Does light accelerate

[timharvey027]

Consider a photon emitted by a photon emitter, it travels through a vacuum and then into a block of glass, it travels slower through this than the vacuum right? It leaves the block of glass and continues through the vacuum until it hits its target.

Q1. When it leaves the block of light where does it get the energy to accelerate? 

Q2.  Where does it accelerate? If it does not how can it go from one speed to another without doing so? 

Q3. What is it's speed when it is halfway out of the block of glass?

Q4. Consider a beam of light as a wave instead of a photon, same questions.

[StringJunky]

10 minutes ago, timharvey027 said:

Consider a photon emitted by a photon emitter, it travels through a vacuum and then into a block of glass, it travels slower through this than the vacuum right? It leaves the block of glass and continues through the vacuum until it hits its target.

Q1. When it leaves the block of light where does it get the energy to accelerate? 

Q2.  Where does it accelerate? If it does not how can it go from one speed to another without doing so? 

Q3. What is it's speed when it is halfway out of the block of glass?

Q4. Consider a beam of light as a wave instead of a photon, same questions.

The apparent slowdown is caused by the photon accumulating a delay time by briefly interacting with and being emitted from electrons as it passes through but at all times it is transmitted it travels at c.... there is never any acceleration.

 

Edited February 11, 2020 by StringJunky

[iNow]

No. The photon does not accelerate or slow down. It always travels at c, by definition. The "slowdown" is only in your perception due to (as StringJunky highlighted) the photon bouncing around and interacting with more stuff in the glass... It travels farther, basically.

[timharvey027]

 

 

Prof Merrifield and Moriaty say it does, confused now.

[iNow]

40 minutes ago, timharvey027 said:

Q1. When it leaves the block of light where does it get the energy to accelerate? 

Nowhere. It doesn't accelerate

40 minutes ago, timharvey027 said:

Q2.  Where does it accelerate? If it does not how can it go from one speed to another without doing so? 

Nowhere. It doesn't accelerate

41 minutes ago, timharvey027 said:

Q3. What is it's speed when it is halfway out of the block of glass?

c. It's speed is invariant. It's speed is c

41 minutes ago, timharvey027 said:

Q4. Consider a beam of light as a wave instead of a photon, same questions.

Irrelevant

If I travel from my home to the store in a straight line at 30 mph, it takes 10 minutes to get there. If I later travel from my home to the store using back roads and take several turns and go up and down some hills, but still do so at a speed of 30 mph, it takes me 20 minutes to get there. I was always traveling at 30 mph, though. I just went a greater distance to get there the 2nd time.

[Strange]

31 minutes ago, timharvey027 said:

Prof Merrifield and Moriaty say it does, confused now.

As it is a video, I have no idea who Merrifield and Moriaty are, what they say, whether it is correct or misleading. 

On the other hand, the comments made by other posters here appear to be more or less correct.

[StringJunky]

Photons int

46 minutes ago, timharvey027 said:

Prof Merrifield and Moriaty say it does, confused now.

To elaborate: Photons interact/get absorbed permanently  with electrons when they have the right wavelength. Those that do will not pass through if they have, rendering the material opaque at those wavelengths when observed. With transparent material, like glass, photons in the visible spectrum don’t have the right wavelength to be absorbed by the electrons, so they are only absorbed briefly and then. re-emitted for a brief but finite time. The important thing to realise is that as it travels in between electrons it is moving at c. Each time it bumps  into an electron its total travel time is extended, hence, giving the appearance of slowing down.

Edited February 11, 2020 by StringJunky

[swansont]

I am not going to watch a 16 minute video that you claim supports your view. The video may be mistaken, or you may be misunderstanding what they say in their alleged support.

 

1 hour ago, timharvey027 said:

Consider a photon emitted by a photon emitter, it travels through a vacuum and then into a block of glass, it travels slower through this than the vacuum right? It leaves the block of glass and continues through the vacuum until it hits its target.

Q1. When it leaves the block of light where does it get the energy to accelerate? 

Q2.  Where does it accelerate? If it does not how can it go from one speed to another without doing so? 

Q3. What is it's speed when it is halfway out of the block of glass?

Q4. Consider a beam of light as a wave instead of a photon, same questions.

