"Science begets knowledge, opinion ignorance.
It is easily overwhelmed by Earth's air water vapour, which affects the red end of the spectrum very badly. The KH jump looks reliable, so the redshift is OK, but the rest is rubbish. Once the redshift is established, the software just puts labels where it knows the peaks must be, according to the redshift.

Galaxy Color and Redshift Chart » Talk - Zooniverse

I doubt there would be enough water vapour in space, but it is a fair question.

Cat :)
No, red shift is caused by the recession velocity of a source. Light has the same speed all the time, in every medium, as viewed by any observer in an inertial frame of reference.
In substances, such as air and water, with refractive indices larger than 1.000 the light is delayed by each atom it passes by energizing an electron to a higher orbital, pausing for a moment, and then emerging to continue on at the speed of light to the next atom, where it pauses again. During the period of time it is travelling between atoms it is going at c.
Sep 11, 2020
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Thank you what about.
Think of an elastic stretched between 2 posts 10’ apart that is a straight line. Now let’s add some weights and some helium balloons so that you end up with a sawtooth pattern. How far is it from one post to the other if you follow the elastic.

Now think of the elastic as a photon, the weights as filaments/galactic clusters and the voids as helium balloons.

Is the universe expanding as fast as we think or is some of it just deformation of spacetime.
Yes, photons can be slightly diverted from a straight path due to gravitational influences. This is what causes gravitational lensing. The path length is increased and can cause photons from the same source, emitted at the same time, to arrive at different times due to path length differences.
The space time stretching by “weights” actually happens. Concentrated mass, stars and galaxies, bends space time. Light passing through a galaxy will both blueshift (traveling into the galaxy) and redshift (traveling out). The net is essentially 0 change in redshift.

There are several factors for redshifts: Doppler explains velocity differences based on motions through space; cosmological redshift due to expansion of space time; scattering effects due to gas and dust.
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In substances, such as air and water, with refractive indices larger than 1.000 the light is delayed by each atom it passes by energizing an electron to a higher orbital, pausing for a moment, and then emerging to continue on at the speed of light to the next atom, where it pauses again. During the period of time it is travelling between atoms it is going at c.

Bill, that explanation seems to be counter to the tenants of quantum mechanics, in that an electron in any particular atom is confined to very specific energy levels (orbitals) and cannot absorb just any passing photon to go to a higher energy level that is specific to the photon energy plus the electrons previous quantum energy level. And, there is also an issue with conservation of momentum unless the electron happens to release the photon along exactly the same path as it absorbed it from.

There is a known process for the scattering of photons by electrons - Rayleigh scattering. See

Note that it sounds similar to what you posted, but is frequency dependent and causes changes in photon direction, because it is elastic scattering.

Do you have a source for your explanation (for the speed of light decrease in masses) that I can look at?
Here is a blurb from wiki talking about absorption and reemission. The source is:

Cromie, William J. (24 January 2001). "Researchers now able to stop, restart light". Harvard University Gazette.

"Denser media, such as water, glass, and diamond, have refractive indexes of around 1.3, 1.5 and 2.4, respectively, for visible light. In exotic materials like Bose–Einstein condensates near absolute zero, the effective speed of light may be only a few metres per second. However, this represents absorption and re-radiation delay between atoms, as do all slower-than-c speeds in material substances."

I don't know how a photon of arbitrary energy can be absorbed and reemitted though. Rayleight scattering may be the cause. It occurs when a photon causes the atom's bonds to stretch and bend. Maybe that can catch, hold and reemit a photon thus taking up some time.
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I have the impression that scattering and atom absorption/emission are models of the same event.

The first model was a mechanical one but the scattering profile was off. Once EM explained how it worked, the predicted profile matched the results.
Rayleigh scattering is not the same thing Bill is posting about. If you read the link I posted, you will see that is the case. What Bill posted does not change the direction of the photons as they interact with each atom, so the light does not diffuse but is slowed until it exits the medium, at which point it is not dispersed. The light makes a straight passage through the medium. By contrast, Rayleigh scattering of sunlight in the atmosphere is mainly in shorter visible wave lengths and scatters the light at high angles - making the sky look blue, instead of black, like the sky looks on the moon during the moon "day".

As for the Bose Einstein condensates in Bill's link, those are extremely low temperature materials that are quantum coupled atoms with unusual properties. They are not like your typical glass lens or gas cloud.

