# Properties of photons

1. May 27, 2006

### pmerrill

A beginner question (at least not since my university physics course 20 years ago).

My understanding is thus:
• light is made of photons
• photons travel at speed c
• energy of light is related to it's colour

So what's the difference between a blue photon and a red photon at a fundamental level?

I'm familiar with e=hv, the wave/particle duality but I not looking for the equations to explain the observations but the fundamentals. How can a "particle" with no mass travelling at c be different?

2. May 27, 2006

### Mariko

I would assume if its red or redshifted its moving away from you and if its blue its heading your way.

3. May 27, 2006

A Blue Photon has more Energy than a Red Photon. Other than that, they may have different phase and polarization. Fundementally , Energy is the most obvious measurable quantity which distinguishes these two photons in particular.

4. May 27, 2006

### actionintegral

I concur with Super Nade. According to the inventor of the photon (Einstein), a photon has the property of frequency. Frequency implies that something must varying in time. The rate at which that "something" varies determines the observed "color" of the photon.

5. May 27, 2006

### Reshma

Let's view this in a simplified manner.
The relation between the frequency $f$ and the wavelength $\lambda$ is given by:
$$v=\lambda f = c$$
For photons, the velocity "c" remains constant and the frequency and the wavelength vary. The colours that we see (some we don't ) are due do this variation between the frequency and wavelength.

6. May 27, 2006

I must also add that "color" has no physically precise meaning if taken out of a Mathematical context. It is a relative term attributed to individual experience. Physically and Mathematically speaking "color" implies Energy. :)

7. May 27, 2006

### pmerrill

I might have a glimmer of hope here. So what's the "something"? I'm now wondering what the manfestation of the frequency we speak about. So now I've got two photons, one has a higher "frequency" and thus more energy, I trust that frequency does not mean vibration in the sense of a string. If people did not think that light was a wave early on, maybe this property would not have been called frequency but something else?

Do we have any idea as to what a blue photon is doing faster than a red photon at a quantum level? Are we now into string theory and the photon is made of a quantum energy string that "vibrates" at a certain "frequency"?

8. May 27, 2006

### masudr

If one solves the Schrodinger equation, given that the wavefunction is a product of a function of time co-ordinates and a function of space co-ordinates (for a single particle):

$$\hat{H}\psi\left(x,t\right) = i\hbar\frac{\partial}{\partial t}\psi\left(x,t\right) \mbox{, where } \psi\left(x,t\right)\equiv \phi\left(x\right) T\left(t\right)\,$$

one will see that the T function comes out as

$$T(t) = e^{-iEt/\hbar} \mbox{, where } \hat{H}\phi\left(x\right) = E\phi\left(x\right).$$

Here, E is the separation constant, which can be identified as energy through the Einstein-de Broglie relation $p=h/ \lambda$. (The second equation above is often elevated to being called the time independent Schrodinger equation, but note that it's nothing more than the eigenstate-eigenvalue equation for the $\hat{H}$ operator, i.e. the Hamiltonian, and is primarily useful in determining energy eigenstates which are constant in time since the Hamiltonian generates time translations).

So the thing that is happening faster at the quantum level for the blue photon is that its phase factor is changing quicker than for the red photon.

*** Digression ***

From the above, we can conclude that

$$\psi\left(x,t\right) = \sum_i \phi_i\left(x\right)e^{-iE_it/\hbar}\,$$

since the equation is linear.

Last edited: May 27, 2006
9. May 28, 2006

### HallsofIvy

Staff Emeritus
No, saying that a photon is "red" has nothing to do with whether it is "redshifted" or "blueshifted"- and if you see it, then the photon must have been headed your way!

If a photon is originally well into the "ultra-violet" and comes from a star that is moving away from you it might be "redshifted" just enough for you to see it as blue.

10. May 28, 2006

Actually, there are an excellent set of articles published in OPN Trends asking the exact same question you did. You'd be surprised to know that there is no universeally accepted answer to this question :)

http://www.osa-opn.org/abstract.cfm?URI=OPN-14-10-49

11. Jun 8, 2006

### UglyDuckling

I’m glad you said invented. I think its an invention that’s managed to survive long past its sell by date.

Although the idea of a photon seems to have captured the public imagination the actual evidence for its existence is scant! True it provides an explanation for the photoelectric effect and Compton scattering but beyond that there are many characteristic of a world where it fails to provide an explanation and in some cases the idea is indirect contradiction to the observable evidence ie interference of light.. You need to be very selective in your choice of experiment to support the idea that there is such a thing as a particle called a photon?

