Jump to content

Does light accelerate


timharvey027
 Share

Recommended Posts

19 minutes ago, between3and26characterslon said:

You could watch the video and you would hear what they say

Or maybe I can't.

Or maybe I don't want to. Because it is a video and therefore a truly terrible way of communicating information. What next, interpretive dance?

Or maybe I watch it and then say, "no you are both wrong, what they actually say is that the photons get eaten by dragons".

If you can't find a proper source to support what you claim, then I am not going to take your claims seriously.

22 minutes ago, between3and26characterslon said:

I said they don't bounce around and are not absorbed and re-emitted

And that would be a strawman argument.

Link to comment
Share on other sites

19 hours ago, Leon1961 said:

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.

I have no problem with this.
Photons or EM waves are neither classical particles nor waves.
But they do have properties of both, depending on what is being measured.
I always use the term quantum particles, which should convey a totally different meaning.
 

 

1 hour ago, between3and26characterslon said:

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

There is a cross-section for the interaction which depends on wavelength and atomic/molecular make-up ( for the purpose of this discussion ).
Otherwise any material could be used for fiber optic cable.
 

 

58 minutes ago, between3and26characterslon said:

As far as I'm aware photons do not have a memory and so could not resume a path parallel to their original path.

No memory.
But they do have momentum; and that is  required to be conserved.

Link to comment
Share on other sites

1 hour ago, between3and26characterslon said:

 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.

If there is no real transition for them, how can they do otherwise?

As MigL has pointed out, momentum must be conserved. Energy, too. If none is imparted to the atom (no real transition) then how can the photon's energy and momentum change? Where does it go?

Link to comment
Share on other sites

21 minutes ago, swansont said:

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.

Not something I'd considered/knew. In one of the videos they say that any of the component waves could be c but the super position of waves would be less than c. Does that mean the speed of the photon is c but the speed of light is <c

I don't understand this.

 

21 minutes ago, Strange said:

Or maybe I can't.

Or maybe I don't want to. Because it is a video and therefore a truly terrible way of communicating information. What next, interpretive dance?

Or maybe I watch it and then say, "no you are both wrong, what they actually say is that the photons get eaten by dragons".

If you can't find a proper source to support what you claim, then I am not going to take your claims seriously.

And that would be a strawman argument.

And you are under no obligation to watch it, I thought it would help in understanding the OP's reasoning.

The link I posted, as I stated, is By Dr Don Lincoln from Fermilab. This is verifiable by going to the Fermilab website which is what I did. So I think watching a video from an expert in his field from a world renowned establishment at the forefront of scientific thinking is a reasonable source for a discussion on a forum.

I don't believe I provided a strawman argument.

If you put a pencil in a glass of water it appears to be broken due to the refraction of light in/by water, the direction light that leaves the glass is parallel to the path it would have taken if the glass was not there. Their explanation was that if light bounced between atoms or was re-emitted you would not see a clear image of the pencil in the water, the light would be scattered and the image defuse.

This makes sense to me but if it is wrong perhaps you could explain how light can leave the glass parallel to its original path and still showing a clear image of the pencil

14 minutes ago, MigL said:

There is a cross-section for the interaction which depends on wavelength and atomic/molecular make-up ( for the purpose of this discussion ).

Otherwise any material could be used for fiber optic cable.
 

 

No memory.
But they do have momentum; and that is  required to be conserved.

I understand that cheese would not make a good fiber optic cable. If glass is transparent to red light and red light is slower in glass than in vacuo then it cannot be slower because it is bouncing around otherwise it would be defuse would it not? it would be like cheese.

Does conserving momentum equate to conserving direction? If a photon can bounce around in any direction when within the medium how does it acquire a very specific direction when it leaves the medium?

Link to comment
Share on other sites

1 hour ago, between3and26characterslon said:

I don't believe I provided a strawman argument.

No one says that the reason that light is slower in a material is because it is "absorbed and re-emitted".

1 hour ago, between3and26characterslon said:

Does conserving momentum equate to conserving direction?

yes, because momentum is a vector quantity.

 

1 hour ago, between3and26characterslon said:

If a photon can bounce around in any direction when within the medium how does it acquire a very specific direction when it leaves the medium?

A small amount of light will be scattered because no material is perfectly transparent. But that is not relevant to the subject of the thread.

