So as I approach the event horizon, my motion will slow to a subatomic crawl AND I will fade from view. But at least it'll be a colorful show in the meanwhile!

But here's the rub: RELATIVITY. We're talking here about what you see, but I'd like to talk about *me.* What's it like for ME to fall into a black hole? There are a couple cool points to be made, but obviously, the short answer = "not good."

But specifically, if we (briefly) ignore the fact that I'm going to be shredded into a thin stream of my constituent atoms , let's talk about how this goes down according to my clock.

It's important to understand that clocks don't run slower in gravitational fields. 1 second is one second. General relativity tells us that clocks in a strong gravitational field run slowly *relative to* clocks in a weak gravitational field. It's not about experience of time.

So I'm not going to experience the weirdness of time slowing down. I won't detect myself slowing down; I won't see myself changing colors. What I will see is myself rapidly accelerating toward a black hole. This won't end well.

In fact, it won't end well, and quickly! While you could in principle watch me approach the event horizon for all eternity, by my watch the end will come swiftly!

How swiftly? Well, for a typical stellar-mass black hole (10x the mass of the sun), it would take about 66 microseconds to fall in from the event horizon and hit the singularity. Human reaction time is ~4000x longer!! That's barely enough time for a neuron to fire ~15 times!

It's so fast that I would never notice it. You could wait for thousands and thousands of years for me to reach the event horizon, but I'll be torn to bits and crushed into oblivion in a tiny fraction of a second.

But that's a small . What about a big one? Let's take Sgr A*, the supermassive black hole at the center of our Milky Way galaxy (a prime target for the @ehtelescope). Sgr A* has a mass 4.1 million times that of the sun! But it still only takes ~30 seconds to hit the bottom!

Ok, fine, go big or go home: how about M87, the first black hole the EHT ever imaged? A monster 6.5 billion times the mass of the sun, it's larger than our solar system. https://twitter.com/ehtelescope/status/1118978385647144963?s=20

Imagine this: it took @NASANewHorizons 9 years to reach Pluto. Falling from the event horizon of M87 to its singularity, I'd rocket farther than that in under two hours!!!

Now I want to show this in animated form. First, let's get on the same page. Imagine I drop a baseball. I can represent this in two ways: with the baseball falling and a clock ticking, or with the baseball's position over time.

Now let's plot my distance over time according to you: remember, you see me approach the event horizon more and more slowly until I disappear completely, so my path shown this way gets shallower over time. It takes "forever" to see me fall in!

But from my perspective in my rocket, this all happens very fast. What takes an infinite amount of time to your eyes is over and done before, and I'm obliterated before I have much time to think about it. Thank goodness for small favors, amirite?

Note that in principle once I'm inside the black hole I could try to fire my rocket engines to get out, but inside, gravity is so strong that at the speed of light it's not possible to travel away from the singularity or even maintain a constant distance from it.

So that's it, that's the thread. Hope you like the videos; please share and reply with questions/comments!