Some questions about light

[Catthedragon]

Light is, to my knowledge, made up of particles that behave in a wave-like manner (just like all matter in the universe, technically). Going by that, should it not be possible to make these particles speed up or slow down? Movement is energy, and I can think of three energetic forces that might have an impact on light:

 

1. Heat. Higher temperatures make all other forms of matter speed up, I don't see why light would be an exception. I'm not sure whether this has been tested, but logically there should be some kind of difference in speed between the light being released from an immensely hot light source (let's say a star) and the light lost out in freezing space. Furthermore, what state of matter is light exactly? Plasma?

 

2. Distance. I know this is wrong, but I don't understand how waves (or wave-like particles) maintain 100% of their energy when crossing the absolute emptiness of space. Movement is energy, including wave-like movement, and therefore some of that energy should be expended. Especially in the case of mass, when it is solid, physical matter that must move back and forth. There is something going on here that I am not aware of, I'm sure. But I felt like I should mention it.

 

3. Force. As with any object, when light hits something, some of its energy should be transferred into the other object. According to this, light should slow down a bit each time it hits something, for instance an atmospheric molecule.

 

Sorry to go off topic, and sorry for asking such a childish question, but thinking about point #3, where exactly does light go? When I turn the light off in my room, why don't latent photons keep bouncing around, at least for a little while? Why does light stop? What is the end of the life of a photon? Does it just rest on something, and if so, should this not cause them to build up over lengths of time? Furthermore, what is the structure of a photon? Just some things I'm curious about.

Edited April 28, 2014 by Catthedragon

[studiot]

You need to distinguish carefully between the wave and the vibrating particles or elements carrying that wave.

 

The motion of any wave is not the same as the motion of the particles.

 

The speed of any wave is independent of the speed of the source.

 

This includes the speed of light waves however light wave are unique in that the their speed is also independent of the speed of the observer.

 

A medium is an array of linked vibrating particles.

 

Once a wave has left the source and is being transmitted by the medium its speed is normally a constant determined by the physical properties of the medium.

[Catthedragon]

You need to distinguish carefully between the wave and the vibrating particles or elements carrying that wave.

 

The motion of any wave is not the same as the motion of the particles.

 

The speed of any wave is independent of the speed of the source.

 

This includes the speed of light waves however light wave are unique in that the their speed is also independent of the speed of the observer.

 

A medium is an array of linked vibrating particles.

 

Once a wave has left the source and is being transmitted by the medium its speed is normally a constant determined by the physical properties of the medium.

Okay. Basically what I'm asking is... why. Why is the speed of a wave independent of the speed of a source? Why is the speed of a wave a constant determined by the medium? What do you mean by "source", actually? The particles forming the wave? Or where the wave originated from? Almost nothing you said made sense without context.

[studiot]

 

Almost nothing you said made sense without context.

 

 

Yeah, that's true. I was trying to establish some context before answering your 3 specific questions.

 

I don't know how much you know but the subject of wave motion takes up whole chapters of physics or applied maths texts at every level from elementary to postgraduate. There are even complete books about it.

 

The particles forming the wave?

 

 

The particles do not form the wave. Notice I said that the elements or particles 'carry' the wave.

 

A wave is not like traffic on a road. The traffic is made up (formed) from lots of individual vehicles.

 

 

You need to distinguish carefully between the wave and the vibrating particles or elements carrying that wave.

 

This is the first and most fundamental thing to know about waves, nothing else will make sense until you have understood this point.

 

Imagine a single weight hung on a spring bouncing up and down.

That is a single oscillator or vibrating element or 'particle'.

It is not a wave, and not executing wave motion.

 

Now suppose we have a bunch of similar weights on springs.

 

They could be made to bounce up and down completely randomly, with no connection to each other.

 

That is still not a wave.

 

But suppose that they have a mechanical connection so each weight will nudge its neighbour and impart some of its motion.

 

A wave is then formed that passes down the line of weights as the motion is handed on from weight to weight, like a chain of buckets.

 

But the weights themselves do not move along the line.

 

I can draw a sketch later if that helps.

