# How does light speed up when it exits a denser material?

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#### thedoc

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##### How does light speed up when it exits a denser material?
« on: 16/06/2015 12:38:38 »
How does light speed up when it exits a denser material?
« Last Edit: 16/06/2015 17:07:07 by Georgia »

#### syhprum

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##### Re: How does light speed up when it exits a denser material?
« Reply #1 on: 13/03/2014 20:58:50 »
Photons never move at other than light speed the apparent slowing down is due to the tortuous path they take being absorbed and re-emitted by atoms en route.
When they leave the materiel this effect ceases and their normal speed becomes apparent.
I know that this a simplified version but the gist is correct.
« Last Edit: 14/03/2014 21:23:55 by syhprum »
syhprum

#### annie123

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##### Re: How does light speed up when it exits a denser material?
« Reply #2 on: 14/03/2014 19:43:51 »
Re the speed of light in general - I have asked elsewhere on this forum about the scientist who has slowed down light and even stopped it at one point, and why this is not being celebrated/discussed/questioned/debated/explored more. Is this scientist's discovery (Lene Haus) not that important after all? I thought that showing Einstein was wrong would be a big deal.

#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #3 on: 15/03/2014 01:44:32 »
Quote
I thought that showing Einstein was wrong would be a big deal.
Einstein's theories have been remarkably resilient at large scales. (Relativity does not address the quantum scale, although his work on the photoelectric effect does.)

We routinely slow down light slightly by shining it through a transparent medium like glass - this is how lenses work.

Slowing light to a walking pace or slower does not violate Einstein's theory of relativity, which takes the speed of light in a vacuum as a speed limit for matter.

These demonstrations of slowing light uses light of a very specific wavelength, and a medium which is carefully primed to put it in the right quantum state.
It is definitely not a vacuum...

#### thedoc

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##### Hear the answer to this question on our show
« Reply #4 on: 16/06/2015 16:00:39 »
We discussed this question on our  show
We put this question to Cambridge University physicist Zephyr Penoyre...
Zephyr - When it goes into the glass, it’s moving slower. But also, because the frequency has to be the same, the same waves have to be coming into the glass at the same rate as they're coming out, otherwise you’ve lost waves somewhere along the way…
Chris - And it’s changed colour.
Zephyr - And that’s because wavelength has changed. So, the energy of the light is to do with the wavelength of the light. So, as it slows down, the energy gets higher - because the wavelength has shrunk - so the total amount of energy passing through the glass is exactly the same as it was going through the air, it's just moving at a slightly different speed.
Click to visit the show page for the podcast in which this question is answered. Alternatively, [chapter podcast=1001073 track=15.06.16/Naked_Scientists_Show_15.06.16_1003774.mp3] listen to the answer now[/chapter] or [download as MP3]
« Last Edit: 01/01/1970 01:00:00 by _system »

#### PmbPhy

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##### Re: How does light speed up when it exits a denser material?
« Reply #5 on: 16/06/2015 18:03:03 »
Quote from: evan_au
Relativity does not address the quantum scale, although his work on the photoelectric effect does.
What do you mean? There is a relativistic theory of quantum mechanics.

#### Colin2B

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##### Re: How does light speed up when it exits a denser material?
« Reply #6 on: 16/06/2015 19:07:19 »
Re the speed of light in general - I have asked elsewhere on this forum about the scientist who has slowed down light and even stopped it at one point, and why this is not being celebrated/discussed/questioned/debated/explored more. Is this scientist's discovery (Lene Haus) not that important after all? I thought that showing Einstein was wrong would be a big deal.
The work of Lene Hau is being celebrated but it would be a mistake to conclude that it shows Einstein to be wrong. It is also a mistake the think that Einstein said light speed is constant in all circumstances, the constancy of light speed as described by Einstein is for a vaccum. Dr Hau's work was on light in dense, indeed very cold, media eg sodium, for which she has received significant and well deserved accolades.
and the misguided shall lead the gullible,
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#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #7 on: 17/06/2015 11:45:03 »
Quote from: Zephyr
So, as (light) slows down, the energy gets higher
This seems to be talking about the energy density of a beam of light, eg measured  Joules/m3 or similar

However, when you are talking about an individual photon, its energy remains the same in vacuum, air or glass.

Unlike objects having a rest mass, light does not gain or lose energy when its speed changes, so it doesn't need to gain energy when it exits the glass.

#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #8 on: 17/06/2015 11:55:29 »
Quote from: PmbPhy
There is a relativistic theory of quantum mechanics.

Yes, quantum mechanics has been adapted to consider relativistic effects such as c being a speed limit, time dilation and gravitational red shifting, etc. All these can be tested on Earth, for example by accelerating subatomic particles to nearly the speed of light in a particle accelerator, or by flying atomic clocks around the Earth in a GPS satellite.

However, I have not seen a well-tested quantized theory of relativity. To test this would require close study of the behaviour of subatomic particles close to the event horizon of a black hole. If scientists attempted experiments on a reasonable-sized black hole here on Earth, I fear that they would not live long enough to publish the paper.

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #9 on: 17/06/2015 13:32:51 »
Quote
How does light speed up when it exits a denser material?
It doesn't. Light which exits the material it's not the same light which enters and which is inside: the em field which exits is generated by the last charges inside the material.

--
lightarrow

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #10 on: 18/06/2015 00:29:30 »
At the event horizon of a black hole the gravitational field exhibits a critical density like a 'perfect' medium. Perfect in respect of its ability to stop light. The photon's energy is effectively canceled exactly at the horizon. At this point the electromagnetic and gravitational fields are at equilibrium. Not in the sense of a stationary point but in the balance of energies. This does not help unless observational data can be collected from the immediate environment surrounding a black hole. A collision with the horizon of Sag A* is the best chance of obtaining this data.

#### Bill S

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##### Re: How does light speed up when it exits a denser material?
« Reply #11 on: 18/06/2015 15:13:40 »
Quote from: Lightarrow
Light which exits the material it's not the same light which enters and which is inside: the em field which exits is generated by the last charges inside the material.