It provides no insight into the outcome of the Michelson Moreley experiment, totally flounders with respect to interference/diffraction and is at a loss when considering the violation of Bell’s inequality in light correlation experiments.

Physics does not have any understanding of what light is; only at set of rules for making predictions about observable outcomes for given experimental situations. And even the elements of the logic underpinning the rules seem, to contradict themselves, relative to our intuitive perception of the universe.

12. Jun 8, 2006

### Farsight

UglyDuckling: if you're saying we ought to be calling bosons something other than particles, I'd go along with that. But Physics does not have any understanding of what light is is maybe a little sweeping.

13. Jun 8, 2006

### ZapperZ

Staff Emeritus
Can you point to me where is the evidence that interference contradicts the photon picture? Are you totally ignorant of the Marcella paper in EJP in which a painful derivation of ALL interference effects, starting from the single slit, to the double slit, up to multiple slits, was derived?

Using your logic, interference effects also "contradicts" the existence of electrons, protons, neutrons, buckyballs, etc. This is nonsense.

Just because we keep using the "wave" picture to describe typical wave phenomena does NOT mean that the standard QM treatment of using photons is unable to produce the same thing. It certainly can! We don't subject students to it especially at the undergraduate level simply because it is a lot easier mathematically to use the wave picture, the same way one has to be totally insane to use relativistic equations to build a house!

There is a DIFFERENCE between claiming something to be contradictory to saying something providing "no insight", whatever that is. You seem to be confusing the two. There's no contradiction here between the MM experiment and the photon picture.

Show me something that you think you understand, and I'll show you evidence that if you dig deeper, all you have is nothing more than a basic set of DESCRIPTION. So I can play this game too. We understand it ENOUGH to make you put your LIFE and the life of your loved ones to depend on it. Don't believe me? Check and see how we verified the properties of the semiconductor in the electronics that you are using, especially next time you decide to fly in an airplane.

When one can make a reproducible prediction of the properties and behavior of something, that implies one has made an understanding of what it is. This is the criteria that we all use to claim that we "understand" something. It is bogus to claim that we know nothing of what it is even AFTER that.

Zz.

P.S. I would also strongly suggest that you do not hijack this thread into another "photon bashing" discussion. If you believe there is a "contradiction" to the current idea, submit it to the IR forum (you obviously don't care for peer-reviewed journals) and we'll handle it there per our guidelines. So don't come back and tell me you have not been notified of this.

Last edited: Jun 8, 2006
14. Jun 8, 2006

### vanesch

Staff Emeritus
That's about the correct definition of what physics (or science) is, in general. Usually it is indeed a set of rules (a paradigm) for making predictions of observable outcomes for given experimental situations, and often it is in contradiction with our intuitive perception of the universe.

15. Jun 26, 2006

### UglyDuckling

Its sometime since I read Marcella’s paper but from recollection its purpose was to demonstrate, to students, that quantum mechanics could be rigorously applied to the mediation of light. I do seem to recall, light being referred to as a free particle, However, I suspect this is merely a convenient expression drawn from popular culture and often used in text books, but not a serious attempt to describe the object/process that is mediating the exchange.

Again we are simply in the business of predicting measurable outcomes and not describing the underlying process. The methodology still contains the “apparent” logical contradictions and accommodating deterministic discontinuity inherent in quantum mechanics.

Just like the Copenhagen Convention?

I think this point is reflected in my earlier answer.

I don't recall saying that the idea of the photon and MM were contradictory

Understanding how something will behave, is different from understanding
what that something is and the processes that controls its behaviour.

Understading how something will behave under a particular set of circumstances does not imply you have made an understanding of what it is.

Understood?

16. Jun 26, 2006

### ZapperZ

Staff Emeritus
And I keep asking, what "apparent" contradictions? Remember, YOU were the one who claimed that...

Your reply above has shown nothing to that effect. I provided you with a source in which the quantum picture of light can provide EQUALLY, if not better, description of experimental observations, including a citation in a recent thread that I made on here on the interference of independent photons that according to Mandel ".. has no classical analogue.."!

So your claim that the photon picture can't explain observation is false. I can, however show you several experimental observations that the old classical picture of light cannot explain. THIS was the type of evidence that I was asking for, and the only type that I'm interested in, from you when you make such statements.

Zz.

17. Jun 26, 2006

### Staff: Mentor

Yes. The former is physics, and the latter is metaphysics or philosophy.

18. Jun 27, 2006

### UglyDuckling

Recognising the boundaries between the two is the problem?