[Rahul08]

 

 

1. Heat. Higher temperatures make all other forms of matter speed up, I don't see why light would be an exception. I'm not sure whether this has been tested, but logically there should be some kind of difference in speed between the light being released from an immensely hot light source (let's say a star) and the light lost out in freezing space. Furthermore, what state of matter is light exactly? Plasma?

I agree with you that high temperatures can increase the kinetic energy of a matter. But one should not forget the basic fact that light is not a matter, but its just a form of energy. So it can only be converted into one form to another, such as heat.

According to laws of physics, Speed of light remains constant irrespective of the speed of source. One would observe the speed of light to be the same at all state (motion or rest). The speed of light only changes when it enters from one medium to another of different optical densities, a phenomenon called as refraction.

 

 

 

2. Distance. I know this is wrong, but I don't understand how waves (or wave-like particles) maintain 100% of their energy when crossing the absolute emptiness of space. Movement is energy, including wave-like movement, and therefore some of that energy should be expended. Especially in the case of mass, when it is solid, physical matter that must move back and forth. There is something going on here that I am not aware of, I'm sure. But I felt like I should mention it.

Now, this question can be answered from one of the points from above. As I have said, light consists of energy. I agree with you that some of energy is required to move a mass object. But, light particles, called as photons are massless (m=0). Then how would you expect to expend its own energy for movement.

 

 

 

3. Force. As with any object, when light hits something, some of its energy should be transferred into the other object. According to this, light should slow down a bit each time it hits something, for instance an atmospheric molecule.

Again, you are right. Yes, it is true that when light hits an object it transfers some of its energy to the sub-atomic particles present in the object. We can take example of the compton scattering effect. After hitting a charged particle, say electron, the energy of photon decreases and its wavelength increases ( such as those of gamma rays and x rays). I am not sure, but i think these again recombine with an other beam of light in appropriate proportion. In this way, energy will be conserved and would not contradic the law of conservation of energy.

I would prefer any expert in this forum to explain this thing more correctly.

 

 

And I'll answer your last questions soon, as its my time to lunch. So I am posting these all now

Edited April 28, 2014 by Rahul08

[swansont]

Light is, to my knowledge, made up of particles that behave in a wave-like manner (just like all matter in the universe, technically). Going by that, should it not be possible to make these particles speed up or slow down? Movement is energy, and I can think of three energetic forces that might have an impact on light:

 

1. Heat. Higher temperatures make all other forms of matter speed up, I don't see why light would be an exception. I'm not sure whether this has been tested, but logically there should be some kind of difference in speed between the light being released from an immensely hot light source (let's say a star) and the light lost out in freezing space. Furthermore, what state of matter is light exactly? Plasma?

 

The frequency spectrum (blackbody spectrum) will shift to higher values for higher temperatures. The energy of light is frequency-dependent,rather than depending on speed.

 

 

2. Distance. I know this is wrong, but I don't understand how waves (or wave-like particles) maintain 100% of their energy when crossing the absolute emptiness of space. Movement is energy, including wave-like movement, and therefore some of that energy should be expended. Especially in the case of mass, when it is solid, physical matter that must move back and forth. There is something going on here that I am not aware of, I'm sure. But I felt like I should mention it.

There's nothing to dissipate the energy. The varying E and B fields induce each other, so unless there is an interaction, there's no mechanism for energy loss.

 

3. Force. As with any object, when light hits something, some of its energy should be transferred into the other object. According to this, light should slow down a bit each time it hits something, for instance an atmospheric molecule.

 

 

This result happens — there is a phenomenon called radiation pressure. But the photon is typically absorbed, meaning it's gone. You can scatter photons off of e.g. electrons, and the resulting photon will have a lower energy. But again, this is a frequency change rather than a speed change.

 

Sorry to go off topic, and sorry for asking such a childish question, but thinking about point #3, where exactly does light go? When I turn the light off in my room, why don't latent photons keep bouncing around, at least for a little while? Why does light stop? What is the end of the life of a photon? Does it just rest on something, and if so, should this not cause them to build up over lengths of time? Furthermore, what is the structure of a photon? Just some things I'm curious about.

