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Another thought confuses the issue, but I think it is relevant to the discussion.

Some time ago, if I remember correctly, when we were discussing the speed of light through water, and other media, we established that light travels at c through any medium, and that the apparent slowing results from the absorption and re-emission of the photons by atoms in the medium.

If this is the case, why are astronomers able to see the emission/absorption spectra of distant stars? Why are these spectra not converted into the emission spectra of the atoms in the lenses of their telescopes?


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Originally Posted By: Bill S.
Correct me if I am wrongly interpreting you, but you seem to be saying that an emitting source that is accelerating relative to a beam of light will observe that light as travelling at less than "c".
If that is the case, and the source were able to accelerate to the speed of light, would it not observe the light as stationary?

I had intended to refer to the following as a response to your OP but carelessly let the chance go by.

**************************

In their book 'The Matter Myth' Paul Davies and John Gribbin wrote (110, Viking, 1991):-

"As the recession speed of galaxies grows with distance, there comes a point at which this speed is so great that it exceeds the speed of light."

(and, page 111)

"This elasticity of space, a feature of general relativity, allows galaxies to effectively separate from one another faster than the speed of light."

and in his book 'Einstein's Universe' Nigel Calder writes (92, BBC, 1979) "If you are accelerating towards a source of light, its speed [i.e. the speed of the light it emits] seems greater. If you are accelerating away from it, its speed seems diminished."

And, whilst wearing his science hat, Isaac Asimov wrote (104, 'Understanding Physics, Light, Magnetism and Electricity', Mentor, 1966)

To put it briefly, it is possible to deduce from Einsteins assumption of the constant measured velocity of light that the velocity of any moving body will always be be measured as less than the velocity of light.

and, in a footnote -

This is often expressed as a body cannot move faster than light but that is not quite right. It is only the measured velocity that is less than the measured velocity of light. It is quite conceivable that there are objects in the universe that are travelling at velocities (relative to ourselves) that are greater than the velocity of light, but we could not see such bodies or sense them in any way and therefore could not measure their velocities.

**************************

So, in relation to your question - yes - if a source were able to accelerate to the speed of light it would observe previously emitted light as being stationary then, as the above references suggest, as a result of further acceleration that light would be moving away from it thus would not be observable or detectable.

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Ok, I'm not absolutely sure about this. I don't have enough in depth knowledge of QM (quantum mechanics) to be sure, but here are my thoughts on the matter.
Whoops! There, I got myself so tangled up trying to think this through that I had to back up and start over.

I guess I could always say, because, but that isn't a very satisfying answer.

Originally Posted By: Wikipedia
At the microscale, an electromagnetic wave's phase speed is slowed in a material because the electric field creates a disturbance in the charges of each atom (primarily the electrons) proportional to the permittivity of the medium. The charges will, in general, oscillate slightly out of phase with respect to the driving electric field. The charges thus radiate their own electromagnetic wave that is at the same frequency but with a phase delay. The macroscopic sum of all such contributions in the material is a wave with the same frequency but shorter wavelength than the original, leading to a slowing of the wave's phase speed. Most of the radiation from oscillating material charges will modify the incoming wave, changing its velocity. However, some net energy will be radiated in other directions (see scattering).


So I copied this quote out of the Wikipedia article on Refractive index. What this says is a bit different from the idea that the atoms actually absorb and re-emit the photon. This seems to be saying that it absorbs some of the energy from the photon and re-emits it with a slight delay, so that the probability wave of the photon is pulled back slightly. That is a bit different from our usual idea of what happens when an atom absorbs a photon and the outer electron jumps up to a higher energy level. For a better understanding you might want to read the whole article.

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Originally Posted By: Bill S.
Another thought confuses the issue, but I think it is relevant to the discussion.

I don't believe that it is relevant but it's a subject that intrigues me.

Originally Posted By: Bill S.
Some time ago, if I remember correctly, when we were discussing the speed of light through water, and other media, we established that light travels at c through any medium, and that the apparent slowing results from the absorption and re-emission of the photons by atoms in the medium.

I question the decision that light travels through any medium at c. It is my understanding that it does not.

