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Jeff Root
2014-Jul-17, 03:44 PM
From the thread "Black Holes Ain't":



the gravitational field of [a black hole] is so strong that
escape velocity exceeds "c"...and that will render it black.


... is a horribly inaccurate and misleading analogy.


I can see how it might be misleading. We can get to that later.

How is it inaccurate?

How is it horribly inaccurate?

How is it an analogy?


Black holes are not "black" because of escape velocity.
If that were the reason then, for example, light could get
some distance away from the surface before falling back
(in the same way you can throw a ball in the air at less
than escape velocity).
That is a non sequitur. Saying that the principle of
escape velocity applies to light and black holes does not
imply that light behaves like massive objects. There
simply is no reason to make that inference.

The fact that light and other massless things always move
at a constant speed -- which is radically unlike the behavior
of massive things -- is in no way contradicted by saying that
escape velocity at the event horizon of a black hole is the
speed of light, and that is why light cannot escape.



For that reason, I don't think the (Newtonian) concept of
escape velocity is even meaningful at (or within) the event
horizon. That makes it an analogy.
I do not see any way that it does not apply. Nothing can
escape the event horizon of a black hole, because escape
velocity at the event horizon is the speed of light, and
nothing moves faster than that speed. There is no reason
to call that an analogy.



And also misleading because people who get their science
articles from popular media may think that light can escape
the surface, or you could escape if you moved faster than
light or ....
I suggested that we leave this until after we agree on
whether Pete's statement was actually inaccurate or not.

But since you say it ... The event horizon is the surface
which bounds the region from which light cannot escape.
Below the surface, light cannot escape. Above the surface,
light can escape. What are you saying is wrong about the
idea that "light can escape the surface"? Although I would
probably not express it that way, I see nothing wrong with
the physics. The horizon is the surface marking the lowest
level in the gravity well from which light can escape.

And ... If you could move faster than light, then yes, you
would be able to escape.

So I disagree with all of your objections.

-- Jeff, in Minneapolis

Strange
2014-Jul-17, 05:16 PM
That is a non sequitur. Saying that the principle of
escape velocity applies to light and black holes does not
imply that light behaves like massive objects. There
simply is no reason to make that inference.

I don't understand your logic. Escape velocity has a clear definition. If light is affected by gravity such that it is constrained by escape velocity then why wouldn't anything we say about it not be the same as we say about anything else regarding escape velocity. (Sorry if that sounds confused but I have no idea what you are trying to say. The "escape velocity" analogy is the thing making the correspondence with massive objects, not me. I am pointing out drawing an analogy with massive objects in low energy domains is inappropriate.)


The fact that light and other massless things always move
at a constant speed -- which is radically unlike the behavior
of massive things -- is in no way contradicted by saying that
escape velocity at the event horizon of a black hole is the
speed of light, and that is why light cannot escape.

I do not see any way that it does not apply. Nothing can
escape the event horizon of a black hole, because escape
velocity at the event horizon is the speed of light, and
nothing moves faster than that speed. There is no reason
to call that an analogy.

But, I say again, if it were due to the escape velocity then light and/or massive objects could escape temporarily at less than the speed of light. (And they can't.)

Maybe you are redefining escape velocity to mean "nothing can escape unless it exceeds the speed of light"? But if so, that is another reason it is misleading: using a term with one established meaning to mean something else.

Also, if that is what you mean then it may be true, although perhaps only in Painlevé-Gullstrand coordinates. I don't believe it is true in Schwarzschild coordinates. (And the reason I don't think there is a problem with the two metrics giving a different answer is that it is impossible!)

I may be wrong about that so we will have to see if someone who knows what they are talking about joins the thread. :)


What are you saying is wrong about the
idea that "light can escape the surface"? Although I would
probably not express it that way, I see nothing wrong with
the physics. The horizon is the surface marking the lowest
level in the gravity well from which light can escape.

OK, for surface read "the interior of the black hole".

kzb
2014-Jul-17, 05:37 PM
Maybe it is more correct to say its wavelength is stretched to infinity, in which case it is an exercise in philosophy to say whether it has escaped or not.

Is quantum tunneling allowed across an event horizon?

Would there not be some quantum uncertainty in the position of the horizon?

Strange
2014-Jul-17, 05:42 PM
Maybe it is more correct to say its wavelength is stretched to infinity, in which case it is an exercise in philosophy to say whether it has escaped or not.

I was thinking more about things escaping the black hole, rather than light from outside the black hole.


Is quantum tunneling allowed across an event horizon?

Would there not be some quantum uncertainty in the position of the horizon?

Once you start bringing quantum effects in, then the whole picture probably changes completely (see the thread this one was split from).

Jeff Root
2014-Jul-17, 07:14 PM
That is a non sequitur. Saying that the principle of
escape velocity applies to light and black holes does not
imply that light behaves like massive objects. There
simply is no reason to make that inference.
I don't understand your logic. Escape velocity has a
clear definition.
I will describe it as the speed required to completely
escape from a specific depth in a gravity well, given
that the thing trying to escape doesn't run into
anything which slows or stops it.

