# Why aren't black holes the brightest objects in the universe?

1. May 4, 2013

### DARKSYDE

I've seen many examples that an observer witnessing an object approach the event horizon will see it slow down to a point where it never seems to cross. Why then do we not see "everything" that has crossed into this horizon frozen in time?

2. May 4, 2013

### mathman

The light from the "frozen" object gets red shifted as time passes. Sooner or later the radiation will be in the radio wave spectrum.

3. May 4, 2013

### DARKSYDE

Then radio telescopes would be blinded by them if that were true?

4. May 5, 2013

### mrspeedybob

We'd be talking about wavelengths many orders of magnitude larger then the observable universe. I don't think any radio telescopes yet developed detect these.

5. May 5, 2013

### Nabeshin

Regardless of the wavelength, the power observed by an external observer will just go asymptotically down to zero as the time dilation increases. The wavelength shift just means that this radiation (what little amount there is after an appreciable time) leaves the detectable regime that much faster.

6. May 7, 2013

### DARKSYDE

But wouldn't there be a constant source of energy entering this horizon? Such as photons ??

7. May 7, 2013

### Chronos

You are not seeing the big picture here.

8. May 7, 2013

### Nabeshin

This was way more humorous than it had any right to be, Chronos... :)

Darksyde: In order for you to SEE photons they have to enter your eye! If they're falling into the black hole, they're clearly not going to enter your eye, now are they?

9. May 7, 2013

### DARKSYDE

Nabeshim,

That's the point, the observer never fully sees an object cross the horizon. The photons will reach your eye untill that point.

10. May 7, 2013

### Bandersnatch

Applying my limited understanding of the subject:

If a source of light falls into a black hole, it appears to have its time dilated in such a way so as to never reach the event horizon. But it also means that any two events happening on the source become infinitely stretched in time, as seen by an external observer.
If the two events are two consecutive emissions of a photon, then as it asymptotically approaches the event horizon, the time between the emissions becomes inifitely long. As a result the source appears to grow dimmer(flux goes down) across all wavelengths, even before taking into account the gravitational redshift.

11. May 9, 2013

### Chronos

You still fail to 'see' what is happening in conformal time. If you draw it out in a Penrose diagram, it will become obvious.

12. May 9, 2013

### DARKSYDE

-Chronos

Would conformal time have any effect if we were to observe black holes in our own galaxy? Since there the expansion of the universe does not apply to this kind of observation, would the Hubble constant be somewhat diminished?

13. May 9, 2013

### Chronos

The Hubble effect is too small to measure over distances less than millions of light years.

14. May 9, 2013

### DARKSYDE

That is what I thought as well. Why did you mention it then?

15. May 14, 2013

### Chronos

You are confusing conformal time with distance. It also applies to gravitational wells.

16. May 17, 2013

### mrspeedybob

I just got introduced to quasars.

It would seem that under certain circumstances black holes are among the brightest objects in the universe, though not for the reason the O.P. had in mind.