Crossing an event horizon

[Shag]

Assumptions: Both Traveller and Observer are industructible and immortal. There is nothing else falling into the hole that obscure observers view.


An observer sees the traveller become redshifted and slow down as they approach the event horizon but the traveller never quite reaches it.

The traveller speeds ever faster towards the hole, he looks up and sees massively blue shifted light, but notices the stars visibly moving, some are going nova, some are being born. As he reaches the horizon the universe ends, time ends. Basically a black hole is a one way time machine. The only possible esape for traveller is the off chance that the hole dies before the end of time through losing energy through hawking radiation.

So actually it would be a great prison for immortals.

 

[Umbran]

Maybe the Observer never sees the Traveler entering the Event Horizon because the *image* of the Traveller (ie, light bouncing off the Traveller and into the Observer's eyes) can't escape the Event Horizon, thus freese-framing the Traveller's image?

And the Traveler actually slips through unnoticed, leaving behind an immaterial ghost/hologram for all eternity? No.

Let me reiterate - as far as the Observer is concerned the clock the Traveler is carrying slows down as she falls. He floats up there, getting old as the years pass, and the Traveler is as young as when she first dropped.

Here's the kicker - at the last moment of his aged life, if the Observer could yank the Traveler back up, she'd still be young! It isn't just an image, it's her actual body not changing as time hasn't passed for her.

 

[Klaus]

No wonder Dr. Sheldon Cooper is so well-adjusted...

 

[Bullgrit]

Is gravity instantaneous? I mean, is there travel time between the gravity of an object and its effects on another object a long distance away?

Say we create a black hole out beyond pluto space. Would the Solar System (planets, Sun) react immediately? Would "sensors" on Earth detect it immediately? Or would there be a delay as the gravity "wave" swept in?

Bullgrit

 

[jonesy]

Is gravity instantaneous?

Nope.

According to the measurements done by the National Radio Astronomy Observatory, and the University of Missouri, gravity travels at the speed of light.

 

[Someone]

AFAIK gravity "travels" at the speed of light. If some mass magically appeared X light minutes, light hours or light years away from us we'd notice it's gravitational pull X minutes, hours or years after that.

 

[Umbran]

According to the measurements done by the National Radio Astronomy Observatory, and the University of Missouri, gravity travels at the speed of light.

Correct. This also matches the best available theory.

Say we create a black hole out beyond pluto space. Would the Solar System (planets, Sun) react immediately? Would "sensors" on Earth detect it immediately? Or would there be a delay as the gravity "wave" swept in?

"Gravity wave" is just the sort of thing you should think, yes. Just like light, gravity moves in waves (and particles - the photon carries electromagnetism, the graviton carries gravity), that move at the speed of light (so, like the photon, the graviton should not have any mass).

Black holes orbiting each other should radiate such waves, and the the Laser Interferometry Gravitational-wave Observatory (or LIGO) is designed to detect such things.

 

[Bullgrit]

I've read A Brief History of Time several times, but the last was a couple/few years ago, so I may be misremembering. But I thought Hawking at least suggested in that book that gravity affects happen immediately -- that gravity warps space around the source. I think the textual illustration was spheres sitting on and indenting a piece of black cloth.

Just like light, gravity moves in waves (and particles - the photon carries electromagnetism, the graviton carries gravity), that move at the speed of light (so, like the photon, the graviton should not have any mass).

Wait, huh? Gravitons travel away from their source, and cause what they "hit" to move back towards their source? And if gravitons are particles, how do they escape a black hole? And if photons have no mass, how are they pulled into a black hole?

Honestly, until you mentioned it here, I thought gravitons were a science fiction construct. No lie, the only time I remember reading about gravitons was in a Superman comic -- he supposedly manipulated gravitons to fly. This is why I haven't bothered looking up the term on Wikipedia or elsewhere. I'm going to look it up now, though.

I'm all confusalled. (If someone tells me that gravitons are talked about in A Brief History of Time, I'm going to be embarrassed. Really, it's one of my favorite books.)

Bullgrit

 

[jonesy]

Brief History of Time is 20 years old next year. A lot has happened since. When he talked about it the experiments hadn't happened yet.

Gravitons are so far hypothetical particles, but the waves themselves have been measured.

 

[Umbran]

But I thought Hawking at least suggested in that book that gravity affects happen immediately -- that gravity warps space around the source. I think the textual illustration was spheres sitting on and indenting a piece of black cloth.

It has been ages since I read it as well. At this time, I don't think Hawking would say gravity works instantaneously.

The usual analogy is that spacetime is a rubber sheet, and masses rest on the sheet, and deform it. Now, if you do that with a real rubber sheet, it doesn't deform instantaneously. It actually takes time for the sheet to change shape. If you shake one end of the sheet, it takes time for that shaking to get to the other side of the sheet. Same thing here.

Wait, huh? Gravitons travel away from their source, and cause what they "hit" to move back towards their source?

They cause what they interact with to move back towards the source. It isn't exactly like colliding billiard balls here.

There's nothing new about it, though. Electromagnetic forces are transferred by photons. If you bring a positive and negative electric charge near each other, they exchange photons, and are drawn together, too.

And if gravitons are particles, how do they escape a black hole?

They don't come from inside the black hole. You can think of them being emitted from the outer surface, if that helps. The real answer is that sometimes they act like particles, and sometimes they act like waves (again, just like light), and for most concerns, emission from a garden variety black hole is when they're more like waves.

And if photons have no mass, how are they pulled into a black hole?

The photons have energy, and as Einstein told us, mass and energy are equivalent. Light is bent by heafy objects, occasionally leading to what astronomers call "gravitational lensing" (something else for you to look up).

I'm all confusalled. (If someone tells me that gravitons are talked about in A Brief History of Time, I'm going to be embarrassed. Really, it's one of my favorite books.)

I don't believe they are mentioned there, so you're safe. Remember that A Brief History of Time is brief, a short explanation. It doesn't contain everything.