Explanations often say, or imply, that it is not a single photon that travels through the medium (e.g. glass); it is a succession of new photons, created at each new emission from atoms in the glass.  If this were what actually happened, then the absorption spectrum would be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured frequencies.

The reason for this is that a solid is composed of a network of ions and electrons fixed in a "lattice".  They have what is known as "collective vibrational modes", or phonons. These are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon.

If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available; so the lattice does not absorb this photon and it is re-emitted but with a very slight delay.

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #12 on: 20/06/2015 12:35:47 »
Quote from: Lightarrow
Light which exits the material it's not the same light which enters and which is inside: the em field which exits is generated by the last charges inside the material.

Explanations often say, or imply, that it is not a single photon that travels through the medium (e.g. glass); it is a succession of new photons, created at each new emission from atoms in the glass.  If this were what actually happened, then the absorption spectrum would be discrete because atoms have only discrete energy states. Yet, in glass for example, we see almost the whole visible spectrum being transmitted with no discrete disruption in the measured frequencies.

The reason for this is that a solid is composed of a network of ions and electrons fixed in a "lattice".  They have what is known as "collective vibrational modes", or phonons. These are quanta of lattice vibrations, and it is these vibrational modes that can absorb a photon.

If a photon has an energy beyond the phonon spectrum, the solid cannot sustain this vibration, because the phonon mode is not available; so the lattice does not absorb this photon and it is re-emitted but with a very slight delay.

Yes, this is also what I quoted from another forum in this one some years ago. Infact I haven't written that a photon is absorbed by an atom's material then ri-emitted and so on.

Nontheless a photon entering the material is not the same photon exiting from it: the em field, entering the material, let's say glass, excites it and the glass generates in turn another em field, which interacts with the first and with glass itself. Photons are constantly created and destroyed in the resulting field so their individuality is lost at the end of the path.

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lightarrow

#### Bill S

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##### Re: How does light speed up when it exits a denser material?
« Reply #13 on: 20/06/2015 22:36:02 »
Quote from: Lightarrow
Infact I haven't written that a photon is absorbed by an atom's material then ri-emitted and so on.

You are right, of course.  These thing always seem to get more complicated, the closer you look at them.

#### RTCPhysics

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##### Re: How does light speed up when it exits a denser material?
« Reply #14 on: 25/06/2015 10:01:09 »
Reading through the views expressed, there seemed to be three alternative explanations put forward.

1.   The ‘bus stop’ theory, where like a bus stopping at every bus stop, a light photon stops at every electron it meets on its route through the translucent material.
2.   The ‘spring’ theory, where the photon acts like a spring which becomes more tightly coiled on its passage through the translucent material.
3.   The ‘water surfer’ theory, where like a surfer skimming the wave tops of water, the photon is swept through the translucent material carried on the top of the internal wave motion of the material’s ‘electron sea’.

The arguments raised against each particular theory were along the following lines:

The ‘bus stop’ theory needs to explain how all visible light photons are absorbed and emitted by electrons on their passage through the translucent material. Particular elements such as silicon, have a specific absorption and emission spectra, so some visible light frequencies would not be affected and pass through at the speed of light without hindrance.

The ‘coiled spring theory’, raised the question of where the frequency of a photon of visible light changes to a higher value. Is it upon contact with one or multiple electrons on its passage through the translucent material? And interestingly, if an infra-red photon passed through the translucent material would it be visible on its passage as ‘red light’?

The ‘water surfer’ theory makes the assumption that there is a ‘standing wave’ within the ‘electron sea’, which carries the photon along without absorption and emission. (This is articulated much more cogently as the collective vibrational modes (phonons) of the ions and electrons located in their lattice structure.) The speed of passage of the photon through the translucent material riding this vibrational lattice must be presumably less than the speed of light, but the photon itself is unaffected and emerges at the speed of light again.

All three theories are plausible, but it does seem to me that our understanding of the photon and its interaction with the electron is at best sketchy. If we understood the commonality of the energy structures of the photon and the electron, we would know how the photon is absorbed and released by the electron, (bus stop theory), receives and releases energy, (coiled spring theory) and hitches a lift without energy being exchanged with the electron lattice (water surfer theory).

#### chiralSPO

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##### Re: How does light speed up when it exits a denser material?
« Reply #15 on: 25/06/2015 14:25:30 »
And interestingly, if an infra-red photon passed through the translucent material would it be visible on its passage as ‘red light’?

No, the frequency determines the "color," and that does not change from medium to medium. The wavelength must change, but ultimately it is the frequency (energy) that registers as the color in our eye.

If you consider the eye as the detector, then it doesn't matter what material the light passes through, ultimately the refractive index of the eye where the light is absorbed determines the "observed wavelength", but we can design an experiment to probe the color of light in other materials:

Say we have two solutions that each contain the same concentration of a dye that absorbs at a very specific frequency, but the two solutions also contain different amounts of some other dissolved species that changes the refractive index substantially (many additives could allow us to tune the refractive index by 10-20%, without changing the behavior of the dye). If the spectrum of light coming out of the solution is independent of the refractive index, that proves that the frequency determines color, not the wavelength (and if the absorbed light shifts with refractive index, that proves the wavelength is the important factor)

#### RTCPhysics

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##### Re: How does light speed up when it exits a denser material?
« Reply #16 on: 24/07/2015 13:30:18 »
Quote
No, the frequency determines the "color," and that does not change from medium to medium. The wavelength must change, but ultimately it is the frequency (energy) that registers as the color in our eye.