Light is a boson, and the numbers of them are not conserved. They can be created and destroyed in interactions.

 

The reason you don't notice the light bouncing around is that it's fast, and the things in the room aren't wonderful reflectors. If the reflectance is 80%, after 200 bounces you will have reduced it to a few 10^-20 of the original flux. 10²⁰ photons of visible light per second is around 30 watts, so you've taken 30 watts of visible light (which would require several 100 watt incandescent bulbs to produce, because they are incredibly inefficient at giving off visible light) and in 200 bounces you have ~1 photon left. If the light is going 3 meters per bounce, that takes 10 nanoseconds. The entire reduction takes 2 microseconds. That's why you don't notice it dimming. It's way too fast.

——————

 

I agree with you that high temperatures can increase the kinetic energy of a matter. But one should not forget the basic fact that light is not a matter, but its just a form of energy.

 

Light has energy. It is not, itself, energy. Energy is a property, not a substance.

[Fred Champion]

I think one of the biggest problems many people have when considering light is that light is modeled as a particle in particle physics and that particle is called a photon. Now, treating a photon as a particle may be fine in particle physics, where everything is treated as a particle (whether or not it really is), but carrying that concept outside the models of particle physics leads to confusion.

 

The term "photon" refers to a quantity of light, the smallest observable amount or "unit" of light. Why there is a smallest observable quantity of light is explained by the way we observe light. The observation is accomplished only by interaction with matter. That interaction produces a reaction, a change in the matter, when light encounters matter. We actually never observe light; we recognize a reaction.

 

Why there is a smallest reaction is probably best explained (in very general terms) by recognizing that in every sort of change some external activity provides additional excitation up to a threshold amount at which the matter reacts. (Consider increaasing pressure on the trigger of a weapon. There is no observable reaction until the pressure reaches and then just barely overcomes the resistance of the mechanism and the hammer falls.)

 

We have no way of determining whether the threshold in the matter is reached in one step or many. Instant change (a single step) may not seem reasonable, but our observations are made with matter that reacts in the same way as the matter we are observing. When it comes to determining what is real and what is the result of limited ability to observe, we are left with Hume's notion that we can know only what we experience.

[swansont]

I think one of the biggest problems many people have when considering light is that light is modeled as a particle in particle physics and that particle is called a photon. Now, treating a photon as a particle may be fine in particle physics, where everything is treated as a particle (whether or not it really is), but carrying that concept outside the models of particle physics leads to confusion.

 

 

Whether some people find it confusing is not a criterion for consideration. It's whether the model explains how nature behaves, and the answer to that is "yes". Light's energy is indeed quantized, and there are a number of experiments (and outside of the realm of particle physics) that confirm this.

[Fred Champion]

 

Whether some people find it confusing is not a criterion for consideration. It's whether the model explains how nature behaves, and the answer to that is "yes". Light's energy is indeed quantized, and there are a number of experiments (and outside of the realm of particle physics) that confirm this.

I suggest the reason we find (measure) the energy from light quantized is because the receptors we have for gathering that energy will respond to light in only a quantized manner.

 

Consider an experiment in which we have a source emit a single photon. If we set up receptors around the source, how many receptors will recieve enough energy to respond and let us determine that a photon was actually emitted?

[swansont]

I suggest the reason we find (measure) the energy from light quantized is because the receptors we have for gathering that energy will respond to light in only a quantized manner.

No, that's not the case. In e.g. the photoelectric effect (or a semiconductor with its bandgap) there is an energy threshold, but above the threshold the response is continuous. But even that is evidence that the energy is quantized in the photons, because the ionization/ ability does not depend on the intensity. You can have more than enough energy present and have no ionization if the frequency is too small, but have plenty of ionization even with lower intensity if the frequency is above the ionization frequency.

 

If that's solely due to the atom's response, you have the large task ahead of you of re-forming atomic theory.

 

Consider an experiment in which we have a source emit a single photon. If we set up receptors around the source, how many receptors will recieve enough energy to respond and let us determine that a photon was actually emitted?