Light is always able to escape from any gravity well that
is not a black hole, because its speed is always greater
than escape velocity.

A massive object is able to escape from such a gravity
well if its speed is greater than the escape velocity,
regardless of the direction it is headed. If it is headed
downward in the gravity well, its speed increases as it
falls, so that it continues to have escape velocity even
though it moves into a region of stronger gravity. If it
doesn't run into anything it will rise out of the gravity
well again on a hyperbolic trajectory.

Black holes are a complication in that the direction of
motion makes a difference. If a massive object goes
closer than three Schwarzschild radii -- the location of
the last stable orbit -- it will experience a net loss of
speed, and may no longer have escape velocity when
it returns to 3 R.

If light goes closer than 1.5 R -- the location of the
photon sphere -- it will spiral down into the black hole.

Light which is emitted from less than 1.5 R can escape
only if it is emitted upward. Light emitted horizontally
just above the photon sphere will spiral away from the
black hole, while light emitted horizontally just below
the photon sphere will spiral into the black hole. Light
emitted horizontally exactly in the photon sphere could
theoretically "orbit" the black hole.

The closer to the event horizon light is emitted, the
higher the angle it must have to escape. Light emitted
just above the event horizon at 1 R must be emitted
straight upward to escape.



If light is affected by gravity such that it is constrained
by escape velocity then why wouldn't anything we say
about it not be the same as we say about anything else
regarding escape velocity.
Why *would* it be the same? You have given no reason
to think that light must change speed as it falls and rises
in a gravity well if the principle of escape velocity applies.



(Sorry if that sounds confused but I have no idea what you
are trying to say. The "escape velocity" analogy is the thing
making the correspondence with massive objects, not me.
I am pointing out drawing an analogy with massive objects
in low energy domains is inappropriate.)
I'm saying that it isn't an analogy. The principle of escape
velocity applies to light just as well as it applies to massive
objects. Massive objects speed up and slow down as they
enter and leave a gravity well on a hyperbolic trajectory.
Light does not speed up or slow down relative to observers
local to it, but it does follow a hyperbolic trajectory, and is
constrained by escape velocity.

Light emitted straight upward just above the event horizon
takes longer to reach a distant observer than does light
emitted from a location away from the black hole, because
the space near the black hole is stretching, increasing the
distance it has to travel to reach the observer.



But, I say again, if it were due to the escape velocity then
light and/or massive objects could escape temporarily
at less than the speed of light.
Why?

-- Jeff, in Minneapolis

Jeff Root
2014-Jul-17, 07:38 PM
I forgot to add this informational tidbit:

Artist's illustrations of what a black hole would look like
if seen up close show a black circle surrounded by a haze
of distorted background stars. The edge of the black circle
is the photon sphere, not the event horizon.

-- Jeff, in Minneapolis

Strange
2014-Jul-17, 07:45 PM
Why?

You said: "I will describe it as the speed required to completely escape from a specific depth in a gravity well"

At speeds below the escape velocity, things can temporarily escape. That is not true of a black hole.

The fact we are having this discussion at all is evidence of how misleading the description is....

Jeff Root
2014-Jul-17, 08:21 PM
You said: "I will describe it as the speed required
to completely escape from a specific depth in a
gravity well"

At speeds below the escape velocity, things can
temporarily escape. That is not true of a black hole.
It isn't true of anything.

At speeds below the escape velocity, massive objects can
rise some distance higher in a gravity well, but not escape
completely. They lose speed as they rise. If they lose all
their speed without escaping, they will start to fall. Light
doesn't change speed like that, so what you are saying
doesn't apply. But the speed anything would need to have
to escape from a black hole approaches the speed of light
at locations closer and closer to it. At some point, the
escape speed *is* the speed of light. For light which is
emitted straight up, that location is the event horizon.



The fact we are having this discussion at all is evidence of
how misleading the description is....
I don't think your view has any merit. The claim that light
would have to behave like massive objects if the principle
of escape velocity applied to it is unfounded.

-- Jeff, in Minneapolis

Strange
2014-Jul-17, 08:49 PM
At speeds below the escape velocity, massive objects can
rise some distance higher in a gravity well, but not escape
completely.

And that is not true for a black hole.


The claim that light
would have to behave like massive objects if the principle
of escape velocity applied to it is unfounded.

Who claimed that?

Jeff Root
2014-Jul-17, 09:55 PM
At speeds below the escape velocity, massive objects can
rise some distance higher in a gravity well, but not escape
completely.
And that is not true for a black hole.
Of course it is. A massive object can orbit a black hole
exactly the same as any other gravitating body as long as
it doesn't go lower than "the last stable orbit" at 3 R, as I
described in post #5. If an object goes lower than that it
will lose some speed. As long as it doesn't lose too much
speed it will be able to rise some distance again, or even
escape. The speed it needs to have in order to escape
from below 3 R depends on both its location and direction.
If the direction is toward the photon sphere or lower, then
it won't be able to regain enough speed to rise away again,
and spirals in. Light going below the photon sphere also
spirals in, but without gaining or losing speed.