If you consider the eye as the detector, then it doesn't matter what material the light passes through, ultimately the refractive index of the eye where the light is absorbed determines the "observed wavelength", but we can design an experiment to probe the color of light in other materials:

Say we have two solutions that each contain the same concentration of a dye that absorbs at a very specific frequency, but the two solutions also contain different amounts of some other dissolved species that changes the refractive index substantially (many additives could allow us to tune the refractive index by 10-20%, without changing the behavior of the dye). If the spectrum of light coming out of the solution is independent of the refractive index, that proves that the frequency determines color, not the wavelength (and if the absorbed light shifts with refractive index, that proves the wavelength is the important factor)

I disagree. The explanation that the reduction of the velocity of visible light in a translucent material is effected by a reduction in wavelength but not frequency, implies that the sinusoidal wave model of a single photon is incorrect. With the sinusoidal model, the wavelength and frequency are related, one being the inverse of the other.

The only way I can see to explain the reduction of light speed upon entering a translucent material from say, air, is if the amplitude of the sinusoidal wave is increased at the boundary between the media, but its frequency and wavelength remain unchanged. The higher amplitude increases the three dimensional distance travelled by the photon's new sinusoidal wave form and hence its velocity through the medium. On emerging from the translucent medium back into air, the amplitude reverts to its previous level, giving the photon the energy it needs to return to its previous velocity.

Reading the results of experimental tests upon the functioning of the eye, their results all refer to the wavelength of the visible light as being the important factor in the colours that the brain observes, not frequency.

Absorption and refraction are two different phenomenon, not to be confused. Refraction or bending of light occurs at the surface boundary between the two media. Absorption of a particular frequency of light by electrons can happen anywhere within the receiving medium. In the experiment you described, adding the dye to the change of refractive index would only shift the whole light spectrum to one side. The absorption frequency of the translucent solution would be unchanged within this spectrum, proving nothing. Except that the added dye does not absorb light of the particular frequency used in the experiment. I'm assuming that the absorption line did not move!

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #17 on: 24/07/2015 13:51:29 »
I have never read anywhere that wave amplitude is a factor. Do you have any references for that?

#### chiralSPO

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##### Re: How does light speed up when it exits a denser material?
« Reply #18 on: 24/07/2015 15:29:00 »

I disagree. The explanation that the reduction of the velocity of visible light in a translucent material is effected by a reduction in wavelength but not frequency, implies that the sinusoidal wave model of a single photon is incorrect. With the sinusoidal model, the wavelength and frequency are related, one being the inverse of the other.

They are not quite inverses, they are inverse AND related by the velocity: wavelength = speed / frequency (wavelength * frequency = speed ; meters per cycle times cycles per second equals meters per second) Therefore, to maintain a fixed frequency when light slows down, the wavelength must decrease. This can be (and has been) experimentally shown by performing interferometry (which depends on the wavelength, not the frequency) in media other than vacuum, and comparing against the energy of the photon (which is a function of frequency, not wavelength).

The only way I can see to explain the reduction of light speed upon entering a translucent material from say, air, is if the amplitude of the sinusoidal wave is increased at the boundary between the media, but its frequency and wavelength remain unchanged. The higher amplitude increases the three dimensional distance travelled by the photon's new sinusoidal wave form and hence its velocity through the medium. On emerging from the translucent medium back into air, the amplitude reverts to its previous level, giving the photon the energy it needs to return to its previous velocity.

I don't think amplitude has anything to do with this. The sinusoidal wave is not the "path" that the photon takes. It is a representation of the electromagnetic oscillation of the photon. More photons = greater amplitude (if they're in phase).

Reading the results of experimental tests upon the functioning of the eye, their results all refer to the wavelength of the visible light as being the important factor in the colours that the brain observes, not frequency.

Almost everybody talks about the wavelength of light rather than frequency, but it is implied that they mean the wavelength in a vacuum. The wavelength changes based on the refractive index of the medium the light is in (see above), but the frequency (energy) remains constant. The eye detects light by virtue of rhodopsin, which is the adduct of an enzyme (opsin) and a small molecule called retinal. The light isomerizes retinal, causing a conformational change in the rhodopsin, which sends a signal to the nerve, etc. etc. There are a few different variants of opsin (called photopsins) that can modulate the energy required to isomerize the retinal (this is how we see different colors). The key here is that the differentiation of different "wavelengths" of light is due to little protein devices that respond very selectively to certain energies (frequencies) of light.

(https://en.wikipedia.org/wiki/Rhodopsin)
(https://en.wikipedia.org/wiki/Photopsin)

Absorption and refraction are two different phenomenon, not to be confused. Refraction or bending of light occurs at the surface boundary between the two media. Absorption of a particular frequency of light by electrons can happen anywhere within the receiving medium. In the experiment you described, adding the dye to the change of refractive index would only shift the whole light spectrum to one side. The absorption frequency of the translucent solution would be unchanged within this spectrum, proving nothing. Except that the added dye does not absorb light of the particular frequency used in the experiment. I'm assuming that the absorption line did not move!

This is exactly my point! If the absorption line did not move, and the whole spectrum shifts to one side, the light that gets absorbed depends on how much of a shift there was!!!  []

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #19 on: 25/07/2015 09:22:04 »
I disagree. The explanation that the reduction of the velocity of visible light in a translucent
"Translucent"? it's not the correct term.
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material is effected by a reduction in wavelength but not frequency, implies that the sinusoidal wave model of a single photon is incorrect
"sinusoidal wave model of a single photon"? It doesn't exist. Maybe you intended something else.
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. With the sinusoidal model, the wavelength and frequency are related, one being the inverse of the other.
No, as others already wrote, in an electromagnetic wave, not only wavelength λ and frequency f are related, but radiation velocity (precisely phase velocity vp) too:

λ = vp/f

frequency f doesn't change inside the material, so if vp decreases, also does λ.

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The only way I can see to explain the reduction of light speed upon entering a translucent material from say, air, is ...
... your personal and wrong theory.

