Photon - do we need these?
08/04/08 16:53 Filed in: Physics
PHOTON
This word comes up quite often in physics. What does it mean? What do people use it for? How did this word come about? Do we EVEN NEED this word? Surprisingly, the answer to the last question is NO. You can mostly blame Einstein, but also Gilbert Lewis on the Nobel Prize committee - whoever they are called.
(the following is a summary of arguments summarized by my colleague David Norwood - so you can argue with him when you do not agree with this).
Before I go further in my summary, here is the paper from Norwood that I am basing this post on - The Use and Abuse of the “photon” in Nanomechanics (pdf)
In case you don't like to read much, here is the whole point:
This word comes up quite often in physics. What does it mean? What do people use it for? How did this word come about? Do we EVEN NEED this word? Surprisingly, the answer to the last question is NO. You can mostly blame Einstein, but also Gilbert Lewis on the Nobel Prize committee - whoever they are called.
(the following is a summary of arguments summarized by my colleague David Norwood - so you can argue with him when you do not agree with this).
Before I go further in my summary, here is the paper from Norwood that I am basing this post on - The Use and Abuse of the “photon” in Nanomechanics (pdf)
In case you don't like to read much, here is the whole point:
- Einstein's explanation of the photoelectric effect says that light is like a particle
- Gilbert Lewis comes up with the term "photon"
- Textbooks from thence forth use the concept of the particle nature of light
- It turns out that the wave nature of light explains the photoelectric effect (and many other effects that people use to proclaim photon-ness of light)
- There is perhaps a need for a quantum theory of radiation, but this is different than the particle-nature of light.
General Concept of Photon
Suppose a textbook says something about photons, what comes to most peoples' mind? It is likely something like this:
This is a typical picture you could find in a typical textbook. Here it is depicting light as little balls called photons. A more sophisticated image you will also encounter is this:
Nonetheless, in general a photon is considered to be:
Origins of the word (and concept)
Most people associate the origin of the concept of the photon comes from Einstein's explanation of the photo-electric effect. In short, the photo electric effect is the phenomena associated with the electrons being ejected from a material in the presence of light. Here is a fairly good simulation of the photo electric effect (http://phet.colorado.edu/new/simulations/sims.php?sim=Photoelectric_Effect)
Here are some other explanations of the photoelectric effect
The main point from the experiment is that the eject electrons (or as chemists like to call them - photoelectrons) have energy that depends on the frequency of the incident light. The typical conclusion is that energy of light is based on the frequency - seems obvious, doesn't it?
What did Einstein say about this? He basically says that light consists of independently moving points of quantized energy at a location in space. He did not call them photons. But clearly he said light is localized (like a particle). Next, Lewis Gilbert coined the term "photon" by saying:
"I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon."
(more evidence for this can be found at http://nobeliefs.com/photon.htm)
Then, in 1921, Einstein was awarded the Nobel Prize which explicitly mentioned the photoelectric effect (but not his awesome work on relativity).
Since then, most texts have used the idea of the photon.
So what is wrong?
The photoelectric effect does not mean that light is a particle, but rather it gives further evidence that the electronic states of matter are quantized. Well, then why does the ejected energy seem to depend on the frequency of the incident light and not the intensity of the light? This can be explained by looking a the quantum model of matter. Light incident on an electron can be considered a perturbation to its wavefunction. Only perpturbations of a particular frequency can cause the electron to transition to a new level. If you want a better explanation, ask David Norwood. (or read his paper)
What about light carrying momentum? Isn't a photon needed for that?
No. It is pretty straight forward to show that oscillating electric and magnetic fields can "transfer" both momentum and energy to a charged particle. Since most matter is made up of charged particles, this really isn't a problem.
What about (fill in some other effect usually attributed to the particle nature of light)?
Norwood addresses many of these in his paper. You should read it.
Ok fine, maybe I agree about the photon thing, but what do we do next?
The next step is to stop writing textbooks that say light is a particle and wave. As Yoda said: "you must unlearn what you have learned".
[UPDATE]: It has just been brought to my attention that if we stop using "photon" to represent the particle nature of light, we will have to find another use for this word. It does sound pretty cool.
Suppose a textbook says something about photons, what comes to most peoples' mind? It is likely something like this:
This is a typical picture you could find in a typical textbook. Here it is depicting light as little balls called photons. A more sophisticated image you will also encounter is this:
Nonetheless, in general a photon is considered to be:
- The particle incarnation of light
- Has no mass
- Travels at the speed of light
- Has momentum
- Has energy associated with its frequency
- An elementary particle responsible for the electromagnetic interaction
- Small and localized
Origins of the word (and concept)
Most people associate the origin of the concept of the photon comes from Einstein's explanation of the photo-electric effect. In short, the photo electric effect is the phenomena associated with the electrons being ejected from a material in the presence of light. Here is a fairly good simulation of the photo electric effect (http://phet.colorado.edu/new/simulations/sims.php?sim=Photoelectric_Effect)
Here are some other explanations of the photoelectric effect
The main point from the experiment is that the eject electrons (or as chemists like to call them - photoelectrons) have energy that depends on the frequency of the incident light. The typical conclusion is that energy of light is based on the frequency - seems obvious, doesn't it?
What did Einstein say about this? He basically says that light consists of independently moving points of quantized energy at a location in space. He did not call them photons. But clearly he said light is localized (like a particle). Next, Lewis Gilbert coined the term "photon" by saying:
"I therefore take the liberty of proposing for this hypothetical new atom, which is not light but plays an essential part in every process of radiation, the name photon."
(more evidence for this can be found at http://nobeliefs.com/photon.htm)
Then, in 1921, Einstein was awarded the Nobel Prize which explicitly mentioned the photoelectric effect (but not his awesome work on relativity).
Since then, most texts have used the idea of the photon.
So what is wrong?
The photoelectric effect does not mean that light is a particle, but rather it gives further evidence that the electronic states of matter are quantized. Well, then why does the ejected energy seem to depend on the frequency of the incident light and not the intensity of the light? This can be explained by looking a the quantum model of matter. Light incident on an electron can be considered a perturbation to its wavefunction. Only perpturbations of a particular frequency can cause the electron to transition to a new level. If you want a better explanation, ask David Norwood. (or read his paper)
What about light carrying momentum? Isn't a photon needed for that?
No. It is pretty straight forward to show that oscillating electric and magnetic fields can "transfer" both momentum and energy to a charged particle. Since most matter is made up of charged particles, this really isn't a problem.
What about (fill in some other effect usually attributed to the particle nature of light)?
Norwood addresses many of these in his paper. You should read it.
Ok fine, maybe I agree about the photon thing, but what do we do next?
The next step is to stop writing textbooks that say light is a particle and wave. As Yoda said: "you must unlearn what you have learned".
[UPDATE]: It has just been brought to my attention that if we stop using "photon" to represent the particle nature of light, we will have to find another use for this word. It does sound pretty cool.
