<p>For simplicity, let's say that the black hole is isolated and non-rotating (and uncharged), so that the situation is described by the comparatively simple Schwarzschild spacetime. Let's also suppose that the camera free-falls radially into the black hole.</p>

<p>What is the camera looking at? Suppose it is looking at some stationary object that does something with a known frequency. Your question is basically how at what frequency it will be observed on the video feed emitted by the camera.</p>

<p>Without loss of generality, we can suppose that the camera is looking at <em>us</em>, and that we're shining a laser beam at it: the 'doing something at a known frequency' would be the oscillations in the electromagnetic wave of the laser beam. We can do this because time dilation affects every physical process, so we might as well pick one that is more convenient to think about. </p>

<p>At this point, it is straightforward why the camera feed will not show any time dilation: being equivalent to a reflected laser beam, the gravitational blueshift when going inward will be cancelled by the gravitational redshift going outward.</p>

CC BY-SA 3.0
2015-12-29T20:35:29.260Z
2015-12-29T20:35:29.260Z
Stan Liou
<blockquote>
  <p>If I travelled near a black hole, my time would progress slower relative to someone on Earth. This is clear enough. </p>
</blockquote>

<p>Yes, no problem with the gravitational time dilation. </p>

<blockquote>
  <p>However, what if we sent a probe with a camera to a black hole? When we watch the screen, would we see time through the camera's perspective — that is, would the Universe appear to progress faster as the probe got closer and closer to the black hole?</p>
</blockquote>

<p>No. We'd see the universe progressing normally, because we aren't subject to that gravitational time dilation. (I presume we're at a safe distance). Let's suppose it was a TV camera that took a picture 25 times a second and sent it back to us, adequately catering for redshift. The camera starts off taking 25 pictures a second as measured by us. But after a while we notice we're only getting 24 pictures a second, then 23, and so on. We see events in the wider universe progressing at their normal rate, but eventually the movie starts getting jerky as the frame-rate reduces. In the end the frame-rate reduces to zero, and that's the end of the show. </p>

CC BY-SA 3.0
2015-12-30T08:32:45.800Z
That is some quality answer.
Nico
CC BY-SA 3.0
2015-12-30T10:48:32.430Z
@Nico : thanks. I'm not sure the quality is all that great myself, but nevermind. If it delivers a correct answer in a succinct clear simple  fashion, it'll do. Because there's this saying attributed to Einstein _"if you cannot  explain it to your grandmother, you do not understand it yourself"_, IMHO it's important to be able to explain something, it helps your _own_ understanding. Provided it's the correct explanation of course!
John Duffield
CC BY-SA 3.0
2015-12-30T12:29:35.197Z
Explaining something with your own words like if you were explaining it to your grandmother surely helps a LOT the understanding of a subject. But I don't think that many grandmothers are interested about sending a camera into a black hole !
Nico
CC BY-SA 3.0
2015-12-31T02:38:03.107Z
@JohnDuffield Er...it's "if you can't explain it to a six year old, you do not understand it yourself."
http://www.goodreads.com/quotes/19421-if-you-can-t-explain-it-to-a-six-year-old
Sir Cumference
CC BY-SA 3.0
2015-12-31T11:35:06.270Z
It's hazy, Sir Cumference. You get all sorts of hits when you google [explain it to your grandmother](https://www.google.co.uk/#q=if+you+can%27t+explain+it+to+your+grandmother), but there's no actual citation. This [skeptics page](http://skeptics.stackexchange.com/questions/8742/did-einstein-say-if-you-cant-explain-it-simply-you-dont-understand-it-well-en) looks about right. A child is mentioned, and a barmaid too. @Nico : the camera might not even make it to the event horizon, which would mean my answer isn't quite right.
John Duffield
CC BY-SA 3.0
2015-12-31T11:52:57.857Z
@JohnDuffield Of course it wouldn't reach the event horizon. But the answer would have been far less interesting if we were speaking about things that can actually happen :p
Nico
CC BY-SA 3.0
2015-12-31T12:22:31.087Z
@Nico : actually, I think it's _more_ interesting to talk about why that camera doesn't make it to the event horizon. See stuff like [this](http://www.znaturforsch.com/aa/v56a/56a0889.pdf). It's a black hole "firewall" proposed in 2001 by Friedwardt Winterberg. See an old version of the [Wikipedia firewall article](https://en.wikipedia.org/w/index.php?title=Firewall_(physics)&oldid=587309921). It isn't in the new version, there's some kind of priority dispute going on.
John Duffield
CC BY-SA 3.0
2015-12-31T12:26:49.213Z
@JohnDuffield this is quite complex for the French non-physicist I am. My assumption about throwing a camera in a black hole is that the gravity variations would just destroy the camera. I remember reading about "spaghetification"...
Nico
CC BY-SA 3.0
2015-12-31T12:33:11.407Z
@Nico : I don't think it's complex at all. Not when you understand it. Why don't you ask a question about it? And I'll see if I can _explain it to my grandmother_.  The question could refer to this one and say _If I dropped a TV camera into a black hole, will it be destroyed before it reaches the event horizon, and if so, why?_
John Duffield
CC BY-SA 3.0
2015-12-31T12:39:21.690Z
@JohnDuffield Here you go, let's assume I got the physics knowledge of a six years old grandma ;) http://astronomy.stackexchange.com/questions/13035/camera-reaching-the-event-horizon
Nico
CC BY-SA 3.0
2015-12-31T12:44:10.863Z
@Nico : good stuff. I'm afraid I have to go out now, so I'll answer later. It might have to be tomorrow I'm afraid, seeing as it's New Year's Eve. Bonne année!
John Duffield
CC BY-SA 3.0
2015-12-29T16:13:50.013Z
2015-12-29T16:13:50.013Z
John Duffield
<p>If I travelled near a black hole, my time would progress slower relative to someone on Earth. This is clear enough. However, what if we sent a probe with a camera to a black hole? When we watch the screen, would we see time through the camera's perspective — that is, would the Universe appear to progress faster as the probe got closer and closer to the black hole?</p>

<gravity><black-hole><general-relativity><space-time><time-dilation>
A camera and time dilation?
Sir Cumference
# A camera and time dilation?