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lightarrow
« Last Edit: 25/07/2015 09:24:43 by lightarrow »

#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #20 on: 26/07/2015 08:15:03 »
Quote from: ChiralSPO
The wavelength changes based on the refractive index of the medium the light is in (see above), but the frequency (energy) remains constant
I heard a very practical example of the fact that the wavelength of light changes when it enters a material of higher refractive index:
• Obtaining the light spectrum of stars and planets is a powerful tool for astronomers - it allows them to determine their chemical composition without actually traveling there to take a sample.
• One of the common ways to break light up into a spectrum is to use a diffraction grating - very fine lines etched on a flat surface, which breaks up the light into a rainbow of its constituent colors.
• Astronomers studying the infra-red need to put their instruments above the atmosphere, ideally in a satellite, but volume is at a premium in a satellite.
• Some instrument makers recently "reversed" the usual diffraction grating design - instead of shining light onto the diffraction grating in a vacuum, they shone the light through the back of a silicon wafer onto the diffraction grating, so the light was passing through silicon, rather than through a vacuum when it hit the grating.
• The refractive index of silicon is about 3-5 times higher than a vacuum in the infra-red region, so the wavelength of light is correspondingly shorter.
This allowed them to separate the wavelengths as effectively as an instrument about 4 times longer (and wider & high), so they reduced the volume of the instrument by a factor of about 60, which is a significant saving in a spacecraft!

Quote from: Lightarrow
a photon entering the material is not the same photon exiting from it: the em field, entering the material, let's say glass, excites it and the glass generates in turn another em field, which interacts with the first and with glass itself. Photons are constantly created and destroyed in the resulting field so their individuality is lost at the end of the path.
Maybe I'm just nitpicking here, but...
I would call a photon an em field which propagates through space, as per Maxwell's equations.

To me, it doesn't matter whether the em field at point X is the same em field that exists 1 meter (or 1 micrometer) away. What matters is that it is the same quantum of energy that is propagating (as per Einstein's explanation of the photoelectric effect). To me, the photon that enters a glass block is the same photon that exits a glass block (assuming it isn't absorbed in-between).

#### Thebox

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##### Re: How does light speed up when it exits a denser material?
« Reply #21 on: 26/07/2015 08:29:00 »
Presuming light has a speed and it is not something in space that is being energized at a speed such as CBMR.

#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #22 on: 26/07/2015 12:13:09 »
Quote from: Thebox
Presuming light has a speed
Light definitely has a speed, as measured by Ole Rømer in 1676, Fizeau & Focault around 1848 and many more modern measurements.
In Physics, we call this speed "c", and it is about 300 thousand km/s in a vacuum.

See: https://en.wikipedia.org/wiki/Speed_of_light#Measurement

Quote
Presuming light ...is not something in space that is being energized at a speed such as CBMR.
Visible light happens in space because of things like stars, bouncing off things like planets, Moons and clouds of dust. It does not rely on the CMBR to power it (the CMBR is far too weak to have much effect on visible light).
« Last Edit: 26/07/2015 14:46:01 by chiralSPO »

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #23 on: 26/07/2015 20:23:53 »
Quote from: Lightarrow
a photon entering the material is not the same photon exiting from it: the em field, entering the material, let's say glass, excites it and the glass generates in turn another em field, which interacts with the first and with glass itself. Photons are constantly created and destroyed in the resulting field so their individuality is lost at the end of the path.
Maybe I'm just nitpicking here, but...
I would call a photon an em field which propagates through space, as per Maxwell's equations.
Aha! 90 yars of quantum mechanics thrown away! 😊
Sorry but a photon is something much more complicated than that!
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To me, it doesn't matter whether the em field at point X is the same em field that exists 1 meter (or 1 micrometer) away. What matters is that it is the same quantum of energy that is propagating (as per Einstein's explanation of the photoelectric effect).
Then an electron or a photon is the same, for you 😊
Quote
To me, the photon that enters a glass block is the same photon that exits a glass block (assuming it isn't absorbed in-between).
Prove it, if you can.

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lightarrow

#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #24 on: 26/07/2015 22:40:47 »
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Then an electron or a photon is the same, for you
No, an electron does not obey Maxwell's equations (in particular, it cannot travel at c); a photon does.

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Prove it, if you can.
I don't think it is a matter of proof. I think it comes down to how it is defined.

I don't know what would be an authoritative definition, but in this one: "a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy", ..the focus is on the photon as a quantum of energy, rather than on the photon as a localized disturbance in the em field.

More authoritative definitions & interpretations are welcome!
« Last Edit: 27/07/2015 10:53:00 by evan_au »

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #25 on: 27/07/2015 08:04:50 »
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Then an electron or a photon is the same, for you
No, an electron does not obey Maxwell's equations (in particular, it cannot travel at c); a photon does.
"An electron does not obey Maxwell's equations..a photon does"? Not very correct, because it mixes classical and quantum description.
An electron AND a photon are described by QED = Quantum ElectroDynamics.
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Prove it, if you can.
I don't think it is a matter of proof. I think it comes down to how it is defined.
"A state with n = 1 of the quantistic hamiltonian of the electromagnetic field in absence of charges" is a definition nearer to the concept than your definition.
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I don't know what would be an authoritative definition, but in this one: "a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy", ..the focus is on the photon as a quantum of energy, rather than on the photon as a disturbance in the em field.
My God! That would be "an authoritative definition"? It seems a popular book!
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More authoritative definitions & interpretations are welcome!
At this point you are perfectly right! But it's very difficult to find a correct definition of "photon" because it's a very technic concept.

