Thread: Does a light shone from a hovering object outside of a black hole redshift?

1. Does a light shone from a hovering object outside of a black hole redshift?

Does a light shone from a hovering object outside of a black hole redshift?

I take a spaceship and hover it ( relative to us ) just outside of a black hole.

Shine a light to an external observer ( us ) is it redshifted, blueshifted, or not shifted at all?

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Redshifted.
Standard gravitational redshift, since the source isn't in motion relative to the observer.

(BTW: I thought standard American English was "shined", not "shone".)

Grant Hutchison

3. You like your Black Holes.

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Originally Posted by captain swoop
Me? Tommac? Both?

Grant Hutchison

5. Originally Posted by grant hutchison
Redshifted.
Standard gravitational redshift, since the source isn't in motion relative to the observer.

(BTW: I thought standard American English was "shined", not "shone".)

Grant Hutchison
However the source is in motion relative to the gravitational source. Wouldnt we need to really be travelling at close to the speed of light relative to the singulary?
If we were falling into the black hole while shining the light would there be more redshift?

BTW I think I agree with you but had a few doubts about it, just wanted to clear my thought patterns. I would think there would be a difference in redshift by the object that was falling into the BH at near the speed of light would that effect be compounded on top of the gravitation ? That is the part that I got confused on. Is the red shift from the relative movement of the receeding space-time in the area ... or is it from the extra energy needed to fight through space time?

6. OK ... here is a whacky thought experiment.

Lets say we had a spaceship that could travel at just under the speed of light we took it by a black hole ( EH ) and orbited it at as fast as we could go. In fact the tangential speed reached is near the speed of light.

Now we turned on our headlights. If this was in empty space and we were travelling at the speed of light directly towards an observer we would see a massive blue shift, however since we are at the EH the natural tendency would be a redshift. Wouldnt these two effects balance out when the spaceship velocity was directly in our direction at the speed of light?

7. Originally Posted by tommac
However the source is in motion relative to the gravitational source. Wouldnt we need to really be travelling at close to the speed of light relative to the singulary?
If we were falling into the black hole while shining the light would there be more redshift?
Gravitational red shift is just that, gravitational redshift caused by the source being in a gravitational well. In case the source moves, then yes, there will be extra Doppler effects in the direction of motion.

We would NOT have to travel near the speed of light, just at the regular Keplerian velocity around the hole (maybe slightly modified because of the relativistic effects) and what singularity do you mean? I bet NOT the (non-exisiting) "singularity" at the centre of the black hole.

If you were falling, you would be accelerating and then naturally the red shift would be altered. Haven't these questions been answered before?

Originally Posted by tommac
BTW I think I agree with you but had a few doubts about it, just wanted to clear my thought patterns. I would think there would be a difference in redshift by the object that was falling into the BH at near the speed of light would that effect be compounded on top of the gravitation ? That is the part that I got confused on. Is the red shift from the relative movement of the receeding space-time in the area ... or is it from the extra energy needed to fight through space time?
Lots of stuff is combinational, you just have to add it all in the correct way, but for that you would have to read some physics books.

8. Originally Posted by grant hutchison
Me? Tommac? Both?

Grant Hutchison
Tommac of course, we get several new ones a week.

Tommac, I am sure that if you asked then there are posters here that could give you advice on a good 'Black Hole' book that would answer a lot of these questions.

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Originally Posted by tommac
...however since we are at the EH the natural tendency would be a redshift...
tommac, it's only redshifted to an observer further away from the gravity source. It's the difference in the gravimetric potential between the sender and the receiver that determines which way it's shifted, and by how much.

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Originally Posted by captain swoop
Tommac, I am sure that if you asked then there are posters here that could give you advice on a good 'Black Hole' book that would answer a lot of these questions.
Or, we could all write a book using the answers to his questions... Might even pay for the cost the board.

11. Originally Posted by mugaliens
tommac, it's only redshifted to an observer further away from the gravity source. It's the difference in the gravimetric potential between the sender and the receiver that determines which way it's shifted, and by how much.
Can you please post a source for this? How would this be able to be proven?

What I am trying to understand is why there can be redshift from a spaceship that is travelling at the speed of light in our direction.