From Bill's link:
"Using sodium atoms and two laser beams, they made a new kind of medium that entangles light and slows it down. The laser beams glow yellow-orange like sodium streetlights, and the cigar-shaped cloud of atoms is about eight-thousandths of an inch long and about a third as wide. "
[the researchers]"kept tweaking the atoms until they completely stopped laser light. This happens when a second laser beam directed at right angles to the cloud of atoms is cut off. When that laser is switched on again, it abruptly frees the light from the trap and it goes on its way."
"... light entering the atomic entanglement transfers its energy to the atoms. Light energy raises the atoms to higher energy levels in ways that depend on the frequency and intensity of the light. The laser illuminating the cloud at right angles to the incoming beam acts like a parking brake, stopping the beam inside the cloud when it is shut off. When it is turned on again, the brake is released, the atoms transfer their energy back to the light, and it leaves the end of the cloud at full speed and intensity."
". . . stopped light for one-thousandth of a second. Atomically speaking, "this is an amazingly long time,"

So, what Bill is talking about is a quantum entangled material that can stop a specific light frequency when it is carefully tuned to the energy of photons that are being introduced. It sounds more akin to a laser than anything else.

This is not what happens when white light enters a solid piece of glass at room temperature, slows by an amount dependent on its wavelength as it passes through, and then resumes the speed it had at entry as it leaves. There are no steps in the effects for various wavelength, so quantum effects of the atoms do not seem relevant. When absorption is active, we see dark lines in the spectrum of light exiting a material, and when stimulated emission of radiation (the ser in laser - light amplification by stimulated emission of radiation) is occurring, it is making a very specific, narrow frequency range of light.

So, I am still waiting for an explanation of how white light is slowed by electron absorption and re-emission in a regular piece of glass, without any observable quantum effects like dark bands or bright bands in the exiting spectrum.
Yes, absorption and reemission is definitely there. Scattering preserves the momentum vector of the photon somehow.


Scattering, by definition, means that the direction of the photons is changed.

To conserve momentum, something else must move away from the original path direction of the light after interacting with the photon. See .

What Bill is suggesting for electrons absorbing and re-emitting light in the exact same direction at the exact same wavelength is not "scattering".

Scattering, by definition, means that the direction of the photons is changed.

To conserve momentum, something else must move away from the original path direction of the light after interacting with the photon. See .

What Bill is suggesting for electrons absorbing and re-emitting light in the exact same direction at the exact same wavelength is not "scattering".
Scattering does include scattering forward. For Mie Scattering, forward scattering dominates the distribution. Graph here

I wish I could be of more help but quantum physics is not an area I have spent time studying.
Bill, Mie scattering is still scattering, not transmission of all of the light in a straight line. And it does not mention photons slowing by absorptions and reemission in the same direction.

I asked the question about a reference to your post about that because first, I had never heard it before, and second, it seemed to be inconsistent with quantum mechanics. I am still not seeing anything that helps me understand where you got the idea or how it can work at the quantum level by electron absorption.
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The Feynman Lectures on Physics Vol. I Ch. 31: The Origin of the Refractive Index (

I waded through this as best I could. Most of it is over my head but I can grasp the basics. Feynman says that in a transparent medium the electrons are held to the atoms by tiny springs. The incident photon moves the electron up and down but is unable to cause it to jump a quantum. If it did that the material would be opaque and the light dissipated as heat. Instead, the electron rebounds and reemits the photon on its journey, in the same direction, but with a phase change. The accumulated phase changes add up to a delay equal to the index of refraction.
Bill, I read through the Feynman lecture in you link. It uses wave theory, not quantum mechanics or photons. It uses math that works with electric fields of electromagnetic waves and oscillating electrons. I don't think the word "photon" appears in the whole piece.

The behaviors of electrons assumed in the math is "assume that the atoms are little oscillators, that is, that the electrons are fastened elastically to the atoms, which means that if a force is applied to an electron its displacement from its normal position will be proportional to the force." That is distinctly different from assuming that electrons are limited to specific energy levels by quantum mechanics. And, the electromagnetic waves are modeled as if they have a uniform electric field across the surface of the material, not as discrete packets (phtons) of light that can individually interact with photons.

So, there is no photon absorption by single electrons and reemission of the same energy/direction of photons by electrons in this article.

The problem with light is that we really don't know how it can be a wave that travels through space without some medium to travel in, but we can't find a medium in space. So, we try to think of light as quasi particles, but can't explain how those particles can behave like waves in experiments where they seem to do that.

