# The Naked Scientists Forum

### Author Topic: Can the gravitational field be considered like a medium?  (Read 2579 times)

#### jeffreyH

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##### Can the gravitational field be considered like a medium?
« on: 08/03/2015 03:01:46 »
Sounds like a very strange question. Since we can calculate how light behaves in different mediums and determine its velocity through those mediums then why not with the gravitational field? Near the event horizon of a black hole things slow down. In a super cooled medium light can be slowed down radically. Why are these two situations different? If not the field itself and its energy then what about the density of force carriers? At the event horizon the velocities should be relativistic so a remote observer viewing the scene is identical to the observer viewing the light in a super cooled medium. Am I wrong?

#### Ethos_

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##### Re: Can the gravitational field be considered like a medium?
« Reply #1 on: 08/03/2015 03:39:43 »
At the event horizon the velocities should be relativistic so a remote observer viewing the scene is identical to the observer viewing the light in a super cooled medium. Am I wrong?
I'm not sure we could categorize them as the same Jeff. From the reference frame of a distant observer, the sequence of events as seem from that position would appear to slow down at an infinitely declining rate but never reach a point of absolute motionlessness. And of course, the light we see coming from the event horizon would also be red shifted to an ever increasing extent.

Light coming to us from this event would differ from what I have come to understand about experiments involving the retardation of light through various low temperature mediums

In experiments where we have slowed the speed of light, it appears that, what is really happening is; The actual velocity hasn't really changed but has just taken a more tortured route through the medium. At least, this is my understanding from what I've read about the phenomenon. And I'm also not sure about any red shifting occurring during these experiments either.

Naturally, the information I've read about these experiments might be in error. Nevertheless, I can see a distinct difference between the two scenarios you've suggested. One involving gravitational influences and the other; involves quantum interactions and electromagnetic forces.

« Last Edit: 08/03/2015 03:56:45 by Ethos_ »

#### PmbPhy

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##### Re: Can the gravitational field be considered like a medium?
« Reply #2 on: 08/03/2015 05:53:54 »
Quote from: jeffreyH
Sounds like a very strange question. Since we can calculate how light behaves in different mediums and determine its velocity through those mediums then why not with the gravitational field? Near the event horizon of a black hole things slow down. In a super cooled medium light can be slowed down radically. Why are these two situations different? If not the field itself and its energy then what about the density of force carriers? At the event horizon the velocities should be relativistic so a remote observer viewing the scene is identical to the observer viewing the light in a super cooled medium. Am I wrong?
All you have to do is to determine precisely what is meant by medium and then determine what it means to be considered like in this context and you'll have your answer. In my opinion, no. You can't think of it like that.

#### jeffreyH

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##### Re: Can the gravitational field be considered like a medium?
« Reply #3 on: 08/03/2015 18:18:01 »
Quote from: jeffreyH
Sounds like a very strange question. Since we can calculate how light behaves in different mediums and determine its velocity through those mediums then why not with the gravitational field? Near the event horizon of a black hole things slow down. In a super cooled medium light can be slowed down radically. Why are these two situations different? If not the field itself and its energy then what about the density of force carriers? At the event horizon the velocities should be relativistic so a remote observer viewing the scene is identical to the observer viewing the light in a super cooled medium. Am I wrong?
All you have to do is to determine precisely what is meant by medium and then determine what it means to be considered like in this context and you'll have your answer. In my opinion, no. You can't think of it like that.

I didn't think you could but had to ask the question. I needed to sort that one out in my head.

#### jeffreyH

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##### Re: Can the gravitational field be considered like a medium?
« Reply #4 on: 08/03/2015 18:18:59 »
At the event horizon the velocities should be relativistic so a remote observer viewing the scene is identical to the observer viewing the light in a super cooled medium. Am I wrong?
I'm not sure we could categorize them as the same Jeff. From the reference frame of a distant observer, the sequence of events as seem from that position would appear to slow down at an infinitely declining rate but never reach a point of absolute motionlessness. And of course, the light we see coming from the event horizon would also be red shifted to an ever increasing extent.

Light coming to us from this event would differ from what I have come to understand about experiments involving the retardation of light through various low temperature mediums

In experiments where we have slowed the speed of light, it appears that, what is really happening is; The actual velocity hasn't really changed but has just taken a more tortured route through the medium. At least, this is my understanding from what I've read about the phenomenon. And I'm also not sure about any red shifting occurring during these experiments either.