The basic idea is that at any distance from a gravitating
body there is a minimum speed needed to escape, and
since the speed of light is the maximum speed possible,
nothing can escape from the distance where the escape
speed is equal to or greater than the speed of light.

Just because some special things happen very close to
black holes doesn't mean escape velocity doesn't apply.




The claim that light would have to behave like massive
objects if the principle of escape velocity applied to it
is unfounded.
Who claimed that?
You initially complained about Pete's description of what
happens to light near a black hole. The question is about
the behavior of light. Then in your last post you said:



At speeds below the escape velocity, things can
temporarily escape. That is not true of a black hole.
By "temporarily escape" you clearly mean that massive
objects can rise away from a gravitating body on an
elliptical trajectory. You are implying that what massive
objects do, light must also do if the principle of escape
velocity applies to it.

-- Jeff, in Minneapolis

Reality Check
2014-Jul-18, 01:14 AM
The actual problem with "the gravitational field of [a black hole] is so strong that escape velocity exceeds "c"...and that will render it black" is that this is not a black hole!
This is a dark star (http://en.wikipedia.org/wiki/Dark_star_(Newtonian_mechanics))

A dark star is a theoretical object compatible with Newtonian mechanics that, due to its large mass, has a surface escape velocity that equals or exceeds the speed of light. How light is affected by gravity under Newtonian mechanics is questionable but if it were accelerated the same way as projectiles, any light emitted at the surface of a dark star would be trapped by the star’s gravity, rendering it dark, hence the name.

A GR description is that the gravitational field of a black hole is so strong that it curves spacetime so that the future light cone of a photon always includes the singularity, no matter what direction it is travelling.
Tipping the Light Cone: Black Holes (http://asymptotia.com/2008/03/10/tipping-the-light-cone-black-holes/)

Strange
2014-Jul-18, 07:48 AM
Of course it is. A massive object can orbit a black hole ...

I did not say anything about orbiting. Or most of the other things you comment on.

As you keep puttiing words in my mouth and claiming I have said things I didn't, I will say no more.

Strange
2014-Jul-18, 10:42 AM
A GR description is that the gravitational field of a black hole is so strong that it curves spacetime so that the future light cone of a photon always includes the singularity, no matter what direction it is travelling.

Exactly. And as there is no "outward" direction at or within the event horizon then the concept of "escape velocity" (a speed in the radial direction) is meaningless.

kzb
2014-Jul-18, 03:22 PM
When you think about it, it is not good. Escape velocity is an initial kick given to a body that enables it to escape from a gravity well.

As the body ascends it slows down (whereas light always travels at c)

The escape velocity decreases with altitude.

A body can escape without ever getting to surface escape velocity, as long as it has thrusters on the way up.

Jeff Root
2014-Jul-18, 04:54 PM
Of course it is. A massive object can orbit a black hole ...
I did not say anything about orbiting.
Yes, you did, whether you knew it or not. I said:



At speeds below the escape velocity, massive objects can
rise some distance higher in a gravity well, but not escape
completely.
That is an orbit. You replied:



And that is not true for a black hole.
Obviously, massive objects *can* orbit a black hole.
So whatever you meant, what you said was wrong.

-- Jeff, in Minneapolis

tusenfem
2014-Jul-18, 06:04 PM
That is an orbit. You replied:



Jeff Root you seem to have a very special personal vocabulary, in this case "orbit" which you did not say in the part that you quoted from yourself and which it does not describe.
As often you seem to have your own views on how physics is supposed to function, which do not agree with the general mainstream views.
Also in this discussion you are mixing up massive and massless particles which have different qualities, specifically about the speed they are moving at.
Now you are just trying to argue in any way, to prove that you are right. I think it is time to stop that right now.

Jeff Root
2014-Jul-18, 07:57 PM
tusenfem,

What I described when I said:


At speeds below the escape velocity, massive objects can
rise some distance higher in a gravity well, but not escape
completely.
clearly and unambiguously refers to those massive objects
being in orbit. Nothing about it is in any way nonstandard
or not mainstream physics.

As far as mixing up the properties of massive particles with
those of massless particles, Strange is the one doing that.
I have pointed out each time that they are different, from
the very first post in this thread. Strange kept insisting that
if the principle of escape velocity applied to light, then light
would have to behave like massive particles in how they
move in response to gravity: Slowing as they rise away from
a gravitating body and speeding up as they fall. In other
words, orbiting. I insisted that that is not so, and it isn't.

I don't understand why you have a problem with this. It is
elementary and completely well-known.

-- Jeff, in Minneapolis

tusenfem
2014-Jul-18, 08:11 PM
closing this thread for moderator discussion

slang
2014-Jul-19, 04:40 PM
Infraction issued for arguing moderation in thread.