# Falling into a black hole doesn't reveal the future, but does that mean it's not happening?

In a previous answer it was said that you wouldn't see the universe speed up as you fell into a black hole, which I accept as true.

However, is being unable to see or interact with the outside universe relevant?

That is, we already know that gravity induced time dilation exists (e.g. diverging clocks on satellites), and that it reaches infinity at an event horizon.

The only difference is we can't return the probe just before it hits the horizon, and any message sent out would not only take eons, but would be extremely red shifted.

So as a thought experiment imagine if the black hole suddenly vanished just before the probe crossed the event horizon. According to the probe's clock only 1 hour of proper time elapsed (the time needed to reach the horizon), but the spaceship's clock, having experienced far less gravity induced time dilation, would have measured countless eons of proper time.

Because of this I would argue that even though the probe can't see the outside universe speed-up, it none the less is. So if the black hole radiates away in a finite amount of time as seen from the outside universe, then everything that fell into it, including the probe, would radiate away before crossing the event horizon.

• Thought experiments only really make sense if you are discussing a physically reasonable situation (it might not be completely correct, but it should be, on the face of it, reasonable). A black hole suddenly disappearing is not going to happen. – Harry Wilson Aug 30 '15 at 23:14
• Fair enough, but how about if a particle quantum tunneled to safety just before hitting the horizon? No law of physics would be broken? and it would find itself eons into the future (non-relativistic proper time, thanks to near infinite coordinate time dilation) with no black hole in sight. Or perhaps the particle was just about to hit a small primordial hole before it exploded? and so on. – Paul Sep 1 '15 at 1:14
• You might be interested to read the answers to my question Does someone falling into a black hole see the end of the universe? – John Rennie Sep 7 '15 at 17:08

You do see the universe speed up. You just don't see the whole future of the universe play out before you cross.

If you cross there are some things you never see. You can take the of your crossing and look at it's past light cone and this will include a last moment that you see before you cross. It's actually the view you see in the sky the moment you cross. Anything that didn't reach your eyes by then won't be seen before you cross.

It's not an infinite speed up, but it is a speed up.

The only difference is we can't return the probe just before it hits the horizon, and any message sent out would not only take eons, but would be extremely red shifted.

You can come back any nonzero distance before you cross. The horizon is defined as the point of no return, so by definition you can return if you haven't crossed yet.

To be fair it is possible that we are already inside an event horizon. If a giant shell of matter was contracting around us then we wouldn't notice and if the mass of that shell was M and it contracts to a surface area of $4\pi(2GM)^2$ then we might be inside and just don't realize it. Life doesn't magically act different just because you are inside an event horizon.

What happens is that shell hitting that surface area could be a no return region so for the things inside the no return region forms when you no longer have time to reach that shell before it hits the critical value. So you can imagine some points appear where they are just too far from where the shell is going to form, for instance at the center of the shell and then at the speed of light the event horizon expands from those points until it hits that shell right as they form the critical radius.

The irony is that shell could be configured with rockets that could keep the shell from getting too small and whether or not an event horizon forms Herr now could be related to whether those rockets fire. And that's the point. Before the event horizon forms you could actually send a signal to prevent it from happening but afterwards you can't it it either happens or not but you have no say.

But the point I'm making is that you feel nothing as the event horizon shoots past you at the speed of light and it happens or not n depending on future events that haven't happened or not bit once it crosses you can't control it.

And if that shell is huge you might live and is inside he event horizon and not known since it takes so long for it to contract to where you are.

OK, but you said you you can't return from just before it hits the horizon, but you can. By definition. So let's say you happen to get out of dodge right before the event horizon but just barely.

Then you need to rush away at bear night speed which means you immediately start seeing in that direction go faster after all the light from those things is rushing towards that event horizon at light speed and you are rushing away at near light speed. So it definitely looks like watching a movie on faster forward.

But this is normal for two paths by hat start and stop at the same time and place to have their clocks measure different durations. It is as natural as two paths from the same endpoints having different lengths.

There way to age the most is to move on geodesics. But there can be multiple geodesics between two events that aren't sufficiently close. So you can't know that you aged the most just because you moved on a geodesic.

But if you didn't move on a geodesic then you did not age the most. And so if you fired your rockets then you know you will be younger than you twin that stayed far away if your twin didn't fire any rockets.

So you might see your twin speed up as you rush towards them but that is just you seeing light signals that were already in route. You really did age less than your twin that stayed away from the event horizon as you went in close and came back right before you crossed.

But that is normal, you could do the same in flat spacetime by firing your rockets.

• In regards to the rocket trip (or twin paradox), is it true that if you travel at near c you can cross the galaxy in a day of your proper time (be only a day older) while the non-relativistic galaxy aged a million years? And if so, isn't approaching an event horizon the same (your last second of proper time = eons of proper time on earth)? – Paul Sep 1 '15 at 1:29
• @Paul That's correct, you can age as little as you want if you are prepared to go fast enough. But just the CMR will become lethal and every stray molecule or atom in your way will hurt a great deal. And near the event horizon has a similar effect compared to staying far away. In the shell example the "far away" might be very far away and so we don't notice that they already will never see us. – Timaeus Sep 1 '15 at 2:21
• @Timaeus Technically: You will age at the same pace you always have aged. One hour near a black hole and you have aged one hour. So stricktly speaking ,you can't age as little as you want - but you can make everybody else age more instead. Just a matter of perspective though. – frodeborli Aug 24 '16 at 9:35
• @Paul There is no limit to how fast you can travel (c is not a limit for the traveler). You can travel across the universe in one hour. But you can't see anybody else travel across the universe in one hour unless you travel with them - it would take billions of years limited by the speed of light. This makes it impossible for anybody to travel to another galaxy and come back to tell about it. Also, you can't be sure what you'll see after you've traveled a billion light years - the destination will have changed a lot. – frodeborli Aug 24 '16 at 9:36