# Photons = Wavelike, Particle-like, or Both?

1. Jan 20, 2015

### teetar

I've heard that basically photons can exhibit wave-like or particle-like based on what kind of experiment you perform to prove one or the other. However, are they hence both waves and particles at the same time? I'm not sure if this is a proper question, or if I'm misunderstanding something, but I'm basing this on the fact that when exciting atoms such as the hydrogen atom, it will produce a spectrum of light. If an atom can produce a spectrum of light, then that light would have to be wave-like, right? Otherwise, it would be matter, and hence if you were to continue exciting the particle and it were to continue releasing the light from that spectrum, then shouldn't it eventually (after a long time) lose all it's mass to the photon? Thanks for any enlightenment anyone can offer me!

Edit: Mistyped Photon as Proton. Thanks, Berkeman!

Last edited: Jan 20, 2015
2. Jan 20, 2015

### PWiz

Light exhibits wave-particle duality, so if you consider it as photons, then you're considering its "particle" aspect. And because photons have 0 invariant mass (since they are a type of luxon), the object emitting them does not lose mass (but it does lose energy).

3. Jan 20, 2015

### teetar

Alright, but in order for something to be a particle, doesn't it have to have mass? Or is that matter? (On a side note, are particles and matter the same thing?). Sorry to be asking so many questions, I seem to have a very limited understanding of the subject as of right now. I'm going to have to look into what invariant mass is, and what luxons are. Anyways, thanks for your answer! It would seem that I was thinking of it from the wrong perspective to start.

4. Jan 20, 2015

### Raghav Gupta

Photons have mass. But there relativistic mass is zero with respect to us.
Have you studied Einstein's theory of relativity?
There is a formula there $M=M_o\sqrt{(1-v^2/c^2)}$
As photons have v=c therefore M=0.

5. Jan 20, 2015

### PWiz

Matter - anything that occupies space and has mass.
Particle - any object with attributable properties.

It would help to think of mass as a property of an object.
Photons have 0 rest mass, that's why they travel at the speed of light. If they had any invariant mass at all, they would just approach the speed of light, but never attain it. V=c because $M_0=0$, not the other way around (as you've posted).

Last edited: Jan 20, 2015
6. Jan 20, 2015

### Raghav Gupta

Yeah I have made mistakes. In place of where I have written M should be M0 and in place of M0 should be M. Yes then rest mass is 0. Agree with you PWiz.
Actually I have read an article and found it interesting.
This would explain some other concepts also to you Teetar.

Last edited by a moderator: May 7, 2017
7. Jan 20, 2015

### Raghav Gupta

But that is interesting, how come I not see your sentence @PWiz in your post "V=c because M0=0, not the other way round(as you've posted)." But when I have quoted you I can see it.
Have you used some Ninja techniques as depicted by your pic of Naruto and Sasuke?

8. Jan 20, 2015

### teetar

Thanks for this formula! I haven't had any formal studying on Einstein's theory of relativity yet, and I'm not sure how in depth my current physics course may go into it. I hate to bug you, but do you know of any websites/resources where one can learn more about the theory of relativity? Thanks!

9. Jan 20, 2015

### PWiz

@Raghav Gupta The post and the quote are the same (no ninjutsu application here). Perhaps the latex hasn't loaded on your page yet?

10. Jan 20, 2015

### Raghav Gupta

Yeah I can see now. I see you may have edited and that time lapse might be the factor.

11. Jan 20, 2015

### Raghav Gupta

See these interacting videos
The guy Doc Schuster really explains in a nice and fun way.

12. Jan 20, 2015

### teetar

Thank you, @PWiz and @Raghav Gupta! Your help has been fantastic. Thanks for the explanations, the formulas, and the links. I'll have to look into it all much more, but this has shown me at least what I should start looking into. Thank you very much!

13. Jan 20, 2015

### Raghav Gupta

This is the first time I have helped a member of PF , otherwise I usually ask questions.
Yippee this is my century of messages I have made.
I suggest @PWiz also to make a century here as he has also made 99 messages I see.
You really have some type of ninjutsu involved PWiz as I can't track you what thread you were seeing etc. :)

Last edited: Jan 20, 2015
14. Jan 20, 2015

### Staff: Mentor

This is a common question here at PF and one that requires a little delicacy when thinking about. First and foremost, you have to understand that the primary theory dealing with photons is called Quantum Electrodynamics (QED). From wiki: http://en.wikipedia.org/wiki/Quantum_electrodynamics

(QED) is the relativistic quantum field theory of electrodynamics. In essence, it describes how light and matter interact and is the first theory where full agreement between quantum mechanics and special relativity is achieved.QED mathematically describes all phenomena involving electrically charged particles interacting by means of exchange of photons and represents the quantum counterpart of classical electromagnetism giving a complete account of matter and light interaction.

QED has a very particular way of explaining things that you've probably never heard of before. It is a type of theory known as a Quantum Field Theory, meaning that it explains all matter and light as being excitations of an underlying field. Photons are excitations of the electromagnetic field, electrons are excitations of an electron field, etc. Within QED, there is no separation between the particle-like and wave-like properties. By this I mean that QED does not say that something acts like a particle sometimes and acts like a wave at other times. Nor does it does say that an object is both a particle and a wave at the same time. The particle-like properties (definite position upon measurement, quantized energy, etc) are fully described by saying that the particle is an excitation of the field. The wave-like properties (self-interference, wavelength, etc) are also fully described at the same time. You can say that in QED, objects are explained as being quantum objects with properties that don't correspond to normal, everyday language and intuition, such as being able to have both wave-like and particle-like properties. So, is a photon a wave, a particle, or both? The answer is that it is none of those things. It is a quantum object exhibiting both types of properties.

No. By exciting the hydrogen atom you are giving it the energy it needs to release as electromagnetic radiation. Note that Einstein's equation, E=Mc2, means that energy has mass. So when you excite the atom you give it energy and it becomes more massive as a result. When it releases this energy in the form of light, it becomes less massive since it has lost energy. In relativity, there are several different types of mass. A photon has zero rest mass, but has something else called relativistic mass. Relativistic mass takes into account the energy and momentum a system possesses, and since a photon has momentum (and energy), it has relativistic mass even though it has zero rest mass. Basically this means that if I have a box with perfects mirrors on the inside, full of light, it is more massive than the same box without the light. The relativistic mass of the photons all add up to give the system as a whole (box + light) more mass than just the box itself.

15. Jan 20, 2015

### teetar

@Drakkith thanks for this answer! This is pretty thorough! First off, I'd like to apologize for this thread then, as it sounds like I could've done some more searching and eventually found more forums relating to this. Secondly, thanks for introducing me to all these theories. I found it very enlightening to see the view on photons from the perspective that you used. Thirdly, thanks for answering my subsidiary question. I didn't even think about the relation between mass and energy at the time that I was thinking of this question. On a side note (sorry for asking even MORE questions) do you know of any good resources that explain the equivalence between mass and energy? I'm slightly confused and I've had a tough time trying to get explanations that I can understand at the moment. Thanks if you can!
Thanks a bunch, Drakkith, your post is pretty awesome!

16. Jan 20, 2015

### Staff: Mentor

The only thing I can suggest is to either do a search here on PF for the topics your interested in, or make a new thread. I don't personally have many references to good sources, especially in the areas your asking about.

17. Jan 20, 2015

### teetar

Alright, I'll look around. Thanks to everyone who was able to help me out with this!