# Black Holes

1. May 11, 2010

### iRaid

Hello everyone, I'm interested in knowing something about black holes. I was wondering how time can speed up when you're near a black hole? Do you feel it or is it actually faster.. Can someone explain this because I cant wrap my head around how it can speed up time. It has to do with the event horizon where it just seems like someone stopped there, but is it actually that slow for something that goes into it?

I do know that large masses can make time speed up or look like it, but please explain better.

Thanks.

2. May 11, 2010

### Chronos

Time is relative. Near a black hole, the photons emitted by an approaching object are time dilated in the opposite direction. Redshift is the other consequence. Redshift and time dilation are different manifestations of the same underlying effect.

3. May 12, 2010

### libbon

Actually time is slowing down. The closer you get to the speed of light, the more time slows down, this is called time dilation, the reason time slows down is because since the speed of light is the universal speed limit, the only way to prevent you from passing it is for time to slow down. So the faster you go the more time slows down. And also the higher the gravitational field the more slower time ticks relative to other regions of time. So for example as a theoretical experiment, let's say you put four people in a space ship and put them next to a black hole just outside the event horizon but not so close so that they get pulled in. If you have them orbit it for a year, when they come back to earth, it will be the future, now depending on the mass of the black hole and the time spent next to it i think how much farther in the future it will be depends. But in this case i think it will be about 10 years in time will have passed on earth versus the people in the spaceship next to the black hole for a year.
I don't think i dwelled too much into the black hole part but i hoped i helped a little, check out this:

http://en.wikipedia.org/wiki/Gravitational_time_dilation

4. May 12, 2010

### libbon

Also to answer your questions more clearly, to the people near the black hole, even if time is slowing down, it seems regular to them. But to the observer, if the spaceship full of people went towards the event horizon it would looked like there frozen in time. And as in the doppler effect and blue shift and redshift, if you don't already know it try looking it up, to the observer it might look like there going slow or faster as they orbit around the black hole. But then that also goes into the observer effect, look into that too. You tell me if i took it too far discussing this, sorry its so long :D

5. May 12, 2010

### iRaid

Thanks for the answer, I never knew about the gravitational redshift, but now that I do, it seems to be clearer.

Thank you.

6. May 13, 2010

### A Dhingra

Chronos : " Redshift and time dilation are different manifestations of the same underlying effect . "

well i thought that according to doppler effect if a body moves with the speed of the wave and in the direction opposite to that of the wave ....then the wave can never reach the observer...so the same should apply for light........but according to relativity light still reaches the observer due to the observer.....
now can u explain what u said.......sorry if i am going against any rules by asking like this........... thnks still

7. May 14, 2010

### Chronos

Redshift is not unlike a soccer ball caught by a retreating goalie. The velocity of the retreating goalie makes the energy of the ball appear less than the kick velocity.

8. May 14, 2010

### libbon

Here you go, redshift and time dilation are much more different, what is very similar is the doppler effect and redshift, it's just that one has to do with sound and one has to do with light. In the future if you want to look something up, i reccomend using wikipedia, thats where i find a lot of my physics ideology
Here:
http://en.wikipedia.org/wiki/Redshift

http://en.wikipedia.org/wiki/Doppler_effect

9. May 14, 2010

### libbon

Here you go, redshift and time dilation are much more different, what is very similar is the doppler effect and redshift, it's just that one has to do with sound and one has to do with light. In the future if you want to look something up, i reccomend using wikipedia, thats where i find a lot of my physics ideology
Here:
http://en.wikipedia.org/wiki/Redshift

http://en.wikipedia.org/wiki/Doppler_effect

10. May 25, 2010

### DR_henegar

Black holes are one of my favorite things to ask about. If you were about to go into a black hole, a couple things may happen. One: you will be stretched heavily (and probably ripped apart) due to the immense gravity to a certain point. Two: You will then be squished together, if anything of you is left from being stretched. and Three: You will be sucked and destroyed by the black hole, never to return. In the event horizon is where you are out of luck my friend. Actually, getting anywhere close to a black hole even before the event horizon is going to do this, because with our speed of rockets, we could not escape if we got too close.

11. May 27, 2010

### Chronos

Pardon my failed attempt to provide an analogy in classical physics. I am aware of the differences. GR applies complex corrections to its classical equivalent in the high energy realm.

12. Jun 18, 2010

### emergent

as it is understood to me time is relative/temporal. time is more a tool of the mind to organize/rationalize movement or change.

Last edited: Jun 18, 2010
13. Jun 23, 2010

### seto6

the time actually does not slow down with respect to the person entering the black hole.
but the person waiting from out side sees it as the person enter event horizon but never will see the person enter the hole because after the event horizon the light con not escape and the wave length of light is increased

14. Jun 28, 2010

### neurotikfisch

Are you saying that, in addition to the observer finding that the subject on the event horizon appears to move slowly, the subject on the event horizon will also appear to be redder? If that's true, will the observer appear to be blue from the subject's point of view, since photons will be entering the gravitational field?

15. Jun 28, 2010

### Chronos

The outside universe will indeed appear blue shifted and time contracted to the infalling observer.