(Asked by: username_0 on 2015-12-29)

If I travelled near a black hole, my time would progress slower relative to someone on Earth. This is clear enough. However, what if we sent a probe with a camera to a black hole? When we watch the screen, would we see time through the camera's perspective — that is, would the Universe appear to progress faster as the probe got closer and closer to the black hole?

**Tags:** gravity, black-hole, general-relativity, space-time, time-dilation

---

## 2 answers

### Answer by username_2 on 2015-12-29. Score: 7 | Accepted answer

> If I travelled near a black hole, my time would progress slower relative to someone on Earth. This is clear enough.

Yes, no problem with the gravitational time dilation.

> However, what if we sent a probe with a camera to a black hole? When we watch the screen, would we see time through the camera's perspective — that is, would the Universe appear to progress faster as the probe got closer and closer to the black hole?

No. We'd see the universe progressing normally, because we aren't subject to that gravitational time dilation. (I presume we're at a safe distance). Let's suppose it was a TV camera that took a picture 25 times a second and sent it back to us, adequately catering for redshift. The camera starts off taking 25 pictures a second as measured by us. But after a while we notice we're only getting 24 pictures a second, then 23, and so on. We see events in the wider universe progressing at their normal rate, but eventually the movie starts getting jerky as the frame-rate reduces. In the end the frame-rate reduces to zero, and that's the end of the show.

> **Comments:**
> **username_3**: That is some quality answer.  
> **username_2**: @Nico : thanks. I'm not sure the quality is all that great myself, but nevermind. If it delivers a correct answer in a succinct clear simple  fashion, it'll do. Because there's this saying attributed to Einstein _"if you cannot  explain it to your grandmother, you do not understand it yourself"_, IMHO it's important to be able to explain something, it helps your _own_ understanding. Provided it's the correct explanation of course!  
> **username_3**: Explaining something with your own words like if you were explaining it to your grandmother surely helps a LOT the understanding of a subject. But I don't think that many grandmothers are interested about sending a camera into a black hole !  
> **username_0**: @JohnDuffield Er...it's "if you can't explain it to a six year old, you do not understand it yourself." http://www.goodreads.com/quotes/19421-if-you-can-t-explain-it-to-a-six-year-old  
> **username_2**: It's hazy, Sir Cumference. You get all sorts of hits when you google [explain it to your grandmother](https://www.google.co.uk/#q=if+you+can%27t+explain+it+to+your+grandmother), but there's no actual citation. This [skeptics page](http://skeptics.stackexchange.com/questions/8742/did-einstein-say-if-you-cant-explain-it-simply-you-dont-understand-it-well-en) looks about right. A child is mentioned, and a barmaid too. @Nico : the camera might not even make it to the event horizon, which would mean my answer isn't quite right.  
> **username_3**: @JohnDuffield Of course it wouldn't reach the event horizon. But the answer would have been far less interesting if we were speaking about things that can actually happen :p  
> **username_2**: @Nico : actually, I think it's _more_ interesting to talk about why that camera doesn't make it to the event horizon. See stuff like [this](http://www.znaturforsch.com/aa/v56a/56a0889.pdf). It's a black hole "firewall" proposed in 2001 by Friedwardt Winterberg. See an old version of the [Wikipedia firewall article](https://en.wikipedia.org/w/index.php?title=Firewall_(physics)&oldid=587309921). It isn't in the new version, there's some kind of priority dispute going on.  
> **username_3**: @JohnDuffield this is quite complex for the French non-physicist I am. My assumption about throwing a camera in a black hole is that the gravity variations would just destroy the camera. I remember reading about "spaghetification"...  
> **username_2**: @Nico : I don't think it's complex at all. Not when you understand it. Why don't you ask a question about it? And I'll see if I can _explain it to my grandmother_.  The question could refer to this one and say _If I dropped a TV camera into a black hole, will it be destroyed before it reaches the event horizon, and if so, why?_  
> **username_3**: @JohnDuffield Here you go, let's assume I got the physics knowledge of a six years old grandma ;) http://astronomy.stackexchange.com/questions/13035/camera-reaching-the-event-horizon  
> **username_2**: @Nico : good stuff. I'm afraid I have to go out now, so I'll answer later. It might have to be tomorrow I'm afraid, seeing as it's New Year's Eve. Bonne année!  

---

### Answer by username_1 on 2015-12-29. Score: 4

For simplicity, let's say that the black hole is isolated and non-rotating (and uncharged), so that the situation is described by the comparatively simple Schwarzschild spacetime. Let's also suppose that the camera free-falls radially into the black hole.

What is the camera looking at? Suppose it is looking at some stationary object that does something with a known frequency. Your question is basically how at what frequency it will be observed on the video feed emitted by the camera.

Without loss of generality, we can suppose that the camera is looking at *us*, and that we're shining a laser beam at it: the 'doing something at a known frequency' would be the oscillations in the electromagnetic wave of the laser beam. We can do this because time dilation affects every physical process, so we might as well pick one that is more convenient to think about.

At this point, it is straightforward why the camera feed will not show any time dilation: being equivalent to a reflected laser beam, the gravitational blueshift when going inward will be cancelled by the gravitational redshift going outward.


---