While I'm looking for a link with that definition, have a look at:
http://www.sheffield.ac.uk/polopoly_fs/1.14183!/file/photon.pdf

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lightarrow

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #26 on: 27/07/2015 13:12:53 »
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Then an electron or a photon is the same, for you
No, an electron does not obey Maxwell's equations (in particular, it cannot travel at c); a photon does.
You are mixing classical and quantum concepts here. If you want to talk of Maxwell's equations, then forget the word "photon".
If you want to call for a quantum description, then BOTH electron and photon are described by QED: quantum electro dynamics.
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Prove it, if you can.
I don't think it is a matter of proof. I think it comes down to how it is defined.
I don't know what would be an authoritative definition, but in this one: "a photon is a discrete bundle (or quantum) of electromagnetic (or light) energy",
And from that how can you deduce that the photon which exits a piece of glass is the same which entered?
If it were so simple, Einstein wouldn't have had the need to say something like: "50 years of thoughts about the subject didn't bring me closer to understand what a light quantum is...but nowadays every Bob and Alice thinks to know what it is..."
A photon is a much complicated concept. Have a look here:
http://www.sheffield.ac.uk/polopoly_fs/1.14183!/file/photon.pdf
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..the focus is on the photon as a quantum of energy, rather than on the photon as a localized disturbance in the em field.
More authoritative definitions & interpretations are welcome!
It's very difficult to find one because it's a very technical subject, despite the fact we always talk about it as if we knew it well...
Here is one from an extract of a discussion in other forum (and I admit not to have completely grasped it []):
Given a free electromagnetic field (that is without electric charges) a photon is (something alike) the subspace with n = 1 of the hilbert space of quantized field's states, where n are eigenvalues of the operators "number of particles" N which gives the hamiltonian of the field when multiplied by ħω, which gives the field energy when applied to the state.

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lightarrow

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #27 on: 27/07/2015 17:16:02 »

#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #28 on: 28/07/2015 22:10:53 »
When you calculate the path and speed of a photon through a block of glass, you must do a calculation at the boundaries of the glass block, because the conditions change (ie glass has a higher refractive index than air), and this affects the speed of the photon. It does not matter whether you are using classical optics or infinite-dimensional Hilbert spaces to do the calculation.

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You are mixing classical and quantum concepts here. If you want to talk of Maxwell's equations, then forget the word "photon".
The fact that QED is able to unify the behaviour of photons & electrons makes it better than classical physics; maybe one day we will have a theory that includes gravity too, and that will be even better than QED.

But the existence of a "better" theory does not necessarily render the old theory invalid. The new theory is only considered valid because in the "classical limit", it predicts the same results as the experiments which had already been successfully explained by the classical theory.

So in selecting a method to solve a particular problem, it is important to know how accurate an answer do you need?

All design calculations are approximations; will the approximations of a given method yield an answer which is "good enough" for the problem at hand? This means you must understand the domain of applicability of the different theories:
• Where they will give essentially identical answers
• Where they will give answers that are "close enough", and you can build a prototype and tweak it to correct the discrepancies that occur because real objects are never exactly the same as even the best theoretical models.
• Where they give divergent answers, and you must use the more complex theory (but you still need to build the prototype!).

In cases 1 & 2, the selection is essentially made on pragmatic grounds like simplicity and accuracy (the proverbial "back of the envelope calculation" may be all you can afford in the middle of a discussion).

For myself, my maths classes did not go beyond eigenvectors on finite dimensional vector spaces, so I don't have all the mathematical tools needed for determining the eigenvalues of infinite-dimensional Hilbert spaces.

However, I would argue that you do not need these advanced mathematical tools to calculate the path and speed of a photon through a glass block, because this is in the domain of classical optics. The answers are "close enough", provided the size of the block of glass is much larger than the wavelength of light, and the size of the atoms in the glass is far smaller than the wavelength of the light. You just need to recognize that the classical term "a ray of light" is an approximation to "a stream of photons" in quantum theory.

Undoubtedly, a grand unified theory which includes gravity (or even a small extension of current theories which is able to explain why calculations on Feynman diagrams work so well) will require additional, even more complex mathematical tools in your toolkit.

You don't always need a sledgehammer to crack a nut...
« Last Edit: 29/07/2015 03:52:38 by evan_au »

#### McQueen

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##### Re: How does light speed up when it exits a denser material?
« Reply #29 on: 29/07/2015 05:18:01 »
One thing that is significant about the answers given here is that they either tend to ignore physical facts as in the posts favouring the propagation of light through a substance in accordance with  Maxwell’s equations or they  give static answers which ignore the dynamics present. Too often (almost exclusively) the emission and absorption of photons by electrons in an atom is treated as a one off case, an electron within the atom absorbs and then emits an electron, this does not fit in with the observed data Firstly the definition of spectroscopy is as follows:

Spectroscopy is a scientific measurement technique. It measures light that is emitted, absorbed, or scattered by materials and can be used to study, identify and quantify those materials.

In order for the above to be true it is implicit that electrons in the atoms comprising the substance must be absorbing and emitting light of a given frequency at precisely that same frequency. (i.e., If green light at 540 THz is being observed it means that the electrons are oscillating at that same frequency of $$540\times 10 ^{12}$$ THz  and absorbing and emitting electrons in those same numbers.

Next examining Lene Hau’s experimental results on the slowing down of light, it is found that while the results wholly substantiate the emission and absorption theory the results equally completely rule out Maxwell’s electromagnetic field, interacting with the fields of the atoms in the material while allowing the light to pass through only slightly slowed down.

Again what makes  an object transparent or opaque? An opaque material such as wood will absorb all or most of the visible spectrum of light releasing the extra energy as heat, while a transparent material such as glass will absorb and re-emit most of the  visible spectrum, resulting in the passage of light .  A photon is characterized by its energy which is directly related to the wavelength of the light when the radiation is considered as a wave. This relation is :

$$E_{ph}= \frac{hc}{\lambda}$$

A large amount of photons make a ray of light, this ray of light can be characterized by its wavelength, intensity and the variation of this intensity over time. This translates in the particle domain to the amount of particles crossing an imaginary surface per unit of time. Particles interact with each other by exchanging particles or energy. A photon interacts with other particles by transferring its energy to the other particle (absorption). This exchange of energy happens if the maximum energy that the photon can bring is at least equal to the smallest quantum of energy that the other particle can accept.