Q1. No energy needed. The total energy involved remained constant. While in the material the photon is interacting with the material, and this cannot be ignored in considering conservation of energy.

Q2. As others have stated, the photon always travels at c. But time is spent interacting (virtually) with the medium.

Q3. Nonsensical question. There is no "halfway out." The border is not well-defined at this scale, and you either have a photon or you don't. It's not a scaled-down version of a submerged beach ball rising out of a pool.

Q4. The classical picture is a little different, but the notion of light interacting with the medium and consequently slowing down the speed of propagation still holds. It just does so in with the bulk medium, rather than on an individual particle basis. The wave outside of the medium travels at c, the wave inside at c/n.

46 minutes ago, StringJunky said:

 To elaborate: Photons interact/get absorbed permanently  with electrons when they have the right wavelength. Those that do will not pass through if they have, rendering the material opaque at those wavelengths when observed. With transparent material, like glass, photons in the visible spectrum don’t have the right wavelength to be absorbed by the electrons, so they are only absorbed briefly and then. re-emitted for a brief but finite time. The important thing to realise is that as it travels in between electrons it is moving at c. Each time it bumps  into an electron its total travel time is extended, hence, giving the appearance of slowing down.

To add to this: it's because the interaction is with a virtual state of the atom in the medium - the photon can't impart energy or momentum to the atom because there is no state to do so with, so the photon must be re-emitted with the same energy and momentum, with none imparted to the atom.

[Ghideon]

3 hours ago, timharvey027 said:

Prof Merrifield and Moriaty say it does, confused now.

Where in the video is something stated that supports that? I can't hear them say acceleration. 

 

The video is actually not totally bad I think. He shows some common "explanations" and why they are incorrect. Then he gives a classical and a quantum mechanical description of speed of light in matter. The end of video there is a quick discussion about polariton* where the mathematics model treat light as having mass while it interacts with matter. Video does not claim light to have mass, only that there are physicists modeling the light that way in certain interactions. But do not find anything about accelerating light. 

 

 

*)  https://en.wikipedia.org/wiki/Polariton. 

[Strange]

13 minutes ago, Ghideon said:

Where in the video is something stated that supports that?

And there is the problem with using video as a source.

[Leon1961]

4 hours ago, timharvey027 said:

Consider a photon emitted by a photon emitter, it travels through a vacuum and then into a block of glass, it travels slower through this than the vacuum right? It leaves the block of glass and continues through the vacuum until it hits its target.

Q1. When it leaves the block of light where does it get the energy to accelerate? 

Q2.  Where does it accelerate? If it does not how can it go from one speed to another without doing so? 

Q3. What is it's speed when it is halfway out of the block of glass?

Q4. Consider a beam of light as a wave instead of a photon, same questions.

A photon propagates in a certain manner and this manner of propagation depends on both the photon and the medium in which it propagates. When the medium changes, the manner of propagation changes.

A1: There is no acceleration in the sense of gaining or losing energy. There is only interaction with the medium causing the manner of propagation to change.

A2: This change happens instantaneous, but only to the part of the photon that crosses from one medium to the next.

A3: When it is half way in, one half propagates differently than the other half.

A4: Light can be considered to have wave properties. This does not mean that light can be considered as a wave any more than I can be considered to be a dog when I exhibit dog properties when I bark like a dog. In any case, the part of the beam in one medium propagates differently to the part of the beam in another medium.

In all cases, energy is preserved.

[StringJunky]

1 hour ago, Leon1961 said:

A photon propagates in a certain manner and this manner of propagation depends on both the photon and the medium in which it propagates. When the medium changes, the manner of propagation changes.

A1: There is no acceleration in the sense of gaining or losing energy. There is only interaction with the medium causing the manner of propagation to change.

A2: This change happens instantaneous, but only to the part of the photon that crosses from one medium to the next.

A3: When it is half way in, one half propagates differently than the other half.

A4: Light can be considered to have wave properties. This does not mean that light can be considered as a wave any more than I can be considered to be a dog when I exhibit dog properties when I bark like a dog. In any case, the part of the beam in one medium propagates differently to the part of the beam in another medium.

In all cases, energy is preserved.

Read swansont's post.

Edited February 11, 2020 by StringJunky

[studiot]

1 hour ago, Leon1961 said:

In all cases, energy is preserved.