 

At most, one.

[Fred Champion]

No, that's not the case. In e.g. the photoelectric effect (or a semiconductor with its bandgap) there is an energy threshold, but above the threshold the response is continuous. But even that is evidence that the energy is quantized in the photons, because the ionization/ ability does not depend on the intensity. You can have more than enough energy present and have no ionization if the frequency is too small, but have plenty of ionization even with lower intensity if the frequency is above the ionization frequency.

 

If that's solely due to the atom's response, you have the large task ahead of you of re-forming atomic theory.

 

 

At most, one.

Photoelectric effect is the greatest thing since sliced bread. Solar power and stealth technology. Can't imagine what's next.

 

Seems to me that the response being dependent on frequency (different frequencies for different materials) reinforces the idea that the measurement is receptor dependent. Do we have a basis for comparing responses with a specific frequency across multiple materials that does not provide just a null?

 

"At most one". Ask a tricky question, get a tricky answer. Conservation of energy?

[swansont]

Photoelectric effect is the greatest thing since sliced bread. Solar power and stealth technology. Can't imagine what's next.

 

Seems to me that the response being dependent on frequency (different frequencies for different materials) reinforces the idea that the measurement is receptor dependent. Do we have a basis for comparing responses with a specific frequency across multiple materials that does not provide just a null?

 

The PEE is not the effect behind solar power; the atom involved in such interactions is not ionized, just promoted to the conduction band. It's excitation, making it similar in behavior: the photon must have an energy exceeding the bandgap.

 

I did not claim there is no effect from the receptor, just that the receptor's behavior does not explain all of the effect, and you need to have quantized energy of the photon to explain it. Quantized energy of the receptor would not matter if it was simply an issue of adding the required amount of energy from a wave, but that's not what we observe. As I said, we have a situation where high intensity of low-frequency light will not cause ionization, but low intensity of high-frequency light, representing less energy, will.

 

"At most one". Ask a tricky question, get a tricky answer. Conservation of energy?

As above, conservation of energy does not explain that. Quantization of energy does.

[MigL]

Are you suggesting, Fred, that quantised measurements are solely dependant on the receptors and that the emitter produces continuous and NOT quantised values of energy ?

 

Congratulations, you've just re-introduced black body UV catastrophy and poor Max Planck is spinning in his grave.

[Fred Champion]

Are you suggesting, Fred, that quantised measurements are solely dependant on the receptors and that the emitter produces continuous and NOT quantised values of energy ?

 

Congratulations, you've just re-introduced black body UV catastrophy and poor Max Planck is spinning in his grave.

Any measurement is always dependent on the receptor to some degree. Simplest example: if we try to measure the photoelectric effect using cardboard we won't succeed even though the cardboard will give evidence of light. Less simple example: flowers present different colors. Same emissions, different receptors, different effeects. What determines the difference, the emitter or the receptor?

 

I suggest the emitter probably does produce its emissions continuously, at least over some number of those emissions. We accept that light from a particular source will have a discernible frequency. So what is it that cycles to produce the frequency? I suggest that each cycle may be thought of as a "pulse", and the individual pulses are the actual emissions.

 

My understanding of the "packets" concept is that they are thought to contain light of a certain frequency. Now, unless these packets contain more than one pulse (more than one cycle) they would be just the one pulse. If they are just one pulse, what is the notion of a packet? If they do present as a series of pulses (Do we know how many would be in one packet?), then either the emitter produces that series continuously, over some interval, or it produces a single emission that induces a cyclical response in the receptor.

 

We have to account for our perception of frequency. The doppler effect argues for frequency produced at the source. The quantized effects argue for a receptor dependent response.

[swansont]

Any measurement is always dependent on the receptor to some degree.

 

Which is not the same as saying that all measurement is dependent on the receptor. That would make the act of measurement moot. If you want to make the case that light is not quantized, you need to explain all of the effects we attribute to quantization. Such as why quantized absorbers are frequency selective, if the total energy of the light is given by the amplitude and not nhv.