Link to comment
Share on other sites

22 hours ago, swansont said:

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.

Correct, you cannot treat a material as having a distinct edge. It is clear to me that my original text gives the impression that I treat them as having a distinct edge, but that is not intended. Anyway, like I tried to explain before, it is not relevant for my original answer if there is a distinct edge or not. What is relevant is that there is difference in propagation depending on the medium.

22 hours ago, swansont said:

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

Yes they are behaviors of waves. My point is that exhibiting behavior of X is not evidence of being X. Take a Zebra for example. They have an interference pattern on their sides, but that clearly was not caused by a wave. Or like I said before: if I bark like a dog then I exhibit dog behavior, but that does not make me a dog. Besides, in this case photons also exhibit particle properties which waves do not, which is a clear indication that photons cannot “be” waves.

22 hours ago, swansont said:

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

The problem here is that in the context of “particle behavior”, “particle” means solid ball or implies solidness or means something else that does not also exhibit wave behavior, while in most other cases “particle” is an object that also exhibits wave behavior. So, I will replace “particle” with “solid ball” to make a clear distinction between these two. But you must read “particle” when I write “solid ball”:

Yes, exhibiting solid ball properties is evidence of solid ball behavior but that is not evidence of being a solid ball. In this case photons also exhibit wave properties which solid balls do not, which is a clear indication that photons cannot “be” solid balls.

 

 

Edited by Leon1961
Link to comment
Share on other sites

42 minutes ago, Leon1961 said:

Correct, you cannot treat a material as having a distinct edge. It is clear to me that my original text gives the impression that I treat them as having a distinct edge, but that is not intended. Anyway, like I tried to explain before, it is not relevant for my original answer if there is a distinct edge or not. What is relevant is that there is difference in propagation depending on the medium.

Yes they are behaviors of waves. My point is that exhibiting behavior of X is not evidence of being X. Take a Zebra for example. They have an interference pattern on their sides, but that clearly was not caused by a wave. Or like I said before: if I bark like a dog then I exhibit dog behavior, but that does not make me a dog. Besides, in this case photons also exhibit particle properties which waves do not, which is a clear indication that photons cannot “be” waves.

So you are pointing out that you can't do something that careful explanations of physics don't do.

Light has wave properties/behaviors, and particle properties/behaviors. You seem to be rebutting a different argument. It's not a strawman, per se, because there are some introductory explanations out there that use that phrasing. But it's strictly for the tourists — people reading a pop-sci article, taking a survey class in physics, or from someone giving an oversimplified explanation. 

 

42 minutes ago, Leon1961 said:

The problem here is that in the context of “particle behavior”, “particle” means solid ball or implies solidness or means something else that does not also exhibit wave behavior, while in most other cases “particle” is an object that also exhibits wave behavior. So, I will replace “particle” with “solid ball” to make a clear distinction between these two. But you must read “particle” when I write “solid ball”:

Yes, exhibiting solid ball properties is evidence of solid ball behavior but that is not evidence of being a solid ball. In this case photons also exhibit wave properties which solid balls do not, which is a clear indication that photons cannot “be” solid balls.

If you've moved past the tourist level you know that classical descriptions don't apply; you don't have classical behavior and you know classical descriptions don't apply. MigL mentioned the phrase "quantum particle" which underscores that point. A quantum particle is not a little ball bearing, nor is it a water wave.

Link to comment
Share on other sites

 

14 hours ago, Strange said:

No one says that the reason that light is slower in a material is because it is "absorbed and re-emitted".

Are you sure? It looked to me that some of the earlier posts were saying exactly that hence my confusion. I cannot understand how a photon can travel in a straight line, enter a medium and bounce around or be absorbed and re-emitted in random directions and then leave the medium parallel to its original direction.

However a photon traveling in a straight line until it reaches a medium, changing direction and going in a straight line through the medium and then resuming a path parallel to its original path does make sense. Interacting with a medium is not the same as bouncing around etc..

14 hours ago, Strange said:

yes, because momentum is a vector quantity.

As above

14 hours ago, Strange said:

A small amount of light will be scattered because no material is perfectly transparent. But that is not relevant to the subject of the thread.

Of course

Edited by between3and26characterslon
Link to comment
Share on other sites

On 2/12/2020 at 12:11 AM, swansont said:

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

That rather depends upon what Tim meant v what Leon meant.