There are a lot of mathematical models that treat light in different ways (waves vs particles) which work well in some situations but fail in other situations. So, we just accept a "duality" and use experience to tell us which models work for which situations.

That works OK until we try to use those models to understand more things that we don't currently understand. If we extrapolate different models for the same situation, we get different answers that conflict. We really don't have a basis to know which, if any, is right, because we don't have a way to test the models in those particular situations. And that leads to a lot of unverifiable speculation, with people "shooting down" each other's theories using tenants of models that may or may not really apply to those situations.

I think that is what we have going on here. And it is going on elsewhere, too. See for example. That is just one of many such discussions that come up when you search "quantum mechanics refraction theory".

The problem is that nobody I have found can actually explain what physically happens and prove that it happens that way with an experiment. It is all conjecture, and mostly by people who (like me) are not recognized experts on the subject. Even recognized experts like Feynman can only assume that light travels along many many paths simultaneously and is only visible due to "constructive interference" at the places we see it. So, that at best get us back to the double slit paradox where somehow what we want to think about as "one photon" sometimes acts like one particle, and sometimes like a distributed wave, depending on how we choose to detect it.

So, let's drop this discussion here. There just does not seem to be sufficient understanding or agreement to help with understanding the original question for this thread.
No, I am not going to "drop this discussion here". I would like to understand refractive index at the atomic level and would like to hear from someone who can explain it. I take the Standard Laws of Physics as "the current best approach" and I seek to understand them, not some "out of the mainstream" theory. You are welcomed to believe whatever you want but I am not interested in anything outside the Standard Laws. If you cannot accept the results of Michelson-Morley and Dirac's wave particle duality, there is nothing I can do for you.

The absorption of an EM wave in a transparent media is by a phonon or lattice vibration. The electron remains in a certain orbital but is forced to vibrate. It does not get enough energy to jump to another orbital. Its ability to absorb and reemit the energy of incident EM waves is continuous not discrete, as orbital changes require.
Bill, I am not finding any actual expert material that says that photons are absorbed by electrons and then are reemitted in the same direction at the same energy is the explanation for light having reduced speed in matter. All I am finding is similar discussions to what we are having between the two of us, here, with no such references in those discussions, either.

So. it is not a matter of whether light does slow down in matter - we know that for a fact by measurements. So all we are discussing is how to understand why it does that in the conceptual framework of light being photons.

And my objection to the idea you posted is in the word "absorbed" for the photon. I am not disagreeing on the idea that the electron clouds in the matter somehow influence the effective energy of the photons, just with the idea that electrons repeatedly completely absorb the entire energy of each photon and then reemit it again with the same energy in the same direction. As you seem to state in your last sentence of your last post, above, you agree that the electrons cannot be changing orbitals up and down as the photons pass, which is what "absorb" means.

The Feynman lecture link in your earlier post that uses macro wave theory might have some parallel in quantum theory, where the orbitals of electrons can be perturbed without changing their orbitals to a higher level, but still have their fields plus the photons fields combine in a manner to produce a velocity reduction (which is a phase shift in wave theory). We do know that electron orbital energy levels are influenced by some factors that seem to warp their spatial probability distributions around their nucleus. Maybe somebody with more chops in quantum physics than you or I have can, or already has developed an electron interaction theory to explain the reduced speed of photons in matter. But, my searches for it have not turned up such a thing, and apparently you have been searching for that too, with the same negative results that I am having. So, I am not accepting that the "Standard Laws of Physics" actually say that photons of light transmitted though matter are repeatedly absorbed and then reemitted in the same direction with the same energy inside the matter.

As for continuing the discussion between the two of us, here, I really don't have anything else to say. I too would like to learn how light gets slower in matter, and would be interested in a link that proposes a mechanism consistent with Quantum Theory about how that happens. But, it is not helping me to read more explanations made with electromagnetic wave theory, because I already understand that. It is just that that understanding is not helping me understand concepts in cosmology that are trying to think of photons as discrete energy packets.
Read the Feynman paper again and ignore the lack of the word photon. He uses the term EM wave which is the same thing. The wave is absorbed by a vibration of a bound electron and reemitted in the direction of travel. Any amount of energy can be transferred in the direction of travel. Ignore the fact that those energy amounts are not quantized as are the amounts that would stimulate a jump to another orbital.