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Europa1969
2010-Mar-01, 08:18 PM
Will light go slower than C when pulled in to a black hole? One thing I have always wondered concerning a Black hole is… when a black hole starts pulling a photon back into it’s self will it “slow down’ (for lack of better words) before it starts pulling it back in or does the photon still travel at C and then in an instant pull it back at C or a slower speed?

So lets say a mass is emitting light in all directions. That mass begins to get sucked into a black hole. At some point the light cannot escape. Will the light that was being emitted in a direction opposite of the black hole goes from being emitted in one direction to getting sucked back into the BH. at some point the photons have to stop and go in the opposite direction. If this is so then c is not constant.

ShinAce
2010-Mar-01, 09:29 PM
C is constant in the vacuum of FLAT space and LOCALLY.

If you're falling with the test 'beacon', you will measure c for light emitted in all directions. If you stay a safe distance, you can no longer measure all emitted photons travelling at c.

Likewise, what's the speed of a photon as it crosses inflationary spacetime?

loglo
2010-Mar-02, 12:36 AM
Will light go slower than C when pulled in to a black hole? One thing I have always wondered concerning a Black hole is… when a black hole starts pulling a photon back into it’s self will it “slow down’ (for lack of better words) before it starts pulling it back in or does the photon still travel at C and then in an instant pull it back at C or a slower speed?

So lets say a mass is emitting light in all directions. That mass begins to get sucked into a black hole. At some point the light cannot escape. Will the light that was being emitted in a direction opposite of the black hole goes from being emitted in one direction to getting sucked back into the BH. at some point the photons have to stop and go in the opposite direction. If this is so then c is not constant.

C is constant, the path ahead of the photon is what is changed by the Black Hole.

astromark
2010-Mar-02, 04:05 AM
C is constant, the path ahead of the photon is what is changed by the Black Hole.

That is correct. The Photon can only do what it does. Thats travel at c. The space its traveling in is being pulled into the BH. Your light emitting object still is emitting that light. It just can not reach you. It is not correct to say the light has slowed. It has not.

adamskiiix
2010-Mar-02, 07:49 AM
but wouldn't the black hole have to eat all space that the photon would travel throughout its whole existence? :confused:

astromark
2010-Mar-02, 08:01 AM
Eat might not be the best word for it, but. Yes. That is what is happening. The Black Hole is collapsing space so the light can never get away. Thats why it is called a black hole. Gravity force exceeding c.

loglo
2010-Mar-03, 12:52 AM
but wouldn't the black hole have to eat all space that the photon would travel throughout its whole existence? :confused:

I think talking of space being eaten or collapsing is very misleading. GR is about curvature. Inside the event horizon all possible paths ahead of the photon are curved inwards towards the singularity, the photon is merely going straight, as usual.

sauron
2010-Mar-03, 02:06 AM
I think talking of space being eaten or collapsing is very misleading. GR is about curvature. Inside the event horizon all possible paths ahead of the photon are curved inwards towards the singularity, the photon is merely going straight, as usual.

It's pretty clear that on the way out of the BH every route is a U turn so to speak. What about the way in. As the photon "takes the turn" and starts back towards the singularity. Is it a straight path down or is space curved in such a way that the photon is caught in a type of spin? Are there theories on how a photon behaves once inside the event horizon?

Tensor
2010-Mar-03, 02:59 AM
It's pretty clear that on the way out of the BH every route is a U turn so to speak. What about the way in. As the photon "takes the turn" and starts back towards the singularity. Is it a straight path down or is space curved in such a way that the photon is caught in a type of spin? Are there theories on how a photon behaves once inside the event horizon?

GR can handle the math all the way down to the singularity. Inside the event horizon, timelike paths become spacelike and spacelike paths become timelike. Photons would travel as they would outside the event horizon, along a null geodesic. It's just that all paths are warped to the point where they fall into the singularity. Hawking and Penrose's singularity theorems showed that black hole singularities are points where world lines end.

TheHalcyonYear
2010-Mar-03, 03:02 AM
A technique developed by Professor Robert Boyd at the University of Rochester
has slowed the speed of light five million-fold—and has done so without the
complex, room-filling mechanisms previously used to slow light. The new
apparatus is small and, in the words of its creator, “ridiculously easy to
implement.” Such a simple design will likely pave the way for slow light, as it is
called, to move from a physical curiosity to a useful telecommunications tool.
I find this really interesting.

sauron
2010-Mar-03, 03:36 AM
singularities are points where world lines end.

Time stops?

BTW, does the photon actually reach the singularity or does it seem to fall on and on forever. And if it doesn't does that mean the singularity is like a hollow shell with everything ever sucked into the the black hole (crossing the event horizon) doing an eternal dive into it? And doing a "cross section" cut of the black hole would be like opening up a time capsule so to speak?

01101001
2010-Mar-03, 04:54 AM
I find this really interesting.

