# Black Holes

1. Nov 1, 2007

### bioquest

Does time slow down/stand still near a black hole? How much? Like, if you were on a spaceship close enough to a black hole would you not age for a long time because time would pass really slowly or would it just SEEM like time would be passing really slowly?

2. Nov 1, 2007

### CompuChip

For you, nothing special would happen while you cross the event horizon and go to the center of the hole (as for the time passing, I mean -- you being pulled apart by immense gravity like a string of spaghetti is a completely different story).
But due to the gravitational forces light will escape from the vicinity of the hole just very slowly, and in fact slower and slower until the point where it cannot escape at all (the event horizon). Since the signals from closer to the horizon would take longer to reach an observer well away from the hole, and the signal from the event horizon would actually take infinitely long, such an observer would see you falling toward the event horizon infinitely without ever crossing it, and see you age more and more slowly.

3. Nov 2, 2007

### daniel_i_l

Actually, no observer sees light going at less than c, but if light goes from close to a BH to an observer far away from the BH the far away observer will measure a lower frequency. As the source of the light gets closer and closer to the horizon of the BH the frequency that this observer measures gets lower and lower until the source gets to the horizon and then he measures 0 frequency - or in other words - no light at all.
The time dilation effects between an observer close to a BH and an observer far away from one have nothing to do with the the speed of signals between them. It has to due with the dependence of the ST metric on the distance from the BH.

4. Nov 2, 2007

### DaveC426913

You do not have to cross the event horizon and get pulled into spaghetti to undergo time dilation. You could go into an orbit around the black hole or simply fly real close to the EH.

In an orbit around the BH you would experience nothing special (timewise at least, there would be some interesting tidal effects within your ship), but when you emerged from the gravitational field, you would find that more time had passed in the universe than you would have experienced personally. You would be younger than your twin brother.

Likewise, if your twin brother were observing you from a nearby planet, he would see you go into orbit and move about your ship slower than normal until you emerged.

5. Nov 2, 2007

### bioquest

How much time would have passed in the universe than you would have experienced personally after emerging from the gravitational field? Could that time in that situation or different be something like hundreds of years/how long could it be?

Last edited: Nov 2, 2007
6. Nov 2, 2007

### Chris Hillman

Correcting some common misonceptions

• Of course not---that wouldn't even make sense. How could time "slow down"?
• This is a FAQ and also a common misconception. For a previous post by myself addressing this misconception, see [post=1181763]this post[/post].
• You are asking about a phenomenon known as "gravitational time dilation"; this term is standard but misleading.
• This effect can and should be understood as an effect of spacetime curvature: two initially parallel null geodesics diverge; when comparing proper times between emission and reception of wavefronts by two observers near a massive object, this can result in a red shift measured by the receiving observer when he recieves signals from the sending observer.

Of course none of your instruments would measure "time slowing down"; that wouldn't even make sense. Rather, if you sent time signals every second as measured by an ideal clock you carry to another observer in another spaceship which is farther away, then generally speaking (ignoring details about the how your spaceship and his are moving) he would recieve the time signals redshifted and separated by more than one second as measured by his own ideal clock. Thus, to this second observer you would in a sense "appear" to be aging more slowly, but of course this wouldn't enable you to have a longer and richer life experience, all other things being equal!

DaveC is speaking carelessly (or has a similar misconception). It is very important to understand that no one observer ever "undergoes time dilation" or "experiences time dilation" or even "measures time dilation". In both str and gtr "time dilation" always involves a pair of observers and is always heavily dependent on the details of the motion of both observers; in gtr, the geometry of spacetime itself is also crucial because as already explained this effects how signals propagate from one observer to another.

No "center".

This refers to coordinate speeds, which characterize our description of the situation rather than what is measured by a given observer. As measured by any observer, light always moves in a vacuum at unit speed (in geometric units in which c=G=1). Be aware that there are multiple operationally significant notions of "distance in the large" and thus "velocity in the large", even in flat spacetime. Failure to recognize invariably engenders needless confusion.

CompuChip is trying to express the same geometric picture in different words, in which two initially parallel radially outgoing null geodesics (modeling the world lines of "photons" emitted by a static observer near the hole, for example) will diverge due to spacetime curvature.

Last edited: Nov 2, 2007
7. Nov 2, 2007

### pervect

Staff Emeritus
A full answer to this question gets rather involved.

One can theoretically arrange a trajectory that passes close by a black hole such that one expreriences a short time following the trajectory, but a long time elapses in the universe outside the black hole.

However, one can also arrange such a trajectory without the black hole, simply by travelling near 'c'.

The black hole is different in that a stationary observer can experience this time dilation effect. But the stationary observer must accelerate to remain stationary if he gets close enough to the black hole for the so-called "gravitational time dilation" to be important. One can orbit a black hole if one doesn't get too close, but inside a certain radius, unpowered orbits are not possible, and one must thrust away from the black hole to avoid being drawn in. Really signficant time dilation occurs only inside this critical radius (called the photon sphere - at the critical radius, light itself has an orbit around the black hole, but nothing with rest mass can travel fast enough to achieve orbit because it must travel slower than light).

If you want to approach, but not cross, the event horizon of a black hole, and return, you will not be able to avoid this "slowdown" effect, so in that sense, time slows down near a black hole.