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Originally Posted by tommac
Now we turned on our headlights. If this was in empty space and we were travelling at the speed of light directly towards an observer we would see a massive blue shift, however since we are at the EH the natural tendency would be a redshift. Wouldnt these two effects balance out when the spaceship velocity was directly in our direction at the speed of light?
This stuff has been worked out and simulated with reference to stars falling into galactic black holes. I'll see if I can find the reference later.
For most of the star's descent into the black hole, there is always a place a distant observer can stand and see blueshifted light: there's a relativistic beaming effect which also strongly brightens the blueshifted light, but concentrates it into a fairly narrow solid angle. When the star gets very close to the black hole (IIRC, as it passes through the photon sphere), it then redshifts very rapidly for all distant observers: the blueshifted "beamed" light is no longer aimed in an outward direction, so distant observers can only see redshifted light.

Grant Hutchison

13. but this star is falling not hovering right? So I would assume that it gets very redshifted.

Originally Posted by grant hutchison
This stuff has been worked out and simulated with reference to stars falling into galactic black holes. I'll see if I can find the reference later.
For most of the star's descent into the black hole, there is always a place a distant observer can stand and see blueshifted light: there's a relativistic beaming effect which also strongly brightens the blueshifted light, but concentrates it into a fairly narrow solid angle. When the star gets very close to the black hole (IIRC, as it passes through the photon sphere), it then redshifts very rapidly for all distant observers: the blueshifted "beamed" light is no longer aimed in an outward direction, so distant observers can only see redshifted light.

Grant Hutchison

14. Emission lines from gas in accretion disk

Some astronomers have tried to calculate the
shape of emission lines due to hot gas which
orbits a black hole. Light emitted by the gas
exhibits features due to Doppler shift -- gas
on one side of the disk comes towards us, gas
on the other side moves away from us --
and gravitational redshift. There are several
parameters involved, such as the mass of the
black hole, the distance of the emitting material
from the event horizon, the orientation of the
disk, and so forth.

You can see a long, technical paper which
discusses these effects by going to astro-ph
and looking at

"Relativistic emission lines from accreting black holes -
The effect of disk truncation on line profiles"

by Andreas Mueller and Max Camenzind. See

http://arxiv.org/abs/astro-ph/0309832

The paper was published in Astron.Astrophys. 413 (2004),
861-878.

If you just look at the illustrations and read the
captions, you'll see quite a range of shapes for
the emission lines. Perhaps this paper may answer

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Originally Posted by tommac
but this star is falling not hovering right? So I would assume that it gets very redshifted.
You were asking about objects moving at relativistic velocity in the near vicinity of a black hole. I was responding to that question.

Grant Hutchison

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Originally Posted by tommac
Lets say we had a spaceship that could travel at just under the speed of light we took it by a black hole ( EH ) and orbited it at as fast as we could go. In fact the tangential speed reached is near the speed of light.

Now we turned on our headlights. If this was in empty space and we were travelling at the speed of light directly towards an observer we would see a massive blue shift, however since we are at the EH the natural tendency would be a redshift. Wouldnt these two effects balance out when the spaceship velocity was directly in our direction at the speed of light?
Further to the above, here are the two papers I was recalling:

Cohen et al.'s Observable blueshifts near compact objects and Čadež & Gomboc's The flashy disappearance of a star falling behind the horizon of a black hole.
Both describe how blueshift is observable in bodies falling towards a black hole (and outside the photon sphere) if the distant observer is positioned in the approximate line of the velocity vector.

Grant Hutchison

17. thanks ... I will take a look.
Originally Posted by StupendousMan
Some astronomers have tried to calculate the
shape of emission lines due to hot gas which
orbits a black hole. Light emitted by the gas
exhibits features due to Doppler shift -- gas
on one side of the disk comes towards us, gas
on the other side moves away from us --
and gravitational redshift. There are several
parameters involved, such as the mass of the
black hole, the distance of the emitting material
from the event horizon, the orientation of the
disk, and so forth.

You can see a long, technical paper which
discusses these effects by going to astro-ph
and looking at

"Relativistic emission lines from accreting black holes -
The effect of disk truncation on line profiles"

by Andreas Mueller and Max Camenzind. See

http://arxiv.org/abs/astro-ph/0309832

The paper was published in Astron.Astrophys. 413 (2004),
861-878.

If you just look at the illustrations and read the
captions, you'll see quite a range of shapes for
the emission lines. Perhaps this paper may answer

18. OK then so wouldnt this extrapolate to a hovering spacecraft? A hovering spacecraft is similar to a spacecraft that is orbiting EXCEPT that it is moving exactly parallel and congruent with gravity.

