Register to reply 
Observing someone falling into a Black Hole. 
Share this thread: 
#1
Feb2412, 10:35 AM

P: 154

I understand that anything falling into black hole would appear to take forever to cross the event horizon relative to an observer at a distance. But what if you were hovering right above the event horizon and you saw someone fall in? Would they still appear to be frozen in time? If so what if you tried to shove them in?



#2
Feb2412, 10:59 AM

P: 1,261

Even then it would appear to take forever. The gravitational timedilation at the event horizon is a proper of the horizon itselfas viewed from any finite (nonzero) distance away, the effect becomes infinite as the infalling object approaches the horizon.
The particular conditions of infall (e.g. whether they are pushed or not) doesn't matter. 


#3
Feb2412, 11:10 AM

P: 154

So what if I was right behind him pushing him in as hard as I could? And also would I be able to communicate with the person falling in? And how long would I be able to communicate with him for since it takes him "forever" to fall in?



#4
Feb2412, 11:13 AM

P: 1,261

Observing someone falling into a Black Hole.
It doesn't matter how hard you push him. As long as you are not falling in with him, he will appear to take forever to go in.
You can communicate with himyou can send signals to him (because signals can go INTO the horizon), but he won't be able to respond (signals can't come out). While he 'appears' to take forever to fall in, he did actually fall infrom his perspective, he cross the event horizon without anything interesting happening. 


#5
Feb2412, 11:23 AM

P: 154




#6
Feb2412, 01:11 PM

P: 1,261

Yes.
The exact details are a little more complicatedbut yes, that is the main idea. Imagine he sends a signal towards you every secondfrom his perspective (his reference frame). From your perspective his signals get farther and farther apart, until there is an infinite amount of time between the signalsin this way, you only end up receiving a certain number of signals. From his perspective, he eventually crosses the horizon, and even though he keeps sending signals every secondonly a certain number of them were sent out before he crossed the horizonand again only a certain number of signals end up getting out to you. In this way, both reference frames agree on the number of signals that are receivedthis is an important aspect of maintaining physical symmetry. 


#7
Feb2412, 03:45 PM

P: 154

So what if I try to grab him at this point? Would he just be a hologram of sort?



#8
Feb2412, 03:51 PM

P: 1,261

You wouldn't see him by the time you got there.
Even just reaching out your arm. 


#9
Feb2412, 04:02 PM

P: 614

a side question to falling into a black hole: the event horizon is a surface, right? If I watched someone fall into it, wouldn't their image become "flat"?



#10
Feb2412, 04:05 PM

P: 1,261




#11
Feb2412, 04:07 PM

P: 614

So if someone were to take a close look at a black hole's event horizon, it would be a cluttered flat image of everything that ever struck the surface, right? Assuming that the black hole existed in an environment with a lot of stuff falling onto it.
whoah and wait, if something at the event horizon appears to take forever to continue falling inwards, what happens to the light that is bouncing off of an object falling onto the event horizon? 


#12
Feb2412, 04:53 PM

P: 126




#13
Feb2412, 05:23 PM

Sci Advisor
PF Gold
P: 9,487

The infalling observer has the same problem as the stationary observer. The stationary observer also becomes frozen from the perspective of the infalling observer as he reaches the event horizon, so, neither can communicate with the other. Photons from outside the EH cannot catch up with the infalling observer as he reaches the EH.



#14
Feb2712, 05:23 PM

P: 154




#15
Feb2712, 05:36 PM

P: 15,319




#16
Feb2812, 03:26 PM

P: 154




Register to reply 
Related Discussions  
Observing the physics of Black Hole interiors  Special & General Relativity  19  
Falling into black hole  Special & General Relativity  24  
Observing an astronaut falling into a black hole  Special & General Relativity  8  
Falling into a black hole  Special & General Relativity  22 