But such a close approach will inevitable also require crushingly large accelerations to avoid crossing the event horizon due to the fact that unpowered orbits do not exist.

Another common scenario is to imagine falling into a black hole, passing through the event horizon, and not returning. In this case, one does *NOT* see the history of the universe play itself out during the trip inward. (This is mentioned in several black hole FAQ's).

8. Nov 2, 2007

### bioquest

accidentally posted twice

9. Nov 2, 2007

### bioquest

One can theoretically arrange a trajectory that passes close by a black hole such that one expreriences a short time following the trajectory, but a long time elapses in the universe outside the black hole. However, one can also arrange such a trajectory without the black hole, simply by travelling near 'c'.

are you saying that...you would only age a little, but the rest of the universe could be like 200 years older than it was before you left? because you would age slowly? Or are you saying tha it would only feel like you were aging slower but you wouldn't really be?

10. Nov 2, 2007

### dibiz116

you wouldn't 'feel' like you were aging any slower or faster than usual, you would be aging normally (from your own perspective), but when you leave whatever situation is causing time dilation and returned to your original state in the universe, 200 years (or whatever amount of time) would have passed for the universe and only say, 2 years would have passed for you. time isn't going more or less slowly for either, more time just passed for the rest of the universe than the amount of time that passed for you.

but if something was observing you from this outside universe, you would appear to them to be aging slowly, just as if you could observe the universe from your position it would appear to be aging faster than normal, but in reality neither observer 'feels' like it is aging any slower or faster than usual.

11. Nov 2, 2007

### pervect

Staff Emeritus
I'm saying that if you had a twin, and you left from some place that was far away from the black hole and took a close pass by the black hole while your twin stayed home, you and your twin could compare ages when you returned from your trip.

When you returned, you would be young, and your twin would be old. This works just the way with a black hole as it does with the SR "twin paradox".

I'm also taking the viewpoint that you and your twin should only compare ages when you are both close to each other, i.e. "at the same place". This is due to something that's called "the relativity of simultaneity", which also important in SR.

12. Nov 3, 2007

### bioquest

sorry double posted I dont know why it did that

Last edited: Nov 3, 2007
13. Nov 3, 2007

### bioquest

okay so

Where would you be able to go and have this happen? (Below) If one of the places this would happen was in/around a black hole, how close would you have to be to the black hole + other details? Also if you spent 2 years there might 200 years/how much time have passed in the rest of the universe?

you wouldn't 'feel' like you were aging any slower or faster than usual, you would be aging normally (from your own perspective), but when you leave whatever situation is causing time dilation and returned to your original state in the universe, 200 years (or whatever amount of time) would have passed for the universe and only say, 2 years would have passed for you.

and how much time would have passed in the universe at most do you think if you were in a place like that for 2 years where you would come and more time would have passed in the universe than for you?

Also would it be the more time spent there (2 years etc) the larger the amount of time dilation ie the larger the amount of difference would be in time passed between you and the rest of the universe?

Last edited: Nov 3, 2007
14. Nov 3, 2007

### pervect

Staff Emeritus
I'm going to assume that you are looking for a large amount of time dilation.

The details are going to depend on the specific black hole you attempt to utilize. For this purpose, "bigger is better". I will utilize the black hole "Gargantua" from Kip Thorne's book, "Black Holes & Time warps". This book opens with several fictionalized accounts (using, however, very good physics) of exploring (or attempting to explore) various sizes of black holes. I would recommend the book to anyone interested in the topic of black holes. Gargantua, as you might guess, is one of the largest:

Skipping on a bit

In this particular situation, one year of ships time will turn out to be equivalent to roughly 100 years of time far away from the black hole. To measure this, you measure the blue shift of infalling light. Thorne doesn't make a point of pointing out that this is the time dilation factor, but that's what it is:

To achieve a higher time dilation factor, you'll either need a bigger black hole (but Gargantua is already very large), or you'll be able to need to stand more than 10g's of acceleration (but that's already quite high).

One more point you might be interested in:

Thorne goes on to explain that with very sensitive instruments, you could detect the tidal force from the black hole.

15. Nov 4, 2007

### bioquest

So...you could actually go to outer space, experience a year or so there (ie. only age a year, only experience a year, etc) and have 200 years or so pass by on earth? (I mean you can make comments clarifying the accuracy of the time difference...ie 3 years...100 years..etc)

16. Nov 4, 2007

### DaveC426913

Yes. But there is no theoretical upper limit on the time dilation - it's asymptotic. It could be thousands or millions of years.

17. Nov 4, 2007

### bioquest

So right now would we have the necessary knowledge to go up into space, near a black hole wherever, stay there, and come back to earth 200 years or more later?

18. Nov 4, 2007

### pervect

Staff Emeritus
19. Nov 4, 2007

### DaveC426913

Apart from actually finding the right kind of BH and getting to it (neither of which is trivial), there's nothing technologically beyond us, no. The big trick is finding one that's large enough that you can get within the radius where time dilation has a noticeable effect without being so close that the tides tear your ship apart (the tides are more gentle around a larger BH).

Last edited: Nov 4, 2007
20. Nov 4, 2007

### bioquest

So are there any black holes that we know of currently that could be used for this purpose?