# What really is a photon?

1. Dec 28, 2007

### Mephisto

I'm confused. It's a particle and it's also a wave; I'm actually starting to think that photon is just a distortion of spacetime or something... It's a small electro-magnetic field, where one always induces the other and thus this ever-changing electromagnetic pulse travels through space. Do physicists actually have a well accepted idea of what a photon ACTUALLY is?

I'm also thinking that as photon comes near an electron, the way it gets absorbed would be that somehow the electric field interacts with the electrons electric field and it then pulls/pushes that electron and converts this energy stored in the photon's electric field into the kinetic energy of an electron. Or something like that. I don't know. I haven't exactly figured out emittance of a photon yet.

How right/wrong am I? What is the accepted view?

2. Dec 28, 2007

### Shackleford

I believe the simplest answer is that a photon is a quantum of energy. The wave-particle duality is just that - a mathematical duality that accurately describes the same physical object's properties, motion, and so forth. Don't think of it as being a wave but actually a particle but actually a wave...ad infinitum. I don't believe anyone knows why light behaves both as a particle and a wave.

I'm just getting back into physics, so I could be a rusty on everything. Correct me, someone, if I am mistaken!

3. Dec 29, 2007

### malawi_glenn

a photon is a photon, an electron is an electron, etc

What properties they have (particle, wave, pointlike, electric charge) etc, its the physicists role to assign, and how they interact etc.

4. Dec 29, 2007

### f95toli

As far as I know the most stringent definition is something like "an excitation of a tempo-spatial mode".
Basically, every system "contains" an infinite number of electromagnetic modes and each mode can be occupied by 0 or more excitations that we call photons
This is somewhat analogues to other phenomena in e.g. acoustics. There is an inifinte number of acoustic modes in a room, when you clap your hands some modes are excited but when the the room is silent no mode is occupied (but they are still there).

5. Dec 29, 2007

### lightarrow

I have the Copyright of that question!

6. Dec 29, 2007

### malawi_glenn

hehe yeah you do ;) Always good to search the forums for old threads before one posts. Certainty when one have questions about quite genreral and fundamental things.

7. Dec 29, 2007

### Mephisto

lightarrow i think its pretty funny that the topic name is exactly the same :D

I went through the topic, but it seems that most of the people are really quite vague in their description of a photon, as if they were afraid to say something definite about what it is. My overall impression then is that they simply don't know. They also like to describe it sometimes using math, but it's in reverse! Math should describe the photon, not the other way around.
Moreover, there is a lot of mentions of a detector, but detector has nothing to do with the nature of a photon in the physical realm... But maybe that's just because of the way you phrased the question, talking about the "revelators".

I was just trying to see what it could mean that a photon is both a wave and a particle, and how can it have 0 mass. The picture I came up with i think makes at least some sense. The best answer I saw so far, (and also the one that seems most physical) was made in your topic by hellfire:
"A photon is an excitation of the free electromagnetic field."
even though I am not exactly sure if i understand it correctly.

8. Dec 29, 2007

### malawi_glenn

Mephisto: A photon is a photon. An electron is an electron. (as far as we know today) Why must the answer be anything else?

Also, how can we find information about what an object is and behaves if we did not use detectors?...

"A photon is an excitation of the free electromagnetic field." Is an abstract mathematical approach to it.

9. Dec 29, 2007

### Bright

If you would ask question: "What my gransma watch is?", you could find one or another answer.

Now, you are looking for a SIMILAR answer to the question "What photon is?"

Unfortunately, there is NO SIMILAR answer. Because in terms of existing physics photon cannot be represented as an entity, consisting of a simpler and easy to understand parts.

In General Physics there is ONE concept of photon.
In Nonrelativistic QM - there is another concept.
In Quantum Fielt Theory - absolutely different from both above.
In String theory - something else.
In Final Theory the photon is just some garbage, when we compare for example photon and electron, which is not garbage.

So, up to you...

10. Dec 29, 2007

### Arham

According to qunatum physics every particle have wave-like properties. Photon is a particle (like electron and ...) and therefore possesses these wave-like properties. But special relativity implies that there is one difference between photon and other ususal particles: Its rest mass is zero.

I hope my english is not so bad!

11. Dec 29, 2007

### Bright

I have NO IDEA what is rest mass of photon, because photon never stops.
However, if photon travels, at speed of light of course, it's mass is not zero already.

Its energy = hw
according to equivalence principle,

hw = m*c^2

So, photon equivalent mass should be

m = hwc^-2

12. Dec 29, 2007

### Arham

Yes. Its relativistic mass is not zero. But according to m = m0 / sqrt(1-v^2/c^2), m0 must be 0, since v = c.

13. Dec 29, 2007

### malawi_glenn

Bright : now you are just talking rubbish.

14. Dec 29, 2007

### Bright

This relationship is good only for v < c

If v = c, it does not have any sence.
Of course you may try introduce any kind of sence to it even if v = c
But, this is just your (and others) misunderstanding... an attempt introduce any sence, when actually there is no sence...

15. Dec 29, 2007

### Bright

Which part of my posts is rubbish?

P.S. Or you think all of them? )

16. Dec 29, 2007

### Arham

Bright: It's simple. Suppose we have a particle of rest mass m0 which is not equal to zero. This particle can't reach to velocity of light, because to do this we must give it energy infinitley. So the m0 of photon must be zero.

Last edited: Dec 29, 2007
17. Dec 29, 2007

### peter0302

Whether a photon has "rest mass" is irrelevant since the photon is never "at rest."

But as Bright's "rubbish," points out the photon does indeed have energy, and therefore momentum, which gives it many of the properties of a massive particle. The question becomes how you define "mass." If mass is energy that obeys inertia, then the photon obviously doesn't have mass. But if mass is simply localized energy, then the photon does indeed have mass.

Here's a more important question! Does light have gravity?

18. Dec 29, 2007

### Staff: Mentor

Ahem. Real Physicists (TM) call it invariant mass and calculate it using

$$(mc^2)^2 = E^2 - (pc)^2$$

which gives a well-defined value of zero for a real (as opposed to a virtual) photon, for which E = pc.

19. Dec 29, 2007

### Mephisto

i appreciate you trying to help me with this, but your answers are probably the most cryptic of them all... I'm asking what a photon is, and you are telling me that a photon is a photon. What kind of an answer is that?! What am I supposed to get from that?

I guess maybe I should rephrase my question... I don't have a clear view right now of the difference between normal matter and a photon... I would be ok with just thinking that photon is a packet of energy, but it seems there is more to it!
Why do they say that light is electromagnetic radiation? Light is made up of photons. So surely photons have to have something to do with electro-magnetism.

20. Dec 29, 2007

### quantumfireball

An harmonic oscillator of frequency v,excited to an energy level n is equivalent to n photons.