# The Naked Scientists Forum

### Author Topic: What happens to the passage of time around a black hole?  (Read 815 times)

#### memoryerase1

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##### What happens to the passage of time around a black hole?
« on: 31/12/2015 23:13:10 »
What happens to the passage of time around a black hole?

For a person in orbit around a black hole, time passes more slowly than for an outside observer. But is this gravitational time dilation rate for a rocket shuttle orbiting the black hole as close to the point where you would become spaghettified (heated up and stretched)?

Could it be possible to orbit closer to the point of the event horizon, and orbit in even stronger gravity for time to speed up a lot more, for outside observers away from the black hole?

Is the figure of 1 year in the black hole, and four years pass for a outside observer, just for a shuttle orbiting in a random point where the gravity is not so strong.

« Last Edit: 01/01/2016 22:12:01 by chris »

#### Space Flow

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##### Re: If you orbit a black hole for a year, four years would have passed for a ?
« Reply #1 on: 01/01/2016 01:53:12 »
If you orbit a black hole for a year, four years would have passed for a outside observer.
I don't know who or what you are quoting with that statement.
It would certainly be true for one particular orbit at a certain radius at the appropriate speed for that radius.
If you change to an orbit at a larger radius than that, your orbital speed would also decrease and your time dilation would be less. Similarly if you decrease the radius of your orbit, you would also increase your speed and the combination of those two actions would be reflected in a larger time dilation.
If you didn't have to worry about an accretion disk trying to turn you into plasma, you could keep decreasing the radius of your orbit, subsequently increasing your orbital speed, to the point of almost totally freezing your clock rate as seen by an outside observer. You could then witness the Universe evolve through millions or even billions of years by an outside observers time, in a matter of weeks, days, hours, minutes, or even seconds.
Although matter can not travel at the speed of light, there exists no restriction on how close to light speed it can go. There are some energy considerations, but here the energy can be supplied by the Huge amount of gravity.
And you can do all this without touching the event horizon.
If you get to this point, between you and the event horizon is still a region where light is trapped in an orbit around your black hole. The radius that corresponds to the orbital speed of light.

Hope that helps.

#### evan_au

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##### Re: If you orbit a black hole for a year, four years would have passed for a ?
« Reply #2 on: 01/01/2016 21:27:43 »
There was a scene in "Interstellar" where some astronauts landed on a planet orbiting close to a black hole.
They stayed there for a fairly short time (hours?), but their colleague orbiting farther out was now an old man.

When the director said he wanted the scene to work like this, the scientific adviser (physicist Kip Thorne) reportedly went away and identified an orbit where a planet could possibly remain in a stable orbit this close to a black hole.

However, it takes an enormous amount of energy for a rocket to drop down into a low orbit around a black hole, and just as much for the rocket to return from the gravitational well. You won't be able to do it with any kind of rocket that we have today.

An easier trip for a rocket would be an hyperbolic path, which takes you close to the black hole, and back out again without having to fire the rockets again. But you don't spend so much time close to the black hole.

#### Space Flow

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##### Re: What happens to the passage of time around a black hole?
« Reply #3 on: 01/01/2016 23:52:07 »
Evan, if you already have a planet in a stable orbit as a destination, couldn't a ship use the gravity of the Black hole to increase it's speed to the planet's orbital speed so it could then be captured by the planet's gravity?
That would surely save some fuel.
Don't know if the gravity assist scenario would work on the way back out, but we have used fly by's the Earth to boost the speed of some of our craft and send them outward.

#### The Naked Scientists Forum

##### Re: What happens to the passage of time around a black hole?
« Reply #3 on: 01/01/2016 23:52:07 »