The potential confusion I refer to is that of light energy v total energy.

The light definitely looses energy in interactions with an attenuating medium.
Clearly that lost energy is gained by something else.

Talking of that loss/gain,

Can it be said that this light that is lost comes to an abrupt stop and therefore decelerates?

Edited by studiot
Link to comment
Share on other sites

23 minutes ago, between3and26characterslon said:

 

Are you sure? It looked to me that some of the earlier posts were saying exactly that hence my confusion. I cannot understand how a photon can travel in a straight line, enter a medium and bounce around or be absorbed and re-emitted in random directions and then leave the medium parallel to its original direction.

You described an inelastic scattering interaction, which implies an absorption into a real state, and re-emission from that state. That's not what others are detailing.

 

 

14 minutes ago, studiot said:

That rather depends upon what Tim meant v what Leon meant.

The potential confusion I refer to is that light energy v total energy.

The light definitely looses energy in interactions with an attenuating medium.
Clearly that lost energy is gained by something else.

Talking of that loss/gain,

Can it be said that this light that is lost comes to an abrupt stop and therefore decelerates?

That's not a description that has much meaning, because we don't know exactly what is happening during the time it takes for a photon to be absorbed in an atomic state, and the photon is gone when the process is done. There's never any point where you can say the photon has stopped or slowed down. It's there, and then it's gone. Heisenberg limits what you can say about the interaction.

Attenuation implies an absorption by a real state, and that's not what's going on with the slowdown in a medium. But the energy and momentum of a photon in a medium is not what it is in a vacuum. What exactly happens has been a long-time issue as described by the Abraham-Minkowski controversy (short version: there are two conflicting ways to describe the momentum of a photon in a medium) 

Light is interacting with the medium even without attenuation. Attenuation is an unnecessary complication to the discussion of slowdown.

Link to comment
Share on other sites

21 minutes ago, swansont said:

That's not a description that has much meaning, because we don't know exactly what is happening during the time it takes for a photon to be absorbed in an atomic state, and the photon is gone when the process is done.

Yes and furthermore that time is variable within the limits of uncertainty.

However we do say that the bottle of champagne that stops abruptly and launches a ship then smashing to a thousand pieces decelerates.

I think an alternative answer is to suggest to Tim that he should distinguish between rays of light and other descriptions.
That may be more productive.
 

Link to comment
Share on other sites

On 2/11/2020 at 4:17 PM, StringJunky said:

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.

 

On 2/11/2020 at 4:19 PM, iNow said:

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.

 

On 2/11/2020 at 4:51 PM, iNow said:

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.

 

On 2/11/2020 at 5:12 PM, 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.

 

6 minutes ago, swansont said:

You described an inelastic scattering interaction, which implies an absorption into a real state, and re-emission from that state. That's not what others are detailing.

I described what I believe others had posted so that I could demonstrate what it was I am not understanding.

"Bouncing, absorbed and re-emitted", all of those things happen and will happen to some light which is traveling through a medium which is transparent to that light i.e. the medium is not perfectly transparent to any wavelength.

But there is some light which travels through the transparent medium, is not bounced around, absorbed and re-emitted, and yet is slowed. The OP's post is that bouncing, absorbing and re-emitting is not a mechanism that adequately explains why light is slowed in a medium. This is explained in the video they posted and supported by the one I found from Fermilab.

Surely light propagates through a medium at the same speed i.e. all photons will traverse the same distance in the same time, two photons entering the medium together will exit together?

Link to comment
Share on other sites

10 minutes ago, studiot said:

Yes and furthermore that time is variable within the limits of uncertainty.

However we do say that the bottle of champagne that stops abruptly and launches a ship then smashing to a thousand pieces decelerates.

I think an alternative answer is to suggest to Tim that he should distinguish between rays of light and other descriptions.
That may be more productive.
 

Apples and oranges. "Abruptly" in the case of a macroscopic object is typically a much larger interval that we see with the HUP. It still takes a measurable amount of time, even if the naked eye can't discern it. 

1 minute ago, between3and26characterslon said:

I described what I believe others had posted so that I could demonstrate what it was I am not understanding.

"Bouncing, absorbed and re-emitted", all of those things happen and will happen to some light which is traveling through a medium which is transparent to that light i.e. the medium is not perfectly transparent to any wavelength.

