- #1

lifeisareaction

- 2

- 0

You are using an out of date browser. It may not display this or other websites correctly.

You should upgrade or use an alternative browser.

You should upgrade or use an alternative browser.

- Thread starter lifeisareaction
- Start date

- #1

lifeisareaction

- 2

- 0

- #2

mathman

Science Advisor

- 8,077

- 547

The effect of black holes is based on the fact that the escape velocity is greater than the speed of light.

- #3

Phobos

Staff Emeritus

Science Advisor

Gold Member

- 1,954

- 6

Welcome to Physics Forums, lifeisareaction!

- #4

- 10,177

- 1,342

If you adopt a reference frame of a "stationary observer" who is hovering at some distance 'r' away from the black hole, it turns out that the speed of any matter falling into the black hole approaches 'c' as r approaches the Schwarzschild radius. One could losely describe this state of affairs as an object reaching the speed of light at the event horizon - after which it continues to accelerate.

However, stationary observers do not exist exactly at the event horizon, or anywhere inside it. Inside the event horizon, all observers are falling into the black hole.

So there is no actual observer who ever sees the infalling matter exceed or even reach 'c'. But it is possible for the rate of change of the Schwarzschild 'r' coordinate with respect to the Schwarzschild 't' coordinate to exceed the speed of light, in fact it turns out that one expects this to happen inside the event horizon. However, that this is a 'coordinate velocity" that no physical observer will ever observe. One might think of this as the velocity that a stationary observer inside the event horizon would measure, if a stationary observer could exist. (But of course, such an observer cannot exist!).

There is some mathematical background on a very closely related question at

https://www.physicsforums.com/showpost.php?p=602558&postcount=29

https://www.physicsforums.com/showpost.php?p=621784&postcount=31

(the first post in mine, the second is another poster, George Jones).

- #5

mathman

Science Advisor

- 8,077

- 547

If you adopt a reference frame of a "stationary observer" who is hovering at some distance 'r' away from the black hole, it turns out that the speed of any matter falling into the black hole approaches 'c' as r approaches the Schwarzschild radius.

It is my impression that, to an outside observer, the object falling into a black hole appears to slow down to a crawl, never actually reaching the event horizon.

- #6

kublai

- 31

- 0

mathman said:It is my impression that, to an outside observer, the object falling into a black hole appears to slow down to a crawl, never actually reaching the event horizon.

hmmm, if that were true how did that black hole get all that matter to make a black hole? If the event horizon was laden with objects that never reached inside it would not be black, would it?

- #7

mathman

Science Advisor

- 8,077

- 547

- #8

- 10,177

- 1,342

mathman said:It is my impression that, to an outside observer, the object falling into a black hole appears to slow down to a crawl, never actually reaching the event horizon.

To the outside observer at

To the outside obsever, stationary with respect to the black hole, the object falling into the black hole approaches a velocity of 'c' as the stationary observer and the object, both assumed to be at the same point in space, approach

For the details, see the links to the previous thread I posted.

As I mentioned before, it requires infinite acceleration to "hover" at the event horion of a black hole, so the velocity of c is never measured, only approached as a limit.

Share:

- Last Post

- Replies
- 7

- Views
- 594

- Last Post

- Replies
- 8

- Views
- 760

- Last Post

- Replies
- 4

- Views
- 1K

- Last Post

- Replies
- 1

- Views
- 582

- Last Post

- Replies
- 2

- Views
- 579

- Replies
- 19

- Views
- 623

- Last Post

- Replies
- 32

- Views
- 3K

- Replies
- 4

- Views
- 663

Stargazing
Light Years To Miles.

- Last Post

- Replies
- 3

- Views
- 2K

- Last Post

- Replies
- 4

- Views
- 441