"When a massive star collapses under its own gravity, it forms a black hole that is so heavy that it captures everything that passes its event horizon. Not even light can escape. At the event horizon, time replaces space and points only forward. The flow of time carries everything towards a singularity furthest inside the black hole, where density is infinite and time ends."
I enjoy reading passages like that because there's a point where I lose comprehension completely - after "Not even light can escape" - and the rest of it is enjoyably poetic gibberish to a non-physicist like me. It reminds me of the beginning of The Four Quartets by TS Eliot:
Time present and time past
Are both perhaps present in time future,
And time future contained in time past.
If all time is eternally present
All time is unredeemable.
What might have been is an abstraction
Remaining a perpetual possibility
Only in a world of speculation.
What might have been and what has been
Point to one end, which is always present.I can assure you that this is also gibberish to me.
These kind of popular science translations are like one-way hash functions.
If you really understand the science, you can probably recognize where they are coming from.
But by themselves they contain 0 information.
The press release is glibly going from "science mode" to "mystical mode" and acting like it's all known by physics, which it's not. (I'm not really complaining ... it's a press release, after all.)
I cannot wrap my mind around what it means, but it’s a savage description.
After crossing the event horizon the radial direction becomes like time. And what used to be time becomes like space.
What this means that you will inevitably travel to the center, just in the same way as you travel into the future in normal life. Inside the event horizon the center is not at any place that you can see, instead it is in your future.
To answer your question in clear terms: No, you cannot move freely in the radial direction. Only towards the center, in the same way you cannot move freely forward or backwards in time.
> In what sense does [time] become spacelike? Can one move back and forth in time inside the event horizon?
This can be seen best in a conformal diagram or Penrose diagram (now that he has a Nobel prize might as well use the name of his creator).
Here is the Penrose diagram of a star collapsing gravitationally into a Black Hole and its evaporation via Hawking radiation: https://fias.uni-frankfurt.de/~hossi/Bilder/BR/bhevap_l.jpg
You should track the r=0 line, initially it points upwards (it's timelike) as it chugs along at the center of the star as it collapses gravitationally. When the BH forms, it becomes horizontal (spacelike) and lies at the future of every test particle that enters into the BH. The singularity is inevitable for anything that crosses the horizon.
The Black Hole will at some point have radiated all its energy in Hawking radiation at which point it disappears and r=0 becomes timelike again.
Unfortunately it's hard to make it easier to understand without indulging in some math.
Closed orbits are impossible though, all test particles finish at the singularity in a finite amount of proper time.
It is the point at which nothing, no matter how fast or massive, can possibly return from the gravitational pull. For instance: light, traveling at the speed of light in a vacuum, cannot escape once it has crossed the event horizon; it's pulled inevitably inward.
Not quite what you seem to be asking, but the event horizon is the point where[0] the escape velocity is equal to the speed of light; the orbital velocity at that distance is greater (by a factor of ln 2 IIRC, so v_orbital ≈ 1.44c). There's a more distant distance, called the innermost stable orbit or the photon sphere, where the orbital velocity is equal to c (so photons will orbit if you emit them tangentially at this height), but the escape velocity is only ~0.69c.
0: It's not quite right to say that that's because the escape velocity is equal to c, though.
How it happens? Normal spacial direction has positive sign. Time has negative. When crossing the horizon the equations become such that t gets a positive sign and the radial direction a negative one. After that they behave as expected.
Delaying your fall is precisely the same as trying to avoid tomorrow by moving around. Moving away from the center is like moving back in time. No matter how you move in the spatial dimensions you can’t do it.
With the exception of faster than light travel. That allows you to move away from the center, and it also allows you to avoid tomorrow. It’s pretty much the same as time traveling in general relativity.
And similar in case of a black hole, you are falling through the event horizon and nothing happens, it is just that calculating what you need to do to escape, you note that it is too late.
To expand a bit on the molecule situation, when one of the atoms is inside the black hole, then parts of its electric field are still outside and influence the other atom. To have the other atom escape to infinity, you would need to break the part of the bond "still outside" and the gravity of the black hole.
You know how the universe is expanding, such that the milky way is moving away from (sufficiently) distant galaxies faster than the speed of light? But chemistry still works fine here? When you fall into a black hole, you accelerate until you're moving away from the rest of the universe faster than light, but all your atoms are still (relatively, assuming the black hole is large enough to have negligible tidal forces) stationary relative to each other. The event horizon is just the point where your speed relative to the effectively-distant[0] outside universe exceeds the speed of light. Any signal you emit can't be moving facter than c relative to you, so it will always be moving away from the outside universe (v<-c implies v+v'<0).