In a transparent material such as glass the photons of all frequencies are transmitted by being absorbed and re-emitted, since this represents a longer path than the light would experience while travelling through air it results in a slowing down of light and a speeding up once again as it exits the medium.  Also since the speed of light undergoes a change it is obvious that the wavelength must also increase according to the relation $$\lambda = \frac{c}{\nu}$$.
« Last Edit: 29/07/2015 16:32:49 by McQueen »
“Sometimes a concept is baffling not because it is profound but because it’s wrong.”

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #30 on: 29/07/2015 12:38:24 »
Good. Let me know what you grasped (or not).

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lightarrow

#### lightarrow

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##### Re: How does light speed up when it exits a denser material?
« Reply #31 on: 29/07/2015 13:06:29 »
When you calculate the path and speed of a photon through a block of glass, you must do a calculation at the boundaries of the glass block, because the conditions change (ie glass has a higher refractive index than air), and this affects the speed of the photon. It does not matter whether you are using classical optics or infinite-dimensional Hilbert spaces to do the calculation.
No, if you use classical optics and/or classical electrodynamics you calculate the path, speed, phase, etc, of a light beam or of an em wavefront, not of a photon, because "photon" is a quantistic concept which have to be treated quantistically. Infact in all of these discussions we could better never talk of photons at all; to talk of photons it's a sort of "trend".
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Quote
You are mixing classical and quantum concepts here. If you want to talk of Maxwell's equations, then forget the word "photon".
The fact that QED is able to unify the behaviour of photons & electrons makes it better than classical physics; maybe one day we will have a theory that includes gravity too, and that will be even better than QED.
But the existence of a "better" theory does not necessarily render the old theory invalid.
Certainly, I'm the first to say that, in most of the cases, we could and should better talk of classical electrodynamics only. But it was you to write "No, an electron does not obey Maxwell's equations (in particular, it cannot travel at c); a photon does". In case it's the opposite...
But anyway is an incorrect statement for what I've already written.
Quote
The new theory is only considered valid because in the "classical limit", it predicts the same results as the experiments which had already been successfully explained by the classical theory.
So in selecting a method to solve a particular problem, it is important to know how accurate an answer do you need?
All design calculations are approximations; will the approximations of a given method yield an answer which is "good enough" for the problem at hand? This means you must understand the domain of applicability of the different theories:
• Where they will give essentially identical answers
• Where they will give answers that are "close enough", and you can build a prototype and tweak it to correct the discrepancies that occur because real objects are never exactly the same as even the best theoretical models.
• Where they give divergent answers, and you must use the more complex theory (but you still need to build the prototype!).
In cases 1 & 2, the selection is essentially made on pragmatic grounds like simplicity and accuracy (the proverbial "back of the envelope calculation" may be all you can afford in the middle of a discussion).
I agree with you, but as I said, we can't answer the question "the photon which enters the glass is the same which exits?" without using QED, at least I'm pretty sure it's impossible to do it with Maxwell's equations only.
Quote
For myself, my maths classes did not go beyond eigenvectors on finite dimensional vector spaces, so I don't have all the mathematical tools needed for determining the eigenvalues of infinite-dimensional Hilbert spaces.
However, I would argue that you do not need these advanced mathematical tools to calculate the path and speed of a photon through a glass block, because this is in the domain of classical optics. The answers are "close enough", provided the size of the block of glass is much larger than the wavelength of light, and the size of the atoms in the glass is far smaller than the wavelength of the light. You just need to recognize that the classical term "a ray of light" is an approximation to "a stream of photons" in quantum theory.
I wish it were enough!
Quote
Undoubtedly, a grand unified theory which includes gravity (or even a small extension of current theories which is able to explain why calculations on Feynman diagrams work so well) will require additional, even more complex mathematical tools in your toolkit.
You don't always need a sledgehammer to crack a nut...
Do you agree, at least, that a photon is an excitation of the em field and that the field outside the glass is not equal to field inside?

--
lightarrow
« Last Edit: 29/07/2015 13:10:34 by lightarrow »

#### RTCPhysics

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##### Re: How does light speed up when it exits a denser material?
« Reply #32 on: 02/08/2015 10:42:34 »
I have never read anywhere that wave amplitude is a factor. Do you have any references for that?

It was the absence of references that caught my attention. All the diagrams of the electro-magnetic (e-m) spectrum, that I have seen, have always shown each photon as having the same amplitude, whatever the wavelength. But if, in practice, photons can vary their amplitude, then it is reasonable to conclude that photons interacting with the electrons on the surface of a denser ‘isotropic’ medium, conserve their energy by exchanging velocity for amplitude upon entry and  amplitude for velocity upon exit, rather than increasing or decreasing their wavelength.

But if it is a fact that the wavelength of a photon changes, then how does this actually happen? Relativity is one possibility. For a photon to hold its energy level constant, measured in cycles per second, upon entering an isotropic medium implies that the intervals of time that it experiences remain unchanged, but the shortening of its wavelength to slow it down implies that it experiences a reduction in the distance intervals of space as it traverses through the isotropic medium, reverting back to its original velocity upon exit. However, this explanation of the reduced velocity by the application of relativity theory to photons is another with no references!

As you will have read by now, there are many more informed and interesting replies which rebut the logic of an amplitude variation in the photon, restricting its role to interference effects, so unless there is a path to rebutting them, amplitude variation remains what it is, a personal idea.

#### evan_au

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##### Re: How does light speed up when it exits a denser material?
« Reply #33 on: 02/08/2015 12:04:49 »
Quote from: RTCPhysics
if it is a fact that the wavelength of a photon changes, then how does this actually happen?
The wavelength of any wave is directly related to the velocity: v=nλ, where:
• v: velocity (=c for light in a vacuum)
• n: frequency in Hz
• λ: Wavelength in m
If the frequency remains the same, then reducing the velocity as a photon enters a block of glass causes a proportional reduction in wavelength inside the glass. This is explainable using classical optics, without reference to relativity.

Quote
rebut the logic of an amplitude variation in the photon
The photon's energy is exchanged between an electric field and magnetic field as it propagates through space (as per Maxwell, applied to a single photon).