I feel this could be a misleading statement.

What about attenuation ?

 

[swansont]

2 hours ago, Leon1961 said:

A photon propagates in a certain manner and this manner of propagation depends on both the photon and the medium in which it propagates. When the medium changes, the manner of propagation changes.

A1: There is no acceleration in the sense of gaining or losing energy. There is only interaction with the medium causing the manner of propagation to change.

A2: This change happens instantaneous, but only to the part of the photon that crosses from one medium to the next.

A3: When it is half way in, one half propagates differently than the other half.

How does this sharp division you propose jibe with the Heisenberg uncertainty principle?

 

2 hours ago, Leon1961 said:

A4: Light can be considered to have wave properties. This does not mean that light can be considered as a wave any more than I can be considered to be a dog when I exhibit dog properties when I bark like a dog. In any case, the part of the beam in one medium propagates differently to the part of the beam in another medium.

Things don’t behave the same at the quantum level as they do in the macroscopic world just because you want them to.

1 hour ago, studiot said:

I feel this could be a misleading statement.

What about attenuation ?

 

The discussion suggests we’re ignoring it for now (it would be an irrelevant complication) and in any event, attenuation conserves energy.

[michel123456]

Questions:

1. Is the entering photon (called A) the same as the one going out of the material , or is it some other photon (called B)?

2. If inside the material the photon travels at c (along a longer path) it means that there is void in the material. And if there is void, how is it possible that a photon cannot travel through the material without encountering obstacles? (and thus travel through the material at c?)

 

[Strange]

52 minutes ago, michel123456 said:

1. Is the entering photon (called A) the same as the one going out of the material , or is it some other photon (called B)?

This is not really a scientific question.

It normally has identical properties. In which case the question doesn't really mean anything. 

Unless the material has some property such as changing polarization or performing parametric down conversion (splitting the photon into two photons with half the energy) or similar. In which case whether you consider it to be the same photon or not depends how you choose to define "same".

55 minutes ago, michel123456 said:

2. If inside the material the photon travels at c (along a longer path) it means that there is void in the material. And if there is void, how is it possible that a photon cannot travel through the material without encountering obstacles? (and thus travel through the material at c?)

There is no void it interacts with the electrons it encounters. As this is an interaction with the field created by all the electrons in the material, I believe this is a continuous process (unlike, for example, the photoelectric effect where a photon is completely absorbed and later emitted by an atom).

[swansont]

2 hours ago, michel123456 said:

Questions:

1. Is the entering photon (called A) the same as the one going out of the material , or is it some other photon (called B)?

No way to tell, since (as Strange ha pointed out) the photons are identical particles.

2 hours ago, michel123456 said:

2. If inside the material the photon travels at c (along a longer path) it means that there is void in the material. And if there is void, how is it possible that a photon cannot travel through the material without encountering obstacles? (and thus travel through the material at c?)

It travels a path in which it undergoes interactions which take time. "along a longer path" is ambiguous. (One interpretation I have makes it wrong, and the other is irrelevant)

The notion of a "void" doesn't really apply here. It can be a uniform material. There are atoms, which have some cross-section for interaction, and the photon.  These aren't tiny ball bearings undergoing collisions. The real question is: how could the photon not undergo an interaction under the conditions we're talking about?

 

 

[Leon1961]

22 hours ago, swansont said:

How does this sharp division you propose jibe with the Heisenberg uncertainty principle?

Things don’t behave the same at the quantum level as they do in the macroscopic world just because you want them to.

1: About Heisenberg: Agreed, half way in and half way out is not something that can ever be said to exist. But exactly how much is inside or outside a certain medium is not relevant for my answer and I am not writing a scientific paper here, so I (over?) simplified things.

2: This is not about me wanting anything. All I am saying is that exhibiting properties of something is not evidence of “being” something. In other words: considering a beam of light as a wave is an unfounded assumption. Mind you, with that I am not saying that light consists of particles because exhibiting particle properties is not evidence of being a particle also.

[swansont]

11 hours ago, Leon1961 said:

1: About Heisenberg: Agreed, half way in and half way out is not something that can ever be said to exist. But exactly how much is inside or outside a certain medium is not relevant for my answer and I am not writing a scientific paper here, so I (over?) simplified things.