Naturally, the information I've read about these experiments might be in error. Nevertheless, I can see a distinct difference between the two scenarios you've suggested. One involving gravitational influences and the other; involves quantum interactions and electromagnetic forces.

Thanks for the answer. It helped a lot.

#### evan_au

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##### Re: Can the gravitational field be considered like a medium?
« Reply #5 on: 08/03/2015 20:11:54 »
Space is filled with an electromagnetic field. This field supports the propagation of quantised waves on this medium, which we call "photons".

Space is filled with a gravitational field. In general relativity, this field should supports the propagation of gravitational waves. So far, there has been no direct detection of gravitational waves, but there has been some indirect evidence from orbiting pulsars.

In quantum theory, we expect that most energy-carrying waves will be quantized, if you look closely enough. We call these hypothetical quantized particles on the gravitational field "gravitons". However, we expect that detection of individual gravitons will be even harder than detecting gravitational waves.

#### JohnDuffield

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##### Re: Can the gravitational field be considered like a medium?
« Reply #6 on: 09/03/2015 10:52:48 »
Can the gravitational field be considered like a medium?
No. Space can be considered to be a medium. See this paper:

"The strong similarities between the light propagation in a curved spacetime and that in a medium with graded refractive index are found. It is pointed out that a curved spacetime is equivalent to an inhomogeneous vacuum for light propagation. The corresponding graded refractive index of the vacuum in a static spherically symmetrical gravitational field is derived. This result provides a simple and convenient way to analyse the gravitational lensing in astrophysics."

Also see Einstein's 1920  Leyden Address:

"...According to this theory the metrical qualities of the continuum of space-time differ in the environment of different points of space-time, and are partly conditioned by the matter existing outside of the territory under consideration. This space-time variability of the reciprocal relations of the standards of space and time, or, perhaps, the recognition of the fact that "empty space" in its physical relation is neither homogeneous nor isotropic, compelling us to describe its state by ten functions (the gravitation potentials gmn), has, I think, finally disposed of the view that space is physically empty..."

Then see Einstein talking about field theory in 1929:

"This theory having brought together the metric and gravitation would have been completely satisfactory of the world had only gravitational fields and no electro-magnetic fields. Not it is true that the latter can be included within the general theory of relativity by taking over and appropriately modifying Maxwell's equations of the electro-magnetic field, but they do not then appear like the gravitational fields as structural properties of the space - time continuum, but as logically independent constructions. The two types of field are causally linked in this theory, but still not fused to an identity. It can, however, scarcely be imagined that empty space has conditions or states of two essentially different kinds, and it is natural to suspect that this only appears to be so because the structure of the physical continuum is not completely described by the Riemannian metric."

A field is "a state of space". We tend to say that an electron has an electromagnetic field and a gravitational field, but note that where an electron is, space does not have two different states.

Sounds like a very strange question. Since we can calculate how light behaves in different mediums and determine its velocity through those mediums then why not with the gravitational field? Near the event horizon of a black hole things slow down. In a super cooled medium light can be slowed down radically. Why are these two situations different?
Because in the latter situation light takes a tortured path, like Ethos said. As for the former situation, see the second paragraph of this

If not the field itself and its energy then what about the density of force carriers?
Do not think of virtual particles as real particles flitting back and forth. They're "field quanta", like you divide the electromagnetic field up into little chunks and say each is a virtual photon. The electron and the proton exchange field such that the hydrogen atom doesn't have much field remaining. But hydrogen atoms don't twinkle, and magnets don't shine.  Also see Matt Strassler's article where he says this: "The best way to approach this concept, I believe, is to forget you ever saw the word 'particle' in the term. A virtual particle is not a particle at all." The notion that virtual particles pop into existence is a popscience myth.

Quote from: [quote author=jeffreyH link=topic=54518.msg451997#msg451997 date=1425783706
At the event horizon the velocities should be relativistic so a remote observer viewing the scene is identical to the observer viewing the light in a super cooled medium. Am I wrong?
Yes. You can't view the light at a place where the coordinate speed of light is zero.
« Last Edit: 09/03/2015 10:54:24 by JohnDuffield »

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##### Re: Can the gravitational field be considered like a medium?
« Reply #6 on: 09/03/2015 10:52:48 »