Inside a block of glass, the permittivity ε≈4ε0, which results in a higher refractive index for glass.

We can draw analogy with another case where ε>ε0, which is a capacitor. The energy stored in a capacitor is E=½CV2; the higher the dielectric ε, the larger the capacitance (for the same physical arrangement of the plates).

If you apply the same amount of energy to a capacitor with vacuum as the dielectric, and also to a capacitor with glass as the dielectric (εr≈4), you find that the strength of the electric field inside the glass dielectric is halved, ie the amplitude of the electric field is halved.

The energy of the photon does not change as it enters, transits or exits a block of glass. If the above analogy holds(?), the amplitude of the photon's electric field is reduced when it enters glass.

On the other hand, normal silica glass is almost non-magnetic, so the permeability μ ≈ μ0, and the amplitude of the magnetic field of the photon is not really affected while propagating through glass.

The velocity of the photon is determined by both the permeability & permittivity of the medium.

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #34 on: 02/08/2015 18:57:21 »
Good. Let me know what you grasped (or not).

--
lightarrow

At the moment I am reading through quantum physics. I now understand the double slit experiment and the relationship between photon wave length and measurement. Also exactly why position and momentum cannot be measured together. I have started on complex vector spaces and bra ket notation and am up to complex conjugates. When I get further I may have questions.

#### McQueen

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##### Re: How does light speed up when it exits a denser material?
« Reply #35 on: 03/08/2015 01:34:52 »
Quote
as a photon enters a block of glass causes a proportional reduction in wavelength inside the glass.
Surely you mean a proportionate increase in wave - length ?
“Sometimes a concept is baffling not because it is profound but because it’s wrong.”

#### chiralSPO

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##### Re: How does light speed up when it exits a denser material?
« Reply #36 on: 03/08/2015 03:14:10 »
Quote
as a photon enters a block of glass causes a proportional reduction in wavelength inside the glass.
Surely you mean a proportionate increase in wave - length ?

no, a reduction of wavelength would be expected when light enters the glass from air or space (or water).

#### Colin2B

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##### Re: How does light speed up when it exits a denser material?
« Reply #37 on: 03/08/2015 08:42:12 »
Quote
as a photon enters a block of glass causes a proportional reduction in wavelength inside the glass.
Surely you mean a proportionate increase in wave - length ?
λ=v/f
f doesn't change, so if the light slows wavelength reduces.
and the misguided shall lead the gullible,
the feebleminded have inherited the earth.

#### timey

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##### Re: How does light speed up when it exits a denser material?
« Reply #38 on: 04/08/2015 00:24:34 »
At the moment I am reading through quantum physics. I now understand the double slit experiment and the relationship between photon wave length and measurement. Also exactly why position and momentum cannot be measured together. I have started on complex vector spaces and bra ket notation and am up to complex conjugates. When I get further I may have questions.

I happened upon this most interesting thread by chance, but nearly fell off my chair when I read this post!  You have made comment on my thread about there being limits to my understanding.  I want you to know that I have long since read through quantum physics, "Quantum" by Manjit Kumar as well as touching on the subject in the dozens of physics books I've read over the last 5 years.  I've also studied complex vector spaces and bra ket notation and complex conjugates with Professor Susskind on YouTube on ""The Theoretical Minimum Lectures, I'm on lecture 8 of the GR lectures at present.  I want you to know that I understand mathematics but I just don't have a flair for the notation in much the same way that I can understand musical notation but am incapable of playing unless by ear.  The concepts of the physics themselves I find really rather simple.

« Last Edit: 04/08/2015 02:09:39 by timey »

#### timey

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##### Re: How does light speed up when it exits a denser material?
« Reply #39 on: 04/08/2015 12:37:10 »
It has been mentioned above as to an exchange of energies between the light and the medium it is passing through affecting frequency.

IF... (I have my ideas, but I'll say 'for whatever reason')... If the length/rate of a second is not a constant then Planck's h constant can actually be considered as a variable.  If velocity is measured by seconds that are not constant then the length measurement is affected.

We know that mass and a gravity field affect the phenomenon of time dilation...

IF we can view the experience of time from the perspective of the quantum world as being of a different length/rate of second in relation to our own experience of time, then Planck's h constant becomes an inadequate tool in the measurement of lengths/frequencies.

IF the quantum world experiences time at a rate much slower to our own experience, Planck's h constant would actually be 'blocking' the view of quantum activity.

The implications of this notion are that the quantum world may not be granular but may in fact be as smooth as the macroscopic world, and if Planck's h constant were to be variable in accordance to the variability of the length/rate of a second... then perhaps it would be possible to measure both location and velocity simultaneously.

Under the remit of this notion, the energies exchanged between the light and the medium of the glass would be time related.

#### Aquarius

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##### Re: How does light speed up when it exits a denser material?
« Reply #40 on: 04/08/2015 14:09:06 »
You are misunderstanding what a photon is and how it travels, if you look on my post TOE fun you will I have demistified it and that includes the slit experiment apparent wavelike effects as well and much more. []

#### timey

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##### Re: How does light speed up when it exits a denser material?
« Reply #41 on: 04/08/2015 14:24:54 »
Erm...Nope,sorry I am not misunderstanding the photon or how it travels.  What I am doing is extrapolating a notion regarding Planck's h constant with regards to measuring frequency and relating it to quantum light theory...but I'll gladly check out your thread.

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #42 on: 04/08/2015 18:05:06 »

IF the quantum world experiences time at a rate much slower to our own experience, Planck's h constant would actually be 'blocking' the view of quantum activity.

The implications of this notion are that the quantum world may not be granular but may in fact be as smooth as the macroscopic world, and if Planck's h constant were to be variable in accordance to the variability of the length/rate of a second... then perhaps it would be possible to measure both location and velocity simultaneously.

Poppycock.

#### timey

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##### Re: How does light speed up when it exits a denser material?
« Reply #43 on: 04/08/2015 19:25:24 »

Poppycock.