And is not what is being discussed. The OP describes light that goies into a medium and makes it out the other side. It is describing light that is not absorbed. IOW, a perfectly transparent medium is a model for this. (if there is loss, we are ignoring it)

1 minute ago, between3and26characterslon said:

But there is some light which travels through the transparent medium, is not bounced around, absorbed and re-emitted, and yet is slowed. The OP's post is that bouncing, absorbing and re-emitting is not a mechanism that adequately explains why light is slowed in a medium. This is explained in the video they posted and supported by the one I found from Fermilab.

You can describe this classically or quantum mechanically. They are two descriptions of the same phenomenon. If the explanation uses waves, it's the classical description. Once you say photon, though, you must use the QM explanation, because you are invoking the particle behavior.

 

1 minute ago, between3and26characterslon said:

Surely light propagates through a medium at the same speed i.e. all photons will traverse the same distance in the same time, two photons entering the medium together will exit together?

Yes, and it takes more time for reasons that have been discussed.

Link to comment
Share on other sites

51 minutes ago, swansont said:
1 hour ago, studiot said:

 

Apples and oranges.

Not really.

Milliseconds or femtoseconds, they are still quantities of time, whereas apples are not oranges.

It has also been claimed that there is no relationship between absorbance and refractive index.

That is not true, the Kramers Konig curves are well used by spectroscopists.

image.png.2130b0dac98803c7525eeb6e57b3d6ce.png

Edited by studiot
spelling
Link to comment
Share on other sites

30 minutes ago, swansont said:

Yes, and it takes more time for reasons that have been discussed.

Sure.

And the terminology you use, "virtual state, interaction, does not impart energy or momentum"

is different from some of the terminology others used, "bounces around, absorbed and re-emitted, bumps into"

 

Link to comment
Share on other sites

57 minutes ago, swansont said:

The OP describes light that goies into a medium and makes it out the other side. It is describing light that is not absorbed.

I don't think so.

Light enters the medium. Agreed.

Some of the light passes through all of the medium and makes it out the other side. Agreed.

But.

Some of the light must make it partway through the block before it is absorbed.

So what is the difference between this light and the light that does not make it that far through the block?

Well some of it is scattered and still makes it out of the block !

That is how Raman spectroscopy works.

 

I said it is complicated and the first thing is for Tim to recognise that he is talking about rays of light not waves or photons and idealised rays at that.

Edited by studiot
Link to comment
Share on other sites

1 hour ago, between3and26characterslon said:

Sure.

And the terminology you use, "virtual state, interaction, does not impart energy or momentum"

is different from some of the terminology others used, "bounces around, absorbed and re-emitted, bumps into"

 

Swansont's description is more accurate but the principle of mine described and others alludes to the same: energy and momentum is not imparted. It's actually much more complicated and you need to understand statistics and probability to describe quantum processes properly, I think.

Edited by StringJunky
Link to comment
Share on other sites

1 hour ago, studiot said:

Not really.

Milliseconds or femtoseconds, they are still quantities of time, whereas apples are not oranges.

The issue at hand is whether or not you can see what's going on. You can see an impact even if it takes milliseconds. You just need a sufficiently fast movie camera. But you aren't going to film a photon absorption.

Quote

It has also been claimed that there is no relationship between absorbance and refractive index.

(Not claimed by me)

 

1 hour ago, between3and26characterslon said:

Sure.

And the terminology you use, "virtual state, interaction, does not impart energy or momentum"

is different from some of the terminology others used, "bounces around, absorbed and re-emitted, bumps into"

 

Not everyone in the discussion has a PhD in atomic physics. It doesn't make them wrong. You also probably don't want the conversation to be phrased as it might if the only participants were PhD physicists. Would it be helpful to you for me to say it's like the intermediate state of a Raman transition (with the expectation that I wouldn't have to do any followup explanation)?

1 hour ago, studiot said:

I don't think so.

Light enters the medium. Agreed.

Some of the light passes through all of the medium and makes it out the other side. Agreed.

But.

Some of the light must make it partway through the block before it is absorbed.

So what is the difference between this light and the light that does not make it that far through the block?

Well some of it is scattered and still makes it out of the block !

That is how Raman spectroscopy works.

Does it "hit its target" after scattering?

Do you think that the OP, or others asking questions, know what Raman scattering is, that they would consider it as part of the problem?

 

Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
 Share

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue.