Can A White Photon Exist?not so easy to Answer

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In summary, White light is composed of different colors of light. When looking at a white light source, you would see photons of all different colors, as all those colors combined create the color white as seen by the human eye. However, there is no "white" photon, as all photons are diffracted to the same spot.
  • #1
chemistryknight
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I read this question in a book. but i think a color of a photon is an expression of it's energy . the white light is consist of 7 colours ( 7 frequencies ) . and the photon can exist in one frequency only ( energy = f * h ) .
if we have white light source extremely dim to produce one photon how could we see it ?
can anyone help me
 
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  • #2
"White" light is not monochromatic. White light is composed of at least the three primary colors of light, as defined by the color-sensitive cells in our eyes: red, green, and blue.

You cannot have a single photon of white light. If you have a very dim white light source, which produces only one photon at a time, you will never see a "white" photon. You will, however, see photons of all different colors, as all those colors combined create the color white as seen by the human eye.

- Warren
 
  • #3
chroot said:
You cannot have a single photon of white light. If you have a very dim white light source, which produces only one photon at a time, you will never see a "white" photon. You will, however, see photons of all different colors, as all those colors combined create the color white as seen by the human eye.

Well, what about a superposition of 1-photon fock states with different momenta ? This is not the same as an n-photon state.

|psi> = Integral dk g(k) |k>

There is of course a semantic issue if we call "|psi>" a one-photon state...


EDIT: also, there's a difference with "white" light as seen as a thermal _mixture_ of one-photon states |k> of different k, which is not a pure state but an ensemble described by a density matrix.


cheers,
Patrick.
 
Last edited:
  • #4
The problem is, a single foton should be detected in three different eye detectors to give white. On other hand, a single stimulation of a non-colour detector in the eye should be possible, then producing sort of grey.
 
  • #5
Hi chemistryknight,

I would take the one-photon light source and let the photons go through
a prism, then I would look at how the photons are diffracted.
If the white light consists of different photons, then
you would get dots, that are spread due to dispersion.

-Edgardo
 
  • #6
Edgardo said:
Hi chemistryknight,

I would take the one-photon light source and let the photons go through
a prism, then I would look at how the photons are diffracted.
If the white light consists of different photons, then
you would get dots, that are spread due to dispersion.

-Edgardo

All this doesn't distinguish between a pure superposition of pure-momentum 1-photon states and a mixture, because the measurement apparatus (the eye, or the prism with position detector etc...) measures pure momentum states.

cheers,
Patrick.
 
  • #7
First of all,

yeah, understand what you say vanesch.
I personally thought that a white source consists of photons, whose state is already determined before measurement (more like in classical physics).

In your opinion, a white light source-photon-state is a superposition of all frequency-states,
like you said with the integral? Could also be. But it's quite interesting, if you think
of the (rather naive?) picture of an electron "jumping to another shell" to make the transition from E1 to E2. This jump in the atom is not determined until you measure
the energy of the photon.

Secondly,

I understood chemistryknight's question in another way, namely, if there's
a white photon, that means, if white is a colour. If so, then all photons would
be diffracted to the same spot.
 
  • #8
Edgardo said:
Secondly,

I understood chemistryknight's question in another way, namely, if there's
a white photon, that means, if white is a colour. If so, then all photons would
be diffracted to the same spot.

Then the question is moot. If there IS a "white" photon, then it means that there is a SINGLE WAVELENGTH associated with the color "white" that our eyes perceive. This would be no different than a "red" photon, a "blue" photon, etc... i.e. all the colors that can be associated with a single, monochromatic wavelength. If that's a case, a white photon is of no mystery and this question would not have been asked.

But no matter how much we want to force is, there is no "white" photon. We should not let our eyes (which is a very limited and easily unreliable detector) be the primary source of things like this.

Zz.
 
  • #9
Edgardo said:
First of all,

yeah, understand what you say vanesch.
I personally thought that a white source consists of photons, whose state is already determined before measurement (more like in classical physics).

That is a possibility: it is a statistical mixture of "pure color" photons. That is not a pure quantum state, but an ensemble described by a density matrix. It is probably the best description for "sunlight".

In your opinion, a white light source-photon-state is a superposition of all frequency-states,
like you said with the integral?

No, it is probably a state which is much harder to make, but it is what I thought came closest to "a white photon". In fact, the perfect "white photon" would be by a non-destructive POSITION measurement. But I don't know how to do that with photons. It is easier with other particles, such as electrons. Indeed, a position state is a superposition of many momentum states.

cheers,
Patrick.
 
  • #10
Even if we did have a superposition of different energy states, the photon would still not be white. What we consider "white" is merely an interpretation of our brain, when neighbouring retinal cells receive photons of varying energies; and hence there is nothing physical about the colour white.

When a photon is detected by the retina, no matter what it's state is, it will end up in an energy eigenstate (as the cell will perform an energy measurement) and hence end up with a definite energy and a definite frequency (assuming nondegenerate eigenstates).

There is no such thing as a white photon. We can have a superposition of different energies, but that doesn't make the photon white.

vanesch said:
Indeed, a position state is a superposition of many momentum states.

Yes. Any state is generally a superposition of the eigenstates of some operator (that is what we mean by a complete set of states).
 
  • #11
It seems to me that the question can mean one of the following two :

1. Can a single photon excite multiple color sensors in our eye such that we see "white"?

or

2. Can the incertainty of a single photon's frequency cover the entire visible spectrum?
 
  • #12
Gonzolo said:
It seems to me that the question can mean one of the following two :

1. Can a single photon excite multiple color sensors in our eye such that we see "white"?

or

2. Can the incertainty of a single photon's frequency cover the entire visible spectrum?

If it is the 2nd one, then it is a complete misunderstanding of the uncertainty principle. It isn't our inability to accurately measure a single photon's frequency. It is our ability to know what the next one, and the next one, and the next one, etc. is going to be. That is what is contained in the uncertainty principle. The width of a spectrum peak is meaningless when one measures just a single photon. One can obtain this with arbitrary precision not limited by the uncertainty principle. It is the collection of identically-prepared photons that would contribute to the line width of the spectrum. I believe I have tried to explain this in my application of the uncertainty principle in the case of a single-slit measurement.

Zz.
 
  • #13
masudr said:
Even if we did have a superposition of different energy states, the photon would still not be white. What we consider "white" is merely an interpretation of our brain, when neighbouring retinal cells receive photons of varying energies; and hence there is nothing physical about the colour white.

Ah, well. I thought as a way of talking, "white" meant "contains all wavelengths with equal intensity/amplitude..."

You have white noise, pink noise and red noise in electronics.

We have beams of white neutrons or X-rays and also monochromatic beams.
...

cheers,
Patrick.
 
  • #14
I can only echo Zz's message about the HUP. It is not to do with the position (or momentum or energy) of a single particle. It is to do with the variance these things. And that requires repeated measurements of identical systems.

1. Can a single photon excite multiple color sensors in our eye such that we see "white"?

No. I believe the retinal cell absorbs the photon once detected. So a single photon will never be seen as white. Anyway, it requires about ten photons for a cell to register the light.
 
  • #15
The retina has several different opsin molecules, three types for the average man, and four for the average woman. Each type responds, by flexing, to a particular narrow band of frequencies. The photon interacts with the molecule, raising it to a higher energy state, which in the case of opsin, causes it to flex. So the number of opsin molecules flexing at a particular time is a (distorted) measure of the photon flux at those energies. The flexing causes a neural pulse to be sent to the visual cortex in the back of the brain. This is the ONLY information the brain receives about the frequency distribution of the ambient light. All our color sensations are constructed in the brain from this information.

The question is then, could a single photon interact with more than one opsin molecule.
 
  • #16
Thanks selfAdjoint for your knowledge on this. Well it's clearly obvious that once the opsin molecule has been raised to a higher energy state, the original photon is gone from the universe never to come back. Of course when the opsin molecule drops to a lower energy state, it will emit radiation.

In any case, the original question of there being a white photon has been resolved long ago. No photon can be white. A photon could (possibly) excite several different opsin molecules by the process of absorption and emission. To determine these frequencies we will need an approximate solution to the opsin molecule. And so, it may well be possible that a single photon could cause the brain to register the colour white.

So we have the following conclusion: A photon can be in a superposition of different energy states that correspond to "white"; when detected by the human optical system, it will collapse into a definite energy eigenstate. However, by the process of absorption and reabsorption we could register other colours such that we perceive white.

I'm guessing that the human optical system is designed to ignore the emission bands of opsin. Or maybe we have gotten used to seeing colours like this, so we call the excitations caused by a "blue" photon blue only because the colour we see when a photon of such energy enters our eye is a actually a combination of all the secondary effects of opsin emission.
 
  • #17
The secondary layer of the visual cortex does subtractions on the electrical impulses coming from the opsin molecules. The source type of the impulses is known (which axon they came in on) and coded differences are generated and passed on to the deeper layers.

BTW, I don't think White exists outside human brains. I don't think any color does. Just bouncing photons of different frequencies.
 
  • #18
Yes I agree with what you say 100%. White is just merely an interpretation of the frequency/energy of incoming photons.
 
  • #19
When we say that white light (as we see it) is the superposition of many photons, how exactly is this wave and frequency described?
 
  • #20
White light is not the superposition of many photons. We perceive white light when neighbouring retinal cells register photons of colours from across the spectrum.

When a photon is in a superposition of many states (nothing to do with white photons) it doesn't have a single frequency etc. Upon measurement, it drops into an eigenstate which has a defined energy/frequency.
 
  • #21
masudr said:
White light is not the superposition of many photons. We perceive white light when neighbouring retinal cells register photons of colours from across the spectrum.

When a photon is in a superposition of many states (nothing to do with white photons) it doesn't have a single frequency etc. Upon measurement, it drops into an eigenstate which has a defined energy/frequency.

When measuring in a certain basis (here, in the "color" or photon momentum basis) you cannot distinguish between a superposition and a statistical mixture, because Born's rule transforms the former in the latter.
You would have to change measurement basis in order to find out the difference. So as long as you stick to a human eye, which measures "color", you won't see any difference.

But you can easily conceive photons in a superposition of momentum states, and I don't see why we can't call them 'white photons'.
I'm wondering (I'm not sure) if ultra-short pulse lasers don't produce photons in a superposition of momentum states instead of a "mixture". In fact, I think they do, because of their timing: because you have ultra-short pulses in time, you know their position in time (for a given value of t, you know pretty well where they are along the beam) and the only way to do so is to have a *superposition* of momentum states. But ok, they are only 'white' over a very limited bandwidth, probably not the bandwidth of white light.

cheers,
Patrick.
 
  • #22
Patrick, I completely agree with you. Apart from being easier to say, I don't think there is any reason why we should call a photon white for being in a superposition of states, since we do not perceive that as white. On the other hand a statistical mixture is perceived as white, but that requires several photons to be registered as white so we can't call any single photon white.

Why are you trying to ascribe a colour to a quantum particle?
 
  • #23
masudr said:
Why are you trying to ascribe a colour to a quantum particle?

I'm probably a lunatic, but the first reaction I have to "white" is not to associate it with something visual, but to "flat spectrum" and "all momenta".

As I said before, "white" noise, "white" neutron beams, pre-whitening filter,...

:bugeye:
Patrick.
 
  • #24
vanesch said:
When measuring in a certain basis (here, in the "color" or photon momentum basis) you cannot distinguish between a superposition and a statistical mixture, because Born's rule transforms the former in the latter.
You would have to change measurement basis in order to find out the difference. So as long as you stick to a human eye, which measures "color", you won't see any difference.

Interesting related news:

Optics enters the single-cycle regime

http://physicsweb.org/articles/news/9/2/4/1
http://physicsweb.org/articles/news/9/2/4/1/050204

"Since the duration is extremely short, the pulse contained wavelengths
between 410 and 1560 nanometres - a range of 1.9 octaves"


Regards, Hans
 
  • #25
Hans de Vries said:
"Since the duration is extremely short, the pulse contained wavelengths
between 410 and 1560 nanometres - a range of 1.9 octaves"

Wow ! :bugeye:

Patrick
 
  • #26
vanesch said:
I'm probably a lunatic, but the first reaction I have to "white" is not to associate it with something visual, but to "flat spectrum" and "all momenta".

So if i said a white proton, you would think "superposition of various momentum eigenstates"? You are a lunatic! :tongue: No, to be fair we all are, especially if we study physics.
 
  • #27
masudr said:
So if i said a white proton, you would think "superposition of various momentum eigenstates"? You are a lunatic! :tongue: No, to be fair we all are, especially if we study physics.

If you said a white proton, I would be in doubt between:

- indeed a superposition of momentum eigenstates
- or the QCD meaning as a singlet state of the color charge

Whoops ! The men in white coats are coming :rofl: :rofl:

:bugeye:
Patrick.
 
  • #28
vanesch said:
or the QCD meaning as a singlet state of the color charge


:smile: I should have expected that one...
 
  • #29
vanesch said:
When measuring in a certain basis (here, in the "color" or photon momentum basis) you cannot distinguish between a superposition and a statistical mixture, because Born's rule transforms the former in the latter.
You would have to change measurement basis in order to find out the difference. So as long as you stick to a human eye, which measures "color", you won't see any difference.

But you can easily conceive photons in a superposition of momentum states, and I don't see why we can't call them 'white photons'.
I'm wondering (I'm not sure) if ultra-short pulse lasers don't produce photons in a superposition of momentum states instead of a "mixture". In fact, I think they do, because of their timing: because you have ultra-short pulses in time, you know their position in time (for a given value of t, you know pretty well where they are along the beam) and the only way to do so is to have a *superposition* of momentum states. But ok, they are only 'white' over a very limited bandwidth, probably not the bandwidth of white light.

cheers,
Patrick.


But can the photon, however configured, interact with more than one retinal cone? The opsin molecule in that cone is only capable of telling the brain "I was excited by a wavelength somewhere in a distribution about X", where X = 556 nm, 535 nm, or 44 nm, depending on the molecule. It's like putting a complex frequency pattern through a band-pass filter. See http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/V/Vision.html , for example. This signal is then input to the visual cortex which does a lot of data processing on it befor the color sensation is generated. So it appears to me that one cole can't generate the sensation white.

As for thinking of a photon in a superposition of frequencies as white, surely Patrick, that is an abus de langage caused by ignoring the complex relation between the physical observable frequency (or wavelength) and the living sensation color.
 
  • #30
Yes, sA, that's exactly what I was trying to say. We ruled out secondary re-emissioin effects before, and so it's pretty clear that one photon can't be seen as white.

And again, I agree, that otherwise calling a photon white is just a case of language; the term could be accepted, if by common usage, as long as people knew that we didn't mean the photon would look white one absorbed by the retinal cones. But such an interpretation will ulikely become common usage, and hence we can't call objects white simply because they exist in a superposition of various momentum states.
 
  • #31
selfAdjoint said:
But can the photon, however configured, interact with more than one retinal cone? The opsin molecule in that cone is only capable of telling the brain "I was excited by a wavelength somewhere in a distribution about X", where X = 556 nm, 535 nm, or 44 nm, depending on the molecule.

There are two possible responses to this, according to one's view on QM.

The first one ("traditional") is: if you consider that this interaction is a *measurement* then the answer is of course no. You apply the Born rule to your photon superposition in the momentum basis, and thus you pick out a probability for each of those components to be chosen.

My pet interpretation, however, which is more MWI-like is:
This interaction of the EM quantum field and those opsine molecules is just a unitary process described in QED language. This then means that you simply entangle the different states of the different opsine molecules with the photon superposition. This is what a quantum chemist should tell you, after he has written out his hamiltonian of the interaction of photons with his molecules.
So yes, this then entangles with different nerve states (K/Na balances in superposition) etc... until your consciousness has to make the ultimate choice, using the Born rule, whatever that may mean. Upon making this choice, you can then track back that this came down to exactly one excitation of one molecule (in the chosen branch).

In practice however, there is of course no observational difference because all these entanglements are so hopelessly complicated that we will never be able to use them to perform interference experiments, discriminating between superpositions and stochastical mixtures.

So, if our eye were sensitive to a single-photon impact, you would get A SINGLE color sensation, red, green or blue, upon the impact of a white photon.

But as our real eye needs several photons in order to do so, and as each one will give rise to a different sensation, this will probably never be experienced.

A single photon: you don't see anything (because we need more)
Several photons: we will get stimuli from different types of molecules and experience "white".

cheers,
Patrick.
 
  • #32
vanesch said:
There are two possible responses to this, according to one's view on QM.

The first one ("traditional") is: if you consider that this interaction is a *measurement* then the answer is of course no. You apply the Born rule to your photon superposition in the momentum basis, and thus you pick out a probability for each of those components to be chosen.

My pet interpretation, however, which is more MWI-like is:
This interaction of the EM quantum field and those opsine molecules is just a unitary process described in QED language. This then means that you simply entangle the different states of the different opsine molecules with the photon superposition. This is what a quantum chemist should tell you, after he has written out his hamiltonian of the interaction of photons with his molecules.
So yes, this then entangles with different nerve states (K/Na balances in superposition) etc... until your consciousness has to make the ultimate choice, using the Born rule, whatever that may mean. Upon making this choice, you can then track back that this came down to exactly one excitation of one molecule (in the chosen branch).

In practice however, there is of course no observational difference because all these entanglements are so hopelessly complicated that we will never be able to use them to perform interference experiments, discriminating between superpositions and stochastical mixtures.

So, if our eye were sensitive to a single-photon impact, you would get A SINGLE color sensation, red, green or blue, upon the impact of a white photon.

But as our real eye needs several photons in order to do so, and as each one will give rise to a different sensation, this will probably never be experienced.

A single photon: you don't see anything (because we need more)
Several photons: we will get stimuli from different types of molecules and experience "white".

cheers,
Patrick.

The issue here is whether a photon can be in a superposition of several different wavelengths, at least within the setting of this problem. I haven't seen any convincing argument that it can!

Thus, if we go by this, a "white light" isn't a superposition of a photon having different wavelengths, but rather a composition of photons, each having a spectrum of wavelengths. What's my justification for this? Look at this light using a spectrometer. You'll see a band of wavelengths, not one or the other, or the other, or the other... So the question of whether a single photon is responsible for the excitation of all our optic nerves is moot.

Zz.
 
  • #33
ZapperZ said:
Thus, if we go by this, a "white light" isn't a superposition of a photon having different wavelengths, but rather a composition of photons, each having a spectrum of wavelengths. What's my justification for this? Look at this light using a spectrometer. You'll see a band of wavelengths, not one or the other, or the other, or the other... So the question of whether a single photon is responsible for the excitation of all our optic nerves is moot.

You're right (I already said this before) that something like _sunlight_ is just a statistical mixture. But have a look at ultra-short laser pulses. I do not see how you can get around having them in a superposition of momentum states in order to make these short wave packets.
Such light, when NOT analysed in the time domain, but by spectral analysis, should give you identical results as with "thermal" white light.

cheers,
Patrick.
 
  • #34
vanesch said:
You're right (I already said this before) that something like _sunlight_ is just a statistical mixture. But have a look at ultra-short laser pulses. I do not see how you can get around having them in a superposition of momentum states in order to make these short wave packets.
Such light, when NOT analysed in the time domain, but by spectral analysis, should give you identical results as with "thermal" white light.

cheers,
Patrick.

But to be able to do a "spectral analysis", you need more than just ONE photon. In fact, you need a conglomorate of photons in a single pulse. How you truncate the pulse in the time domain dictates the Fourier components within that pulse. This is something that I've had to deal with since I work with 8 ps laser pulses that impinges on a photocathode.

But it still doesn't mean that a single photon has that superposition. Single photon sources do not operate via such truncation in time.

Zz.
 
  • #35
ZapperZ said:
But to be able to do a "spectral analysis", you need more than just ONE photon. In fact, you need a conglomorate of photons in a single pulse.

We both agree that using spectral analysis on a population cannot distinguish between a statistical mixture of "single color" photons and pure state "superpositions of different wavelength photons". They give you both a broad spectrum.

How you truncate the pulse in the time domain dictates the Fourier components within that pulse. This is something that I've had to deal with since I work with 8 ps laser pulses that impinges on a photocathode.

What do you understand by "the Fourier components of that pulse" else than a superposition of momentum states (which are nothing else but these Fourier components) ?

If you had an incoherent mixture of individual photons of different wavelengths, and not a single coherent superposition, you wouldn't have the correct phase relationships in order to get a sharp pulse in time, but you'd just have white noise (white thermal light).

cheers,
Patrick.
 
<h2>1. Can a white photon exist?</h2><p>The concept of a "white photon" is not well-defined in physics. Photons are particles of electromagnetic radiation that have a range of energies and frequencies, but the color white is a perception based on the combination of different wavelengths of light. So, while a photon can have a wide range of colors, it cannot be considered "white" in the traditional sense.</p><h2>2. How is the color of a photon determined?</h2><p>The color of a photon is determined by its frequency, which is related to its energy. Higher frequency photons have shorter wavelengths and are perceived as having a bluer color, while lower frequency photons have longer wavelengths and are perceived as having a redder color. The entire spectrum of colors, including white, can be created by combining different frequencies of photons.</p><h2>3. Can white light be made up of only one type of photon?</h2><p>No, white light is made up of a mixture of photons with different frequencies. In fact, a single photon only has one specific frequency and therefore one specific color. White light is typically a combination of many different colors of photons, such as those found in sunlight or produced by a light bulb.</p><h2>4. Is it possible to create a white photon?</h2><p>As mentioned before, the concept of a "white photon" is not well-defined in physics. However, it is possible to create a beam of light that appears white to our eyes by combining photons of different colors. This can be achieved through a process called color mixing, where different colored lights are combined to create the perception of white light.</p><h2>5. Are there any real-life applications for white photons?</h2><p>Since white photons are not a well-defined concept, there are no specific applications for them. However, the ability to create white light using a combination of different colored photons has many practical applications, such as in lighting, photography, and display technologies. Additionally, understanding the properties of photons and their interactions is crucial in fields such as quantum mechanics and particle physics.</p>

1. Can a white photon exist?

The concept of a "white photon" is not well-defined in physics. Photons are particles of electromagnetic radiation that have a range of energies and frequencies, but the color white is a perception based on the combination of different wavelengths of light. So, while a photon can have a wide range of colors, it cannot be considered "white" in the traditional sense.

2. How is the color of a photon determined?

The color of a photon is determined by its frequency, which is related to its energy. Higher frequency photons have shorter wavelengths and are perceived as having a bluer color, while lower frequency photons have longer wavelengths and are perceived as having a redder color. The entire spectrum of colors, including white, can be created by combining different frequencies of photons.

3. Can white light be made up of only one type of photon?

No, white light is made up of a mixture of photons with different frequencies. In fact, a single photon only has one specific frequency and therefore one specific color. White light is typically a combination of many different colors of photons, such as those found in sunlight or produced by a light bulb.

4. Is it possible to create a white photon?

As mentioned before, the concept of a "white photon" is not well-defined in physics. However, it is possible to create a beam of light that appears white to our eyes by combining photons of different colors. This can be achieved through a process called color mixing, where different colored lights are combined to create the perception of white light.

5. Are there any real-life applications for white photons?

Since white photons are not a well-defined concept, there are no specific applications for them. However, the ability to create white light using a combination of different colored photons has many practical applications, such as in lighting, photography, and display technologies. Additionally, understanding the properties of photons and their interactions is crucial in fields such as quantum mechanics and particle physics.

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