2: This is not about me wanting anything. All I am saying is that exhibiting properties of something is not evidence of “being” something.

The point is you can't treat a material as if it has a distinct edge at the quantum level, or a particle like it's a hard sphere. You did so, and without any scientific justification.

 

11 hours ago, Leon1961 said:

In other words: considering a beam of light as a wave is an unfounded assumption.

Unfounded? Do they not exhibit diffraction and interference? Are these not behaviors of waves? 

 

11 hours ago, Leon1961 said:

Mind you, with that I am not saying that light consists of particles because exhibiting particle properties is not evidence of being a particle also.

On the contrary, exhibiting particle properties is evidence of particle behavior. It's a tautology.

[between3and26characterslon]

 

On 2/11/2020 at 4:27 PM, timharvey027 said:

**removed link to video as you can scroll up if you want to watch it**

Prof Merrifield and Moriaty say it does, confused now.

I saw this just a few days ago which gives a similar explanation to the video you posted but with a little more detail.  06:15 for the explanation to start

 

https://www.youtube.com/watch?v=CUjt36SD3h8

[Country Boy]

On 2/11/2020 at 10:27 AM, timharvey027 said:

 

 

Prof Merrifield and Moriaty say it does, confused now.

Did you listen carefully to this?  The first person clearly say "light appears to slow down" while the other two  are referring to the average speed of light through the substance.  They are all saying the same thing everyone here is saying:  Light moves from one atom to another at speed c but takes some time interacting with each atom or molecule so that the average speed through the substance is lower than c. 

[between3and26characterslon]

18 minutes ago, Country Boy said:

Did you listen carefully to this?  The first person clearly say "light appears to slow down" while the other two  are referring to the average speed of light through the substance.  They are all saying the same thing everyone here is saying:  Light moves from one atom to another at speed c but takes some time interacting with each atom or molecule so that the average speed through the substance is lower than c. 

He literally said, "I want to prove to you that light really does slow down" (01:12) and he did not mention the average speed of light through the substance.

light does not bounce around between atoms nor is it absorbed and re-emitted otherwise fiber optics and prisms would not work. Let's remember the speed of light is c in vacuo

 

He gave 3 explanations

1. Light is a wave and therefore oscillates, there are EM oscillations of the atoms in the medium, the super position of these two waves propagates through the medium at less than c

2. light travels at the average speed of all possible paths through the medium (Young's' slits was used as an example)

3. Light ceases to be a photon and becomes a Polariton

[Strange]

2 minutes ago, between3and26characterslon said:

He literally said, "I want to prove to you that light really does slow down" (01:12) and he did not mention the average speed of light through the substance.

And that is why videos are such a terrible source. How can I know what they said in the video. But I do know what physics says, and it says that what you claim they said is wrong.

3 minutes ago, between3and26characterslon said:

light does not bounce around between atoms nor is it absorbed and re-emitted otherwise fiber optics and prisms would not work

Fibre optics and prisms only work because the photons do interact with the atoms in the material.

 

[between3and26characterslon]

2 minutes ago, Strange said:

And that is why videos are such a terrible source. How can I know what they said in the video. But I do know what physics says, and it says that what you claim they said is wrong.

Fibre optics and prisms only work because the photons do interact with the atoms in the material.

 

You could watch the video and you would hear what they say, it may not be a great source but it is what the OP is referring to.

The link I posted is a video from Fermilab with Dr Don Lincoln who is easily found on the Fermilab website so there is some pedigree.

I didn't say photons do not interact with atoms, the super position of EM oscillations is after all interaction, I said they don't bounce around and are not absorbed and re-emitted (to clarify, some wave lengths are and some are not, depending on the medium). As far as I'm aware photons do not have a memory and so could not resume a path parallel to their original path.

With fiber optics and prisms I'm not referring to total internal reflection at the boundary, I'm referring to the propagation of light through the medium before it gets to the boundary. If light bounced or was re-emitted it would go in all directions.

[swansont]

55 minutes ago, between3and26characterslon said:

light does not bounce around between atoms nor is it absorbed and re-emitted otherwise fiber optics and prisms would not work. Let's remember the speed of light is c in vacuo

You have to be careful, here. "light" and "photons" are not synonymous with each other, and don't mix quantum and classical models.

The speed of light is smaller in a medium. The speed of photons is still c.