Observing someone falling into a Black Hole.

In summary: It only approaches infinity. In theory, it would become infinitely red-shifted after an infinite amount of time, but since nothing can escape from the event horizon, we would never observe this.In summary, at the event horizon of a black hole, an object falling in would appear to take forever to cross the horizon from the perspective of an observer at a distance. This is due to the gravitational time-dilation at the horizon. It doesn't matter how hard the object is pushed or if it is being communicated with, it will still appear to take forever to cross the horizon. The same is true for someone trying to reach out and grab the object. As the object approaches the horizon, its image would become flat and eventually infinitely red-shift
  • #1
Flatland
218
11
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?
 
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  • #2
Even then it would appear to take forever. The gravitational time-dilation at the event horizon is a proper of the horizon itself---as viewed from any finite (non-zero) 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
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
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 him---you 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 in----from his perspective, he cross the event horizon without anything interesting happening.
 
  • #5
zhermes said:
You can communicate with him---you 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).

So initially I would receive his responses but not after he falls into the event horizon even though he appears to still be falling in?
 
  • #6
Yes.

The exact details are a little more complicated---but yes, that is the main idea.

Imagine he sends a signal towards you every second---from 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 signals----in 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 second---only a certain number of them were sent out before he crossed the horizon---and 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 received---this is an important aspect of maintaining physical symmetry.
 
  • #7
So what if I try to grab him at this point? Would he just be a hologram of sort?
 
  • #8
You wouldn't see him by the time you got there.
Even just reaching out your arm.
 
  • #9
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
SHISHKABOB said:
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"?
Yes it would.
 
  • #11
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
SHISHKABOB said:
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?

If I'm remembering correctly off the top of my head, this is Steven Hawking's current argument for why information is not destroyed when it falls into the BH.
 
  • #13
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
zhermes said:
You wouldn't see him by the time you got there.
Even just reaching out your arm.

I'm not quite sure I understand this. I'm assuming you mean that the light is being red shifted but wouldn't you be able to see him with sensitive enough equipment?
 
  • #15
Flatland said:
I'm not quite sure I understand this. I'm assuming you mean that the light is being red shifted but wouldn't you be able to see him with sensitive enough equipment?

He would eventually be infinitely red-shifted. i.e. the photon's energy would approach zero, its wavelength would approach infinity.
 
  • #16
DaveC426913 said:
He would eventually be infinitely red-shifted. i.e. the photon's energy would approach zero, its wavelength would approach infinity.

Does it actually become infinitely red-shifted after some finite time or does it only approach infinity?
 

1. What exactly is a black hole?

A black hole is a region in space where the gravitational pull is so strong that nothing, including light, can escape from it. It is formed when a massive star collapses in on itself.

2. How does someone fall into a black hole?

As a person or any object gets closer to a black hole, the gravitational force increases. Eventually, the force becomes so strong that the object is unable to overcome it and will be pulled into the black hole.

3. What happens to someone who falls into a black hole?

Once someone crosses the event horizon, or the point of no return, of a black hole, they will be stretched and compressed by the intense gravitational forces. This is known as "spaghettification." Eventually, the person will reach the singularity, a point of infinite density, where they will be crushed.

4. Can anyone survive falling into a black hole?

No, it is not possible for someone to survive falling into a black hole. The intense gravitational forces would rip apart any known material, including the human body.

5. Can we observe someone falling into a black hole?

No, it is not currently possible to observe someone falling into a black hole. Black holes are invisible, and once someone crosses the event horizon, no information can escape, including light. However, we can observe the effects of someone falling into a black hole, such as the emission of x-rays and gravitational waves.

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