0: "Nothing can move facter than c" refers to relative motion of objects near each other, which is why the Hubble expansion of the universe doesn't contradict that either.
There is a classic analogy of light traveling through space-time being like ants crawling on a balloon. A black hole is a spot where the balloon is being sucked in. Faster and faster as you get closer. The event horizon is where the speed at which the balloon moves matches the speed of the ant. There is nothing special from the ant's immediate perception about this line, but if the ant crosses, it is doomed.
I thought the size only affected the "acceleration" of the gravitational pull based on the distance from the singularity? A supermassive black hole means you will take longer to be spaghettified, but it will still happen and it will happen before you cross the horizon, won't it?
The reason light cannot escape from past the event horizon is that that is the point where space-time is so warped that what you consider a perfectly normal time axis is, from the perspective of an outside observer, now pointing in a spatial dimension directly towards the center of the black hole.
In contrast, regardless of how small you are, there is some point where you do experience tidal forces and feel yourself getting spaghetti. For a small black hole, this is before you pass the horizon, for a large black hole, it is after.
[0] The distinguishing feature between a real force like gravity, and a truly fictitious force like the centrifugal force, is that for the later, you can construct a coordinate system where the force disappears at all locations. For a real force, you can construct a coordinate system where the force goes away at any given point; but there will always be some point with an apparent force.
I hope it helps. I think this is the cleanest explanation with actual visual representation.
Edit: Sorry this one is even better.
How Time Becomes Space Inside a Black Hole | Space Time
If you think of spacetime as a 4 dimensional geometry, there is no particularly reason to pick out any specific shape of the "time" coordinate. For a given interval in space-time, there is notion of "proper time" that is invariant to coordinate system transformations, so a given observer does have an "ideal" coordinate to use for time. The "proper time" of a given observer could be the space axis of another coordinate system.
Assuming you have enough background in math, what I think is a reasonably understandable explanation is here:
https://www.youtube.com/watch?v=yv2-KPagCQs&feature=youtu.be...
Once he gets through explaining the hyperbolic coordinate sytstem, you can skip ahead to
https://www.youtube.com/watch?v=yv2-KPagCQs&feature=youtu.be...
Space is time line:
https://www.youtube.com/watch?v=yv2-KPagCQs&feature=youtu.be...
A somewhat clarifying picture of it taking "infinite" time to cross the event horizon, without anything interesting happening near the event horizon:
https://www.youtube.com/watch?v=yv2-KPagCQs&feature=youtu.be...
[0] As with any gravitational object, there are tidal forces. However, due to the increasing radius of the event horizon, the strength of the tidal forces at the event horizon decrease as the mass of the blackhole increases.
In practice it just means that wherever you try to go you'll end up in the same place, basically you're going down a funnel and the exceptional thing is that the funnel is made out of spacetime itself.
http://visualrelativity.com/LIGHTCONE/schwarzschild.html
And here's a more technical one:
The waste land is at times gibberish, though, in his own words.
Even worse, it is frequently explained with an analogy to escape velocity. You have a certain escape velocity at the surface of the Moon, the analogy starts, and your rocket needs to exceed that to get out. Gravity is stronger at the surface of Earth, so escape velocity there is higher, and rockets need to go even faster.
At the event horizon, gravity is strong enough that escape velocity is the speed of light, and so nothing can get out because you can't go faster than light.
Great--except the escape velocity thing only applies to trying to get out ballistically. (Current rockets are essentially ballistic because they expend most of their fuel while still deep in Earth's gravity well).
Escape velocity explains why someone inside the event horizon can't throw things out of the black hole using a catapult, or using a rocket that expends all its fuel before reaching the event horizon.
It doesn't explain why something with continuous thrust could not get out. It doesn't explain why someone inside couldn't build a ladder that extends past the event horizon and simply climb out. It doesn't explain why someone outside could not lower a tether in and pull someone out.
Eventually being a really long time.
But the "radiation" happens right at the event horizon, the particles that get emerge have been "created" on the outer side.
What have I missed?
https://math.ucr.edu/home/baez/physics/Relativity/BlackHoles...
http://www.scholarpedia.org/article/Hawking_radiation#The_or...
Highly recommend the illustrations on the press release for a quick overview
Cross section of a black hole (pdf) - https://www.nobelprize.org/uploads/2020/10/fig2-phy-en-cross...
The Milky Way - https://www.nobelprize.org/uploads/2020/10/fig3-phy-en-milky...
Stars closest to the centre of the Milky Way - https://www.nobelprize.org/uploads/2020/10/fig4-phy-en-stars...