[imatfaal]

Are you suggesting, Fred, that quantised measurements are solely dependant on the receptors and that the emitter produces continuous and NOT quantised values of energy ?

 

Congratulations, you've just re-introduced black body UV catastrophy and poor Max Planck is spinning in his grave.

 

"poor Max Planck is spinning in his grave" - I presume quantised in multiples of [latex]\hbar[/latex]

[Fred Champion]

I don't know whether "light" is a wave or a particle. I suspect we may never know. As far as I know we have no way of observing light being emitted nor transmitted; we observe only the effect of light encountering matter (and responding to gravity).

 

The statement that "quantized absorbers are frequency selective" certainly implies that the effect may be due to characteristics of the absorber. The observation that the effect varies from one absorber to another seems convincing does it not?

 

I did not introuduce amplitude of light into the thread. Seems to me that amplitude should be constant for a specific frequency.

[swansont]

I don't know whether "light" is a wave or a particle. I suspect we may never know. As far as I know we have no way of observing light being emitted nor transmitted; we observe only the effect of light encountering matter (and responding to gravity).

 

The statement that "quantized absorbers are frequency selective" certainly implies that the effect may be due to characteristics of the absorber. The observation that the effect varies from one absorber to another seems convincing does it not?

Not to me, and not to the physics community in general, for some of the reasons already discussed. Personal incredulity isn't a particularly effective argument.

 

I did not introuduce amplitude of light into the thread. Seems to me that amplitude should be constant for a specific frequency.

It's trivially not. I've had plenty of lasers in the lab where I can dial up or down the amplitude and keep the frequency constant; that's known because it's servo-locked to an atomic transition.

[Fred Champion]

Not to me, and not to the physics community in general, for some of the reasons already discussed. Personal incredulity isn't a particularly effective argument.

 

It's trivially not. I've had plenty of lasers in the lab where I can dial up or down the amplitude and keep the frequency constant; that's known because it's servo-locked to an atomic transition.

I am quite impressed that someone who is able to speak for the "physics community in general" is willing to converse with me. Wow. I must ask if you accept that personal credulity is "a particularly effective argument"?

 

 

Sorry about the third paragraph of post 17. Was interrupted. Couldn't continue. Did not have time to develop the thought. Should not have hit the "post" button. Yes of course amplitude is variable, at least to some extent.

 

In trying to look into the amplitude of light (and sound too), I haven't been able to find out if there are limits. Most published info I have found deals with noise as the factor limiting fidelity (in sound and light) and doesn't address physical (perhaps theoretical) limits of amplitude. Do you know if there are upper and/or lower limits on the amplitude of light?

 

The reason I ask is because I suspect the most revealing part of light amplitude will be the limits (if there are any) and they might shed some light (pun intended) on the nature of the transmission process. I am speculating only that something might be learned, not on what might be learned.

[swansont]

I am quite impressed that someone who is able to speak for the "physics community in general" is willing to converse with me. Wow. I must ask if you accept that personal credulity is "a particularly effective argument"?

Sarcasm and straw-man arguments aren't particularly effective, either. I can read textbooks, and presumably you can, too, to observe what is mainstream in physics. It should be trivial to find that this has been the accepted view in physics for quite a while, along with the theory and some of the experiments that back this up.

 

 

Sorry about the third paragraph of post 17. Was interrupted. Couldn't continue. Did not have time to develop the thought. Should not have hit the "post" button. Yes of course amplitude is variable, at least to some extent.

 

In trying to look into the amplitude of light (and sound too), I haven't been able to find out if there are limits. Most published info I have found deals with noise as the factor limiting fidelity (in sound and light) and doesn't address physical (perhaps theoretical) limits of amplitude. Do you know if there are upper and/or lower limits on the amplitude of light?

 

The reason I ask is because I suspect the most revealing part of light amplitude will be the limits (if there are any) and they might shed some light (pun intended) on the nature of the transmission process. I am speculating only that something might be learned, not on what might be learned.

This is a very different question from whether or not the amplitude can change at the same frequency. Is that an agreement on your part that it does?