Originally Posted By: Bill S.
If this is the case, why are astronomers able to see the emission/absorption spectra of distant stars? Why are these spectra not converted into the emission spectra of the atoms in the lenses of their telescopes?

My argument is that the light from distant stars passes through the medium that is our atmosphere before it even reaches those lenses.

My reason for rejecting that subject's relevance is that the light emitted by OP objects that are moving away from us at superluminal velocities does not arrive here.

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Originally Posted By: Bill 6
In their book 'The Matter Myth' Paul Davies and John Gribbin wrote (110, Viking, 1991):-

"As the recession speed of galaxies grows with distance, there comes a point at which this speed is so great that it exceeds the speed of light."

Well, that isn't quite the way it is. In fact as the recession speed grows it approaches the speed of light (C). Of course as the galaxies approach C their clocks slow down in accordance with relativity, and the frequency of the light they emit is red shifted so much at its frequency approaches 0. Thus, from our point of view, they disappear. In actuality, again from our point of view, they become so short that they appear as 2 dimensional objects. So anything at the edge of the observable universe appears to be pasted to the sky at that distance. They effectively disappear to us, but they aren't moving faster than C. In fact if you could be magically transported to the edge of the observable universe you would look around and everything would look just the same as it does here. And if you could find our galaxy, it would seem to be disappearing from sight into the edge of the universe.

And as far as your quote from Isaac Asimov is concerned. I greatly admire Dr. Asimov's writing, but keep in mind that his PhD was in biochemistry, not physics. So he didn't always get it just exactly right.

Bill Gill


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Originally Posted By: Bill6
My reason for rejecting that subject's relevance is that the light emitted by OP objects that are moving away from us at superluminal velocities does not arrive here.


You are absolutely right, of course, but I defend it on the grounds of "thread drift". smile

I have found an explanation at http://www.physicsforums.com which even I can understand. This is a small quote from it:

"A solid has a network of ions and electrons fixed in a "lattice". Think of this as a network of balls connected to each other by springs. Because of this, they have what is known as "collective vibrational modes", often called phonons. These are quanta of lattice vibrations, similar to photons being the quanta of EM radiation. It is these vibrational modes that can absorb a photon. So when a photon encounters a solid, and it can interact with an available phonon mode (i.e. something similar to a resonance condition), this photon can be absorbed by the solid and then converted to heat (it is the energy of these vibrations or phonons that we commonly refer to as heat). The solid is then opaque to this particular photon (i.e. at that frequency). Now, unlike the atomic orbitals, the phonon spectrum can be broad and continuous over a large frequency range. That is why all materials have a "bandwidth" of transmission or absorption. The width here depends on how wide the phonon spectrum is."


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Getting back to the thread - in your OP you referred to the idea that ...light from an object that was moving away from us at superluminal speed would never reach us. and A distant object is moving away, relative to Earth, at superluminal speed...

You then write At a given point (say 15 billion l y away....) it emits a photon. This photon should reach Earth 15 billion years later.

As pointed out by Davies and Gribbin, light emitted by an object that is moving away from us faster than the speed of light never reaches us.

Clearly we could not observe galaxies that recede faster than light, for their radiation would never reach us. (111, 'The Matter Myth', Viking, 1991)

It is my understanding that Professors Davies and Gribbin are recognised authors of physics books and articles who would not be prepared to publish something that is not accepted by the scientific community in general ergo I prefer to accept their word as to what takes place.

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Quote:
It is my understanding that Professors Davies and Gribbin are recognised authors of physics books and articles who would not be prepared to publish something that is not accepted by the scientific community in general ergo I prefer to accept their word as to what takes place.


Absolutely! I have enjoyed, and learned from books by both. In fact it was one of Gribbin's books that sparked my interest 20 years ago.

In asking questions like this I am not so much arguing with the experts, as admitting that I don't understand what they are claiming. I cannot pretend to understand something I don't really understand, so I tend to keep at it until others become too exasperated to respond. Continuing to do that is one of the privileges of growing old. smile


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Bill 6

Ok, I have done some research on the web and find that there are some problems with what I have been saying. For one thing Special Relativity (SR) does not apply to objects at the edge of the observable universe. You have to handle them with General Relativity (GR).

I have spent some time trying to digest the information I got from a paper I found on ARVIX Expanding Confusion: common misconceptio...ineweaver, 2003 .
I'm still not sure I understand it all, but here I go trying to write out what I think it said.

The major point in the paper is that there is no sharp cutoff of what we can see at the Hubble Radius. The Hubble Radius is the point at which galaxies are receding from us at C (light speed). The light emitted by an object that is beyond the Hubble Radius moves away from the source at C. Some of it moves away from us, some of it moves toward us. At the time of emission the object is outside the Hubble Radius, so we cannot receive that light. But the Hubble Radius is expanding with time, so that light that was emitted outside of the Hubble Radius when is was emitted may well come within the Hubble Radius. In fact there seem to be many visible galaxies that have recession velocities greater than C.

The red-shift of light due to the expansion of the universe is not due to the doppler effect caused by the speed of the source. The shift is due to the expansion of the universe. Therefore the light is not red-shifted to a 0 frequency, or at least not at the Hubble Radius.

I am still trying to get my mind wrapped around what the authors had to say in the paper. In part that is because it is a technical paper, with a bunch of differential equations in it. I had 1 semester of differential equations over 30 years ago, and promptly forgot all about it when I got out of college. And I suspect that the math in their world is a bit above what I would have learned in a first course.

Anyway, that is what I have come up with so far. If anybody has something that will supplement what I have there, please feel free to reply.

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Originally Posted By: Bill
The red-shift of light due to the expansion of the universe is not due to the doppler effect caused by the speed of the source. The shift is due to the expansion of the universe. Therefore the light is not red-shifted to a 0 frequency, or at least not at the Hubble Radius.


This gives me the opportunity to the accuracy of a small extract from my past notes.

"Having used the Doppler Effect, with its resultant red shift, to establish that the Universe is expanding, it has to be said that this cosmological redshift which I used as proof is not, strictly, an example of the Doppler effect at all. Although the galaxy groups are separating, they are not moving through space; space itself is expanding and carrying the material of the Universe with it. Therefore the stretching of the waves does not occur only at the source, as is the case with the sound waves from a receding car; it happens over the whole distance because it is spacetime that is expanding while the light waves are passing through it. Waves are stretched throughout their journey the further they travel the more they are red shifted."

Does that sound right?


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That sounds just about right. Having read that paper about fallacies in the expansion of the universe I have realized that the far away galaxies aren't really moving any faster than nearer galaxies. It is just that there is more space between them and us and the amount of space is growing faster.

Bill Gill


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Originally Posted By: Bill
It is just that there is more space between them and us and the amount of space is growing faster


Presumably each "unit" of space grows at the same rate, but the overall growth rate is proportional to the number of "units".


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Originally Posted By: Bill S.
Although the galaxy groups are separating, they are not moving through space; space itself is expanding and carrying the material of the Universe with it.

This is similar to the comment made by Davies and Gribbin in 'The Matter Myth' however whilst I referred to their agreement with the OP this does not mean that I accept that idea.

For 'space' to be able to transport the material of the universe (be able to push the galaxies apart) it would need to be more than simply a void but would have to possess some form of a physical characteristic.

This not only involves the ex nihilo creation of matter but also the continual creation of an amount of energy that is increasingly greater than the infinity of the universe.

I believe in the tired light explanation for the greater redshift of the more distant galaxies and also agree with Asimov that superluminal (undetectable in some RF's) motion is attainable.

Responses pointing out that the big bang expanding universe as well as SR's light barrier have been ratified will be reminded of the principle of physics that whilst a theory may appear to be supported by numerous experiments it only takes one (repeatable) experiment to invalidate any theory.

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Originally Posted By: Bill6
For 'space' to be able to transport the material of the universe (be able to push the galaxies apart) it would need to be more than simply a void but would have to possess some form of a physical characteristic.


This bothered me for some time, but then I thought that if you imagine two objects in space; if space is static, these objects will remain at the same distance apart unless some force acts on them. A force is needed to move them through space, so if space is expanding, the objects must move with it, otherwise they are effectively moving through space; movement being relative, and all that.