See topic Backward light (http://www.bautforum.com/space-astronomy-questions-answers/51151-backward-light.html)


You probably found Light travels backward and faster than light (http://www.bautforum.com/showthread.php?t=20338) or maybe Light So Fast It Actually Goes Backwards (http://www.bautforum.com/showthread.php?t=41470).

Or was it: Backwards Traveling Light (http://www.bautforum.com/showthread.php?t=49239)?

You sure we need another thread on this?

See also:

Another speed of light (c) post (http://www.bautforum.com/showthread.php?t=20338)
Scientists get light to travel faster than...light? (http://www.bautforum.com/showthread.php?t=20337)

Keywords: NEC Scientists Doctor Dr. Lijun Wang Dr. Alexander Kuzmich and Dr. Arthur Dogariu FTL faster than light 62 nanoseconds Professor Prof. Robert Boyd University of Rochester backward light negative speed of light group velocity pulse wave photon

Wikipedia: Group Velocity (http://en.wikipedia.org/wiki/Group_velocity)


However, superluminal communication is not possible in this case, since the signal velocity remains less than the speed of light. It is also possible to reduce the group velocity to zero, stopping the pulse, or have negative group velocity, making the pulse appear to propagate backwards. However, in all these cases, photons continue to propagate at the expected speed of light in the medium.

PS: And some others about group velocity (not photon velocity):
What the heck is this? [FTL claim] (http://www.bautforum.com/science-technology/95302-what-heck-ftl-claim.html)
Light Barrier Has been Broken! (http://www.bautforum.com/science-technology/55072-light-barrier-has-been-broken.html)
Yet another FTL claim. (http://www.bautforum.com/science-technology/55022-yet-another-ftl-claim.html)

DrRocket
2010-Mar-03, 05:12 AM
GR can handle the math all the way down to the singularity. Inside the event horizon, timelike paths become spacelike and spacelike paths become timelike. Photons would travel as they would outside the event horizon, along a null geodesic. It's just that all paths are warped to the point where they fall into the singularity. Hawking and Penrose's singularity theorems showed that black hole singularities are points where world lines end.

Then you have a different set of references for the singularity theorems than I do. In The large-scale structure of spacetime Hawking is very careful in his handling of singularities, and adopts critieria that are rather subtle. The singularity is not a point and is not even actually in the spacetime manifold. Rather he adopts the definition that spacetime is singular if it fails to be time-like geodesically complete. This is a very technical condition and difficult to describe in any other than precise and somewhat archane mathematical terminology.

publius
2010-Mar-03, 05:42 AM
The way I think of geodesic completeness, which may only grasp part and not be precise enough is this.

To be geodesically complete, any two points on the manifold must joined by a geodesic (that exists on the manifold), and any geodesic must be infinitely extendable. I think the former implies the latter, but I'm not sure.

At any rate in Schwarzschild, time-like paths that cross the horizon just terminate at the singularity. They cannot be extended. In space-times, we have the time-like vs space-like distinction about geodesics, and I'm sure that complicates things. But a space-time is incomplete if some of the time-like geodesics can't be infinitely extended.

-Richard

Tensor
2010-Mar-03, 01:57 PM
Then you have a different set of references for the singularity theorems than I do. In The large-scale structure of spacetime Hawking is very careful in his handling of singularities, and adopts critieria that are rather subtle. The singularity is not a point and is not even actually in the spacetime manifold. Rather he adopts the definition that spacetime is singular if it fails to be time-like geodesically complete. This is a very technical condition and difficult to describe in any other than precise and somewhat archane mathematical terminology.


The references are probably the same, how we might explain things is what is different. First off, I freely admit it was a great struggle in following the theorems. About half of it was just a bit over my head. But here and there I would grasp what they were doing. Since that is the case, I wouldn't disagree with the definition of a singular spacetime you showed above. But, as publius points out, all world lines that cross the horizon end at the singularity. I would say I greatly simplified it. It's my mental picture and analogy, it may not be someone elses.

Tensor
2010-Mar-03, 02:00 PM
Time stops?

No, time doesn't stop. We just don't know what happens at the singularity. Our equations simply don't work.


BTW, does the photon actually reach the singularity or does it seem to fall on and on forever.

It reaches the singularity. Everything that crosses the event horizon reaches the singularity.

Strange
2010-Mar-03, 03:02 PM
BTW, does the photon actually reach the singularity or does it seem to fall on and on forever. And if it doesn't does that mean the singularity is like a hollow shell with everything ever sucked into the the black hole (crossing the event horizon) doing an eternal dive into it? And doing a "cross section" cut of the black hole would be like opening up a time capsule so to speak?

This, or something very like it, was discussed (at length) in the how can a black hole form (http://www.bautforum.com/space-astronomy-questions-answers/100620-how-can-black-hole-form.html) thread.

rodin
2010-Mar-06, 06:06 PM
Since lightspeed is constant would not the light lower in frequency to some limit in extreme gravity rather than velocity?