 

There is no theoretical limit to amplitude; photons are bosons and you can have as many in a volume as you want. There are practical limitations, because light interacts with things, and you can't generate arbitrary amplitudes in empty space.

[Fred Champion]

I don't know much about amplitude. Don't know enough to agree or disagree about amplitude changing at a frequency. That's why I asked the question. What I've found so far seems to imply that amplitude may be more variable at lower frequencies. If that is so, then there is some reason for it.

 

Since the photoelectric effect is seen to be frequency dependent, could it be that amplitude at those frequencies is so close to being constant and is so small that it is just not significant (in terms of the energy imparted)? I believe higher amplitude is credited with greater energy transmission at the lower frequencies. I was thinking that if there is that relationship there might be limits.

 

If I read your second paragraph correctly you are saying that intensity and amplitude are the same thing (the number of photons). I didn't get that from what I've read. I thought the light in a photon has both frequency and amplitude. And I don't understand your statement about generating arbitrary amplitudes in empty space at all. Not a criticism, I just don't get it.

 

To be honest, amplitude makes sense to me only in a wave model.

[swansont]

Amplitude in the wave model is strength of the E and B fields. In the quantum picture it is the number of photons. It gets you to the same result in a lot of cases, but where it doesn't is in certain interactions where the electron is excited or ionized. There are cases where the wave model simply does not explain what is happening.

 

The issue here is that you are not just asking - you have been insisting one answer is correct, and it's not.

[Mordred]

I don't know whether "light" is a wave or a particle. I suspect we may never know. As far as I know we have no way of observing light being emitted nor transmitted; we observe only the effect of light encountering matter (and responding to gravity).

 

The statement that "quantized absorbers are frequency selective" certainly implies that the effect may be due to characteristics of the absorber. The observation that the effect varies from one absorber to another seems convincing does it not?

 

I did not introuduce amplitude of light into the thread. Seems to me that amplitude should be constant for a specific frequency.

actually we do have the technology to emit a single photon and detect a single photon, as well as entangle two photons together

 

http://www.toshiba-europe.com/research/crl/qig/quantumdots.html

http://www.toshiba-europe.com/research/crl/qig/singlephotondetection.html

http://www.toshiba-europe.com/research/crl/qig/singleelectronspingeneration.html

http://www.toshiba-europe.com/research/crl/qig/singlephotonsource.html

http://www.toshiba-europe.com/research/crl/qig/entangledled.html

 

its pretty obvious just from that alone that photons do exist, in the same way any particle exists with an energy/mass relation. Photons mediate the electromagnetic force. That includes light.

 

• Photons are the force carriers of the electromagetic force
• W and Z bosons are the force carriers which mediate the weak force
• Gluons are the fundamental force carriers underlying the strong force

in other words their interactions with their environment is how the force is carried. with photons their interactions has wave like properties. So it can be treated mathematically as both a particle and a wave

Edited May 9, 2014 by Mordred

[Fred Champion]

actually we do have the technology to emit a single photon and detect a single photon, as well as entangle two photons together

 

http://www.toshiba-europe.com/research/crl/qig/quantumdots.html

http://www.toshiba-europe.com/research/crl/qig/singlephotondetection.html

http://www.toshiba-europe.com/research/crl/qig/singleelectronspingeneration.html

http://www.toshiba-europe.com/research/crl/qig/singlephotonsource.html

http://www.toshiba-europe.com/research/crl/qig/entangledled.html

 

its pretty obvious just from that alone that photons do exist, in the same way any particle exists with an energy/mass relation. Photons mediate the electromagnetic force. That includes light.

 

• Photons are the force carriers of the electromagetic force
• W and Z bosons are the force carriers which mediate the weak force
• Gluons are the fundamental force carriers underlying the strong force

in other words their interactions with their environment is how the force is carried. with photons their interactions has wave like properties. So it can be treated mathematically as both a particle and a wave

Thanks for the links. I will study the info. Got to go back to solitary for a while. I bet Swansot will miss me.

[swansont]

I miss proper spelling of my user name even more, Ferd.