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Originally Posted By: Bill 6
Responses pointing out that the big bang expanding universe as well as SR's light barrier have been ratified will be reminded of the principle of physics that whilst a theory may appear to be supported by numerous experiments it only takes one (repeatable) experiment to invalidate any theory.

But so far there have been a great many experiments that ratify SR, and none that don't. Likewise the expanding universe seems to be readily handled by GR. And of course once again GR has been ratified by a great many experiments, and there have been no experiments that don't ratify it. After a while you have to give up and accept that the probability that they are wrong is extremely low. When that happens it is generally accepted that they meet the requirements to be called natural law.

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Originally Posted By: Bill6
This not only involves the ex nihilo creation of matter but also the continual creation of an amount of energy that is increasingly greater than the infinity of the universe.


Once again we run into the equivocal nature of infinity. Presumably we are dealing with a mathematical infinity, or that kind of pseudo infinity for which the surface of a sphere is often used as an illustration. This 2D surface grows as the radius of the higher dimensional sphere increases, so there is only the first part of the problem to deal with: the ex nihilo creation of matter. Solve that, and increasing energy will follow E=mc^2.

If the infinity is mathematical, there is no problem with the increasing energy being greater than it, because, as Cantor showed, there is an infinite range of sizes of infinity.


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Originally Posted By: Bill S.
Once again we run into the equivocal nature of infinity. Presumably we are dealing with a mathematical infinity, or that kind of pseudo infinity for which the surface of a sphere is often used as an illustration.

'We' are not dealing with a mathematical infinity.

I agree with Einstein that as far as mathematical propositions are certain, they do not refer to reality.

An infinite universe has no boundaries - no shape.

Originally Posted By: Bill S.
This 2D surface grows as the radius of the higher dimensional sphere increases, so there is only the first part of the problem to deal with: the ex nihilo creation of matter. Solve that, and increasing energy will follow E=mc^2.

And until somebody solves that particular problem it is a work of pure fiction to imply that the universe is expanding as the result of the ex nihilo creation of a greater than infinite force of energy.

On the basis of the claim that the galaxies are all receding from each other and the fact that this would require an increasing, greater than infinite, force of energy...expanding universe proponents are implying that this marvel of creation already exists - has already been solved - ergo the first law of thermodynamics should be scrapped or at least amended.

Originally Posted By: Bill S.
If the infinity is mathematical, there is no problem with the increasing energy being greater than it, because, as Cantor showed, there is an infinite range of sizes of infinity.

The infinity to which I refer is not mathematical however the idea that "If the infinity is mathematical, there is no problem with the increasing energy being greater than it." complies with Einstein's comment regarding the questionable veracity of self-consistent ('certain') mathematical propositions.

It is a skillful sleight of hand that allows one of the so-called proofs of a theory to contradict a primary law of physics namely that energy cannot be created or to be able to show that "...there is no problem with the increasing energy being greater than..." an infinite universe.

The very basis of the 'big bang' theory - galactic redshift - as well as the increasing redshift of the more distant galaxies can be much more easily explained (on an Occam's razor basis) in accordance with a universe of infinite time and space as can other of the so-called 'proofs' of that theory but 'simple' explanations do not impress the general public nor do they sell books so Occam's razor as well as Einstein's appeal to keep things as simple as possible are conveniently ignored by physicists.

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Originally Posted By: Bill6
I agree with Einstein that as far as mathematical propositions are certain, they do not refer to reality.


We certainly agree on this point; and who better to have in our corner than Einstein?

Before we risk getting back into infinite discussions, I would appreciate your comments on my post #48432.


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Originally Posted By: Bill S.
Before we risk getting back into infinite discussions, I would appreciate your comments on my post #48432.

I got the impression that this was merely an agreement with my post #38429 - that the newly created space must have some form of material substance which forces the galaxies apart.

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Bill6; perhaps I didn't express it well. What I was saying was that if an object is static in space, and space is static, a force is needed to move that object through space. Therefore, if an object is static in space, and space is moving, the object must move with it, otherwise it is moving through space with no apparent force causing it to move. Relativity tells us that space moving past an object, and an object moving through space must be considered as the same thing.

I still may not have expressed it well, but its 2.15am. and I've done all my thinking for today.


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