I do so hate it when people quote a portion of your presentation as if that portion was going to make sense when not attached to the rest...and then insult it without providing an explanation of their discordance.

JefferyH, are you saying that there is no varition in the length of a second?  Or are you saying that Plancks h constant takes into account variations in the length of a second?

Oh wait...that's right, you're not really saying anything at-all are you?

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #44 on: 04/08/2015 20:28:56 »

IF the quantum world experiences time at a rate much slower to our own experience, Planck's h constant would actually be 'blocking' the view of quantum activity.

The implications of this notion are that the quantum world may not be granular but may in fact be as smooth as the macroscopic world, and if Planck's h constant were to be variable in accordance to the variability of the length/rate of a second... then perhaps it would be possible to measure both location and velocity simultaneously.

Poppycock.

No it was the statement that you may be able to measure position and velocity or more properly momentum simultaneously. You say you understand quantum physics and yet your statements indicate the opposite.

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #45 on: 04/08/2015 20:37:07 »

#### timey

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##### Re: How does light speed up when it exits a denser material?
« Reply #46 on: 04/08/2015 22:06:58 »
As far as I am aware JefferyH, I am not questioning the mathematical structure of the Dirac delta function or of the momentum operator.  These are  considerations that were formulated after Plancks discovery of quanta and his dirivation of the h constant which is an action of a time or a length, whereas Planck tried, to no avail, to iron out the quantum nature of his findings.

Now if you could actually relate the first half of the post that you didn't quote, to the part that you did quote - then we might even be in danger of having a proper conversation!
« Last Edit: 04/08/2015 22:10:38 by timey »

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #47 on: 04/08/2015 23:00:05 »
It has been mentioned above as to an exchange of energies between the light and the medium it is passing through affecting frequency.

IF... (I have my ideas, but I'll say 'for whatever reason')... If the length/rate of a second is not a constant then Planck's h constant can actually be considered as a variable.  If velocity is measured by seconds that are not constant then the length measurement is affected.

You may be confusing proper time with coordinate time. Proper time runs at a constant rate and is always in a local frame of reference. Coordinate time changes when viewed from remote frames. Planck's constant is just that, a constant. You cannot consider it as a variable. That is what the term constant means.

We know that mass and a gravity field affect the phenomenon of time dilation...

I have no idea why you should choose the word affect in this context as they are the very reason for time dilation.

IF we can view the experience of time from the perspective of the quantum world as being of a different length/rate of second in relation to our own experience of time, then Planck's h constant becomes an inadequate tool in the measurement of lengths/frequencies.

If you are writing in terms of Planck's constant then the second is the wrong base. You then need to be thinking in units of Planck time. You can't mix the two and hope to get meaningful answers.

IF the quantum world experiences time at a rate much slower to our own experience, Planck's h constant would actually be 'blocking' the view of quantum activity.

The implications of this notion are that the quantum world may not be granular but may in fact be as smooth as the macroscopic world, and if Planck's h constant were to be variable in accordance to the variability of the length/rate of a second... then perhaps it would be possible to measure both location and velocity simultaneously.

Under the remit of this notion, the energies exchanged between the light and the medium of the glass would be time related.

The rest is unsubstantiated speculation.

#### timey

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##### Re: How does light speed up when it exits a denser material?
« Reply #48 on: 05/08/2015 00:14:41 »
Nope, I'm not confusing these relativity/space time related considerations of proper time or co-ordinate time.  I do have my own ideas on time - but this thread is not my thread, nor is it in the new theories section...therefore for the purposes of this conversation I think it within the remit of accepted physics to say that we have time as a measurement and, as a separate issue, that we observe the occurrence of the phenomenon of time... and that this time is subject to change in the rate of its occurrence due to changes in the gravity field.

Of course my post/posts are entirely speculative, , is there something wrong with that?

IF we can view gravity fields on a smaller scale, as in our own personal gravity field, ie: if we get fatter we feel heavier, if we go to the moon we feel lighter... on the basis that a black hole will change the rate of time fairly drastically, 'maybe' the gravity fields of the quantum world are 'differently oriented' in relation to to time dilation.

The smallest quantum package is delivered at an unbelievably small fraction of a 'second'.  Clearly Planck has used a time measurement factor in the derivation of this constant... But IF the quantum world is operating within a time structure that is different, ie not occurring at the same rate as our own, the time measurement that Planck has used to derive these units of Planck time as constant would be rendered inadequate in measuring the entirety of the quantum world.  The gaps between the quantum leaps being the point of relevance here.

#### jeffreyH

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##### Re: How does light speed up when it exits a denser material?
« Reply #49 on: 05/08/2015 01:12:30 »
Nope, I'm not confusing these relativity/space time related considerations of proper time or co-ordinate time.  I do have my own ideas on time - but this thread is not my thread, nor is it in the new theories section...therefore for the purposes of this conversation I think it within the remit of accepted physics to say that we have time as a measurement and, as a separate issue, that we observe the occurrence of the phenomenon of time... and that this time is subject to change in the rate of its occurrence due to changes in the gravity field.

Of course my post/posts are entirely speculative, , is there something wrong with that?

IF we can view gravity fields on a smaller scale, as in our own personal gravity field, ie: if we get fatter we feel heavier, if we go to the moon we feel lighter... on the basis that a black hole will change the rate of time fairly drastically, 'maybe' the gravity fields of the quantum world are 'differently oriented' in relation to to time dilation.

The smallest quantum package is delivered at an unbelievably small fraction of a 'second'.  Clearly Planck has used a time measurement factor in the derivation of this constant... But IF the quantum world is operating within a time structure that is different, ie not occurring at the same rate as our own, the time measurement that Planck has used to derive these units of Planck time as constant would be rendered inadequate in measuring the entirety of the quantum world.  The gaps between the quantum leaps being the point of relevance here.

Well now you are clearer in meaning and actually saying something interesting.