What would happen if I were to turn on a flashlight inside a black hole facing directly outward from the singularity? Since the velocity of light isn’t affected by gravity, it couldn’t stop and then return towards the singularity. Since I’m facing directly out it couldn’t curve around the singularity like light normally would. Would it continue outwards until it had no energy? If yes, does that mean that it would be redshifted to an infinite wavelength? Or is there something about black holes that makes facing out this way impossible?
There may be a physicist to come along to give you a detailed answer to this, but since I happen to be the first one here I’ll just say this. You seem to have the concept of a flashlight as being some kind of a photon gun, where you’re shooting light out in some direction. The light is emitted from the source in all directions and a flashlight just tries to reflect it into a useful direction. But if there is a singularity nearby, it’s going to suck all the light (and the flashlight and you, by the way) into it. The light doesn’t “turn around,” it never gets started in that direction. Once a photon is emitted it is heading straight for the singularity.
Also, by “inside the black hole” I assume you mean inside the Schwarzschild radius. From the standpoint of your experience, there is nothing special about this radius other than that is the point of no return for incoming light (or anything else).
The other problem with your question is that you would have been torn apart by tidal forces long before it could occur to you to do such an experiment.
Yes, that’s what a black hole is. All paths taken by photons inside the black hole are closer to the center. It’s not like throwing a baseball up and watching it rise and then fall back to the ground.
And also note that when you use your clock inside the black hole to measure the speed of the photons emitted by your flashlight you don’t see anything out of the ordinary. The photons travel at c, just like always. The only problem is that your clock wouldn’t agree with any other clock in the universe, which you would discover if you could compare them, which you couldn’t.
The tidal forces depend on the size of the black hole. AFAIU, if the black hole is something on the order of the Saggitarius A* (the black hole at the center of Milky Way) the tidal forces would not be big enough to tear you apart as you cross the radius.
I like to have a stab at these before someone more knowledgeable shows up.
We can’t really model the inside of a black hole; most of our equations end up as infinity or imaginary numbers or whatever.
That was the justification for all that weirdness in Interstellar: “You can’t say what it’s like in there, so…magic bookshelves + daughter bonding?”
It’s not quite true to say the velocity of light is not affected by gravity, as gravity distorts space-time and velocity includes direction.
Essentially, on, or inside, the event horizon, space is so distorted that there is no “out” direction for light to go.
ETA: I really thought I’d be first response. :smack:
Couldn’t you exist inside a really massive black hole? Our entire universe could be inside a super massive black hole and we wouldn’t necessarily know it.
You can’t face directly outwards from the singularity, any more than outside of a black hole you can face towards yesterday. The two are exactly analogous: Inside of a black hole, r is a timelike coordinate (and t is a spacelike coordinate): The outside is just as inaccessible as last Thursday, and the singularity at the center is just as inevitable as tomorrow.
Yes, if our universe had enough matter to cause a Big Crunch then it would have turned out that our entire universe was one big black hole.
But it looks like that’s not the case, our universe probably won’t have a Big Crunch, and so we probably don’t live inside a black hole.
I’m aware that the experiment is impossible to survive, and you couldn’t communicate the results even if you did. I’m not sure that I understand the idea of there being no “away from” though. Gravity is pulling everything in a given direction; I emit a photon in the opposite direction. Or is space twisted in such away that that direction doesn’t exist? And yes, you’re right Mijin, I should have said speed, not velocity.
Just saw the other replies. So there is no way to face directly outward from the singularity. Thank you.
I am also a tyro.
In agreement with everyone above (“What he said!”)
Also, to state explicitly a point brought up by Cooking and Lemur: If there is anything I’ve learned from reading the physics threads on this board, from your personal point of view, you can’t really tell when you’ve crossed the event horizon. This basically says that your local physics experience should seem unchanged to you.
This assumes that you have not been stretched or vaporized, etc.
also, maybe things would be different if you put the flashlight on a treadmill… 
To follow up (and not to create a new thread) - according to the current theory, for an outside observer looking at it, time runs slower and slower as an object approaches the Schwarzschild boundary, and stops right at it.
So as I, sitting at a safe distance away, observe someone falling into a (let’s say large, so that there is no spaghettification) black hole, I would see him freeze right as he crosses the radius - and stay there? Is that description too simplified, and not taking into account the behavior of light (with which I am observing) “bouncing” off the person?
It’s a simplification from the standpoint of the fact that you couldn’t actually observe anything, since light from the object would be red-shifted to infinity. But as the object approached the event horizon, time dilation would make it appear to move slower and slower as it become more red-shifted, and by extrapolation one concludes that theoretically, if you could still see it, you would see it frozen forever at the event horizon.
We had an interesting discussion in another GQ thread about the paradoxical contradiction between this “frozen forever” scenario, and the fact that from the proper time in the reference frame of the infalling object, it would cross the event horizon in a perfectly ordinary way and reach the singularity in a finite and indeed rather short amount of time. If you work through the numbers, an object falling into the black hole at the center of our galaxy, which has about 3.7 million solar masses and a radius of about 11 million km, would transit the black hole’s radius in a maximum proper time of no more than 57 seconds.
Ah, this is the thread I was thinking of, and these posts in particular – #22 and #25.
There’s different ways of looking at, but I think the easiest way to think about it is that when you’re in the black hole the event horizon is receding from you FTL, so in fact even light being shone towards the event horizon gets further away from it with time.
As I understand it, there was a point a few decades ago when it was considered a plausible possibility that the universe was closed, and at some point in the future would collapse back in upon itself in a ‘big crunch’.
Had this turned out to be the case, we would have been living in what would appear from the outside to be an observable universe-sized black hole.
There are a number of answers to this, as this is unsettled science but the most common relativistic thought experiment could have the following implications.
If you are past the event horizon pointing a flashlight outside the black hole just no longer becomes an option, and that “direction” simply doesn’t exist for you. Think of your current situation, time is a “dimension” for you but what direction would you point your flashlight to light up the past?
Right now you know that this past exists, but it is inaccessible to you as a direction.
I don’t know horse pucky about black holes but I am forever going to refer to my flashlight as a photon gun!
Maybe I’m missing some basic understanding of this hypothesis, but the presumed spacetime geometry in a black hole is completely different from that of the observable universe. The universe has no center, but a black hole is defined by having a singularity at its center. The radial dimension r inside a black hole is presumed to be timelike, whereas no such geometry exists in the observed universe. In the event that a Big Crunch were to happen, it would be the result of the aggregate gravitational attraction of all the dispersed matter overcoming the balance of dark energy, which seems to be a completely different dynamic than that inside a BH.
The confusions you’re having cancel each other out. In a Big Crunch universe, the central singularity is the Big Crunch singularity. In other words, it’s where you end up if you follow the timelike direction for long enough. And for the purpose of this discussion, we’re ignoring dark energy, because we’re talking about the models as they existed a few decades ago (wow, I guess it really has been that long since the cosmology revolution). Black holes with dark energy are much more complicated.