Originally Posted by grant hutchison
Further to the above, here are the two papers I was recalling:

Cohen et al.'s Observable blueshifts near compact objects and Čadež & Gomboc's The flashy disappearance of a star falling behind the horizon of a black hole.
Both describe how blueshift is observable in bodies falling towards a black hole (and outside the photon sphere) if the distant observer is positioned in the approximate line of the velocity vector.

Grant Hutchison

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Originally Posted by tommac
OK then so wouldnt this extrapolate to a hovering spacecraft? A hovering spacecraft is similar to a spacecraft that is orbiting EXCEPT that it is moving exactly parallel and congruent with gravity.
No, a hovering object is accelerating in order to remain where it is. An object on a ballistic trajectory is in freefall. Two different sets of coordinates, two different results.

Grant Hutchison

20. Originally Posted by tommac
OK then so wouldnt this extrapolate to a hovering spacecraft? A hovering spacecraft is similar to a spacecraft that is orbiting EXCEPT that it is moving exactly parallel and congruent with gravity.
Let me add that for the purposes of the crew, a spacecraft which is hovering near a black hole would be the functional equivalent of the surface of a planet with the same mass, and with a radius equal to the spacecraft's distance from the center. The state of being at rest on a planet is the general relativity equivalent of undergoing a forced acceleration, such as aboard a rocket, in the absence of a nearby massive object. Whether or not the object is a black hole is immaterial as long as we are not too close to the event horizon.

As alway, Grant's words of wisdom are most welcome.

21. Originally Posted by grant hutchison
No, a hovering object is accelerating in order to remain where it is. An object on a ballistic trajectory is in freefall. Two different sets of coordinates, two different results.

Grant Hutchison
But it is really moving relative to the gravitational field. light moves at the speed of light right? So at the EH light is travelling at exactly the same speed as space-time is receeding, right? Although it is hovering relative to the external observer it is travelling really fast relative to everything local.

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I've found some relatively simple info on Black Holes;

First page by Dr. Glyn George,

Leads to another page by Dr. Robert Nemiroff,

http://antwrp.gsfc.nasa.gov/htmltest/rjn_bht.html

It's a must visit. "A stimulating, relativistically accurate trip!" according to Kip Thorne
The Feynman Professor of Theoretical Physics, California Institute of Technology, Author of "Black Holes and Time Warps - Einstein's Outrageous Legacy"

And finally, the last page by Dr. Andrew Hamilton.

I think you'll find the answers to all your blackhole questions within those pages. :-)

23. is redshift and blue shift the direction the light is traveling?

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Originally Posted by tommac
But it is really moving relative to the gravitational field. light moves at the speed of light right? So at the EH light is travelling at exactly the same speed as space-time is receeding, right? Although it is hovering relative to the external observer it is travelling really fast relative to everything local.
Yes, light moves at light speed relative to local observers. I doubt if it's reasonable to consider the gravitational field a "local observer".

Grant Hutchison

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Originally Posted by johnathan
is redshift and blue shift the direction the light is traveling?
No, it's the measured change in energy (and therefore frequency and wavelength) when light is received in a different reference frame from the one it was emitted in. It can arise because of differences in velocity, gravitational potential, or cosmic expansion.

Grant Hutchison

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Originally Posted by johnathan
is redshift and blue shift the direction the light is traveling?
All light sources have some features that vary with wavelength.
For example, some have bright spectral lines. A spectral line is a
particular wavelength (or frequency) of light emitted or absorbed
by a particular atom. For example, oxygen in a certain temperature
range emits light with a wavelength of 500.7 nanometres, which
is green. If you are moving toward some of this glowing oxygen
gas, the wavelength that you measure will be shorter, so the color
will shift closer to the blue end of the spectrum. If you are moving
away from the glowing oxygen gas, the measured wavelength is
longer, so the color shifts closer to the red end of the spectrum.

By measuring the amount of shift of a spectral line, you can
determine your speed toward or away from the light source.

-- Jeff, in Minneapolis

27. Does it always? I thought that if two photons of light are travelling next to each other than the photons dont travel at the speed of light away from each other.

Originally Posted by grant hutchison
Yes, light moves at light speed relative to local observers. I doubt if it's reasonable to consider the gravitational field a "local observer".

Grant Hutchison

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Originally Posted by tommac
Does it always? I thought that if two photons of light are travelling next to each other than the photons dont travel at the speed of light away from each other.
Originally Posted by grant hutchison
Yes, light moves at light speed relative to local observers. I doubt if it's reasonable to consider the gravitational field a "local observer".

Grant Hutchison
There are no "local observers" in the rest frame of a photon.

Grant Hutchison